Project Title: Quantifying Slip Rates Over Time: Estimating Long-Term Deformation Rates Using Cosmogenic Isotopes
Mendenhall Fellow: Ari Matmon, (650) 329-5552, amatmon@usgs.gov
Duty Station: Menlo Park
Start Date: May 5, 2002
Education: Ph.D., 2001, Earth Sciences, Hebrew University, Jerusalem, Israel
Research Advisors: David Schwartz (650) 329-5651, dschwartz@usgs.gov; Wayne Thatcher, (650) 329-4810, thatcher@usgs.gov; R.J. Ryerson, Lawrence Livermore National Laboratory, (925) 422-6170, ryerson@llnl.gov
Project Description: Geologic data provide information on deformation rates on two temporal scales: 1) short-term intervals varying from hundreds to ten thousands of years deduced from paleoseismic studies, such as trenching, and 2) long-term information over millions of years deduced from deformed pre-extensional stratigraphic markers. Geodetic data record strain accumulation rates on the order of years or tens of years. In some instances (particularly along strike-slip faults in the San Andreas Fault system) the geodetically inferred fault slip rates agree well with available geologic slip rates. In other regions (e.g., the Basin and Range province or the New Madrid seismic zone) there are significant discrepancies between long-term geologic rates and short-term geodetic rates of deformation. Quantification of slip rates over long and varying time intervals will help determine the relevant deformation rate to use as input into the National Seismic Hazard Map and in other probabilistic seismic hazard assessments. Quantification of these rate changes over time will also provide critical constraints on the physical mechanisms controlling the cycle of earthquake strain accumulation and release.Recently, activities of in-situ produced 26Al,10Be, and 36Cl have been measured and used to estimate the rates and distribution of bedrock erosion, basin scale sediment production, soil production, and timing and rates of Quaternary displacement. Cosmogenic nuclide activities are interpreted using models dependent on a variety of assumptions. On noneroding surfaces, cosmogenic nuclide activities may be used to calculate exposure ages. Studies show that measuring 36Cl in samples collected from Ca or Cl-rich bedrock scarps is an effective method for determining the age of motion on normal faults. Any planar, uneroded bedrock scarp with a simple exposure history (without burial or nontectonic exhumation) is a candidate for cosmogenic nuclide dating. Provided that exposure histories are simple and scarps are in good condition, cosmogenic dating will determine scarp ages more precisely and more accurately than measuring relative weathering characteristics. Detailed and precise nuclide dating of fault scarps will greatly improve models of fault segmentation and fault behavior. Data show that in appropriate settings, cosmogenic nuclides can be used to provide the temporal and spatial dating resolution needed to understand better the long-term behavior of individual normal fault systems in complex extensional environments.
Ari aims to quantify slip rates along faults in the western United States on a time scale of 103 to 105 years by measuring the activities of cosmogenic 10Be and 26Al or 36Cl on fault scarps, alluvial fans and fluvial terraces. Currently, there are very few data of this type available, even for the major faults in the United States. A major scientific challenge in assessing earthquake hazard is defining slip rates for major faults and the appropriate recurrence interval for expected earthquakes. The proposed study is directly related to a current USGS study of slip rate measurements along faults in the western United States and the assessment of seismically hazardous regions.
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