海洋地质与环境重点实验室知识库

俯冲带大地震及其引发的海啸会给人类社会带来惨重的灾难。马尼拉俯冲带是地震和海啸发生的高风险区域，时刻威胁我国和周边国家的安全，然而其潜在的最大地震震级尚不清楚，因此，研究马尼拉俯冲带大地震的潜在性具有重要意义。本文将借助传统的热-岩石学方法和最新的断层几何结构分析方法，分别研究马尼拉俯冲带北段逆冲大断层的发震范围及其滑动模式（粘滑或蠕滑），进而评估其孕震潜力，并为俯冲带逆冲大地震的研究提供重要实例和新的视角。

俯冲带逆冲大断层的孕震潜力的主要控制因素之一是断层的脆性破裂范围。为了确定马尼拉俯冲带北段逆冲大断层的脆性破裂范围，我们在南海古洋脊以北的俯冲区域选取了四条垂直于海沟方向的剖面，分别建立了二维热模型，通过有限元数值计算方法，并以地表实测热流为约束，获取了俯冲断层温度结构。基于热-岩石学中地震破裂面范围的温度约束条件（破裂面上边界温度约为150℃、下边界温度约为350℃），圈定了潜在地震发生的破裂面上下边界位置。结果表明俯冲断层地震潜在破裂面宽度为60±4 km，结合南海古洋脊以北的俯冲区域南北长度约为410 km，计算得到俯冲断层的潜在最大破裂面积约为24700 km²；进一步假定同震平均滑移距为10 m，剪切模量为30 GPa，估算研究区的潜在最大地震震级可达M_W8.52。

俯冲带逆冲大断层的孕震潜力，不仅受控于断层的脆性破裂范围，还取决于断层的滑动模式（粘滑或蠕滑）。马尼拉俯冲带所记录到的最大板间地震震级仅为M_W6.3，远小于热-岩石学约束下的潜在最大地震震级。为探究这一理论计算结果与观测结果的巨大差异，我们进一步通过分析俯冲断层的几何形态来推断断层的滑动模式。多道地震资料显示研究区俯冲前海底地形因覆盖较厚的沉积物而表现为光滑，但俯冲后大量沉积物被刮擦，致使崎岖的基底显露出来，从而改变了其光滑状态。与俯冲前海底地形相比，俯冲界面的粗糙度更能反映实际孕震断层的几何结构。我们借助傅里叶频谱分析法，首次定量计算了马尼拉俯冲带北段俯冲界面的粗糙度值，并与板块俯冲前海底地形的粗糙度值进行对比，结果表明前者的粗糙度明显高于后者。同时，热模型结果显示该俯冲带断层强度较高，进一步表明俯冲界面较为粗糙。通常粗糙界面会造成孕震断层的各向不均一性并发生蠕滑，因此，马尼拉俯冲带逆冲大断层的滑动模式以蠕滑为主，不易于形成大地震。

为了探究马尼拉俯冲带在全球范围的粗糙水平，揭示俯冲界面粗糙度与逆冲大地震震级的关系，我们收集了全球范围内三十五条横跨海沟的地震深度剖面，据此勾勒俯冲界面的几何形态，并借助傅里叶频谱分析方法，定量计算了这些俯冲界面的粗糙度。结果表明马尼拉俯冲带的俯冲界面在全球范围内相对粗糙。同时，对这些剖面所在位置记录到的最大逆冲型地震震级进行统计，发现俯冲界面粗糙度与最大地震震级存在相关关系：已发生8级以上大地震的俯冲断层粗糙度数值均低于0.1，而M_W＜6.5的俯冲断层粗糙度数值多高于0.5，进一步说明马尼拉俯冲带发生大地震的可能性较低。

综上所述，本文通过结合实测地质与地球物理资料，对马尼拉俯冲带北段进行了热结构数值模拟，据此圈定了温度约束下该区俯冲断层的潜在发震范围，推断该区具备孕育8级以上大地震的潜力；另外，借助多道地震资料与傅里叶频谱分析法，首次定量分析了俯冲界面的粗糙度，结果表明马尼拉俯冲带北段浅部俯冲界面倾向于粗糙，这与该俯冲带高断层强度结果一致。如果孕震深度的粗糙程度与浅部一致，则俯冲断层的滑动模式倾向为蠕滑，会降低该区域逆冲大地震发生的可能性。

