Abstract: 3-D V_P and V_S images of southern Philippines at the 0~100km depths are generated by inverting a large number of travel-time data from the International Seismological Centre(1960-2017)through seismic tomography method. The results show lateral variation exists in the crust and upper mantle:High V_P and V_S anomalies emerge in mid-west Mindanao and Bohol Island, which might be caused by the combined action of huge magmatism and ophiolite accretion in the lower crust; low velocity anomalies of the upper mantle in the west of Mindanao are consistent with locations of volcanoes on the surface. It, thus, could be inferred that the low velocity anomaly is closely related to magmatic activity. The dense earthquake distribution along plate margin extending to 100km coincides with the strong activity of the Philippine Sea Plate which is located in the northeast and southeast of Mindanao. Relative weak activity of Sulawesi Sea Basin is presented simultaneously. The subduction of the Philippine Sea Plate is mostly concentrated in the crust and the top of the uppermost mantle.
Our tomographic images show that lateral heterogeneities exist in the crust and uppermost mantle of the southern Philippines. Low V_P and V_S anomalies emerge in Philippine Trench and Cotabato Trench, in contrast, high V_P and V_S anomalies appear in shallow crust of land area where a large number of earthquakes and magmatic activities develop. This may reflect strong tectonic processes between the Philippine Sea Plate and Philippine Mobile Belt.
Low V_P and V_S anomalies in the crust of eastern Mindanao coinciding with the location of volcanoes on the surface may show partial melting of crust material caused by dehydration of the subducting Philippine Sea Plate. Such a similar phenomenon can be also seen in the south of Negros Island and Cotabato Trench. Thus we infer that active tectonic behaviors are constrained within the crust of the Philippine Sea Plate, Sulu Sea Basin and Sulawesi Basin.Low V_P and V_S anomalies of the mantle in the mid-west of Mindanao island are associated with magmatic activity which may be caused by a collision between the east and west part of Mindanao at 5Ma. The fracture system in the west of Mindanao provides the possible passage ways of mantle hot material upwelling, coinciding with the model of geothermal distribution in this area. According to the geochemical analysis, ophiolite observed in Sanbaoyan and the western part of Mindanao could indicate material composition from crust to upper mantle on Eurasian continental margin which may show the evidence of rapid expansion environment of mid-ocean ridge. High V_P and V_S anomalies in the mantle of northeast and southeast of Mindanao coinciding with the distribution of massive earthquake along boundaries show a well agreement with the shape of the Philippine Sea Plate. Dense earthquake distribution in south Mindanao at 100km shows the Philippine Sea Plate has strong activity and stress accumulation in the upper mantle. On the contrary, the seismicity in southwest Mindanao and Cotabato Trench reduces rapidly at the depth from 50km to 100km, revealing weak subduciton and stress release of Sulawesi Basin in the mantle.

Key words: seismic tomography, P- and S-wave, Philippine Sea Plate, Sulawesi Sea Basin

摘要： 文中搜集整理了国际地震中心1960—2017年公开的地震数据，使用地震层析成像的方法反演了南菲律宾地区0～100km的P波和S波三维速度结构，成像结果揭示了南菲律宾地区地壳及上地幔速度结构的横向变化：1）棉兰老岛中西部—保和岛一带在地壳浅部出现的P波和S波高速异常特征很有可能是巨量的岩浆活动及蛇绿岩在下地壳增生共同作用的结果；2）棉兰老岛西部在地幔中出现的低速异常同地表出露的火山位置十分吻合，推测该低速异常与岩浆活动密切相关；3）菲律宾海板块在棉兰老岛的东北部和东南部活动性较强，板块边缘在100km的深度仍然有着较为密集的地震分布；4）苏拉威西海盆在棉兰老岛西南侧的活动性较弱，其俯冲活动主要集中在地壳及上地幔顶部。

关键词: 地震层析成像, P波和S波, 菲律宾海板块, 苏拉威西海盆