  | |
| UTC time | 1920-06-05 04:21:35 |
|---|---|
| ISC event | 912519 |
| USGS-ANSS | ComCat |
| Local date | 5 June 1920 |
| Local time | 12:21:35 TT |
| Magnitude | 8.2 Mw 8.0 Ms [1] |
| Depth | 20 km (12 mi) |
| Epicenter | 23°41′17″N121°57′14″E / 23.688°N 121.954°E |
| Areas affected | Hualien County, Taiwan |
| Max. intensity | VII (Very strong) |
| Casualties | 8 dead, 24 injured |
On June 5, 1920, a shallow magnitude 8.2 earthquake struck offshore Hualien County, Empire of Japan (now Taiwan). It is currently the largest earthquake in Taiwan's modern history.
Taiwan is located within a complex zone of convergence between the Philippine and Eurasian Plates. Off the island's east coast, these plates converge at a rate of 75 mm per year. To the south of Taiwan, oceanic crust of the Eurasian Plate is subducting beneath the Philippine Plate, creating an island arc, the Luzon Arc. At Taiwan the oceanic crust has all been subducted and the arc is colliding with continental crust of the Eurasian Plate. To the north of Taiwan the Philippine Sea Plate is in contrast subducting beneath the Eurasian Plate, forming the Ryukyu Arc. [2]
The earthquake had a moment-magnitude of 8.2 according to the International Seismological Centre. Lower-end estimates of the moment magnitude yielded a value of Mw 7.7 ± 0.2, which would still make it the largest in Taiwan. The USGS ShakeMap shows all areas of Taiwan felt shaking of between V and VII on the Modified Mercalli intensity scale. The shaking was also felt in southeast China, the island of Luzon in the Philippines, and southwest Japan. [3]
The source fault responsible for the earthquake has not been conclusively identified, although the epicenter location suggest it was associated with the southern portion of the Ryukyu subduction zone. Based on analyzing stations in Taiwan that recorded the earthquake, the hypocenter was determined to be within the accretionary wedge at 20–35 km (12–22 mi) focal depth. The likely source is a rupture propagating down-dip and laterally along a splay fault associated with the subduction zone. [4]
Eight deaths and 24 injuries were reported. [5] 1,530 houses were affected in the cities of Taipei, Taitung, Hsinchu and Taoyuan, [5] of which 273 were destroyed, and 277 others were severely damaged. [1]
Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.
A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.
The Anatolian Plate is a continental tectonic plate that is separated from the Eurasian plate and the Arabian plate by the North Anatolian Fault and the East Anatolian Fault respectively. Most of the country of Turkey is located on the Anatolian plate. Most significant earthquakes in the region have historically occurred along the northern fault, such as the 1939 Erzincan earthquake. The devastating 2023 Turkey–Syria earthquake occurred along the active East Anatolian fault at a strike slip fault where the Arabian plate is sliding past the Anatolian plate horizontally.
Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthquakes are the planet's most powerful, with moment magnitudes (Mw) that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes.
The Ryukyu Trench, also called Nansei-Shotō Trench, is a 1398 km (868 mi) long oceanic trench located along the southeastern edge of Japan's Ryukyu Islands in the Philippine Sea in the Pacific Ocean, between northeastern Taiwan and southern Japan. The trench has a maximum depth of 7460 m (24,476 ft). The trench is the result of oceanic crust of the Philippine Plate obliquely subducting beneath the continental crust of the Eurasian Plate at a rate of approximately 52 mm/yr. In conjunction with the adjacent Nankai Trough to the northeast, subduction of the Philippine plate has produced 34 volcanoes. The largest earthquake to have been recorded along the Ryukyu Trench, the 1968 Hyūga-nada earthquake, was magnitude 7.5 and occurred along the northernmost part of the trench on 1 April 1968. This earthquake also produced a tsunami.
The Manila Trench is an oceanic trench in the Pacific Ocean, located west of the islands of Luzon and Mindoro in the Philippines. The trench reaches a depth of about 5,400 metres (17,700 ft), in contrast with the average depth of the South China Sea of about 1,500 metres (4,900 ft). It is created by subduction, in which the Sunda Plate is subducting under the Philippine Mobile Belt, producing this almost N-S trending trench. The convergent boundary is terminated to the north by the Taiwan collision zone, and to the south by the Mindoro terrane. It is an area pervaded by negative gravity anomalies.
The islands of Japan are primarily the result of several large ocean movements occurring over hundreds of millions of years from the mid-Silurian to the Pleistocene, as a result of the subduction of the Philippine Sea Plate beneath the continental Amurian Plate and Okinawa Plate to the south, and subduction of the Pacific Plate under the Okhotsk Plate to the north.
