Hi All. “Far Field Aftershock” (FFA) from major quakes is a controversy to say the least. There are some who support it, but for the most there are many who don’t.

The biggest problem with the FFA theory is that it can’t be proven that the quakes wouldn’t have occurred were it not for a major event shortly before they occurred. There is always the possibility that they would have occurred anyway, but it can also be said they occurred when they did as to opposed to a later time was because the distant major event moved the time schedule ahead.

Speaking at the annual meeting of the Seismological Society of America in San Francisco today (4/18/01), Aron Meltzner and David Wald, researchers at Caltech and the USGS in Pasadena, said aftershocks which took place within the first two days after the San Francisco mainshock, rattled areas of southern California, western Arizona, western Nevada and south-central Oregon. The largest of these events were a magnitude 6.1 earthquake near the Salton Sea, in California’s Imperial Valley, and a magnitude 5.0 earthquake under or near Santa Monica Bay.

Traditionally, scientists have thought that earthquakes do not trigger other earthquakes more than one rupture length away from the fault rupture. That is, if the part of the fault that slipped in the 1906 earthquake was 250 miles long, the earthquake would not trigger other earthquakes more than 250 miles away from that part of the fault. However, after the 1992 Landers earthquake triggered earthquakes near Mammoth Lakes, California, Yucca Mountain, Nevada, and in Yellowstone National Park, the scientific community became more aware of these “distant aftershocks.” Looking back in time, Meltzner and Wald wanted to see if the same phenomenon took place after 1906.

“What we found,” said Meltzner, “is that earthquakes started popping up all over southern California and in Arizona and Oregon that were well beyond 250 miles from the fault. That doesn’t include two earthquakes east of Reno, Nevada that were barely within the 250-mile radius, and countless others that may have taken place in remote locations where no one was around to report them. These distant aftershocks started within an hour of the San Francisco earthquake and continued for about a day and a half.”

Meltzner and Wald rule out that the earthquakes in southern California, Arizona, and Oregon which were all more than 250 miles from the fault occurred at about the same time by coincidence. “Not only is it highly unusual for so many earthquakes in different places to cluster together like that in time,” explained Meltzner, “but some of them occurred in places that rarely experience earthquakes.” They calculated the odds at about 4 billion to one that all the so-called distant aftershocks were unrelated and happened within the same two-day period by chance alone. “If it all happened by chance, we could expect something like that to occur once every 20 million years,” added Meltzner. “Obviously, since it happened in the 48-hour period immediately following the 1906 earthquake, it wasn’t just chance.”

The biggest problem with the FFA theory is that it isn’t possible to pick a specific location where a quake might be triggered, but general locations can be found by using past major quakes.

For the most part triggered quakes appear to occur more often at the edges of the “shadow zone.” This is 103-107° and 142-146° from the epicenter of the major event, but isn’t limited exclusively to that zone. For instance when a major quake occurs in the area of Alaska the area most likely to be affected is the portion of southeastern United States. When a major quake occurs in the Solomon Islands area the areas to be most likely affected will be Southern California, Mexico and Central American. However as with everything this isn’t a done thing. Just because there is a major quake this doesn’t mean there will be quakes at the previous mentioned locations, only that the chances are greater that there will be.

I don’t think anyone really has an idea as to why this occurs. If they did they could be well on there way to solving the problem of earthquake prediction. Then again maybe not. For every question answered there are only more questions that need answers. Take Care…Don in creepy town

Reference.

U.S. Department of the Interior
U.S. Geological Survey

NOAA

Dr. Lowell Whiteside

Aron Meltzner

David Wald