Fluid Distribution Along the Nankai-Trough Megathrust Fault off the Kii Peninsula

Inferred from Receiver Function Analysis

verfasst von: Dr. Takeshi Akuhara

Verlag: Springer Singapore

Buchreihe : Springer Theses

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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This thesis explores fluid distribution along the Nankai-Trough megathrust fault around the Kii Peninsula of Japan, where devastating earthquakes are expected to occur in the near future. Exploring fluid distribution along subduction zones is an important issue because the fluid is considered to control the occurrence of earthquakes. One of the effective strategies to estimate fluid content is retrieving receiver functions (RFs) from seismograms, but in the case of ocean-bottom seismometers (OBSs), noisy P-wave reverberations within the seawater column make such an analysis difficult. The author therefore developed a novel technique to suppress the water reverberations, which allows obtaining the fluid distribution data along a wide depth range on the plate interface.

This thesis first presents the new technique, called the water layer filter method, and demonstrates its efficiency by using both synthetic and observation data. Then, using the method, a receiver function image of the Philippine Sea Plate is constructed to reveal dehydration processes of the subducting oceanic crust around the Kii Peninsula. Finally, the author performs high-frequency receiver function inversion analysis. The results indicate the presence of a thin fluid-rich sediment layer along the megathrust fault off the Kii Peninsula that acts as a pathway of fluid.

Nowadays, the number of offshore observations is increasing worldwide. In this respect, the attempt to better analyze OBS data employing the new method will become more important in future studies.

Inhaltsverzeichnis

Frontmatter

Chapter 1. General Introduction

Abstract

The study of fluid distribution along megathrust faults at subduction zones is important because such fluid controls the slip behavior of the megathrust faults. A number of studies have directed their efforts toward understanding the fluid distribution, exploiting both active- and passive-source seismic surveys. Receiver function (RF) analysis is a powerful method for this purpose. The method aims to extract P-to-S conversion phases from seismograms, which are useful for constraining physical properties along subsurface discontinuities such as subducting plate interfaces. While megathrust faults are located beneath offshore regions, RF analysis has hardly been applied to data recorded by ocean-bottom seismometers (OBSs) because seismograms recorded at offshore observatories are dominated by water reverberations. This thesis develops a new technique for solving this problem and then uses the technique to reveal the subsurface structure and fluid distribution along the megathrust fault off the Kii Peninsula, southwestern Japan. This chapter provides the background to this study, the tectonic setting of the study area, and descriptions of the data used in this thesis.

Takeshi Akuhara

Chapter 2. Inverse Water-Layer Filter Method

Abstract

Teleseismic waveforms recorded by ocean-bottom seismometers (OBSs) are affected by water reverberations. In this case, conventional methods, i.e., deconvolution of horizontal-component records by vertical-component records, fail to calculate receiver functions (RFs) correctly since the source wavelets cannot be approximated by the vertical-component records. We formulate a frequency filter termed the water-layer filter (WLF), which represents the impulse response of the water layer. With the inverse of the filter, we can remove the water reverberations from the vertical-component records. We demonstrate the effectiveness of the new method by conducting synthetic experiments.

Takeshi Akuhara

Chapter 3. Application of Inverse Water-Layer Filter Method

Abstract

Here, we address several issues regarding the application of an inverse water-layer filter (IWLF) method to observed data. Initially, we present a nonlinear inversion method to determine IWLF parameters. We use vertical components of teleseismic P waveforms as inputs for the inversion and simultaneously solve for a source wavelet, two-way travel times within the water layer, and reflection coefficients on the seafloor. A simulated annealing algorithm is employed for this optimization. We then investigate the validity of the IWLF method using autocorrelation functions. Finally, we observe that radial receiver functions calculated using the IWLF method show clearer phases than those calculated by a typical method.

Takeshi Akuhara

Chapter 4. Receiver Function Image of the Subducting Philippine Sea Plate

Abstract

We construct receiver function (RF) image using onshore and offshore observatories located around the Kii Peninsula, southwest Japan, to investigate fluid distribution along the subducting Philippine Sea Plate. We calculate RFs at offshore sites using the inverse water-layer filter method. Resultant RF amplitudes suggest the existence of low-velocity zones directly beneath the plate interface of both onshore and offshore regions. We interpreted this as evidence of hydrous oceanic crust, extending from 5–35 km depth to the plate interface. We attribute the reduction of RF amplitudes beneath the Kii Peninsula to the dehydration of the oceanic crust, which increases the seismic velocity of the oceanic crust. Furthermore, we suggest that the densification caused by the dehydration makes the plate interface permeable. This permeable plate interface may indicate the location of non-volcanic tremors, somewhat contrasting with the situation of long-term slow slip events, which are considered to occur along the impermeable plate interface.

Takeshi Akuhara

Chapter 5. A Fluid-Rich Layer Along the Megathrust Fault Inferred from High-Frequency Receiver Function Inversion Analysis

Abstract

In this chapter, we conduct receiver function inversion analysis to investigate detailed seismic properties near the megathrust fault using ocean-bottom seismometers deployed off the Kii Peninsula, southwest Japan. RFs were calculated at high frequencies (up to 4 Hz), removing the effect of water reverberations from the vertical-component records. Our inversion was performed in two steps: first, we modeled the sediment layer by a simple stacking method; second, we determined deeper structure using a waveform inversion. The results indicate the presence of a thin low-velocity zone (LVZ) of a 0.2–1.2 km thickness with an S-wave velocity of 0.7–2.4 km/s along the plate interface. We interpret this LVZ as a thin fluid-rich sediment layer between the overriding and subducting plates, which acts as a pathway for fluid migration.

Takeshi Akuhara

Chapter 6. General Discussion

Abstract

In the previous chapters, we developed a new method for conducting receiver function (RF) analysis using ocean-bottom seismometer (OBS) data and estimated the hydrous state of the subducting Philippine Sea Plate. In this chapter, we briefly discuss the following topics, which are not mentioned much in the previous chapters, but are worthy of studying in the future: (1) along-dip variation of LVZ thickness along the subducting plate, (2) a comparison of different subduction zones regarding slow earthquakes, and (3) an expected perspective based on high-frequency RF analysis using OBS data. All of these topics may be addressed in the future by applying our method, which will lead to advanced knowledge relating to the physical properties of megathrust faults.

Takeshi Akuhara

Chapter 7. Conclusion

Abstract

In this thesis, we developed a method to calculate receiver functions (RFs) from ocean-bottom seismometer (OBS) data, in which water reverberations on vertical-component records were efficiently removed by applying a frequency domain filter. The filter can be expressed with two parameters: a two-way travel time within the water layer and a reflection coefficient on the seafloor. We presented a nonlinear waveform inversion analysis to determine these parameters from observed data. Then, by applying this method, we investigated the hydrous state of the subducting Philippine Sea Plate around the Kii Peninsula, southwestern Japan.

Takeshi Akuhara

Backmatter

Titel: Fluid Distribution Along the Nankai-Trough Megathrust Fault off the Kii Peninsula
verfasst von: Dr. Takeshi Akuhara
Verlag: Springer Singapore
Electronic ISBN: 978-981-10-8174-3
Print ISBN: 978-981-10-8173-6
DOI: https://doi.org/10.1007/978-981-10-8174-3