Constraints on the Rupture of the October 21, 1868, Hayward Earthquake Determined From the Distribution of Modified Mercalli Intensity
The October 21, 1868, Hayward earthquake was the most damaging earthquake to occur in California in the half- century following the 1848 annexation. The earthquake shattered the city centers of Oakland and San Francisco, and cracked brick buildings as far away as Santa Rosa and Gilroy. We have re-evaluated MMI intensites at the 124 sites with damage or felt reports compiled by Toppozada et al. (1981), and added 26 sites where we obtained reports from newspapers and historical narratives. We used the 1878 Thompson and West Atlas of Alameda County to locate most of the specific buildings that were reported as damaged. The resulting ShakeMap interpolates the distribution of intensity along the Hayward fault, and the extent of shaking throughout the greater Bay Area and the San Joaquin Delta. Surprisingly, the highest intensities (MMI 8-9 to 9) are clustered near the middle of the fault rupture, in Hayward, San Leandro, and San Lorenzo. The intensities are lower (MMI 7- 8) at the ends of the fault rupture, in Berkeley and Warm Springs. The lack of strong shaking at either end of the fault rupture makes it hard to discern the rupture direction: the intensities observed at regional distances suggest that the rupture was stronger to the northwest towards Petaluma (MMI 7) and Martinez (MMI 7) than to the southeast towards Calaveras Valley (MMI 6-7) and Gilroy (MMI 6-7). The relatively low intensities in Oakland and Berkeley (MMI 7-8) suggest that the shallow locked zone near Piedmont, which Simpson et al. (2001) infer from the distribution of fault creep, did not rupture in the earthquake. This result appears to contradict Yu and Segall's (1996) conclusion that the fault slipped > 1 m in Berkeley. Given the large proportion of aseismic slip on Hayward fault, both observed geologically at the surface and inferred geodetically at depth, it is natural to propose that the rupture process of the 1868 earthquake comprised a series of disjoint asperity ruptures with variable rupture directions, and a substantial amount of dynamically forced slip.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 7212 Earthquake ground motions and engineering seismology