Bungo Channel Research Articles

1854年安政南海地震による愛媛県最南端（愛南町）での地震動·津波被害·地下水位変化

The 1854 Ansei Nankai Earthquake caused widespread damage from Kii peninsula to Kyushu region with Japanese seismic intensities of V to VI. A large amount of descriptions related to this earthquake are found in the archives “New Collection of Material for the History of Japanese Earthquakes”. The no. 5 of supplements to vol. 5 has collected documents related to the Ansei Tokai Earthquake (Nov. 4th, Ansei 1 in the old (lunisolar) calendar, or Dec. 23rd, 1854 in the new (Gregorian) calendar), Ansei Nankai Earthquake (Nov. 5th), and the largest aftershock in the Bungo channel (Nov. 7th), whose total number of pages amounts to 2,528. These collection books have quoted from many historical documents published by local governments, and reprinted from original historical material related to the earthquakes. However, when the published books were used, no reprint of original material was made. Purpose of this study is to reprint photocopy of an original private record handed down to Warabioka family who was a head of Masaki village of Ainan-town in the southernmost of Ehime Prefecture in Southwestern Japan. Furthermore, we compare our reprinted text with published ones, and make detailed investigation on natural phenomena and damage by the Ansei Nankai earthquake. It is identified that a lack of description about natural phenomena and damage and an error of the number of the dead in the texts adopted in the “New Collection of Material for the History of Japanese Earthquakes”. In particular, it is clear that the number of the dead in Fukaura village is not 101 but only one. This is consistent with the record of the necrology and gravestone. Preliminary estimation for the crustal deformation indicates a lowering of groundwater level in Masaki region by volume dilatation associated with the earthquake. This result is consistent with the record that the groundwater level was lowered. Furthermore, we compared our reprinted text with seven historical documents of Uwajima-Date family who had governed this area at that time in order to know the difference of information in the historical documents of the two families. As a result of comparison, there is no contradiction about descriptions of tsunami damage. However, the comparison shows no description about inland natural phenomena and damage such as well water dried up and rock fall in the historical documents of Uwajima-Date family, whereas it is written in the record of Warabioka family. These results show that the private record written by the village head has also valuable information on the earthquake.

1854年安政南海地震による愛媛県最南端（愛南町）での地震動·津波被害·地下水位変化

The 1854 Ansei Nankai Earthquake caused widespread damage from Kii peninsula to Kyushu region with Japanese seismic intensities of V to VI. A large amount of descriptions related to this earthquake are found in the archives “New Collection of Material for the History of Japanese Earthquakes”. The no. 5 of supplements to vol. 5 has collected documents related to the Ansei Tokai Earthquake (Nov. 4th, Ansei 1 in the old (lunisolar) calendar, or Dec. 23rd, 1854 in the new (Gregorian) calendar), Ansei Nankai Earthquake (Nov. 5th), and the largest aftershock in the Bungo channel (Nov. 7th), whose total number of pages amounts to 2,528. These collection books have quoted from many historical documents published by local governments, and reprinted from original historical material related to the earthquakes. However, when the published books were used, no reprint of original material was made. Purpose of this study is to reprint photocopy of an original private record handed down to Warabioka family who was a head of Masaki village of Ainan-town in the southernmost of Ehime Prefecture in Southwestern Japan. Furthermore, we compare our reprinted text with published ones, and make detailed investigation on natural phenomena and damage by the Ansei Nankai earthquake. It is identified that a lack of description about natural phenomena and damage and an error of the number of the dead in the texts adopted in the “New Collection of Material for the History of Japanese Earthquakes”. In particular, it is clear that the number of the dead in Fukaura village is not 101 but only one. This is consistent with the record of the necrology and gravestone. Preliminary estimation for the crustal deformation indicates a lowering of groundwater level in Masaki region by volume dilatation associated with the earthquake. This result is consistent with the record that the groundwater level was lowered. Furthermore, we compared our reprinted text with seven historical documents of Uwajima-Date family who had governed this area at that time in order to know the difference of information in the historical documents of the two families. As a result of comparison, there is no contradiction about descriptions of tsunami damage. However, the comparison shows no description about inland natural phenomena and damage such as well water dried up and rock fall in the historical documents of Uwajima-Date family, whereas it is written in the record of Warabioka family. These results show that the private record written by the village head has also valuable information on the earthquake.

