From: alden@netcom.com (Andrew L. Alden)
Subject: Parkfield
Organization: Netcom - Online Communication Services  (408 241-9760 guest) 
Date: Tue, 22 Dec 1992 19:37:14 GMT


I'm finally sifting through my notes from the American Geophysical Union
meeting the other week.  In earthquake news, it seems there were two big
events in 1992, the southern California quakes and the "failure" of the
Parkfield experiment.  The Landers/Big Bear quakes spawned too many
presentations for me to make sense of, so I'll talk about Parkfield.

The Parkfield segment of the San Andreas fault, it was officially
forecast in 1984, would almost certainly have a large, magnitude 6 or
greater, earthquake on it before the end of 1992.  The stories around
that time said that the fault had had regular M5-1/2 or M6 earthquakes,
almost like clockwork, for the entire historical period of record--every
22 years from 1857 to 1966, if you call the 1934 M6 event a "premature"
quake that should have occurred in 1944.  Continuing the trend into the
future appeared to mean another similar event in the 1988-1993 window.

This simple model was, it seems, quietly discarded a while back.  Jim
Savage (USGS) told the meeting that if 1934 is accepted at face value,
the window would run until October 1991, not January 1993.  T.R.
Toppozada (CDMG) described Parkfield earthquakes that weren't on the
list in 1984; in fact, he said, the Parkfield segment shows a pattern of
decreasing activity since the great 1857 quake, not a steady state of
"characteristic events," a term I haven't seen as much as I used to.  At
a December 10 press conference, Bill Bakun and Al Lindh (USGS), authors
of the clockwork hypothesis on which the Parkfield prediction was based,
defended it stoutly but in a rather generic way: hypotheses are useless
if they cannot fail, and the "regular recurrence hypothesis" will fail
on December 31 if a M6 has not struck Parkfield; so science has marched
a step further, and our new hypotheses are even better.

So now we know that even on perhaps the world's straightest, cleanest
stretch of strike-slip fault, you don't ever get a nice, regular set of
stress releases like a violin string under a well-rosined bow (or a
perfect planar fault in an infinite half-space).  Although the plate
movement at depth in the lithosphere is in fact smooth and inexorable,
the crust above it always responds in a complex, irregular way.  That's
nice scientific progress...thing is, we didn't need the whole Parkfield
experiment to learn that.

The benefits from the project were great and worth the public money. 
Bakun pointed to our better knowledge of which instruments work best for
monitoring faults and progress in informing the public about earthquake
hazards.  Lindh noted that scientists, policymakers, emergency agencies,
media, and the public are now talking together in a useful way.  Andy
Michael and many others spoke up for it too, to good effect.

I don't begrudge a dime paid for the experiment, even though the
hypothesis ostensibly being tested was ridiculously simple.  My question
is, was that hypothesis used because even Congress could understand it?
Was the test question ignorantly simple or disingenuously simple?