Seismotectonics
Encyclopedia
Seismotectonics is the study of the relationship between the earthquakes, active tectonics
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
evidence. This information can then be used to quantify the seismic hazard
of an area.
and seismic reflection profiles, where available, augmented by other geophysical
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
. 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.
In the last 30 years, it has been possible to routinely calculate focal mechanism
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.
s or tsunami deposit
s.
s and related fault generated folds
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
es of Turakirae Head recording the history of coseismic uplift of the Rimutaka Range
due to displacement on the Wairarapa Fault
in North Island
, New Zealand
.
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
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
Geomorphology is the scientific study of landforms and the processes that shape them...
evidence. This information can then be used to quantify the seismic hazard
Seismic hazard
Seismic hazard refers to the study of expected earthquake ground motions at the earth's surface, and its likely effects on existing natural conditions and man-made structures for public safety considerations; the results of such studies are published as seismic hazard maps, which identify the...
of an area.
Methodology
A seismotectonic analysis of an area requires the integration of a large number 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 structureStructural geology
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 information about the history of deformation in the rocks, and...
and seismic reflection profiles, where available, augmented by other geophysical
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
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
Stress field
A stress field is a region in a body for which the stress is defined at every point. Stress fields are widely used in fluid dynamics and materials science....
. 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.
Instrumentally recorded events
Since the early 20th century, sufficient information has been available from seismometers 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 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
Focal mechanism
The focal mechanism of an earthquake describes the inelastic 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 is also known as a fault-plane solution...
s from teleseismic data. Catalogues of events with calculated focal mechanisms are now available online, such as the searchable catalogue from the NEIC
National Earthquake Information Center
The National Earthquake Information Center is part of the United States Geological Survey located on the campus of the Colorado School of Mines in Golden, Colorado. The NEIC has three main missions:...
. 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 long-term 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 seismiteSeismite
Seismites are sedimentary beds disturbed by seismic shaking. The German paleontologist Adolf Seilacher first used the term in 1969 to describe a variety of post-depositional effects of seismic shocks on unconsolidated sediments...
s or tsunami deposit
Tsunami deposit
A tsunami deposit is a sedimentary unit deposited as the result of of a tsunami. Such deposits may be left onshore during the inundation phase or offshore during the 'backwash' phase. Such deposits are being used to identify past tsunami events and thereby better constrain estimates of both...
s.
Geomorphology
Seismically active faultActive fault
An active fault is a fault that is likely to have another earthquake sometime in the future. Faults are commonly considered to be active if there has been movement observed or evidence of seismic activity during the last 10,000 years....
s and related fault generated folds
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary 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
Raised beach
A raised beach, marine terrace, or perched coastline is an emergent coastal landform. Raised beaches and marine terraces are beaches or wave-cut platforms raised above the shore line by a relative fall in the sea level ....
es of Turakirae Head recording the history of coseismic uplift of the Rimutaka Range
Rimutaka Range
The Rimutaka Range is one of several mountain ranges in the North Island of New Zealand which form a ridge running parallel with the east coast of the island between East Cape and Wellington.The ridge is at its most pronounced in the southern part of the island, where it consists of the Ruahine,...
due to displacement on the Wairarapa Fault
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 strike-slip fault with a component of uplift to the northwest as expressed by the Rimutaka Range...
in North Island
North Island
The North Island is one of the two main islands of New Zealand, separated from the much less populous South Island by Cook Strait. The island is in area, making it the world's 14th-largest island...
, New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
.