Cape Shionomisaki Research Articles

Nankai Forearc Structural and Seismogenic Segmentation Caused by a Magmatic Intrusion off the Kii Peninsula

AbstractThe causes for forearc basin and megathrust rupture zone segmentation are controversial. The Nankai forearc, Japan, is separated into five domains based on topography: Enshu, Kumano, Muroto, Tosa, and Hyuga. The boundaries of these domains correspond to the rupture limits of large earthquakes. We examined the geologic structure of the boundary region between the Kumano and Muroto domains off the Kii Peninsula using multichannel seismic reflection data to evaluate the role of upper plate composition in controlling segmentation. The results suggest that thick cover sediments and underlying accretionary prism are obliquely thrust landward over the igneous basement complex rock in the region of offshore of Cape Shionomisaki and separate the forearc basin. The igneous basement complex rocks directly overlying the plate interface in the hypocentral regions of 1944 Tonankai and 1946 Nankai earthquakes. The 1944 earthquake originated at the base of the complex, and the rupture extent slipped past its basement boundary, whereas the 1946 event nucleated at the updip boundary of the basement complex. The dense igneous rocks might have worked as a heavily loaded barrier on the seismogenic megathrust and separated the rupture area of both the earthquakes. Upper plate geology may be an important factor in controlling seismogenesis in the Nankai Trough and may serve as an example for understanding the controls on megathrust slip in other subduction zones.

Seismicity distribution in the Tonankai and Nankai seismogenic zones and its spatiotemporal relationship with interplate coupling and slow earthquakes

We conducted seismic tomography to estimate the seismic velocity structure and to evaluate the spatiotemporal distribution of interplate earthquakes of the Kii Peninsula, central Honshu, Japan, where the Tonankai and Nankai megathrusts are located. Microearthquakes were quantitatively detected by using the data from a cable-type seafloor seismic observation network, completed in 2015. Our velocity model was consistent with the previous 2-D active-source surveys, which reported the areal extent of key structures: a high-velocity zone beneath Cape Shionomisaki, a subducted seamount off Cape Muroto, and the subducted Paleo-Zenith Ridge. The absence of any other subducted seamount with the same or larger spatial scale, than the identified key structures, was confirmed. Our velocity model also revealed that there was not a simple relationship between areas of large coseismic slip or strong interplate coupling and areas of high velocity in the overriding plate. Relocated hypocenters widely ranged from the upper plate to within the slab, while the most active region was attributed to the oceanic crust in the aftershock region of 2004 off-Kii earthquake. Compared with the results from the land-based observation network, the accuracy of the focal depth estimation was substantially improved. Furthermore, we identified the seismic activity in the vicinity of the plate boundary and determined 14 locations for interplate seismicity areas. They were primarily distributed in the range of seismogenic zone temperature (150–350 °C) along the plate boundary and were located outside of the strong interplate coupling zone. Several active areas of interplate earthquakes exhibited clustered activity during the periods of slow-slip events, observed and accompanied with shallow very-low-frequency earthquakes. Thus, regular interplate microearthquakes became active at the plate boundary in the conjunction with slow slip. In summary, as regular earthquakes provide a more accurate source location than slow earthquakes and can detect events of smaller magnitude, monitoring such interplate earthquakes may reveal spatiotemporal variations in the stick–slip conditions on the plate boundary.

Stress Map of Japan: Detailed Nationwide Crustal Stress Field Inferred From Focal Mechanism Solutions of Numerous Microearthquakes

