Earthquake of M6.9 Strikes in the East China Sea off Taiwan

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Location of the November 8 2011 Taiwan earthquake (image from USGS)

Location of the November 8 2011 Taiwan earthquake (image from USGS)

A major submarine earthquake which struck in the northwest of the Pacific Ocean on 8 November, 2011, was caused by the movement of the earth’s Pacific plate against the Eurasian continent. The quake took place in the East China Sea, roughly 300 miles east of Taipei and some 700 miles south of Seoul. For simplicity, it is referred to in this article as the Taiwan earthquake.

The Taiwan Earthquake of 8 November, 2011

At a magnitude of 6.9, the November 8 tremor constitutes a significant event, though by no means an unusual one. The earthquake occurred deep in the earth’s crust (at a depth of around 130 miles) and at the time of writing no damage or injuries had been reported. An information bulletin issued by the NOAA indicated that no tsunami was expected as a result of the earthquake.

The Tectonic Setting of the Taiwan Earthquake

Today’s M6.9 earthquake occurred on the Pacific Ring of Fire, a notable belt of seismic activity on the boundary of the Pacific tectonic plate, at a part of the earth’s surface where the jigsaw of plate movements is particularly complex. The event was caused by one of these plates being subducted (forced beneath another).

Put simply, the relatively small Philippine plate is moving approximately WNW against the Eurasian plate. Behind it, the Pacific plate is moving (more rapidly) in the same direction. The Pacific plate is being subducted beneath the Philippine plate in the east, while in the west the Philippine plate is being subducted beneath the Eurasian plate: it is here that the November 8 earthquake occurred.

Earthquakes increase in depth away from plate boundaries (image from USGS)

Earthquakes increase in depth away from plate boundaries (image from USGS)

As one plate is subducted beneath another, the plane of contact between the two dips at an angle (which varies from plate to plate). The zone along which earthquakes occur is called a Wadati-Benioff Zone. The further away from the actual plate boundary an earthquake appears, the deeper its epicentre will be – a pattern clearly demonstrated by plotting the earthquakes and their depth on a graph.

Historic Seismicity and the Honshu Earthquake of 2011

The 8 November Taiwan earthquake was by no means unusual. Seismic data from USGS show that a significant earthquake (M5.0 or greater) is to be expected in this area on an annual basis. Prior to 8 November, the earth had experienced over 20 tremors of M6.9 or greater in 2011 – including an immediate precursor of the major M9.0 event to strike Honshu.

The Taiwan earthquake occurred relatively close to the location of the Honshu tremor of March 2011, though in a different tectonic setting (the Philippine plate peters out to the south east of Japan, and to the north, the Pacific plate is being forced directly below the Eurasian plate). Some scientists speculate that the Honshu earthquake may have increased pressure upon the Philippine plate to the south – though it’s unclear whether the Taiwan earthquake may be regarded as an expression of this increased pressure.

Resources

Black, Richard and Amos, Jonathan. “What chance of a ‘big one’ in Tokyo?” BBC news online. 21 March 2011. Accessed 8 November 2011

NOAA. Pacific Tsunami Warning Centre Bulletin. 8 November 2011. Accessed 8 November 2011.

Open University. Physical and Chemical Properties of the Earth. 2001.

USGS. “Magnitude 6.9: North East of Taiwan.” Accessed 8 November 2011.

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