Hi Michael. I think what Petra wants to know is how many people who read this board feel about the probability of quake occurring on another fault at, or near the same time of a major event in the area of Parkfield.

When the quake of 1857 occurred the surface rupture caused by the quake was extensive. The San Andreas fault broke the surface continuously for at least 350 km (220 miles), possibly as much as 400 km (250 miles), with an average slip of 4.5 meters (15 feet), and a maximum displacement of about 9 meters (30 feet) (possibly greater) in the Carrizo Plain area. Kerry Sieh (1978) noted that the Elkhorn Thrust fault, a low-angle thrust fault near the San Andreas, may have slipped simultaneously in the 1857 quake -- an observation that a team of researchers (1996) have recently used to support the idea that future movements along the San Andreas fault zone might produce simultaneous rupture on thrust faults in and near the Los Angeles area, causing a terrible "double earthquake".

During the 1989 Loma Prieta quake the Monte Vista Thrust fault at the base of the foothills on the western side of the Santa Clara Valley slipped. This is one of the most dangerous Bay Area thrust faults, because of its location near an urban area. However, this fault has a long recurrence interval for large earthquakes - on the order of several thousand years. As with other thrust faults, they know generally where the fault is located, but it is difficult to identify the actual surface trace.

The pressure from the Joshua Tree earthquake was believed by the scientists to have triggered the Landers quake. The scientists found the southern part of the Landers quake fault was moved during the Joshua Tree earthquake, and these two earthquakes were connected. The Landers quake then triggered the Big Bear quake just three hours later.

On October 16, 1999 the powerful 7.1, Hector Mine earthquake struck only 15 miles from the location of the Landers quake seven years earlier. Like the previous quakes in this sequence, the Hector Mine quake was in the desert and did little damage; however, its power was felt from Los Angeles to Phoenix. It generated over 11,000 aftershocks. Like the Landers quake, this one also triggered earthquakes far from its own zone. The Landers quake triggered earthquakes exclusively north of the epicenter while this one triggered the quakes southward as far as the Mexican border. The recent flurry of quakes in the Baja area could be the results of the Hector Mine quake.

What really troubled the scientists was that the Hector Mine quake triggered a series of quakes very near the San Andreas Fault some 130 miles away. They referred to this as the faults "talking" with each other. After two weeks, the quakes near the San Andreas fault stopped.

The scientists believe that the area where these earthquakes are occurring have been inactive for several thousand years. The sudden occurrence of four major quakes in seven years is an indication that they are all related. One earthquake is setting the stage for the next. Instead of the massive Landers quake decreasing pressure, it actually increased the pressure that helped trigger the Hector Mine quake. In the May 10, 2001, issue of Nature, Dr. Andrew M. Freed states the following:

"Stress changes in the crust due to an earthquake can hasten the failure of neighboring faults and induce earthquake sequences in some cases. The Hector Mine earthquake in Southern California M7.1 occurred only 20 km from, and 7 years after, the 1992 Landers earthquake M7.3. This suggests that the Hector Mine earthquake was triggered in some fashion by the earlier event." Of course this theory along with many others are very hard to prove and in some cases can never be proven until the next major quake, and the next major quake after the next major quake.

Just some late night, or early morning thoughts. Take Care…Don in creepy town