Improvements of the Maximum Intersection Method for 3D Absolute Earthquake Locations

We present an improvement of the maximum intersection (MAXI) method in which absolute earthquake location is defined by the maximum intersection number of hyperbolic equal differential time (EDT) volumes (one EDT volume being described as all grid nodes satisfying the arrival-time differences between two stations, +/- a tolerance value known as TERR). This 3D technique is well adapted to a strongly heterogeneous environment, avoids the depth versus origin-time trade-off (even using P arrivals alone), and objectively filters possible erroneous arrival times. Improvements consist of introducing an iterative multiscale approach on the TERR parameter and conducting a hierarchy of grid discretization to refine the preliminary solution (multigrid algorithm). The outlier cleaning process is strengthened by conducting statistical analyses based on the number of EDT volume intersections per station, rather than on travel-time residues. Processing two series of synthetic arrival times computed in the 3D seismological configuration of the southernmost Ryukyu subduction zone, offshore Taiwan, allows us to (1) confirm MAXI robustness, even when arrival times are highly disturbed or azimuthal coverage largely reduced, and (2) show that MAXI, by limiting trade-offs between origin time and depth and between epicentral position and depth, is less affected by a wrong velocity model than a technique that minimizes travel-time residues. To conclude, we apply the MAXI method to locate earthquakes recorded by the combined Central Weather Bureau of Taiwan (CWB) and the Japanese Meteorological Agency (JMA) networks (1991-2008) within a georealistic a priori 3D P-velocity model. Hypocenter determinations are selected based on MAXI confidence factors, and earthquake clustering along well-known tectonic features witnesses hypocenter determination accuracy.