For the branch of the science of seismology concerned with the specification of the time, location, and magnitude of future earthquakes within stated limits, see Earthquake prediction.
Earthquake forecasting is a branch of the science of seismology concerned with the probabilistic assessment of general earthquake seismic hazard, including the frequency and magnitude of damaging earthquakes in a given area over years or decades.[1] While forecasting is usually considered to be a type of prediction, earthquake forecasting is often differentiated from earthquake prediction, Earthquake forecasting estimates the likelihood of earthquakes in a specific timeframe and region, while earthquake prediction attempts to pinpoint the exact time, location, and magnitude of an impending quake, which is currently not reliably achievable.Wood & Gutenberg (1935). Kagan (1997b, §2.1) says: "This definition has several defects which contribute to confusion and difficulty in prediction research." In addition to specification of time, location, and magnitude, Allen suggested three other requirements: 4) indication of the author's confidence in the prediction, 5) the chance of an earthquake occurring anyway as a random event, and 6) publication in a form that gives failures the same visibility as successes. Kagan & Knopoff (1987, p.1563) define prediction (in part) "to be a formal rule where by the available space-time-seismic moment manifold of earthquake occurrence is significantly contracted ...."</ref>[2] Both forecasting and prediction of earthquakes are distinguished from earthquake warning systems, which, upon detection of an earthquake, provide a real-time warning to regions that might be affected.
In the 1970s, scientists were optimistic that a practical method for predicting earthquakes would soon be found, but by the 1990s continuing failure led many to question whether it was even possible.[3] Demonstrably successful predictions of large earthquakes have not occurred, and the few claims of success are controversial.[4] Consequently, many scientific and government resources have been used for probabilistic seismic hazard estimates rather than prediction of individual earthquakes. Such estimates are used to establish building codes, insurance rate structures, awareness and preparedness programs, and public policy related to seismic events.[5] In addition to regional earthquake forecasts, such seismic hazard calculations can take factors such as local geological conditions into account. Anticipated ground motion can then be used to guide building design criteria.[citation needed]
Methods for earthquake forecasting
Methods for earthquake forecasting generally look for trends or patterns that lead to an earthquake. As these trends may be complex and involve many variables, advanced statistical techniques are often needed to understand them, therefore these are sometimes called statistical methods. These approaches tend to have relatively long time periods, making them useful for earthquake forecasting.
Elastic rebound
Even the stiffest of rock is not perfectly rigid. Given a large force (such as between two immense tectonic plates moving past each other) the Earth's crust will bend or deform. According to the elastic rebound theory of Reid (1910), eventually the deformation (strain) becomes great enough that something breaks, usually at an existing fault. Slippage along the break (an earthquake) allows the rock on each side to rebound to a less deformed state. In the process, energy is released in various forms, including seismic waves.[6] The cycle of tectonic force being accumulated in elastic deformation and released in a sudden rebound is then repeated. As the displacement from a single earthquake ranges from less than a meter to around 10 meters (for an M 8 quake),[7] the demonstrated existence of large strike-slip displacements of hundreds of miles shows the existence of a long-running earthquake cycle.[8]
Characteristic earthquakes
The most studied earthquake faults (such as the Nankai megathrust, the Wasatch fault, and the San Andreas Fault) appear to have distinct segments. The characteristic earthquake model postulates that earthquakes are generally constrained within these segments.[9] As the lengths and other properties[10] of the segments are fixed, earthquakes that rupture the entire fault should have similar characteristics. These include the maximum magnitude (which is limited by the length of the rupture), and the amount of accumulated strain needed to rupture the fault segment. Since continuous plate motions cause the strain to accumulate steadily, seismic activity on a given segment should be dominated by earthquakes of similar characteristics that recur at somewhat regular intervals.[11] For a given fault segment, identifying these characteristic earthquakes and timing their recurrence rate (or conversely return period) should therefore inform us about the next rupture; this is the approach generally used in forecasting seismic hazard. Return periods are also used for forecasting other rare events, such as cyclones and floods, and assume that future frequency will be similar to observed frequency to date.
