PLANETARY TREMORS: Major Seismic Uptick Along The Pacific Ring Of Fire – Two Strong 6.2 And 6.1 Earthquakes Hit Southwest Of Yonakuni, Japan; Just Hours After A Powerful 6.8 Rocked Taiwan, Shaking Buildings And Killing One Person; Seismologist Warns Of Stronger Quakes To Come! [PHOTOS + MAPS + TECTONIC SUMMARY]

Earthquake 3D map.


April 20, 2015 – TAIWAN/JAPAN
 The southern parts of Japan’s Okinawa island prefecture were affected by two strong quakes that hit off Taiwan Monday morning.

The first quake was measured at 6.2 magnitude by the China Earthquake Networks Center. The epicenter was located off the east coast of Hualien in Taiwan at 70km southwest of Yonakuni, Japan. It had a depth of 35.6km or 22.1 miles.

USGS shakemap intensity of the 6.2 magnitude quake.

The second quake hit just minutes after, a second tremor initially registered as 6.5 struck the same area at 66km southwest of Yonakuni. The temblor was later revised down to 6.1 by the U.S. Geological Survey.

Several hours earlier, a stronger quake hit the same area. The Japan Meteorological Agency measured the quake at 6.8 magnitude and issued tsunami warnings around Okinawa’s Miyako and Ishigaki islands.

The Japanese agency said that estimated 1-meter (3.2 feet) high tsunami would affect areas around Japan the islands at around 11:30 a.m. local time.

One man died and another was hospitalised Monday after a fire caused by a powerful quake off Taiwan that set buildings shaking in the capital Taipei and sparked a short-lived tsunami warning in far southwestern Japan.

Japanese forecasters had warned the 6.8 magnitude earthquake could cause a tsunami as high as one metre (three feet) affecting several islands in the Okinawa chain. But they lifted the alert around an hour later, with no abnormal waves recorded.
No damage was reported in Japan, but a four-storey apartment building in New Taipei City caught fire after an electrical box outside the block exploded in the quake.
A 65-year-old man who lived in the building “showed no signs of life” at the scene, the fire service said.
Another 18-year-old resident remains in hospital with smoke inhalation but is not in a serious condition, the fire service said.
Residents and office workers were evacuated from a building in central Taipei because of a feared gas leak and vehicles in a nearby multi-storey carpark were overturned, but no one was injured.

Three more quakes rocked the island in the evening. The Seismology Center said one with a magnitude of 5.8 and another at 5.7 — both considered to be aftershocks — shook buildings in Taipei.

A staff member of the Seismology Center points at a graphic showing the earthquake at the central Weather Bureau in Taipei on April 20, 2015. One man died
and another was hospitalized in a fire caused by a powerful quake off Taiwan that also set buildings shaking in the capital Taipei and
sparked a short-lived tsunami warning in far southwestern Japan. AFP PHOTO / Sam Yeh

Another quake with a magnitude of 5.5, with its epicenter 42 km (27 miles) east of the eastern city of Hualien, also jolted the island. This was not seen as an aftershock.

There were no immediate reports of damage.

In Japan, local authorities urged people to move away from the coast and seek higher ground, in a drill that has become fairly regular in a country prone to powerful earthquakes and occasional devastating tsunamis.

“We are issuing warnings via the radio,” Satoshi Shimoji of the Miyako City government told NHK. “We want residents to get as far as possible from the sea.”

Boats were seen sailing out to sea — common practice when a tsunami warning is issued because away from the coast a tsunami is little different from a swell.

However, an hour after the quake, the Japan Meteorological Agency cancelled the warning.

The US Geological Survey said the 6.6 magnitude quake, which Japanese authorities had originally put at 6.8, struck 71 kilometres (44 miles) east of Hualien at 0143 GMT.

Cars are overturned in a carpark in Taipei on April 20, 2015 after an earthquake rocked the Taiwanese capital (AFP Photo/)

In a photo provided by the Taipei Fire Department, vehicles are seen piled on themselves in an automated parking tower after the lift system failed during the 6.8
magnitude earthquake that struck off the island’s eastern coast in Taipei, Taiwan, Monday, April 20, 2015. No injuries resulted in the accident. The date that this
photo was taken, seen in bottom right, is from the camera of the Taipei Fire Department. (Taipei Fire Department via AP)

Workers repair part of a damaged wall on a tower following an earthquake that shook buildings in Taipei on April 20, 2015 (AFP Photo/Sam Yeh)

In Tokyo, Chief Cabinet Secretary Yoshihide Suga told reporters officials were still collecting information, but that the quake did not appear to have done serious damage.

Japan sits at the confluence of four of the earth’s tectonic plates and records more than 20 percent of the planet’s most powerful earthquakes every year.

Strict building codes and a long familiarity with the dangers mean quakes that might cause devastation in other parts of the world are frequently uneventful in Japan.

However, more than 18,000 people were killed by a huge tsunami that smashed into the northeast coast in 2011 after a huge 9.0 magnitude earthquake.

Kuo Kai-wen, chief of Taiwan’s Seismology Centre, warned there could be more quakes on the island.

“This was the third quake measuring more than 6.0 magnitude in Taiwan so far this year — we would not rule out the likelihood that there might be more strong quakes of this scale.” – Yahoo.

