Background and Scope - Tohoku, Japan Earthquake and Tsunami of 2011 - page 17

Most of the seismic activity in Japan occurs in a region near the trenches and above
70 kilometers in depth, indicated by the red colored circles in Figure 6. More
infrequent, deeper earthquakes occur along the edges of the subduction zones at
progressively deeper depths indicated by green (70 - 200 km) and blue (below 300
km) circles. The inset at the upper left of Figure 6 shows the distribution of seismic
activity in transect A-A' across the epicenter of the Great East Japan event. This
graphic shows earthquakes greater than magnitude 3.0 that have occurred since 1990.
It displays a cluster of shallow earthquakes (in red) with both crustal and subduction
zone origins, and a pattern of deeper subduction zone events (in green). The Great
East Japan earthquake lies in the shallow part of the subduction zone. Depending on
their magnitude, shallow events can cause the most damage at the earth's surface.
Figure 6. Historical seismicity of Japan - 1900 to present, events magnitude 7.0 and
larger; depth profile of earthquakes of magnitude greater than 3 since 1990 (Adapted
from Rhea et al, USGS 2011)
The Japan Trench subduction zone has generated nine events of magnitude 7 or
greater since 1970, excluding the six events associated with the Great East Japan
earthquake. All nine shallow subduction zone earthquakes had focal depths between
10 and 68 km. In this recent period, a 7.7 magnitude event in 1978 in Sanriku resulted
in 23 deaths, 10,000 injuries, and over $800 million in damage. In 1994, a 7.8