AbstractCrustal stress is a crucial parameter for understanding tectonics and assessing the occurrence of large earthquakes. Stress at seismogenic depths over a wide area has been inferred from focal mechanism solutions. The existence of focal mechanism solutions determines the area that can be potentially studied. Therefore, we studied the crustal stress orientations in Japan from focal mechanism solutions of small earthquakes. First, we obtained about 220,000 focal mechanisms using the P‐wave first‐motion polarities picked by a deep learning model. Next, we performed a linearized inversion on the nationwide stress orientation model at a grid of 0.2‐degree intervals. The estimation errors reflect the quality and quantity of focal mechanism solutions. Overall, the results indicate that Japanese islands are under an east–west compressional stress, whereas some areas along the Pacific coast of northeast Japan are under east–west extensional stress. A closer look shows the spatial trends of stress orientation may vary near the geological borders, including the Hinagu and Futagawa faults in Kumamoto Prefecture (Kyushu Island) and the Median Tectonic Line in Shikoku Island. Our stress model also revealed the existence of stress anomalies around Cape Shionomisaki in the Kii peninsula. Most of the active faults are at approximately optimal orientation for the derived stress orientation and stress ratio, although uncertainties in the fault geometry, particularly the dip angle, substantially affect this evaluation. The stress map helps understand the overall vision of the crustal stress in Japan and further study the local stress distribution.

High‐Density Seismic Refraction Imaging of Plate‐Boundary Structures in the Slow Earthquake Gap Zone off Western Kii Peninsula, Nankai Trough

AbstractDespite recent observations of slow earthquakes along the Nankai subduction zone, none have been reported in the central Nankai Trough between the Kii Channel and Cape Shionomisaki. In November 2018, a very dense array of 96 ocean‐bottom seismometers were deployed by JAMSTEC to acquire active‐source seismic refraction dataset (supplemented by a multichannel seismic reflection profile) from the seaward side of the subduction trough to the accretionary prism off Cape Shionomisaki. We applied traveltime tomography to the refraction data to constrain the P wave velocity down to the upper mantle, coordinating with a migrated seismic reflection profile to confirm the depth of the Moho and interpret shallower structural features. From a comparison with a transect across the Kumano basin, we conclude that structural and physical differences between these two locations, especially the geometry of the subducting plate surface, lead to different slow earthquake activities.

Unusually high sea level at the south coast of Japan in September 2011 induced by the Kuroshio

Using a coastal assimilation model, generation mechanism of unusually high sea level (UHSL) at the south coast of Japan in September 2011 is investigated. Both model results and tide gauge observations indicate that sea level rise associated with the UHSL event occurred twice in the middle and end of September. The first one, which is localized around the eastern part of the Seto Inland Sea, is caused by a cyclonic circulation in the Kii Channel formed as a result of northward migration of the Kuroshio axis toward Cape Shionomisaki. The second sea level rise, which is the main contributor to this UHSL event, is observed in wide areas not only at the south coast of Japan, but also at the coast of the Japan Sea. It is brought about by a coastal trapped wave (CTW) induced as a result of a fluctuation of the Kuroshio path to the south of the Boso Peninsula. The CTW with positive SSH anomalies propagates westward along the south coast of Japan, and then goes into the coast of the Japan Sea. Sensitivity experiments and a modal characteristic analysis indicate that the CTW is mainly characterized by the first mode baroclinic Kelvin wave. The phase speed for the first mode is calculated at 2.96 m s^{-1}, which compares well with that estimated by tide gauge observations.

High-resolution modeling of the Kuroshio current power south of Japan

Ocean current power near the Japanese coast was estimated using a high-resolution, three-dimensional ocean model. Two experiments were conducted to distinguish the impact of the Kuroshio large meander. The simulation results were validated by comparison with measurements. To choose the proper site for ocean current power generation experiment, we quantitatively analyzed the variation of the current field for three candidate areas: around the Tokara Strait, south of Cape Ashizuri (AS), and south of Cape Shionomisaki (SH). The results show that the current around the Tokara Strait is the weakest among the three candidates. The current south of SH is strongest for the non-large-meander period. The area south of AS is promising for ocean current power generation, because the current is stable and strong regardless of the large meander, and the average ocean current power density can reach 1 \(\hbox {kW/m}^{2}\).