Short-term slow slip events along the Ryukyu Trench, southwestern Japan, observed by continuous GNSS

In this study, short-term slow slip events (SSEs) along the Ryukyu Trench, southwestern Japan were systematically examined using continuous global navigation satellite system (GNSS) data. In total, 130 probable and 93 possible short-term SSEs in the moment magnitude (M w ) range of 5.6 to 6.8 were identified from January 1997 to November 2013 by GNSS time series offset monitoring and elastic dislocation modeling with a rectangular fault located on the subducting Philippine Sea Plate. The detected short-term SSEs were found to have a variety of characteristic recurrence intervals, magnitudes, durations, and coincidental seismic activities. Short-term SSEs without identified low-frequency earthquakes (LFEs) and low-frequency tremors (LFTs) were found to be common along the Ryukyu Trench. The total slip distributions and SSE numbers were heterogeneous and mostly between 10 and 60 km in depth. Although shallow (depth ≤20 km) short-term SSEs have never been detected along the Nankai Trough, it was notable that such SSEs often occur on the shallow plate interface along the Ryukyu Trench. This may be related to the incomplete interplate locking estimated by various geodetic studies. A band of short-term SSEs in the 20 to 40 km depth range extends from west Shikoku through the Bungo Channel to mid-Kyushu and then fades away around the subducted Kyushu-Palau Ridge. SSEs with accompanying LFEs and LFTs were found to be limited to western Shikoku and the Bungo Channel. We found several distinctive clusters of short-term SSEs, in addition to a cluster previously identified in the Yaeyama Islands. The study also identified a cluster northeast of Kikaijima consisting of 20 repeated SSEs at depths in the vicinity of 10 km near the trench where the Amami Plateau subducts, as well as another cluster southeast of Okinawa Island consisting of 29 M w ≤ 6.0 SSEs. The results suggest that the distribution of short-term SSEs, as well as that of large earthquakes, is affected by the topography of the subducting plate.

Spatial distributions of and diurnal variations in low molecular weight carbonyl compounds in coastal seawater, and the controlling factors

We studied the spatial distributions of and the diurnal variations in four low molecular weight (LMW) carbonyl compounds, formaldehyde, acetaldehyde, propionaldehyde, and glyoxal, in coastal seawater. The samples were taken from the coastal areas of Hiroshima Bay, the Iyo Nada, and the Bungo Channel, western Japan. The formaldehyde, acetaldehyde, and glyoxal concentrations were higher in the northern part of Hiroshima Bay than at offshore sampling points in the Iyo Nada and the Bungo Channel. These three compounds were found at much higher concentrations in the surface water than in deeper water layers in Hiroshima Bay. It is noteworthy that propionaldehyde was not detected in any of the seawater samples, the concentrations present being lower than the detection limit (1nanomole per liter (nM)) of the high performance liquid chromatography (HPLC) system we used. Photochemical and biological experiments were performed in the laboratory to help understand the characteristic distributions and fates of the LMW carbonyl compounds. The primary process controlling their fate in the coastal environment appears to be their biological consumption. The direct photo degradation of propionaldehyde, initiated by ultraviolet (UV) absorption, was observed, although formaldehyde and acetaldehyde were not degraded by UV irradiation. Our results suggest that the degradation of the LMW carbonyl compounds by photochemically formed hydroxyl radicals is relatively insignificant in the study area. Atmospheric deposition is a possible source of soluble carbonyl compounds in coastal surface seawater, but it may not influence the carbonyl concentrations in offshore waters.

