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Research about Japan

Source-linked reports with geographic coverage including Japan.

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USGS2008

A slab fragment wedged under Tokyo and its tectonic and seismic implications

The Philippine Sea and Pacific plate slabs both subduct beneath Tokyo, and so their configuration and seismic potential have been subject to intensive study . Previous work suggests that the Philippine Sea slab extends up to 100 km northwest of Tokyo and subducts to a depth of 90 km beneath the Kanto basin, where it is folded against the underlying Pacific slab. Here we evaluate seismic data in three dimensions and delineate a distinct 25-km-thick and 100-km-wide body beneath the Kanto basin that has hitherto been considered to be part of the Philippine Sea slab. We find that several of its characteristics—such as its high seismic velocity and the presence of a double seismic zone—point instead to an affinity with the Pacific slab, implying that the Philippine Sea slab penetrates to depths of only 35–40 km beneath the Kanto basin. We propose that the body is a fragment of Pacific lithosphere that formed by the collision of two intersecting seamount chains with the Japan Trench 2–3 million years ago. We suggest that such slab fragments may not be uncommon, especially where seamount subduction deforms trenches and near triple junctions.

Tokyo, Japan Trench↗
USGS2008

Aeromagnetic evidence for a buried Early Cretaceous magmatic arc, northeast Japan

Positive aeromagnetic anomalies, recent drilling, and models constructed from these data delineate the plutonic roots of the Early Cretaceous Kitakami magmatic arc in northeast Japan. Buried plutons, mostly offshore, produce belts of positive magnetic anomalies. These anomalies and magnetotelluric data suggest that the plutons form a batholith 70–120 km wide, nearly 800 km long, and 10–15 km thick. The batholith may mark the location of the main Kitakami arc. Most of the exposed Kitakami plutons are 2–20 km in diameter; some are 3 km thick. The small plutons line up along NW trending faults; some may have been satellite vents that tapped into the magma supply of the main arc. The batholithic roots of the main arc now compose almost half of the modern Japan forearc basement. Steep magnetic gradients, offset anomalies, and basin stratigraphy portray extensive faulting of the Kitakami batholith during oblique subduction in the Late Cretaceous and rifting in the Miocene. The eastern boundary of the Kitakami batholith lies between 90 and 140 km west of the modern trench, much closer than the 300-km distance between the active arc and trench. The Early Cretaceous forearc basin and accretionary prism may underlie the modern forearc basin east of the batholith, but clear evidence is lacking. Much of the Early Cretaceous margin, including most of its forearc therefore is missing. How the material was removed is unknown: it could have been strike-slip faulted, eroded by subduction-related processes, or both.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

Introduction to the special section northeast Japan: A case history of subduction

Subduction-related tectonic events such as strike-slip faulting, uplift of high-pressure metamorphic rocks in the forearc, back arc spreading, arc magmatism, and continental collisions have shaped northeast Japan, making it a case history for subduction. Much of the record of these events is preserved and can be used to reconstruct the tectonic history of the region. Although many geological, geophysical, and geochemical data have been collected in Japan for the last 30 years, interpretation of these data into a plate tectonic context did not really begin until about 10 years ago. Much of the data and interpretation on northeast Japan have been published in Japanese or as individual local studies. The primary goal of this special section is to present new investigations covering a wide range of topics and scales and place them in a tectonic framework. The papers in this special section focus on three main subjects: Mesozoic aspects of northeast Japan; the relation of Japan Sea opening with the tectonics and geochemistry of northeast Japan in the Tertiary; and the geophysical setting. The purpose of this overview is to link these subjects and provide a tectonic framework for north- east Japan and eastern Asia since the Mesozoic.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

