Seismotectonics

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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. [1]

Contents

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 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. [2]

Earthquakes

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 mechanisms from teleseismic data. Catalogues of events with calculated focal mechanisms are now available online, such as the searchable catalogue from the NEIC. [3] 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. [4] : viii  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. [5]

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 seismites [6] or tsunami deposits. [7]

Geomorphology

Seismically active faults 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 beaches 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. [8]

See also

Related Research Articles

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Tectonics are the processes that result in the structure and properties of the Earth's crust and its evolution through time.

The moment magnitude scale is a measure of an earthquake's magnitude based on its seismic moment. Mw  was defined in a 1979 paper by Thomas C. Hanks and Hiroo Kanamori. Similar to the local magnitude/Richter scale (ML ) defined by Charles Francis Richter in 1935, it uses a logarithmic scale; small earthquakes have approximately the same magnitudes on both scales. Despite the difference, news media often says "Richter scale" when referring to the moment magnitude scale.

<span class="mw-page-title-main">1946 Aleutian Islands earthquake</span> Earthquake near the Aleutian Islands, Alaska

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<span class="mw-page-title-main">1990 Manjil–Rudbar earthquake</span> 1990 severe earthquake centered in northeastern Iran

The 1990 Manjil–Rudbar earthquake occurred on Thursday, 21 June 1990 at 00:30:14 local time in the Caspian Sea region of northern Iran. The shock had a moment magnitude of 7.4 and a Mercalli Intensity of X (Extreme). Devastation occurred in a 20,000 km2 (7,700 sq mi) area, causing extensive damage in several cities. A large aftershock also added to the destruction. Between 35,000 and 50,000 people died in the earthquake; another 60,000–105,000 were injured.

<span class="mw-page-title-main">Focal mechanism</span> Process that generates seismic waves in an earthquake

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 is also known as a fault-plane solution. Focal mechanisms are derived from a solution of the moment tensor for the earthquake, which itself is estimated by an analysis of observed seismic waveforms. The focal mechanism can be derived from observing the pattern of "first motions", whether the first arriving P waves break up or down. This method was used before waveforms were recorded and analysed digitally, and this method is still used for earthquakes too small for easy moment tensor solution. Focal mechanisms are now mainly derived using semi-automatic analysis of the recorded waveforms.

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<span class="mw-page-title-main">Tacoma Fault</span> Active east–west striking north dipping reverse fault

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The 1953 Suva earthquake occurred on 14 September at 00:26 UTC near Suva, Fiji, just off the southeast shore of Viti Levu. This earthquake had an estimated magnitude of Ms 6.8 and Mw 6.4. The earthquake triggered a coral reef platform collapse and a submarine landslide that caused a tsunami. Eight people were reported killed; a wharf, bridges, and buildings were severely damaged in Suva.

The 2002 Kalehe earthquake occurred on October 24 at 06:08 UTC. This earthquake had a magnitude Mw 6.2, and the epicenter was located in Democratic Republic of the Congo, near Lake Kivu. Two people were reported dead. Building damage was reported in Goma, Lwiro, Kalehe, and Mugeri. The seismicity, volcanism, and uplift in the basin of Lake Kivu delimits the rift of a tectonic plate. Lake Kivu belongs to the western branch of the East African Rift System (EARS). The western branch is usually divided into several segments, and Lake Kivu belongs to the northern segment.

The 1989 Ungava earthquake occurred at 09:24 local time on 25 December to the north of Lac Bécard in a remote part of the Ungava Peninsula in northern Quebec. It had a magnitude of 6.3 on the surface wave magnitude scale and 6.2–6.5 on the moment magnitude scale and a maximum felt intensity of only IV (Light) on the Mercalli intensity scale, due to its remoteness from any inhabited areas. The mainshock was preceded by a magnitude 5.1 foreshock ten hours earlier. It was the first earthquake in eastern North America known to be associated with ground rupture.

<span class="mw-page-title-main">1819 Rann of Kutch earthquake</span> Earthquake and tsunami in Gujarat, India

The 1819 Rann of Kutch earthquake occurred at about 18:45 to 18:50 local time on 16 June 1819. It had an estimated magnitude ranging from 7.7 to 8.2 on the moment magnitude scale and a maximum perceived intensity of XI (Extreme) on the Mercalli intensity scale. It triggered a tsunami and caused at least 1,543 deaths. The earthquake caused an area of subsidence that formed the Sindri Lake and a local zone of uplift to the north about 80 km (50 mi) long, 6 km (3.7 mi) wide and 6 m (20 ft) high that dammed the Koree / Kori / Puran / Nara river. This natural dam was known as the Allah Bund.