Tectonic Summary – Seismotectonics of the Philippine Sea and Vicinity

The Philippine Sea plate is bordered by the larger Pacific and Eurasia plates and the smaller Sunda plate. The Philippine Sea plate is unusual in that its borders are nearly all zones of plate convergence. The Pacific plate is subducted into the mantle, south of Japan, beneath the Izu-Bonin and Mariana island arcs, which extend more than 3,000 km along the eastern margin of the Philippine Sea plate. This subduction zone is characterized by rapid plate convergence and high-level seismicity extending to depths of over 600 km. In spite of this extensive zone of plate convergence, the plate interface has been associated with few great (Magnitude greater than 8.0) ‘megathrust’ earthquakes. This low seismic energy release is thought to result from weak coupling along the plate interface (Scholz and Campos, 1995). These convergent plate margins are also associated with unusual zones of back-arc extension (along with resulting seismic activity) that decouple the volcanic island arcs from the remainder of the Philippine Sea Plate (Karig et al., 1978; Klaus et al., 1992).

South of the Mariana arc, the Pacific plate is subducted beneath the Yap Islands along the Yap trench. The long zone of Pacific plate subduction at the eastern margin of the Philippine Sea Plate is responsible for the generation of the deep Izu-Bonin, Mariana, and Yap trenches as well as parallel chains of islands and volcanoes, typical of circum-pacific island arcs. Similarly, the northwestern margin of the Philippine Sea plate is subducting beneath the Eurasia plate along a convergent zone, extending from southern Honshu to the northeastern coast of Taiwan, manifested by the Ryukyu Islands and the Nansei-Shoto (Ryukyu) trench. The Ryukyu Subduction Zone is associated with a similar zone of back-arc extension, the Okinawa Trough. At Taiwan, the plate boundary is characterized by a zone of arc-continent collision, whereby the northern end of the Luzon island arc is colliding with the buoyant crust of the Eurasia continental margin offshore China.

USGS plate tectonics for the region.

Along its western margin, the Philippine Sea plate is associated with a zone of oblique convergence with the Sunda Plate. This highly active convergent plate boundary extends along both sides the Philippine Islands, from Luzon in the north to the Celebes Islands in the south. The tectonic setting of the Philippines is unusual in several respects: it is characterized by opposite-facing subduction systems on its east and west sides; the archipelago is cut by a major transform fault, the Philippine Fault; and the arc complex itself is marked by active volcanism, faulting, and high seismic activity. Subduction of the Philippine Sea Plate occurs at the eastern margin of the archipelago along the Philippine Trench and its northern extension, the East Luzon Trough. The East Luzon Trough is thought to be an unusual example of a subduction zone in the process of formation, as the Philippine Trench system gradually extends northward (Hamburger et al., 1983). On the west side of Luzon, the Sunda Plate subducts eastward along a series of trenches, including the Manila Trench in the north, the smaller less well-developed Negros Trench in the central Philippines, and the Sulu and Cotabato trenches in the south (Cardwell et al., 1980). At its northern and southern terminations, subduction at the Manila Trench is interrupted by arc-continent collision, between the northern Philippine arc and the Eurasian continental margin at Taiwan and between the Sulu-Borneo Block and Luzon at the island of Mindoro. The Philippine fault, which extends over 1,200 km within the Philippine arc, is seismically active. The fault has been associated with major historical earthquakes, including the destructive M7.6 Luzon earthquake of 1990 (Yoshida and Abe, 1992). A number of other active intra-arc fault systems are associated with high seismic activity, including the Cotabato Fault and the Verde Passage-Sibuyan Sea Fault (Galgana et al., 2007).

Relative plate motion vectors near the Philippines (about 80 mm/yr) is oblique to the plate boundary along the two plate margins of central Luzon, where it is partitioned into orthogonal plate convergence along the trenches and nearly pure translational motion along the Philippine Fault (Barrier et al., 1991). Profiles B and C reveal evidence of opposing inclined seismic zones at intermediate depths (roughly 70-300 km) and complex tectonics at the surface along the Philippine Fault.

Several relevant tectonic elements, plate boundaries and active volcanoes, provide a context for the seismicity presented on the main map. The plate boundaries are most accurate along the axis of the trenches and more diffuse or speculative in the South China Sea and Lesser Sunda Islands. The active volcanic arcs (Siebert and Simkin, 2002) follow the Izu, Volcano, Mariana, and Ryukyu island chains and the main Philippine islands parallel to the Manila, Negros, Cotabato, and Philippine trenches.

Seismic activity along the boundaries of the Philippine Sea Plate (Allen et al., 2009) has produced 7 great (Magnitude greater than 8.0) earthquakes and 250 large (Magnitude greater than 7) events. Among the most destructive events were the 1923 Kanto, the 1948 Fukui and the 1995 Kobe (Japan) earthquakes (99,000, 5,100, and 6,400 casualties, respectively), the 1935 and the 1999 Chi-Chi (Taiwan) earthquakes (3,300 and 2,500 casualties, respectively), and the 1976 M7.6 Moro Gulf and 1990 M7.6 Luzon (Philippines) earthquakes (7,100 and 2,400 casualties, respectively). There have also been a number of tsunami-generating events in the region, including the Moro Gulf earthquake, whose tsunami resulted in more than 5000 deaths.

More information on regional seismicity and tectonics

USGS.

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