seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

, an isoseismal map is used to show lines of equally felt seismic intensity, generally measured on the

Modified Mercalli scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

. Such maps help to identify

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

epicenter The epicenter, epicentre () or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Surface damage Before the instrumental pe ...

s, particularly where no

instrumental An instrumental is a recording normally without any vocals, although it might include some inarticulate vocals, such as shouted backup vocals in a big band setting. Through semantic widening, a broader sense of the word song may refer to inst ...

records exist, such as for

historical earthquakes Historical earthquakes is a list of significant earthquakes known to have occurred prior to the beginning of the 20th century. As the events listed here occurred before routine instrumental recordings, they rely mainly on the analysis of writte ...

. They also contain important information on ground conditions at particular locations, the underlying

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...

, radiation pattern of the

seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy ...

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 noninstrumental data. The shape and size of the isoseismal regions can be used to help determine the

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

, focal depth, and focal mechanism of an earthquake.

History

The first known isoseismal map was produced for the 1810 earthquake in Mór in

Hungary Hungary ( hu, Magyarország ) is a landlocked country in Central Europe. Spanning of the Pannonian Basin, Carpathian Basin, it is bordered by Slovakia to the north, Ukraine to the northeast, Romania to the east and southeast, Serbia to the ...

, and published by Kitaibel and Tomtsányi in 1814. The first, six-level intensity scale was proposed by Egen in 1828 for an earthquake in

Rhineland The Rhineland (german: Rheinland; french: Rhénanie; nl, Rijnland; ksh, Rhingland; Latinised name: ''Rhenania'') is a loosely defined area of Western Germany along the Rhine, chiefly its middle section. Term Historically, the Rhinelands ...

Robert Mallet Robert Mallet (3 June 1810 – 5 November 1881) was an Irish geophysicist, civil engineer, and inventor who distinguished himself in research on earthquakes and is sometimes called the father of seismology. His son, Frederick Richard Mallet was ...

coined the term "isoseismal" and produced a map for the

1857 Basilicata earthquake The 1857 Basilicata earthquake (also known as the Great Neapolitan earthquake) occurred on December 16 in the Basilicata region of Italy southeast of the city of Naples. The epicentre was in Montemurro, on the western border of the modern provi ...

with a three-fold intensity scale and used this and other information to identify the epicentral area (a term he also coined). Later studies made use of similar techniques, the main changes being to the actual seismic intensity scale employed.

Methodology

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 generally separate zones of broadly similar felt intensity, while containing areas of both higher and lower degrees of shaking.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 In statistics, originally in geostatistics, kriging or Kriging, also known as Gaussian process regression, is a method of interpolation based on Gaussian process governed by prior covariances. Under suitable assumptions of the prior, kriging giv ...

, rather than relying on visual interpolation.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.

Use

Locating the epicenter

In most earthquakes, the isoseismals define a single clear area of maximum intensity, which is known as the epicentral or meizoseismal area. In some earthquakes, 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 the area that contains the epicenter.

Measuring the magnitude

The magnitude of an earthquake can be estimated by measuring the area affected by intensity level III or above in km² and taking the logarithm. A more accurate estimate relies on the development of regional calibration functions derived using many isoseismal radii. Such approaches allow magnitudes to be estimated for historical earthquakes.

Estimating the focal depth

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.

Confirming the focal mechanism

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

Testing seismic hazard assessments

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.

References

{{Authority control Seismology