| 摘要: | 本篇有兩個研究區域，分別是宜蘭平原和明多洛島(Mindoro)。宜蘭平原位於台灣的東北方，也位在沖繩海槽的西南末端，該地區正處於由弧陸碰撞轉為弧後張裂的過渡帶，地震活動頻繁。另外，弧後張裂伴隨的岩漿活動提供了良好的地熱環境(Tong et al., 2008)，清水地熱發電廠從民國69年至82年曾開採地下熱水及蒸氣來發電。
菲律賓群島是由巴拉旺陸塊(Palawan Continental Block, PCB)和菲律賓移動帶(Philippine Mobile Belt, PMB)在中新世早期發生弧陸碰撞所形成，本研究第二個研究地區明多洛島即位於碰撞的中心。明多洛島北方的左移斷層Sibuyan Verde Passage Fault (S.V.P.F.)為菲律賓斷層的一個分支，在斷層周遭可觀測到密集的地震，包括發生在2017年4月初主震Mw 5.1及5.9的Batangas地震序列。
宜蘭平原的地體構造複雜，本研究室於2013年11月底在宜蘭平原近山區共架設10個寬頻地震站以偵測地震；為了研究弧陸碰撞的地震特性，我們自2013年4月起也在明多洛島的西北部架設了寬頻地震站。匹配定位法(match and locate method, M&L, Zhang & Wen, 2015)使用模板事件(template event)的S波來尋找具有類似波形的小地震，本研究利用此方法偵測宜蘭平原及明多洛島周遭的地震，第一是探討清水地熱區和三星斷層的地震活動度，第二是藉由偵測完整的Batangas地震序列以進一步分析其擴散及餘震隨時間的衰減。
在宜蘭平原的偵測結果中，三星斷層周遭的原始目錄共有404筆地震，M&L額外搜尋了110筆事件，我們進一步探討地震發生頻率-規模的關係(b value)，原始目錄的完整規模(magnitude of completeness, Mc)是0.9，斜率b值為1.00；偵測結果目錄的完整規模0.2，b值0.80；最後則是聯集目錄，其完整規模是0.9，斜率b值上升至1.01，說明M&L可加強原始目錄偵測小規模地震的能力。另外，從聯集目錄中地震的分佈可約略看出地震多集中在三星斷層末端及向東北傾約45 ̊ 的斷層面。
M&L在Batangas地震序列中共偵測到3155筆事件，聯集目錄則有3222筆，其數量約為原始目錄的8倍。根據聯集目錄中的Batangas地震序列，4/4餘震序列的地震分布為向東北傾的斷層面，說明主斷層面的走向即震源機制解中的339 ̊，推測4/4就在此破裂面上發生右移地震。至於4/8餘震序列，其空間分佈較4/4序列寬廣且與震源機制解315 ̊ 的走向相符，4/8可能於此斷層面上發生了右移地震。
聯集目錄內餘震序列的地震數目較多且完整，因此可以藉由餘震的衰減速率來討論本研究寬頻地震網的偵測能力，及該區域的地體構造特性。我們使用修正-大森法則(modified Omori’s law)及其積分形式分析聯集目錄中4/4和4/8餘震序列每小時地震個數n(t)的衰減情形，得到平均K值為2619.25、c值8.64、p值則是1.48。c值代表地震網偵測早期餘震序列的能力，c值8.64表示M&L在主震發生後約8小時可完整偵測到餘震；p值1.48在全球平均值0.9-1.5中是較大的，推測該地區的地殼溫度高，因此累積應力釋放快、餘震衰減也快。
;There are two research areas in this study, namely the Ilan Plain and Mindoro Island. Ilan plain in NE Taiwan is on the southwest end of Okinawa trough, which is undergoing a period of transition from arc-continent collision to post-arc rifting with frequent earthquakes. In addition, the magmatic activity associated with the back-arc extension provides a good geothermal environment (Tong et al. 2008). The Qingshui geothermal power plant used underground hot water and steam to generate electricity from 1980 to 1993.
The Philippines archipelago was formed by arc-continent collision between the Palawan Continental Block and Philippine Mobile Belt in the early Miocene. The second study area of this study, Mindoro, is located at the center of the arc-continent collision. The Silbuyan Verde Passage Fault (S.V.P.F.) in the north of Mindoro is a branch of Philippine Fault. We can observe intensive earthquakes around this fault, including the Batangas earthquake sequence that occurred in early April 2017.
The tectonic structure of Ilan Plain is complex, so we set up 10 broadband seismic stations on Ilan plain to detect earthquakes at the end of November 2013. In order to study the seismic characteristics of arc-continent collision, we also set up broadband seismic stations in the northwestern of Mindoro. The match and locate method (M&L, Zhang & Wen, 2015) uses the S wave of the template event to detect small earthquakes with similar waveforms. In this study, we used M&L to detect earthquakes around the Ilan Plain and Mindoro Island. First, we discuss the seismicity of the Qingshui geothermal area and Sanxing fault, then detect the complete Batangas earthquake sequence to further analyze the migration and decay of aftershocks.
In Ilan Plain, there were 404 earthquakes in the original catalogue around Sanxing fault. M&L detected 110 additional events in the same period. We further discuss the frequency-magnitude relationship of earthquake catalogs. The magnitude of completeness (Mc) of original catalog is 0.9 and b value is 1.00. The Mc of detection result catalog is 0.2 and b value is 0.80. The Mc of collective catalog is 0.9 and b value rises to 1.01, indicating that M&L can strengthen the ability of detecting small earthquakes. In addition, according to the distribution of earthquakes in collective catalogue, we can find that the earthquakes are mostly concentrated at the end of Sanxing fault and the dip of fault plane is about 45 degrees to the northeast.
M&L detected 3,155 events during the Batangas sequence and there were 3,222 events in the collective catalogue, which is about 8 times the number of original catalog. According to the seismic distribution of collective catalogue, the dip of 4/4 rupture plane is to the northeast, indicating that the strike of fault plane is 339 degrees. As for the 4/8 aftershock sequence, the spatial distribution is consistent with the strike of 315 degrees.
The amount of earthquakes in collective catalogue is large and complete. Therefore, we can discuss the detection ability of broadband seismic network and the tectonic characteristics of study area based on the attenuation rate of aftershock. We use modified Omori′s law and integral form to analyze the attenuation rate of 4/4 and 4/8 aftershock sequences. The average K value is 2619.25, c value is 8.64 and the p value is 1.48. The c value represents the ability of seismic network detecting early aftershock sequences. The c value is 8.64 indicating that M&L can detect complete aftershocks about 8 hours after the main shock. The p value is 1.48, which is large within global average of 0.9-1.5, indicating that the crust temperature of this area is high so the cumulative stress released quickly. |
