Seismic Trenching for the Pulse of the Earthquake Engine

University Of Utah, Salt Lake City UT

Investigators

Abstract

EAR-0207372: SEISMIC TRENCHING FOR THE PULSE OF THE EARTHQUAKE ENGINE Ronald L. Bruhn and Gerard Schuster Department of Geology and Geophysics University of Utah Salt Lake City, UT 84112-01 11 PROJECT ABSTRACT The purpose of this research project is to study how earthquakes occur on normal faults. The patterns of earthquake recurrence in space and time provide the 'pulse' of the earthquake engine. Investigating these patterns is revealing the complex processes that interact to generate earthquakes within the earth. Successfully understanding the nature of past heartbeats is the key to predicting the future behavior of large faults that generate earthquakes. Until now, trenching across fault scarps has been the primary tool for revealing the past history of earthquakes. By carefully logging the sedimentary deposits in trenches, geologists have been successful in deducing the sizes and recurrence intervals of past earthquakes. However, there are three shortcomings with the trenching method: it is expensive and environmentally intrusive, it is limited to depths no deeper than about 30 feet, and yields only a vei7tical cross-section of the fault. These limitations mean that the earthquake heartbeat is poorly sampled in both space and time, which can lead to erroneous conclusions about the nature of earthquakes. To overcome these problems, and to answer fundamental questions about the heartbeat of normal faulting, University of Utah earth scientists are conducting more than two dozen shallow seismic experiments over fault scarps in the Oquirrh, Stansbury and Wasatch faults. These are three large normal fault zones located in the Wasatch Front of Utah. The first arrival times of the recorded seismic waves yield velocity tomograms, or images, that delineate both the shallow and deep patterns of sedimentary 'colluvial' wedges that formed in response to the offset of the earth's surface during past earthquakes. When identified, the colluvial wedges to a depth of 50 m or so are cored by a powerful Odex drill, and samples of the deposits surrounding and within the wedges are dated by a variety of dating methods. By this procedure the science team is extending our knowledge of earthquake behavior beyond 100,000 years, and significantly extending the history of earthquakes along the Wasatch Fault and other faults in the eastern Basin and Range Province. This combined use of seismic tomography, drilling and dating methods is referred to as seismic trenching. The extended earthquake history being determined during this study provides new information for predicting the future history of earthquakes in the region, and for creating a new generation of mechanical models of processes of faulting. The research has significant impact on the applied field of earthquake hazards and the basic science of earthquake mechanics.