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How Good Is Your Location? Comparing and Understanding the Uncertainties in Locations of a Sequence of Events in Nevada

Description: 

Event location is an essential function of seismology. In this study, we are motivated to better understand the uncertainties of event location as part of the Source Physics Experiments (SPE). The SPE were designed to improve our physics-based understanding and modeling capabilities of explosion seismic sources. Phase III of SPE will provide a direct comparison of earthquake and explosion sources by co-locating a chemical explosion with the hypocenter of a shallow earthquake and recording both events at a common set of sensors. For this endeavor, a shallow sequence of earthquakes that occurred in 1993 at the Rock Valley Fault Zone on the Nevada National Security Site (formerly the Nevada Test Site) has been targeted. 12 events in the sequence were large enough to be recorded both locally and regionally at distances of 200 km or greater. A temporary seismic station deployed directly over the sequence by the University of Nevada, Reno recorded short S-P arrival times confirming the shallow origins of the sequence.

In order to select a site for drilling, best estimates of the earthquakes locations and depths were required, as well as an understanding of the uncertainties involved. To address these issues, we examined relocations of nine of the events using four different location algorithms, multiple independent sets of phase picks, and a variety of regional and local velocity models. Results using different combinations of the algorithms, picks, and velocity models generally gave locations within approximately 1-2 km of each other. Narrowing to a single set of picks from correlation alignment and a common station constellation typically produced epicentral differences on the order of a few hundreds of meters and depth differences of a couple of kilometers for any single velocity model. S-P times at the closest station were used to further refine depth estimates to within one kilometer, depending on the velocity model.

M. L. Pyle portion prepared by LLNL under Contract DE-AC52-07NA27344.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Session: It’s All About Relocation, Relocation, Relocation

Type: Oral

Date: 4/20/2023

Presentation Time: 04:45 PM (local time)

Presenting Author: Ting Chen

Student Presenter: No

Invited Presentation: 

Authors