Extrapolation from the Parkfield earthquakes of 1857, 1881, 1901, 1922, 1934, and 1966 led to a forecast of an earthquake around 1988, or before 1993 at the latest (at the 95% confidence interval), based on the characteristic earthquake model.[12] Instrumentation was put in place in hopes of detecting precursors of the anticipated earthquake. However, the forecasted earthquake did not occur until 2004. The failure of the Parkfield prediction experiment has raised doubt as to the validity of the characteristic earthquake model itself.[13]
Seismic gaps
At the contact where two tectonic plates slip past each other, every section must eventually slip, as (in the long-term) none get left behind. But they do not all slip at the same time; different sections will be at different stages in the cycle of strain (deformation) accumulation and sudden rebound. In the seismic gap model, the "next big quake" should be expected not in the segments where recent seismicity has relieved the strain, but in the intervening gaps where the unrelieved strain is the greatest.[14] This model has an intuitive appeal; it is used in long-term forecasting, and was the basis of a series of circum-Pacific (Pacific Rim) forecasts in 1979 and 1989–1991.[15]
However, some underlying assumptions about seismic gaps are now known to be incorrect. A close examination suggests that "there may be no information in seismic gaps about the time of occurrence or the magnitude of the next large event in the region";[16] statistical tests of the circum-Pacific forecasts shows that the seismic gap model "did not forecast large earthquakes well".[17] Another study concluded that a long quiet period did not increase earthquake potential.[18]
California (outlined in white) and buffer zone showing the 2,606 fault subsections of UCERF 3.1. Colors indicate probability (as a percentage) of experiencing an M ≥ 6.7 earthquake in the next 30 years, accounting for the stress accumulated since the last earthquake. Does not include effects from the Cascadia subduction zone (not shown) in the northwest corner.
The 2015 Uniform California Earthquake Rupture Forecast, Version 3, or UCERF3, is the latest official earthquake rupture forecast (ERF) for the state of California, superseding UCERF2. It provides authoritative estimates of the likelihood and severity of potentially damaging earthquake ruptures in the long- and near-term. Combining this with ground motion models produces estimates of the severity of ground shaking that can be expected during a given period (seismic hazard), and of the threat to the built environment (seismic risk). This information is used to inform engineering design and building codes, planning for disaster, and evaluating whether earthquake insurance premiums are sufficient for the prospective losses.[19] A variety of hazard metrics[20] can be calculated with UCERF3; a typical metric is the likelihood of a magnitude[21] M 6.7 earthquake (the size of the 1994 Northridge earthquake) in the 30 years (typical life of a mortgage) since 2014.
↑ Zoback 2006 provides a clear explanation. Evans 1997, §2.2 also provides a description of the "self-organized criticality" (SOC) paradigm that is displacing the elastic rebound model.
↑ For a list of evaluation metrics available as of 2013 see Table 11 in Field et al. 2013, p.52.
↑ Following standard seismological practice, all earthquake magnitudes here are per the moment magnitude scale. This is generally equivalent to the better known Richter scale.
Field, Edward H.; Biasi, Glenn P.; Bird, Peter; Dawson, Timothy E.; Felzer, Karen R.; Jackson, David D.; Johnson, Kaj M.; Jordan, Thomas H.; Madden, Christopher; Michael, Andrew J.; Milner, Kevin R.; Page, Morgan T.; Parsons, Tom; Powers, Peter M.; Shaw, Bruce E.; Thatcher, Wayne R.; Weldon, Ray J. II; Zeng, Yuehua (2013), Uniform California earthquake rupture forecast, version 3 (UCERF3) – The time-independent model, vol.Open-File Report 2013–1165, United States Geological Survey. Also California Geological Survey Special Report 228, and Southern California Earthquake Center Publication 1792.
Lomnitz, Cinna; Nava, F. Alejandro (December 1983), "The predictive value of seismic gaps.", Bulletin of the Seismological Society of America, 73 (6A): 1815–1824.
