Characterization of Large Earthquakes on the San Andreas Fault in the Carrizo Plain: Implications for Fault Mechanics and Seismic Hazard

Geology and Geophysics

Grant, Lisa Baugh ; Sieh, Kerry E. (advisor)

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Geology and Geophysics;   

Others  :  https://thesis.library.caltech.edu/7373/1/Grant_lb_1993.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Despite the widespread use of geomorphic offset measurements forcalculating earthquake probabilities, little attention has been paid to either theuncertainties in the interpretation of offset geomorphic features, or the effectsof these uncertainties on fault models and estimates of seismic hazard.Interpretation of offsets along the San Andreas fault in the Carrizo Plain havebeen the basis of hypotheses of a strong Carrizo fault segment which regularlybreaks in great earthquakes several centuries apart with dextral surface slip onthe order of 10 m per event.

The smallest geomorphic offset measurements along a 6 km stretch ofthe fault southeast of Wallace Creek vary between ~6.5 m and ~10m. A 3-Dexcavation of alluvial deposits at the Phelan fan shows that at least 6.6 to 6.9m of dextral slip occurred during the 1857 Fort Tejon earthquake, and that thepenultimate earthquake occurred several centuries prior to 1857. Thus, eitherthe amount of surface slip varied several meters over a 2-3 km stretch of thefault in 1857, or 2 to 3 meters of slip in a penultimate earthquake wasfollowed by ~7 m of slip in 1857.

Two monuments from an 1855 survey which spans the San Andreasfault in the Carrizo Plain have been displaced 11.0 ± 2.5m right-laterally by thegreat earthquake of 1857. This magnitude of offset is consistent withgeomorphic indications that slip across the fault during the 1857 earthquakeand associated foreshocks and aftershocks varied from 6.6 to 10 m over 2.6 kmalong this section. Comparison of recent geodetic measurements with thelate Holocene slip rate at Wallace Creek shows that fault slip rates determinedfrom short-term wide aperture measurements are indistinguishable fromrates determined from long-term narrow aperture measurements. Usingradiocarbon dates of the penultimate large earthquake and measurements ofslip in 1857, we calculate an average slip rate for the last complete earthquakecycle that is at least 25% lower than the late Holocene slip rate on the mainfault trace. This suggests that variation in fault slip during the 1857earthquake left a slip deficit in at least the upper 1km of crust at WallaceCreek. Slip in future earthquakes may compensate this deficit.

Three trenches across the San Andreas fault on the Bidart fan in theCarrizo Plain record evidence of 7 previous earthquakes. Radiocarbon datingindicates five earthquakes, including the 1857 earthquake, have occurredsince A.D. 1218. The penultimate earthquake, event B, occurred between 1405and 1510 A.D. Several centuries before 1857, events B, C, D and E occurred ina temporal cluster after approximately 1218 A.D. and prior to 1510 A.D. Theaverage recurrence interval within this cluster ranges from 73 to 116 years,depending on assumptions. Events B and D may correlate with prehistoricearthquakes recorded in sediments elsewhere along the southern SanAndreas fault. Events C and E appear to have ruptured locally in smallermagnitude earthquakes. Surface slip from either event B, or events B and Ccombined, totals 7 to 11 m.

If fault strength is defined by long earthquake repeat time, then theCarrizo segment of the San Andreas is not inherently stronger than theMojave segment. The temporal and spatial distribution of large earthquakeson the San Andreas fault is difficult to reconcile with slip-based theories ofsegmentation of strike-slip faults. Temporal patterns of seismicity may bemore robust than spatial trends. Clusters of large earthquakes analogous tosequences of foreshocks, mainshocks and aftershocks may occur on longertime scales of seismic "supercycles."