Possible slip history scenarios for the Hyuga‐nada region and Bungo Channel and their relationship with Nankai earthquakes in southwest Japan based on numerical simulations

AbstractThe Nankai earthquakes are great interplate earthquakes with recurrence intervals of 100–200 years, occurring along the Nankai Trough, southwestern Japan. New geological evidences suggest that the rupture area of the 1707 earthquake (one of the largest) extended farther westward than previously thought, and similar magnitude events may have recurrence intervals of several hundred years. West of the Nankai earthquake segments, in the Bungo Channel region, slow slip events (SSEs) have occurred with recurrence intervals of several years since 1980. Farther westward, no M ≥ 7.5 interplate earthquakes have occurred except for one in 1968 in the northern Hyuga‐nada region, near the Bungo Channel. We numerically simulated the generation cycles of Nankai earthquakes, Hyuga‐nada earthquakes, and Bungo Channel SSEs to obtain possible slip histories in these regions, particularly during 1707–1980. We showed that the Nankai and Hyuga‐nada earthquakes occur mostly independently, with average recurrence intervals of 151 and 117 years, respectively. An SSE patch slipped coseismically during both the Nankai and Hyuga‐nada earthquakes. The rupture of the Nankai earthquakes extended to the Hyuga‐nada region every ~500–600 years and was followed by quiescence of SSEs for more than 200 years. Subsequently, SSEs occurred repeatedly every 8 years. Current observational findings are generally consistent with the characteristics obtained from our scenarios, except regarding the recurrence intervals of Hyuga‐nada earthquakes, for which few observations are available. Further evidence of the slip history in these regions will be required to use information from such simulations to forecast future earthquakes.

A long-term slow slip event from 1996 to 1997 in the Kii Channel, Japan

We present a long-term slow slip event from 1996 to 1997 in the Kii Channel along the Nankai Trough in southwestern Japan that has not been reported previously. The long-term slow slip event had a moment magnitude Mw 6.7 and a duration of 1 to 1.5 years. The magnitude of the event was smaller than those of the Tokai (Mw 7.1) and the Bungo Channel (Mw 7.0 to 7.1) events along the Nankai Trough. The slip was located slightly shallower than the depth of the low-frequency earthquakes around the Kii Channel. Long-term slow slips have been identified in various regions along the Nankai Trough, except in the Kii Peninsula. Unveiling the history of long-term slow slips is expected to contribute to an understanding of plate boundary characteristics as well as to the prediction of large earthquakes along the Nankai Trough.

Time‐dependent modeling of slow slip events and associated seismicity and tremor at the Hikurangi subduction zone, New Zealand

We present a time‐dependent slip model of 12 slow slip events (SSEs) occurring in the Hikurangi margin of New Zealand during 2010 and 2011. This model is obtained by inverting daily GPS solutions from GeoNet's continuous GPS network on the North Island and northern South Island. We compare the properties of these SSEs to observations in Japan, Cascadia, and Mexico and find that Hikurangi SSEs have comparatively large amounts of slip (up to 27 cm), high slip rates (up to 1.4 cm/d), and a large range of depths (10–40 km), durations (7–270 days), and sizes (Mw 5.9–6.9). We further investigate the relationship between the Cape Turnagain SSE and an associated seismic swarm and find that observations are consistent with stress triggering outside the slowly slipping region; however, other explanations cannot be ruled out. We also compare slip during the long‐term Manawatu SSE with the tremor epicenters found by Ide (2012) and note that tremor locations are offset in the downdip direction relative to the slipping region, similar to observations in the Bungo Channel of Japan and Guerrero, Mexico.

Comprehensive model of short‐ and long‐term slow slip events in the Shikoku region of Japan, incorporating a realistic plate configuration

We numerically simulate slow slip events (SSEs) in the Shikoku region of Japan, incorporating the configuration of the subducting plate. We adopt a rate‐ and state‐dependent friction law with cutoff velocities, assuming a frictional parameter distribution based on observed long‐term SSEs and nonvolcanic tremors that reflects the slip of short‐term SSEs. Our model reproduces recurrences of long‐ and short‐term SSEs and segments of short‐term SSEs. In our simulation, short‐term SSEs' transition from episodic to continuous slip is reproduced. This feature is consistent with tremor activity recently reported in both Shikoku and Cascadia. In addition to the long‐term SSEs in the Bungo Channel, our model also reproduces newly found long‐term SSEs in central Shikoku and predicts that these SSEs recur during the interseismic period between megathrust earthquakes. Our model comprehensively reproduces various SSEs and their characteristics as reported in the Shikoku region.