Seismic structure off the Kii Peninsula, Japan, deduced from passive- and active-source seismographic data

We conduct seismic tomography to model subsurface seismicity between 2010 and 2012 and structural heterogeneity off the Kii Peninsula, southwestern Japan, and to investigate their relationships with segmentation of the Nankai and Tonankai seismogenic zones of the Nankai Trough. In order to constrain both the shallow and deep structure of the offshore seismogenic segments, we use both active- and passive-source data recorded by both ocean-bottom seismometers and land seismic stations. The relocated microearthquakes indicate a lack of seismic activity in the Tonankai seismogenic segment off Kumano, whereas there was active intraslab seismicity in the Kii Channel area of the Nankai seismogenic segment. Based on comparisons among the distribution of seismicity, age, and spreading rate of the subducting Philippine Sea plate, and the slip-deficit distribution, we conclude that seismicity in the subducting slab under the Kii Channel region nucleated from structures in the Philippine Sea slab that pre-date subduction and that fluids released by dehydration are related to decreased interplate coupling of these intraslab earthquakes. Our velocity model clearly shows the areal extent of two key structures reported in previous 2-D active-source surveys: a high-velocity zone beneath Cape Shionomisaki and a subducted seamount off Cape Muroto, both of which are roughly circular and of 15–20 km radius. The epicenters of the 1944 Tonankai and 1946 Nankai earthquakes are near the edge of the high-velocity body beneath Cape Shionomisaki, suggesting that this anomalous structure is related to the nucleation of these two earthquakes. We identify several other high- and low-velocity zones immediately above the plate boundary in the Tonankai and Nankai seismogenic segments. In comparison with the slip-deficit model, some of the low-velocity zones appear to correspond to an area of strong coupling. Our observations suggest that, unlike the Japan Trench subduction zone, in our study area there is not a simple correspondence between areas of large coseismic slip or strong interplate coupling and areas of high velocity in the overriding plate.

Short-term fluctuations south of Japan and their relationship with the Kuroshio path: 8- to 36-day fluctuations

To detect short-term fluctuations south of Japan, we applied wavelet analysis to ocean reanalysis data of the Japan Coastal Ocean Predictability Experiment 2 with a horizontal resolution of 1/36°. It was found that fluctuations of the 8- to 36-day period band appear as frontal waves in the Kuroshio Current. The amplitude of the fluctuations increases toward the downstream of Cape Shionomisaki. The fluctuations have a wavelength of about 300 km, and the signals propagate eastward. The fluctuations of the 8- to 36-day period band are stronger during the period of the nearshore non-large-meander Kuroshio path than during the period of the offshore non-large-meander Kuroshio path. We suggest that the 8- to 36-day fluctuation is a result of the instability of the accelerated velocity of the Kuroshio Current downstream of Cape Shionomisaki.

Structure and dynamics of the sudden acceleration of Kuroshio off Cape Shionomisaki

A sudden acceleration of the Kuroshio jet appears off Cape Shionomisaki in the high-resolution (horizontal resolution of 1/36°) JCOPE 2 ocean reanalysis data. Using this dataset, we investigated the structure of the Kuroshio acceleration. The increase in the velocity of the current is accompanied by a downstream flow separation from the coast and an outcrop of cold temperature inshore. The acceleration of Kuroshio appears when it takes a near-shore path. Cape Shionomisaki amplifies the responses to the Kuroshio flow by creating the zonal velocity acceleration toward the downstream region when the Kuroshio flows closer to the coast. The Kuroshio acceleration coincided with the topographic ridge on the continental shelf near Cape Shionomisaki. This relation suggests that the dynamics of the acceleration is linked to the topographic feature. We proposed an explanation of the Kuroshio acceleration using a hydraulic control theory. An analytical solution was applied to the coastal topography around the Kii Peninsula. The solution captured some aspects of the Kuroshio acceleration.

