Download Magnitude 6.9, Japan and Magnitude 7.0 El Salvador Monday, 21 st

Earthquakes and tsunamis in the last week
Magnitude 6.9, Japan and Magnitude 7.0 El Salvador
Monday, 21st November at 20:59:49 UTC and Thursday, 24th November
at 18:43:48 UTC
Within 3 days two tsunamigenic
earthquakes with similar magnitudes
occurred in Japan and El Salvador.
Both events, plotted with green stars,
were generated in subduction zones
that are part of “The Pacific ring of
fire”.
The Ring of Fire is a major area in the
basin of the Pacific Ocean where a
large number of earthquakes and
volcanic eruptions occur, because of
the subdcution of oceanic crust.
Magnitude 6.9, Japan and Magnitude 7.0 El Salvador
Monday, 21st November at 20:59:49 UTC and Thursday, 24th November
at 18:43:48 UTC
Earthquake locations & overview
A magnitude 6.9 (Mw) earthquake
struck the coast of central Japan
(near Fukushima), with depth 11.4
km. Within a week, a similar
magnitude (Mw 7.0) earthquake
struck off the coast of El Salvador,
depth 10.3 km.
Shortly after each main shock
tsunami warnings were issued to
those on the closest coastal regions,
were tsunamis hit shortly after
(within 2 hours) of each event.
5 KM
Images courtesy of USGS
Magnitude 6.9, Japan and Magnitude 7.0 El Salvador
Monday, 21st November at 20:59:49 UTC and Thursday, 24th November
at 18:43:48 UTC
Shaking caused
There was Moderate (V) to Strong (VIII) shaking experienced after during
the Japanese Earthquake (left) and Moderate (V) shaking on the Nicaraguan
coast. Both events show relatively weak motion by the time the seismic
energy had propagated to land.
Magnitude 6.9, Japan and Magnitude 7.0 El Salvador
Monday, 21st November at 20:59:49 UTC and Thursday, 24th November
at 18:43:48 UTC
Tectonic setting
Both Japan and El Salvador earthquakes are related with plate
subduction at very complicated tectonic areas, involving several major
plates and leading to high rate of seismicity and a active volcanoes.
In the Salvadorian area, the Cocos and
Caribbean plate create a convergent
margin with rates between 72 to 81
mm/yr, leading to seismicity of all types
including large extensional event like
the occurred on the 24th November.
In the Japanese area, the Pacific plate
subducts beneath the North American
plate at rates of 83-90 mm/yr causing
abundant inter-plate and intra-plate
seismicity such as the crustal event
seen last week.
Magnitude 6.9, Japan and Magnitude 7.0 El Salvador
Monday, 21st November at 20:59:49 UTC and Thursday, 24th November
at 18:43:48 UTC
Focal mechanisms
Both events have extensive mechanism but different causes. The Japan earthquake occurred
on a normal fault in the overriding plate, that accommodates the extension due to the role
back of the subducting pacific plate. The El Salvador event was generated by an extensional
fault created due to the bending of the plate at the outer-rise, where the plate bend into the
subduction zone.
Magnitude 6.9, Japan and Magnitude 7.0 El Salvador
Monday, 21st November at 20:59:49 UTC and Thursday, 24th November
at 18:43:48 UTC
Tsunamis
Below are plots of the tsunami travel times for the respective
events. The tsunami as it hit the Japanese coast was relatively
small and had waves of up to 1.4 m in some coastal regions. In El
Salvador waves reached up to about 1 m as it hit the coast.
Japan
El Salvador
Magnitude 6.9, Japan and Magnitude 7.0 El Salvador
Monday, 21st November at 20:59:49 UTC and Thursday, 24th November
at 18:43:48 UTC
Seismic waves recorded in the UK
Japan Earthquake (Mw 6.9)
Seismic waves from both of
these earthquakes were
recorded in the UK. The figure on
the write shows the arrivals from
the Japan earthquake (top) and
Nicaraguan earthquake (below)
recorded at a seismic station in
the Scottish borders.
S-wave
El Salvador Earthquake (Mw 7.0)
P-wave
Due to the shallow depth of both
of these earthquakes, the
surface waves have a much
greater amplitude than the body
waves (i.e. P and S waves).
Surface waves
P-wave
S-wave
Surface waves
Magnitude 6.9, Japan and Magnitude 7.0 El Salvador
Monday, 21st November at 20:59:49 UTC and Thursday, 24th November
at 18:43:48 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 earthquake
http://earthquake.usgs.gov/earthquakes/eventpage/us10007c4w#executive – Nicaragua
http://earthquake.usgs.gov/earthquakes/eventpage/us10007b88#executive – Japan
@seismologyUoL
[email protected]