Eastern Shikoku Research Articles

Anisotropic Velocity Structure Beneath Shikoku, Japan: Insights From Receiver Function and Shear Wave Splitting Analyses

AbstractA combination of receiver function (RF) analysis and shear wave splitting can reveal the anisotropic properties of the Earth's velocity structure. The RF profile exhibits harmonic patterns partially generated from the discontinuities of anisotropic media near the receiver. The anisotropic properties are determined using splitting parameters, which include the fast polarization direction (FPD) and split time. We utilized the seismograms recorded by stations on Shikoku Island, Japan. Also, we applied the Bayesian information criterion to control the spike count of each RF, which consists of a series of spikes generated by time‐domain iterative deconvolution. Anisotropic differences arose between northern and southern regions along the 30‐km iso‐depth line of the top surface of the Philippine Sea Plate. In the southern part of Shikoku Island, the FPDs are sub‐perpendicular to the plunge of the subducting slab, while in the northern part, they are sub‐parallel to the plunge of the slab. Our results indicate that anisotropic strengths are weaker in the tectonic‐tremor band in northwestern Shikoku and stronger in the northern part of central and eastern Shikoku around the no tectonic‐tremor area. These anisotropic variations may characterize Shikoku Island’s geological structure.

Complex Interaction, Striations, and Periodicity of Deep Tremor Surrounding a Tremor Gap in the Central Nankai Subduction Zone, Japan

AbstractThe Nankai subduction zone is characterized by a belt of deep tremor extending from western Shikoku to the Tokai region and a tremor seismicity gap beneath the Kii Channel between Shikoku and the Kii Peninsula. Tremor activities near the gap are not well understood because of poor seismic network coverage. In this study, we conducted an observation campaign in eastern Shikoku to investigate the detailed tremor activity across the tremor gap. The new tremor catalog shows high detectability and resolution, with tremor clusters in both eastern Shikoku and the western Kii Peninsula having distinct shapes. We discovered characteristic time intervals between tremor activity on both sides of the gap, suggesting the occurrence of undetected slow slip events (SSEs) within the gap that propagated approximately 10 km/day. We also found a relationship between the tremor activities and a shallower long‐term SSE. Our examination of the detailed distribution of tremor events showed a distinct striation pattern that may be associated with the history of the subducting plate motion and the plate geometry, and our examination of their periodicity showed spatial variations in the dominant period of tremor activity that likely reflect the heterogeneous physical properties of the plate interface.

Probabilistic tsunami hazard assessment based on the Gutenberg–Richter law in eastern Shikoku, Nankai subduction zone, Japan

Earthquake and tsunami predictions comprise huge uncertainties, thus necessitating probabilistic assessments for the design of defense facilities and urban planning. In recent years, computer development has advanced probabilistic tsunami hazard assessments (PTHAs), where hazard curves show the exceedance probability of the maximum tsunami height. However, owing to the lack of historical and geological tsunami records, this method is generally insufficient for validating the estimated hazard curves. The eastern coast of Shikoku in the Nankai subduction zone, Japan, is suitable for validation because tsunami records from historical Nankai Trough earthquakes are available. This study evaluated PTHAs by comparing the tsunami hazard curves and exceedance frequencies of historical Nankai Trough tsunamis. We considered 3480 earthquake scenarios representing the rupture patterns of past Nankai earthquakes and calculated all tsunamis. The probability of earthquake occurrence was based on the Gutenberg–Richter law. We considered uncertainty in tsunami calculations with astronomical tide variations. The estimated tsunami hazard curves are consistent with the exceedance frequencies obtained from historical tsunamis. In addition, sensitivity tests indicate the significance of the earthquake slip heterogeneity and tsunami defense facilities in PTHAs. We also extended the PTHAs to tsunami inundation maps in high resolution and proposed an effective new method for reducing the tsunami computation load.Graphical

Development of a detection method for short-term slow slip events using GNSS data and its application to the Nankai subduction zone

