Background and Scope - Tohoku, Japan Earthquake and Tsunami of 2011 - page 5

1.3.2TimeandShakingEffectsofSeismicEvent
Following a strong foreshock of magnitude 7.3 at 11:45 local time on March 9, 2011
the earthquake motion of the main shock began at 14:46 local time on March 11,
2011 (5:46 UTC, Coordinated Universal Time). Figure 1 shows the reported shaking
intensities in Central and Northern Honshu as well as in Hokkaido. Within the figure,
a star indicates the epicenter of the main shock with the Japan Trench visible at the
right. Shaking intensities correspond to values on the Modified Mercalli Intensity
(MMI) scale, which evaluates the intensity of an earthquake in terms of its effects on
people and structures. On the MMI scale, an intensity of I represents no observable
consequences from an event; an intensity of XII implies complete catastrophic
damage. Table 1 provides an abbreviated description of this intensity gradation.
Figure 1. Shaking intensity (Modified Mercalli Index, MMI) and affected coastal
areas from the March 11, 2011 Great East Japan earthquake and tsunami. (Adapted
from NASA Earth Observatory 2011)
As one would expect from a very large magnitude earthquake, the intense shaking
occurred over a several hundred kilometer (km) radius. Shaking intensities in excess
of IV extended over 500 km southwest and over 350 km north of the earthquake
epicenter. People in parts of the Miyagi prefecture close to the earthquake source felt