Recent damaging earthquakes in Japan, 2003-2008

During the last six years, from 2003-2008, Japan has been struck by three significant and damaging earthquakes: The most recent M6.6 Niigata Chuetsu Oki earthquake of July 16, 2007 off the coast of Kashiwazaki City, Japan; The M6.6 Niigata Chuetsu earthquake of October 23, 2004, located in Niigata Prefecture in the central Uonuma Hills; and the M8.0 Tokachi Oki Earthquake of September 26, 2003 effecting southeastern Hokkaido Prefecture. These earthquakes stand out among many in a very active period of seismicity in Japan. Within the upper 100 km of the crust during this period, Japan experienced 472 earthquakes of magnitude 6, or greater. Both Niigata events affected the south-central region of Tohoku Japan, and the Tokachi-Oki earthquake affected a broad region of the continental shelf and slope southeast of the Island of Hokkaido. This report is synthesized from the work of scores of Japanese and US researchers who led and participated in post-earthquake reconnaissance of these earthquakes: their noteworthy and valuable contributions are listed in an extended acknowledgements section at the end of the paper. During the Niigata Chuetsu Oki event of 2007, damage to the Kashiwazaki-Kariwa nuclear power plant, structures, infrastructure, and ground were primarily the product of two factors: (1) high intensity motions from this moderate-sized shallow event, and (2) soft, poor performing, or liquefiable soils in the coastal region of southwestern Niigata Prefecture. Structural and geotechnical damage along the slopes of dunes was ubiquitous in the Kashiwazaki-Kariwa region. The 2004 Niigata Chuetsu Earthquake was the most significant to affect Japan since the 1995 Kobe earthquake. Forty people were killed, almost 3,000 were injured, and many hundreds of landslides destroyed entire upland villages. Landslides were of all types; some dammed streams, temporarily creating lakes threatening to overtop their new embankments and cause flash floods and mudslides. The numerous landslides resulted, in part, from heavy rain associated with Typhoon Tokage. The earthquake forced more than 100,000 people into temporary shelters, and as many as 10,000 displaced from their upland homes for several years. Total damages was estimated by Japanese authorities at US$40 billion, making this the second most costly disaster in history, after the 1995 Kobe earth-quake. The 2003 Tokachi-Oki earthquake was the third event of magnitude 8.0+ to strike the southeastern portion of Hokkaido in the last 50 years. The event produced tsunami run-ups along the shoreline of southern Hokkaido that reached maximum heights of 4 meters. Accelerations recorded by seismic networks of Hokkaido indicated a high intensity motion region from Hiroo area to Kushiro City, with a PGA values in the range of 0.35 to 0.6g. Despite high acceleration levels, the observed ground failure, liquefaction, structural, port, and lifeline damages were remarkably light.

Conference Paper↗
USGS2008

Google earth mapping of damage from the Nigata-Ken-Chuetsu M6.6 earthquake of 16 July 2007

We describe the use of Google Earth during and after a large damaging earthquake thatstruck the central Japan coast on 16 July 2007 to collect and organize damage information and guide the reconnaissance activities. This software enabled greater real-time collaboration among scientists and engineers. After the field investigation, the Google Earth map is used as a final reporting product that was directly linked to the more traditional research report document. Finally, we analyze the use of the software within the context of a post-disaster reconnaissance investigation, and link it to student use of GoogleEarth in field situations

Conference Paper↗
USGS2007

A global search inversion for earthquake kinematic rupture history: Application to the 2000 western Tottori, Japan earthquake

[1] We present a two-stage nonlinear technique to invert strong motions records and geodetic data to retrieve the rupture history of an earthquake on a finite fault. To account for the actual rupture complexity, the fault parameters are spatially variable peak slip velocity, slip direction, rupture time and risetime. The unknown parameters are given at the nodes of the subfaults, whereas the parameters within a subfault are allowed to vary through a bilinear interpolation of the nodal values. The forward modeling is performed with a discrete wave number technique, whose Green's functions include the complete response of the vertically varying Earth structure. During the first stage, an algorithm based on the heat-bath simulated annealing generates an ensemble of models that efficiently sample the good data-fitting regions of parameter space. In the second stage (appraisal), the algorithm performs a statistical analysis of the model ensemble and computes a weighted mean model and its standard deviation. This technique, rather than simply looking at the best model, extracts the most stable features of the earthquake rupture that are consistent with the data and gives an estimate of the variability of each model parameter. We present some synthetic tests to show the effectiveness of the method and its robustness to uncertainty of the adopted crustal model. Finally, we apply this inverse technique to the well recorded 2000 western Tottori, Japan, earthquake ( Mw 6.6); we confirm that the rupture process is characterized by large slip (3-4 m) at very shallow depths but, differently from previous studies, we imaged a new slip patch (2-2.5 m) located deeper, between 14 and 18 km depth.