Using global navigation satellite system (GNSS) data to detect millimeter-order signals of short-term slow slip events (S-SSEs) and to estimate their source parameters, especially duration, is challenging because of low signal-to-noise ratio. Although the duration of S-SSEs in the Nankai subduction zone has been estimated using tiltmeters, its regional variation has never been quantitatively studied. We developed an S-SSE detection method to estimate both the fault model and duration with their errors based on the detection methods developed by previous studies and applied it to a 23-year period of GNSS data in the Nankai subduction zone. We extracted S-SSE signals by calculating correlation coefficients between the GNSS time series and a synthetic template representing the time evolution of an S-SSE and by computing the average of correlation coefficients weighted by the predicted S-SSE signals. We enhanced the signals for duration estimation by stacking GNSS time series weighted by displacements calculated from the estimated fault model. By applying the developed method, we detected 284 S-SSEs from 1997 to 2020 in the Nankai subduction zone from Tokai to Kyushu and discussed their regional characteristics. The results include some newly detected S-SSEs, including events accompanying very low-frequency earthquakes and repeating earthquakes in offshore Kyushu. Our study provides the first geodetic evidence for synchronization of S-SSEs and other seismic phenomena in offshore Kyushu. We estimated the cumulative slip and duration, and their error carefully. We also estimated the average slip rate by dividing the cumulative slip by the cumulative duration. This study clarified that the average slip rate in western Shikoku was approximately twice as that in eastern Shikoku and Kyushu. These regional differences were statistically significant at the 95% confidence interval. Multiple factors can influence the regional characteristics of S-SSEs, and we speculate that the subducting plate interface geometry is one of the dominant factors.Graphical

Right-lateral offset associated with the most recent earthquake on the Ikeda fault of the Median Tectonic Line, southwest Japan, revealed by ground-penetrating radar profiling

The Median Tectonic Line (MTL) is an arc-parallel strike-slip fault that accommodates much of the arc-parallel component of the oblique convergence of the Philippine Sea and Eurasian plates at the Nankai Trough. The MTL in Shikoku is one of the fastest-slipping faults in Japan, with a late Quaternary right-lateral slip rate of 5–10 mm/yr. To estimate the right-lateral slip amounts of the past faulting events on the MTL, we acquired 2D and pseudo-3D ground-penetrating radar (GPR) sections across the ENE-trending Ikeda fault of the MTL in eastern Shikoku. We conducted the GPR surveys at the Higashi-Miyoshi site, where two terrace riser offsets mark the active fault trace. The 2D lines were about 28–64 m long, and the pseudo-3D data were sized 20 m × 30 m with a 0.5-m inline spacing. We used 50 MHz GPR antennas and conducted wide-angle measurements to estimate the electromagnetic wave velocity. We identified three paleochannels on the final depth-converted GPR sections, and two of them are deflected by the fault. A paleochannel at 0.6–1.4 m depth is observed on all inline sections of the pseudo-3D GPR data. We built a 3D model of this paleochannel and estimated the right-lateral and vertical displacements of ~ 3.5 m and ~ 0.5 m, respectively. This paleochannel offset is probably caused by the most recent surface-rupturing earthquake on the Ikeda fault, which may be the 1596 Keicho-Fushimi earthquake. This study demonstrates the usefulness of the GPR surveys to identify geological features displaced laterally and vertically by the most recent surface-rupturing earthquake.

New Insights Into the Slip Budget at Nankai: An Iterative Approach to Estimate Coseismic Slip and Afterslip

AbstractThe slip budget at a subduction zone megathrust is met by a spectrum of slip behaviors including coseismic slip from great earthquakes, afterslip, episodic tremor and slip (ETS), long‐term slow slip events (SSEs), and quasisteady interseismic creep. Long‐term geodetic monitoring at the Nankai subduction zone in southwest Japan has recorded deformation observations spanning nearly a complete earthquake cycle, making this region the best study area for obtaining a full understanding of how the slip budget is partitioned along the interface and how the rheological conditions evolve through space and time. In this work, we conduct an iterative inversion of vertical surface displacement data to jointly estimate coseismic slip during the 1944/46 Tonankai and Nankai earthquakes, spatiotemporal afterslip history following those earthquakes, and postseismic mantle flow. A suite of postseismic models with variable elastic slab and plate geometry and mantle viscosity all show at least 30 years of sustained afterslip within the ETS zone and the gap between the top of the ETS zone and the fully coupled zone. This result demonstrates that stable, long‐duration afterslip is possible in the ETS zone. Summing the estimated cumulative slip from this study with estimates from previous studies of cumulative slip during the interseismic period, including episodic SSEs, we find that eastern Shikoku island has nearly met all of its slip budget for a 150‐year interseismic cycle, whereas western Shikoku island has a slip deficit of nearly half the budget (4 out of 8.25 meters) above 20 km depth on the subduction interface.

