Earthquake - Definition and Facts

Unique identifier / Notation: GH0001

Synonyms: Earth tremor.

Type: Geohazards

Cluster: Seismogenic (Earthquakes)

Coordinating agency or organisation: Global Earthquake Model Foundation (GEM).

Earthquake is a term used to describe both a sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the Earth (USGS, no date).

Primary reference(s)

USGS, no date. Earthquake glossary. United States Geological Survey (USGS). Accessed 14 October 2020.

Additional scientific description

Earthquake hazards are the physical phenomena that result from the occurrence of an earthquake. Primary earthquake hazards are those phenomena that occur most directly from an earthquake: ground shaking, landslides (and debris flow), liquefaction, surface rupture (and fissures), and subsidence/uplift. Secondary earthquake hazards are those that are caused by primary hazards, and include tsunami, seiche, flooding, fire, and ground gases (PNSN, no date).

Metrics and numeric limits

Earthquake magnitudes are given using one of several broadly equivalent scales. The ‘Moment magnitude’ (Hanks and Kanamori, 1979) scaling is the preferred measure of an earthquake’s size, as it quantifies the energy released by the earthquake and unlike other scales, does not saturate for large-magnitude events. Magnitude scales are a logarithmic scale; each increase of 1 magnitude unit (i.e., 4.3 to 5.3) represents an order of magnitude (factor of 10) increase in the amplitude of seismic measurements, and a factor of 32 increase in the energy release of an earthquake (USGS, no date a).

Earthquakes of magnitude 7.0 and above can be expected to cause widespread, intense ground shaking as well as other primary and secondary hazards; earthquakes of magnitudes 6.0 to 6.9 may cause local damage, while smaller earthquakes can cause damage to vulnerable structures at near-source distances. Note that damage may be more severe and widespread for an earthquake of a given magnitude and other characteristics in regions of fragile buildings, high-density populations or regions with local soil conditions that promote the amplification of ground shaking.

There are many different metrics for measuring the effects of earthquakes at a particular location. Qualitative intensity measures, like the Modified Mercalli intensity (MMI) scale (Wood and Neumann, 1931), and similar scales such as the Medvedev-Sponheuer-Kárník (MSK) scale or the European Macrointensity Scale (EMS-98) (Grünthal, 1998), describe the severity of an earthquake in terms of its effects on the Earth’s surface, the infrastructure and the population (USGS, no date b). Modified Mercalli intensity values range from I (not felt) to XII (Total Damage), and the threshold for structural damage begins at VI, although this varies according to the fragility of buildings in a given region. For some earthquake reporting agencies, MMI XI and XII are no longer assigned and MMI X is available but has not been applied in recent times. Since 1931, it has become clear that many of the phenomena described by Wood and Neumann (1931) were less related to ground shaking, and more to other factors that would promote widespread destruction (Dewey et al., 1995).

Some of the other quantitative measures of ground shaking by seismic instruments include: the global map of earthquake hazard and risk produced by the Global Earthquake Model Foundation (GEM, 2018), the metric ‘European Macroseismic Scale’ for measuring the effects of earthquakes at a particular location (Grünthal, 1998), and ShakeMap®, developed by the U.S. Geological Survey (USGS, no date b).

Key relevant UN convention / multilateral treaty

Sendai Framework for Disaster Risk Reduction 2015-2030 (UNDRR, 2015).

Examples of drivers, outcomes, and risk management

Earthquakes and associated (primary and secondary) hazards killed nearly 750,000 people between 1994 and 2013, more people than all other natural hazard disasters combined (CRED, 2015).

While technology does not yet exist for reducing earthquake hazards, the risk to buildings and infrastructure and human population can be mitigated by seismic retrofitting of existing buildings, improved compliance with seismic safety building guidelines, and avoidance of building on cliff faces, soft soils, or next to an active fault.

The most common and effective measure for mitigating earthquake risk is by implementing building codes with provisions for earthquake safety. For example, the US Federal Emergency Management Agency (FEMA, 2020) hosts a useful website on Seismic Building Codes.

The Global Earthquake Model Foundation recently produced a global map of earthquake hazard and risk (GEM, 2018) and is releasing the underlying national and regional models. Many of GEM’s hazard models have been developed by or in collaboration with national governments for seismic design regulations in building codes.

