9:15 AM - 9:30 AM
[S13-1-04] Non double couple components of Mw>4.5 events in The Geysers geothermal field, California revealed by a hierarchical Beyesian inversion
Earthquakes occurring in geothermal and volcanic environments can have significant non double couple (non-DC) components related to fluid movement. However, significant non-DC components can also emerge as a consequence of not adequately (or not at all) accounting for the noise in the data in routine waveform inversions for earthquake source parameters. Here, we apply a hierarchical Bayesian moment tensor inversion method to a number of earthquakes from The Geysers field in California using data from broadband stations at regional distances. This probabilistic technique yields parameter uncertainties that indicate the reliability of the non-DC components. Furthermore, we allow the noise to determine the level of data fit by treating it as a parameter in the inversion, and account for interdependence of data errors using an empirically estimated noise covariance matrix.
The structure of The Geysers field involves stacks of permeable and impermeable highly fractured rocks, probably underlain by a magma body. Seismicity is dominated by microearthquakes, half of which have significant non-DC components. We have analysed several Mw>4.5 events using two different assumptions about the noise (two different noise covariance matrices).
The double couple components of the sources are consistent with strike-slip or normal faults present in the geothermal field, and the non-DC components show a variety of values. Inversions with the empirically estimated cosine covariance matrix result in a slightly lower isotropic component than inversions with a pure diagonal covariance matrix. One event is predominantly a double couple, while the others have significant non-DC components. They can be explained as a combination of shear faulting and crack opening, but at least one event requires an additional mechanism that reduces the isotropic component, such as fluid extraction.
The structure of The Geysers field involves stacks of permeable and impermeable highly fractured rocks, probably underlain by a magma body. Seismicity is dominated by microearthquakes, half of which have significant non-DC components. We have analysed several Mw>4.5 events using two different assumptions about the noise (two different noise covariance matrices).
The double couple components of the sources are consistent with strike-slip or normal faults present in the geothermal field, and the non-DC components show a variety of values. Inversions with the empirically estimated cosine covariance matrix result in a slightly lower isotropic component than inversions with a pure diagonal covariance matrix. One event is predominantly a double couple, while the others have significant non-DC components. They can be explained as a combination of shear faulting and crack opening, but at least one event requires an additional mechanism that reduces the isotropic component, such as fluid extraction.