Oceanic structure in the vicinity of Cape Shionomisaki and distribution of sea level height

Oceanic structure in the vicinity of Cape Shionomisaki and distribution of sea level height

A Regional Biogeography of Shore Molluscs: Influence of the Kuroshio Current and the Two Capes

The regional pattern of shore molluscan assemblages was investigated along a 650-km coastline of the Kii Peninsula in central Japan. Hydrography of the coastal waters differed between three subareas, the Inner Kii Channel (IKC), the Outer Kii Channel (OKC), and the Sea of Kumano (SOK), which are demarcated by the two prominent capes, Cape Hinomisaki and Cape Shionomisaki. The water of OKC was warm, saline, and transparent compared to that of the other subareas, due to the influence of the intruding Kuroshio current. Corresponding to this hydrographic difference, the composition of shore malacofauna differed between OKC and the other subareas. Species diversity and relative abundance of the southern species (< 35° N) increased southward and were higher in the OKC than in the other subareas. Multivariate analysis on various environmental and biotic parameters detected significant relationships between the temperature and geographic groups of species, and between substratum geology and trophic groups. The present and previous results elucidated the following biogeographic process in the study area. The Kuroshio washes the coast of the OKC, and enriches the southern species by increasing the temperature and providing their larvae; thus the diversified warm-water malacofauna of the OKC intervenes between the colderwater malacofaunas of the IKC and SOK. This major pattern is further modified by shore geology and the degree of wave exposure at local scales. The present survey method, a semi-quantitative timed search for intertidal assemblages required less time than, and yet yielded results similar to, the widely used dispersed quadrat survey for the parameters selected in the present analysis.

Favorable conditions for cold-water intrusion from the Kuroshio intermediate layer into Osaka Bay

From 1980 to 1995, in August, the bottom layer of Osaka Bay was occupied by cold, nutrient-rich water compared with that observed during both previous and subsequent decades. To investigate the mechanisms for the intrusion of bottom-layer cold water into Osaka Bay, the intrusion into Osaka Bay via the Kii Channel is simulated using a finite-volume coastal ocean model with unstructured triangular cell grids. The initial conditions, boundary conditions, and surface temperature given to the model are obtained from daily reanalysis data provided by the Japan Coastal Ocean Predictability Experiment. The model shows that cold water uplifted on the eastern side of the Kii Peninsula is propagated westward at 1.0 m/s as a coastal boundary current; it reaches the Kii Channel mouth when the Kuroshio axis is located around 74 km south of Cape Shionomisaki. However, the modeled cold water mass at the Kii Channel mouth does not intrude further to the north of the Kii Channel; therefore, another mechanism is required to explain the cold-water intrusion into the bottom layer of Osaka Bay. A plausible explanation is the estuarine circulation established by the freshwater supply at the bay head. When the river runoff is included in the model without forced vertical mixing, the temperature in Osaka Bay decreases 6.6 days later than the temperature decreases at the Kii Channel mouth. Furthermore, the shoreward current speed in the bottom layer of the modeled estuarine circulation is 15 cm/s, which provides the mechanism required for the cold water mass to pass the Kii Channel.

Intrusion of Kuroshio-Derived bottom cold water into Osaka Bay and its possible cause

The bottom layer in Osaka Bay was occupied in August from 1980 to 1995 by a water mass of relatively low temperature and rich in nutrients in comparison to previous and following decades. The relationship between Osaka Bay bottom temperature and Kuroshio axis location south of Kii Peninsula has been investigated using the dataset archived by Osaka Research Institute of Environment of Agriculture and Fisheries and axis-location data provided by Marine Information Research Center. The correlation between bottom temperature in the bay and Kuroshio-axis distance from Cape Shionomisaki indicates that the bottom temperature in the bay decreases when the Kuroshio axis is a long distance from the cape, and vice versa. To investigate why the temperature distribution depends on the axis location, composite temperature maps are depicted using summer temperature data from 1970 to 2005 archived in the Japan Oceanographic Data Center (JODC) after dividing all data into two groups with different axis locations. These temperature maps and temperature-salinity plots using the same JODC data suggest a scenario: cold water in the Kuroshio intermediate layer is first upwelled on the eastern side of the Kii Peninsula and thereafter moves westward as a coastal boundary current in the Kelvin wave sense of the Northern Hemisphere when the Kuroshio axis is located around 74-km distance from Cape Shionomisaki. This scenario is validated using internal Froude number maps depicted using the JODC-archived hydrographic data and geostrophic current fields. In addition, the reanalysis daily data provided by Japan Coastal Ocean Predictability Experiment are used for the validation.