Megathrust earthquakes in the subduction zone and related tsunamis could bring great disaster to human society. The Manila Trench subduction zone takes a high risk of earthquakes and tsunamis, which poses a great threat to China and surrounding countries. However, the maximum magnitude of its megathrust earthquake is still unclear, so it is of great significance to study the mechanism and risk of large earthquakes. In this study, we will estimate the potential seismogenic area and slip modes (stick-slip or creeping) of the megathrust in the Manila Trench subduction zone, using the well-established thermo-petrological modeling and the new fault geometrical analysis method respectively. Therefore, we will assess its megathrust seismogenic potential, and provide an important case and a new perspective for the seismogenic research of the subduction zones.

One main factor of megathrust seismogenic potential of subduction zone is the range of brittle rupture area. We constructed four two-dimensional thermal models along-strike north of the fossil ridge in the South China Sea, using the finite element code, constrained by measured heat flow, to obtain thermal structures of fault. Based on thermo-petrology (the up- and down-dip thermal limits of rupture area are ~150℃ and ~350℃), we inferred the brittle rupture area of the northern Manila Trench subduction zone. The results show that the rupture width of the potential seismogenic area is 60±4 km, and the rupture length is ~410 km north of the ~18 ° N fossil ridge. We inferred a maximum potential seismogenic area of ~24700 km2 and a potential earthquake magnitude of M_W ~ 8.52 with 10 m coseismic slip and 30 GPa shear modulus.

The megathrust seismogenic potential is not only constrained by brittle rupture area, but also by slip modes of fault (stick-slip or creeping). The maximum recorded magnitude of megathrust earthquake in the Manila Trench subduction zone is only M_W=6.3, which is much smaller than the calculated magnitude constrained by thermo-petrology. In order to figure out this difference between theory and observation, we further analyzed the geometrical structure of subduction faults which largely affect their slip modes. The multi-channel seismic profiles in this area show that the subducted South China Sea plate is very smooth because it is covered with very thick sediments, but rugged igneous basement is exposed after most sediments peeling off at the deformation front. The décollement roughness is more realistic to reflect the geometrical structure of faults compared with sea floor roughness. We quantified the roughness of the subduction interface in the northern Manila Trench subduction zone by Fourier spectrum analysis method for the first time and compared it with the roughness of incoming sea floor. The results show that the décollement is significantly rougher than the sediment-covered sea floor. Besides, thermal model results show that the fault strength here is high, which also indicates that the subduction interface is rough. The rough interface always causes the anisotropy of seismogenic fault to creep, so the slip mode of the Manila Trench subduction zone may be dominated by creep, which is not easy to form a large earthquake.

In order to explore the global roughness level of the northern Manila Trench subduction zone and the relationship between the roughness of décollement and megathrust earthquake magnitude, we made statistics on global subduction zones with 35 seismic depth profiles across the trench showing the décollement geometrical structure and quantified the décollement roughness with Fourier spectrum analysis method. The subduction interface in the Manila Trench subduction zone is rough globally. Besides, there is a good correlation between the maximum magnitude of megathrust earthquakes and the décollement roughness. Roughness values of the faults with M_W≥8 earthquakes are all less than 0.1, while those with M_W＜6.5 are mostly higher than 0.5. It further illustrates that the likelihood of large earthquakes in the Manila Trench subduction zone is diminished.

In summary, we modeled the thermal structure of the northern Manila Trench subduction zone based on geological and geophysical data. The potential seismogenic zone is limited by thermal control, and it shows that this area can host a megathrust earthquake with magnitude of M_W≥8. In addition, with multi-channel seismic data and Fourier spectrum analysis, the roughness of subduction interface is quantified for the first time. It is found that the shallow part of the subduction interface in the northern Manila Trench subduction zone is rough globally, which is consistent with high fault strength here. If the roughness of the seismogenic depth is the same as that of the shallow part, the slip mode is inclined to creep and the likelihood of great megathrust earthquakes occurrence is diminished.