Location of the Focal Region and Hypocenter of the California Earthquake of April 18, 1906

Anthony Lomax

Anthony Lomax Scientific Software, Mouans-Sartoux, France - www.alomax.net

March, 2006

Definition of 1906 focal volume

Lomax (2005) determined probabilistic hypocentral locations for the foreshock and mainshock of the 1906 California earthquake through reanalysis of arrival-time observations in conjunction with modern wave-speed models and event location techniques. The Lomax (2005) “preferred” mainshock location has a large uncertainty volume (pdf), but is consistent with the association of initial rupture of the 1906 earthquake with an extensional right-bend (e.g. Lawson, 1908) or stepover (e.g. Zoback et al., 1999) in the submerged San Andreas Fault (SAF) system offshore of the Golden Gate.

Since the observed surface rupture for the 1906 earthquake occurred along the SAF (e.g. Lawson, 1908), we may assume that the focal region for the 1906 mainshock is located within the SAF zone. Then, an upper bound on a likely focal area is given by the intersection of: a) the SAF zone (as defined by the area between the SAF and the Golden Gate Fault in Bruns et al., 2002), b) the locations of recent micro-earthquakes in the area, and c) the uncertainty volume (pdf) for the Lomax (2005) “preferred” mainshock location. This area is bounded approximately by a polygon with corners at 37.79ºN-122.61ºW, 37.82ºN-122.57ºW, 37.71ºN-122.48ºW, 37.67ºN-122.53ºW, and center near 37.75ºN-122.55ºW (Figure showing 1906 focal area). This likely focal area extends northwestwards from offshore of Lake Merced to offshore of the southernmost Marin Peninsula. The southeast end of this area is a few kilometers to the northwest of the 1906 epicenter near Daly City of Bolt (1968), and near the proposed SAF stepover of Zoback et al. (1999).

We can constrain a likely focal volume, including depth, by assuming that the 1906 mainshock hypocenter is within or at the bottom of the seismogenic zone defined by recent micro-earthquakes, as has been found for a number of recent, large earthquakes on the San Andreas Fault system, e.g. Morgan Hill, 1984 (Cockerham and Eaton, 1984), Loma Prieta, 1989 (Dietz and Ellsworth, 1990), and Parkfield, 2004 (Langbein, et al., 2005). With this assumption, the depth range of recent micro-earthquakes in the likely focal area implies a 1906 focal depth between a few kilometers and about 13 km (Figure showing 1906 focal volume).

1906 hypocenter

The likely focal volume defined above gives the best available description of the location of the 1906 earthquake focus. The center point of this volume and an associated uncertainty give a best estimate for the hypocenter of the 1906 California earthquake:

Epicenter: 37.75ºN, 122.55ºW (+/-7km along strike of SAF, +/-2.5km perpendicular to strike of SAF)

Depth: 8km (+/-5km)

Figure showing 1906 epicenter

Maps showing 1906 epicenter: (Google - MapQuest)

For Google Earth users:

to view the 1906 epicenter click here (or save the Placemark kmz file available here and drag it into My Places).
to view the the 1906 source area, faults and recent earthquakes click here (or save the Image Overlay kmz file available here and drag it into My Places).