Session: New Approaches to Geophysical Research Using Dense Mixed Sensor and Broadband Seismology Arrays [Poster]

Type: Poster

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

Geophysical surveying is one of the Earth science disciplines which is used for exploration of geothermal energy. Magnetotelluric (MT) survey was conducted in Ashute Geothermal prospect area acquiring 28 soundings. North-South of the major hydrothermal manifestations in the Ashute field and to make the suggest cross-section maps and 1D inversion model, to study the subsurface resistivity structure of the geothermal system at the Ashute, Butajira field, its relation to the geothermal field marked Aligned volcanic cones and the group of thermal spring in the axial part of Debre zait Selti graven suggest the main NE – SW Debre zait Selti fault system and transverse NW – SE older rift structure beyond to control Magmatism (heat source), water recharge and permeability of the system. The trend of high and low resistivity anomaly and the discontinuities (NE-SW alignment) structure and transverse NW-SE faults and opening fissures, that indicated the possibility of the connection of the Ashute, Butajira geothermal systems. In Addition to this MT soundings towards NE-SW Ashute geothermal field and BTJ-068 in the Ashute field and reinterpretation of result using TEM data were recommended for a better definition of the anomalies and achieve the stated objectives.

Presenting Author: Tsegaye W. T. Tadesse

Additional Authors

Session: Numerical Modeling of Earthquake Ground Motion, Seismic Noise, Rupture Dynamics and Seismic Wave Propagation [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

We study the 3D nonlinear basin effects in Kathmandu Valley, Nepal during the 2015 Mw 7.8 Gorkha earthquake using a hybrid broadband ground motion simulation approach. We are particularly interested in understanding the source of frequency and amplitude characteristic, and the reasons they pertain to the lack of high frequency components. In order to do that, we couple a kinematic source model with a realistic 3D geometry of Kathmandu basin, which accounts for nonlinear behavior in unconsolidated sediments, to capture low frequency component of ground motion. Moreover, a stochastic approach is used to include the high frequency portion of ground motion. We finally combine the resultant time-histories in corresponding frequency ranges to investigate the behavior of Kathmandu basin during the mainshock. Despite the scarcity of material properties, stratigraphy information, and geotechnical databases, our results show that our model significantly improves ground motion prediction compared to rudimentary 1D and 2D basin analyses. Our goal is to use the validated model to derive a synthetic Ground Motion Prediction Equation (GMPE) for the capital and the most populated region of Nepal.

Presenting Author: Peyman Ayoubi

Additional Authors

Session: State of Stress and Strain in the Crust and Implications for Fault Slip Based on Observational, Numerical and Experimental Analysis [Poster]

Type: Poster

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

Mapping the contemporary stress field orientation in the Mediterranean can provide fundamental insights on the complexity of plate tectonic forces in this region at different depths. Despite increased data availability and methodological improvements, most recent stress field characterization across the entire Mediterranean dates back to 1995. We consistently map stress field orientations and related parameters for the Mediterranean utilizing all focal mechanisms from the World Stress Map database release 2016. Main goals are (1) to resolve the regional stress field orientation at unprecedented finer scale, (2) to evaluate the performance of our stress inversion methodology in one of the most tectonically complex regions on Earth, (3) to compare different types of stress and strain observations covering the entire depth range from surface (e.g. GPS data) to mantle (e.g. shear wave splitting). The obtained stress distributions generally capture correctly the main seismotectonic features of each area, including tectonically complex settings such as the Alpine Orogeny or the Ionian Sea. S_HMax and stress regimes tend to be depth-uniform within uncertainties, but larger stress heterogeneity is resolved for the upper 10 km. Both coseismic elastic strain field from potency tensors and strain field from GPS data are highly consistent with the stress field orientation, indicating that stress and strain maintain a linear relationship in this area and that inelastic strain release is consistent with the elastic stress or strain accumulation. The Italian Peninsula displays the largest discrepancies between these parameters, potentially indicating stress/strain changes with depth, a prominent role of aseismic deformation, and/or a non-linear relation between stress and strain. Increasing discrepancy between stress field orientation and fast shear wave propagation is found from eastern (sub-parallel) to western (sub-perpendicular) Mediterranean, eventually indicating different causes of anisotropy. Implications of these results with respect to seismic hazard will be discussed.

Presenting Author: Patricia Martinez-Garzon

Additional Authors

Session: New Approaches to Geophysical Research Using Dense Mixed Sensor and Broadband Seismology Arrays [Poster]

Type: Poster

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

We used Vp/Vs ratios and guided waves to locate the northern extension of the West Napa Fault in St. Helena, California. We acquired two high-resolution, P- and S-wave seismic profiles (Profiles 1 and 2) using multiple hammer hits to generate both P- and S-wave seismic energy along Profile 1 (215 m long) at 109 shot points, each spaced 2 m apart and co-located with the 109 geophones. We also generated additional P-wave energy along Profile 1 at every fifth geophone using a 227-kg accelerated weight drop (AWD). Profile 2 was a shorter (75 m), higher resolution (1 m shot and geophone spacing) profile located about 70 m northward and parallel to Profile 1. For both profiles, we recorded P-wave data using 40-Hz, vertical-component geophones and S-wave data using 4.5-Hz, single-component geophones. The resulting data sets allowed us to develop P- and S-wave refraction tomography and reflection models along the seismic profiles. From the P- and S-wave tomography models, we developed Vp/Vs and Poisson’s ratio models that delineate the fault most clearly at about 15 m depth as areas of very high ratios. In addition, we generated guided waves within the fault zone 220 m north of Profile 1 and 150 m north of Profile 2 using the AWD source, and we recorded them with the S-wave geophones. We observed high-amplitude guided waves on both profiles in the vicinity of the fault trace, as shown by the tomography and Vp/Vs ratio models. Our data indicate that the West Napa Fault extends northward into the St. Helena area, but the fault may not extend to shallow (< 10 m) depths, as suggested by the trenching results of (Philibosian et al., 2019).

