Tuesday, March 22, 2011

Mercalli intensities for the Tohoku earthquake

RMS is a provider of products, services, and expertise for the quantification and management of catastrophe risk. Below is a Mercalli intensity map produced by RMS for the Tohuku earthquake.

The Mercalli intensity map indicates damages to structures and human reactions to the event. Mercalli intensities tend to be highest near the epicenter, and decrease with distance.

[I mis-read my colors, so this paragraph is not true: In this case, the highest intensities of VI are not at the epicenter. This is presumably because of regional geology; unconsolidated rocks amplify the groundshaking, so the intensity pattern depends in part on the geology of the region.]

The Mercalli scale is described by the USGS. The description for Mercalli intensity VI is:
V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned.
VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
Because of the earthquake-resistant building construction in Japan, the groundshaking of the quake did not do much damage. The tsunami was to blame for most of the destruction.

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