TECTONIC STRESS FIELD AND RECENT MOVEMENTS OF THE EARTH’S CRUST IN THE MANILA SUBDUCTION ZONE AND ADJACENT FAULTS D.Q.VAN Institute of Geophysics, VAST, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam N.V.DUONG Institute of Geophysics, VAST, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam N.V.LUONG Institute of Marine Geology and Geophysics , VAST, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam Abstract Analysis Four models of average stress states have been calcu- lated on the base of focal mechanism data and 25 focal The method of determination of average stress pattern mechanisms of the largest earthquakes belong to differ- n ent segments of Manila subduction zone and adjacent = xi faults are chosen. In order to identify the recent move- xTB ∑ ment pattern in the studied fault systems, some consent i=1 n criteria of classifying focal mechanisms and average stress states have been drawn out. Based on comparative analysis of the special correlation between the stress dis- • The average stress pattern have been defined for the systems/or segments tribution patterns with kinematic-geometric parameters of the active faults of faults, characteristics of average tectonic stress field • The average tress field is based on the average of kinematic-geometric pa- and recent tectonic movements have been defined for the rameters of faults systems/or segments of the active faults in the studied re- The double—couple model for seismic source gion. The criteria of classifying focal mechanisms and average stress states Introduction 0 0 The Manila Trench subduction zone is an active convergent plate margin between the South China Sea and Strike-slip fault: 0 ≤ Ψ ≤ 20 0 0 the northern Philippines. No earthquakes larger than Mw 7.6 have been observed in the past 100 years in 160 ≤ Ψ ≤180 this region, which implies a high probability of larger earthquakes in the near future. 0 0 Understanding of the stress field of the Manila Trench subduction zone is very important for studying the Oblique fault: 20 < Ψ < 70 tectonic movement, seismic and tsunami hazards in South China Sea. In this study, we use the focal mecha- 110 0 < Ψ <160 0 nism parameters of the large earthquakes occurred in the past to classify the difference of the tectonic movements of the active faults in the Manila Trench and Luzon island. The results will be useful for the in- 0 0 vestigations of tsunami sources in South China Sea. Reverse fault: 70 ≤ Ψ ≤ 110 0 ≤ Ψ ≤ 0 70 110 Results H < 20 km 20 km <H < 50 km H> 50 km Conclusions The stress field of Manila Trench and the faults in Luzon island varying in depth -In the shallow part, H< 20km, the Manila trench is characterized by extensional faulting in the northern part, whereas by compressional faulting in the southern part of Manila Trench. However, the strike-slip faulting is dominant in the Luzon island. - In the intermediate part, 20 km < H < 50 km, the earthquake epicenters are moved to the east. The reverse faulting is dominant for whole region. The compressional stress fields are NW-SE direction in the northern part and NE-SW direction in the southern part of Manila. - In the deep part, H > 50 km, we found that the earthquakes almost occurred in the south of Manila Trench. The stress field is characterized by compressional faulting. The di- rections of stress field are almost in NE-SE. .