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Seismologists sometimes refer to a pair of similarly sized earthquake shocks that occur relatively closely spaced in time and location as an earthquake "doublet." This occurrence is distinct from the normal pattern of earthquake aftershocks. Aftershocks gradually diminish in magnitude and generally come from the same origin as a mainshock, whereas doublet earthquakes originate from a place other than the original earthquake rupture area. The first earthquake can be a considerable distance and time away from the second earthquake. The magnitude of the second quake may even be slightly larger than the first.
Frequency and Hazard
Earthquakes identified as doublets occur once or twice globally every year, making them far rarer than more typical earthquake patterns. In earthquake prone regions officials do not plan for doublet earthquakes occurring because they are so rare. However, when they do occur, they have a high disaster potential.
Some large earthquakes may be closely spaced doublet earthquakes, where the shaking from the rupture of the first fault or fault section overlaps with shaking from the rupture of a second fault section nearby. The two separate sources for these earthquakes are usually only identified by seismologists after extensive analysis, not in real time. Closely spaced doublet earthquakes could enhance tsunami risk or complicate tsunami warning efforts.
- 1915 Imperial Valley earthquakes: Two magnitude 6.25 shocks occurred ~1 hour apart. Six people died and several were injured in the second quake at Mexicali, located just inside the Mexican border. Unstable banks of the New and Alamo Rivers caved in many places.
- A relatively recent doublet earthquake occurred late in 2006 and early in 2007 in the Kuril Islands of Russia, which had not had a large-scale earthquake since 1915. The first earthquake occurred on November 15 and had a magnitude of 8.3. Shortly after this, seismic activity began in the Pacific plate where the second earthquake occurred on January 13, with a magnitude of 8.1. There was only 1 reported injury from the first earthquake and no reported injuries from the second earthquake since they both originated in the ocean. However, each earthquake caused a tsunami and the earthquake on November 15 created a tsunami that reached the coast of California, causing $500,000- $1,000,000 in damage. There may also be examples of triplet earthquakes, such as the 2010 Mindanao earthquakes.
- 2006 Hengchun earthquakes: On 26 December 2006, two magnitude 7.0 earthquakes occurred only 8 minutes apart off the southwest coast of Taiwan in the Luzon Strait.
- 2012 East Azerbaijan earthquakes: Quakes of magnitude 6.4 and 6.3 occurred 11 minutes apart on August 11, 2012 near the cities of Ahar and Varzeqan.
- "Magnitude 7.8 – VANUATU #Summary". USGS. 2009-10-07. Retrieved 2009-10-09.
- "How One Big Earthquake Triggers Another". Live Science. Retrieved 2016-10-26.
- "Doublet Earthquakes And Earthquake Dynamics". scientificblogging.com. January 30, 2008. Retrieved 2010-02-03.
- "Doublet Earthquakes And Earthquake Dynamics". Science 2.0. 2014-08-27. Retrieved 2016-11-01.
- Gray, Richard. "Is the world at risk from HIDDEN earthquakes? 'Doublet' quakes are difficult to detect and can produce larger, more devastating tsunamis". Daily Mail: Science and Tech. Daily Mail. Retrieved 29 April 2018.
- "Central Kuril Island Tsunami in Crescent City and California.html". www.usc.edu. Retrieved 2016-11-01.
- Worldwide doublets of large shallow earthquakes
- A great earthquake doublet and seismic stress transfer cycle in the central Kuril islands
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