H. Dragert | Geology Reports

USGS2008

Characterizing an "uncharacteristics" ETS event in northern Cascadia

GPS and borehole strainmeter data allowed the detection and model characterization of a slow slip event in northern Cascadia in November 2006 accompanying a brief episode of seismic tremor. The event is much smaller in area and duration than other well-known ETS events in northern Cascadia but is strikingly similar to typical ETS events at the Nankai subduction zone. The 30-45 km depth range and the 2-3 cm slip magnitude as interpreted for this event appear to be common to most ETS events in these two subduction zones, regardless of their sizes. We infer that the Nankai-type small ETS events must be abundant at Cascadia and that ETS event at the two subduction zones are governed by a similar physical process. Copyright 2008 by the American Geophysical Union.

Geophysical Research Letters↗

USGS2008

Understanding slow slip, tremor, and quakes

[No abstract available]

Conference Paper↗

USGS2004

Earthquake nucleation by transient deformations caused by the M = 7.9 Denali, Alaska, earthquake

The permanent and dynamic (transient) stress changes inferred to trigger earthquakes are usually orders of magnitude smaller than the stresses relaxed by the earthquakes themselves, implying that triggering occurs on critically stressed faults. Triggered seismicity rate increases may therefore be most likely to occur in areas where loading rates are highest and elevated pore pressures, perhaps facilitated by high-temperature fluids, reduce frictional stresses and promote failure. Here we show that the 2002 magnitude M = 7.9 Denali, Alaska, earthquake triggered wide-spread seismicity rate increases throughout British Columbia and into the western United States. Dynamic triggering by seismic waves should be enhanced in directions where rupture directivity focuses radiated energy, and we verify this using seismic and new high-sample GPS recordings of the Denali mainshock. These observations are comparable in scale only to the triggering caused by the 1992 M = 7.4 Landers, California, earthquake, and demonstrate that Landers triggering did not reflect some peculiarity of the region or the earthquake. However, the rate increases triggered by the Denali earthquake occurred in areas not obviously tectonically active, implying that even in areas of low ambient stressing rates, faults may still be critically stressed and that dynamic triggering may be ubiquitous and unpredictable.

Nature↗