We determine probabilistic hypocentral locations for the foreshock and mainshock of the Great 1906 California earthquake through reanalysis of arrival-time observations in conjunction with modern velocity models and advanced event-location techniques. We obtain two additional observations for the mainshock and one for the foreshock that were not used in previous location studies. Using a robust likelihood function for event location, we generate a usable subset of the predominantly unreliable teleseismic readings and determine new wave-type identifications for some of the local and teleseismic readings. Our locations are much better constrained than those of earlier studies, even though we do not assume that the epicenter lies on the San Andreas fault, as did previous authors.
We confirm the conclusions of earlier studies that the local and teleseismic arrival-time observations can be explained by a single foreshock focus and a single mainshock focus on the San Andreas fault, and that there is no single, unique hypocenter that satisfies all available local observations. The maximum-likelihood point (Latitude, 37.78° N; Longitude, 122.51° W) for our “preferred” mainshock location indicates a hypocenter to the west of San Francisco, close to the San Andreas fault zone. This hypocenter has a 68% confidence error of about ±8 km parallel to the San Andreas fault and about ±24 km perpendicular to the fault, and a depth in the midcrust of about 12 ± 7 km. The closest point on the San Andreas fault to this hypocenter lies about 10 km to the northwest of the widely accepted mainshock epicenter of Bolt (1968). Our mainshock location is consistent with the association of initial rupture of the 1906 mainshock with a dilatational right-bend or step-over in the submerged San Andreas fault system offshore of the Golden Gate. Our foreshock location is less well constrained than our mainshock location but is consistent with the foreshock hypocenter being at the same location as the mainshock hypocenter.
Online material: Visualization of 3D probabilistic hypocentral locations associated with the 1906 earthquake.