Presenting Author: Mark R. Goldman

Additional Authors

Session: Machine Learning in Seismology [Poster]

Type: Poster

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Currently, many traditional methods are used to detect arrivals in three-component seismic waveform data collected at various distances. Accurately establishing the identity and arrival of these waves in adverse signal-to-noise environments is vital in detecting and locating seismic events. Autocorrelation and template matching techniques are just a few of the various methods that may be used, yet, each have their own limitations.

In this work, we present updated results using convolutional neural networks (CNNs). CNNs have been shown to significantly improve performance at local distances under certain conditions such as induced seismicity. In this work we expand the use of CNNs to more remote distances and lower magnitudes. We explore the advantages and limits of a certain architecture of CNN and update results previously presented.

We describe in detail performance results of our method tuned on a new dataset with expert defined arrival picks. The dataset used is from the Dynamic Network Experiment 2018 (DNE18) and comes from sensors in Utah. We demonstrate the ability to train the CNN on events from the dataset and achieve significantly higher test set performance than standard methods. Furthermore, we validate performance on streaming data, including very low magnitude expert picked arrivals.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Presenting Author: Robert Forrest

Additional Authors

Session: Structural Seismology: From Crust to Core [Poster]

Type: Poster

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

We analyze the teleseismic travel time data recorded by 70 temporary stations from the Y2 and ANTILOPE-1 arrays using the radial anisotropy tomography to investigate the upper mantle structure beneath western Tibet. Two distinct regions are separated by a layered belt along 80°E, which is interpreted as the subducted Indian plate. In the eastern part, a high-velocity zone with positive radial anisotropy has been detected, which is consistent with our previous results. To the west, we observed a layered radial anisotropic structure although the velocity is relatively high from 70 km to 250 km depth. Combing our results with geological, geochemistry and GPS study, we propose that such local feature (the layered belt) may result in the tearing of northward advancing Indian plate. In addition, the different features between the western and eastern side of our study region indicate the existence of remnant Eurasian lithosphere.

Presenting Author: Heng Zhang

Additional Authors

Session: Earthquake Source Parameters: Theory, Observations and Interpretations [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

There has been elevated earthquake activity in the Permian Basin since 2008. Previous studies had suggested a link between hydrocarbon extraction, waste-water disposal, and increased seismicity for parts of West Texas. In 2015, a statewide seismic network known as TexNet has been established to monitor the earthquake activities. To date, there has been over seven thousand small earthquakes (ML<3) reported by TexNet across the state of Texas. In the Permian Basin, these earthquakes are unevenly distributed throughout the basin, often occurring in clusters. One of the main tasks of TexNet is to determine earthquakes' source mechanisms on a daily basis. In general, there is no uniform focal mechanism pattern for the Permian Basin as a whole. However, the source mechanism patterns shift from normal faulting in the Midland Basin to a mixed strike-slip and vertical-dip-slip faulting in the Delaware Basin. In the Delaware Basin, these clusters are interpreted to delineate well-oriented basement-rooting faults, which have formed as a result of the accumulation of several extensional and compressional deformation events since the Proterozoic. A vast majority of events that have been located are shown to occur below the sediment interface, corroborating the interpretation of modern reactivation along preexisting basement-rooting faults. Further integration of seismicity, source mechanisms and available geologic data, have resulted in the generation of a new 3D structural framework of the Delaware Basin. This framework has enabled basin-scale fault characterization to better understand potential controlling fabrics that affect local stress-states. First-order observations from this model highlight areas where heterogeneities in source mechanisms may be due to preexisting, basement-rooting fault patterns while the primary structural is a NW-SE oriented high-angle thrust fault. Secondary features include fault-propagation folds and smaller scaled oblique-slip sub-vertical fault zones that trend WNW-ESE. They are likely to accommodate normal and transtensional reactivation.

Presenting Author: Dino Huang

Additional Authors

Session: Methods for Site Response Estimation

Type: Oral

Date: 4/26/2019

Time: 11:00 AM

Room: Pine

Southern California Edison (SCE) provides electricity to more than 15 million people in 15 southern California counties by maintaining a vast network of transmission lines and substations, many of which are susceptible to strong ground shaking from earthquakes. To better understand the potential for amplified ground shaking at several of those sites with co-located strong motion recording stations, we evaluated V_S30, the time-averaged shear-wave velocity in the upper 30 m of the subsurface. Ground motion models and ground motion prediction equations (GMPEs) use V_S30 as a parameter to account for site amplification and site conditions; the parameter is also used to classify soil stiffness, which is widely used in building codes. To evaluate lateral variability in V_S30 at each site, we conducted 2-D active-source seismic surveys, recording body and surface waves along linear profiles (118 to 174 m long) near the strong motion recording stations. We used sledgehammer (3.5-kg) and accelerated weight-drops (45-kg angled and 226-kg vertical) at each shot point co-located with 4.5-Hz (vertical- and horizontal-component) geophones at 2-m spacing along the profiles. We used 2-D refraction tomography and Multichannel Analysis of Surface Waves (MASW) methods to evaluate shear-wave velocities, from which we calculated V_S30 at every meter along the seismic profiles. Our results show that V_S30 varies by as little as 30 to as much as ~ 200 m/s along individual seismic profiles. We also found that V_S30 nearest to strong motion stations can be up to 46% lower or 41% higher than V_S30 within 50 m of the strong motion stations, suggesting 1-D V_S30 may not accurately capture true site conditions at many strong motion sites due to lateral changes in subsurface materials and structures, which imply SCE substations may experience greater or smaller shaking than indicated by the co-located strong motion stations.

