The , also referred to as the Shionihara Fault, is an active
earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
fault system located in
Fukushima Prefecture
Fukushima Prefecture (; ja, 福島県, Fukushima-ken, ) is a prefecture of Japan located in the Tōhoku region of Honshu. Fukushima Prefecture has a population of 1,810,286 () and has a geographic area of . Fukushima Prefecture borders Miya ...
of
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
, to the west of
Iwaki city
is a city located in Fukushima Prefecture, Japan. , Iwaki had a population of 337,765 in 143,500 households, and population density of 270 persons per km2. The total area of the city is , making it the largest city in the prefecture and the 10th ...
. It mainly consists of a trace of three separate
striations.
Structure
The fault was first mapped by the Active Fault Research Group in 1991 as a complex of north-northwest-striking inactive traces of fault in the
Hamadōri
is the easternmost of the three regions of Fukushima Prefecture, Japan, the other two being Nakadōri in the central area of the prefecture and Aizu in the west. Hamadōri is bordered by the Abukuma Highlands to the west and the Pacific Ocean t ...
region. It has since been compartmentalized into separate
striations near Tabito-cho west of
Iwaki city. The northernmost and largest of the faultlines, the North Fault, was identified in 2009 and extends roughly 24 km (15 mi) from the southeast to the northwest (N45˚W). To its southwest, two parallel faultlines, the East and Shionihara faults, extend from the south-southeast to the north-northwest (N10˚W). The faultlines are separated by 1 km (0.6 mi) and span roughly 23 km and 22 km (14 mi), respectively. The westernmost of the two, the Shionihara Fault lies near
Tabito-cho and borders the small village of Shionohira, after which it was named.
The main structural trend is north-northwest–south-southeast, with sinking observed only to the south on the east side of the fault.
Metamorphic rock
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
and
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
, as well as
granite
Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...
and
epidiorite
Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky ...
are distributed in the region; the fault is described as a limit to the distribution of
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...
from the
Neogene Period.
Yunodake Fault
To the northeast of the Idosawa Fault complex lies a separate normal fault trace, which was named the Yunodake Fault (also Yunotake) in 2011. Distanced approx. 50 km (30 mi) from the
Fukushima Daiichi Nuclear Power Plant, the fault had been dormant for 120,000–130,000 before it ruptured during the
magnitude 7.1 Mw Fukushima Hamadori earthquake on 11 April 2011.
[Press Conference Handouts Archive: November 2011]
/ref> Several geological surveys have since been conducted in its vicinity.
/ref> Evidence of sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
layers deposited after the Late Pleistocene
The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division of ...
beneath the fault suggests that the Yunotake Fault had been in the active in the past.
Notable earthquakes
* March 19, 2011: A magnitude 6.1 earthquake struck near Ibaraki Prefecture
is a Prefectures of Japan, prefecture of Japan located in the Kantō region of Honshu. Ibaraki Prefecture has a population of 2,871,199 (1 June 2019) and has a geographic area of . Ibaraki Prefecture borders Fukushima Prefecture to the north, ...
, with its hypocenter
In seismology, a hypocenter or hypocentre () is the point of origin of an earthquake or a subsurface nuclear explosion. A synonym is the focus of an earthquake.
Earthquakes
An earthquake's hypocenter is the position where the strain energy s ...
located near the Idosawa Fault. The earthquake occurred as a result of normal-faulting.Ishikawa, Fukushima
file:Fukushimaprefectural road No.40.JPG, 270px, downtown Ishikawa
is a Towns of Japan, town located in Fukushima Prefecture, Japan. , the town had an estimated population of 15,511 in 5690 households, and a population density of 130 persons per k ...
. The quake occurred along the Shionihira Fault and caused a surface rupture of approx. 7 km (4 mi). In addition, a vertical displacement of 0.8–1.8 m was observed.
See also
* Strike-slip tectonics
* Structural geology
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informatio ...
References
{{East Asia plates
Seismic faults of Japan