Hi All. I spent the better part of the day looking for the link to the Yellowstone Park quakes of 1992. I haven’t found it yet, but have found a number of references to it.
I did however find some interesting data pertaining to the Loma Prieta quake.
In the months after the quake, researchers noticed what appeared to be a pattern in which large earthquakes were tossed like footballs back and forth between the San Andreas and Hayward faults.
First there was an 1836 quake on the northern Hayward Fault, followed by one in 1838 on the Peninsula section of the San Andreas. Then there was an 1865 quake on the Peninsula, followed three years later by one on the southern Hayward that was strong enough to cause extensive damage in the burgeoning city of San Francisco.
Could these pairs of quakes be related?
It sounded like a long shot. At the time, scientists didn't think the minuscule changes in stress involved could have a significant effect on a fault so far away. But the question was intriguing.
Simpson and Paul Reasenberg of the USGS used a computer model to simulate the effect of Loma Prieta on surrounding faults. They concluded that far from stressing the Hayward Fault and triggering new earthquakes, the quake should have relaxed its neighbor to the east.
On the other hand, they found that Loma Prieta would have increased the stress on the parts of the San Andreas to either side of it. This was troubling, because the stretch of the fault to the north runs up the Peninsula and is thought to be ripe for a major earthquake.
Looking for some kind of physical connection between the Hayward and San Andreas faults, other teams used sound waves to take the equivalent of an ultrasound picture of the rock layers deep beneath the fault system. So far those studies have been inconclusive -- although the latest one, by Tom Parsons and Patrick Hart of the USGS in Menlo Park, found that the two faults do bend slightly toward one another, starting about 11 miles down.
They don’t actually join but they're pointing that way.''
In the meantime, the connection between the pairs of historical quakes sort of melted away. It turns out that the 1836 quake was not on the Hayward Fault; but so far have been unable to find which fault it was on. There is however two faults it may have occurred on. The first is the Pinole fault, and the other is the Rodgers Creek fault going on the assumption that the 1836 quake was at the extreme northern end of the Hayward fault. So far nothing has been found that would indicate that it was either of these faults. It also appears the 1865 quake may not have been on the San Andreas, but on a network of buried faults along the foothills west of San Jose. For those who are familiar with the area this is around the Alma Reservoir.
But the basic idea -- that faults can swap stress and influence each other -- has persisted and grown stronger.
The thinking as of now is that it’s still not clear whether the stress generated by an earthquake triggers only quakes that were about to happen anyway (making them occur a few years earlier) or if it can create new quakes, perhaps by awakening long-dormant faults. I asked that question of a couple of geologist a few years back. I never received an answer, but I did get one heck of an argument started. I think they’re still friends. Take Care…Don in creepy town.