McCann, W. R.; Nishenko, S. P.; Sykes, L. R.; Krause, J. (1979), "Seismic gaps and plate tectonics: Seismic potential for major boundaries", Pure and Applied Geophysics, 117 (6): 1082–1147, Bibcode:1979PApGe.117.1082M, doi:10.1007/BF00876211, S2CID129377355 .
Reid, Harry Fielding (1910), "The Mechanics of the Earthquake.", The California Earthquake of April 18, 1906: Report of the State Earthquake Investigation Commission, vol.2.
Scholz, Christopher H. (2002), The Mechanics of earthquakes and faulting (2nded.), Cambridge Univ. Press, ISBN0-521-65223-5 .
Schwartz, David P.; Coppersmith, Kevin J. (10 July 1984), "Fault Behavior and Characteristic Earthquakes: Examples From the Wasatch and San Andreas Fault Zones", Journal of Geophysical Research: Solid Earth, 89 (B7): 5681–5698, Bibcode:1984JGR....89.5681S, doi:10.1029/JB089iB07p05681.
An earthquake – also called a quake, tremor, or temblor – is the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, and wreak destruction across entire cities. The seismic activity of an area is the frequency, type, and size of earthquakes experienced over a particular time. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume.
The San Andreas Fault is a continental right-lateral strike-slip transform fault that extends roughly 1,200 kilometers (750 mi) through the U.S. state of California. It forms part of the tectonic boundary between the Pacific Plate and the North American Plate. Traditionally, for scientific purposes, the fault has been classified into three main segments, each with different characteristics and a different degree of earthquake risk. The average slip rate along the entire fault ranges from 20 to 35 mm per year.
Earthquake prediction is a branch of the science of seismology concerned with the specification of the time, location, and magnitude of future earthquakes within stated limits, and particularly "the determination of parameters for the next strong earthquake to occur in a region". Earthquake prediction is sometimes distinguished from earthquake forecasting, which can be defined as the probabilistic assessment of general earthquake hazard, including the frequency and magnitude of damaging earthquakes in a given area over years or decades.
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 thousands of instrumentally detectable aftershocks, which steadily decrease in magnitude and frequency according to a consistent pattern. In some earthquakes the main rupture happens in two or more steps, resulting in multiple main shocks. These are known as doublet earthquakes, and in general can be distinguished from aftershocks in having similar magnitudes and nearly identical seismic waveforms.
The 1857 Fort Tejon earthquake occurred at about 8:20 a.m. on January 9 in central and Southern California. One of the largest recorded earthquakes in the United States, with an estimated moment magnitude of 7.9, it ruptured the southern part of the San Andreas Fault for a length of about 225 miles, between Parkfield and Wrightwood.
Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthquakes are the planet's most powerful, with moment magnitudes (Mw) that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes.
Coulomb stress transfer is a seismic-related geological process of stress changes to surrounding material caused by local discrete deformation events. Using mapped displacements of the Earth's surface during earthquakes, the computed Coulomb stress changes suggest that the stress relieved during an earthquake not only dissipates but can also move up and down fault segments, concentrating and promoting subsequent tremors. Importantly, Coulomb stress changes have been applied to earthquake-forecasting models that have been used to assess potential hazards related to earthquake activity.
Episodic tremor and slip (ETS) is a seismological phenomenon observed in some subduction zones that is characterized by non-earthquake seismic rumbling, or tremor, and slow slip along the plate interface. Slow slip events are distinguished from earthquakes by their propagation speed and focus. In slow slip events, there is an apparent reversal of crustal motion, although the fault motion remains consistent with the direction of subduction. ETS events themselves are imperceptible to human beings and do not cause damage.
The 2007 Alum Rock earthquake occurred on October 30 at 8:04 p.m. Pacific Daylight Time in Alum Rock Park in San Jose, in the U.S. state of California. It measured 5.6 on the moment magnitude scale and had a maximum Mercalli intensity of VI (Strong). The event was then the largest in the San Francisco Bay Area since the 1989 Loma Prieta earthquake, which measured 6.9 on the moment magnitude scale, but was later surpassed by the 2014 South Napa earthquake. Ground shaking from the Alum Rock quake reached San Francisco and Oakland and other points further north. Sixty thousand felt reports existed far beyond Santa Rosa, as far north as Eugene, Oregon.
