Background and Scope - Tohoku, Japan Earthquake and Tsunami of 2011 - page 6

the highest intensities of VIII and IX; however, intense shaking also occurred over a
long distance southward. Shaking values up to V occurred in areas south of Tokyo.
Within the Tokyo metropolitan area, intensities ranged from V to VII. The Sendai
area experienced an average intensity of VII followed by mass evacuations from
buildings and varying amounts of structural damage, including substantial damage to
tsunami research facilities at Tohoku University. At the town of Fukushima, the
reported shaking intensity reached a level of VIII (USGS 2011).

Table 1. Summary descriptions of Modified Mercalli Intensity levels. (Adapted from
Richter, C.F., 1958. Elementary Seismology . W.H. Freeman and Company, San
Francisco, pp. 135-149; 650-653.)
Modified Mercalli Scale (summarized)

I Detected only by sensitive instruments

Felt by few persons at rest usually on upper floors, suspended objects may
swing

II

Felt noticeably indoors, but not always recognized as earthquake, vibration like
passing truck

III

Felt indoors by many, outdoors by a few; dishes windows, doors disturbed;
autos rock

IV

Felt by most people; breakage of dishes, windows, plaster; disturbance of tall
objects

V

Felt by all, fright and evacuation outdoors; falling plaster and chimneys, damage
small

VI

Everybody runs outdoors, damage to buildings variable depending on
construction

VII

Fall of walls, monuments, chimneys; sand and mud ejected, drivers of autos
disturbed

VIII

Buildings leaning, cracked, shifted from foundations; ground cracked,
underground pipes broken

IX

X Most masonry and frame structures destroyed, rails bent, landslides

XI Few structures left standing, ground fissures

XII Damage is total, waves seen on ground

Seismograms throughout Japan recorded the magnitude and duration of the
earthquake. In Tokyo, 390 km from the source, shaking started about 90 seconds after
the initial rupture and lasted for approximately 6 minutes with a peak acceleration of
0.16 g (TKY026-EW, g = acceleration of gravity). About 30 seconds after the start of
the main shock, Japan's Earthquake Early Warning system issued a warning to Tokyo
area residents who then had about a minute to seek shelter. Figure 2 shows an
accelerometer record from Miyagi Prefecture, located 180 km from the epicenter. In
this figure, the abscissa shows the time elapsed from the start of the main shock at the