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Unformatted text preview: 1917 D uring an earthquake, rupture propa- gates along the fault plane within a few tens of seconds. Much slower rupture, lasting for weeks or months, has recently been observed in slip transients or slow earthquakes ( 1 , 2 ). These events are also dubbed silent earthquakes, because seismometers cannot sense any seismic waves during rupture. Silent earthquakes share their source region with that of low- frequency seismic waves ( 3 5 ), akin to the seismic tremor known to occur in volca- noes where it is attributed to fluids trapped in cracks or conduits. Silent earthquakes and seismic tremor do not cause strong, sudden ground motion, and are hence not considered hazardous. However, they occur in subduction zones where 90% of Earths destructive seismic energy is released in large-magnitude ( M > 7.0) megathrust earthquakes. Monitoring and interpreting such events may improve our understanding of the stress build-up in subduction zones and help in forecasting large future earthquakes ( 6 ). The docu- mented examples of this activity are in re- gions where megathrust events are expect- ed: the Nankai subduction zone in Japan and, most recently, the Cascadia subduction zone in the Pacific, off Washington state and western Canada ( 7 ). In Japan, low-noise seismometer arrays have discovered deep nonvolcanic seismic tremor in the Nankai subduction zone, where at least nine great ( M > 8.0) earth- quake sequences have occurred in the his- torical record at intervals of one or two centuries, with devastating consequences. The tremor is attributed to water that has been liberated by metamorphism of the subducting Philippine sea plate and is trapped under the forearc crust ( 3 ). Intraslab earthquakes have been linked to such metamorphism ( 8 ). Seismic explo- ration has also elucidated the interplate fault region and its possible water content ( 9 , 10 ). For example, high pore-fluid pres- sure has been imaged in the Tokai segment ( 6 ) and suggested as a cause of the silent earthquake detected there. In the Cascadia subduction zone, a silent earthquake was detected ( 1 ) with space-geodetic, Global Positioning System (GPS) arrays, which sense the slow motion of Earths surface over several hundred kilometers. Seismic tremor occurred in the same time span, from sources in the region where the silent earthquake slip occurred. This activity, called episodic tremor and slip (ETS), was predicted to recur in Cascadia every 14 months, with the latest event predicted for July 2004 ( 11 ). The ex- pected ETS event was observed from 8 to 24 July, with the slip migrating northward from Puget Sound, Washington, to Vancouver Island at the northern end of the Cascadia subduction zone. Two significant ( M = 5.8 and 6.4) earthquakes were also detected off Vancouver Island. The event was preceded by another, unexpected episode of tremor and slip beginning in late April; this event may have moved southward into northern California and ter- minated at the southern end of the sub-...
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This note was uploaded on 07/23/2008 for the course GEOSC 203 taught by Professor Anandakrishnan during the Fall '07 term at Pennsylvania State University, University Park.
- Fall '07