Presenting Author: Joanne H. Chan

Additional Authors

Session: Modeling and Understanding of High-Frequency Ground Motion [Poster]

Type: Poster

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Numerous accelerometric stations are assumed to be installed in free field conditions and hence, considered to be able to record the true ground motion without any alteration. However, recent experiments showed large differences in high frequency content between stations located very close to each other (less than 2 m) depending on whether the station is buried and firmly coupled to soil (classical ‘post seismic’ survey setup) or whether the station in anchored on a small concrete slab inside a shallow manhole (classical permanent network setup).

With respect to soil site conditions and based on the analysis of earthquake records, we observed large amplification above 10 Hz and up to a factor of 3 at 20 Hz for a station installed on concrete slab with respect to a collocated buried one. The differences between buried and slab-anchored stations seem to be much smaller in case of rock site conditions.

Stations may also be installed at shallow depths (few meters) within a seismic vault or cave and still considered as “free field”. In such configuration, the down-going waves interference with the up-going ones may induce important deamplification of the signal within a frequency band, which, although in the high frequency part of the earthquake spectrum, is still of importance for earthquake engineering.

The effect of the sensor installation practices on recorded high-frequency ground motion need to be better characterized and pertinent information should be disseminated as part of the station’s metadata in order to be taken into account for a better use of records. Indeed, the high frequency content of ground motion records has become an increasingly important topic within the framework of site-specific seismic hazard studies. A lot of studies, involving the “kappa” parameter concept, scattering analysis, etc., are using the high frequency part of the accelerometric signals provided by seismological networks.

Presenting Author: Fabrice Hollender

Additional Authors

Session: Observations of Volcanism in the Three Spheres: Land, Air and Sea [Poster]

Type: Poster

Date: 4/26/2019

Time: 06:00 PM

Room: Fifth Avenue

Shallow submarine volcanoes pose unique scientific and monitoring challenges. The interaction between water and magma creates violent explosions just below the surface, but the inaccessibility of submerged volcanoes means they are typically not instrumented. This both increases the risk to marine and aviation traffic and leaves the underlying eruption physics poorly understood. The 2016-2017 submarine eruption of Bogoslof volcano, Alaska, is a unique example of a shallow submarine eruption that produced more than 70 explosive events over 9 months, many of which were captured on infrasound, seismic, and hydroacoustic stations. The nearest infrasonic and seismic monitoring stations are on islands more than 50 km from Bogoslof, making detection and characterization of eruptive events challenging. A campaign hydrophone was installed in May of 2017 near the volcano and captured the second half of the eruption sequence. The eruption infrasound is dominated by low frequency energy, and modeling of discrete low frequency signals associated with many of the explosions indicate large magmatic gas bubbles expanding at the water-air interface were responsible for generating these signals. Seismic signals recorded during unrest on nearby islands showed varying T phase amplitudes, which are used as a proxy for source depth. The single hydrophone recorded a wide variety of events, including high-amplitude signals during explosions. Some of the strongest explosions produced clear signals on all three sensors. Travel-time corrected onsets for some of the strongest explosions show a surprising result: infrasound signals arrive first, followed by hydroacoustic and then seismic. These data support the development of a new, top-down explosion model in cases where gas-rich magmas are erupted from shallow submarine vents.

Presenting Author: John J. Lyons

Additional Authors

Session: Observations of Volcanism in the Three Spheres: Land, Air and Sea [Poster]

Type: Poster

Date: 4/26/2019

Time: 06:00 PM

Room: Fifth Avenue

We present the first continuous observations of the evolution of the magmatic melt system beneath Axial Volcano. We analyzed data from December 2014 through May 2018 from two cabled broadband ocean-bottom seismometers with collocated absolute pressure sensors to estimate seafloor compliance as a function of both frequency and time. The April 2015 submarine eruption induced dramatic changes in shear structure that were primarily concentrated in the lower crust (deeper than 2.5 km). We propose that the nearly 20% drop in shear velocities in the lower crust over the course of 10-12 weeks following the eruption represents the fracturing of the lower crust and the intrusion of high aspect-ratio melt sills. The subsequent rate of healing suggests that the sills were on the order of 1 meter thick. The absence of a signal on the eastern flank of the caldera indicates that the lower crustal melt pathway is relatively narrow in cross section (<1.2 km2) compared to the overlying melt chamber (≥42 km2). The lower crustal melt must also be concentrated beneath the center or to the west of the surface caldera. We find that the melt chamber and the lower crust contain minimum melt fractions of 14% and 4%, respectively, though the true melt fractions may be significantly higher. Our images demonstrate the promise of using continuous data to understand submarine volcanism and crustal accretion processes.

Presenting Author: Adrian K. Doran

Additional Authors

Session: Advances, Developments and Future Research into Seismicity in Natural and Anthropogenic Fluid-Driven Environments [Poster]

Type: Poster

Date: 4/26/2019

Time: 06:00 PM

Room: Fifth Avenue

Fluid-injection into the subsurface in the frame of reservoir-engineering activities for hydrocarbon and geothermal energy production has resulted in a dramatic increase of induced seismicity during the last 10 years. This includes four M>5 earthquakes in Oklahoma and Kansas (US) through the reactivation of previously unknown critically stressed and thus hazardous faults in the basement. We investigate seismic recordings of relocated events with local magnitudes ML in the range [1.9, 5.2] from a regional seismic network deployed in southern Kansas since 2014 including 19 broadband stations and 5 accelerometers. Determination of seismic moment tensors and subsequent refinement was done employing the hybridMT package (Kwiatek, 2016). HybridMT inversion is based on the P-waves first ground displacement amplitudes from vertical components and provides unconstrained full, deviatoric, and double-couple constrained moment tensors. The results of moment tensor inversion are refined and suppresses the influence of local path, site, and sensor effects. In this study, we also implemented the use of the horizontal components, thereby increasing the input data by a factor of 3. The refined methodology was tested and tuned on synthetic datasets based on the shear-tensile source model (Vavrycuk, 2001). This model describes the source kinematics by four fault plane parameters (strike, dip, rake) and a tensile angle representing the fault opening. We find that focal mechanisms from the 3-component inversion are generally consistent with the generated synthetic fault plane parameters, signifying the correct performance of the new approach. Furthermore, moment tensor inversion of several tens of events with already reported focal mechanisms (Rubinstein, 2018) appear to be generally consistent between applied methods. In this contribution, first results from statistically significant full moment tensors as well as double-couple solutions from initial ~2,300 seismic events are presented and discussed in the context of their spatial and temporal changes within the investigated area.

