Source characteristics of micro-earthquakes at the Northwest Geysers
Geothermal Field, California
GRC Transactions, Geothermal Resources Council, 34, page 1265--1272. Davis, California, Geothermal Resources Council, (2010)Abstract
We investigate the source parameters of micro-earthquakes (<M3) at
the Northwest Geysers, near an injection well, before and during
water injection, looking for temporal and spatial variations. Our
objective is to understand the relation among injection, production
and source mechanisms of micro-earthquakes. To determine the source
parameters of the micro-earthquakes, such as fault radius, stress
drop, seismic moment and radiated energy, we use two techniques:
the Empirical Green's Function (EGF) method; and the NetMoment method,
and compare the source parameters results of micro-earthquakes used
in both studies. Our results indicate that micro-earthquakes at the
Northwest Geysers have on average small radius and large stress drops
(median of 27.7 MPa). The stress drops of these induced micro-earthquakes
are comparable to the ones of deeper, large (>M5.5), natural occurring
tectonic earthquakes. Possible explanations for the high stress drop
are, for example, fault re-strengthening by geochemical alteration
or the fracturing of intact rock induced by the injection of cold
water. This source information has implications for understanding
the physics of faulting and for seismic potential assessment in areas
of ongoing geothermal exploration.