Background and Scope - Tohoku, Japan Earthquake and Tsunami of 2011 - page 19

In the case of the 2004 Sumatra earthquake, the USGS calculated surface energy (1.1
x 10 17 joules) was less than a quarter of the USGS Great East Japan value. As a
further comparison, the 2010 Maule Chile earthquake, with a seismic moment about
40% of the Great East Japan event, had a USGS-calculated surface energy of 2 x 10 16
joules, less than one twentieth of that calculated for Japan 2011. This data suggest the
Great East Japan earthquake, based on data from previous large earthquakes,
produced significantly higher surface energy release than expected.
The last column of Table 5 shows the values for tsunami magnitude, Mt, developed
by Abe (1979). Abe first used wave height and distance from the wave source to
obtain an event scale analogous to that used for earthquakes. Abe then applied that
event scale to evaluate the magnitude of tsunamis. Taken together with seismic
moment, the value of Mt measures the efficiency of the source as a generator of
tsunami waves. An earthquake with a much smaller moment magnitude than the
observed tsunami, where proportionately more of the energy goes into wave
generation, has been termed a tsunami-earthquake (Kanamori 1972). Kanamori
suggests that for certain earthquakes, the size of the generated tsunami far exceeds the
earthquake's size as measured by the surface wave magnitude, Ms. He cites two such
events, the 1946 Aleutian Islands tsunami and the 1896 Meiji Sanriku tsunami. The
1896 Meiji Sanriku tsunami, discussed in Section 1.5.1, occurred in the same region
as the 2011 Great East Japan event. More recent events have also met the criteria for
tsunami-earthquakes including tsunamis of 1992 in Nicaragua and 1996 in Java. For
the Great East Japan tsunami, the computed value of Mt = 9.1 agrees fairly well with
the moment magnitude Mw of 9.0. A great deal of the concentrated energy from this
event, however, occurred at shallow depths and at low frequencies (Simons 2011).
The discussion remains open on whether this was or was not a tsunami-earthquake.
1.4.4GreatEastJapanEarthquakeSourceandSlipDistribution
Several significant foreshocks preceded the March 11 Great East Japan earthquake.
On March 9, three additional shocks with magnitude greater than 6.0 accompanied a
7.3 magnitude event. The large 7.3 foreshock occurred about 40 kilometers from the
March 11 mainshock at a moderate depth of 32 kilometers. Between the Great East
Japan event, and July 31, 2011, the Japanese Meteorological Agency (JMA, 2011)
has reported at least 500 events greater than Mw = 5.0; 80 events greater than 6.0;
and four aftershocks of over 7.0 in magnitude. Figure 7 shows the temporal and
spatial distribution of the fore and aftershock distribution from March 8 th through the
16 th with a red line showing the location of the Japan Trench off the Japanese coast.
In addition to many smaller events, two additional large aftershocks greater than
magnitude 7.0 occurred on April 7 and July 10. These aftershocks defined the rupture
zone approximately 500 kilometers along the trench axis (strike axis) and 200
kilometers perpendicular to the trench (down-dip direction). As expected with a
subduction zone earthquake, the focal depth for the aftershocks increases with
distance from the trench. The epicenter of the main shock lies off the Oshika
peninsula.