Presenting Author: Patricia Martinez-Garzon

Additional Authors

Session: U.S. Geological Survey National Seismic Hazard Model Components [Poster]

Type: Poster

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Earthquake Model of the Middle East (EMME) project has paved the road to a state of the art seismic hazard assessment in the region, with Georgia being one of the key participants.

After EMME project, it was decided to initiate the revision of the nation seismic hazard model of Georgia, in light of the new findings of the EMME projects, new harmonized datasets, fully aligned with the probabilistic framework promoted by Global Earthquake Model (GEM).

In this contribution we are presenting the main elements of the newly developed seismic hazard model of Georgia. The starting point is the updating of the regionally harmonized datasets (i.e. earthquake catalogues, active faults) with focus on data that become available within the recent years. For declustering the earthquake catalogues a window-based procedure was used. Completeness of the earthquake catalogue within the sub-regions was also investigated.

The seismogenic source model was built upon various ingredients including the earthquake catalogue, the location of active faults, the main geological strctures and the active tectonic information. The seismogenic source model consists of two models: an area source model and an active fault combined with background seismicity. Earthquake catalogue were used to characterize the earthquake rate forecast on the are source, while the slip-rates were converted to seismic productivity of active faults .

The seismogenic sources of Caucasus region were categorized in three seismo-tectonic classes (i.e. shallow crust, volcanic sources and deep seismicity). Given this classification, four ground motion models were selected and used to quantify the inherent uncertainties of ground motion.

Finally, the probabilistic seismic hazard was estimated for the entire region using OpenQuake engine (Pagani et al 2014) and the results consists of a full range of results, including mean and quantiles of hazard maps, hazard curves and hazard spectra.

The work presented here was supported by the Shota Rustaveli National Science Foundation (SRNF) (Project 216758).

Presenting Author: Nino S. O. Tsereteli

Additional Authors

Session: Non-Traditional Application of Seismo-Acoustics for Non-Traditional Monitoring [Poster]

Type: Poster

Date: 4/26/2019

Time: 06:00 PM

Room: Fifth Avenue

This talk discusses on-going research at Georgia Tech Research Institute (GTRI), the applied research arm of Georgia Institute of Technology, in non-traditional application of Seismo-acoustics for Non-traditional Monitoring. In particular it deals with connection between seismic activity related to industrial machinery and Infrasound. In particular, results of a study that compared a number of commercially available infrasound sensors with and without a wind screen loaned to GTRI by NASA Langley will be presented. Sources of producing controlled infrasound under consideration at GTRI will also be discussed. These include a sonic boom simulator, a low frequency acoustic driver, oscillating jets, and door opening and closing. Each source was most effective in a given frequency range. Controlled infrasound at 0.1 Hz was obtained by varying the exit Mach number of exit Mach number of a cold plume from very low Mach number to a high Mach number at a nominal frequency of 0.1 Hz. It is expected that the amplitude of this signal will increase by heating the jet. Preliminary results of successful attempts at removing wind noise using Wavelet methodology are also shown. Characteristics of seismic signals from a fan and a compressor in a building together with related infrasound will be discussed. The implication of such research on monitoring industrial machinery activity will be summarized.

Presenting Author: Krishan K. Ahuja

Additional Authors

Session: Advances in Intraplate Earthquake Geology [Poster]

Type: Poster

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

The east margin of the Basin and Range extensional province is defined by the Wasatch fault zone (WFZ), whose central segments have been trenched for decades. The Levan segment (LS) and Fayette segment (FS) in central Utah are the southern segments of the WFZ and have little paleoseismic data available. Presence of evaporites at depth and a large discrepancy between regional geologic and geodetic rates could implicate salt tectonics as a contributor to displacement on the LS and FS. The Utah Geological Survey in cooperation with the U.S. Geological Survey excavated paleoseismic trenches on the LS and FS to determine Holocene earthquake timing and investigate if any fault displacement may be attributed to salt tectonics. A trench on the LS across a 3-m high scarp showed evidence for 1 – 2 Holocene surface-faulting earthquake(s). A trench on the FS across a 1-m high scarp revealed evidence for a single post-Lake Bonneville surface-faulting earthquake. The LS trench showed ~2.6-m of vertical offset and a complex 4.4-m wide zone of tilted, overturned and sheared blocks of alluvial-fan strata suggesting strike slip and horizontal extension. A soil covered by scarp colluvium on the hanging wall provides evidence for the surface-faulting earthquake that created the deformation zone. Scarp-derived colluvium overlying soil developed on older colluvial wedge suggests a second, smaller displacement event. Well-defined stratigraphic and fault relationships exposed in the trench across the FS scarp indicate a single surface-faulting earthquake with ~1 m of vertical displacement. Preliminary dating results for the FS trench site indicate it ruptured at 5.4 ± 0.1 ka (2s). A single Holocene surface-faulting earthquake on the FS is consistent with an expected lower slip rate and longer recurrence interval at the southern end of the WFZ. The deformation exposed in both trenches is consistent with discrete, rapid, meter-scale displacements and indicative of seismogenic earthquakes.

