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Group 2 – Tectonic Geodesy | Poster Abstracts
2-008
LARGE EARTHQUAKES, AFTERSHOCKS, AND BACKGROUND SEISMICITY:
ANALYSIS OF INTERSEISMIC AND COSEISMIC SPATIAL SEISMICITY PATTERNS
IN SOUTHERN CALIFORNIA Hauksson E
We associate waveform-relocated background seismicity and aftershocks with the 3D shapes of late
Quaternary fault zones in southern California. Major earthquakes that can slip more than several
meters, aftershocks, and near-fault background seismicity mostly rupture different surfaces within
these fault zones. Major earthquakes rupture along the mapped traces of the late Quaternary faults,
called the principal slip zones (PSZs). Aftershocks occur in the immediate vicinity of the PSZs,
within ±2 km wide fault zones. In contrast, the near-fault background seismicity is accommodated
on a secondary heterogeneous network of small slip surfaces and forms spatially decaying
distributions extending out to distances of ±10 km away from the PSZs. We call the regions where
the enhanced rate of background seismicity occurs, the seismic damage zones. One possible
explanation for the presence of the seismic damage zones and associated seismicity is that they
develop as faults accommodate bends and geometrical irregularities in the PSZs. The seismic
damage zones mature and reach their finite width early in the history of a fault, during the first few
kilometers of cumulative offset. Alternatively, the similarity in width of seismic damage zones
suggests that most fault zones are of almost equal strength, although the amount of cumulative
offset varies widely. It may also depend on the strength of the fault zone, the time since the last
major earthquake as well as other parameters. In addition, the seismic productivity appears to be
influenced by the crustal structure and heat flow, with less active seismic damage zones in area of
low heat flow and thick crust.
2-009
DETAILED P- AND S-WAVE VELOCITY MODELS ALONG THE LARSE II
TRANSECT, SOUTHERN CALIFORNIA Murphy JM, Fuis GS, Ryberg T, Lutter WJ,
Catchings RD, and Goldman MR
Structural details of the crust determined from P-wave velocity models can be improved with Swave
velocity models, and S-wave velocities are needed for model-based predictions of strong
ground motion in southern California. We picked P- and S-wave travel times for refracted phases
from explosive-source shots gathers of the Los Angeles Region Seismic Experiment, Phase II
(LARSE II), and we developed refraction velocity models from these picks using two different
inversion algorithms. Vp/Vs ratios were calculated from the resulting P- and S-wave models
where both models are constrained by ray coverage. The two P-wave velocity models are
compared to each other and to results from forward modeling. Generally, the P-wave inverse and
forward models agree well for velocities lower than 5.0 km/s but only broadly agree with each
other for velocities above 5.0 km/s. Similarly, the S-wave inverse models agree well with each
other for velocities lower than 2.5 km/s but only broadly agree for velocities higher than 2.5 km/s.
The most prominent structures in our S-wave models are two north-dipping low-velocity zones in
the Central Transverse Ranges that we interpret as faults. These low-velocity zones differ
somewhat between the two inversion models, but the Vp/Vs models (one model for each
technique) show these features to be remarkably similar. Interestingly, both Vp/Vs models have
several features that are not visible in either the P- or S-wave models alone. Two of these features
(relatively high Vp/Vs ratios) occur in the vicinity of wells that bottom in “granitic” rocks and we
interpret these high Vp/Vs ratios to indicate that the granitic rocks are highly fractured or even
brecciated. Finally, to evaluate the Southern California Earthquake Center (SCEC) Community
Velocity Model (CVM), which predicts Vs based on the Vp model, we compare data from our Vp
and Vp/Vs models to empirical formulas that relate P- to S-wave velocities (see Brocher, 2005).
These empirical curves provide an adequate average relationship between Vp and Vs, but our
2008 SCEC Annual Meeting | 145