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.

Question on Irish earthquake

A question was posted to an old post on the largest earthquake near Ireland, about where it was felt.

So, here is the map to answer that question, from the British Geological Survey:

Read about the European intensity scale numbers here.

Chile earthquake intensities

Below is another clip from the USGS Chile earthquake poster showing the Modified Mercalli intensities from the recent 8.8 magnitude quake. Recall that magnitude is a seismograph-based estimate of the energy released by a quake and will be more or less the same for all seismic stations that record the waves. Intensities are evaluated according to human reactions and effects on buildings, are highest near the epicenter, and fall off with distance. I was surprised to see that Santiago has almost 5 million residents, so lots of people that are at risk.  The intensity in Santiago was 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") on a scale of I-XII.

 

This map of the intensity data is a little easier to read:

No doubt about it, this was an intense event.

Jones Geological Services and Dillsburg

Jeri Jones of Jones Geological Services is often mentioned in media accounts of the Dillsburg earthquakes. His company's website has a piece on the Dillsburg series of small events on its web page, mostly focused on the activity last Fall. The isoseismal map contours values of Modified Mercalli intensity from the magnitude 2.1 earthquake of October 19, 2008.

Jones poses the following question:

What makes this swarm of shallow earthquakes interesting to investigate?

1. The number of tremors felt over the last ten weeks
2. The regular “boom” that accompanies many of these tremors
3. A sulfur smell associated with the larger tremors and sometimes within the groundwater

How intense is that? Lancaster earthquake intensities

Notice now that the Lancaster quake NEIC report now lists it as magnitude 3.4 . Re-evaluation of the data?

An earthquake magnitude is an objective measure of the energy released. It is based on seismograph readings, and is a single number that characterizes the size of the earthquake. The first methods for determining magnitudes were derived by seismologist Charles Richter, so although different algorithms are now used, these are still often called Richter magnitudes.

Intensities are another way of indicating the nature of seismic ground motion in a particular area, and the response of people and structures to that ground motion. It is somewhat subjective; intensities vary from being highest near the epicenter, to decreasing values at greater distances. The Modified Mercalli Intensity Scale is the one in common usage today.

click above for larger image
from http://neic.usgs.gov/neis/bulletin/neic_bcah.html

Notice the intensities for the Lancaster quske as shown above are all Roman numerals II and III - compare these to the description above for the Modified Mercalli Scale.