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Group 2 – Tectonic Geodesy | Poster Abstracts
2-002
TESTING COMMUNITY VELOCITY MODELS FOR SOUTHERN CALIFORNIA
USING THE AMBIENT SEISMIC FIELD Ma S, Prieto GA, and Beroza GC
We correlate the vertical component of ambient seismic noise data recorded on 56 broadband
stations with dense coverage in the greater Los Angeles area, to determine station-to-station
Green’s functions. These Green’s functions provide an important test of community velocity
models (SCEC CVM 4.0 and CVM-H 5.2) used for strong ground motion prediction for future
scenario earthquakes in southern California. Comparisons of the ambient-noise Green’s functions
for nearly 300 paths, with those calculated by the finite-element method in the community velocity
models reveal a strong waveform similarity for the dominant surface waves between 0.1 and 0.2
Hz. We find a mean correlation coefficient between the ambient-noise and finite-element Green’s
functions of 0.62 for the CVM 4.0 and 0.49 for the CVM-H 5.2, indicating stronger waveform
similarity for CVM 4.0. We also find that for 77% of the paths, the surface waves in the finiteelement
Greens’ functions for CVM 4.0 arrive early, suggesting that the CVM 4.0 has velocities in
the upper 10 km that are too fast along these paths. The same bias is evident for CVM-H 5.2, but is
substantially smaller, with only 61% of the paths too fast. For 67% of the paths CVM 4.0 has
velocities faster than CVM-H 5.2. The time lags we obtain between the ambient-noise and finiteelement
Green’s functions provide key information for improving future community velocity
models.
2-003
RESOLVING 3D FAULT GEOMETRY AT DEPTH ALONG ACTIVE STRIKE-SLIP
FAULTS: SIMPLE OR COMPLEX? Nicholson C, Hauksson E, Plesch A, and Shearer PM
A primary concern for SCEC in terms of understanding earthquake rupture and seismic hazard is
resolving active 3D fault geometry at seismogenic depths. This is particularly important for
properly extrapolating near-surface observations to depth where principal ruptures occur. This
collaborative project is using recently developed seismicity and focal mechanism catalogs to
identify the geometry and sense of slip of active fault segments at depth, and to help evaluate the
SCEC Community Fault Model (CFM). This comparison forms the basis for identifying and
developing improved fault representations for CFM, as well as identifying possible systematic
biases in hypocentral location or fault models. In several places, aligned hypocenters are
systematically offset from CFM 3D fault surfaces. This offset could be the result of: a) incorrect
extrapolation of mapped surface fault traces to depth; b) earthquake mislocation owing to velocity
models or location procedures used; or possibly c) existence of previously unidentified active
subparallel fault strands. Along the Elsinore fault, active fault segments exhibit dips that vary
along strike from about 80°SW to near-vertical to 70°NE. However, in CFM this fault is presumed
to be vertical. Similar changes in dip along strike are found for the adjacent Agua Tibia-Earthquake
Valley fault. Where the Elsinore fault is multi-stranded, fault segments separated by only a few
kilometers can either remain subparallel throughout the seismogenic zone, or merge at depth to
form y-shaped structures and intervening half-grabens, as appears to be the case beneath Lake
Elsinore. Farther north, where the fault zone splays to form the Whittier and Chino faults, the
seismicity exhibits a more complex 3D fault structure that includes intersecting high- and lowangle
faults, as reflected in the recent 2008 Chino Hills earthquake sequence. At issue is whether
these complex fault geometries defined by the microearthquakes are necessarily representative of
the primary, through-going strike-slip fault that will likely produce the major slip and seismic
moment release, or whether these structures illuminated by the seismicity only represent adjacent
secondary faults. This issue is particularly important along the San Andreas fault, where various
sections are largely aseismic and where models of the fault geometry at depth--being either
2008 SCEC Annual Meeting | 143