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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS006 Oral Presentation 1933
A New Approach to Paleoseismic Event Correlation -
Dr. Glenn Biasi
Seismological Laboratory University of Nevada Reno
Ray J. Weldon
When there is more than one paleoseismic site on a fault, the issue arises of correlating events between
them. As the number of paleoseismic sites increases, the number of possible correlations and
combinations among them becomes unmanageable. Correlation based on dating evidence alone
remains uncertain even with high quality radiocarbon evidence. These realities seriously complicate the
use of paleoseismic data for estimation of recurrence intervals and seismic hazard. We present a new
approach that permits estimates of seismic hazard and multiple-site earthquake recurrence without
having to solve the correlation problem per se. The most important input data for our approach are
reasonably complete paleoseismic records and approximate dates for the events. To begin we develop a
complete list of all possible single and multiple-site ruptures allowed within the event dating
uncertainties. Each individual paleoseismic event is counted as a rupture. We compare each event with
the neighboring site chronology, and make another rupture of any which overlap with the first. If two
overlap in time, both become new ruptures. Each rupture is extended in turn to a third site chronology
to make new rupture(s). Lack of dating overlap keeps this process from expanding as the outer product
of the number of events. Ruptures are also extended through sites with no evidence of the rupture, but
penalized for contradicting the paleoseismic record there. The pool of all possible ruptures thus includes
all the available event data for the fault. Paleoseismic displacement estimates or displacements drawn
from an average displacement model are used to extrapolate ruptures beyond the outside sites in each
rupture. Rupture length is used with regression relations to assign each rupture an average
displacement and simple displacement profile. To constrain the rupture history of the fault we construct
rupture scenarios by drawing from the pool of all possible ruptures. Sampling is constrained so that
every event is included each scenario exactly once. The scenario becomes a possible rupture history for
the fault at some level of probability. A large suite of scenarios is developed in this manner. To extract
likely scenarios from the suite, scenarios are graded for consistency with dating evidence (do dates
overlap strongly or weakly) and by comparing the displacement totaled over the ruptures at any given
point on the fault with the total predicted independently from the geodetic or geologic slip rate and
elapsed time since the oldest event. Other grading criteria may also be applied, such as by using perevent
displacement measurements. Ensembles of scenarios become more or less likely actual histories
of the fault. They may then be used directly in seismic hazard assessment using ground motion
prediction methods, or indirectly to estimate the likely fractions of shorter or longer ruptures, frequency
of multiple segment ruptures, etc. We illustrate the method with paleoseismic data from the southern
San Andreas fault in California. Results have been provided as inputs to the most recent assessments of
the Working Group on California Earthquake Probabilities.
Keywords: paleoseismology, seismic hazard, san andreas fault