| 摘要: | 當地震發生時，能量隨著距離衰減，故震度會隨著震央距的增加而減少，此研究我們發現於台灣東北外海的中深源地震，其震度分佈沒有相同的現象，較大震度的地區反而是位於震央距較遠的區域(約為北緯24度之區域)，該地區為菲律賓海板塊開始隱沒的區域，因此該現象很有可能是因為隱沒帶構造所造成的異常震度分布。同樣的觀測現象在日本及紐西蘭隱沒帶地區皆有發現，Phillips and McVerry (2003) 對紐西蘭隱沒帶的研究，認為是因為隱沒帶Q值較大，沿著隱沒帶上來的波線能量衰減較小，以致於地表上觀測最大震度產生位移的現象; Furumura and Kennett (2003) 對日本隱沒帶地震的研究也同樣觀察到該現象，利用有限差分法模擬探討測站接收到的異常高頻尾波。本研究利用發生於2002年9月16日的東北外海的中源地震為例，藉由二維有限差分法模擬計算，探討隱沒帶對於地震震度分布之影響。結果顯示，Q值的差異的確會對該現象有影響，但隱沒帶速度變化造成能量產生聚焦的效應較為顯著；藉由板塊形貌的測試發現，隱沒角度影響最大震度出現的位置及能量聚集的程度，傾角越小，震波能量越能傳遞到愈遠處，相對高區的位置越遠離震央，觀測資料也顯示了相同的效應；根據震源位置測試，隱沒帶邊界型地震的能量較不易沿著隱沒帶方向傳遞，內部型地震則較容易，其原因是因為隱沒帶是個相對高速的構造，波線易往低速移動，藉由此測試我們可推測2002_0916_0003地震事件應為隱沒帶內部型的地震。 模擬測試的結果顯示異常震度分佈並沒有如觀測資料與四周的差異那麼大，推測可能原因如模擬模型並沒有把場址效應放入考慮、背景速度模型較為粗糙，以及本研究沒有考慮到三維的構造…等等因素。雖然如此，藉由二維模擬的計算，我們仍將異常震度分佈完整的重現，顯示該異常震度分佈的確是受到隱沒帶與四周速度差異所影響。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. |
