Seismotectonics is the study of the relationship between the

earthquakes 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 ...

, active

tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

and individual faults of a region. It seeks to understand which faults are responsible for seismic activity in an area by analysing a combination of regional tectonics, recent instrumentally recorded events, accounts of historical earthquakes and

geomorphological Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or n ...

evidence. This information can then be used to quantify the

seismic hazard A seismic hazard is the probability that an earthquake will occur in a given geographic area, within a given window of time, and with ground motion intensity exceeding a given threshold. With a hazard thus estimated, risk can be assessed and inclu ...

of an area.

Methodology

A seismotectonic analysis of an area often involves the integration of disparate datasets.

Regional tectonics

An understanding of the regional tectonics of an area is likely to be derived from published geological maps, research publications on the

geological structure Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informati ...

and

seismic reflection Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic ...

profiles, where available, augmented by other

geophysical Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' so ...

data. In order to understand the seismic hazard of an area it is necessary not only to know where potentially active faults are, but also the orientation of the

stress field A stress field is the distribution of internal forces in a body that balance a given set of external forces. Stress fields are widely used in fluid dynamics and materials science. Consider that one can picture the stress fields as the stress cr ...

. This is normally derived from a combination of earthquake data, borehole breakout analysis, direct stress measurement and the analysis of geologically young fault networks. The World Stress Map Project provides a useful online compilation of such data.

Earthquakes

Instrumentally recorded events

Since the early 20th century, sufficient information has been available from

seismometers A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, Types of volcanic eruptions, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to f ...

to allow the location, depth and magnitude of earthquakes to be calculated. In terms of identifying the fault responsible for an earthquake where there is no clear surface trace, recording the locations of

aftershocks In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousand ...

generally gives a strong indication of the strike of the fault. In the last 30 years, it has been possible to routinely calculate

focal mechanism The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and ...

s from teleseismic data. Catalogues of events with calculated focal mechanisms are now available online, such as the searchable catalogue from the NEIC. As focal mechanisms give two potential active fault plane orientations, other evidence is required to interpret the origin of an individual event. Although only available for a restricted time period, in areas of moderate to intense seismicity there is probably sufficient data to characterise the type of seismicity in an area, if not all the active structures.

Historical records

Attempts to understand the seismicity of an area require information from earthquakes before the era of instrumental recording. This requires a careful assessment of historical data in terms of their reliability. In most cases, all that can be derived is an estimate of the location and magnitude of the event. However, such data is needed to fill the gaps in the instrumental record, particularly in areas with either relatively low seismicity or where the repeat periods for major earthquakes is more than a hundred years.

Field investigations

Information on the timing and magnitude of seismic events that occurred before instrumental recording can be obtained from excavations across faults that are thought to be seismically active and by studying recent sedimentary sequences for evidence of seismic activity such as

seismite {{Earthquakes Seismites are sedimentary beds and structures deformed by seismic shaking. The German paleontologist Adolf Seilacher first used the term in 1969 to describe earthquake-deformed layers. Today, the term is applied to both sedimentar ...

s or

tsunami deposit A tsunami deposit (the term tsunamiite is also sometimes used) is a sedimentary unit deposited as the result of a tsunami. Such deposits may be left onshore during the inundation phase or offshore during the 'backwash' phase. Such deposits are u ...

Geomorphology

Seismically

active fault An active fault is a fault that is likely to become the source of another earthquake sometime in the future. Geologists commonly consider faults to be active if there has been movement observed or evidence of seismic activity during the last 10, ...

s and related fault generated

folds Benjamin Scott Folds (born September 12, 1966) is an American singer-songwriter, musician, and composer, who is the first artistic advisor to the National Symphony Orchestra at the John F. Kennedy Center for the Performing Arts, Kennedy Center in ...

have a direct effect on the geomorphology of a region. This may allow the direct identification of active structures not previously known. In some cases such observations can be used quantitatively to constrain the repeat period of major earthquakes, such as the

raised beach A raised beach, coastal terrace,Pinter, N (2010): 'Coastal Terraces, Sealevel, and Active Tectonics' (educational exercise), from 2/04/2011/ref> or perched coastline is a relatively flat, horizontal or gently inclined surface of marine origin, ...

es of

Turakirae Head Turakirae Head is a promontory on the southern coast of New Zealand's North Island The North Island, also officially named Te Ika-a-Māui, is one of the two main islands of New Zealand, separated from the larger but much less populous South ...

recording the history of coseismic uplift of the

Rimutaka Range The Remutaka Range (spelled Rimutaka Range before 2017) is the southernmost range of a mountain chain in the lower North Island of New Zealand. The chain continues north into the Tararua, then Ruahine Ranges, running parallel with the east c ...

due to displacement on the

Wairarapa Fault The Wairarapa Fault is an active seismic fault in the southern part of the North Island of New Zealand. It is a dextral (right lateral) strike-slip fault with a component of uplift to the northwest as expressed by the Rimutaka Range. It forms par ...

North Island The North Island, also officially named Te Ika-a-Māui, is one of the two main islands of New Zealand, separated from the larger but much less populous South Island by the Cook Strait. The island's area is , making it the world's 14th-larges ...

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 List of islands of New Zealand, smaller islands. It is the ...

.McSaveney, M.J., Graham, I.J., Begg, J.G., Beu, A.G., Hull, A.G., Kyeong, K. & Zondervan, A. 2006. Late Holocene uplift of beach ridges at Turakirae Head, south Wellington coast, New Zealand. New Zealand Journal of Geology and Geophysics abstracts, 49, 337–358

References

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External links

Presentation on Seismotectonics from Department of Geophysics, University of Munich
Seismology Tectonics Structural geology