Continuous GPS stations operating over the last decade have begun to detect transient deformation signals from aseismic processes that can occur over hours to days. Deviations from uniform motion of GPS stations in Washington and British Columbia led to the detection of an aseismic 1-2 week slip event on the deeper (25- to 45-km) northern Cascadia subduction zone interface (Dragert et al., 2001). Similar events repeat at 13- to 16-month intervals (Miller et al., 2002), and are also spatially and temporally correlated with pulsating, tremorlike seismic signals with 1-5 Hz frequency content that do not have impulsive onsets (Rogers and Dragert, 2003). One possible explanation for these episodic tremor and slip (ETS) events is that fluids generated by dehydration processes from the slab are playing an important role in regulating the deep slip on the interface (Obara, 2002).
Recent evidence suggest that ETS activity may be occurring along the entire Cascadia subduction zone (Szeliga et al., 2004). We are assessing our GPS and seismic observations to provide better constraints on this behavior in northern California.
The daily position time series at Yreka (YBHB) (Figure 27.1) shows evidence for cm-level transient deformation lasting 1-2 weeks that is similar to the behavior detected in northern Cascadia. We have developed a detection algorithm, based on a smoothed, 10-day running median of hourly power spectral density estimates in the 1-5 Hz range, and have applied this algorithm to Yreka (YBH) seismic data from 2000-2004. The high peaks (Figure 27.1) correspond to periods when the tremor noise dominates the signals, and these peaks are highly correlated with the episodic westward GPS displacements. The peaks have a 10.92.0 month period (shorter than the 14 month period found near Puget Sound). The correlations are less clear on the north GPS component. Episodic displacements at these frequencies have not been observed on the coastal continuous GPS stations, so currently the YBHB GPS observations and the corresponding tremors observed at YBH and other northern California stations provide the only evidence for episodic slow earthquakes in the southern Cascadia subduction zone. We are currently studying tremors detected on other seismic instruments and improving our GPS regional filtering techniques to better characterize the possible deformation signals.
Dragert, H., K. Wang, and T. S. James, A silent slip event on the deeper Cascadia subduction interface, Science, 292, 1525-1528, 2001.
Miller, M. M., T. Melbourne, D. J. Johnson, W. Q. Sumner, Periodic slow earthquakes from the Cascadia subduction zone, Science, 295, 2423, 2002.
Obara, K., Nonvolcanic deep tremor associated with subduction in southwest Japan, Science, 296, 1679-1681, 2002.
Rogers, G., and H. Dragert, Episodic tremor and slip: The chatter of slow earthquakes, Science, 300, 1942-1944, 2003.
Szeliga, W., T. I. Melbourne, M. M. Miller, and V. M. Santillan, Southern Cascadia episodic slow earthquakes, Geophys. Res. Lett., 31, L16602, doi:10.1029/2004GL020824, 2004.
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