Annual Meeting - SCEC.org

Group 2 – Seismology | Poster Abstracts
alternating intervals of course-grained sands, gravels, and cohesive, fine-grained silts, clays and
organic-rich clays. The main source of these sediments was most likely the San Gabriel River,
which is located ~2 km East of the study site, with material possibly coming from the Los Angeles
River, ~7 km to the West. In order to corroborate our cross-borehole correlations, which were based
on visual comparison, we are using high-resolution records of magnetic susceptibility (Chi) to
correlate stratigraphy between four boreholes that appear to span the steepest part of the young
fold. Specifically, we are focusing on cores 7, 10, 11 and 13, which are 30 to 100 m apart from one
another. We measured the magnetic susceptibility every 2.5cm in all boreholes using a Bartington
probe sensor. We have attempted to correlate between pairs of boreholes under a working
assumption that the Chi records should be correlatable. Our initial results suggest that this
hypothesis is largely correct, but we have also encountered segments of the cores in which the Chi
data do not correlate between cores. These results demonstrate that magnetic susceptibility is a
useful tool in correlating stratigraphic horizons that might otherwise be difficult to correlate with
other methods.
2-123
A BRIEF HISTORY OF LUMINESCENCE GEOCHRONOLOGY AS IT HAS BEEN
APPLIED TO USGS PALEOSEISMOLOGY ASSESSMENTS AND MAPPING
PROJECTS Mahan SA
The aim of paleoseismology is to assess the probability and severity of future earthquakes using
the geologic record of past earthquakes. To this end, geology and geomorphology provide a
context for the location and amount of past fault ruptures; geochronology provides the critical
temporal context for the occurrence and sequencing of earthquakes prior to the historic record.
Traditionally, carbon-14 dating has been employed for geochronologic purposes; however,
materials containing carbon are not always associated with earthquake-related deposits, and the
technique’s dating range is limited to around 40,000 years. Because it is not subject to these
restrictions, luminescence dating (Thermoluminescence [TL] and optically stimulated luminescence
[OSL]), has been used to directly date the last time sediment associated with fault movement was
re-deposited and thus exposed to sunlight. Prescott and Robertson (1997) review comparisons
between luminescence dating methods as well as other Quaternary geochronological methods, and
this discussion is included in the poster presentation. A paper by M. Fattahi (“Dating Past
Earthquakes and Related Sediment by Thermoluminescence Methods; a Review”; in press, QI)
provides an excellent history of early luminescence work.
The U.S. Geological Survey (USGS) has used luminescence dating techniques since 1996, in order to
more fully elucidate past fault movements as identified in trenches. Luminescence data and
research pertaining to paleoseimology has been published for such locations as southern
California, Nevada, Utah, Indiana, and Colorado in USGS publications, National Earthquake
Hazards Reduction Program reports or refereed journal publications. In this poster three
luminescence dating studies will be presented in detail: (1) Paleoliquifaction sites within the New
Madrid Seismic Zone, located along the Wabash River of Indiana; (2) A 95-mile long lineament in
northeastern Colorado near the town of Anton, CO, where a trench was excavated to determine
whether the lineament formed due to Pleistocene faulting; and (3) fine-grained layers within
alluvial-fan deposits from southern Death Valley that were dated, and the resulting implications of
those ages for climate-driven sedimentation along a tectonically active mountain front.
2008 SCEC Annual Meeting | 207