Hi All. Whenever Petra starts thinking, or has a question that needs to be answered it usually means work for me. Here is her question (s)

Before the 6.8 subduction zone EQ in Olympia, WA there were a number of 5.0
plus eq's near Victoria BC, out in the ocean a bit. So, I've been trying to understand if its possible those eq's were on the northern end of the block that moved, or if they are unrelated. Part two of this question is, when subduction zone quakes occur, does it make it anymore likely that the block that is north or south would then move?

She sent this out by way of e-mail, but the answer she got didn’t help any, but did set my brain in motion that the answer may be found in the Seismic Gap Theory which was suggested in the reply she received.

While the seismic gap theory has not been proven there does appear to be some very strong evidence to support it in some cases. The Loma Prieta quake is one of those cases as well as the Parkfield quake, which did not happen.

The reason the Parkfield quake didn’t occur was that the strain needed to cause a quake of the expected magnitude was not there and may not be there for sometime to come if the current slippage rate continues.

The Loma Prieta quake occurred because the strain needed for just such was there. This could be seen by the lack of small quakes for a long period of time before the quake. When viewing an earthquake map with all of the quakes on the map the next seismic gap is just north of Portola Valley, which is just south of San Francisco. If the seismic gap theory is correct then the next large quake on the San Andreas Fault should be just north of Portola Valley.

There is another area where the seismic gap can be applied. That is the southern portion of the Rodgers Creek Fault, which is where the next expected large quake could occur. There have been quakes north of it, south of it, east of it and west of it. The Mogi Doughnut theory could be applied here. There are some good theories (although not proven) to work with. The proof for the theories will be in the form of large quakes where they are expected. Of course there will always be some large quakes where they are not expected, because there just isn’t enough history for every given area.

This is not be the only answer, but it is something to think about. Take Care…Don in creepy town.