Interaction between the trigger meander of the Kuroshio and the abyssal anticyclone over Koshu Seamount as seen in the reanalysis data

[1] The JCOPE (Japan Coastal Ocean Predictability Experiment) reanalysis data is analyzed to examine the transition from the non-large meander path to the large meander path of the Kuroshio that occurred in 2004. It was preceded by the generation of “trigger meander” off the southeastern coast of Kyushu, which then propagated eastward. After passing Cape Shiono-misaki, the trigger meander amplified rapidly over Koshu Seamount (located about 200 km to the south of Cape Shiono-misaki) where the accompanying abyssal anticyclone was significantly enhanced. Strong downwelling occurred upstream of the meander trough where the abyssal anticyclone crossed the Kuroshio, causing the vertical vortex stretching (shrinking) in the upper (lower) ocean; the resulting enhanced abyssal anticyclone further amplified the meander trough in the upper ocean. All of these features agree well with the model results suggesting the importance of baroclinic instability over Koshu Seamount in the formation of the large meander path.

Maturation and spawning of Spratelloides gracilis Clupeidae in temperate waters off Cape Shionomisaki, central Japan

The reproductive ecology of Spratelloides gracilis was investigated in the temperate waters off Cape Shionomisaki, central Japan. Cape Shionomisaki is located at the northern margin of the distribution range of this species. Females of S. gracilis with gonadosomatic index (GSI)≥4.0 were defined as mature based on the relationship between GSI and the histological maturity phases of their ovaries. More than 50% of females greater than 60 mm standard length (SL) were mature. Hatch dates of larvae and juveniles collected in the study area were determined by otolith daily ring counts and found to extend from April to November. The size at maturity of females (60 mm SL) and the spawning season of S. gracilis in the temperate waters off Cape Shionomisaki (April–November) was larger and shorter, respectively, than those in tropical waters in the western Pacific. The reproductive traits observed for S. gracilis off Cape Shionomisaki appear to be adaptive to northern temperate waters with seasonal changes in environmental conditions.

Prediction of slope water intrusion into the Kii channel in summer

Intrusions of the warm, oligotrophic surface slope water (SSW) and the cold, nutrient-rich bottom slope water (BSW) from the continental slope influence the annual variations in water temperature and nutrient concentrations in the Kii Channel in August. In order to evaluate the relationships between both these intrusions and the distance of the Kuroshio axis from Cape Shionomisaki (Kuroshio distance), a Distance-Intrusion-Diagram (DID) for temperature, which can reproduce the vertical temperature profile of the channel, was constructed by analyzing the temperature and Kuroshio distance records in August for 1967–2001. DIDs for nutrients (nitrate and phosphate) are also constructed by using the relationship between the nutrient concentration and water temperature. The only explanatory variable in the DIDs is the Kuroshio distance. The DID for temperature predicts that the SSW occupies almost the entire water column when the Kuroshio approaches Cape Shionomisaki (Kuroshio distance = 18.5 km). When the Kuroshio distance lies in the range 18.5–74 km, the BSW thickness increases proportionally to the Kuroshio distance increment while the SSW thickness decreases. The BSW occupies the largest portion of the channel when the Kuroshio distance is 74 km. Further, beyond 74 km, the BSW thickness reduces gradually. Yearly variations in the temperature and concentrations of nitrate and phosphate were hindcast with the DIDs. The results revealed that the Kuroshio distance contributes 70%, 35%, and 30% of the variances in temperature, nitrate concentration, and phosphate concentration, respectively.

Buried large block revealed by gravity anomalies in the Tonankai and Nankai earthquakes regions, southwestern Japan

Abstract We confirmed existence of a buried large block beneath seafloor off Cape Shionomisaki, Kii Peninsula, southwestern Japan, by improving marine gravity data. We found that the 1944 Tonankai and 1946 Nankai earthquakes occurred close to this block. The block is characterized by high gravity anomalies of 80 mGal larger than surrounding area with a diameter of about 40 km. Center of the anomaly is located 30 km off Cape Shionomisaki, southern end of the Kii Peninsula. This high gravity anomaly can be explained by existence of a higher density body with approximate thickness of 8 km just beneath the seafloor. This block coincides with the reported high velocity zone from recent seismic refraction survey in this area. Judging from seismic velocity, density, and geologic evidences, the block presumably consists of intruded acidic rocks, not seamount.

