Hi All. Do aftershocks tell us where the next quake might be? In most case probably not. Two models, or theories were looked at although I’m sure there are others.

According to Dr. Mark Zoback of Stanford University who used two models one being the asperity model and the other the barrier model. The so-called asperity model, suggested that stress along an earthquake zone is heterogeneous before a quake, and because of the quake becomes more homogeneous. The second model, called the barrier model, claims the reverse: the zone is homogeneous to start with and becomes more diverse after the main quake.

However, the Loma Prieta quake, which happened along an area where the Pacific tectonic plate meets the North American plate didn’t seem to follow either model. It showed that the motion of its aftershocks was not in the same direction as the main quake.

"There was reverse slip in which the Pacific plate moved up with respect to North America but many aftershocks occurred in which the Pacific plate moved down," Zoback said. "In addition to the Pacific plate moving north with respect to North America, which is the motion one expects, there were aftershocks that moved south."

Mark Zoback. “Using an analogy, think of bending a pencil almost but not quite to the point at which it would break. If the pencil eventually snaps, it is because either you somehow added just a little more pressure or because the stress weakened it enough that it could not withstand the same pressure. The latter is what appears to have happened with Loma Prieta aftershocks.”

This seems to me that the aftershocks occurred not because of the transfer of stress, but because the area was weakened. This of course doesn’t answer the question as to why some quakes near the same magnitude and the same type of fault have more aftershocks then others.

In the case of the Loma Prieta quake it could be that the quakes that were occurring south of the main event was that the area was playing catch-up with everything in the area of the main event, or further north.

The Coulomb Stress Transfer shows that the stress from the Loma Prieta quake moved south into the San Juan Bautista area. This was most likely the cause of the M>5.4 quake in August of 1998. This isn’t to say that the Loma Prieta quake triggered the San Juan Bautista quake. It added additional stress that could have mover the timetable forward 10 or 15 years. The quake was going to occur anyway, it just did it sooner.

I wonder what the Quien Sabe Fault east of Hollister is thinking about. Haven’t heard from it for a spell now. Take Care…Don in creepy town