Annual Meeting - SCEC.org

TUESDAY | Meeting Program
evolution of earthquake rupture and on the induced strong ground motions. One of the key issues raised by the recent
Tohoku-Oki earthquake is the possibility of a very different frequency content of slip in different areas of a fault. I will
offer an interpretation of this observation, although non-unique, in terms of rupture proagation through the residual
stress field left by previous seismicity.
11:00 Earthquake Focal Mechanisms Imply Homogeneous Stress at Seismogenic Depths (J. Hardebeck, USGS)
Stress at seismogenic depths is often thought to be heterogeneous, based on the variety of earthquake focal mechanisms
present in some catalogs. However, when differences in tectonic regime and the sizable focal mechanism uncertainties
are accounted for, the focal mechanisms appear more uniform. On average, the closer two events are in space, the more
similar their focal mechanisms, so comparing events over a large area increases the apparent heterogeneity.
Additionally, even relatively well-constrained focal mechanisms may have uncertainties of 25° or more; these errors
increase the apparent focal mechanism variability, especially for similar mechanisms. It has been proposed that
homogeneous focal mechanisms could be compatible with an extremely heterogeneous crustal stress field. The
hypothesis is that the stress field contains significant heterogeneity in stress orientation, and earthquakes preferentially
occur where the local stress tensor aligns with the stressing rate tensor, which loads the faults that are well-oriented in
those areas. Earthquake focal mechanisms would reflect the stressing rate tensor, and uniform tectonic loading would
produce relatively uniform focal mechanisms. A testable prediction of this model is that focal mechanisms before and
just after a large earthquake should align with different stress fields: the tectonic loading and the mainshock static
stress perturbation, respectively. I test this prediction using aftershocks of the Landers earthquake. The observed
rotation of focal mechanisms near the mainshock rupture could represent a true coseimic stress rotation, so I limit my
test to aftershocks far enough away from the mainshock that the stress change is too small to rotate the stress field. I
identify the aftershock zone as the region where post-Landers events are significantly more consistent with triggering
by the Landers-induced static stress perturbation than the pre-Landers events. I find that these aftershocks also align
with the same stress field as the pre-Landers mechanisms, contradicting the heterogeneous stress model. The aftershocks
occurred on faults that were well oriented for failure in the pre-Landers background stress field and further loaded by
the Landers-induced static stress change. The heterogeneous stress model, if it applies anywhere, should apply to the
complex active fault system in southern California. This counterexample is evidence that the heterogeneous stress
model is not widely applicable.
11:30 Strain-Based Interpretation of Southern California Focal Mechanism Data and Implications for a
Community Stress Model (I. Bailey, USC)
The observations based on recorded motion (e.g. seismograms and geodetic data) that are typically used to constrain the
crustal stress field are more directly related to strain. Studying these data directly in terms of strain can help to better
understand the degree to which we can constrain crustal stress. Here, we examine characteristics of focal mechanisms
of small earthquakes (ML