Magnitude 7.1 EAST COAST OF JAPANFriday, 25 October, 2013 at 17:10:18 UTC

A magnitude 7.1 earthquake occurred 350 km east of the Japanese coastline, at a depth 26 km (61 miles). This event was likely an aftershock of the March 2011 magnitude 9.0 Tohoku-Oki earthquake.

A tsunami alert was given by the Japanese authorities. But due to the earthquake’s size, only a 40 cm high tsunami was observed at the coast.

Shaking intensity

Since the earthquake occurred around 350 km from the coastline of Japan, no strong shaking was felt.

I. Instrumental

Not felt by many people unless in favourable conditions.

II. Weak Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may swing.

III. Slight Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many to do not recognise it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.

IV. Moderate

Felt indoors by many people, outdoors by a few people during the day. At night, some awakened.

V. Rather Strong

Felt outside by most, may not be felt by some people in non-favourable conditions. Dishes and windows may break and large bells will ring. Vibrations like train passing close to house.

VI. Strong Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.

VII. Very Strong

Difficult to stand; furniture broken; damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by people driving motor cars.

VIII. Destructive

Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved.

IX. Violent General panic; damage considerable in poorly designed structures, well designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Intense Some well build wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent.

XI. Extreme Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Cataclysmic

Total destruction – everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock move position. Landscape altered, or leveled by several meters. In some cases, even the routes of rivers are changed.

Magnitude 7.1 EAST COAST OF JAPANFriday, 25 October, 2013 at 17:10:18 UTC

USGS PAGER alertPopulation Exposed to Earthquake Shaking

Overall, the population in this region resides in structures that are resistant to earthquake shaking, though some vulnerable structures exist.

Recent earthquakes in this area have caused secondary hazards such as tsunamis, landslides, and fires that might have contributed to losses.

Green alert level for shaking-related fatalities.

Green alert level for economic losses. There is a low likelihood of casualties and damage.

Images courtesy of the US Geological Survey

Magnitude 7.1 EAST COAST OF JAPANFriday, 25 October, 2013 at 17:10:18 UTC

Magnitude 7.1 EAST COAST OF JAPANFriday, 25 October, 2013 at 17:10:18 UTC

Earthquake and historic seismicity

The earthquake epicentre (purple dot) is plotted on the map with regional seismicity greater than M6.5 since 1973. This includes aftershocks from the great March 2011 earthquake.

Subduction zones at the Japanese island arcs are geologically complex due to its location at the intersection of four major tectonic plates. The region within 300 km of the October 25 earthquake has hosted more than 40 events of M6.5 or greater. The March 2011 Tohoku earthquake is the largest one with M9.0. Since then, two large events of M7.7 and M7.3 have occurred in the vicinity of the October 25 earthquake. The M7.7 event, on March 11, 2011, was also a normal faulting event occurred 95 km north of the October 25 event. The M7.3 event, on December 7, 2012, was a more complex earthquake resulting from thrust motion near the trench 100 km to the northwest of the October 25 earthquake.

150 km

M7.1 - Off the east coast of Honshu, JapanFriday, October 25 2013 17:10:18 UTC

Tectonic summary

The earthquake occurred as the result of normal faulting in the shallow oceanic crust of the Pacific plate. The Pacific plate moves westwards with respect to the North America plate at a rate of 83 mm/yr before subducting beneath the island of Honshu.

The figure on the left shows the style of faulting (focal mechanism), and the right one shows schematically how blocks associated with the normal faulting move.

Eurasia Plate

North America Plate

Pacific Plate

83 mm/yr

Philippine SeaPlate

6

Aftershock seismicity

So far, 8 aftershocks have been recorded by the USGS. The largest of these was a magnitude 5.5 earthquake, 4 hours after the mainshock.

Most aftershocks appear to have occurred directly to the north and to the south of the mainshock epicentre (large orange circle).

Magnitude 7.1 EAST COAST OF JAPANFriday, 25 October, 2013 at 17:10:18 UTC

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Seismogram recordings by the BGS seismometer network

First body wave arrivals Surface waves

Time of earthquakeIt took just 11 minutes for the first seismic waves to arrive

in the UK!See http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/12/Traveltime_background.pdf for an

explanation of these seismic phases

Magnitude 7.1 EAST COAST OF JAPANFriday, 25 October, 2013 at 17:10:18 UTC

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Seismogram recordings by the UK school seismometer network

See http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/12/Traveltime_background.pdf for an explanation of these seismic phases

Magnitude 7.1 EAST COAST OF JAPANFriday, 25 October, 2013 at 17:10:18 UTC

Surface waves

DEOS, University of Liverpool

STED (St. Edwards School,Poole

Time of earthquake

9

Find out more….

• BGS (British Geological Survey) – seismology and earthquakes – frequently asked questions http://www.earthquakes.bgs.ac.uk/education/faqs/faq_index.html

• IRIS (Incorporated Research Institutions for Seismology) – learning about earthquakes http://www.iris.edu/hq/programs/education_and_outreach/students

• UK School Seismology Project – classroom activities, videos and support documents http://www.bgs.ac.uk/schoolseismology/home.html

• USGS (United States Geological Survey) – FAQs, glossary, posters, animations http://earthquake.usgs.gov/learn/

• USGS summary of the Japan earthquakehttp://earthquake.usgs.gov/earthquakes/eventpage/usc000kn4n#summary

• BBC News report and video from 25th October 2013 http://www.bbc.co.uk/news/world-asia-24677578

Magnitude 7.1 EAST COAST OF JAPANFriday, 25 October, 2013 at 17:10:18 UTC