Hi All. Every now and then I come across something that catches my attention. The following is from a USGS abstract. However this isn’t what got my attention. I’m using it to give a little background to an interesting discovery I made by chance.

Stress Transfer by the 1988-89 M=5.3, 5.4 Lake Elsman Foreshocks to the Loma Prieta Fault: Unclamping at the Site of Peak Mainshock Slip
Hugo Perfettini
Laboratoire de Géologie, Ecole Normale Supérieure, Paris

Ross S. Stein and Robert W. Simpson
U.S. Geological Survey, Menlo Park

Massimo Cocco
Istituto Nazionale di Geofisica, Rome

“Abstract. We study the stress transferred by the 27 June 1988 M=5.3 and 8 August 1989 M=5.4 Lake Elsman earthquakes, the largest events to strike within 15 km of the future Loma Prieta rupture zone during 74 years before the 1989 M=6.9 Loma Prieta earthquake. We find that the first Lake Elsman event brought the rupture plane of the second event 0.3-1.6 bars (0.03-0.16 MPa) closer to Coulomb failure, but that the Lake Elsman events did not bring the future Loma Prieta hypocentral zone closer to failure. Instead, the Lake Elsman earthquakes are calculated to have reduced the normal stress on (or ‘unclamped’) the Loma Prieta rupture surface by 0.5-1.0 bars (0.05-0.10 MPa) at the site where the greatest slip subsequently occurred in the Loma Prieta earthquake. This association between the sites of peak unclamping and slip suggests that the Lake Elsman events did indeed influence the Loma Prieta rupture process. Unclamping the fault would have locally lowered the resistance to sliding. Such an effect could have been enhanced if the lowered normal stress permitted fluid infusion into the unclamped part of the fault.”

“Although less well recorded, the ML=5.0 1964 and ML=5.3 1967 Corralitos events struck within 10 km of the southwest end of the future Loma Prieta rupture. No similar relationship between the normal stress change and subsequent Loma Prieta slip is observed, although the high-slip patch southwest of the Loma Prieta epicenter corresponds roughly to the site of calculated Coulomb stress increase for a low coefficient of friction. The Lake Elsman-Loma Prieta result is similar to that for the 1987 M=6.2 Elmore Ranch-M=6.7 Superstition Hills earthquakes, suggesting that foreshocks might influence the distribution of mainshock slip rather than the site of mainshock nucleation.”

Now that you have a general idea of the area surrounding the 1989 Loma Prieta quake keep the occurrence of the two Corralitos quakes when you read the link provided. Take Care…Don in creepy town

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