| Summary: | 碩士 === 國立中央大學 === 地球物理研究所 === 100 === When earthquakes occur, the waveforms which recorded by stations are influenced by three effects: source, path, and site effect. If seismic wave didn’t pass through any special structure, the shake intensity would decrease with the epicentral distance.
In this study we discover the intensity distribution which caused by middle depth earthquakes in North-eastern Taiwan does not follow this trend; the largest shake intensity don’t locate in the station with smallest epicentral distance, but move to the 24。N Latitude and this region is just the location which Philippine Sea plate start to subduct down to the Eurasian lithosphere. So, this pattern is probably caused by the slab structure.
The same phenomena are also observed in Japan and New Zealand subduction zone areas. Phillips and McVerry (2003) considerred that the cause of the anomalous intensity pattern is the slab path has high Q, and non-slab path has low Q, so the energy for non-slab path attenuates more than slab-path. Furumura and Kennett (2003) used the finite difference modeling to explaining the guided wave which is recorded in the station in high intensity area. In this study we take an earthquake occurred in north-eastern Taiwan in 2002, for example event “2002_0916_0003”, and also use finite difference modeling to discuss slab structure’s effect on intensity distribution. The result shows that the main reason which causes intensity anomalies is slab laminated velocity variation, not Q value difference; According to slab structure’s shape testing, we find the high intensity location is dependent on the slab bending angle, and we also found the same effect from observation; According to source location testing, we found the slab structure is easy to transmit intraplate events’ energy than that of interplate events, and we infer the sample event 2002_0916_0003 is intraplate event.
Our modeling result is not apparent like observation. It is due to our modeling doesn’t consider site effect, 3D structure, and the background velocity is too rough; However, we still use 2D finite difference method to simulate the intensity anomalies, it prove that this phenomenon is actually caused by slab velocity variation.
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