1. Earthquakes and tsunamis in the last week
Monday, 21st November at 20:59:49 UTC and Thursday, 24th November at 18:43:48 UTC
Magnitude 6.9, Japan and Magnitude 7.0 El Salvador
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.

2. 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
Earthquake locations & overview
Images courtesy of USGS
5 KM
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.

3. Shaking caused 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.

4. Tectonic setting 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 mm/yr causing abundant inter-plate and intra-plate seismicity such as the crustal event seen last week.

5. Focal mechanisms 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
Japan trench
North American plate
(Okhotsk microplate)
Pacific plate
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.

6. Tsunamis Magnitude 6.9, Japan and Magnitude 7.0 El Salvador Japan
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

7. Seismic waves recorded in the UK El Salvador Earthquake (Mw 7.0)
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.
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).
S-wave
Surface waves
P-wave
El Salvador Earthquake (Mw 7.0)
P-wave
S-wave
Surface waves

8. Find out more…. 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
IRIS (Incorporated Research Institutions for Seismology) – learning about earthquakes
UK School Seismology Project – classroom activities, videos and support documents
USGS (United States Geological Survey) – FAQs, glossary, posters, animations
USGS summary of the earthquake
– Nicaragua
– Japan

9. @seismologyUoL