Japan (Tohoku) M 9.0 Subduction Earthquake March 2011
John Elliott, Richard Walters, Barry Parsons
On the 11th March, the largest earthquake to have occurred in Japan’s more than a century-long instrumental record, occurred 100 km off the east coast of Honshu (Figure 1). The earthquake was generated from the slip on a fault where the plate, on which Japan lies, moved suddenly over the Pacific Oceanic plate. The near instantaneous uplift of the seabed in the earthquake generated a tsunami which struck the island of Japan within tens of minutes and spread right across the Pacific Ocean in the following hours.
This event is now listed as the fourth largest earthquake to be recorded in over a century (Table 1). These very large earthquakes and associated tsunami occur when one oceanic plate moves suddenly beneath another plate by tens of metres along a region hundreds of kilometres long. Such earthquakes occur at places where the surface plates sink back into the earth at deep-sea trenches like around the Pacific Ocean – the “ring of fire” – as well as in the Indian Ocean, the location of the 2004 Sumatra earthquake and tsunami.
The Pacific plate moves westward towards Japan at a rate of about 9 cm per year (Figure 1). However, at the edge of this plate, the motion is locked by friction at the subduction zone as the Pacific plate is pulled beneath Japan. This locking along the subduction zone as the plates continue to move relative to one another bends the overlying plate, building up stress on the fault until eventually it is large to produce movement on the fault resulting in an earthquake (and potentially a tsunami if the motion on the fault extend to close to the seafloor).
The aftershocks in the days after the main earthquake were typically 1000 times smaller, but numbered many hundreds (Figure 2). They stretch down the east coast of Japan for over 500 km, indicating the likely region of slip in the earthquake.
Using the distribution of slip on the fault as calculated by the USGS we have determined the regions of the subduction fault that have been brought closer to failure (Figure 3). Whilst it is not possible to predict when the next earthquake will strike the region, the analysis shows that the portion of the subduction fault east of Tokyo (indicated by the red patches in Figure 3) has now been brought closer to failure (as well as the portion of fault to the north).
Table 1: Globally largest 5 earthquakes recorded since 1900 (source USGS).
Figure 1: Location map of the 2011 Japan (Tohoku) M 9.0 subduction Earthquake with the approximate fault outline shown in pink. Previous large earthquakes in the epicentral region are shown in yellow and occurred in 1896 and 1933. The Pacific plate is subducting beneath Japan at about a rate of 9 cm per year. Plate boundaries are marked by red lines. Volcanoes are shown as red triangles. Cities with populations greater than 250,000 are shown as red squares.
Figure 2: Aftershock distribution in the seven days following the mainshock with circles indicating earthquake location and size. Magnitude 9.0 earthquakes release 1,000 times more energy than magnitude 7.0 events. Plate boundaries are marked by red lines. Volcanoes are shown as red triangles. Cities with populations greater than 250,000 are shown as red squares.
Figure 3: Changes in stress along the subduction fault as a result of the slip in the mainshock based upon the USGS finite fault solution (http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/finite_fault.php). The areas shown in blue are portions of the subduction zone that have seen a reduction in the stress on the fault bringing it further away from failure as a result of the recent earthquake. However, regions at the two ends of the fault rupture shown in red have experienced an increase in stress that has now been pushed slightly closer to failure. Plate boundaries are marked by red lines. Volcanoes are shown as red triangles. Cities with populations greater than 250,000 are shown as red squares.
For a more detailed description of the earthquake please download the COMET briefing paper here.