Some success has also been achieved in the development of early warning systems, which detect earthquakes close to the source or fault rupture, and trigger warnings to more distant locations, providing seconds to minutes of advance warning (Gasparini et al., 2007). Examples include the warning system for Japan’s bullet trains, and Mexico City’s warning system for evacuating vulnerable buildings.

Glossarium of Earthquake

• Landslide or Debris Flow (Earthquake Trigger): Landslide is the downslope movement of soil, rock and organic materials under the effects of gravity, which occurs when the gravitational driving forces exceed the frictional resistance of the material resisting on the slope. Landslides could be terrestrial or submarine (Varnes, 1978).
• Ground Gases (Seismogenic): Ground gases generated in the ground from magma (molten or semimolten natural material derived from the melting of land or oceanic crust) include carbon dioxide, sulphur dioxide, hydrogen sulphide and hydrogen halides (adapted from IVHHN, 2020 and USGS, no date).
• Earthquake: Earthquake is a term used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the Earth (USGS, no date).
• Ground Shaking (Earthquake): Earthquake ground shaking is the movement of the Earth’s surface produced by seismic waves that are generated when an earthquake occurs (adapted from USGS, no date).
• Liquefaction (Earthquake Trigger): Soil liquefaction occurs when soil is transformed from a solid to a liquid state as a result of increased pore pressure and reduced effective stress. It is typically caused by rapid loading of the soil during earthquake shaking (AGI, 2017).
• Earthquake Surface Rupture, Fissures, and Tectonic Uplift/Subsidence: Earthquake surface ruptures and fissures are localised ground displacements that develop during and immediately after an earthquake, where the fault which hosted the earthquake intersects the Earth’s surface. Surface ruptures represent the upward continuation of fault slip at depth, while fissures are smaller displacements, or more distributed deformation in and around the rupture area (adapted from USGS, no date and PNSN, no date).
• Subsidence and Uplift, Including Shoreline Change (Earthquake Trigger): Tectonic uplift and subsidence are the distributed vertical permanent ground deformations (warping) that result from earthquake displacements on a dipping (inclined) fault (Styron, 2019). This includes changes to the shoreline as a result of uplift and subsidence.
• Tsunami (Earthquake Trigger): Tsunami is the Japanese term meaning wave (‘nami’) in a harbour (‘tsu’). It is a series of travelling waves of extremely long length and period, usually generated by disturbances associated with earthquakes occurring below or near the ocean floor (IOC, 2019).

This article was originally published in the United Nations Office for Disaster Risk Reduction (UNDRR) on this link: https://www.undrr.org/understanding-disaster-risk/terminology/hips/gh0001

References

CRED, 2015. The Human Cost of Natural Disasters: A global perspective. Centre for Research on the Epidemiology of Disasters (CRED).

Dewey, J.W., B.G. Reagor, L. Dengler and K. Moley, 1995. Intensity distribution and isoseismal maps for the Northridge, California, earthquake of January 17, 1994. U.S. Geological Survey Open-File Report 95-92. doi: 10.3133/ofr9592.

FEMA, 2020. Seismic building codes. Last updated 13 October 2020. US Federal Emergency Management Agency (FEMA). Accessed 14 October 2020.

Gasparini, P., G. Manfredi and J. Zschau (eds.), 2007. Earthquake Early Warning Systems. Springer.

GEM, 2018. Global earthquake hazard and risk maps. Global Earthquake Model Foundation (GEM). Accessed 14 October 2020.

Grünthal, G. (ed), (1998). The European Macroseismic Scale EMS. Accessed 14 October 2020.

Hanks, T.C. and H. Kanamori, 1979. A moment magnitude scale. Journal of Geophysical Research, 84: 2348-2350.

PNSN, no date. Earthquake Hazards Overview. Pacific Northwest Seismic Network (PNSN). Accessed 14 October 2020.

UNDRR, 2015. Sendai Framework for Disaster Risk Reduction 2015-2030. United Nations office for Disaster Risk Reduction (UNDRR). Accessed 14 October 2020.

USGS, no date a. Earthquake glossary. United States Geological Survey (USGS). Accessed 14 October 2020.

USGS, no date b. ShakeMap scientific background. United States Geological Survey (USGS). Accessed 14 October 2020.

Wood, H.O. and F. Neumann, 1931. Modified Mercalli intensity scale of 1931. Bulletin of the Seismological Society of America, 21:277-283.