Spatial‐temporal patterns of seismic tremors in northern Cascadia

Abstract

[1] We study in detail the two consecutive episodic tremor-and-slip (ETS) events that occurred in the northern Cascadia subduction zone during 2003 and 2004. For both sequences, the newly developed Source-Scanning Algorithm (SSA) is applied to seismic waveform data from a dense regional seismograph array to determine the precise locations and origin times of seismic tremors. In map view, the majority of the tremors occurred in a limited band bounded approximately by the surface projections of the 30-km and 50-km depth contours of the plate interface. The horizontal migration of tremor occurrence is from southeast to northwest with an average speed of 5 km/d. In cross section, tremors in both sequences span a depth range of over 40 km across the interface, with the majority occurring in the overriding continental crust. In particular, 50-55% of them are located within 2.5 km from the strong seismic reflector bands above the plate interface. The lack of vertical migration implies that a slow diffusion process in the vertical direction cannot be responsible for tremor occurrences. The source spectra of tremors clearly lack high-frequency content (>5 Hz) relative to local earthquakes. We propose two possible models to explain the relationship between slip and tremors. The first one regards ETS tremors as the manifestation of hydroseismogenic processes in response to the temporal strain variation associated with the episodic slip along the lower portion of the plate interface downdip from the locked zone. In the second model, tremors and slip are associated with the same process along the same structure in a distributed deformation zone across the plate interface. Neither model can be dismissed conclusively at this stage.

Non-volcanic Tremor: A Window into the Roots of Fault Zones

“…LFEs are only rarely identified in regions with tremor outside of Japan (e.g. Kao et al, 2006, Sweet et al, 2008. It is unclear if this difference represents a real variation in tremor activity or simply a limitation in the observation capabilities of networks outside of Japan.…”

“…Within an individual ETS episode, highlysimilar bursts of tremor repeat many times, suggesting that tremor radiates from an individual location many times . From ETS episode to ETS episode, tremor also typically occurs in the same locations Kao et al, 2006), whereby much of the area where tremor occurs is the same from event to event. Ambient tremor occurring outside ETS events is typically found in these same locations as well.…”

Low-frequency earthquakes and tomography in western Japan: Insight into fluid and magmatic activity

“…Recently it is found that LF events take place constantly in the uppermost mantle under the non-volcanic areas in western Japan, possibly related to the dehydration reactions of the subducting Philippine Sea plate (e.g., Ide et al, 2007;Ito et al, 2007;Obara, 2009;Schwartz and Rokosky, 2007;. Such LF events are also found to occur in the forearc region of other subduction zones, such as Cascadia (Kao et al, 2006;Schwartz and Rokosky, 2007). It is generally considered that the LF events reflect the fluid and magmatic activities in the crust and uppermost mantle under a tectonically active region.…”

Detecting Deep Tectonic Tremor in Taiwan with a Dense Array

“…In Cascadia, when the multibeam-backprojection method is applied to the array data (Ghosh et al, 2012), nearly continuous tremor activities were detected. The array-based results provide detailed spatiotemporal evolutions of tremor activities when compared with the network-based methods, such as the WECC method (e.g., Wech and Creager, 2008), the source-scanning algorithm (Kao et al, 2006), and the waveform-matching technique (Shelly et al, 2007a).…”