9:30 AM - 9:45 AM
[S13-1-05] Determination of high precision microseismic source mechanism by iterative relative moment tensor inversion
Source mechanism of microearthquakes provides fundamental information about the physical process at the source, in-situ stress field, etc. However, their reliable estimate is still a difficult task due to our poor knowledge of the underground structure. Dahm (1996) developed a novel method of source mechanism determination for earthquake clusters called relative moment tensor inversion (RMTI), in which relative body-wave amplitudes for two earthquakes recorded at a common station are used to eliminate the effect of un-modeled propagation paths. If the source mechanism of one of those earthquakes is known a priori, the other source mechanisms can be determined without a computation of Green's function. A difficulty in this method is that errors in the mechanism of reference events may lead to biased solutions for other events.
To resolve this problem, we propose a method that iteratively applies the RMTI to source clusters improving each moment tensor as well as their relative accuracy. The procedure is as follows:
(1) Sample co-located multiple earthquakes with focal mechanisms, as initial solutions, determined by an ordinary method.
(2) Apply the RMTI to estimate the source mechanism of each event relative to those of the other events.
(3) Repeat the step 2 for the modified source mechanisms until the reduction of total residual converges.
Numerical tests showed that the solutions were successively improved by iteration and almost reached the input mechanisms. Even in the case of poor observation condition, we could obtain reliable estimates by imposing a DC constraint. Application to natural earthquakes revealed a small-scale heterogeneity which could not be identified by earthquake locations. The proposed method also has a potential to resolve real non-DC components, so our next plan is to apply the method to induced microseismic events in oil, gas, and geothermal fields, in which the relation between fluid injection and non-DC components is an important issue.
To resolve this problem, we propose a method that iteratively applies the RMTI to source clusters improving each moment tensor as well as their relative accuracy. The procedure is as follows:
(1) Sample co-located multiple earthquakes with focal mechanisms, as initial solutions, determined by an ordinary method.
(2) Apply the RMTI to estimate the source mechanism of each event relative to those of the other events.
(3) Repeat the step 2 for the modified source mechanisms until the reduction of total residual converges.
Numerical tests showed that the solutions were successively improved by iteration and almost reached the input mechanisms. Even in the case of poor observation condition, we could obtain reliable estimates by imposing a DC constraint. Application to natural earthquakes revealed a small-scale heterogeneity which could not be identified by earthquake locations. The proposed method also has a potential to resolve real non-DC components, so our next plan is to apply the method to induced microseismic events in oil, gas, and geothermal fields, in which the relation between fluid injection and non-DC components is an important issue.