Outcrop of a fault damage zone of the Ikeda fault, the Median Tectonic Line, in eastern Shikoku, southwest Japan

Outcrop of a fault damage zone of the Ikeda fault, the Median Tectonic Line, in eastern Shikoku, southwest Japan

Detailed Spatial Slip Distribution for Short‐Term Slow Slip Events Along the Nankai Subduction Zone, Southwest Japan

AbstractMany short‐term slow slip events (S‐SSEs) occur at the transition zone along the Nankai subduction zone, southwest Japan. Because crustal deformation due to a single S‐SSE is small, the source fault is often represented using a planar uniform single‐fault slip model, resulting in little constraint on the spatial heterogeneity in amounts of slip. To comprehensively investigate the detailed cumulative spatial distribution of S‐SSEs in the entire Nankai subduction zone, we adopted a stacking approach of Global Navigation Satellite System data by referencing low‐frequency earthquakes. We extracted the cumulative displacements due to 12, 11, and 14 S‐SSEs that occurred between 2004 and 2009 in eastern Shikoku, Kii, and Tokai; coherent displacements nearly opposite to the direction of plate subduction were obtained. The inverted slip indicated significant slip laterally elongated along the transition zone at ~30–35 km depth. Small slip patches were inferred in the shallow portions at ~15–20 km depth in eastern Shikoku and at ~10–15 km depth in Tokai, respectively. The shallow patch in eastern Shikoku was located on the downdip edge of the coseismic slip area of the 1946 Nankai earthquake, while the Tokai small slip was located on the shallower side of the anticipated source area of a large earthquake. Large slip patches of S‐SSEs tend to avoid the spatially dense low‐frequency earthquake areas; in major S‐SSE areas, the number of low‐frequency earthquakes is small. This spatial dependence of fault slip style within the transition zone provides new insights on the generation mechanism of slow earthquakes.

Determination of Trace Elements in Aerosols at a Rural Mountainous Area and a Local City of Eastern Shikoku Region, Japan.

A simple digestion method for a mixed cellulose membrane filter in order to analyze trace elements in aerosols was examined. The determination of Cu, Fe and Ni in the certified reference materials of China loess (CJ-1) could be conducted using the digestion method. Trace elements in aerosols smaller than 10 μm size collected at Tokushima City and near mountain areas for each season were determined as a water-soluble fraction and all components by the digestion method. Back trajectory analysis implied that the contribution from the Asian continent was larger in the winter and spring than the summer. Systematic changes in the trace element compositions for each season were not observed, except for Fe, Th and U. Water-soluble components in aerosols at Tokushima City showed higher concentrations of all trace elements than those of the mountain areas. For aerosols in Tokushima City and near to the mountain areas, As, Bi, Cd, Cu and Pb were mainly derived from anthropogenic sources, whereas Ba, Co, Cs, Ga, Mn, Rb, Sr, Th, U and V were derived from natural crustal sources. The origin of Pb and Cd was considered based on the Pb/Cd ratio.

LA–ICP–MS zircon U–Pb ages of felsic tuffaceous beds in the Takikubo andHorita formations, Izumi Group, Ikeda district, eastern Shikoku, southwestern Japan

LA–ICP–MS zircon U–Pb ages of felsic tuffaceous beds in the Takikubo andHorita formations, Izumi Group, Ikeda district, eastern Shikoku, southwestern Japan

Dietary Overlap But Spatial Gap between Sympatric Japanese Serow (Capricornis crispus) and Sika Deer (Cervus nippon) on Eastern Shikoku, Japan

Spatial separation and dietary differences may allow the coexistence of similar sized ungulate species. Over the past two decades, populations of the sika deer (Cervus nippon) have increased markedly and overlapped with the habitats of the Japanese serow (Capricornis crispus). Recently, the Japanese serow population has declined and shifted its habitat from higher to lower altitudes in Shikoku, Japan. To examine the potential for the coexistence between the Japanese serow and the sika deer, seasonal dietary and habitat overlaps were investigated in a warm temperate area in Shikoku. Although fecal analyses showed a high dietary overlap between the Japanese serow and the sika deer, the camera trap results revealed the differences in habitat uses between the two species: the Japanese serow utilized steep rocky slopes, whereas the sika deer appeared more frequently in grasslands. Artificially created grasslands on the present study area would contribute to the difference of habitat utilization between the two species.