<span class="mw-page-title-main">1983 Sea of Japan earthquake</span> 1983 earthquake and tsunami centered off the coast of Akita Prefecture, Japan

The 1983 Sea of Japan earthquake occurred on May 26, 1983 at 11:59:57 local time. It had a magnitude of 7.8 on the moment magnitude scale. It occurred in the Sea of Japan, about 100 km west of the coast of Noshiro in Akita Prefecture, Japan. Out of the 104 fatalities, all but four were killed by the resulting tsunami, which struck communities along the coast, especially Aomori and Akita Prefectures and the east coast of Noto Peninsula. Images of the tsunami hitting the fishing harbor of Wajima on Noto Peninsula were broadcast on TV. The waves exceeded 10 meters (33 ft) in some areas. Three of the fatalities were along the east coast of South Korea. The tsunami also hit Okushiri Island, the site of a more deadly tsunami 10 years later.

The 1981 Playa Azul earthquake occurred on October 24, 1981, at 21:22 local time. It was located near Playa Azul, Michoacán, Mexico. The magnitude of the earthquake was Mw 7.2, or Ms 7.3. Three deaths were reported, two from Michoacán and one from Mexico City. Some buildings were damaged in both Michoacán and Mexico City. A small tsunami was registered in Acapulco with a maximum height of 9 cm.

The 1956 Amorgos earthquake occurred at 03:11 UTC on July 9. It had a magnitude of 7.7 on the moment magnitude scale and a maximum perceived intensity of IX on the Mercalli intensity scale. The epicentre was to the south of the island of Amorgos, the easternmost island of the Cyclades in the Aegean Sea. There was significant damage on Amorgos and the neighbouring island of Santorini. It was the largest earthquake in Greece in the 20th century. It was followed 13 minutes later by a magnitude 7.2 earthquake near Santorini. It triggered a major tsunami with a maximum run-up of 30 m. The combined effects of the earthquake shaking and the tsunami caused the deaths of 53 people with a further 100 injured.

<span class="mw-page-title-main">2017 Aegean Sea earthquake</span> Earthquake in Greece and Turkey

In 21 July 2017, a large earthquake measuring 6.6 on the moment magnitude scale struck right near Bodrum, a popular town of tourism in Turkey, killing 2 and injuring hundreds. Mostly referenced as the 2017 Bodrum–Kos earthquake, this earthquake generated a tsunami which was one of the largest tsunamis in the Mediterranean Sea region.

The 1963 Marj earthquake occurred on February 21 in northern Libya. The earthquake occurred at 18:14:36 local time with a moment magnitude of 5.6 and a maximum Mercalli intensity of VIII (Severe). Financial losses totaled $5 million USD, with 290–375 deaths, 375–500 injuries, and 12,000 homeless.

The Advanced National Seismic System (ANSS) is a collaboration of the U.S. Geological Survey (USGS) and regional, state, and academic partners that collects and analyzes data on significant earthquakes to provide near real-time information to emergency responders and officials, the news media, and the public. Such information is used to anticipate the likely severity and extent of damage, and to guide decisions on the responses needed.

References

  1. ↑ E. Boschi, E.; Mantovani, E.; Morelli, A., eds. (2012). "A review of the Eastern Alps - Northern Dinarides Seismotectonics". Recent Evolution and Seismicity of the Mediterranean Region. Springer. ISBN   9789401120166.
  2. ↑ Website for the World Stress Map Project
  3. ↑ NEIC Moment Tensor and Broadband Source Parameter Search
  4. ↑ Ambraseys, Nicolas; Melville, C.P. (1982). A History of Persian Earthquakes (PDF). Cambridge University Press. ISBN   9780521021876.
  5. ↑ Historical data on earthquakes and active faulting. The contribution of IRRS and IC to EC project FAUST (Contract ENV4-CT97-0428)
  6. ↑ Migowski, C.; Agnon A.; Bookman R.; Negendank J.F.W.; Stein M (2004). "Recurrence pattern of Holocene earthquakes along the Dead Sea transform revealed by varve-counting and radiocarbon dating of lacustrine sediments" (PDF). Earth and Planetary Science Letters. 222 (1): 301–314. Bibcode:2004E&PSL.222..301M. doi:10.1016/j.epsl.2004.02.015 . Retrieved 2009-12-29.
  7. ↑ Luque, L.; Lario J.; Zazo C.; Goy J.L.; Dabrio C.J.; Silva P.G. (2001). "Tsunami deposits as paleoseismic indicators: examples from the Spanish coast". Acta Geológica Hispánica. 36 (3–4): 197–211. Retrieved 2009-12-29.
  8. ↑ 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.
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