Location and Fault Mechanics of the Focal Region of the
California
Earthquake of April 18, 1906
Anthony Lomax
Anthony Lomax Scientific Software, Mouans-Sartoux, France
www.alomax.net
Abstract
We investigate the location and tectonics of the focal region
of the 18 April 1906 California Earthquake. Applying modern,
probabilistic relocation to arrival observations from the Lawson
report, we place the 1906 hypocenter to the west of San Francisco near
the offshore San Andreas Fault system; we associate this focus with a
dilatational right-bend in the San Andreas Fault system, in agreement
with previous work. Using seismological, geological and marine evidence
from the Lawson report and from later studies, we propose a
10-15 km long focal region in a zone of sea floor subsidence and
complex faulting extending northwestwards from offshore Daly City to
offshore of the Golden Gate. We characterize the geometry of 1906
rupture in this region by normal faulting on a steeply west-dipping
structure trending about 20º clockwise to the San Andreas Fault
zone, and by strike-slip rupture along the San Andreas
Fault system around and away from the west-dipping structure: rupture
to the north on a vertical, currently a-seismic fault structure under
the Golden Gate Fault, and rupture to the south under the San Francisco
Peninsula along a steeply
southwest-dipping structure showing present day extensional tectonism.
All three of these structures extend from the near-surface to about 10
km depth. We also identify a 20 km long, linear trend of clustered
seismicity at 10-13 km depth which connects the northern part of the
proposed focal region with the south end of the southwest-dipping, San
Francisco Peninsula structure. We suggest that
this trend reveals faulting at the base of a brittle, upper crust in
response to an underlying shear zone in a ductile, lower crust. Motion
across this hypothesized, deep shear zone, rotated about 6º
clockwise relative to the strike of the San Andreas Fault, may explain
the present day extensional tectonism found along the northern San
Francisco Peninsula. These new interpretations on the focal region of
the 1906 earthquake have implications for seismotectonic understanding,
earthquake monitoring and seismic hazard assessment in the San
Francisco Bay Area.
Location of the 1906 focal region and hypocenter
Fault mechanics of the focal region
3D visualization using the Java Seismicity
Viewer and screen images of NonLinLoc (NLL) location
results. Each Java visualization and screen image shows the NLL
relocation for
the corresponding figure in the printed paper. ...
(Important Note: The 3D Java visualizations require the Java
plug-in (v1.4.2 or higher) in your browser and that your network and
browser security allow Java applets and active content. To download the
Java plug-in visit: http://www.java.com)
Viewing tips:
- To use multiple viewers
simultaneously, open each Figure link in a new browser window.
- For more information,
select the Seismicity Viewer menu
option Help->Help.
[ Back ]
|