Evaluation of crustal inhomogeneity parameters in the southern Longmenshan fault zone and adjacent regions

Xiao-Ping Fan,Yi-Cheng He,Cong-Jie Yang,Jun-Fei Wang

doi:10.1007/s10950-020-09949-w

Xiao-Ping Fan, Yi-Cheng He + Show 2 more

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https://doi.org/10.1007/s10950-020-09949-w

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Abstract

Broadband teleseismic waveform data from 13 earthquakes recorded by 70 digital seismic stations were selected to evaluate the inhomogeneity parameters of the crustal medium in the southern Longmenshan fault zone and its adjacent regions using the teleseismic fluctuation wavefield method. Results show that a strong inhomogeneity exists beneath the study region, which can be divided into three blocks according to its structure and tectonic deformation features. These are known as the Sichuan-Qinghai Block, the Sichuan-Yunnan Block, and the Mid-Sichuan Block. The velocity fluctuation ratios of the three blocks are approximately 5.1%, 3.6%, and 5.1% in the upper crust and 5.1%, 3.8%, and 4.9% in the lower crust. The inhomogeneity correlation lengths of the three blocks are about 10.1 km, 14.0 km, and 10.7 km in the upper crust and 11.8 km, 17.0 km, and 11.8 km in the lower crust. The differences in the crustal medium inhomogeneity beneath the Sichuan-Yunnan Block, the Sichuan-Qinghai Block, and the Mid-Sichuan Block may be related to intensive tectonic movement and material flow in the crust and upper mantle.

Highlights

Many seismological observations have revealed that heterogeneity in the earth’s crust can cause the scattering of seismic waves, which can carry a considerable amount of geological structural information
It can be seen from Eqs. (2), (3), and (4) that if ⟨ε3⟩ ≪ 1, the medium has a relatively weak inhomogeneity, while if ⟨ε3⟩ ≫ 1, the medium has a relatively strong inhomogeneity
Most of the high values of γ are in zone I, and most of the low values are in zone III, with mid-values in zone II

Summary

Introduction

Many seismological observations have revealed that heterogeneity in the earth’s crust can cause the scattering of seismic waves, which can carry a considerable amount of geological structural information. Large-scale inhomogeneities can be described using traditional structural parameters (e.g., seismic velocity, conductivity, and density), but smallscale inhomogeneities are described using the parameters σ and a, where σ is the velocity fluctuation ratio and a is the correlation length of a heterogeneity. In a Gaussian heterogeneous media (pðmÞ 1⁄4 σ2 π3α3e−m2α2=4, where m is wave length), a higher value of σ means a stronger inhomogeneity in the lithospheric medium, and a higher value of a means a larger-scale inhomogeneity (Sato 1984, 1989; Saito et al 2002). The teleseismic fluctuation wavefield method (TFWM) is currently accepted as an accurate means of evaluating values for σ and a (Ritter et al 1998; Ritter and Rothert 2000)

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