Hi All. With the recent swarm of quakes in the area of Tracy, California I thought it would be interesting to see what has occurred in the past in this area.

The quakes (7 at this time) appear to be occurring on the northern end of the San Joaquin fault. This would make it a part of the Great Valley fault, which for the most part is a thrust fault.

The San Joaquin fault is located at the eastern front of the Diablo Range. The fault is associated with the Coast Range-Sierran Block boundary zone.

The largest quake to have occurred within a 5 mile radius of 37.600N/121.449W the epicenter of the most recent quake was a 3.5Ml on 05/06/1976 at 37.6120N/121.4105W.

Most of the recent quakes show strike slip with some thrust faulting. There is also an indication that a couple of the quakes are strike slip on a dipping fault plane.

In 1997 a paleoseismic trenching investigation where the San Joaquin fault appears to offset two alluvial terraces of Lone Tree Creek was performed. Two trenches were excavated across the steepest portion of the scarp, one on each of the two deformed terraces. The trenches were oriented perpendicular to the scarp and approximately parallel to the downstream direction of the terrace. Four test pits were also excavated on the undeformed portions of the terraces, both above and below the scarp, to help document alluvial stratigraphy and soil-profile development.

The San Joaquin fault does not reach the surface at this site. The researchers have interpreted this to be a blind thrust or reverse fault at depth whose presence is indicated by surface deformation in the form of a monoclinal fold scarp.

At this preliminary stage in the investigation, the timing and frequency of seismic activity is not well constrained. The presence of a two-to-three-times greater slope and vertical offset on the older terrace compared to the younger terrace suggests at least two seismic events have taken place since the formation of the older terrace. Take Care…Don in creepy town