Frequency-Dependent Moment Tensors of Induced Microearthquakes

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

Moment tensors (MTs) of 984 microearthquakes in The Geysers geothermal reservoir are calculated using the P-wave amplitudes extracted from signals filtered in different frequency bands defined by seven upper cutoff frequencies (2, 3, …, 8 Hz). The frequency dependence of the double-couple (DC), compensated linear vector dipole (CLVD), and isotropic (ISO) percentages, and the RMS as well as the P/T-axes is analyzed. The DC, CLVD and ISO percentages vary significantly with the frequency band of the signal, while the P/T-axes are more stable. It means that the increasing bandwidth significantly changes the relative energy distribution of different MT components, but the fault plane geometry is less affected. The frequency dependence of the RMS is consistent with the spectral distribution of noise recorded by the network, which implies a strong influence of noise level on the MT inversion in different frequency bands. The percentage of the DC and ISO components tends to decrease and increase, respectively, as the upper cutoff frequency increases. This suggests that the shear rupture along the main fault plane radiates energy at lower frequencies than the tensile rupture.

Since the ISO component can be directly related to the injection and migration of fluids, the frequency dependence of the ISO component is further evaluated to separate events with a weak or strong frequency dependence. Most of events with a strongly varying ISO are characterized by an increase of the ISO with increasing frequency band of the analyzed signal. These events are mainly located within the same depth interval as most of the injection points and do not migrate downward as the other events. This might be explained by the presence of small tensile cracks adjacent to the main shear fracture created by strong thermoelastic effects in the vicinity of injection wells rather than by the poroelastic effects that are more important at larger distance.

Presenting Author: Vaclav Vavrycuk

Authors