In the geology of the Philippines, the Philippine Mobile Belt is a complex portion of the tectonic boundary between the Eurasian Plate and the Philippine Sea Plate, comprising most of the country of the Philippines. It includes two subduction zones, the Manila Trench to the west and the Philippine Trench to the east, as well as the Philippine Fault System. Within the Belt, a number of crustal blocks or microplates which have been shorn off the adjoining major plates are undergoing massive deformation.
The Philippine Fault System is a major inter-related system of geological faults throughout the whole of the Philippine Archipelago, primarily caused by tectonic forces compressing the Philippines into what geophysicists call the Philippine Mobile Belt. Some notable Philippine faults include the Guinayangan, Masbate and Leyte faults.
The island of Taiwan was formed approximately 4 to 5 million years ago at a convergent boundary between the Philippine Sea Plate and the Eurasian Plate. In a boundary running the length of the island and continuing southwards, the Eurasian Plate is sliding under the Philippine Sea Plate. In the northeast of the island, the Philippine Sea Plate slides under the Eurasian Plate. Most of the island comprises a huge fault block tilted to the west.
The 1966 Hualien earthquake occurred on March 13 at 00:31 local time of Taiwan. The epicenter was located in the offshore area between Yonaguni Island, Japan and Hualien, Taiwan.
Subduction polarity reversal is a geologic process in which two converging plates switch roles: The over-lying plate becomes the down-going plate, and vice versa. There are two basic units which make up a subduction zone. This consists of an overriding plate and the subduction plate. Two plates move towards each other due to tectonic forces. The overriding plate will be on the top of the subducting plate. This type of tectonic interaction is found at many plate boundaries.
At 23:50 (UTC+8) on 6 February 2018, an earthquake of magnitude 6.4 on the moment magnitude scale hit Taiwan. The epicenter was on the coastline near Hualien, which was the most severely affected area, with a maximum felt intensity of VIII (Severe) on the Mercalli intensity scale. At least 17 deaths were reported, with 285 injured. The maximum foreshock was recorded on 4 February 2018, at 21:56:40. The epicenter was located at Hualien County, Taiwan, reaching a scale of ML 5.8.
At 14:52 local time on 31 March 2002, an earthquake of magnitude 7.1 on the moment magnitude scale hit Taiwan. The epicenter was offshore from Hualien, which was the most severely affected area with a maximum felt intensity of VII on the Mercalli intensity scale. At least 5 deaths have been reported, with a further 213 injured.
The subduction tectonics of the Philippines is the control of geology over the Philippine archipelago. The Philippine region is seismically active and has been progressively constructed by plates converging towards each other in multiple directions. The region is also known as the Philippine Mobile Belt due to its complex tectonic setting.
The 1879 Surigao earthquake occurred on June 30 at 18:38 02:55 local time on the northeastern tip of Mindanao. The earthquake with a moment magnitude (Mw ) of 7.4 struck with an epicenter just south of Lake Mainit. Extensive damage occurred but there were no reports of casualties.
The East Luzon Trough is an oceanic trench north of the Philippine Trench and east of the island of Luzon. The trench is located near the Philippine orogeny and located in the southeastern region of the Philippine Sea Plate. The depth of the trough is 5,700 meters. The East Luzon Trough formed during the Eocene and Oligocene epoch, 40–24 million years ago.
The 1867 Keelung earthquake occurred off the northern coast of Taiwan on the morning of December 18 with a magnitude of 7.0. It produced strong shaking that seriously damaged the cities of Keelung and Taipei. A tsunami, thought to be the only confirmed destructive of its kind in Taiwan, drowned hundreds and had a run-up exceeding 15 m (49 ft). The total death toll was estimated to be 580 while more than 100 were injured. It was followed by aftershocks that were felt on average ten times a day.
The Ryukyu Arc is an island arc which extends from the south of Kyushu along the Ryukyu Islands to the northeast of Taiwan, spanning about 1,200 kilometres (750 mi). It is located along a section of the convergent plate boundary where the Philippine Sea Plate is subducting northwestward beneath the Eurasian Plate along the Ryukyu Trench. The arc has an overall northeast to southwest trend and is located northwest of the Pacific Ocean and southeast of the East China Sea. It runs parallel to the Okinawa Trough, an active volcanic arc, and the Ryukyu Trench. The Ryukyu Arc, based on its geomorphology, can be segmented from north to south into Northern Ryukyu, Central Ryukyu, and Southern Ryukyu; the Tokara Strait separates Northern Ryukyu and Central Ryukyu at about 130˚E while the Kerama Gap separates Central Ryukyu and Southern Ryukyu at about 127 ˚E. The geological units of the arc include igneous, sedimentary, and metamorphic rocks, ranging from the Paleozoic to Cenozoic in age.