|MadSci Network: Earth Sciences|
The concept of earthquakes triggered by other strong ones was under debate for many years. Nowadays it is accepted that it can appear in areas where there are many faults associated with the occurrence of strong earthquakes. It must be pointed out that the concept of "triggering" applies only to short-range and never to long-range earthquakes (i.e. a rupture in North Anatolian Fault can not trigger one in San Andreas Fault). With the term short-range we refer to different parts of the same fault (for example parts of the North Anatolian Fault Zone) or to areas that are seismo-tectonically connected (for example Anatolia-Aegean). The occurrence of a strong earthquake in an area causes changes in the stress field. Thus, it is possible for an earthquake in that broader seismotectonic area to occur earlier (in terms of years) than it would initially. It is believed that this was the case for the 7-9-1999 Athens earthquake (Mw = 5.9), triggered by the destructive 17-8-1999 Turkey earthquake (Mw = 7.8). Of course there is no direct evidence to establish the theory of earthquake triggering. However, it has been observed that after a strong earthquake in the North Anatolian Fault Zone, the broader Aegean region shows an increase in seismic activity. This can be attributed to the kinematic interaction of the Aegean and Anatolian plates (for a review see: "Present-day kinematics of the Middle East and Eastern Mediterranean", Journal of Geophysical Research, Volume 99, pages 12,071- 12,090, 1994). The North Anatolian Fault also provides a very good example for earthquake triggering within the same fault zone. Let us refer to the case of the Erzincan earthquake (Ms = 7.9) of 26 December, 1939, which is the greatest earthquake occurred in Turkey since 1668. This earthquake causing 32962 deaths produced ground breakage of 360 km long with an average displacement of 4.5 m, extending from Erzincan through Erbaa to the south of Amasya (Ketin 1976). It has played role as a trigger to the other earthquakes of the 1939-1967 sequence. As a result of triggering, ruptures propagates westward (see: "Seismotectonics of Turkey", Demirta & Yilmaz). Moreover, such a triggering relationship is proposed for the 12-11-1999 Turkey rupture (Mw = 7.1) following the 17-8-1999 rupture (Mw = 7.8) (Professor Leonardo Seeber, of the University of Columbia, New York, has worked in that subject). Considering the case of southern California, which exhibits great seismic activity along many faults, an interaction amongst these faults is likely to exist. It would be interesting to examine the possible effect of earthquakes occurring in the three major fault systems of the area: the San Andreas fault, the Hayward fault and the Calaveras fault. However, it must be pointed out that in the case of earthquakes one must be very cautious in any deductions. Therefore the occurrence of an earthquake could simply imply the relief in the stress field of an area, instead of a trigger in some neighboring faults (it is very easy to cause panic with the "domino" hypothesis). Some selected references are sited below for further elaboration in the subject: 1. Nalband S.S., Hubert A., King G.C.P., "Stress coupling between earthquakes in northwest Turkey and the north Aegean Sea", Journal of Geophysical Research B: Solid Earth, Volume 103, B10, pages 24,469-24,486, 1998. 2. Armijo R., Meyer B., Hubert A., Barka A., "Westward propagation of the North Anatolian fault into the northern Aegean: Timing and kinematics", Geology, Volume 27, Issue 3, pages 267-270, 1999. 3. Hatzfeld D., Ziazia M., Kementzetzidou D., Hatzidimitriou P., Panagiotopoulos D., Makropoulos K., Papadimitriou P., Deschamps A., "Microseismicity and focal mechanisms at the western termination of the North Anatolian Fault and their implications for continental tectonics", Geophysical Journal International, Volume 137, Issue 3, pages 891-908, 1999. 4. Anderson J., Brune J., Louie J., Zeng Y., Savage G., Yu G., Chen Q., DePolo D., "Seismicity in the western Great-Basin apparently triggered by the Landers, California, earthquake, 18 June 1992", Bull. Seism. Soc. Am., 84(3), pages 863-891, 1994. 5. Brodsky E.E., Sturtevant B., Kanamori H., "Eartquake, volcanoes and rectified diffusion", Journal of Geophysical Research, Volume 103, pages 23,827-23,838, 1998. 6. Hill D., et al., "Seismicity remotely triggered by the magnitude 7.3 Landers, California, earthquake", Science, 260(5114), pages 1617-1623, 1993. 7. Demirta R., Yilmaz R., "Seismotectonics of Turkey", http://angora.deprem.gov .tr/jpg/Turkey.htm. 8. Allen C.R., "The tectonic environments of seismically active and inactive areas along the San Andreas fault system", Stanford University, Publ., Geol. Sci. 11, pages 70-82, 1968. And the references therein. Hoping to be helpful!
Try the links in the MadSci Library for more information on Earth Sciences.