Tottori↗
USGS2007

Anomalous deepening of a belt of intraslab earthquakes in the Pacific slab crust under Kanto, central Japan: Possible anomalous thermal shielding, dehydration reactions, and seismicity caused by shallower cold slab material

[1] A belt of intraslab seismicity in the Pacific slab crust parallel to iso-depth contours of the plate interface has been found beneath Hokkaido and Tohoku. Hypocenter relocations have shown that this seismic belt does not run parallel to but obliquely to the iso-depth contours beneath Kanto, deepening toward the north from ∼100 km to ∼140 km depth. The depth limit of the contact zone with the overlying Philippine Sea slab is located close to and parallel to this obliquely oriented seismic belt, suggesting that the deepening of the seismic belt there is caused by the contact with the overlying slab. The contact with this cold slab hinders the heating of the Pacific slab crust by hot mantle wedge, which would cause delay of eclogite-forming phase transformations and hence deepening of the seismic belt there. The depth limit of the subducting low-velocity crust also deepens toward the north, supporting this idea.

Geophysical Research Letters↗
USGS2007

Investigation of the M6.6 Niigata-Chuetsu Oki, Japan, earthquake of July 16, 2007

The M6.6 mainshock of the Niigata Chuetsu Oki (offshore) earthquake occurred at 10:13 a.m. local time on July 16, 2007, and was followed by a sequence of aftershocks that were felt during the entire time of the reconnaissance effort. The mainshock had an estimated focal depth of 10 km and struck in the Japan Sea offshore Kariwa. Analysis of waveforms from source inversion studies indicates that the event occurred along a thrust fault with a NE trend. The fault plane is either a strike of 34 degrees with a dip of 51 degrees or a strike of 238 degrees with a dip of 41 degrees. Which of these two planes is associated with the mainshock rupture is unresolved, although attenuation relationship analysis indicates that the northwest-dipping fault is favored. The quake affected an approximately 100-km-wide area along the coastal areas of southwestern Niigata prefecture. The event triggered ground failures as far as the Unouma Hills, located in central Niigata approximately 50 km from the shore and the source area of the 2004 Niigata Chuetsu earthquake. The primary event produced tsunami run-ups that reached maximum runup heights of about 20 centimeters along the shoreline of southern Niigata Prrefecture.

Niigata↗
USGS2006

Remote sensing observations of landslides and ground deformation from the 2004 Niigata Ken Chuetsu earthquake

In recent years, major developments in remote sensing have made it possible to use these technologies to document the effects of earthquakes. Specifically, high-resolution satellite imagery and three-dimensional laser scanning (LIDAR) can provide important observations of earthquake damage that supplement traditional observations from field reconnaissance. The 2004 Niigata Ken Chuetsu earthquake provided an opportunity to use remote sensing to document the distribution of landslides in the epicentral region through the use of high-resolution satellite imagery and to document the detailed three-dimensional geometries of several failures using LIDAR. The satellite imagery was acquired the day after the earthquake, but at very large acquisition angles that resulted in image distortion. Nonetheless, the satellite imagery accurately identified the landslide distribution in the epicentral region, although the total area of landslides was underestimated by about 25% as compared with traditional aerial reconnaissance because of the large acquisition angle for the satellite imagery. Terrestrial LIDAR was used to collect three-dimensional data at several failure sites, including two large rock slides and a railroad tunnel portal affected by ground deformation. The LIDAR data allowed for precise measurement of failure deformations and geometries, and provided digital terrain models that could be archived and used in future analyses. In the future, satellite imagery and LIDAR, as well as other remote sensing technologies, will play an increasing role in documenting and understanding the effects of earthquakes.

Niigata Prefecture↗
USGS2004

Interpretation of the Miyakejima 2000 eruption and dike emplacement using time animations of earthquakes

The seismic sequence of over 12,000 earthquakes accompanying the 2000 eruption of Miyakejima volcano has been studied by viewing time animations of the earthquakes beneath transparent topography. Seismic activity began on the evening of June 26 extending southwest from Miyakejima’s summit. A few hours later the seismicity abruptly shifted to the WNW and a submarine eruption occurred off the West Coast of Miyakejima on the morning of June 27. Phreatic eruptions at Miyakejima’s summit in July were accompanied by the formation of a new caldera. Following caldera formation explosive eruptions occurred in August. The eruption ended with minor explosions in September. The seismic activity that began with a low-magnitude swarm beneath Miyakejima grew to a major swarm with hundreds of events of M＞4 extending more than 40km WNW from Miyakejima. Lesser numbers of earthquakes occurred on two N-S trending lines extending south and north of the main seismic trend. The seismicity has been interpreted as evidence for emplacement of a massive dike on the main trend that triggered additional earthquakes on the two cross trends. Our interpretation involves more restricted dike emplacement west of Miyakejima, including the possibility of additional submarine eruption, following cracking of the Philippine Sea plate. The seismic activity associated with explosive eruptions in August helps to define Miyakejima’s magma plumbing. A shallow reservoir beneath the southwest slope is defined by concentrations of earthquakes at 4-6km depth, and a deeper source is suggested by a smaller number of earthquakes extending to 10km vertically beneath the shallow source. Seismic activity preceding and accompanying eruptions at Miyakejima’s summit are defined by seismic swarms extending from 4km depth to the surface along a path connecting the summit with the shallow reservoir. Away from Miyakejima shallow (＜1 km) earthquake swarms at minimum rates of 1 event per hour extending over several hours occur within restricted areas of diameter less than 3km and define possible additional sites of undersea eruption or intrusion. Beneath sites west of Miyakejima the seismicity at depths of less than 4km occurs earlier and toward Miyakejima, consistent with magma transport from Miyakejima’s shallow reservoir. Shallow swarms extending 15km to the WNW strongly suggest that additional intrusion and possibly eruption may have occurred on June 27-28. Between June 27 and July 12 along the main seismic trend, and beneath the shallow sites, progressively deeper earthquake swarms occur at progressively later times, a pattern inconsistent with magma transport and interpreted here as the Philippine Sea plate cracking downward. The initial shallow cracking guided magma to the June 27 undersea eruption site. Subsequent cracking to the west allowed very rapid lateral withdrawal of magma from the Miyakejima reservoir allowing a new caldera to form. The deep cracking of the plate may have triggered additional magma sources, including a deep source suggested by the modeling of regional ground deformation data.

Miyakejima↗
USGS2003

Basin-centered asperities in great subduction zone earthquakes: A link between slip, subsidence, and subduction erosion?

Published areas of high coseismic slip, or asperities, for 29 of the largest Circum-Pacific megathrust earthquakes are compared to forearc structure revealed by satellite free-air gravity, bathymetry, and seismic profiling. On average, 71% of an earthquake's seismic moment and 79% of its asperity area occur beneath the prominent gravity low outlining the deep-sea terrace; 57% of an earthquake's asperity area, on average, occurs beneath the forearc basins that lie within the deep-sea terrace. In SW Japan, slip in the 1923, 1944, 1946, and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350??C isotherm on the plate boundary, the inferred downdip limit of the locked zone. Basin-centered coseismic slip also occurred along the Aleutian, Mexico, Peru, and Chile subduction zones but was ambiguous for the great 1964 Alaska earthquake. Beneath intrabasin structural highs, seismic slip tends to be lower, possibly due to higher temperatures and fluid pressures. Kilometers of late Cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion. The deep-sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes. Basin-centered asperities could indicate a link between subsidence, subduction erosion, and seismogenesis. Whatever the cause, forearc basins may be useful indicators of long-term seismic moment release. The source zone for Cascadia's 1700 A.D. earthquake contains five large, basin-centered gravity lows that may indicate potential asperities at depth. The gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore, except in northwestern Washington, where the low extends landward beneath the coast. Transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes. Published in 2003 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗
USGS2003

Life history, diversity and distribution: A study of Japanese pteridophytes

Many studies address the relationships between diversity or distribution and attributes of the physical environment. However, how these relationships are connected to variation in life history is poorly understood. This is particularly true in the case of pteridophytes. Japanese ferns and their allies comprise one of the best-known pteridophyte floras in the world. We analyzed ca 600 species of Japanese pteridophytes for which there is detailed information on distribution, reproduction, and chromosome number. Species richness was greatest in groups with a single reproductive mode (sexual, followed by apogamous), but distribution was greatest in species groups with multiple reproductive modes: sexual plus either sterile (irregular in meiosis) or apogamous. Geographical ranges varied greatly among species with small chromosome numbers but were uniformly small among species having high chromosome numbers. Seasonally green (mostly summer green) species had significantly larger distribution ranges than evergreen species. Endemic species had higher proportions of apogamy and sterility than non-endemic species. Seasonally green species had significantly larger distributional ranges, and a smaller proportion of species with apogamous reproduction, than evergreen species. There was no clear relationship between distribution and spore size, either among endemic species, non-endemic species, or all species combined. There was no relationship between spore size and chromosome number when all species were combined. However, positive relationships were detected within three of the nine largest genera, suggesting potential phylogenetic effects. We concluded that habitat availability, rather than dispersability, may be the limiting factor for the distribution of pteridophytes in Japan.

Ecography↗
USGS2003

Detailed fault structure of the 2000 Western Tottori, Japan, earthquake sequence

We investigate the faulting process of the aftershock region of the 2000 western Tottori earthquake (Mw 6.6) by combining aftershock hypocenters and moment tensor solutions. Aftershock locations were precisely determined by the double difference method using P- and S-phase arrival data of the Japan Meteorological Agency unified catalog. By combining the relocated hypocenters and moment tensor solutions of aftershocks by broadband waveform inversion of FREESIA (F-net), we successfully resolved very detailed fault structures activated by the mainshock. The estimated fault model resolves 15 individual fault segments that are consistent with both aftershock distribution and focal mechanism solutions. Rupture in the mainshock was principally confined to the three fault elements in the southern half of the zone, which is also where the earliest aftershocks concentrate. With time, the northern part of the zone becomes activated, which is also reflected in the postseismic deformation field. From the stress tensor analysis of aftershock focal mechanisms, we found a rather uniform stress field in the aftershock region, although fault strikes were scattered. The maximum stress direction is N107??E, which is consistent with the tectonic stress field in this region. In the northern part of the fault, where no slip occurred during the mainshock but postseismic slip was observed, the maximum stress direction of N130??E was possible as an alternative solution of stress tensor inversion.

Tottori↗
USGS2002

Temporal evolution of a hydrothermal system in Kusatsu-Shirane Volcano, Japan, inferred from the complex frequencies of long-period events

We present a detailed description of temporal variations in the complex frequencies of long-period (LP) events observed at Kusatsu-Shirane Volcano. Using the Sompi method, we analyze 35 LP events that occurred during the period from August 1992 through January 1993. The observed temporal variations in the complex frequencies can be divided into three periods. During the first period the dominant frequency rapidly decreases from 5 to 1 Hz, and Q of the dominant spectral peak remains roughly constant with an average value near 100. During the second period the dominant frequency gradually increases up to 3 Hz, and Q gradually decreases from 160 to 30. During the third period the dominant frequency increases more rapidly from 3 to 5 Hz, and Q shows an abrupt increase at the beginning of this period and then remains roughly constant with an average value near 100. Such temporal variations can be consistently explained by the dynamic response of a hydrothermal crack to a magmatic heat pulse. During the first period, crack growth occurs in response to the overall pressure increase in the hydrothermal system caused by the heat pulse. Once crack formation is complete, heat gradually changes the fluid in the crack from a wet misty gas to a dry gas during the second period. As heating of the hydrothermal system gradually subsides, the overall pressure in this system starts to decrease, causing the collapse of the crack during the third period.

Kusatsu-Shirane Volcano↗
USGS2002

Evidence from the AD 2000 Izu islands earthquake swarm that stressing rate governs seismicity

Magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits 1,2,3,4 that cannot be associated with the diffusion of pore fluids or heat 5 . Such ‘swarm’ seismicity also migrates with time, and often exhibits a ‘dog-bone’-shaped distribution 3,4,6,7,8,9 . The largest earthquakes in swarms produce aftershocks that obey an Omori-type (exponential) temporal decay 10,11,12 , but the duration of the aftershock sequences is drastically reduced, relative to normal earthquake activity 7,13 . Here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion 8,11,14,15 . A 1,000-fold increase in seismicity rate and a 1,000-fold decrease in aftershock duration occurred during the two-month-long dyke intrusion. We find that the seismicity rate is proportional to the calculated stressing rate, and that the duration of aftershock sequences is inversely proportional to the stressing rate. This behaviour is in accord with a laboratory-based rate/state constitutive law 16 , suggesting an explanation for the occurrence of earthquake swarms. Any sustained increase in stressing rate—whether due to an intrusion, extrusion or creep event—should produce such seismological behaviour.

Izu Islands↗
USGS2002

Integrated biostratigraphic and radiometric study on the Lower Cenomanian (Cretaceous) of Hokkaido, Japan

New 40 Ar- 39 Ar total fusion ages have been obtained on sanidine from tuffs in the zone of Mantelliceras saxbii (equivalent to the same subzone of the European ammonite zonation) and the zone of Graysonites wooldridgei (equivalent to the European subzone of Neostlingoceras carcitanense ). Ages of 98.98±0.38 Ma and 99.16±0.37 Ma, for these zones, respectively, place new constraints on the age of the AlbianlCenomanian boundary. If the base of the Cenomanian is to be defined, as currently proposed, as the FAD of the planktonic foraminifera Rotalipora globotruncanoides then the Albian/Cenomanian boundary will be at least as old as 99.2±0.4 Ma. Some 50±10m of undated sediments lie below the lowest find of R . globotruncanoides and above bono fide uppermost Albian strata.

Hokkaido↗