Re: Why don't strong earthquakes trigger other earthquakes?

     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!