Crustal Deformation Near the Mendocino Triple Junction
M H Murray
(Geophysics Dept, Stanford University, CA 94305;
415-723-9594; mhmurray@pangea.stanford.edu);
M Lisowski
(Hawaii Volcano Observatory, USGS, Hawaii National Park,
HI 96718; 808-967-8843; mlisowski@tako.wr.usgs.gov)
Abstract Cape Mendocino in northern California is one of the few
places in the world where three plate boundaries meet on or
close to land. Since 1981, the USGS has made geodetic
measurements to study how compressional deformation
associated with the Cascadia subduction zone is replaced by
strike-slip deformation across the San Andreas fault system
as the Mendocino triple junction migrates northward
along the California margin. Geodolite trilateration
measurements of interseismic deformation between 1981 and
1989 reveal a complex transition from right-lateral shear
distributed across the San Andreas fault system
accommodating 25 mm/yr (about 50% of the total predicted
Pacific--North America relative plate motion), to about 15
mm/yr of uniaxial contraction across the Eel River and Mad
River fault systems in the onshore accretionary wedge, in
general agreement with geologic studies. Since 1989, GPS
measurements near Cape Mendocino have also measured
deformation caused by the 1991 Honeydew, 1992 Cape
Mendocino, and 1994 Mendocino Fault earthquakes. The
April 25, 1992 M=7 Cape Mendocino earthquake, the first
well-documented rupture associated with the Cascadia
megathrust, caused significant crustal deformation
including a meter-level uplift of the coast revealed by
the die-off of intertidal marine organisms. We compare
the coseismic deformation predicted by dislocation models
estimated from the observed displacements
with geologic rates, such as 3-4 mm/yr Holocene uplift
derived from marine terrace studies. These comparisons
suggest that seismic events such as the 1992 earthquake, if
repeated every 100-300 years, can account for nearly all
the long-term deformation observed near Cape Mendocino.
Crustal deformation near Cape Mendocino, California.
Red vectors show measured horizontal rates of displacement
with 95% confidence region ellipses. Blue vectors show
rates predicted from a simple dislocation model comprised
of 5 vertical strike slip faults (green lines) and 1 dipping
thrust fault (green rectangle).
Presented at the American Geophysical Union meeting, December, 1995.