Friday, February 22, 2008

What about 22 years before 1857?

Plenty of people are curious about Parkfield. The 6.0 in 2004 simply leaves the question of when the next one will be, how it will relate to the timing of all the others. It's still an excellent place to look at high-magnitude repeating earthquakes, and probably still one of the best places to poke around for some hints toward the holy grail of earthquake prediction.

I think that stuff is interesting, and that these are good questions, but what really makes me go hmmm about Parkfield is what happened there before 1857. All of the lists that I've ever seen of major events there with the 1857 Fort Tejon earthquake, and that average recurrence interval of 22 years is calculated from there on out. But what about beforehand? Has anybody looked into this?

The SAFOD station is about getting current earthquake information from Parkfield, but has anyone done any paleoseismological digging at the site? That would be something I'd like to see. I don't know how well a 22-year recurrence interval would show up in cross section, since that doesn't give much time for new material to accumulate on the old break, but it would answer whether or not the same pattern was going on before 1857.

I've heard a theory that the whole chain of Parkfield events is still part of the Fort Tejon aftershock sequence. That's a tantalizing thought that would accommodate the increasingly longer time between the events. If paleoseismology showed that Parkfield was quiet before 1857, it could add credence to this theory - if they're not aftershocks, something about that quake changed the conditions to make them ripe for repeated earthquakes. But if not aftershocks, that brings up the question of what specific change initiated the sequence.

But if these regular 6-pointers were just as regular before 1857, what made them tick? Surely Parkfield's location between the locked and creeping sections of the fault have something to do with it, but how so? And why did 1857's quake get so much bigger? How many 6s do we get before the next Fort Tejon?

Once I'm done with my music thesis, this is a subject about which I would like to read more. I'm hoping I can find things addressing Parkfield's past when so much attention is given to its future (if any of you know of any good ones, please let me know!), but if there isn't much out there, that's a study that really ought to be done.

1 comment:

kdinger said...

Have a look at: Paleoseismic and Postseismic Observations of Surface Slip along the Parkfield Segment of the San Andreas Fault,
Nathan A. Toké, J Ramón Arrowsmith, Jeri J. Young and Christopher J. Crosby, Bulletin of the Seismological Society of America; September 2006; v. 96; no. 4B; p. S221-S238; DOI: 10.1785/0120050809.

Abstract at:

You'll need to be a member of SSA to get access to the full paper (if you're not currently a member you should join; it is cheap, $20?, for students).

I don't know much about paleo trenching and haven't read the paper carefully, but it looks like M6 events are difficult to see as discrete events in trenches; i.e. a series of M6s looks similar to continuous creep. They find no evidence for meter-scale surface displacements (which would indicate M > 7 events). They take the total vertical deformations over a couple of radiocarbon dated intervals and compare them to the offset observed at their trench site due to the 2004 Parkfield event to say that if these observed vertical deformations were accumulated in 2004-like events, then the average recurrence interval of such events is between 8 and 188 years. (A large part of this uncertainty is due to uncertainty in scaling up their measurements from the first few days after the 2004 event to account for afterslip. Presumably they could go back at some later time and remeasure the total offset including afterslip)

Since they have no discrete events to date, they can't comment on any possible regularity of pre-1857 events.