The 1596 Keicho Earthquake, a 5‐Day, 300‐km‐Long Sequential Rupture Event in the Median Tectonic Line Fault Zone, Southwestern Japan

AbstractHistorical accounts and geological surveys suggest that a sequential rupture event occurred in 1596 CE in the Median Tectonic Line fault zone (MTLFZ) in southwestern Japan. During this event, known as the 1596 Keicho earthquake, the rupture probably propagated in two or three sequential stages, similar to a series of events that occurred along the North Anatolian fault during 1939–1999. The detailed rupture process of the 1596 earthquake event is unknown, however, because coseismic seismological, geological, and geodetic observation data are not available. To reconstruct the rupture processes, we integrated data from Coulomb stress change analyses, fault geometrical analyses, historical accounts, and rupture history. The results suggest that the rupture initiated on the Kawakami fault on 1 September 1596 and propagated bilaterally, to the Beppu Bay fault system, which ruptured on 4 September, and to the MTLFZ in eastern Shikoku and the Rokko‐Awaji fault zone and Arima‐Takatsuki Tectonic Line in the Kinki area, which ruptured on 5 September. However, there is a small possibility that the rupture initiated at the western end of the MTLFZ and propagated eastward unilaterally. The propagation of ruptures might be sensitively controlled not only by stress perturbation and fault geometry but also by rupture history. Evaluation of fault rupture behavior from various viewpoints can contribute to the mitigation of future seismic disasters in fault zones where detailed coseismic observation data from the past are lacking.

OH defects in quartz as a provenance tool: Application to fluvial and deep marine sediments from SW Japan

Hydrogen-related point defects (OH defects) in the crystal lattice of detrital, monocrystalline quartz have been proposed as an indicator to distinguish different sediment sources, making it a potential tool for provenance analysis. This study tests this novel technique on samples from SW Japan by comparing the results of OH defect analyses with those of “classical” provenance studies. OH defects of 188 detrital, monocrystalline quartz grains from five rivers (Fuji, Tenryu, Nagara, Yoshino, and Shimanto) and four offshore wells drilled in the Nankai Trough area (ODP site 1177; IODP sites C0002, C0007, C0012) are characterized using Fourier-transform infrared (FTIR) spectroscopy. Considerable differences in OH defect concentrations are identified between individual river samples. A general increase of values from east to west is observed. Possible links to the respective source geology are established. Very low OH defect contents (<1 wt. ppm water) of sediment from the Fuji and Tenryu rivers may be a result of high deformation and exhumation in the Izu-Honshu collision zone. Moderate to high OH defect contents of Nagara and Yoshino river samples (up to 50 wt. ppm water) are similar to the values of the global average in continental crust, possibly due to a mixing of various source lithologies. High and very diverse OH defect contents of Shimanto River sediments (up to 211 wt. ppm water) are likely an effect of extensive sediment reworking and mixing in the Nankai accretionary wedge. The OH defect signal is shown to be traceable through the offshore sedimentary record. In the western Shikoku Basin (ODP site 1177), turbidite deposits at 15 Ma and 7 Ma show a very similar signal as recent Shimanto river sediment, whereas eastern Shikoku Basin sediments (IODP Site C0012), at 15 Ma and 6 Ma strongly resemble present day Nagara and Yoshino river sediments. Sediments deposited in the Nankai trench wedge at 1 Ma (IODP Site C0007) indicate a shift towards lower OH defect concentrations. This may be due to increased, trench parallel sediment flux from the low-OH defect Fuji and Tenryu river sources, as proposed by previous studies. This study presents the first OH defect dataset in detrital quartz from rivers and offshore drill sites in a subduction zone environment, introducing OH defects as a new useful tool for sediment provenance analysis in active convergent margin settings.

