| Abstract |
TOMO4D is a seismic tomography program that inverts multiple earthquake arrival-time data sets simultaneously, imposing constraints to minimize the structural differences calculated for different epochs. This approach is similar to that of seeking models that resemble an a priori assumption, and it can be solved using a regularization method similar to damped least squares. During 2015 we applied TOMO4D extensively to real data for the first time. In the light of this experience, programming enhancements were made that include model-steepness and roughness optimization and the use of regional-earthquake data. Long Valley caldera in eastern California has been the site of ongoing seismic activity since 1978. In particular, a region to the south of the resurgent dome (the “south moat”) has been the site of multiple swarms involving hundreds of thousands of earthquakes. In volcanic and geothermal systems, the distribution of fluids, cracks and pressurized gases can cause rapid changes in the elasticity of the host rocks that are potentially detectable using time-dependent tomography. We applied TOMO4D to sets of data from 1997 and 2009/10. Differencing independently determined inversions obtained with the widely used program SIMUL2000A shows changes in structure. These changes are also detected by TOMO4D, but with lower amplitudes. These results suggest that the amplitudes of structural changes determined by differencing independent inversions using SIMUL2000A are exaggerated in their magnitude. The commercially exploited Coso geothermal area in southern California is highly seismogenic, with thousands of earthquakes occurring each year. It is monitored by the Geothermal Program Office of the US Navy. We studied each of the years 1996 - 2008, plus 2010 and 2012. As for Long Valley, we determined changes in structure from year to year by comparing independent inversions obtained using SIMUL2000A. Our preliminary results show that inversions conducted using TOMO4D also show changes but again, they are weaker than those obtained using simul2000A. Mt. Etna, a persistently active volcano in Sicily, poses a significant volcanic hazard, and any methods that might provide early warning of activity are potentially important. We selected and relocated about 400 local earthquakes having at least 5 P and 5 S observations from a database provided by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) Osservatorio Etneo. We compared epochs that correspond to pre-eruptive, co-eruptive and post-eruptive periods between 2000 and 2006. Current results suggest that, while epoch results obtained independently using SIMUL2000A show changes in structure, TOMO4D reveals that these changes are probably not required by the observations. |