This project was inspired by theUC Berkeley GeologySummer Field course, members of the UCB Active Tectonics Group and on going seismic activity in and around the Long Valley Caldera.
Seismicity and Deformation South of the Caldera
      The block of Sierrian basement rock south of the Long Valley Caldera, known here as the Hilton Block, has been host to a barrage of seismic activity in recent years.  Several earthquakes  >M6 were recorded in the H.B. in 1980 and ~M5 quakes are not uncommon, including several in May of 1999. Much of this activity appears to occur in 3 primary lineations that run NE-SW at an azimuth of ~20 degrees, and moment tensor solutions indicate sinistral strike-slip motions. These orientations and motions appears somewhat anomalous, as most of the mapped faults in the area have a NW-SE orientation and a combination of dextral and normal dip-slip motions dominate in the Walker Lane-Eastern California Shear Zone. Regardless, the area is a complex region, which occupies the interface between the Long Valley Caldera, Basin and Range extension and The Sierra Nevada Batholith.
     The Hilton Block and its episodes are of seismological  interest. The area offers a high rate of activity in a highly monitored region, unique moment tensor solutions (non-double-couple, and unexpected sinistral), as well as, some interesting  temporal distributions of events relative to caldera inflation. Efforts are currently underway to get high quality 3D locations for all of the events in the area. Determination of moment tensor solutions is also of priority, in order to better understand seismic source mechanisms. As indicated from the preliminary source data, some of the activity in the Hilton Block shows a volumetric expansion and may be due to fluid motions and possible dike injection into the area.
      Investigations into surficial fault expressions of the seismically active region offers a geologically interesting relationship to the history of the region. Aerial photography of the Hilton Block shows several clear lineations that match the seismic patterns. But, is not clearly understood if we have the reactivation of ancient structures in response to the stresses created by the inflation within the caldera, or the birth of new structural features. If further investigations indicate that the seismicity coincides well with the observed lineations, reactivation will most likely provide the best story.


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Peter Higbee
Department of Geology and Geophysics
University of California Berkeley
Berkeley, CA 94720-4767