利用多重地震觀測網之寬頻波形資料改進臺灣海峽及琉球隱沒帶南段地區之震源機制

Abstract

臺灣由歐亞板塊與菲律賓海板塊聚合斜向碰撞而成，地震活動活躍，發生於臺灣島內的中小規模地震可利用台灣地震網資料加以約束震源機制解，但臺灣外海地區的地震往往面臨測站方位角包覆上的限制。為了更瞭解地體構造，本研究使用更確切的震源位置、較適合區域構造之速度模型及更廣的測站方位角包覆性，以區域波形逆推法計算臺灣海峽及琉球隱沒帶南段兩個外海地區的震源機制。 針對臺灣海峽地區，本研究系統性分析1996年至2018年芮氏規模大於3.5的地震事件，包含2018年11月25日 Mw 5.7的臺灣灘地震序列，同時使用福建與臺灣地震觀測網的波形資料，共提供21筆新的震源機制解。在琉球隱沒帶南段地區則分析規模5以上之中深層地震，額外加入日本觀測網的YNG和IGK測站波形資料進行逆推，同時也針對速度模型中的上部地函速度層做測試。在兩個區域的結果均顯示，以加站方式擴大觀測網的包覆性之時，對CLVD有明顯改善，而波形誤差值則是略為增高。調整速度模型則對少數地震的CLVD產生影響，誤差值卻不變。最後的震源機制與初始相比，旋轉角在海峽較明顯(平均差26度)，南琉球隱沒帶變化約20度。 研究結果顯示深度大於190 km深的隱沒帶地震適用地函速度層更接近IASP91的速度模型，鄰近臺灣濱海地區與較淺的地震則可用原始簡易三層速度模型(莫荷面位於40 km)。震源機制可知板塊內部應力發生改變，深度80-120 km呈現地震沿板塊傾沒方向一致的伸張應力(down-dip extension)，而深度超過230 km的地震則全部轉為傾沒壓縮應力(down-dip compression)。針對海峽地區，海峽中部地區使用莫荷面位於30或35 km速度模型得到整體較佳之最終解；而震央位置較靠近臺灣及臺灣沿岸地區之地震，則大多使用莫荷面40 km。 本研究綜合前人結果彙整臺灣海峽地區震源機制44筆，主要屬於歐亞被動大陸邊緣的地殼內淺震。其中苗栗外海一帶有與造山變形前緣有關的極淺逆衝地震；新竹臺中外海以帶走滑的逆衝為主；西南濱海地區則有走滑，以及與早期歐亞大陸邊緣因荷重增加產生撓曲並在附近造成的正斷層；臺南盆地是海峽地震活躍區，主要為正斷層兼有少許走滑移，可能為西北傾的斷層面；臺灣灘地震序列幾乎皆為走滑型態淺震(深度11-21 km)，較符合福建地震局目錄的餘震分布，斷層面應為東西走向；濱海斷裂系統附近則有正斷層及走向滑移，型態較零散。在構造應力方面，苗栗外海主壓縮應力軸為西北－東南向，臺南濱海地區為東北－西南向或東－西向臺灣灘主餘震序列之主應力軸方向則主要為西北－東南向。

Taiwan is a seismically active region formed by the oblique convergence between Philippine Sea Plate and Eurasia Plate. The focal mechanisms of most small-moderate sized earthquakes can be well constrained by the local seismic array of Taiwan, except for those occurred offshore Taiwan where azimuthal coverage is limited. To better understand the tectonic structures, it is desirable to improve the focal mechanisms using reasonable velocity models and the best available stations for well-located events based on waveform inversion. In this study we focus on the shallow earthquakes in Taiwan Strait and the intermediate-depth earthquakes in the southernmost Ryukyu subduction zone. For Taiwan Strait region, we systematically studied earthquakes from 1996 to 2018, including the Mw5.7 Taiwan Shoal sequence happened on 2018/11/25. A total of 21 new moment tensors (MTs) were resolved by combining Fujian and Taiwan seismic networks from either side of the strait. As for the southern Ryukyu subduction zone, we evaluate the MTs of M≥5 intermediate-depth earthquakes of the Ryukyu subduction zone by including waveforms of stations YNG and IGK from Japan network. In the inversion, we test models with different upper mantle velocities as well. The overall results show that as azimuthal coverage improved by enlarging the network, the compensated linear vector dipole (CLVD) components can be reduced significantly but the misfit value is increased slightly. Modifying the velocity has little effect on the CLVD and the average misfit is almost unchanged. When compared to the original MTs, our final solutions are generally rotated by ~26 degrees for events in the Taiwan Strait and 20 degrees in the southern Ryukyu subudution zone. Our study indicates that the events deeper than 190 km in the subduction zone prefer model with upper mantle velocity closer to that of the IASP91. In contrast, the shallower events near the Taiwan’s coastal area can be well simulatedwith the simple 3-layer model with Moho at 40 km and a half-space upper mantle originally designed for Taiwan region. The resulting focal mechanisms reveal that stress in the subducting slab varies with depth. Between depth of 80-120 km, the earthquakes show extension axis aligned with the dip of slab (down-dip extension). When depth is greater than 230 km, the stress of all earthquakes turn 180 degrees to down-dip compression. By combining results of previous studies, we compile a total of 44 well-constrained focal mechanisms in Taiwan Strait region. These events are mainly shallow crustal earthquakes under the passive margin of Eurasia. There is a sequence of extremely shallow thrusting events in the offshore Miaoli area of Taiwan that are associated with the deformation front formed by collision. Further to the south, earthquakes in offshore Hsinchu and Taichung are dominant in strike-slips. In offshore region of southwestern Taiwan, both strike-slip and normal events are present. The latter is related to bending caused by gravity load on the continental margin. In Taiwan Strait areas, Tainan Basin is relatively active in seismicity. Earthquakes there are mostly normal faulting with minor strike-slip component. The causal faults likely dip to the north. Events near the Binhai Fault system show complex focal mechanisims. The 2018 Taiwan Shoal earthquake sequence show high angle strike-slips and shallow centroid depth of 11-21 km, more consistent with aftershock distribution determined by Fujian seismic center. The inferred fault plane is E-W striking, also consistent with the previous study. Overall, the orientation of P axis are NW－SE off shore Miaoli, NE－SW in the offshore area of southwestern Taiwan, and NW－SE in Taiwan Shoal.