Earthquake forecasting is a branch of the science of geophysics, primarily
seismology
Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...
, 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. While
forecasting
Forecasting is the process of making predictions based on past and present data. Later these can be compared with what actually happens. For example, a company might Estimation, estimate their revenue in the next year, then compare it against the ...
is usually considered to be a type of
prediction
A prediction (Latin ''præ-'', "before," and ''dictum'', "something said") or forecast is a statement about a future event or about future data. Predictions are often, but not always, based upon experience or knowledge of forecasters. There ...
, earthquake forecasting is often differentiated from
earthquake prediction
Earthquake prediction is a branch of the science of geophysics, primarily seismology, concerned with the specification of the time, location, and magnitude of future earthquakes within stated limits, and particularly "the determination of par ...
, 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.. 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. define prediction (in part) "to be a formal rule where by the available space-time-seismic moment manifold of earthquake occurrence is significantly contracted ...." Both forecasting and prediction of earthquakes are distinguished from
earthquake warning system
An earthquake early warning (EEW) system is a system of accelerometers, seismometers, communication, computers, and alarms that is devised for rapidly notifying adjoining regions of a substantial earthquake once one begins. This is not the same ...
s, 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. Demonstrably successful predictions of large earthquakes have not occurred, and the few claims of success are controversial. 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.
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.
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 , 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. 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), the demonstrated existence of large
strike-slip
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
displacements of hundreds of miles shows the existence of a long-running earthquake cycle.
Characteristic earthquakes
The most studied earthquake faults (such as the
Nankai megathrust, the
Wasatch fault, and the
San Andreas Fault
The San Andreas Fault is a continental Fault (geology)#Strike-slip faults, right-lateral strike-slip transform fault that extends roughly through the U.S. state of California. It forms part of the tectonics, tectonic boundary between the Paci ...
) appear to have distinct segments. The ''characteristic earthquake'' model postulates that earthquakes are generally constrained within these segments. As the lengths and other properties 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. For a given fault segment, identifying these characteristic earthquakes and timing their recurrence rate (or conversely
return period
A return period, also known as a recurrence interval or repeat interval, is an average time or an estimated average time between events such as earthquakes, floods, landslides, or river discharge flows to occur.
The reciprocal value of return p ...
) 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 earthquake
Parkfield earthquake is a name given to various large earthquakes that occurred in the vicinity of the town of Parkfield, California, United States. The San Andreas fault runs through this town, and six successive magnitude 6 earthquakes occurre ...
s 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. 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.
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. 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.
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"; statistical tests of the circum-Pacific forecasts shows that the seismic gap model "did not forecast large earthquakes well". Another study concluded that a long quiet period did not increase earthquake potential.
Precursor Energy Signals: Electromagnetic Changes and Total Electron Count (TEC)
There have been numerous independent reports on the electromagnetic changes possibly associated with earthquakes. But due to the complicated seismogenic process and the limited understanding of the above phenomena, there are active debates on the electromagnetic signals and their relationship with earthquakes. A ten year longitudinal study of magnetic field changes in California from 2010 to 2020 reported a "...statistical signal (is) of modest size" 24 to 72 hours prior to nineteen earthquakes of M 4.5+. An analysis of ten years of Swarm satellite data reported "A significant correlation was found between the magnetic field and TEC anomalies preceding earthquake occurrences". for earthquakes of M4.0+ one to seven days prior to the earthquake event.
Notable forecasts
UCERF3

The 2015 Uniform California Earthquake Rupture Forecast, Version 3, or UCERF3, is the latest official earthquake rupture forecast (ERF) for the state of
California
California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
, 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. A variety of hazard metrics can be calculated with UCERF3; a typical metric is the likelihood of a magnitude M 6.7 earthquake (the size of the
1994 Northridge earthquake
The 1994 Northridge earthquake affected Greater Los Angeles, California, on January 17, 1994, at 04:30:55 PST. The epicenter of the moment 6.7 () blind thrust earthquake was beneath the San Fernando Valley. Lasting approximately 8 seconds ...
) in the 30 years (typical life of a mortgage) since 2014.
UCERF3 was prepared by the ''Working Group on California Earthquake Probabilities'' (WGCEP), a collaboration between the
United States Geological Survey
The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on Mar ...
(USGS), the
California Geological Survey
The California Geological Survey, previously known as the California Division of Mines and Geology, is the California state geology, geologic agency.
History
Although it was not until 1880 that the California State Mining Bureau, predecessor to ...
(CGS), and the
Southern California Earthquake Center (SCEC), with significant funding from the
California Earthquake Authority (CEA).
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Notes
Sources
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Earthquakes
Forecasting