Study of the inhomogeneous structure in the subduction zone based on the analysis of seismogram envelopes

俯冲带附近的弹性构造表现出较强的不均匀性。针对随机非均匀构造中地震记录包络随着传播距离的增加而衰减的问题，建立了数学模型，并从实际分析中估计了东海、关东和东北地区楔形地幔的随机性特征。(1)包络分析：我们提出了一种反演方法，从整个地震记录包络估计衰减和随机性。从火山锋弧后侧包络面展宽大于弧前侧的证据可以看出，弧后侧的随机性短波分量大于弧前侧的随机性分量。(2)频率相关衰减：通过对比井眼地震记录的入射相位和反射相位，定量估算了沉积层S波衰减。在传统尾线归一化方法的基础上，提出了一种同时测量纵波和纵波衰减的方法。(3)散射衰减与本征吸收比值的测量：假设各向同性散射，基于蒙特卡罗模拟计算S波能量密度的时间迹。将模拟数据与观测数据进行比较，定量估算了关东-东海地区的散射与本征衰减比。(4)数学建模：基于能量输运理论，建立了包含P-S转换的多重各向同性散射模型。(5)野外观测：我们在东北地区的三个地点进行了野外观测，发现包络的形状明显依赖于它们的射线路径。

The elastic structure near around the subduction zone shows relatively strong inhomogeneities. Focusing on the decay of seismogram envelopes with increasing travel distance in the randomly inhomogeneous structure, we have constructed mathematical models, and estimated the characteristics of randomness in the wedge mantle beneath Tokai, Kanto, and Tohoku area from the practical analysis.(1) Envelope analysis: We proposed an inversion method to estimate the attenuation and randomness from the full seismogram envelope. From the evidence that the envelope broadening is larger at the back arc side of the volcanic front compared with those in the fore arc side, we found short wavelength components of the randomness are larger in the back arc side than those in the fore arc side.(2) Frequency dependent attenuation: From the comparison of the incident and the reflected phases in borehole seismograms, we quantitatively estimated S wave attenuation in the sediment layer. Extending the conventional coda normalization method, we proposed a method for the simultaneous measurement of P and S wave attenuation.(3) Measurement of the ratio of scattering attenuation and intrinsic absorption: Supposing isotropic scattering we calculated the time trace of S wave energy density based on the Monte-Carlo simulation. Comparing with the simulation and the observed data, we quantitatively estimated the ratio of scattering to intrinsic attenuation in the Kanto-Tokai area.(4) Mathematical modeling: We have formulated the multiple isotropic scattering model including P-S conversions based on the energy transportation theory.(5) Field observation: We started field observation at three sites in Tohoku, and found that the envelope shapes clearly depend on their ray paths.