Presenting Author: Greg N. McDonald

Additional Authors

Session: Current and Future Challenges in Engineering Seismology [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Building the source model is the first step in a probabilistic seismic hazard study. The source model is intended to describe the frequencies of occurrences of future earthquakes in the area. In regions of low to moderate seismicity, the source model most often relies on an earthquake catalogue associated to a definition of seismogenic sources in space. In France, two earthquake catalogues are currently published and made public: the catalogue from the 2013 European Seismic Hazard model project (EHSM13, Woessner et al. 2015) and the FCAT catalogue (Manchuel et al. 2017). Besides, two propositions for delineation of seismogenic sources are available: again the one used in ESHM13, partly based on the Autran et al. (1998) model for inner France, and the Baize et al. (2013) model. The aim of the present study is to explore the variability of hazard levels depending on the source model uncertainty. Combining the available catalogue and seismogenic source definitions, alternative source models are built. Additionally, uncertainties related to the different steps involved in the modeling of earthquake recurrence are quantified as precisely as possible: uncertainties in the declustering, in the estimation of completeness time periods and in the estimation of Gutenberg-Richter parameters. One major difficulty to address is the quantification of uncertainties in regions of low seismicity where data is scarce. To determine probabilistic seismic hazard, a set of recent ground-motion prediction equations potentially adapted for the different French tectonic contexts and consistent with the ongoing update of the 2020 European hazard map (ESHM20) is considered. Our results at the country level as well as for selected French cities enable to better understand (1) how much the choices made to build the source model can impact hazard levels, and (2) what is the overall variability on hazard levels for different return periods considering the current state-of-the art knowledge in France.

Presenting Author: Celine Beauval

Additional Authors

Session: Explore the Fault2SHA Paradigms Across the Ponds [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Forecasting surface fault rupture is necessary for many engineering projects. In addition to the probability of the mean and maximum displacement along strike, the distribution of off-fault displacement perpendicular to fault strike is also important, particularly for reverse events with their hanging and footwall characteristics. Reverse surface faulting data was evaluated from 11 earthquakes (5.4≤M≤7.9) around the world. The relationship between the width of rupture zone (WRZ) and other parameters was evaluated. The data was first smoothed for Bias Weighting using Brown’s simple exponential moving average (EMA) which helped to eliminate unusual large width of rupture zone. Many probability distributions were evaluated against the data and we found that a power function (axb+c) provided the best to the WRZ data. On-fault surface rupture displacement has been found to occur mostly near the center of the fault, and we expected to observe similar results for off-fault surface ruptures but we found no clear correlation between the magnitudes of the WRZ and the along strike fault distance ratio, X/L. Also, no distinct relationship was found between the WRZ and on-fault vertical displacement. However, a relationship between principle vertical off-fault displacements and distributed vertical off-fault displacements was observed.

Presenting Author: Kourosh Younesi

Additional Authors

Session: Observations of Volcanism in the Three Spheres: Land, Air and Sea [Poster]

Type: Poster

Date: 4/26/2019

Time: 06:00 PM

Room: Fifth Avenue

Mauna Kea is a large postshield-stage volcano that forms the highest peak on Hawaii Island. The 4,205-meter high volcano erupted most recently between 6,000 and 4,500 years ago and exhibits relatively low rates of seismicity, which are mostly tectonic in origin resulting from lithospheric flexure under the weight of the edifice. Here we identify repeating, deep long-period earthquakes (DLPs) occurring beneath the summit of Mauna Kea. These earthquakes, which are not part of the Hawaiian Volcano Observatory’s regional network catalog, were initially detected through a systematic search for coherent seismicity using envelope cross-correlation, and subsequent analysis revealed the presence of a long-term, persistent source. The events have energy concentrated at 2-7 Hz, and can be seen in filtered waveforms dating back to the earliest continuous data from a single station archived at IRIS from November 1999. We use a single-station (horizontal components) match-filter analysis to create a catalog of the repeating earthquakes for the past 18 years. Using templates created through the stacking of thousands of sta/lta-triggers, we find > 1 million earthquakes repeating every 7-12 minutes throughout this time period, in addition to many smaller events occurring in between. The earthquakes occur at 28-31 km within a conspicuous gap in seismicity depth directly beneath the summit that corresponds to a low-velocity anomaly in regional tomography model. Magnitudes (~Ml 1.3-1.6) and periodicity are remarkably stable over time periods of years but do vary slightly on shorter time scales, and the DLPs can be triggered or modified by regional tectonic earthquakes. Based on the events’ frequency content and location, we infer a volcanic source distinct from the regional tectonic seismicity responding to the load of the island.

Presenting Author: Aaron Wech

Additional Authors

Session: Science, Hazards and Planning in Subduction Zone Regions [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Repeating earthquakes (REs) rupture identical fault patches allowing variations in the mechanical behavior of specific areas to be interrogated over the earthquake cycle. We study several families of repeating earthquakes that reveal fault weakening after a large earthquake in Costa Rica followed by fault recovery. We find shorter RE recurrence intervals and increased RE rupture areas immediately following the large earthquake that both gradually return to pre-earthquake values. This is consistent with other studies and the interpretation that an increase in post seismic loading rate speeds up the time to RE failure and drives conditionally stable regions surrounding REs to slip unstably increasing their rupture areas. However, unlike most other observations that report an increase in post seismic RE magnitudes (expected if RE rupture areas increase), our postseismic REs show no change in magnitude. This requires a reduction in RE slip that we interpret as evidence of fault weakening. This is consistent with experimental work showing slip amplitudes and stress drop decrease with loading rate.

