A Study of Early High-frequency Precursors by Physical Modeling

Young-Fo Chang,Bor-Shouh Huang

doi:10.3319/tao.2002.13.1.1(t)

Abstract

Early high-frequency precursors, which arrive before the onset of P waves, have been observed at some subducted zones. Seismologists considered this phenomenon to be the result of high-frequency seismic waves propagation in a high-velocity layer (HVL). Although some numerical simulations have been used to explain the existence of early high-frequency precursors, a high frequency 3-D elastic model has not been considered. . In this high frequency 3-D physical modeling study, thin duralumin plates of different thicknesses, embedded in epoxy blocks, were used to study the elastic wave propagation in a thin HVL. The experimental results reveal that high-velocity and high-frequency precursors can be generated using a thin HVL model, and the high-frequency precursors arrive earlier than low-frequency ones. It was also observed that the thinner the HVL, the smaller the amplitudes and higher the frequency of the precursors. Based on the travel times and amplitudes of the precursors, it can be concluded that P-to-P (P converts to P) interactions playa major role in controlling the propagation of precursors. The precursors are the P waves resonance in the HVL; therefore, the curvature and the period of the holes (the local low values) in the dispersion curve may be used to estimate the thickness of the HVL.

Highlights

TAO, Vol 13, No I, March 2002 subduction zones (Ansell and Gubbins 1986; Gubbins and Snieder 1991; Galea 1992; Smith et al 1994; Stuart et al 1995; Van der Hilst and Snieder 1996). The mechanism of these anoma lous phases has been inferred to be short-wavelength waves traveling within a thin high-veloc ity layer (HVL), while the longer wavelengths are not affected by the HVL and tr vel at nor mal mantle speed
The thin HVL is interpreted as an eclogite layer in which subduct ed oceanic crust is changed with high-pressure and high-temperature conditions (Gubbins et al 1994)
The arrival times of these early high-frequency precursors, relative to the Jeffreys-Bullen travel time, were used to explain how the precursors result from the waves refracted from the thin HVL using the ray method (Huppert and Frohlich 1981; Chiu et al 1985; Ansell and Gubbins 1986; Galea 1992)

Summary

Introduction

TAO, Vol 13, No I, March 2002 subduction zones (Ansell and Gubbins 1986; Gubbins and Snieder 1991; Galea 1992; Smith et al 1994; Stuart et al 1995; Van der Hilst and Snieder 1996). The mechanism of these anoma lous phases has been inferred to be short-wavelength waves traveling within a thin high-veloc ity layer (HVL), while the longer wavelengths are not affected by the HVL and tr vel at nor mal mantle speed. The arrival times of these early high-frequency precursors, relative to the Jeffreys-Bullen travel time, were used to explain how the precursors result from the waves refracted from the thin HVL using the ray method (Huppert and Frohlich 1981; Chiu et al 1985; Ansell and Gubbins 1986; Galea 1992). Smith et al (1994) analyzed the travel times and polarizations of the early high-frequency precursors, and concluded that the early high-frequency precur·sors travel through a high-velocity layer close to the top of the subducted plate and refract up...vard to the station

Objectives

Methods

Results

Conclusion