Stress Drops of Intermediate‐Depth and Deep Earthquakes in the Tonga Slab
Stress Drops of Intermediate‐Depth and Deep Earthquakes in the Tonga Slab
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DOI：
10.1029/2022jb025109
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发表时间：
2022-09
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影响因子：
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通讯作者：
D. Tian;S. S. Wei-S.;W. Wang;Fan Wang
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作者：
D. Tian;S. S. Wei-S.;W. Wang;Fan Wang
已经提出了多种物理机制来解释中深地震和深地震的原因，但它们仍然存在争议。应力降等震源参数有可能提供对其物理机制的深入了解。我们开发了一种改进的谱分解方法来分析复杂俯冲带中临时陆基和海底地震仪的1年地震数据。通过应用这种方法调查汤加板块的1，083个中深地震和深地震，我们成功地解决了743个MW 2.6-6.0地震的震源谱和应力降。尽管绝对应力降取决于源模型参数的选择，但相对应力降可以更可靠地求解。汤加地震的中值应力降不随震级变化，但在70-250和400-600 km两个独立的深度范围内随深度减小2-3倍，分别对应于中深地震和深源地震。中值应力降显示出空间变化，有两个高应力降（比环境值高五倍）区域，与汤加板块弯曲或撕裂的强烈局部变形相一致。在深度为120-300 km的汤加双地震带中，下平面的中值应力降似乎小于上平面，这表明下平面区域的破裂速度较慢或流体含量较高。我们的研究结果表明，在汤加板块的中深和深地震一般遵循地震自相似模型，有利于流体相关的脆化假说的两组地震。
Multiple physical mechanisms have been proposed to explain the cause of intermediate‐depth and deep earthquakes, but they are still under debate. Source parameters such as stress drop, have the potential to provide insight into their physical mechanisms. We develop a modified spectral decomposition method to analyze 1‐year seismic data from temporary land‐based and ocean bottom seismographs in a complex subduction zone. By applying this method to investigate 1,083 intermediate‐depth and deep earthquakes in the Tonga slab, we successfully resolve the source spectra and stress drops of 743 MW 2.6–6.0 earthquakes. Although the absolute stress drops are subject to the choices of source model parameters, the relative stress drops are more reliably resolved. The median stress drop of Tonga earthquakes does not change with respect to magnitude but decreases with depth by 2–3 times in two separate depth ranges of 70–250 and 400–600 km, corresponding to intermediate‐depth and deep earthquakes, respectively. The median stress drops show spatial variations, with two high‐stress‐drop (five times higher than the ambient value) regions, coinciding with strong local deformation where the Tonga slab bends or tears. In the Tonga double seismic zone at 120–300 km depths, the median stress drop appears smaller in the lower plane than in the upper plane, suggesting a slower rupture velocity or a higher fluid content in the lower‐plane region. Our results suggest that intermediate‐depth and deep earthquakes in the Tonga slab generally follow the earthquake self‐similar model and favor the fluid‐related embrittlement hypothesis for both groups of earthquakes.