HR: 1340h
AN: T23A-0574    [Abstracts]
TI: Rupture Process and Energy Budget of Some Reservoir-Induced Earthquakes
AU: * Tomic, J
EM: tomic@moho.ess.ucla.edu
AF: UCLA, Dept. of Earth and Space Sciences, Los Angeles, 90095 United States
AU: Abercrombie, R E
EM: rea@bu.edu
AF: Boston University, Dept. of Earth Sciences, Boston, 02215 United States
AU: do Nascimento, A F
EM: aderson@dfte.ufrn.br
AF: Univrsidade Federal do Rio Grande do Norte, Departmento de Fisica Teorica e Experimental, Natal, RN 1596 Brazil
AU: Houston, H
EM: heidi@moho.ess.ucla.edu
AF: UCLA, Dept. of Earth and Space Sciences, Los Angeles, 90095 United States
AB: The study of earthquake rupture process and energy budget through the analysis of seismic waveforms can yield insights into the effect of such factors as hypocentral depth and the presence of fluids on rupture process. A long-standing issue is whether the rupture processes of induced earthquakes differ from those of tectonic earthquakes. For example, Abercrombie and Leary (1993) noted that hydro-fractures, mining and reservoir-induced earthquakes appear to have lower average stress drop than natural tectonic earthquakes. This difference might be a result of a different tectonic setting or the shallower hypocentral depths of induced earthquakes. Alternatively, simple source models and the assumption of constant rupture velocity might have resulted in underestimated stress drops. We analyze earthquakes induced by seasonal oscillations in the water level in the A\c{c}u Reservoir, NE Brazil for comparison with previous studies of tectonic and induced earthquakes. 286 earthquakes 0$\leq$M$\leq$2.2, were recorded at 200 sps, by 8 3-component digital seismographs. We use 3 different approaches to calculate the source parameters of the 6 largest earthquakes (M$\geq$1.8). First, we fit the individual spectra using an $\omega^{-2}$ source model to find corner frequency ($f_{c}$), frequency-independent $Q$, and long-period amplitude. Second, we use collocated small earthquakes as empirical Green's functions of the large one and calculate the spectral ratios. We fit the spectral ratios solving for the $f_{c}$ of the largest earthquake. Third, we use relative source time functions (i.e. pulse widths) to determine source radius ($r$), and the rupture velocity. Estimates of source duration and $f_{c}$ imply stress drops in the range of 10 to 100 MPa. These are similar to tectonic earthquakes suggesting that hypocentral depth and the presence of water did not strongly affect stress drop. The source time functions vary systematically with azimuth implying a rupture velocity of $\geq$0.6$\beta$, consistent with that of large tectonic earthquakes. To continue the analysis, seismically radiated energy can be estimated from the source spectra; this information bears on whether the ratio of energy to moment changes with moment. Such a change may indicate a difference in the physics of large and small earthquakes and has been seen in some studies, but remains controversial. The radiated energies, considered together with rupture velocities can provide scarce constraints on the efficiency of rupture. It is unusual to have source spectra, static stress drops, radiated energies, and rupture velocities all available for small earthquakes.
DE: 7299 General or miscellaneous
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting