Japan in a subduction zone
Earthquakes
Fig. 6
Distribution of earthquake focuses [
]
Earthquakes very often occur in borders between plates. In Japan, over 1300 felt earthquakes were observed in 2010, but more than 2000 earthquakes were felt in some years. The M≥5 aftershock frequency of M 9 earthquake in eastern Japan in 2011 is over 400 two weeks after the main shock.
In areas of plate margins, the crust is stressed by plate movement. The
rocks are broken and energy is released when the stress exceeds their
strength. The rupture generally occurs along faults, which are
considered seismic sources. Therefore, the rapid slip of rocks along a
fault results in an earthquake. Volcanic activity also causes
earthquakes. There are principally three types of fault motion:
normal dip-slip, reverse dip-slip, and strike-slip (left or
right). Fault movements may include a component of strike-slip and
dip-slip (see also "Neodani fault"). For the magnitude 9.0 eastern Japan earthquake
in 2011, the type of earthquake source
fault is the reverse fault, the size of fault is about 450 km long and
200 km wide, the fault slippage is 20 to 30 meters (maximum), and the
duration of main failure is about three minutes, according to Japan
Meteorological Agency (The 2011 off the Pacific Coast Tohoku Earthquake* [28th report]).
* This name was given by Japan Meteorological
Agency.
Fig. 7 Types of fault motion
Although shallow-focus earthquakes happen at many places on the Earth, earthquakes whose focuses are at more than 70 km deep only occur in subduction zones. Focuses in the subducting plate are distributed with increasing depth at a constant angle from a trench toward an island arc as shown Figure 6. This focus zone is called Wadati-Benioff zone, which corresponds with the leading edge of a descending plate.
The distribution of epicenters in and around Japan is parallel to the
two series of trenches (the eastern Japan arc system and the western
Japan arc system). Figure
4 showing the subducting depths of the leading
edge of the Pacific and the Philippine Sea Plates is made on the basis of
depths of focuses. Deep-focus earthquakes (>30 km) caused by the
subduction of the Pacific Plate occur most frequently off the Pacific
coast of northeastern Honshu and Hokkaido. The depths of focuses
constantly increase with distance from the trenches as shown in
Figure 4. As for earthquakes related to the subduction of the Philippine Sea
Plate, seismic activity is vigorous off eastern Kyushu but less in and
off Shikoku and Kinki. The deepest focuses are about 200 km deep, much
shallower than those of earthquakes related to the Pacific Plate (about
700 km). (The distribution map of epicenters is available from a website
mentioned below or
a page in
the website of Japan Meteorological Agency [figure with a green
frame, "マグニチュード" means magnitude and "震源の深さ" means depths of focuses].)
Shallow-focus earthquakes occur mainly on the landward side of trenches
(near the boundaries of continental plates) and around active volcanoes
and faults. Shallow-focus earthquakes as well as relatively deep-focus
earthquakes also happen off the western coast of northeastern Honshu and
southwestern coast of Hokkaido. This earthquake zone is along the
boundary between the North American Plate and the Eurasian Plate.
The website of National
Research Institute for Earth Science and Disaster Prevention
provides seismic data. The distribution map of epicenters that occurred
in Japan in the last 30 days is available in
this page of the site.
If there are few epicenters on the map, change the setting of duration
for displaying epicenters; click a button at [期間] to show a pull-down
menu and select [最新30日間]. Moreover, a 3D map of the distribution of
focuses is available in
this page (in
Japanese). To show the map, click [表示] or [ダウンロード] under
a line displaying [3D震源分布の閲覧]. You can control a view point on the 3D
map as moving and rotating the map. You need a VRML plug-in such as
Cortona VRML
Client to see the 3D map.
Accretionary prisms
Accretionary prisms are characteristic sedimentary bodies of subduction zones, formed on the landward side of trench. In Japan, accretionary prisms are developing in the Nankai Trough, and most of the basement is composed of accretionary complexes and metamorphosed accretionary complexes. An accretionary complex is defined as former accretionary prisms, characterized by a mix of ocean-floor basalt, pelagic and hemipelagic sediments and terrigenous sediments (turbidite) with complex structures such as mélange and duplex.
Fig. 8
Oceanic plate stratigraphy []
Oceanic plates are produced by basaltic magma upwelling at mid-ocean ridges and move away from the ridges toward trenches. Magma ejected onto a seafloor forms pillow lavas. In temperate and tropical regions, shells of calcareous nannoplanktons deposit on the seafloor shallower than carbonate compensation depth (CCD) to form limestone. As the plate is more densified with increasing distance from the ridge due to cooling, the seafloor becomes deeper. When the depth of seafloor exceeds CCD, calcareous shells no longer deposit because they are dissolved, and only siliceous shells of radiolarian settle on the seafloor. Chert, therefore, is formed on limestone beds (pelagic sediments). As the plate comes close to the land, mud and tuff from the land deposit on the chert beds to be siliceous shale (hemipelagic sediments). In trenches, clastics including sand and mud flow down to the bottom of trench on the landward slope as turbidity current and deposit (trench-fill sediments). This sedimentation makes alternating beds of sand and mud. Accordingly, oceanic plate stratigraphy, consisting of basalt (pillow lavas), limestone, chert (pelagic sediments), siliceous shale (hemipelagic sediments), and alternating beds of sand and mud (trench-fill sediments) in order from the bottom, is formed on the plate travelling from the mid-ocean ridge to the trench.
 |
 |
| Photo 1
Bedded chert [] |
Photo 2
Turbidite
[] |
Rocks and sediments composing the oceanic plate stratigraphy are scraped off and accreted to the landward plate by subduction of the oceanic plate in the trench (Figure 9). This accretionary process is not simple, different from a way of scraping off sticky dirt with something like a knife or spatula. Trench-fill sediments are deformed by plate movement and split into an accreted part and subducted part. The border between the parts is called decollement (horizontal slip plane). Thrust faults inclining landward sequentially develop from the decollement and form imbricate fans. The sediments of accreted part are scraped off and accreted to the front of accretionary prism; this process is called “off-scraping accretion”. Under the accretionary prisms, the decollement steps down and subducted sediments are accreted to the bottom of the prisms with forming duplex structure. In addition, part of the oceanic crust subducted underneath the prisms is scraped off and added to the prisms. This accretion is defined as “underplating”. Moreover, thrust faults (out of sequence thrusts) cut throughout the accretionary prisms. Accreted materials including basalt, limestone, chert, siliceous shale, and turbidite are sheared and mixed to be fragments of all sizes in this process. These fragments are found in muddy matrix as mélange. Accretionary prisms that cut by larger out of sequence thrusts overlap each other and become thick. Since an accretionary prism is formed under the pre-existing prisms by plate subduction, the lower accretionary prism is younger than the upper one. (See also "Tei mélange and Muroto")
Accretionary prisms are not always formed in trenches. About 40% of trenches all over the world possess developed accretionary prisms. Tectonic erosion is in active rather than accretion in other trenches. In Japan, accretionary prisms are well developed only in the Nankai Trough, and little in the Kuril, the Japan, the Ryukyu, and the Izu-Bonin Trenches. The formation of accretionary prisms requires large volume of sediments in a trench. Therefore, the onset of accretion depends on the convergence rate of plate and the supply rate of sediments because sediments are taken to under the landward plate if the plate subduction rate is too fast. The conditions required for accretion seem to be the thickness of trench-fill sediments of >1 kilometer and/or convergence rate of <7.6 centimeters per year (Clift and Vannucchi, 2004).
