Passive Site Response Characterization Using Teleseismic Receiver Functions from Wideband Optical Accelerometers

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Since 2018, the Bureau of Reclamation Strong Motion Monitoring Program (SMMP) has deployed 23 ultra-low-noise optical accelerometers at 7 dams across the western US, replacing traditional Force-Balance Accelerometers (FBAs) used in our legacy systems. In contrast to standard FBA sensors, which usually exhibit self-noise exceeding the NHNM at frequencies below ~1Hz, the noise floor of our optical sensors is comparable to a broadband seismometer.

Because the optical sensor is capable of measuring both strong and weak motion accurately, it unlocks new potential for teleseismic imaging techniques which were never before possible on FBA systems. We utilize several such methods in this study to estimate site response and constrain basin properties beneath three SMMP stations at Jackson Lake Dam, WY.

In 1986, crosshole Vp and Vs tomography was performed in the vicinity of SMMP’s Jackson Dam toe station (JKLK1). While Vp and Vs were measured to a depth of 465 feet by the study, boreholes did not penetrate basement, therefore basin depth underlying JKLK1 remains poorly-constrained.

Here, we combine the crosshole velocity model with analysis of teleseismic receiver functions (RFs), body-wave H/V spectral ratios (HVSR) and P-SV polarization to estimate basin depth using a suite of distant earthquakes recorded by the optical sensors.

Comparison of HVSR between freefield and toe stations indicate fundamental basin resonance of ~0.8Hz, corresponding to a basin depth of 500-600ft beneath JKLK1, in general agreement with prior seismic studies conducted in the area.

The first 5s of RFs from JKLK1 closely agree with synthetic data generated using the crosshole model, while fitting subsequent reverberations using a basin model based on HVSR data yields mixed results. Azimuthal variation of RFs suggests the influence of horizontal scattering at basin boundaries. However, the stability of near-surface impulse responses shown here implies that inversion of teleseismic RFs can potentially characterize site responses at other upgraded SMMP stations where shallow shear structure is unknown.

Presenting Author: Justin S. Ball

Authors