HR: 08:35h
AN: T51A-03    [PDF]
TI: Paleoseismic Data Used to Evaluate Long Term Earthquake Behavior
AU: * Scharer, K M
EM: kscharer@darkwing.uoregon.edu
AF: University of Oregon, Dept. of Geological Sciences 1272, Eugene, OR 97403 United States
AU: Weldon, R J
EM: ray@newberry.uoregon.edu
AF: University of Oregon, Dept. of Geological Sciences 1272, Eugene, OR 97403 United States
AU: Fumal, T E
EM: tfumal@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road MS977, Menlo Park, CA 94025 United States
AU: Biasi, G P
EM: glenn@seismo.unr.edu
AF: University of Nevada Reno, Seismological Laboratory MS174, Reno, NV 89557-0141 United States
AB: The Wrightwood, California paleoseismic record can be used to evaluate important behavior parameters for the southern San Andreas fault. A total of 27 paleoearthquakes have been resolved for two time intervals. The "upper section" includes 14 dated earthquakes from 500 AD to the present; each earthquake is paired with a displacement estimate. Intervals between earthquakes average ~105 years but vary from 10 to 224 years. Displacement estimates also vary, from 1 to 7 m of measured slip with an average of 3.2 m. Variation in the displacement estimates may be exaggerated by how deformation is manifest at the site or by preservation of the evidence. The "deep section" of 13 dated earthquakes spans 3000 to 1200 BC, but contains little displacement data. Based on structural similarities we believe that the deep section is similar in overall behavior. Displacements, earthquake ages, and slip rate at Wrightwood suggest that the San Andreas fault exhibits neither time predictable nor slip predictable behavior. While the average slip rate is 3.1 cm/yr, both the deep and upper sections show variation in slip released through time. Both sections have short periods (~5 events) of high slip rates (~9 cm/yr) and longer intervals (7 to 9 events) of low slip rates (~2 cm/yr). We used the average slip rate and displacement data of the upper section to estimate the strain level of the fault through time. Earthquakes are represented by strain drops equivalent to their displacement and strain is accumulated at the average slip rate between earthquakes. The resulting strain history shows periods of net strain accumulation and loss with a range equivalent to ~5 earthquake displacements. Periods of net strain release have both more frequent and larger earthquakes whereas periods of net strain accumulation have both longer intervals and smaller events. The current strain level is approaching a high which as been reached only three times in the past 1500 years. The highest strains are followed by either a very large earthquake or a flurry of average to large sized earthquakes. Strain level does not reliably predict displacement of the subsequent event, but does correlate weakly with the length of the ensuing interval. Periods of higher strain release are associated with shorter intervals. We also evaluated strain accumulation through time assuming a characteristic slip (3.2 m). The resulting strain history still shows periods of net strain release and gain, but the range is reduced to the displacement of ~3 earthquakes. These results indicate that the periods of net strain loss result from both the frequency and size of the earthquakes. When displacement is modeled as uniform, the correlation between strain level and ensuing interval length is weakened. From these relationships, we conclude that: (1) shorter recurrence intervals appear to be more common at high strain levels. (2) displacement data does not correlate with other parameters, or the correlation is obscured by measurement imprecision. (3) approximately one-third of the strain released by the San Andreas fault occurs during short periods of time (15% of the record), characterized by shorter recurrence intervals and possibly larger events. (4) currently, strain exceeds the level of all but 3-5% of the past 1500 years, and strain has never remained at this level for more than 25 years.
DE: 1208 Crustal movements--intraplate (8110)
DE: 7209 Earthquake dynamics and mechanics
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting