Japan is located along the "Ring of Fire" and is home to some of the world's most active and deadly volcanoes. The March 11, 2011 earthquake is a stark reminder that it sits atop where plates collide, making this island extremely vulnerable to geologic hazards. Initial reports estimate that the Japan coastline moved east about 12 feet as a result of the March 11 earthquake.
Subduction Zones and the Japan Trench
The eastern shore of Japan is located immediately west of the Japan Trench. This trench is a result of a subduction zone created by the Pacific plate sinking underneath the North American plate. The northern half of Japan is part of the North American plate and the southern half is part of the Eurasian plate. The Island of Japan is actually cut in half by a major plate boundary where the North American and Eurasian plates meet. Immediately to the south of Japan is the minor Philippine plate. So, very near to Japan, the Pacific plate is sinking underneath both the North American plate and the Philippine plate.
The epicenter of the March 11, 2011 earthquake is slightly to the west of the Japan Trench, so this means that the earthquake originated below the North American plate, where the Pacific plate is pushing its way through on its way down to the mantle. At the latitude of this earthquake, the Pacific plate moves approximately westwards with respect to the North America plate at a velocity of 83 mm/yr. Many earthquakes occur in this area and this subduction zone is also what causes volcanism in Japan.
The Japan Trench has been the location of nine earthquakes of magnitude 7 or greater since 1973. The largest earthquake originating from the Japan Trench, before the March 11, 2011 earthquake, was a magnitude 7.8 earthquake approximately 260 km to the north of the March 11 event, in December 1994, which caused 3 fatalities and almost 700 injuries.
Japan Earthquake Seismic Waves Travel Far and Fast
One of the most deadly consequences of this Japan earthquake has been the resulting tsunamis, which have caused widespread devastation and fatalities. Another type of wave, seismic waves, have also traveled far and fast from the epicenter. The P-wave, or primary wave, travels the fastest and farthest. The S-wave, or secondary wave, travels slower and is responsible for all the destruction. The speed at which P- and S-waves travel, and the strength of those waves, depends on the physical characteristics of the material they are traveling through. So whether the epicenter is located in the upper mantle or crust, makes a difference in the outcome of the earthquake.
Seismic networks located around the world are able to detect the far-reaching P-waves, and this helps to determine the magnitude and location of the earthquake. Initial data suggested that the March 11, 2011 earthquake was a magnitude 8.9, but further analysis of the seismic data resulted in an upgrade to a magnitude 9.0.
Source:
USGS Earthquake Hazards Program - Information for March 11, 2011 Japan Earthquake M 9.0, usgs.gov
Alexandra Matiella Novak - With a PhD in Geology and expertise in science education, Alexandra is passionate about increasing the public's Earth science literacy.
