Macroseismic intensity

Macroseismic intensity is the measure of an earthquake's effects at a given place, read off people, objects and buildings, and graded in Roman numerals. An earthquake has one magnitude; it has as many intensities as places it reaches. On March 11, 2011, the Japan Meteorological Agency recorded the top of its scale, shindo 7, at Kurihara in Miyagi Prefecture, well inland. Three scales matter to an underwriter. Modified Mercalli (MMI), published by Wood and Neumann in 1931, runs from I to XII and is still the US reference. The European Macroseismic Scale (EMS-98) also runs from I to XII, but it sorts buildings into vulnerability classes, from A (rubble stone, adobe) to F (high-level earthquake-resistant design), and defines each degree by the share of buildings in each class reaching a given damage grade. Japan has its own scale from 0 to 7, with two levels each for 5 and 6. Before seismographs, intensity was the only measure there was: historical catalogs in France and Italy rebuild old earthquakes from damage described in the archives, then infer a magnitude. USGS ShakeMaps publish, minutes after the shock, an MMI intensity map estimated from stations and attenuation relationships. In the vulnerability module of a cat model, damage curves are expressed either in intensity or in an instrumental measure, peak ground acceleration (PGA) or spectral acceleration; recent models prefer the latter. A practitioner's caveat: intensity mixes hazard and vulnerability. A town built of unreinforced masonry will show one degree more than its well-built neighbor shaken just as hard. Calibrating a vulnerability curve on observed intensity then counts building quality twice.