A fluid‐rich layer along the Nankai trough megathrust fault off the Kii Peninsula inferred from receiver function inversion

Abstract

Investigation of fluid distribution along megathrust faults is an important issue, since the fluid affects frictional properties and thus slip behavior on faults. Scattered teleseismic phases, or receiver functions (RFs), have made significant contributions to understanding the fluid content of subducting plates beneath the onshore regions but have been rarely applied in offshore settings. In this study, we conducted 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 vertical component records. Our inversion was performed in two steps: first, we modeled sediment layer by a simple stacking method and then solved for deeper structure by a waveform inversion. The results indicate the presence of a thin low‐velocity zone (LVZ) of a thickness of 0.2–1.2 km with a S wave velocity of 0.7–2.4 km/s along the plate interface. We interpret this LVZ as thin fluid‐rich sediment layer between the overriding and subducting plates that acts as a pathway of fluid migration.

Overpressured Underthrust Sediment in the Nankai Trough Forearc Inferred From Transdimensional Inversion of High‐Frequency Teleseismic Waveforms

“…This is due to intense water reverberations dominating the vertical component records. Recent efforts have overcome this difficulty and allowed for investigating subsurface structure using high-frequency receiver functions with data from offshore observatories (Akuhara et al, 2016(Akuhara et al, , 2017(Akuhara et al, , 2019.…”

Structure and mechanics of the subducted Gorda plate: constrained by afterslip simulations and scattered seismic waves

“…Recently several methods have been developed to account for these problems including transfer function modelling (Audet, 2016), inverse water-layer filter (Akuhara and Mochizuki, 2015;Akuhara et al, 2016) and transdimensional inversion (Akuhara et al, 2019). Akuhara et al (2017) applied inverse water-layer filter to OBS stations located in the Nankai trough and found the presence of a thin low-velocity zone (LVZ) of a thickness of 0.2-1.2 km with a S wave velocity of 0.7-2.4 km/s along the plate interface at 15-20 km depth. Using the transdimensional inversion method, Akuhara et al (2020) also found a LVZ ∼1 km thick at the plate interface at ∼6 km depth in the shallow part of Nankai trough forearc.…”

The Signature and Elimination of Sediment Reverberations on Submarine Receiver Functions

“…In principle, the receiver function technique should provide the best resolution on the age‐dependence, depth, and sharpness of a sharp velocity gradient, making it a promising seismic technique for investigating the lithosphere structure of an oceanic plate (Akuhara et al., 2016, 2017; Akuhara & Mochizuki, 2015; Audet, 2016; Hannemann et al., 2017; Janiszewski & Abers, 2015; Kawakatsu et al., 2009; Kumar et al., 2011; Liu et al., 2020; Olugboji, Park, Karato, & Shinohara, 2016; C. A. Rychert et al., 2013, 2018a). However, it has been pointed out that in the seafloor environment they can be complicated by sediment reverberations and should be treated with caution (Audet, 2016; Kawakatsu & Abe, 2016; Olugboji, Park, & Karato, 2016).…”

“…In principle, the receiver function technique should provide the best resolution on the age-dependence, depth, and sharpness of a sharp velocity gradient, making it a promising seismic technique for investigating the lithosphere structure of an oceanic plate (Akuhara et al, 2016(Akuhara et al, , 2017Akuhara & Mochizuki, 2015;Audet, 2016;Hannemann et al, 2017;Janiszewski & Abers, 2015;Kawakatsu et al, 2009;Kumar et al, 2011;Liu et al, 2020;Olugboji, Park, Karato, & Shinohara, 2016;C. A. Rychert et al, 2013C.…”