Effects of tidally induced eddies on sporadic Kuroshio-water intrusion (kyucho)

The sudden intrusion of Kuroshio warm water into the Bungo Channel (kyucho) occurs mainly at neap tides during summer, suggesting that tidal mixing is one of the essential factors regulating kyucho. In order to clarify the physical mechanisms responsible for the regulation of kyucho, we carry out non-hydrostatic three-dimensional numerical experiments allowing Kuroshio warm water to intrude into a strong tidal mixing region. It is shown that the Kuroshio warm water can (or cannot) pass through the tidal mixing regions off the east coast of the Bungo Channel during neap (or spring) tides. The analysis of the dynamic balance off the east coast of the Bungo Channel shows that tidal residual currents generated by tidal flow interaction with complicated land configurations off the east coast of the Bungo Channel can also play an important role in regulating kyucho. In order to assess separately the effects of tidal mixing and tidal residual currents on kyucho, we incorporate the parameterized vertical mixing and tidal stresses into the numerical model instead of tidal currents. It is demonstrated that tidal mixing cannot by itself block the northward intrusion of Kuroshio warm water, and that an additional effect induced by tidal residual eddies equivalent to horizontal mixing is needed to regulate kyucho. This strongly suggests that the basin–ocean water exchange processes in areas with complicated land configurations can only be reproduced by taking into account the effects of tidal residual eddies on a 1-km scale in addition to tidal mixing effects evaluated by microstructure measurements.

Spatial and temporal evolution of the long-term slow slip in the Bungo Channel, Japan

The Global Positioning System network in Japan detected a transient in the Bungo Channel, Japan, in 2009. Time-dependent analysis suggests that between May and September 2009, an aseismic slip might occur beneath the Bungo Channel and southwest Shikoku. From September 2009 to January 2010, the slip area shifted toward southwest Shikoku. Since approximately March 2010, the slip has increased in magnitude and speed while expanding into the Bungo Channel. The estimated rupture process is similar to those of previous Bungo Channel slow slips, in that the slip starts in a shallower region and expands into a deeper region with the acceleration of the slip. The high-speed period of the 2010 event corresponds to a marked increase in the activity of low-frequency earthquakes, similar to that of the 2003 event. The total slip distribution of the 2010 event is similar to that of previous long-term Bungo slow slips, verifying the hypothesis that long-term slow slips are characteristic events in the Bungo Channel.

Numerical simulation of tidally induced eddies in the Bungo Channel: A possible role for sporadic Kuroshio-water intrusion (kyucho)

It is well known that the sudden intrusion of Kuroshio warm water into the Bungo Channel (kyucho) is regulated by spring–neap tidal forcing. In order to clarify the physical background behind this regulation, numerical experiments are carried out using a high-resolution non-hydrostatic three-dimensional model. We first reproduce the strong mixing region off the east coast of the Bungo Channel resulting from tidal flow interaction with complicated land configurations during spring tides; behind islands and headlands, small-scale eddies satisfying an approximate cyclostrophic balance are generated. As a result, averaged over the whole model domain, the tidal-mean energy dissipation rate reaches ≈1.6 × 10−6 W kg−1. The model predicted energy dissipation rates at the location and times of direct microstructure measurements in the Bungo Channel are comparable to the observed values. We next examine whether or not strong tidal mixing thus reproduced can inhibit the northward intrusion of Kuroshio warm water in the Bungo Channel. It is shown that the Kuroshio warm water can (or cannot) pass through the tidal mixing regions off the east coast of the Bungo Channel during periods of weakened (or enhanced) tidal mixing at neap (or spring) tides. This indicates that taking into account the realistic spring–neap modulation of tidal mixing intensity is indispensable to further increase the ability of the existing forecast system for kyucho in the Bungo Channel.

