Annual Meeting - SCEC.org

Poster Abstracts
SPATIAL VARIABILITY OF GROUND MOTION AMPLIFICATION FROM LOW-VELOCITY SEDIMENTS
INCLUDING FRACTAL INHOMOGENEITIES WITH SPECIAL REFERENCE TO THE SOUTHERN
CALIFORNIA BASINS (B-017)
K.B. Olsen and B.H. Jacobsen
Many state-of-the-art area-specific velocity models (e.g., the Southern California Earthquake Center (SCEC) Community
Velocity Model (CVM) V.4.0) include a wealth of geophysical data, such as tomographc results, and gravity, reflection and
well-log data. However, these CVMs usually poorly resolve near-surface small-scale amplification effects. Toward
characterizing the variability of shallow sediment amplification, we have investigated the effects of inhomogeneities with
fractal distributions augmented onto the shallow seismic velocity structure derived from the SCEC CVM V.4.0. Our analysis
used linear 0-2 Hz 3D visco-elastic finite-difference wave propagation with grid spacings of 25 m or less. We find that even
simple and rather weak fractal stochastic inhomogenities imply significant variations in ground motion amplifications (up to a
factor of four), including bands of strong amplification aligned along the average ray path from a horizontally-propagating
SH-wave source. We show that these patterns depend strongly on the incidence angle of the main wavefront. For verticallyincident
planar SH-wave sources we find that the largest contribution to the site effects from small-scale heterogeneities arise
from those included in the upper ~100 m of the sediment column. Finally, it is important to tune the statistical model
(scattering Q) with anelastic attenuation (intrinsic Q), where a tradeoff appears to persist.
COMPARISON OF USGS PAGER EMPIRICAL FATALITY RATES TO RECONNAISSANCE OBSERVATIONS
AND AN ACCOUNTING OF UNCERTAINTIES IN THE FATALITY MODEL (B-079)
A.H. Olsen, E. So, M. Hearne, K. Jaiswal, and D.J. Wald
The U.S. Geological Survey's Prompt Assessment of Global Earthquakes for Response (PAGER) system estimates fatalities and
economic losses typically within thirty minutes of a significant earthquake. Emergency responders, governmental and aid
agencies, and the media are the primary users of this information. The operational PAGER fatality model combines: estimates
of shaking intensities (in the form of a ShakeMap); estimates of the populations exposed to the shaking (from LandScan); and
fatality rates as a function of intensity, specific to a country or set of countries. The fatality-rate model is empirical, developed
from observations of total fatalities in past events of the last several decades. The first part of this study compares the existing
fatality-rate model to additional data on observed fatalities from municipality- and district-level reconnaissance surveys. This
comparison is not straightforward because the intensity value associated with a fatality is not always known or known with
certainty. The second part of this study identifies the sources of uncertainty in the empirical fatality model, and it proposes a
method to combine the sources of uncertainty for a prediction of fatalities. Since this work is in progress, we present here only
our methodology and some preliminary results.
TIMING OF LARGE EARTHQUAKES BETWEEN 400 AD TO PRESENT ALONG THE CLAREMONT FAULT,
NORTHERN SAN JACINTO FAULT ZONE, FROM MYSTIC LAKE, CALIFORNIA. (A-150)
N.W. Onderdonk, T.K. Rockwell, S.F. McGill, and G. Marliyani
Additional trench exposures and radiocarbon dating over the past year have refined the timing of pre-historic earthquakes
along the northern San Jacinto fault zone at the Mystic Lake paleoseimic site. Seven events are recorded in the upper 1.8
meters of strata, which spans approximately 1600 years. The ages of the events are constrained by 50 radiocarbon dates from
detrital charcoal and are as follows:
Event 1: 1738 to 1853 AD
Event 2: 1670 to 1828 AD
Event 3: 1521 to 1616 AD
Event 4: 1349 to 1445 AD
Event 5: 1076 to 1258 AD
Event 6: 807 to 961 AD
Event 7: 579 to 845 AD
The recurrence interval for the last 7 events ranges from 159 to 210 years and based on historical data it has been at least 200
years since the last ground-rupturing earthquake on the northern San Jacinto fault.
212 | Southern California Earthquake Center