[SIO GP Seminars] Today, Ray Weldon, U of Oregon

[Matt Wei]

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Today, May 18, 3:00 PM
(refreshments served at 2:45 PM)
Munk Conference Room

Ray Weldon
University of Oregon

"Past and future earthquakes on the Southern San Andreas fault."

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Abstract:
The concept of the earthquake cycle is so well established that one  
often hears statements in the popular media like, “the Big One is  
overdue” and “the longer it waits, the bigger it will be.”  
Surprisingly, data to critically test the variability in recurrence  
intervals, rupture displacements, and relationships between the two  
are almost nonexistent. To generate a long series of earthquake  
intervals and offsets, we have conducted paleoseismic investigations  
across the San Andreas fault near the town of Wrightwood, California,  
excavating 45 trenches over 18 years, and can now provide some  
answers to basic questions about recurrence behavior of large  
earthquakes.
To date, we have characterized at least 30 prehistoric earthquakes in  
a 6000-yr-long record, complete for the past 1500 yr and for the  
interval 3000–1500 B.C. For the past 1500 yr, the mean recurrence  
interval is 105 yr (31–165 yr for individual intervals) and the mean  
slip is 3.2 m (0.7–7 m per event). The series is slightly more  
ordered than random and has a notable cluster of events, during which  
strain was released at 3 times the long-term average rate. Slip  
associated with an earthquake is not well predicted by the interval  
preceding it, and only the largest two earthquakes appear to affect  
the time interval to the next earthquake. Generally, short intervals  
tend to coincide with large displacements and long intervals with  
small displacements. The most significant correlation we find is that  
earthquakes are more frequent following periods of net strain  
accumulation spanning multiple seismic cycles.
The extent of paleoearthquake ruptures may be inferred by correlating  
event ages between different sites along the San Andreas fault.  
Wrightwood and other nearby sites experience rupture that could be  
attributed to overlap of relatively independent segments that each  
behave in a more regular manner. However, the data are equally  
consistent with a model in which the irregular behavior seen at  
Wrightwood typifies the entire southern San Andreas fault; more long  
event series will be required to definitively outline prehistoric  
rupture extents.

Have a good day.

Matt

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Meng Wei ( Matt )
Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
University of California, San Diego
La Jolla, CA 92093-0225
mwei at ucsd.edu
(858) 822-4347
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