Italian earthquake damages historic buildings Source International Business Times
I may have missed it on the news but I understand, via the great service provided by United Stated Geological Survey (“USGS”), that a sizeable 6.0Mw earthquake hit northern Italy (4km ENE of Camposanto) on 20th May. The earthquake occurred at 02:03 GMT (04:03am local time), causing severe shaking in the Emilia Romagna region and surrounding areas.
According to the USGS, around 1.25 million people live in areas impacted by a Modified Mercalli Intensity (“MMI”) of V (as in five: see below for an explanation) or higher.
Damage has been reported across Emilia Romagna, particularly to historic buildings, commercial properties and agriculture in towns and cities close to the earthquake’s epicentre. Seven people have so far been confirmed dead with a further 50 people injured.
The report I read said that around 11,000 people have been displaced, some having seen their homes destroyed and others too afraid to return to their properties. Several historic (and unreinforced) buildings in the region also collapsed and significant industrial and agricultural damage has also been reported.
As explained by the USGS, the effect of an earthquake on the Earth’s surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally—total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one often used is the Modified Mercalli (MM) Intensity Scale.
It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.
The Modified Mercalli Intensity value assigned to a specific site after an earthquake has, according to the USGS a more meaningful measure of severity to the non-scientist than the magnitude because intensity refers to the effects actually experienced at that place. After the occurrence of widely-felt earthquakes, the Geological Survey mails questionnaires to postmasters in the disturbed area requesting the information so that intensity values can be assigned. The results of this postal canvas and information furnished by other sources are used to assign an intensity value, and to compile isoseismal maps that show the extent of various levels of intensity within the felt area. The maximum observed intensity generally occurs near the epicentre.
The following is an abbreviated description of the 12 levels of Modified
I. Not felt, except by a very few under especially favourable conditions
II. Felt, only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration
IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V. Felt by nearly everyone; many awakened, some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
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.
IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rail bent.
XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Railway lines bent greatly.
XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.