HR: 1340h
AN: S23B-0306    [Abstracts]
TI: Tensor Strain Seismograms: a New Tool for Earthquake Science
AU: Gladwin, M T
EM: mike.gladwin@csiro.au
AF: CSIRO, P.O. Box 883 , Kenmore, Qld 4069 Australia
AU: * Malin, P E
EM: malin@duke.edu
AF: Duke University, Box 90227 , Durham, NC 27708-0227 United States
AB: In the expressions for seismic wave propagation in energy dissipating rock, the terms involving particle acceleration and velocity are balanced by local differences in the elastic forces plus those produced by the earthquake source. In homogeneous rock, the spatial differences in elastic forces depend only on the differences in strain. Thus, if one could measure the particle accelerations, velocities, and strain differences, the nature of the source could be solved for directly. In this presentation, we briefly summarize the steps we are taking to build and test an instrument system to make such measurements. Our current working system consists of a standard tensor strainmeter operating at 100Hz, and standard 3-component spring seismograph sampled at similar or higher rates. These sensors are colocated in a two hundred meter deep borehole. Our next generation prototype, which we will begin testing in the SAFOD Pilot Hole in 2005, will also contain accelerometers. We have used the high sampling rate borehole strainmeter and seismometer combinations to record, for example, the 19May04 M~6 earthquake in Taiwan at offsets of a few tens of kms, plus several other Taiwanese events. Separating the strainmeter record into shear and area strains and rotating the seismograms into dynamic coordinates allows rapid identification of different wave types and direct and secondary phases in the seismograms. The M~6 event produced a surprisingly long coda on the strainmeter. Comparing other similar strain and velocity seismograms has allowed various aspects of their codas to be resolved and identified. We see these results as early steps in full-scale, multi-parameter broadband measurements of tectonic displacements, pre- and post-rupture strains, and the elastic velocities and accelerations produced by small and large events. In the long run our aim is to use such data to measure their source characteristics directly.
DE: 7260 Theory and modeling
DE: 7294 Instruments and techniques
SC: Seismology [S]
MN: 2004 AGU Fall Meeting