On the nature of regional seismic phases—III. The influence of crustal heterogeneity on the wavefield for subduction earthquakes: the 1985 Michoacan and 1995 Copala, Guerrero, Mexico earthquakes
On the nature of regional seismic phases—III. The influence of crustal heterogeneity on the wavefield for subduction earthquakes: the 1985 Michoacan and 1995 Copala, Guerrero, Mexico earthquakes
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DOI：
10.1046/j.1365-246x.1998.00698.x
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发表时间：
1998-12
影响因子：
2.8
通讯作者：
T. Furumura;B. Kennett
中科院分区：
文献类型：
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作者：
T. Furumura;B. Kennett
在150 ~ 1000 km范围内，区域地震波场最显著的特征通常是Lg相。这种到达代表了被困s波在地壳内作为多次反射的叠加传播，其振幅对沿传播路径的地壳横向变化非常敏感。在事件发生在俯冲带的环境中，例如墨西哥西海岸，由于俯冲带的存在，对区域波场的特征产生了相当复杂的影响。发生在墨西哥俯冲带的1985年米却肯大地震（MW=8.1）是现代历史上最具破坏性的地震之一，其显著特征是，在距离震中350多公里的墨西哥城，强烈的地面震动几乎与震中地区的震动相当，持续了几分钟。人们花费了相当大的努力来解释在破坏性地震中造成首都严重破坏的不寻常的观测波的起源。通过使用1995年墨西哥城南部海岸附近的Copala， Guerrero （MW=7.4）地震的详细强震记录，可以部分地理解该地区传播过程的性质，该地震在墨西哥谷也有增强的振幅。利用伪谱方法对墨西哥西部二维和三维非均质地壳模型中的P和S地震波进行数值模拟，通过使用波场快照序列和地表合成地震记录，可以直接了解传播过程的性质。对不同模型进行比较，可以隔离结构不同方面的影响。1985年米却肯地震和1995年科帕拉地震的二维和三维模型清楚地表明，长时间强烈地面震动的起源来自Sn和Lg波列。这些s波到达是由浅层俯冲地震有效地产生的，并且在墨西哥大陆下方的Cocos板块俯冲产生的横向非均匀波导中传播时被强烈增强。与火山区S- p转换相关的低速表层S波放大通过墨西哥火山带的传输，也强烈增强了Lg尾波的振幅和持续时间。大而长的Lg波串的进一步放大，撞击在墨西哥城盆地的浅层结构上，在非常软的土壤下，几乎是刚性的基岩，具有很强的阻抗对比，引起墨西哥俯冲地震的破坏性强地面震动。
Summary The most prominent feature of the regional seismic wavefield from about 150 to over 1000 km is usually the Lg phase. This arrival represents trapped S-wave propagation within the crust as a superposition of multiple reflections, and its amplitude is quite sensitive to the lateral variation in the crust along a propagation path. In an environment where the events occur in a subduction zone, such as the western coast of Mexico, quite complex influences on the character of the regional wavefield arise from the presence of the subduction zone. The great 1985 Michoacan earthquake (MW=8.1), which occurred in the Mexican subduction zone, was one of the most destructive earthquakes in modern history and its notable character was that at Mexico City, located over 350 km from the epicentre, there was strong ground shaking almost comparable to that in the epicentral region that lasted for several minutes. Considerable effort has been expended to explain the origin of the unusual observed waves that caused the severe damage in the capital city during the destructive earthquake. The nature of the propagation process in this region can be understood in part by using the detailed strong-motion records from the 1995 Copala, Guerrero (MW=7.4) earthquake near the coast to the south of Mexico City, which also had an enhanced amplitude in the Valley of Mexico. Numerical modelling of both P and S seismic waves in 2-D and 3-D heterogeneous crustal models for western Mexico using the pseudospectral method provides direct insight into the nature of the propagation processes through the use of sequences of snapshots of the wavefield and synthetic seismograms at the surface. A comparison of different models allows the influences of different aspects of the structure to be isolated. 2-D and 3-D modelling of the 1985 Michoacan and 1995 Copala earthquakes clearly demonstrates that the origin of the long duration of strong ground shaking comes from the Sn and Lg wave trains. These S-wave arrivals are produced efficiently from shallow subduction earthquakes and are strongly enhanced during their propagation within the laterally heterogeneous waveguide produced by the subduction of the Cocos Plate beneath the Mexican mainland. The amplitude and duration of the Lg coda is also strongly reinforced by transmission through the Mexican Volcanic Belt from the amplification of S waves in the low-velocity surficial layer associated withS-to-P conversions in the volcanic zone. The further amplification of the large and long Lg wave train impinging on the shallow structure in the basin of Mexico City, with very soft soil underlain by nearly rigid bedrock with a strong impedance contrast, gives rise to the destructive strong ground shaking from the Mexican subduction earthquakes.