Still waiting for the 1987 Parkfield earthquake 

The San Andreas Fault (SAF) passes through the small town of Parkfield, California, which is situated roughly halfway between Los Angeles and San Francisco.  Parkfield has experienced strong (at least M6) earthquakes six times between 1857 and 1966.  These quakes, have an average repeat interval of 22 years (24, 20, 21, 12 and 32 years).  Excluding the larger and more extensive 1857 earthquake, they have all occurred on almost exactly the same part of the fault.  Furthermore, the 1934 and 1966 quakes have very similar-looking seismographs, and each was preceded by a M5 foreshock 17 minutes before the main shock.  Another similar earthquake was expected to occur at Parkfield by around 1987, but it still hasn’t happened, and the gap is now 36 years.

(For more information on the history of earthquakes at Parkfield see: http://www.johnmartin.com/earthquakes/eqpapers/00000075.htm) 

In the mid 1980s the USGS and several California universities initiated an intensive seismic monitoring program at Parkfield.  The program now includes the following instrumentation: 

Amongst numerous other studies, earth scientists are monitoring water levels in wells and analyzing data from satellites to assess ongoing ground displacement.

(For more information on the research at Parkfield see: http://www.scec.org/instanet/01news/es_abstracts/langbeinES1.pdf) 

This unparalleled research effort is being conducted for two main reasons.  Firstly, the relatively simple geometry of the SAF at Parkfield allows for a clear understanding of strain accumulation and release on the fault.  Secondly, the apparent regularity of the historic earthquakes at Parkfield makes this an ideal site for testing the “time-predictable recurrence model” developed in the 1980s (Shimazaki and Nakata, 1980).

Some of the data gathered at Parkfield over the past few decades have been recently analyzed by geophysicists from Stanford University.  Their goal is to understand why there is now a 36-year gap between major earthquakes at Parkfield.  Murray and Segall (2002) have estimated the rate of strain accumulation on the Parkfield segment of the SAF, and they conclude that the most of the strain released by the 1966 quake had re-accumulated by 1981, and that there is a 95% probability that another large quake should have occurred by 1987.  In carrying out this analysis Murray and Segall recognized that some of the strain at Parkfield could have been relieved by the nearby M6.5 Coalinga quake of 1983, and that this could have delayed Parkfield by about 2 years.  On the other hand, they also calculate that two small earthquakes in the Parkfield area in 1992 and 1994 (around M4) actually increased strain on the Parkfield rupture zone, essentially countering the delaying effect of the Coalinga quake.

The only explanation offered by Murray and Segall is that local variations in pore-water pressure may have affected the tendency for failure on the Parkfield segment – although they have no means of measuring this parameter. 

Murray and Segall go on to argue that the 36 years worth of strain that has now accumulated at Parkfield is substantially more than that which had accumulated prior to the previous six large earthquakes, and therefore if the segment fails soon (eg. in 2002 or 2003) the resulting quake will likely have a magnitude between 6.6 and 6.9 – which would be significantly more damaging than any of the past five Parkfield earthquakes.

References

Murray J and Segall P, Testing time-predictable recurrence by direct measurement of strain accumulation and release, Nature, V. 419, p. 287-291 (September 2002).

Shimazaki K and Nakata T, Time-predictable recurrence model for large earthquakes, Geophysical Research Letters, Vol. 7, P.279-282 (1980)