Fault Plane Determination for Small Earthquakes: Case Study in Taiwan

Abstract:

Based on the method FMNEAR, we propose an approach to discriminate the fault plane (FP) from the auxiliary one for earthquakes down to Mw 3.8 using a minimum of 6 local strong motion records. Faulting characteristics and source time function (STF) are also determined.

In a first step, a precise focal mechanism (FM) is computed with all representative solutions. Then, in a second step, fault length and maximum duration of the STF are tested on all best solutions defined previously. The discrimination of the FP is finally performed with a local grid search on best FM solutions (on strike and dip) using the best fault length and STF duration.

We obtain by this way a refined geometry of the FP with its corresponding fault slip distribution. The resulting slip map gives directly access to rupture dimension and rupture characteristics (directivity, rake variation, slip patches). For each event, location and depth are tested, a 1D velocity model is defined for each station and Mw is provided. Synthetic tests are performed at each step of the approach in order to confirm its reliability.

This approach is tested in the Longitudinal Valley (eastern Taiwan) where a dense network of stations is available and monitor the high activity generated by the plate boundary of the ongoing Taiwanese orogen. The final goal of this approach, will be to retrieve 3D geometry of active and secondary structures thanks to small to moderate earthquakes. In addition to geometry, rupture characteristics will be available for each fault portion as well as any time / location variation of those characteristics.