Temporal variation of coda Q during Hualien earthquake of 1986 in eastern Taiwan

The May 20, 1986, Hualien earthquake sequence occurred on the northeastern coast of Taiwan. The M=6.1 (GS mb) mainshock was followed by a large number of closely-clustered aftershocks with the largest being an M=5.5 event. One seismic station, TWD of the Taiwan Telemetered Seismographic Network, is located in the surface projection of the source region and provides excellent recordings of the entire earthquake sequence. These recordings, plus events occurring in the same source area preceding the mainshock, offer a unique opportunity to study the spatial and temporal variations of coda Q in a region of active subduction. A simple technique is devised that uses the envelope of the coda waveform to enable a quick determination of the coda Q from drum records. For recordings with a peak power at about 8 Hz, the following findings have been obtained: 1. The ambient coda Q near an active subduction region was as low as 145; 2. There was no significant decrease in coda Q within the period beginning one year and four months prior to the mainshock; 3. There was a significant drop of coda Q immediately after the mainshock; this drop lasted approximately two days before returning to the ambient level; 4. Coda Q values varied with focal depth.