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南苏门答腊-爪哇海沟段外缘隆起带挠曲：大地水准面起伏模拟俯冲前缘岩石圈形变
刘洪芹, 孙伟涛, 杨怡宁, 赵俐红, 凌子龙, 吴时国
海洋学研究 ›› 2026, Vol. 44 ›› Issue (2) : 43-50.
PDF(1720 KB)
PDF(1720 KB)
南苏门答腊-爪哇海沟段外缘隆起带挠曲：大地水准面起伏模拟俯冲前缘岩石圈形变
Flexure of the outer rise of the South Sumatra-Java Trench Segment： Lithospheric deformation at the subduction front simulated by geoid undulation
印度洋俯冲带是全球最活跃的板块边缘之一，也是研究大洋俯冲过程的重要区域。尽管该区海沟的构造演化与深部结构已得到广泛研究，但针对海沟洋坡岩石圈挠曲的探讨仍相对有限。为揭示印度-澳大利亚板块俯冲过程中海沟岩石圈挠曲特征及其动力学意义，本文选取印度洋俯冲带的南苏门答腊-爪哇海沟作为研究区，基于研究区的地壳厚度、大地水准面起伏及水深数据，对东、中、西三段洋坡进行挠曲模拟，并通过非线性最小二乘拟合，获得各段岩石圈的挠曲特征与有效弹性厚度（Te）。模拟结果显示，爪哇海沟的挠曲位置约在16~76 km处，挠曲幅度约为68~192 m，有效弹性厚度约为20~34 km。空间上，Te值在中段最大，在东、西段较小，东段略高于西段。海沟挠曲特征在空间上表现出显著的差异性，主要受控于不同的地质构造因素：东段Te值较低主要归因于下地幔热上涌引起的岩石圈软化；中段Te值较高，指示其岩石圈整体刚性较强，但在邻近Roo 隆起及海山侵入等局部构造条件影响下，板块前缘应力集中，导致部分剖面仍表现出较大的挠曲幅度；西段Te值较低则可能与年轻板块、较强构造活动及流体作用导致的岩石圈强度降低有关，但由于该段俯冲速率较低、板块下沉较浅且拉张作用有限，其整体挠曲幅度仍相对较小。
The Indian Ocean subduction zone is one of the most active plate boundaries in the world and an important region for studying plate subduction. While many studies have examined the structure and evolution in the subduction zone， research on lithospheric flexure along the trench’s oceanward slope remains relatively limited. To clarify the characteristics of lithospheric flexure during subduction of the India-Australia Plate and its geodynamic implications， this study takes the South Sumatra-Java Trench as the research area for flexural modeling. Based on crustal thickness， geoid undulation， and bathymetric data， the lithospheric flexures of the eastern， central， and western segments of the South Sumatra-Java Trench were simulated. Using nonlinear least-squares fitting， we obtained the flexural characteristics and lithosphere effective elastic thickness （Te） of each segment’s oceanward slope. The simulation results indicate that the flexure zone lies roughly between 16-76 km from the trench axis， with an amplitude of 68-192 m， and Te of 20-34 km. Te is the largest in the central segment and smaller in the eastern and western segments. Te in the eastern segment is slightly larger than that in the western segment. The flexure of the trench exhibits significant spatial variability， primarily controlled by distinct geological and tectonic factors. The low Te in the eastern segment is mainly attributed to lithospheric weakening caused by thermal upwelling from the lower mantle. The high Te in the central segment indicates that its lithosphere is generally more rigid； however， influenced by local structures such as the Roo Rise and seamount emplacement， stress concentration at the plate forebulge leads to large flexural amplitudes in some profiles. The low Te in the western segment may be related to reduced lithospheric strength associated with a young plate， stronger tectonic activity， and fluid effects. Nevertheless， because this segment is characterized by a lower subduction rate， shallower slab descent， and limited extensional deformation， its overall flexural response remains comparatively weak.
