Large earthquakes cause redistribution of fluid (groundwater) in fault zones far away. Movement of ground water within the fault zones tends to lubricate the faults, which in turn tends to weaken them. Failing of faults ultimately causes earthquakes

Large earthquakes have been found to increase the chances of subsequent quakes across the globe.

Apart from causing earthquake and tsunami, the seismic waves from the 2004 Sumatra-Andaman quake, which had a magnitude of 9.1, led to weakening, and hence increasing the chances of failure of other faults far away. The 1992 Landers earthquake in California, with a magnitude of 7.3, also had a similar effect on other faults across the world.

Several high magnitude (about 8) earthquakes have occurred from 2005 to 2007 after the 2004 Sumatra-Andaman quake. Such clustering of large earthquakes within a limited time has not occurred anytime after 1900. Similarly, a cluster of four earthquakes of magnitude more than 4 occurred at Parkfield in the San Andreas Fault in California after the Landers quake.

Changes in fault strength as a result of a large earthquake have long been suggested to explain the occurrence of small quakes elsewhere. According to Nature, a quake of 7.9 magnitude that struck Alaska in 2002 had caused small earthquakes as far away as Wyoming and California. But the only difference is immediacy of the quakes seen afar after the Alaska quake, and the Landers and Sumatra quakes.

While earthquakes occurred within hours of the Alaska quake, those seen at Parkfield following the 1992 and 2004 quakes occurred months and years later. While the tremors immediately after the Alaska quake were caused by faults giving way due to vibrations, the mechanism by which earthquakes occurred after the 1992 and 2004 quakes is very different.

A paper published online today (October 1, 2009) in the journal Nature has for the first time provided the much needed evidence to understand how this takes place.

Scientists from three institutions in the U.S. studied 20 years of seismic data from the Parkfield area in San Andreas Fault in the U.S., the most closely observed earthquake zone in the world.

The role of groundwater movement

Using 3-kilometre-deep borehole seismometers called the high-resolution seismic network and other geophysical instruments in place at Parkfield, the scientists were able to find that structural changes in the faults happened due to fluid redistribution around the faults.

Movement of fluid (groundwater), in turn, is caused by seismic waves from large earthquakes. Movement of ground water deep within the fault zone tends to lubricate the fault, which in turn tends to weaken it.

The weakening persists as long the water remains in the fault. “The total displacement of the fluids is only about 10 metres at a depth of three kilometres,” notes Fenglin Niu of Rice University, one of the authors of the paper, in a Carnegie Institution release.

The main implications of the study

According to the paper, there are several key implications of the present study. Most importantly, it will be possible to continually monitor changes in fault strength, which is one of the parameters to know when a failure (quake) is likely to happen. But such monitoring cannot happen by using instruments based on the ground.

Third, not all faults are affected to the same extent by a large quake. But large tremors like the 2004 Sumatra quake can affect faults lying as far as 8,000 km. Finally, fluid redistribution has been the mechanism by which many of the active faults have been affected by the 1992 Landers quake and the 2004 Sumatra quake.

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