Ensemble numerical forecasts of the sporadic Kuroshio water intrusion (kyucho) into shelf and coastal waters

The finite volume coastal ocean model downscaling ocean reanalysis and forecast data provided by the Japan Coastal Ocean Predictability Experiment (JCOPE2) are used to forecast sudden Kuroshio water intrusion events (kyucho) induced by frontal waves amplified south of the Bungo Channel in 2010. Two-month hindcast computations give initial conditions of the following 3-month forecasts computations which consist of ten ensemble members. The temperature time series computed by these ten members are averaged to compare with that actually observed in the Bungo Channel, where sudden temperature rises related to kyucho events are remarkable in February, August, and September. Overall, the intense kyucho events actually observed in these months are predicted successfully. However, intense kyucho events are forecasted frequently during the period of May through June even though intense kyucho events are absent during this period in the actual ocean. It is suggested that the present downscaling forecast model requires reliable lateral boundary conditions provided by JCOPE2 data to which numerous Argo data are assimilated to enhance the accuracy. In addition, it seems likely that the model accuracy is reduced by small eddies moving along the shelf break.

Two Species of Caligus (Copepoda: Caligidae) from Amberjacks (Seriola spp.) Cultured in Oita Prefecture, Kyushu, Western Japan

Two species of copepods of the caligid genus Caligus were found parasitic on amberjacks cultured in the Bungo Channel off the coast of Oita Prefecture, Kyushu, western Japan. They are Caligus spinosus Yamaguti, 1939 from the gills of yellowtail amberjack (Seriola lalandi), and Caligus lalandei Barnard, 1948 from the body surface of Japanese amberjack (Seriola quinqueradiata). This is the first confirmed record of C. spinosus from S. lalandi in Japan.

An Adaptation of Diagnostic Numerical Model of Residual Current to Bungo Channel, Japan

An Adaptation of Diagnostic Numerical Model of Residual Current to Bungo Channel, Japan

Repetitive long-term slow slip events beneath the Bungo Channel, southwestern Japan, identified from leveling and sea level data from 1979 to 2008

[1] Leveling and sea level data for the period from 1979 to 2008 around the Bungo Channel, southwestern Japan, were investigated to characterize vertical deformation patterns. We found evidence of discrete events that may have occurred beneath the Bungo Channel before the two most recent long-term slow slip events (SSEs) (1996–1997 and 2003) detected from GPS data. GPS-derived steady state vertical displacements related to ongoing subduction of the Philippine Sea plate were subtracted from the vertical displacements observed by leveling surveys. The spatial pattern of the residual vertical displacements observed by leveling was similar to that observed by GPS in the periods including recent long-term SSEs. This suggests that discrete vertical displacements, which might represent long-term SSEs, have occurred in each of four intervals between leveling surveys before GPS deployment. We calculated differences of sea level between tidal stations near the Bungo Channel after some corrections and then cross-correlated them with the GPS-derived vertical displacements of the recent long-term SSE to estimate the timing of the events. These cross correlations and the discrete vertical displacements derived from leveling suggest that long-term SSEs may have occurred around 1980, around 1985–1986, and around 1991.

Intrusion route from Iyo nada to Suo nada and its contribution to primary production

In recent years, some of researchers have reported that most of nutrients in Suo nada are derived from open ocean through the Bungo Channel and Iyo nada. If these nutrients flow into euphotic zone of Suo nada, they can provide primary production and play an important role in Suo nada. We investigated pathway from Iyo nada to Suo nada by using observation data such as temperature, salinity, density and nutrients. As the result, we confirm existence of inflow from Iyo nada at euphotic zone in Suo nada. However, it is presented that less of contribution to primary production in Suo nada because of low nutrients in Iyo nada.

Ocean Water Intrusions into the Seto Inland Sea in the Numerical Ocean Modeling

Open-ocean water intrusions from the Kuroshio surface (and/or intermediate) layer are considerable processes for the ecosystem of the semi-enclosed coastal sea such as the Seto Inland Sea. To reproduce the ecosystem in the Seto Inland Sea, a numerical ocean model is constructed by using daily reanalysis data demonstrating the Kuroshio path of the open ocean. The model results are verified quantitatively and examined the ocean water intrusion focusing cold water intrusion into the bottom layer of the Bungo Channel, and warm water intrusion in the surface layer of the channel, called kyucho. Modeled temperature explains the cold water intrusion into the bottom layer of the channel, and cannot demonstrate kyucho because of the lack of disturbance derived the Kuroshio frontal eddies in the reanalysis data.

