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Group 1 – SHRA | Poster Abstracts
Seismic Hazard and Risk Analysis (SHRA)
1-025
PRELIMINARY REPORT ON THE 29 JULY 2008 MW5.4 CHINO HILLS, EASTERN
LOS ANGELES BASIN, CALIFORNIA, EARTHQUAKE SEQUENCE Giveon M, Hauksson
E, Hutton K, Kanamori H, Wei S, Hough SE, Felzer K, Given D, Sevilgen V, and Yong A
The 29 July 2008 Mw5.4 Chino Hills earthquake was the largest event to occur within the greater
Los Angeles metropolitan region since the 1994 Northridge earthquake. The earthquake was
widely felt in a metropolitan region with a population of over 10 million people, and was recorded
by hundreds of broadband and strong motion instruments. Preliminary analysis reveals that this
event fits into a two decade long sequence of events in the northern Los Angeles basin.
The initial real-time SCSN moment tensor solution was determined using an automated analysis of
waveforms from six stations. The moment tensor was also determined using the “cut-and-paste”
technique, where we analyzed the broadband waveforms of regional seismograms from 22 stations
with epicentral distances between 100 to 200 km and maximum azimuth gap of 32°. Both
techniques yielded the same mechanism of oblique composite of thrust and left-lateral slip on an
east-northeast or alternatively on a north-northwest striking plane. We relocated the mainshock
and aftershocks, using a 3D velocity model and HypoDD. The relocations revealed three off-fault
clusters and minimal clustering of aftershocks around either mainshock nodal plane. The first
motion focal mechanisms of the two M>3 aftershocks exhibited strike-slip motion on the northwest
or northeast striking nodal planes.
The 2008 Chino Hills earthquake sequence occurred in the depth range of 13 to 16 km, with low
aftershock productivity, a large difference between ML and Mw, and a high mainshock stress drop
revealing the state of stress at depth, near the brittle-ductile transition zone. Portions of the
Whittier and Chino faults were likely brought closer to Coulomb failure by the mainshock. Felt
ground motions appear to correlate with expected basin effects. All these attributes shed more light
on the seismotectonic setting, ground motions, and state of stress of this region.
1-026
NONSTATIONARITIES IN THE CALIFORNIA CATALOG Page MT, Alderson D, Doyle J,
and Michael AJ
We search for several types of nonstationarity in the ANSS California catalog. First, we search for
one type of spatial nonstationarity, variation in maximum magnitude. In areas that have high
seismicity rates and low maximum magnitudes there are some constraints on the maximum
magnitude; we show that furthermore there is resolvable spatial variation in the maximum
magnitude.
We also investigate seismicity near faults in the SCEC Community Fault Model. We search for
anomalously large events that might be signs of a characteristic earthquakes distribution. We find a
null result – seismicity near major fault zones in Southern California is well-modeled by a
Gutenberg-Richter distribution, with no evidence of characteristic earthquakes is observed up to
approximately magnitude 7.
Finally, we search for temporal magnitude nonstationarity in the California catalog. We investigate
the magnitude autocorrelation function, which in the case of a stationary parent distribution
should exhibit no significant deviations from zero. As expected, we find positive magnitude
correlations at short time lags, which are likely the result of short-term aftershock incompleteness.
2008 SCEC Annual Meeting | 81