STATE OF STRESS IN THE LONG VALLEY CALDERA, CALIFORNIA

Earthquake focal-plane mechanisms and well-bore breakouts in the Long Valley caldera, California, indicate that the resurgent dome and calde ra south moat are characterized by a northeast extensional stress fiel d, consistent with geodetically determined extensional strain within t he caldera. Similar data from the western caldera indicate that it is characterized by a markedly different, northwest-trending, extensional stress field. We hypothesize that this localized rotation of the stre ss field is possible because of near-lithostatic pore pressure at dept h. Because an east-west extensional stress field appears to have exist ed in the western caldera at the time of emplacement of the Inyo volca nic deposits (500-1000 yr ago), the state of stress in the Long Valley caldera appears to be both spatially and temporally heterogeneous, mo st likely as a consequence of intracaldera processes related to magmat ic resurgence.