Fluid Distribution Along the Nankai-Trough Megathrust Fault Off the Kii Peninsula: Inferred from Receiver Function Analysis

ISBN-13: 9789811081736 / Angielski / Twarda / 2018 / 94 str.

Takeshi Akuhara

Opis

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.

Spis treści

1.            General Introduction

1.1.         Fluid Distribution Along Megathrust Faults

1.2.         Receiver Function Analysis

1.3.         Tectonic setting of the Nankai Subduction Zone

1.4.         Data

1.4.1.     Seismic Stations

1.4.2.    Teleseismic Event Records

2.            Inverse Water Layer Filter Method

2.1.         Introduction

2.2.         Definition of Water-Layer Filter

2.3.         Synthetic Test

2.3.1.     Methodology

2.3.2.     Results and Discussion

3.            Application of Inverse Water Layer Filter Method

3.1.         Introduction

3.2.         Inversion Analysis for Water-Layer Filter Parameters

3.2.1.     Overview of Inversion Analysis

3.2.2.     Prior Estimation of Phase Arrivals

3.2.3.     Inversion Scheme

3.2.4.     Results and Discussion

3.3.         Quantitative Assessment of Inverse Water-Layer Filter

3.4.         Application to Receiver Function Estimation

4.            Receiver Function Image of the Subducting Philippine Sea Plate

4.1.         Introduction

4.2.         Methodology

4.2.1.     Common-Conversion-Point Stacking

4.3.         Results

4.3.1.     3-D Geometry of the Subducting Philippine Sea Plate

4.3.2.     Island-arc Moho beneath the Kii Peninsula

4.3.3.     Receiver Function Amplitudes Along the Subducting Plate

4.4.         Discussion

4.4.1.     Forward Modeling of Amplitude Reduction Beneath the Kii Peninsula    

4.4.2.     Uncertainty of Relative Depth Between Hypocenters and

the Oceanic Moho

4.4.3.     Hydrous State of the Subducting Oceanic Crust

4.4.4.     Permeability Difference in Source Area of Long-Term Slow Slip Events and Tremors        

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

5.1.         Introduction

5.2.         High-Frequency Receiver Functions

5.3.         Estimation of Sediment Properties by the H-κ Stacking Method

5.3.1.     Methodology

5.3.2.     Results

5.4.         Receiver Function Inversion Analysis

5.4.1.     Methodology

5.4.2.     Results

5.5.         Fluid-Rich Subducting Sediment Layer Along the Plate Interface

6.            General Discussion

6.1.         Along-Dip Variation in Low-Velocity Zone Thickness

6.2.         Comparison to Other Subduction Zones

6.3.         New Perspectives by High-Frequency Receiver Function Analysis Using OBS data

7.            Conclusion

Blurb

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.