Slow earthquake

A slow earthquake is an a discontinuous, earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of earthquakes sensu strictu. First detected using long term strain measurements, most slow earthquakes now appear to be accompanied by fluid flow and related tremor, which can be detected and approximately located using seismometer data filtered appropriately (typically on the order of 6 Hz). That is, they are quiet compared to a regular earthquake, but not "silent" as originally described [1].

Slow earthquakes are probably caused by a variety of stick-slip and creep processes intermediate between asperity-controlled brittle failure and ductile flow. They are best documented from intermediate crustal levels of certain subduction zones (especially those that dip shallowly--SW Japan, Cascadia, Chile), but appear to occur on other types of fault as well, notably strike-slip plate boundaries such as the San Andreas fault and "mega-landslide" normal faults on the flanks of volcanos.

Slow earthquakes can be episodic, and therefore somewhat predictable, a phenomenon termed episodic tremor and slip or ETS in the literature. Several slow earthquake events around the world appear to have triggered major, damaging seismic earthquakes in the shallower crust (e.g., 2001 Nisqually, 1995 Antofagosta). Conversely, major earthquakes trigger "post-seismic creep" in the deeper crust and mantle [2]. Slow earthquakes therefore constitute a "missing link" between deep earth processes and their typically more intermittent and catastrophic effects on the earth's surface. While it is doubtful that slow earthquakes will ever provide a robust tool for earthquake prediction, their relative regularity does serve to remind the public that the forces that generate earthquakes are ongoing and inexorable.

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