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A slow earthquake is a discontinuous, earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of a typical earthquake. 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 in the 1–5 Hz band). That is, they are quiet compared to a regular earthquake, but not "silent" as described in the past.
Earthquakes occur as a consequence of gradual stress increases in a region, and once it reaches the maximum stress that the rocks can withstand a rupture generates and the resulting earthquake motion is related to a drop in the shear stress of the system. Earthquakes generate seismic waves when the rupture in the system occurs, the seismic waves consist of different types of waves that are capable of moving through the Earth like ripples over water. The causes that lead to slow earthquakes have only been theoretically investigated, by the formation of longitudinal shear cracks that were analysed using mathematical models. The different distributions of initial stress, sliding frictional stress, and specific fracture energy are all taken into account. If the initial stress minus the sliding frictional stress (with respect to the initial crack) is low, and the specific fracture energy or the strength of the crustal material (relative to the amount of stress) is high then slow earthquakes will occur regularly. In other words, slow earthquakes are caused by a variety of stick-slip and creep processes intermediated between asperity-controlled brittle and ductile fracture. Asperities are tiny bumps and protrusions along the faces of fractures. 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 faults as well, notably strike-slip plate boundaries such as the San Andreas fault and "mega-landslide" normal faults on the flanks of volcanos.
Faulting takes place all over Earth; faults can include convergent, divergent, and transform faults, and normally occur on plate margins. As of 2013 some of the locations that have been recently studied for slow earthquakes include: Cascadia, California, Japan, New Zealand, Mexico, and Alaska. The locations of slow earthquakes can provide new insights into the behavior of normal or fast earthquakes. By observing the location of tremors associated with slow-slip and slow earthquakes, seismologists can determine the extension of the system and estimate future earthquakes in the area of study.
- low-frequency earthquakes (LFE)
- very low frequency earthquakes (VLF)
- slow-slip events (SSE)
- episodic tremor and slip (ETS)
Episodic tremor and slip
Slow earthquakes can be episodic (relative of plate movement), and therefore somewhat predictable, a phenomenon termed "episodic tremor and slip" or "ETS" in the literature. ETS events can last for weeks as opposed to "normal earthquakes" occur in a matter of seconds. Several slow-earthquake events around the world appear to have triggered major, damaging seismic earthquakes in the shallower crust (e.g., 2001 Nisqually, 1995 Antofagasta). Conversely, major earthquakes trigger "post-seismic creep" in the deeper crust and mantle. Just like regular earthquakes, slow earthquakes can cause devastating tsunamis, such as the Mentawai tsunami in 2010. The earthquake that caused this registered a magnitude of 7.8 and struck offshore the Mentawai islands in western Indonesia, causing more than 400 human casualties. Seismologists characterized it as a slow earthquake due to disproportionately large tsunami waves, rupture duration near 125 seconds, shallow near-trench slip, and deficiencies in energy.
Every five years a year-long quake of this type occurs beneath the New Zealand capital, Wellington. It was first measured in 2003, and has reappeared in 2008 and 2013. It lasts for around a year each time, releasing as much energy as a magnitude 7 quake.
- Michael R. Forrest. "Slow Earthquakes". Scec.org. Retrieved 2010-05-05.
- Brown, Kevin M.; Tryon, Michael D.; DeShon, Heather R.; Dorman, LeRoy M.; Schwartz, Susan Y. (2005). "Correlated transient fluid pulsing and seismic tremor in the Costa Rica subduction zone" (PDF). Earth and Planetary Science Letters. Elsevier. 238 (1–2): 189–203. Bibcode:2005E&PSL.238..189B. doi:10.1016/j.epsl.2005.06.055.
- Timothy I. Melbourne & Frank H. Webb (2003-06-20). "GEOPHYSICS: Enhanced: Slow But Not Quite Silent - Melbourne and Webb 300 (5627): 1886 - Science". Sciencemag.org. doi:10.1126/science.1086163. Retrieved 2010-05-05.
- Aida Quezada-Reyes (2011). "Slow Earthquakes: an Overview" (PDF).
- Teruo Yamashita (1980). "Causes of Slow Earthquakes and Multiple Earthquakes - Teruo Yamashita". Journal of Physics of the Earth.
- Walter Szeliga; Timothy I. Melbourne; M. Meghan Miller & V. Marcelo Santillan (2004). "Southern Cascadia episodic slow earthquakes" (PDF). Geophysical Research Letters.
- Kato, Teruyaki (2011). "Slow earthquake". In Gupta, Harsh K. Encyclopedia of Solid Earth Geophysics (2 ed.). Dordrecht: Springer. pp. 1374–1382. ISBN 978-90-481-8701-0. Retrieved 2013-04-07.
- Timothy I. Melbourne & Frank H. Webb. "Surface Creep Measurements from a Slow Earthquake on the San Andreas Fault Using InSAR". Seismo.berkeley.edu. Retrieved 2010-05-05.
- Andrew V. Newman; Gavin Hayes; Yong Wei & Jaime Convers (2011). "The 25 October 2010 Mentawai tsunami earthquake, from real-time discriminants, finite-fault rupture, and tsunami excitation" (PDF). Geophysical Research Letters. Retrieved 2012-10-16.
- "'Silent' quake gently rocks Wellington". 3 News NZ. May 28, 2013.
- Seismic Wave Demonstration
- slow earthquake on the San Andreas fault
- short article in Discover
- SW Japan slow earthquakes
- ETS in Cascadia
- slow "sector failure" in Hawaii
- West Hawaii Today Slow Earthquake
- Slow Earthquakes, ETS, and Cascadia
- B.C.'s slow earthquakes fueled by fluid