Monitoring of Magnetotelluric Impedance Tensor near Parkfield, CA
Abstract
We analyze electrical resistivity variations recorded by remote referenced magnetotelluric instruments 22 km from the epicenter of the M 6.0 28 September 2004 Parkfield earthquake. Previous work has analyzed data from these instruments sampled at 1 Hz and processed in 1-day blocks. Those magnetotelluric impedance tensor time series suggested that a change in apparent resistivity of several percent in periods shorter than 10s immediately followed the earthquake. Unfortunately, the time resolution provided by 1-day blocks is too coarse to see signals immediately surrounding the time of the earthquake. Additionally, the most interesting signals were in frequency bands very close to the Nyquist frequency. This study addresses these two problems by examining data from the same site during a six-month window enclosing the earthquake, but sampled at 40 Hz. This finer time resolution allows us to analyze higher frequencies and show a time-series of resistivity variations in smaller 2-hour windows. We use our results to place constraints on the times and frequency bands affected by the resistivity variations surrounding the time of the earthquake. Longer term trends are considered in the context of near-surface electrical distortion effects. We attempt to correct for these effects by calculating a time series of real valued distortion tensors which best correct the broadband impedances in each time window to the six-month median apparent resistivity.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T21A2139B
- Keywords:
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- 0699 ELECTROMAGNETICS / General or miscellaneous;
- 5109 PHYSICAL PROPERTIES OF ROCKS / Magnetic and electrical properties;
- 7223 SEISMOLOGY / Earthquake interaction;
- forecasting;
- and prediction;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting