R. Sliter
E.L. Geist
R.C. Jachens
B. E. Jaffe
A. Foxgrover
P. E. Hart
J. McCarthy
T. Parsons
2003
<p><span>A dilatational </span>step<span>-</span>over<span> between the right-lateral </span>Hayward<span> and </span>Rodgers<span> </span>Creek<span> faults lies beneath </span>San<span> Pablo </span>Bay<span> in the </span>San<span> </span>Francisco<span> </span>Bay<span> area. A key seismic hazard issue is whether an earthquake on one of the faults could rupture through the </span>step<span>-</span>over<span>, enhancing its maximum possible magnitude. If ruptures are terminated at the </span>step<span>-</span>over<span>, then another important issue is how strain transfers through the </span>step<span>. We developed a combined seismic reflection and refraction cross section across south </span>San<span> Pablo </span>Bay<span> and found that the </span>Hayward<span> and </span>Rodgers<span> </span>Creek<span> 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 </span>San<span> Pablo </span>Bay<span> indicated a low likelihood of strike-slip transfer faults connecting the </span>Hayward<span> and </span>Rodgers<span> </span>Creek<span> faults. Numerical simulations suggest that it is possible for a rupture to jump across a 4-km </span>fault<span> gap, although special stressing conditions are probably required (e.g., Harris and Day, 1993, 1999). Slip on the </span>Hayward<span> and </span>Rodgers<span> </span>Creek<span> 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 </span>step<span>-</span>over<span> 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 </span>San<span> Pablo </span>Bay<span> that apparently produced a small tsunami. Historical hydrographic surveys gathered before and after 1898 indicate abnormal subsidence of the </span>bay<span> floor within the </span>step<span>-</span>over<span>, possibly related to the earthquake. We used a hydrodynamic model to show that a dip-slip mechanism in north </span>San<span> Pablo </span>Bay<span> is the most likely 1898 rupture scenario to have caused the tsunami. While we find no strike-slip transfer </span>fault<span> between the </span>Hayward<span> and </span>Rodgers<span> </span>Creek<span> faults, a normal-</span>fault<span> 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.</span></p>
application/pdf
10.1785/0120020228
en
Seismological Society of America
Structure and mechanics of the Hayward-Rodgers Creek Fault step-over, San Francisco Bay, California
article