Presenting Author: Esteban J. Chaves

Additional Authors

Session: Structural Seismology: From Crust to Core [Poster]

Type: Poster

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

We investigate the seismic anisotropy of the crust and upper mantle beneath the northeastern margin of the Tibetan Plateau (NETP) at 55 stations from a temporary experiment (ANTILOPE-IV) operated from 2010 to 2013 using teleseismic shear wave splitting (SWS) measurements. Assuming a single-layer anisotropic model, we obtain 631 pairs of SKS splitting parameters represented by fast polarization directions (FPDs) and delay times (DTs). The general orientations of the seismic anisotropy in the NETP indicate WNW-ESE direction, with average FPD of ~92.9° and average DT of ~1.20 s. Most of the FPDs follow the tectonic trends except those in the Qaidam basin and southwestern Qilian orogen. The stacking FPDs in the Altyn Tagh fault and northwestern Qilian orogen is NW-SE trending, parallel or sub-parallel to the azimuthal anisotropy of the Pn wave and the direction of the absolute plate motion, indicating the major source of the anisotropy is the asthenosphere flow. The FPDs beneath the East Kunlun-West Qinling Fault are E-W and/or ENE-WSW trending, parallel or subparallel to the azimuthal anisotropy of the Pn wave and the trend of the regional geological structures. The FPDs beneath the Qaidam basin are NE-SW trending, parallel to the direction of the GPS velocity, inferring that the source of the observed anisotropy is from the crust and the upper mantle lithosphere. The results of the SWS lead to a vertically coherent deformation pattern in NETP.

Presenting Author: Changhui Ju

Additional Authors

Session: Numerical Modeling of Earthquake Ground Motion, Seismic Noise, Rupture Dynamics and Seismic Wave Propagation [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

The large number of pulses recorded during the 1999 Mw7.6 Chi-Chi, Taiwan earthquake provided an important data base for this study. The prediction of near-fault velocity pulses generated by large earthquakes can provide some reference for the Engineering anti-earthquake design. Based on the established source model and velocity structure model, this paper attempts to use the 3D finite difference method to simulate the 33 near-fault large velocity pulses. The conclusions as the following: (1) Two-segment “shovel-like” fault model constructed by 3D bending plane can better describe the characteristics of the underground real fault. (2) The seismic moment and rise time of the 6 asperities determine the peak and period of the velocity pulse. The asperities located at shallow low angle mainly affect the horizontal pulse components, and the asperities at high angle contribute more to the vertical pulse component. (3) The difference in sliding properties between the north and south ends of the fault causes a difference in the horizontal pulse components, reflecting the characteristics of the fling-step effect and directivity effect. (4) The characteristic period of the velocity response spectrum has the maximum near the turning point at the northern end of the fault and shows a very obvious hanging wall effect in the near-fault region, resulting in the large-scale structures have severe damage because of large resonance effect. (5) PGV gradually increases from south to north along the fault and PGV on the hanging wall is significantly larger than PGV on the footwall, and the distribution of the pulse in front of the rupture is wider than that of the rear. Because the simulation results are basically consistent with the real records, this also verifies the feasibility of simulating pulse-like ground motions with the 3D finite difference method.

Presenting Author: Xue Liang Chen

Additional Authors

Session: Earthquake Source Parameters: Theory, Observations and Interpretations [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

Stress drop, an averaged difference between the shear stress on the fault before and after an earthquake, is an important source parameter. The values estimated by typical seismological methods (e.g., the spectral fitting approaches) suggest that the stress drop is moment-invariant with large scatter and that the on-fault rupture duration tw obeys the scaling of tw ∝ M0^(1/3). However, several observations show that microseismic events from the same location can have similar source durations but different seismic moments, violating the commonly assumed scaling. We use numerical simulations of earthquake sequences to demonstrate that strength variations over seismogenic patches provide an explanation of such behavior, with the event duration controlled by the patch size and event magnitude determined by how much of the patch area is ruptured. We find that stress drops estimated by the spectral fitting analyses for the sources simulated in an asperity-like fault model significantly increase with the event magnitude, ranging from 0.006 to 8 MPa. However, the actual stress drops determined from the on-fault stress changes are magnitude-independent at ~3 MPa. Our findings suggest that fault heterogeneity results in local deviations in the moment-duration scaling and in earthquake sources with complex shapes of the ruptured area, for some of which stress drops may be significantly (~100-1000 times) underestimated by the spectral fitting methods. We further apply the stress-drop estimation approach using second moments, which aims to account for rupture directivity and elliptical sources. The second-moment approach indeed works better, providing close estimates of stress drop for sources with rupture areas similar to elliptical ones, but still significantly underestimates, by a factor of up to 30, stress drops for more complex rupture shapes such as ring-like sources. Hence the estimated stress drops still overall increase with the event magnitude, ranging from 0.1 to 2 MPa, but are overall closer to the actual stress drop of ~3 MPa.

Presenting Author: Yen-Yu Lin

Additional Authors

Session: State of Stress and Strain in the Crust and Implications for Fault Slip Based on Observational, Numerical and Experimental Analysis

Type: Oral

Date: 4/26/2019

Time: 04:30 PM

Room: Vashon

Propagating Rayleigh and Love waves from large earthquakes generate considerable dynamic stresses in the Earth’s crust, even in the far field. These stresses occasionally exceed empirical triggering thresholds on faults that are presumed to be critically stressed. Both volumetric and deviatoric stress variations in these dynamic stress tensors can affect the normal stress on a given fault, while shear stress variations on the fault are a result from transient deviatoric stresses from both types of surface waves, which often act on the fault at the same time.

Reports of potentially dynamically triggered tremor or earthquakes on a given fault are frequently accompanied by estimates of dynamic stresses on this fault. However, these reports do either not consider vertical variations in surface-wave displacements or assume a homogeneous crust. Estimates of the variations of dynamic stresses with back-azimuth typically do not include the effects of radiation patterns controlled by varying source mechanisms and depths.

To complement these estimates we calculate full dynamic stress tensors from 3-D and 3-C surface-wave displacement fields for a realistic Earth model. We test our approach on simulated data from the September 8, 2017, Mw 8.2 earthquake near the coast of Chiapas in Mexico. We quantify and visualize spatiotemporal variations in the isotropic stresses and principal stresses from the deviatoric stress tensors.

