Static Stress Changes Caused by the 1978 Diamond 
Valley, California and 1994 Double Spring Flat,
Nevada Earthquakes

  Ichinose G A; J G Anderson and K D Smith
   Seismological Laboratory MS-174, University
   Nevada Reno, Reno, NV, 89557-0141; ph:
   702-784-4260; e-mail: ichinose@seismo.unr.edu

  The  Sierra  Nevada  frontal fault system from
  Reno, Nevada to Mono Lake, California is char-
  acterized  this  century by moderate levels of
  seismicity mainly  along  conjugate  northeast
  and  northwest  striking  strike-slip  faults.
  These faults appear to act as  zones  of  dis-
  tributed  deformation  between the less active
  major normal-slip frontal fault segments.   An
  ongoing earthquake sequence along one of these
  northeast striking zones following the 1994 Mw
  5.8 Double Spring Flat, Nevada, earthquake has
  raised concern about the possibility of larger
  earthquakes along the major Genoa and Antelope
  Valley frontal faults.   We  use  the  Coulomb
  failure  function  (CFF)  to suggest  that the 
  timing of the Double Spring Flat earthquake was
  delayed  by  static  stress changes due to the
  1978 M 5.5  Diamond  Valley  earthquake.   The
  CFF  near  the  hypocentral region of the 1994
  event dropped by 0.5 bar.  Given a stress drop
  of  0.5  bar  and  a  time  of  16 yrs for the
  stresses  to  reaccumulate  between  1978  and
  1994, the regional stressing rate is estimated
  to be > 0.03 bar/yr.  Seismicity from the Dou-
  ble Spring Flat earthquake has continued for 2
  years and migrated south-eastward towards  the
  Antelope Valley Fault along a series of conju-
  gate fault pairs.  A lobe of CFF increase from
  the  1978 and 1994 mainshocks encourages left-
  lateral slip on northeast striking planes  and
  normal  slip on north-south striking east dip-
  ping planes in a zone extending southeast from
  the 1994 mainshock.  Right-lateral strike-slip
  is discouraged for northwest  striking  planes
  in  this  zone.   The  relocated seismicity as
  well as the focal mechanisms of M >  4  earth-
  quakes  correlate  with  this increase in CFF.
  The  combined  effect  of  the  December  1995
  earthquakes  has also increased the CFF south-
  ward along the northern Antelope Valley  Fault
  where  three  M  >  4  earthquakes occurred in
  1996.  We also consider dynamic triggering  as
  an  alternative  mechanism  to  triggering  by
  static stress changes.