The nature of deformation at Lake Pillsbury, CA

Evolution of seismicity in the Lake Pillsbury region from 2000-2007

Project Summary In May of 2000 the Pillsbury Lake region, a seismically quiescent region bounded by the Ma’acama and Bartlett Springs faults, experienced a burst of seismic activity lasting for six months and culminating in an M4.2 event. A similar pattern of earthquake activity occurred in April 2007 with mainshock of M4.8. Previous studies have interpreted this anomalous activity as evidence of magma transport in the crust caused by the northward migration of the Mendocino Triple Junction. However, a magma intrusion does not fully explain the regional geophysical activity and the spatial seismicity patterns associated with the two swarms. The question to be addressed in this study is whether this theory is correct, namely, is the seismicity near Pillsbury Lake due to crustal dike propagation? While there is some supporting evidence, a simpler explanation is that the seismic lineament is a new fault in the juvenile stage of development. This hypothesis is consistent with the regional tectonics as the lineament shares the right-lateral strike-slip motion of the bounding faults, and is located roughly equidistant from the Ma'acama and Bartlett Springs faults, where the internal shear stress is greatest and thus a probable place for fault formation. The objectives of this study are 1) to determine the relative crustal motion across the lineament, 2) to determine if there are any geomorphic expressions of this lineament that are consistent with that of new crack/fault formation from laboratory and field studies 3) to present a unifying model consistent with all observations associated with the seismic activity.


Geographic Location Lake Pillsbury, CA

Group Members Involved Amanda Thomas <Email> <Web Site>
Doug Dreger
Roland Burgmann

Project Duration April 2008 - current