QuakeFinder is a company focused on developing a system for earthquake prediction. QuakeFinder operates as a project of aerospace engineering firm Stellar Solutions, and by subscriptions and sponsorships from the public.
In seismology, doublet earthquakes – and more generally, multiplet earthquakes – were originally identified as multiple earthquakes with nearly identical waveforms originating from the same location. They are now characterized as distinct earthquake sequences having two main shocks of similar magnitude, sometimes occurring within tens of seconds, but sometimes separated by years. The similarity of magnitude – often within 0.4 magnitude – distinguishes multiplet events from aftershocks, which start at about 1.2 magnitude less than the parent shock and decrease in magnitude and frequency according to known laws.
In seismology, a tsunami earthquake is an earthquake which triggers a tsunami of significantly greater magnitude, as measured by shorter-period seismic waves. The term was introduced by Japanese seismologist Hiroo Kanamori in 1972. Such events are a result of relatively slow rupture velocities. They are particularly dangerous as a large tsunami may arrive at a coastline with little or no warning.
The 1943 Alahan Panjang earthquakes occurred on June 8 and June 9 UTC in Sumatra, then under Japanese occupation. This was an earthquake doublet.
The 2008 Uniform California Earthquake Rupture Forecast, Version 2, or UCERF2, is one of a series of earthquake forecasts prepared for the state California by the Working Group on California Earthquake Probabilities (WGCEP), collaboration of the U.S. Geological Survey, the California Geological Survey, and the Southern California Earthquake Center, with funding from the California Earthquake Authority. UCERF2 was superseded by UCERF3 in 2015.
The 2015 Uniform California Earthquake Rupture Forecast, Version 3, or UCERF3, is the latest official earthquake rupture forecast (ERF) for the state of California, superseding UCERF2. It provides authoritative estimates of the likelihood and severity of potentially damaging earthquake ruptures in the long- and near-term. Combining this with ground motion models produces estimates of the severity of ground shaking that can be expected during a given period, and of the threat to the built environment. This information is used to inform engineering design and building codes, planning for disaster, and evaluating whether earthquake insurance premiums are sufficient for the prospective losses. A variety of hazard metrics can be calculated with UCERF3; a typical metric is the likelihood of a magnitude M 6.7 earthquake in the 30 years since 2014.
Jeanne L. Hardebeck is an American research geophysicist studying earthquakes and seismology who has worked at the United States Geological Survey (USGS) since 2004. Hardebeck studies the state of stress and the strength of faults.
Ruth Harris is a scientist at the United States Geological Survey known for her research on large earthquakes, especially on how they begin, end, and cause the ground to shake. In 2019, Harris was elected a fellow of the American Geophysical Union who cited her "for outstanding contributions to earthquake rupture dynamics, stress transfer, and triggering".
The 1955 Zheduotang earthquake, also known as the Kangding earthquake occurred on April 14 at 09:29:02 local time near the city of Kangding in the Garzê Tibetan Autonomous Prefecture, Sichuan. The earthquake had a moment magnitude of 7.0 and a surface wave magnitude of 7.1 and struck at a depth of 10 km. Severe damage occurred in Kangding with the loss of 70 lives.
The earthquake cycle refers to the phenomenon that earthquakes repeatedly occur on the same fault as the result of continual stress accumulation and periodic stress release. Earthquake cycles can occur on a variety of faults including subduction zones and continental faults. Depending on the size of the earthquake, an earthquake cycle can last decades, centuries, or longer. The Parkfield portion of the San Andreas fault is a well-known example where similarly located M6.0 earthquakes have been instrumentally recorded every 30–40 years.
The 1979 Saint Elias earthquake affected Alaska at 12:27 AKST on 28 February. The thrust-faulting Mw 7.5 earthquake had an epicenter in the Granite Mountains. Though the maximum recorded Modified Mercalli intensity was VII, damage was minimal and there were no casualties due to the remoteness of the faulting. Damage also extended across the border in parts of Yukon, Canada.
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