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1 Geomechanics-Rock Fracture
Group
Department of Geological Sciences and Engineering
Mackay School of
Earth Sciences and Engineering (172)
University of Nevada
Reno, Nevada
89557-0138
* Present address: Structure, Traps, and Seals Team, Shell International Exploration & Production Inc., Bellaire Technology Center, P.O. Box 481, Houston, Texas 77001-0481. Scott.Wilkins{at}Shell.com
A 3D, static fracture mechanics model of earthquake rupture that incorporates
cohesive end zones (CEZs), or zones of increased frictional strength,
is tested to determine whether it helps to understand the observed scaling
behavior of average slip with rupture dimensions for shallow (<20 km),
continental, interplate strike-slip earthquakes. Our new compilation of average
source parameters suggests that (1) average slip increases with aspect ratio
(along-strike length/down-dip width), although in decreasing proportions for
progressively larger aspect ratio ruptures, and (2) a gradual scaling change
exists at an aspect ratio of 
6. These general trends match the functional
form predicted by the CEZ model. Despite these general trends,
significant scatter in average slip is apparent among similarly sized ruptures.
We test the hypothesis that the CEZs represent strength
heterogeneities along the rupture surface that result from
velocity-strengthening frictional behavior; and that this heterogeneity in
frictional behavior along the fault is the primary reason for the failure of a
universal (constant stress drop) scaling law. CEZ lengths are
measured from slip and stress drop distributions determined from published
inversions of geophysical data for the 1984 Morgan Hill, 1992 Landers, 1999
Hector Mine, and 1999 
zmit earthquakes, and range from 15 to 40% of
the rupture segment lengths. These lengths are an order of magnitude larger than
that inferred from characteristics of high-frequency seismic radiation (i.e.,
fmax). These data indicate that the ratio of average
coseismic slip to rupture length decreases in the presence of large
CEZs. Measured CEZ lengths, rupture dimensions, and
average slip are used to calculate average resolved shear-driving stresses and
CEZ shear-yield strengths on the order of 10–30 MPa. In
our new model of earthquake rupture, stress drop is predicted to be a small
fraction of fault strength and thus supports a partial stress drop model of
earthquake rupture for strike-slip interplate events.
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