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Probing the Roots of Active Faults in California Through Deep Seismic Tremor: David Shelly


Project Title: Probing the Roots of Active Faults in California Through Deep Seismic Tremor
Mendenhall Fellow: David Shelly (650) 329-4024, dshelly@usgs.gov
Duty Station: Menlo Park, CA
Start Date: March 31, 2008
Education: Ph.D. (2007), Geophysics, Stanford University
Research Advisors: Jeanne Hardebeck, 650-329-4711, hardebeck@usgs.gov; Gary Fuis, 650-329-4758, fuis@usgs.gov; Jessica Murray-Moraleda, 650-329-4864, jrmurray@usgs.gov

Project Description: Non-volcanic tremor is a weak, extended-duration seismic signal observed episodically on some major faults, often in conjunction with slow slip transients seen in geodetic data.  The coupled phenomenon, termed “episodic tremor and slip” has been observed in the Cascadia subduction zone (northwest United States and southwest Canada) (Rogers and Dragert, 2003) as well as in the Nankai subduction zone in southwest Japan (Obara and others, 2004).  Here, tremor accompanies transient slip on the deep extent of the plate interface, downdip from the portion of the fault that generates large earthquakes. Tremor has now also been detected beneath strike slip faults including the San Andreas fault (Nadeau and Dolenc, 2005) in central California and the source fault of the 2000 Western Tottori earthquake in southwestern Japan (Ohmi and others, 2004).

Transient slip events are of considerable interest to earthquake scientists–not only because they represent an intriguing mode of deformation but also because they may transfer stresses to adjacent parts of the fault that are capable of generating earthquakes.  Thus, the likelihood of a large earthquake may be elevated during or shortly after a slow slip transient.  This hypothesis is supported by empirical evidence: aseismic slip events are believed to have preceded some large historical earthquakes, including the 1960 magnitude 9.5 Chile event, the largest earthquake ever instrumentally recorded (Linde and Silver, 1989).  Similarly, slow slip events beneath the south flank of Kilauea Volcano, Hawaii have been seen to trigger a dramatic increase in micro-earthquake activity (Segall and others, 2006).

For this Mendenhall project, I am investigating deep tectonic tremor in California.  This includes studying tremor already identified beneath the San Andreas Fault near Parkfield, as well as investigating other possible tremor source regions of tremor in California.  Results so far have been promising; precise locations of tremor southeast of Parkfield showing a clear alignment of tremor sources with the strike of the San Andreas fault, near the base of the crust at ~25 km depth.  This result is significant: it argues both that shear slip generates this tremor and that the San Andreas fault remains a localized structure well beyond the base of the seismogenic zone (~12 km), at least to the base of the crust.  More details can be found here: http://www.agu.org/pubs/crossref/2009/2008GL036367.shtml

References
Linde, A.T., and Silver, P.G., 1989, Elevation changes and the Great 1960 Chilean Earthquake: Support for aseismic slip: Geophysical Research Letters, vol. 16, no.11, p. 1305–1308, doi:10.1029/GL016i011p01305 .

Nadeau, R.M., and Dolenc, D., 2005, Nonvolcanic tremors deep beneath the San Andreas fault: Science, vol. 307, no. 389; published online 9 December 2004 (10.1126/science.1107142).

Obara, K., Hirose, H., Yamamizu, F., and Kasahara, K., 2004, Episodic slow slip events accompanied by non-volcanic tremors in southwest Japan subduction zone: Geophysical Research Letters, vol. 31, doi:10.1029/2004GL020848.

Ohmi, S.,Hirose, I., and Mor, J.J., 2004, Deep low-frequency earthquakes near the downward extension of the seismogenic fault of the 2000 Western Tottori earthquake: Earth Planets Space, vol. 56, p. 1185–1189.

Rogers, G., and Dragert, D., 2003, Episodic tremor and slip on the Cascadia subduction zone: The chatter of silent slip:.  Science, vol. 300, p. 1942–1943.

Segall, P.,  Desmarais, E.K., Shelly, D.R., Miklius, A., and Cervelli, P., 2006, Earthquakes triggered by silent slip events on Kilauea volcano, Hawaii: Nature, vol. 442,  p. 71–74, doi:10.1038/nature04938.

Map of study region, including stations utilized in this study

 

Figure 1. A, Map of study region, including stations utilized in this study (triangles). Purple triangles are PBO borehole stations. Green ellipses indicate the four “clusters” of stations used together to calculate differential times. Although the HRSN is spatially extensive, it subtends a small angle from the tremor source. Red dots show locations of 148 correlated Cholame tremors on October 6, 2007. Well-located tremors (those retaining more than 45 observations after relocation) from a combination of 5 templates are plotted. Red box outlines the region shown in B. “SAF” is the San Andreas Fault. B, Zoomed map view of tremor locations from A. Black crosses show the LFE “master template” as well as template (32192) and detected event (27068) from the example in figure 3.  C, Across fault cross-section A–A’. Zero on the horizontal axis indicates the location of the SAF surface trace. D, Along-fault cross-section B–B’. Note that these locations represent only those events well correlated with our master template on one day of activity.


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Last modified: 16:08:32 Thu 13 Dec 2012