A Unified Source Model for the 1906 San Francisco Earthquake

Abstract

We reconcile two previously discordant source models of the 1906 San Francisco earthquake and obtain a model that satisfies both triangulation and seismic data by allowing the rupture velocity to exceed the shear-wave velocity. Employing a projection method to remove the dependence on initial station positions allowed us to make use of a more stable triangulation network, including nonrepeated angle observations along the northern San Andreas fault. This strengthens the case for significant slip over the entire northern segment of the San Andreas fault from San Juan Bautista to Cape Mendocino during the 1906 earthquake. We also found that the teleseismic body-wave data can be reconciled with the geodetically derived slip model by allowing supershear rupture. This resolves a longstanding conflict between the two previous slip models (geodetic and seismic) of this earthquake. Supershear rupture has long been recognized as a theoretical possibility for strike-slip faulting, and it has been observed in several recent large strike-slip earthquakes, which raises the prospect that it might be typical for such events. Supershear rupture leads to substantially different strong ground motion, and as a result, may need to be taken into account when developing ground motion prediction relations for large strike-slip earthquakes. Our final slip model has a seismic moment of 7:9 × 10 20 N m, which corresponds to a moment magnitude of M w 7.9 Online Material: A digital version of geodetic displacements and slip distribution.

Revisiting the 1872 Owens Valley, California, Earthquake

“…Wald et al (1993) conclude that if the rupture did continue offshore, this part of the rupture did not release significant seismic radiation. Song et al (2008) show that the geodetic and seismic data can be reconciled if supershear rupture is allowed, estimating M w 7.9. (Song et al (2008) do not consider uncertainties, but report only a preferred value.…”

“…Song et al (2008) show that the geodetic and seismic data can be reconciled if supershear rupture is allowed, estimating M w 7.9. (Song et al (2008) do not consider uncertainties, but report only a preferred value. The event is listed by NEIC as 7.8 (http://neic.usgs.gov/neis/eq_depot/usa/ 1906_04_18.html, last accessed February 2008).…”

“…If SF1906 was as large as M w 7.9, as inferred by Song et al (2008), one is left with the conclusion that either OV1872 was at least this large, or that it produced relatively higher regional ground motions despite a smaller magnitude. Considering the macroseismic data as well as the geological observations, we estimate a preferred magnitude of 7.8-7.9 for OV1872.…”

“…6a) reveals that the values for OV1872 generally cluster toward the top of the range estimated at any given distance for SF1906, although at distances greater than ∼200 km, intensities for the former event are systematically higher. Furthermore, if we one assumed a longer rupture length for SF1906, as inferred by some studies (e.g., Thatcher et al, 1997;Boatwright and Bundock, 2005;Song et al, 2008), one would reduce the near-fault distance for a number of the high intensity values. That is, the SF1906 intensity values shown in Figure 6a would decay even more quickly with distance.…”

“…Because, however, the range of inferred magnitudes for the OV1872 are in the range 7.4-7.8, a key question is how its intensity distribution compares to that of the SF1906, for which we have both an instrumentally determined magnitude of 7.7-7.9 (Sieh, 1978;Wald et al, 1993;Thatcher et al, 1997;Song et al, 2008) and a set of recently reinterpreted intensities (Boatwright and Bundock, 2005). To compare intensity values for OV1872 and SF1906, we first determine MMI as a function of distance to the fault (Fig.…”