Hi Steve. For right now this is what I have been able to find in regards to the “first motion mechanism” for the Lake Elsman quakes.

I have a feeling this won’t be the final word regarding these two quakes, but it appears they were compressional with some strike slip motion. This would be a reverse fault also known as a thrust fault. Take Care…Don in creepy town

Several studies have identified the Lake Elsman earthquakes as rare events that struck within 5 km of the future Loma Prieta rupture plane, and only 11 km from the Loma Prieta hypocenter [Seeber and Armbruster, 1990; Olson, 1990; Olson and Hill, 1993] (Fig. 1). These authors argued that the lake Elsman events occurred on a steeply northeast-dipping oblique reverse fault, distinct from the Loma Prieta plane. Sykes and Jaumé [1990] regarded the Lake Elsman events as ‘long-term foreshocks’ to Loma Prieta, because of their proximity in space and time to the Loma Prieta rupture, and because they occurred on secondary faults, a feature they argue is typical of the seismic buildup to large events. After both Lake Elsman earthquakes, the U.S. Geological Survey and California State Office of Emergency Services issued a joint advisory of a heightened probability of M=6.5 shocks during the succeeding 5 days. The advisory was partly motivated by the observation that the two Lake Elsman events were among the three largest shocks to occur anywhere along the extent of the 1906 San Andreas rupture since 1914. In addition, several studies had proposed that the section of the San Andreas adjacent to these events had a high probability of a large earthquake [see review by Harris, 1998].

• Lake Elsman