大地水准面起伏 / 挠曲 / 有效弹性厚度 / 爪哇海沟 / 南苏门答腊海沟 / 弹性薄板模型 / 板块俯冲 / 外缘隆起带
geoid undulation / flexure / effective elastic thickness / Java Trench / South Sumatra Trench / elastic thin plate model / plate subduction / outer rise belt
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The Java margin is characterized by a distinct variation in lower to upper plate material transfer and recurring catastrophic tsunamogenic earthquakes. Both processes are closely linked to the subduction of oceanic basement relief resulting in varying degrees of fore-arc deformation. Tomographic models of refraction seismic profiles and reflection seismic lines in combination with high-resolution multibeam bathymetric data reveal the variability in the deep structure and deformation of the Java fore-arc. Shallow subduction processes are governed by the sediment supply in the trench as well as by the nature and fabric of the oceanic lithosphere. The deep structure of the fore-arc reveals a shallow upper plate crust–mantle transition, present along the entire Java margin section. The serpentinized fore-arc mantle wedge governs the depth extent of the seismogenic zone here, which is narrower compared to its Sumatran analogue. In addition, offshore central Java, high relief oceanic basement features potentially act as asperities as well as barriers to seismic rupture, limiting the possible magnitude of subduction thrust earthquakes. However, the potential for geohazards, in particular tsunamis, is high along the entire margin. This results from tsunamogenic earthquakes, ubiquitous splay faults and potentially tsunamogenic landslides, which further increase the risk of future tsunamis.
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Relocation of earthquakes recorded by the agency for meteorology, climatology and geophysics (BMKG) in Indonesia and inversions of global positioning system (GPS) data reveal clear seismic gaps to the south of the island of Java. These gaps may be related to potential sources of future megathrust earthquakes in the region. To assess the expected inundation hazard, tsunami modeling was conducted based on several scenarios involving large tsunamigenic earthquakes generated by ruptures along segments of the megathrust south of Java. The worst-case scenario, in which the two megathrust segments spanning Java rupture simultaneously, shows that tsunami heights can reach ~ 20 m and ~ 12 m on the south coast of West and East Java, respectively, with an average maximum height of 4.5 m along the entire south coast of Java. These results support recent calls for a strengthening of the existing Indonesian Tsunami Early Warning System (InaTEWS), especially in Java, the most densely populated island in Indonesia.
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Detailed P-wave speed velocity structure beneath the Sunda arc has been successfully imaged by applying a non-linear approach to seismic tomography. Nearly one million compressional phases from events within the Indonesian region have been used. These include the surface-reflected depth phases pP and pwP in order to improve the sampling of the upper-mantle structure, particularly below the back-arc regions. We have combined a high-resolution regional inversion with a low-resolution global inversion to minimize the mapping of distant aspherical mantle structure into the study region. In this paper, we focus our discussion on the upper mantle structure beneath the eastern part of the Sunda arc. The tomographic images confirm previous observations of a hole in the subducted slab in the upper mantle beneath eastern Java. The images also suggest that a tear in the slab exists below the easternmost part of the Sunda arc, where the down-going slab is deflected in the mantle transition zone. In good agreement with previous studies, the properties of the deflected slab show a strong bulk-sound signature.
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The bathymetry and free‐air gravity data offshore Sunda trench are used here to analyze the flexural forebulge and bending moment variations along the Southeast Asian subduction zone. The observed bathymetry is corrected for various effects such as the sediment loading, lithosphere age, and the gravity‐derived isostatically compensated topography, which gave rise to the flexural deformation surface of the subducting Indo‐Australian plate. From this, 28 across‐trench sections were constructed to model the plate flexural bending along the Sunda trench. We observed that except in the northern Sumatra trench, rest of the Sunda trench is in agreement with the flexural model explained by the bending moment applied by the slab. In the northern Sumatra part of the trench, additional horizontal stresses of ~30–40 MPa are required for better match of the flexural forebulge thereby increasing the coupling with the upper plate. The outcome of this analysis in comparison with the slab depth variation, which abruptly reduces from ~600 km in Java to ~200 km toward northern Sumatra, suggests that very large bending moment and horizontal stresses are anticorrelated with the slab depth. The shorter slab in Sumatra does not effectively pull the incoming plate in the mantle, and therefore, plate convergence is accommodated at shallow depth, increasing the coupling with the upper plate. We propose that horizontal stresses are the result of the lateral propagation of the stronger slab pull from the neighboring deeper southeastern Java slab.
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