A surprising discovery has been made that in some cases the complex, highly scattered Lg wave is found to be similar for clusters of events. We analyze in detail a subset of 28 out of 90 events from the 1999 Xiuyan sequence. Cross correlations provide highly accurate differential travel-time measurements. Their error estimated from the internal consistency is about 7 msec. These travel-time differences are then inverted by the double-difference technique to obtain epicenter estimates that have location precision on the order of 150 m. The locations are computed with waveform data from four to five regional stations 500 to 1000 km away. The epicenter estimates are not substantially affected by the sparseness of stations or large azimuthal gaps. Comparison with a surface trace a few kilometers away and location estimates based on much more dense networks led us to conclude that the absolute positions are accurate to the 5-km level. Regional event locations must often be based on a small number of phases and stations due to weak signal-to-noise ratios and sparse station coverage. This is especially true for monitoring work that seeks to locate smaller magnitude seismic events with a handful of regional stations. Two primary advantages of using Lg for detection and location are that it is commonly the largest amplitude regional wave (enabling detection of smaller events) and it propagates more slowly than P waves or Sn (resulting in smaller uncertainty in distance, for a given uncertainty in travel time).

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