Magnitude 7.1 EAST COAST OF JAPAN Friday, 25 October, 2013 at

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Magnitude 7.1 EAST COAST OF JAPAN
Friday, 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.
Magnitude 7.1 EAST COAST OF JAPAN
Friday, 25 October, 2013 at 17:10:18 UTC
Since the earthquake occurred around 350 km from the coastline of
Japan, no strong shaking was felt.
Shaking intensity
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 JAPAN
Friday, 25 October, 2013 at 17:10:18 UTC
USGS PAGER alert
Population 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 JAPAN
Friday, 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, Japan
Friday, October 25 2013 17:10:18 UTC
North America
Plate
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.
Eurasia Plate
83 mm/yr
Pacific Plate
Philippine Sea
Plate
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.
Magnitude 7.1 EAST COAST OF JAPAN
Friday, 25 October, 2013 at 17:10:18 UTC
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).
6
Magnitude 7.1 EAST COAST OF JAPAN
Friday, 25 October, 2013 at 17:10:18 UTC
Seismogram recordings by the BGS seismometer network
First body wave arrivals
Surface waves
Time of
earthquake
It 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 7
these seismic phases
Magnitude 7.1 EAST COAST OF JAPAN
Friday, 25 October, 2013 at 17:10:18 UTC
Seismogram recordings by the UK school seismometer
network
Surface waves
DEOS, University of
Liverpool
STED
(St.
Edwards
School,
Poole
Time of
earthquake
See http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/12/Traveltime_background.pdf for an explanation of 8
these seismic phases
Magnitude 7.1 EAST COAST OF JAPAN
Friday, 25 October, 2013 at 17:10:18 UTC
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
9
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