Isoseismal map: Difference between revisions

In [[seismology]] an '''Isoseismalisoseismal map''' is used to show lines of equal felt seismic intensity, generally measured on the [[Modified Mercalli scale]]. Such maps help to identify [[earthquake]] [[epicenter]]s, particularly where no [[seismometer|instrumental]] records exist, such as for [[historical earthquakes]]. They also contain important information on ground conditions at particular locations, the underlying [[geology]], radiation pattern of the [[seismic wave]]s and the response of different types of buildings. They form an important part of the macroseismic approach, i.e. that part of seismology dealing with non-instrumental data. The shape and size of the isoseismal regions can be used to help determine the [[Seismic scale#Magnitude scales|magnitude]], [[Focus (earthquake)|focal]] depth and [[focal mechanism]] of an earthquake.<ref name ="Musson"/><ref name ="Linkimer"/>

Firstly observations of the felt intensity need to be obtained for all areas affected by the tremor. In the case of recent earthquakes news reports are augmented by sending out questionnaires or by collecting information online about the intensity of the shaking. For a historical earthquake the procedure is much the same, except that it requires searching through contemporary accounts in newspapers, letters and diaries etc. Once the information has been assembled and intensities assigned at the location of the individual observations, these are plotted on a map. Isoseismal lines are then drawn to link together areas of equal shaking. Due to local variations in the ground conditions isoseismals will generally separate zones of broadly similar felt intensity, while containing areas of both higher and lower degrees of shaking.<ref name ="Musson">[http://nora.nerc.ac.uk/7165/1/HOW_TO_MAP_AN_EARTHQUAKE__2008_.pdf How to map an earthquake, by Roger Musson, BGS]</ref> To make the isoseismals less subjective, attempts have been made to use computer-based methods of contouring such as [[kriging]], rather than relying on visual [[interpolation]].<ref name ="Linkimer">[http://www.geo.arizona.edu/~linkimer/Publications/38-8-Linkimer.pdf Linkimer, L. 2008. Application of the kriging method to draw isoseismal maps of the significant 2002-2003 Costa Rican earthquakes. Revista Geológica de América Central, 38, 119-134119–134.]</ref>