We then project these stresses onto the San Andreas fault and observe that 1) stresses generated at mid- and lower-crustal depths, where triggered events occur, are as large as and occasionally larger than those at the surface, 2) the maximum dynamic stress at these depths is reached at different times than peak ground velocity at the surface, which is used as a proxy for stress, 3) Combined stresses from simultaneous Rayleigh and Love waves create complex dynamic stress histories, 4) The off-shore Chiapas earthquake’s Love waves generated larger stresses on the San Andreas fault than its Rayleigh waves.

Presenting Author: Boris Roesler

Additional Authors

Session: Current and Future Challenges in Engineering Seismology [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Lognormal uncertainty around ground motion prediction equations is ubiquitous in the literature. However, little research into the durability of lognormal uncertainty or its applicability for forward analysis has been performed. By “durability,” I mean the likelihood that the original characterization of uncertainty remains applicable as more validation data are considered. This presentation reports the results of a study based on the NGA-West2 database (with a total of approximately 15,800 records after censoring) which evaluated the stability of the moments of lognormal uncertainty as validation data accumulates, as well as whether or not the original descriptions of model uncertainty continued to accurately describe the distribution of errors. Three initial database sizes were considered: approximately 1,500, 3,000, and 4,500 records. Lognormal uncertainty had stable moments only for relatively large initial databases (4,500 records), and was not durable for any scenario considered. Lognormal uncertainty may therefore not be suitable for use in forward analysis in some situations. Logistic distributions formulated to approximate the original normal distributions around the median were shown to be more likely to survive additional validation data due to their increased tail thickness. Use of alternative probability distributions may make ground motion prediction equations more resilient to possible outliers in the future and may therefore offer a more realistic description of model uncertainty. Implications for probabilistic seismic hazard analysis (PSHA) and performance-based earthquake engineering (PBEE) and the challenges presented will be discussed.

Presenting Author: Zach Bullock

Additional Authors

Session: Current and Future Challenges in Engineering Seismology [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Our objective is to introduce the SIGMA-2 R&D program on seismic hazard assessment (“SeIsmic Ground Motion Assessment, 2nd edition”). Built upon SIGMA project (2011-2016) legacy, this program aims at delivering an improved representation of the seismic ground-motion, adapted to local site conditions, with a particular emphasis on low seismicity areas.

The program is funded by a consortium of industrial partners operating, managing, controlling facilities (including nuclear installations), and sharing a common interest in improving seismic hazard assessment (SHA) practices within an open collaboration framework. The program organization has been elaborated so as to strengthen the link between researchers and seismic/civil engineers, by involving international representative from private and public sectors from the realization of research actions to the Scientific Committee supervisory level. Besides, bi-annual workshops offer the opportunity to conduct extensive technical discussions/reviews with end-users and to anticipate future impacts on SHA studies.

A dedicated Scientific Committee has been constituted to monitor the quality of SIGMA-2 scientific outcomes, and to provide guidance on technical aspects. In addition, project deliverables must comply with a formal review process supervised by this Committee.

SIGMA-2 research actions are organized into 6 Work Packages with expected contributions at every step of the SHA process: Fault & tectonics; Earthquake catalogs & parameters; Characterization of ground-motion; Site response; Probabilistic SHA and Inputs for engineers.

The program will go on up to the end of 2021.

Presenting Author: Guillaume Daniel

Additional Authors

Session: Metamaterials, Resonances and Seismic Wave Mitigation, an Emerging Trend in Seismology [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

The first direct detection of gravitational waves has been possible by constructing a pair of highly sensitive detectors called Laser Interferometer Gravitational-wave Observatory (LIGO) which involve the most accurate measurement of displacements up to the order of 10^-20 m. Since it is a ground-based interferometer, various noise sources limit its sensitivity. Ground vibrations in the range 0.01-10 Hz, generated through both anthropogenic as well as natural sources become an issue. These displacements, in turn, cause a fluctuation in the local gravitational field due to density perturbations of the surrounding geology introducing Newtonian Noise. The seismic isolation systems can suppress the displacements by several orders of magnitude, however, cannot reduce Newtonian noise. To further shield the detector building and also the walls of the vacuum chambers from such noise, we consider the idea of seismic metamaterials in this study.

Lately, using the concepts in metamaterial physics it has been demonstrated that judicious arrangement of either resonators or soil inclusions can largely affect the seismic wave field on the geophysical scale. In this study, the feasibility of using two different seismic metamaterials is explored; one with steel poles over the surface acting as vertical resonators and another with a periodic arrangement of vertical concrete inclusions in the soil medium. We consider the ground model at the site chosen for the upcoming LIGO-India project at Hingoli in India. To shield the ends of the interferometer, we seek to achieve band gaps below 10 Hz. We develop models for realistic cloak designs and perform time-domain simulations in SPECFEM3D, an open-source code which solves the elastodynamic equations using finite difference in time and the spectral-element method (O>4) in space. This interesting approach to shield the LIGO detectors can potentially improve its sensitivity in the low-frequency range and the results from this study can have an impact on the existing and future ground-based detectors.