Influence of mesoscale eddies on variations of the Kuroshio path south of Japan

The influences of mesoscale eddies on variations of the Kuroshio path south of Japan have been investigated using time series of the Kuroshio axis location and altimeter-derived sea surface height maps for a period of seven years from 1993 to 1999, when the Kuroshio followed its non-large meander path. It was found that both the cyclonic and anticyclonic eddies may interact with the Kuroshio and trigger short-term meanders of the Kuroshio path, although not all eddies that approached or collided with the Kuroshio formed meanders. An anticyclonic eddy that revolves clockwise in a region south of Shikoku and Cape Shionomisaki with a period of about 5–6 months was found to propagate westward along about 30°N and collide with the Kuroshio in the east of Kyushu or south of Shikoku. This collision sometimes triggers meanders which propagate over the whole region south of Japan. The eddy was advected downstream, generating a meander on the downstream side to the east of Cape Shionomisaki. After the eddy passed Cape Shionomisaki, it detached from the Kuroshio and started to move westward again. Sometimes the eddy merges with other anticyclonic eddies traveling from the east. Coalescence of cyclonic eddies, which are also generated in the Kuroshio Extension region and propagate westward in the Kuroshio recirculation region south of Japan, into the Kuroshio in the east of Kyushu, also triggers meanders which mainly propagate only in a region west of Cape Shionomisaki.

地震記象から見た１９４６年南海地震の震源過程

Subevents of the 1946 Nankai earthquake (Mjma8.0) were distinguished from seismograms recorded on smoked papers with strong motion seismograms (T0=6s, amplification=2) and seismological data of Japan Meteorological Agency. Location and origin time of each subevent were roughly estimated from a hypocenter determination with phase readings of P and S waves for the subevent. Rupture of the Nankai earthquake started at a point southward about 50km off Cape Shionomisaki (first subevent). The rupture propagated in the NNW direction with a rupture velocity of 1.9 km/s and a second subevent that grew to class M8 started after about 16 sec from the initiation of the first subevent. Then the rupture propagated westward, and third subevent of M8 occurred around Tosa Bay, located about 200 km from the second event, about 53 sec after the initiation of the second event. Seismic waves were excited near Tosa Bay during the 1946 Nankai earthquake.

Numerical study of the generation and propagation of trigger meanders of the Kuroshio South of Japan

We examine the processes underlying the generation and propagation of the small meander of the Kuroshio south of Japan which occurs prior to the transition from the non-large meander path to the large meander path. The study proceeds numerically by using a two-layer, flat-bottom, quasi-geostrophic inflow-outflow model which takes account of the coastal geometries of Kyushu, Nansei Islands, part of the East China Sea, and the Izu Ridge. The model successfully reproduces the observed generation and propagation features of what is called "trigger meander" until it passes by Cape Shiono-misaki; presumably because of the absence of the bottom topography, the applicability of the present numerical model becomes questionable after the trigger meander passes by Cape Shiono-misaki. The generation of the trigger meander off the south-eastern coast of Kyushu is shown to be associated with the increase in the supply of cyclonic vorticity by the enhanced current velocity in the upper layer along the southern coast of Kyushu where the no-slip boundary condition is employed. Thereafter, the trigger meander propagates eastward while inducing an anticyclone-cyclone-cyclone pair in the lower layer. The lower-layer cyclone induced in this way, in particular, plays a crucial role in intensifying the trigger meander trough via cross-stream advection in the upper layer; the intensified trigger meander trough then further amplifies the lower-layer cyclone. This joint evolution of the upper-layer meander trough and the lower-layer cyclone indicates that baroclinic instability is the dominant mechanism underlying the rapid amplification of the eastward propagating trigger meander.