四国東部，四万十帯白亜紀〜古第三紀付加コンプレックスの砕屑性ジルコンU-Pb年代と後背地の時空変遷

四国東部，四万十帯白亜紀〜古第三紀付加コンプレックスの砕屑性ジルコンU-Pb年代と後背地の時空変遷

Modeling of Long-Period Ground Motions in the Nankai Subduction Zone: Model Simulation Using the Accretionary Prism Derived from Oceanfloor Local S-Wave Velocity Structures

The accretionary prism in the subduction zone, which consists of thick low-velocity oceanic sediments, significantly affects the propagation of seismic waves for shallow, offshore earthquakes, including large interplate earthquakes. In order to simulate long-period (> 5 s) ground motions in the Nankai subduction zone, we constructed a three-dimensional (3D) seismic velocity structure model of the accretionary prism by interpolation/extrapolation of local S-wave velocity structures beneath 46 oceanfloor seismic stations (DONET), which are deployed just above the accretionary prism off the southern Kii and eastern Shikoku regions. We modeled local S-wave velocity structures using a simple two-parameter depth-varying velocity function. To investigate the effects of the accretionary prism on ground and seafloor motions, we conducted numerical simulations of seismic wave propagation for three local earthquakes that occurred in southwestern Japan. The simulations reasonably reproduced the observed seismograms, not only for the period ranges of the moment tensor inversion (~ 50 s), but also for the strong, long-period ground motions in the sedimentary basins (~ 5 s), especially in the region where DONET stations are densely deployed. Since depth-varying, local S-wave structures significantly improve the reproducibility of long-period ground motions, our modeling procedure is useful for modeling long-period ground motions of local and regional offshore subduction zone earthquakes.

Late quaternary slip rates and vectors on the Median Tectonic Line active fault zone in eastern Shikoku, southwest Japan

The Median Tectonic Line active fault zone (MTLAFZ) extends for about 190 km through Shikoku, southwest Japan. Though the MTLAFZ is the most significant onshore active tectonic feature in southwest Japan, its late Quaternary slip rate has been estimated at only a few locations with reasonable references and ages. Better information on this feature's recent slip rates is critical to understanding the ongoing tectonic processes in the region and evaluating the seismic risk of this fault zone. In this paper, new estimates of the late Quaternary slip rate are reported from the Ikeda and Chichio faults in the central portion of the MTLAFZ. The author mapped late Pleistocene fluvial terrace surfaces and used tephrochronology and radiocarbon dating to constrain the age, and measured offset of terrace risers. The slip vectors of both faults are similar, as derived from piercing points on the bottoms of the terrace risers. The vertical component of displacement is 2–6% of the horizontal component. Long-term slip rates during the late Quaternary were calculated at 7.8–9.1 mm/yr, which is more precise than those in the previous studies and represents the highest slip rate in the MTLAFZ. This rate is also much faster than previously reported shorter-term slip rates of geodetic study. The long-term seismic risk of large earthquakes (>M8) along the Ikeda and Chichio faults in the next 30 years are evaluated to be more than 0.4–1.9% and 0.1–3%, respectively, much higher than a previous estimate of 0–0.3%.

O-97 四国東部坂州地域の黒瀬川帯およびその周辺の層序と地質構造(11. 地域地質・地域層序,口頭発表,一般発表)

O-97 四国東部坂州地域の黒瀬川帯およびその周辺の層序と地質構造(11. 地域地質・地域層序,口頭発表,一般発表)

O-3 四国東部秩父帯北帯中の緑色岩(1. オフィオライト,口頭発表,一般発表)

O-3 四国東部秩父帯北帯中の緑色岩(1. オフィオライト,口頭発表,一般発表)

四国東部および紀伊半島西部の三波川帯の苦鉄質・超苦鉄質貫入岩について : (その1)四国東部の三成川帯の苦鉄質・超苦鉄質貫入岩について

四国東部および紀伊半島西部の三波川帯の苦鉄質・超苦鉄質貫入岩について : (その1)四国東部の三成川帯の苦鉄質・超苦鉄質貫入岩について

New record of a Japanese troglobiontic millipede, Epanerchodus acuticlivus, from upper hypogean zone in eastern Shikoku

New record of a Japanese troglobiontic millipede, Epanerchodus acuticlivus, from upper hypogean zone in eastern Shikoku