A Study of the Errors in Locating Earthquakes due to the Geometry of the Taiwan Seisimc Network

Abstract

The Taiwan seismic network is operated by the Central Weather Bureau (CWB). It currently consists of 72 three-component stations and covers an area approximately 300 km long in the N-S direction, and 100 km wide in the E-W direction. The elongated geometry may cause a systematic bias in locating earthquakes in the Taiwan area. We conduct numerical experiments to investigate the possible errors caused by this geometry. We use this network to locate artificial earthquakes, and then study the location errors by comparing the computed with the true locations. We also use an artificial network to study the ability of the computer program routinely used by the CWB to locate earthquakes. The real data from the 15 December 1993 Tapu earthquake sequence are used to compare the quality of earthquake locations computed by the whole CWB network with that of those computed by the whole CWB network with that of those computed by excluding distant stations in network.

It is found that the computer program itself will not cause the computed epicenters to shift along a narrow azimuthal range. M ore than 50 % of artificial earthquakes with focal depths at 10 km or 20 km are located by the computer program at deeper depths under favorable situations. The errors caused by the program itself are less than 300 meters in epicenter locations, and less than 200 meters in focal depth determinations. The particular geometry of the CWB network will cause the computed epicenters to systematically shift along a narrow azimuthal range. The errors in epicenter locations are larger for deeper artificial earthquakes. The geometry of the CWB network will cause nearly all artificial earthquakes with focal depth 10 km or 20 km to be located at shallower depths. If there are errors in the plane-layered velocity model, the geometry of the CWB network still causes similar systematic errors in the epicenter locations. In this case, the shift of located epicenters is still confined to a narrow azimuthal range, but with a much larger distance. For artificial data, the distribution of epicenters computed by a sub-network, without distant stations (i.e. a subset of the CWB network), is similar to that computed by the whole CWB network. But those computed by far sub-networks (i.e. subsets of the CWB network) are significantly different from that computed by the whole CWB network. For real data, the epicenters computed by far sub-networks are also quite different from that computed by the whole CWB network. The quality of hypocenter locations computed by excluding distant stations is better than that computed by the whole CWB network. In summary, improvements in the quality of earthquake location can be achieved simply by not using distant stations to locate earthquakes in the Taiwan area.

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