Background and Scope - Tohoku, Japan Earthquake and Tsunami of 2011 - page 16

km away from the coastline. The origins for all subduction zone earthquakes in
eastern Japan lie landward (west) of the Japan Trench with focal depths that increase
with increasing distance from the trench (Barnes 2003).
Two main topographies characterize the Tohoku/Northeast Japan's coastal zone. In
the north, the dendritic coastline comprises river valleys that flow in steep drainages.
The coastal inlets, termed rias (or riasu), result from the drowning of a river valley
terminus by subsidence rather than by another valley process such as glaciation. In
this case, the subsidence results from a drawing down of the coast during subduction
zone earthquakes. Sea bed slopes are relatively steep, greater than 1/100 in many
places, with steep beach faces and a narrow offshore shelf.
Much of this ria-type coastline — called the Sanriku (literally “three lands”) Coast —
is a geographic region stretching 600 km from southeast Aomori prefecture to the
Oshika peninsula in Miyagi prefecture. Its name first appeared in 1869 as a collective
term for the three provinces of Rikuō, Rikuchū, and Rikuzen. The area hosts one of
the world's richest fisheries where the cold Ohkotsk current from the North meets the
warm Japan Current from the South. As a result, small fishing villages and resort
towns populate the shoreline. Larger port facilities, usually with populations in the
thousands or tens of thousands, intersperse these small population centers.
South of Sanriku, the dendritic coastline gives way to the flat coastal plains of Sendai
in Miyagi prefecture and of the Hamadori region in Fukushima prefecture. Sendai,
the Tohoku region's largest city with over one million people, comprises a large retail
and service sector, and is home to major port facilities. The sea bottom slope in the
Sendai area varies from around 1/200 to 1/500 with a much broader shelf than in
Sanriku. Onshore slopes are less than 1/100 for large portions of the region with mild
sloped sandy beaches (Takahashi 2011). Further south, in the industrial and
agricultural Fukushima prefecture, the topography varies with plains alternating
between coastal headlands. Beach and offshore slopes, although steeper than those in
Sendai, still range around 1/200 offshore and less than 1/50 in onshore beach areas.
1.4.2 Tectonic Setting
Japan's location on the Pacific Rim rests over a complex tectonic juncture between
several plates and subplates. Figure 6 (adapted from USGS 2011) shows locations
and depths for known earthquakes of magnitude 7.0 or greater near Japan. A series of
trenches run along the western edges of the Pacific and Philippine plates as they
subduct under plates and subplates of the Asian continent. In the South, earthquakes
are associated with the movement of the Philippine plate at about 44 millimeters per
year westward into the Okinawa and Yangtze plates. At the north end of the Nansei-
Shoto Trench, Tokyo lies at the juncture of the Philippine, Amur, Okhotsk, and
Pacific plates. The deadliest earthquake in Japan's history, the 1923 Great Kanto
earthquake, occurred on the Sagami trough at the Philippine-Okhotsk boundary. It
caused over 100,000 deaths. In the North, a zone of high seismic activity runs along
the Japan and Kuril trenches into Siberia at the juncture of the Pacific and Okhotsk
plates.