Isoseismal map: Difference between revisions

In [[seismology]], an '''isoseismal map''' is used to show [[Contour line|countour lines]] of equalequally 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-instrumentalnoninstrumental 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"/>

The first known isoseismal map was produced for the 1810 earthquake in [[Mór]] in [[Hungary]], and published by [[Kitaibel]] and [[Tomtsányi]] in 1814.<ref name="Varga">{{cite journal|last=Varga|first=P.|year=2008|title=History of Early Isoseismal Maps|journal=Acta Geodaetica et Geophysica Hungarica|publisher=Akadémiai Kiadó|volume=43|issue=2–3|pages=285–307|doi=10.1556/AGeod.43.2008.2-3.15|urls2cid=http://www.akademiai.com/content/q574r11u7242854h/fulltext.pdf|accessdate=8 September 2011128898064}}</ref> The first, six-level intensity scale was proposed by Egen in 1828 for an earthquake in [[Rhineland]].<ref>{{cite journal |url=http://hess.metapress.com/content/h9v2708334745978/ |lastlast1=Oldroyd |firstfirst1=D. |last2=Amador |first2=F. |last3=Kozak |first3=J. |last4=Carneiro |first4=A. |last5=Pinto |first5=M. |year=2007 |title=The Study of Earthquakes in the Hundred Years Following the Lisbon Earthquake of 1755 |journal=Earth Sciences History |volume=26 |issue=3 |pages=321–370 |url-status=dead |archive-url=https://archive.today/20120711180323/http://hess.metapress.com/content/h9v2708334745978/ |archive-date=2012-07-11 |doi=10.17704/eshi.26.2.h9v2708334745978 |bibcode=2007ESHis..26..321O |url-access=subscription }}</ref><ref>{{cite journal |last=Egen |first=P. N. C. |year=1828 |title={{lang|de|Über das Erdbeben in den Rhein-und Niederlanden vom 23. Februar 1828}} |journal=Annalen der Physik und Chemie |volume=13 |issue= 5|pages=153–163 |doi=10.1002/andp.18280890514 |bibcode=1828AnP....89..153E |url=https://zenodo.org/record/2004894 }}</ref> [[Robert Mallet]] coined the term '"isoseismal'" and produced a map for the [[1857 Basilicata earthquake]] with a threefoldthree-fold intensity scale and used this and other information to identify the epicentral area (a term he also coined).<ref>{{cite book | last author=Robert Mallet | first author-link= Robert | authorlink = Robert Mallet | coauthors = | title = Great Neapolitan Earthquake of 1857: The First Principles of Observational Seismology |as publisherDeveloped =in the Report to the Royal Society |of dateLondon =of 1862the |Expedition locationMade =by Command |of pagesthe =Society Into |the urlInterior =of https://books.google.com/books?hl=en&id=4LcQAAAAIAAJthe |Kingdom doiof =Naples, to |Investigate idthe =Circumstances of |the isbnGreat Earthquake of Demember 1857|url=https://archive.org/details/bub_gb_4LcQAAAAIAAJ|year=1862|publisher=Royal Society}}</ref> Later studies made use of similar techniques, the main changes being to the actual seismic intensity scale employed.

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, 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. Because of 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, 119–134.] {{webarchive|url=https://web.archive.org/web/20100806163045/http://www.geo.arizona.edu/~linkimer/Publications/38-8-Linkimer.pdf |date=2010-08-06 }}</ref><ref name=":0">{{Cite journal|lastlast1=Ambraseys|firstfirst1=N. N.|author-link=Nicholas Ambraseys|last2=Douglas|first2=J.|date=2004-10-01|title=Magnitude calibration of north Indian earthquakes|url=https://academic.oup.com/gji/article/159/1/165/576014/Magnitude-calibration-of-north-Indian-earthquakes|journal=Geophysical Journal International|volume=159|issue=1|pages=165–206|doi=10.1111/j.1365-246X.2004.02323.x|issn=0956-540X|bibcode=2004GeoJI.159..165A|doi-access=free}}</ref>

In most earthquakes, the isoseismals define a single clear area of maximum intensity, which is known as the epicentral or meizoseismal area.<ref name="Ambraseys_&_Melville_2005">{{Cite book |last1=Ambraseys |first1=N.N. |title=A History of Persian Earthquakes |last2=Melville |first2=C.P. |publisher=Cambridge University Press |year=2005 |isbn=9780521021876 |pages=xiii}}</ref> In some earthquakes, there is more than one maximum exists because of the effect of ground conditions or complexities in the rupture propagation, and other information is, therefore, required to identify whichthe area that contains the epicenter.

The magnitude of an earthquakesearthquake can be roughly estimated by measuring the area affected by intensity level III or above in square kilometreskm² and taking the logarithm.<ref name ="Musson"/> A more acurrateaccurate estimate relies on the development of regional calibration functions derived using many isoseismal radii.<ref name=":0" /> Such approaches allow magnitudes to be estimated for historical earthquakes.

The depth to the hypocenter can be estimated by comparing the sizes of different isoseismal areas. In shallow earthquakes, the lines are close together, while in deep events the lines are spread further apart.<ref>[http://www.ias.ac.in/currsci/aug102006/356.pdf 2006. {{citation|last1=Mahajan, |first1=A. K., |last2=Kumar, |first2=N. & |last3=Arora, |first3=B. R. |title=Quick lookLook isoseismalIsoseismal mapMap of 8 October 2005 Kashmir earthquake, Earthquake|url=http://www.ias.ac.in/currsci/aug102006/356.pdf|journal=Current Science, |volume=91, |number=3|year=2006|pages=356–361.]|jstor=24094145}}</ref>

Focal mechanisms are routinely calculated using teleseismic data, but an ambiguity remains as there are always two potential fault planes always are possible. The shape of the areas of highest intensity are generally elongateelongated along the direction of the active fault plane.

Because of the relatively long history of macroseismic intensity observations (sometimes stretching back many centuries in some regions), isoseismal maps can be used to test seismic hazard assessments by comparing the expected temporal frequency of different levels of intensity, assuming an assessment is true and the observed rate of exceedance.<ref>{{Cite book|url=https://link.springer.com/chapter/10.1007/978-3-319-58154-5_8|title=An Overview of the SIGMA Research Project|lastlast1=Pecker|firstfirst1=Alain|last2=Faccioli|first2=Ezio|last3=Gurpinar|first3=Aybars|last4=Martin|first4=Christophe|last5=Renault|first5=Philippe|date=2017|publisher=Springer International Publishing|isbn=9783319581538|series=Geotechnical, Geological and Earthquake Engineering|pages=141–146|language=en|doi=10.1007/978-3-319-58154-5_8}}</ref>