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Group 2 – FARM | Poster Abstracts
quite small, stresses concentrated at the rupture front are consistent with typical static (and low
velocity) friction coefficients of 0.6-0.9; this stress concentration is required to initiate slip.
Growing slip pulses have stress drops close to 3 MPa and feature slip increasing with propagation
distance at a rate of about 0.14 m/km. These values are consistent with seismic inferences of stress
drop and field constraints on slip-length scaling. On the other hand, cracks have stress drops of
over 20 MPa, and slip at the hypocenter increases with propagation distance at a rate of about 1
m/km.
2-068
STATISTICS OF EARTHQUAKE STRESS DROPS FOR EVOLVING SEISMICITY ON
A HETEROGENEOUS FAULT IN AN ELASTIC HALF-SPACE Bailey IW, and Ben-Zion Y
Understanding what limits the size of earthquake stress drops has important implications for
estimating ground motion. Theoretical estimates based on laboratory friction data and
measurements of stress in the crust are >100 MPa, yet seismological derivations are typically of
order ~1-10 MPa. The discrepancy may stem from the fact that earthquake stress drops are average
values over a rupture area that may have highly heterogeneous initial stress, while the theoretical
estimates assume an essentially homogeneous stress. We investigate properties of stress drops in
simulations of evolving seismicity and stress field on a heterogeneous fault. The model fault (Ben-
Zion & Rice, 1993) consists of a set of inherently-discrete slip patches surrounded by a 3-D elastic
half-space. The discrete slip patches provide a simple representation of heterogeneities associated
with segmentation and other geometrical complexities. Previous studies have shown that the
model produces many statistical features of seismicity compatible with observations, e.g.,
frequency-size and temporal event statistics, hypocenter distributions, and scaling of source-time
functions. The model simulations allow us to investigate stress drops from a range of initial stress
distributions that are determined by the self-organized stress evolution along the fault over time.
We show that the stress drops are systematically lower than predicted for a homogeneous fault,
and that this effect is stronger as larger events are considered. Events that saturate the seismogenic
zone consistently have stress drops that are ~30% of the predictions based on the average fault
strength, as well as showing less variation than the stress drops of smaller events. We further
investigate how this variation is affected by rheological properties of the model and hypocentral
depth.
2-069
CONSTANT STRESS DROP FROM SMALL TO GREAT EARTHQUAKES IN
MAGNITUDE-AREA SCALING Shaw BE
Earthquakes span a tremendous range of scales, more than 5 orders of magnitude in length. Are
earthquakes fundamentally the same across this huge range of scales, or are the great earthquakes
somehow different from the small ones? We show that a robust scaling law seen in small
earthquakes, with stress drops being independent of earthquake size, indeed holds for great
earthquakes as well. The simplest hypothesis, that earthquake stress drops are constant from the
smallest to the largest events, combined with a more thorough treatment of the geometrical effects
of the finite seismogenic layer depth, gives a new magnitude area scaling which matches the data
well, and better over the whole magnitude range than the currently used scaling laws which have
non-constant stress drop scaling. This has significant implications for earthquake physics and for
seismic hazard estimates.
2008 SCEC Annual Meeting | 177