Integration of transient strain events with models of plate coupling and areas of great earthquakes in southwest Japan

We model the crustal deformation caused by two long-term subduction slip transients in southwest Japan, which we refer to as the 2000–2004 Tokai and the 2002–2004 Bungo Channel slow slip events (SSEs). We use re-analysed GEONET position time-series, and a Kalman filter based network inversion method to image the spatiotemporal slip variation of the two events on the plate interface during the period of 1998–2004.67 and 2000–2005. Both events are found to have complex slip histories with multiple subevents. In addition to a newly identified slip subevent in 2002–2003, we find that the major event in the Bungo Channel SSE initiated in early 2003 beneath the northeastern corner of the region and expanded southwestward, in contrast to the slip characteristics suggested by other studies. The re-analysed GPS data in the Tokai region shows a renewed slip activity for the Tokai SSE in early 2003–2004 at a similar location as in the period of 2001–2002. The equivalent M_w for both the Tokai and Bungo Channel SSEs are about 7.0. Our results show that the Tokai SSE appears to start before the Miyaki-Kozu seismovolcanic event. Integrating plate coupling and SSEs shows that the transient slip zones are located in a region between the locked zones and the epicentres of the low frequency earthquakes (LFEs). At least part of the interseismic slip deficit is released by episodic SSEs beneath the Bungo Channel region. We find excellent temporal correspondence between transient slip and adjacent LFEs for both SSE, suggesting that they are closely related and possibly reflect that long-term slow slip may modulate the occurrence of LFEs.

Hindcast and predictability of sporadic Kuroshio‐water intrusion (kyucho in the Bungo Channel) into the shelf and coastal waters

Prerequisite(s) for ocean circulation models capable of hindcasting “kyucho” occurrence (a sudden coastal temperature rise induced by Kuroshio frontal waves) in the Bungo Channel, Japan, is investigated using long‐term observed temperature and sea level time series, archived hydrographic data, and reanalysis data provided by the Japanese Coastal Ocean Predictability Experiment (JCOPE) group. Anticyclonic mesoscale eddies impinging on the Kuroshio front south of the Bungo Channel enhance the frontal sharpness, frontal wave growth, and activity of kyucho phenomena. A kyucho hindcast is carried out to examine the reliability of a numerical model including realistic anticyclonic eddies propagating south of Japan. The Finite Volume Coastal Ocean Model (FVCOM) for which boundary conditions are given by daily JCOPE2 reanalysis data is adopted in the present study. This numerical model does a reasonable job of hindcasting kyucho occurrence in 2003. It is therefore considered that forecasts of kyucho occurrence up to 2 or 3 months ahead are possible by using the FVCOM in conjunction with JCOPE2 forecast data.

Amplification of tsunami heights by delayed rupture of great earthquakes along the Nankai trough

We investigated the effect of delayed rupture of great earthquakes along the Nankai trough on tsunami heights on the Japanese coast. As the tsunami source, we used a model of the 1707 Hoei earthquake, which consists of four segments: Tokai, Tonankai, and two Nankai segments. We first searched for the worst case, in terms of coastal tsunami heights, of rupture delay time on each segment, on the basis of superposition principle for the linear long wave theory. When the rupture starts on the Tonankai segment, followed by rupture on the Tokai segment 21 min later, as well as the eastern and western Nankai segments 15 and 28 min later, respectively, the average coastal tsunami height becomes the largest. To quantify the tsunami amplification, we compared the coastal tsunami heights from the delayed rupture with those from the simultaneous rupture model. Along the coasts of the sea of Hyu’uga and in the Bungo Channel, the tsunami heights become significantly amplified (>1.4 times larger) relative to the simultaneous rupture. Along the coasts of Tosa Bay and in the Kii Channel, the tsunami heights become amplified about 1.2 times. Along the coasts of the sea of Kumano and Ise Bay, and the western Enshu coast, the tsunami heights become slightly smaller for the delayed rupture. Along the eastern Enshu coast, the coast of Suruga Bay, and the west coast of Sagami Bay, the tsunami heights become amplified about 1.1 times.