Date of Award

2015-01-01

Degree Name

Master of Science

Department

Geophysics

Advisor(s)

Aaron A. Velasco

Abstract

We take advantage of EarthScope’s USArray Transportable Array (TA), regional seismic networks to investigate 154 M ≥ 7 earthquakes over a ten-year period (2004- 2013), in search of remotely triggered seismicity within the continental United States. We conduct an automated search to detect high frequency signals (which may indicate triggering of small local earthquakes) using a time window of 5 hours before and after each mainshock. The automated detection applies a short-term average (STA) to long-term-average (LTA) algorithms, to create a catalogue of detections. Using the catalog we search for an increase in detection rates after each main-shock. Sharp increases in detection rates may indicate triggered seismicity that is either instantaneous or delayed.

Here, we use both STA/LTA detection methods and local earthquake catalogs to investigate remote triggering in the Coso Geothermal Field (CGF); geothermal sites have long been suggested as areas susceptible to remote triggering of seismicity. Of 154 remote mainshocks (M) ≥ 7 studied, we find 34 mainshocks (22%) likely triggered local seismicity in the CGF region as a result of an imposing transfer of dynamic stress from the remote mainshock events. We assume there is a triggered response in the CGF based on the increase in the magnitude and frequency (rate) from pre-mainshock to post-mainshock auto-detection rates and cataloged seismicity. We observe both instantaneous (16 events) and delayed (18 events) triggering of local seismicity within ± 5 hours of the mainshock. We suggest remote triggering in the CGF region is enhanced by the orientation (back-azimuth) of the passing seismic waves. We find several events align favorably with the local stress field and/or orientation of NNE-SSW trending faults in the CGF triggered region. We find triggering and non-triggering mainshocks (in terms of generated peak dynamic stresses and mainshock parameters) display similar patterns. From this we surmise triggering in the CGF region to be strongly dependent on the state of regional stresses. We suggest that triggering is more likely to occur when critically stressed fault exists, this fault can respond to the passage of dynamic stresses in a manner triggering failure of the fault.

Language

en

Provenance

Received from ProQuest

File Size

62 pages

File Format

application/pdf

Rights Holder

Richard A. Alfaro-Diaz

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