Presenting Author: Shoaib Ayjaz Mohammed

Additional Authors

Session: Machine Learning in Seismology [Poster]

Type: Poster

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Machine learning associated with earthquakes viewed as discrete events often involve a combinatorial structure. Two combinatorial optimization methods have been developed to determine the spatial distribution of earthquakes along a single fault or among faults within a fault system. For a synthetic earthquake catalog that spans millennia, the objective is to determine the optimal arrangement of earthquakes that minimizes misfit to target fault slip rates. The first method is a spatial gap-filling method that uses a greedy-sequential algorithm in which earthquakes are sequentially placed in a locally optimal sense. The second method is integer programming (IP) that globally optimizes earthquake placement subject to a number of constraints, including slip-rate uncertainty (minimum and maximum limits). The binary decision vector in the IP model is composed of every possible location that each earthquake in the synthetic catalog can occur. General mixed-integer programming (MIP) software is used that consists of a number of different algorithms, including the simplex algorithm for the initially relaxed binary constraints and the branch-and-cut algorithm to recursively search a tree of binary solutions. We find that the greedy-sequential algorithm efficiently scales to long catalog durations and complex fault systems. Global optimization, in contrast, is likely a nondeterministic polynomial-time (i.e., NP-Hard) problem, similar to the pure knapsack and bin-packing problems well known in operations research. MIP software has greatly advanced in recent years, making fault-scale problems tractable, although high-performance computing platforms are required for the largest problems. We apply both of these methods to both a single fault (Nankai subduction megathrust) and a complex fault system (California transform system) and compare the results. We demonstrate that, in general, combinatorial optimization methods provide a valuable and independent way to determine on-fault earthquake magnitude distributions.

Presenting Author: Eric Geist

Additional Authors

Session: Central and Eastern North America and Intraplate Regions Worldwide [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Central Asian Orogenic Belt (CAOB) is one of the largest Paleozoic orogens on Earth, which evolved from the latest Mesoproterozoic to the late Paleozoic, with the accretion of ophiolites, island arcs, accretionary wedges, oceanic islands and microcontinental fragments. Being the middle-eastern part of the CAOB, Inner Mongolia is characterized by widespread Cenozoic intraplate volcanisms and lithospheric deformation possibly related to the closure of the Paleo-Asian Ocean. However, the mechanism of lithosphere deformation is poorly understood. Seismic anisotropy determined by the splitting of shear waves, especially the core-refracted phases like SKS, is one of the most direct and effective ways to image the deformation in the interior of the Earth. A total of 120 pairs of shear wave splitting measurements and 113 null measurements are obtained at 32 portable seismic stations in Abaga area, Inner Mongolia. Delay times vary from 0.4 s to 1.4 s with an average value of 0.77±0.21 s; while fast directions trend from N101°W to N45°E. One group of fast directions in line with the strike of regional faults, trending N82.0°E±12.3°, is caused by lattice-preferred orientation of mantle crystals; the other group of fast directions observed in the North China Craton with an average value of N146.8°E± 9.5° are parallel to the direction of lithospheric deformation in Early Cretaceous, which may origin from fossil anisotropy remaining in the lithosphere. In addition, solely null measurements in the northern part of study area may suggest small-scale hot mantle upwellings which eroded the fossil anisotropy in lithosphere.

Presenting Author: Zhengyang Qiang

Additional Authors

Session: Facebook and Twitter and Snapchat, Oh My! The Challenges and Successes of Using Social Media to Communicate Science to the Public [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

Puerto Rico and Virgin Islands Region (PR/VI) is the area of seismic responsibility monitored by the Puerto Rico Seismic Networks (PRSN). This area includes Puerto Rico, USVI, BVI and the Eastern Dominican Republic which are located on the tectonic border of the Caribbean and North American plates. Major events in the region throughout history have generated tsunamis that make the population aware of the risks generated by earthquakes. A significant 7.3Mw earthquake in 1918 generated a tsunami that impacted PR/VI region. More recently, a major 6.4Mw earthquake struck the region and reignited public awareness that large events and tsunamis are possible. In order to educate the population and emergency managers, the PRSN has uninterruptedly coordinated annual tsunami response exercises in PR/VI since 2009 with the Lantex Exercise. Since 2013 under initiatives from the Caribe EWS-Intergovernmental Oceanographic Commission (UNESCO), the exercise was jointly carried out as the CaribeWave/Lantex. As of 2016 the tsunami response exercise is coordinated out as the CaribeWave exercise under the IOC UNESCO framework. Some of the exercise objectives are to test and evaluate the operations of the Caribe EWS, validate tsunami response preparedness and assist tsunami-preparedness efforts of Caribbean emergency management agencies. The PRSN performs communications exercise with all the primary emergency agencies in PR/VI to test the messaging and alert distribution systems in place. The exercise also provides opportunity for state and municipal agencies to test their warning systems. In addition, an intense educational campaign is carried out to promote public preparedness for the eventuality of a tsunami. This effort to reach the people involves the use of the existing mass media which has evolved to include platforms such as social media and automated messaging systems. Here we present an analysis of the exercise evolution over the past decade and the impact the exercise on the public and local stakeholders.

Presenting Author: Gisela Báez-Sánchez

Additional Authors

Session: Frontiers in Earthquake Geology: Bright Futures and Brick Walls [Poster]

Type: Poster

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Fluvial terraces are common in the Nujiang River valley, while the third level is widest and mostly distributed. Three granite boulders were taken as samples at the posterior margin of the third terrace of the Nujiang River at Bingzhongluo. The diameters of all boulders are large (>2m) and buried in soil layers. Sample NJ2-1 was collected from one large granite boulder on floodplain, which is used to estimate the inheritance. Sample BZL43 was chiseled about 1.5cm from the surface of a quartz vein, that insets a sandy slate of the third terrace at downstream about 2km. The ¹⁰Be concentrations of BZL43 is highest of the four samples from the third terrace. Weathering processes have a greater impact on the exposure ages of boulder samples according to the relief, environmental features and ¹⁰Be concentration. Assuming the erosion rate of sample BZL43 from the quartz vein as 0.05cm/ka, the erosion rate of granite boulders is represented graphically as 0.3cm/ka by the relationship among effective exposure age, exposure time and erosion rate. The exposure ages of the three granite boulders on the third terrace were obtained after the correction for erosion and ignoring inheritance. Combined with the corrected exposure age of sample BZL43, it is considered that the third terrace of Nujiang River at Bingzhongluo was developed during 150~203ka.

Presenting Author: Yanwu Lv

Additional Authors