| Abstract: | A dilatational step-over between the right-lateral Hayward and Rodgers Creek faults lies beneath San Pablo Bay in the San Francisco Bay area. A key seismic hazard issue is whether an earthquake on one of the faults could rupture through the step-over, enhancing its maximum possible magnitude. If ruptures are terminated at the step-over, then another important issue is how strain transfers through the step. We developed a combined seismic reflection and refraction cross section across south San Pablo Bay and found that the Hayward and Rodgers Creek faults converge to within 4 km of one another near the surface, about 2 km closer than previously thought. Interpretation of potential field data from San Pablo Bay indicated a low likelihood of strike-slip transfer faults connecting the Hayward and Rodgers Creek faults. Numerical simulations suggest that it is possible for a rupture to jump across a 4-km fault gap, although special stressing conditions are probably required (e.g., Harris and Day, 1993, 1999). Slip on the Hayward and Rodgers Creek faults is building an extensional pull-apart basin that could contain hazardous normal faults. We investigated strain in the pull-apart using a finite-element model and calculated a ???0.02-MPa/yr differential stressing rate in the step-over on a least-principal-stress orientation nearly parallel to the strike-slip faults where they overlap. A 1- to 10-MPa stress-drop extensional earthquake is expected on normal faults oriented perpendicular to the strike-slip faults every 50-500 years. The last such earthquake might have been the 1898 M 6.0-6.5 shock in San Pablo Bay that apparently produced a small tsunami. Historical hydrographic surveys gathered before and after 1898 indicate abnormal subsidence of the bay floor within the step-over, possibly related to the earthquake. We used a hydrodynamic model to show that a dip-slip mechanism in north San Pablo Bay is the most likely 1898 rupture scenario to have caused the tsunami. While we find no strike-slip transfer fault between the Hayward and Rodgers Creek faults, a normal-fault link could enable through-going segmented rupture of both strike-slip faults and may pose an independent hazard of M ???6 earthquakes like the 1898 event. |
| Genre: | Article |
| ProdID: | 70025536 |
| Citation Author: | Parsons, T.; Sliter, R.; Geist, E. L.; Jachens, R. C.; Jaffe, B. E.; Foxgrover, A.; Hart, P. E.; McCarthy, J. |
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| Citation End Page: | 2200 |
| Citation Issue: | 5 |
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| Citation Language: | English |
| Citation Larger Work Title: | Bulletin of the Seismological Society of America |
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| Citation Number Of Pages: | 14 |
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| Citation Search Results Text: | Structure and mechanics of the Hayward-Rodgers Creek Fault step-over, San Francisco Bay, California; 2003; Article; Journal; Bulletin of the Seismological Society of America; Parsons, T.; Sliter, R.; Geist, E. L.; Jachens, R. C.; Jaffe, B. E.; Foxgrover, A.; Hart, P. E.; McCarthy, J. |
| Citation Start Page: | 2187 |
| Citation Volume: | 93 |
| Citation Year: | 2003 |
| Type: | citation/reference |
| Text: | Structure and mechanics of the Hayward-Rodgers Creek Fault step-over, San Francisco Bay, California; 2003; Article; Journal; Bulletin of the Seismological Society of America; Parsons, T.; Sliter, R.; Geist, E. L.; Jachens, R. C.; Jaffe, B. E.; Foxgrover, A.; Hart, P. E.; McCarthy, J. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Wed, 1 Jan 2003 00:00 -0600 |
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