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Escudero Ayala, C.R., 2010

Studies of earthquakes stress drops, seismic scattering, and dynamic triggering in North America

Bibliographic Reference

Escudero Ayala, C.R., 2010, Studies of earthquakes stress drops, seismic scattering, and dynamic triggering in North America: University of Texas, El Paso, Ph.D. dissertation, 149 p.


I use the Relative Source Time Function (RSTF) method to determine the source properties of earthquakes in southeastern Alaska-northwestern Canada in a first part of the project, and earthquakes in the Denali fault in a second part. I deconvolve a small event P-arrival signal from a larger event by the following method: Select arrivals with a tapered cosine window, fast fourier transform to obtain the spectrum, apply water level deconvolution technique, and bandpass filter before inverse transforming the result to obtain the RSTF. I compare the source processes of earthquakes in the area to determine stress drop differences to determine their relation to the tectonic setting of the earthquake's location. Results show a consistency with previous results, stress drop independent of moment implying self-similarity, correlation of stress drop with tectonic regime, stress drop independent of depth, stress drop depends on focal mechanism where strike-slip present larger stress drops, and decreasing stress drop as function of time. I determine seismic wave attenuation in the central western United States using coda waves. I select approximately 40 moderate earthquakes (magnitude between 5.5 and 6.5) located along the California-Baja California, California-Nevada, Eastern Idaho, Gulf of California, Hebgen Lake, Montana, Nevada, New Mexico, off coast of Northern California, off coast of Oregon, southern California, southern Illinois, Vancouver Island, Washington, and Wyoming regions. These events were recorded by the EarthScope transportable array (TA) network from 2005 to 2009. We obtain the data from the Incorporated Research Institutions for Seismology (IRIS). In this study we implement a method based on the assumption that coda waves are single backscattered waves from randomly distributed heterogeneities to calculate the coda Q. The frequencies studied lie between 1 and 15 Hz. The scattering attenuation is calculated for frequency bands centered at 1.5, 3, 5, 7.5, 10.5, and 13.5 Hz. Coda Q present a great correlation with tectonic and geology setting, as well as the crustal thickness. I analyze global and Middle American Subduction Zone (MASZ) seismicity from 1998 to 2008 to quantify the transient stresses effects at teleseismic distances. I use the Bulletin of the International Seismological Centre Catalog (ISCCD) published by the Incorporated Research Institutions for Seismology (IRIS). To identify MASZ seismicity changes due to distant, large (Mw ~7) earthquakes, I first identify local earthquakes that occurred before and after the mainshocks. I then group the local earthquakes within a cluster radius between 75 to 200 km. I obtain statistics based on characteristics of both mainshocks and local earthquake clusters, such as cluster-mainshock azimuth, mainshock focal mechanism, and local earthquake clusters within the MASZ. Based on the lateral variations of the dip along the subducted oceanic plate, I divide the Mexican subduction zone into four segments. I then apply the Paired Samples Statistical Test (PSST) to the sorted data to identify increment, decrement, or either in the local seismicity associated with distant large earthquakes passage of surface waves. I identify dynamic triggering for all MASZ segments produced by large earthquakes emerging from specific azimuths, as well as a decrease for some cases. I find no dependence of seismicity changes on mainshock focal mechanism.

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