Editing Earthquake prediction (section)

==== 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.<ref>{{Harvnb|Castellaro|2003}}.</ref> As the lengths and other properties{{efn|1=These include the type of rock and fault geometry.}} 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.<ref>{{Harvnb|Schwartz|Coppersmith|1984}}; {{Harvnb|Tiampo|Shcherbakov|2012|loc=§2.2|p=93}}.</ref> 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. [[UCERF3]] is a notable example of such a forecast, prepared for the state of California.<ref>{{Harvnb|Field et al.|2008}}.</ref> 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.

The idea of characteristic earthquakes was the basis of the [[#Parkfield|Parkfield prediction]]: fairly similar earthquakes in 1857, 1881, 1901, 1922, 1934, and 1966 suggested a pattern of breaks every 21.9 years, with a standard deviation of ±3.1 years.<ref>{{Harvnb|Bakun|Lindh|1985|p=619}}.</ref>{{efn|1=Of course these were not the only earthquakes in this period. The attentive reader will recall that, in seismically active areas, earthquakes of some magnitude happen fairly constantly. The "Parkfield earthquakes" are either the ones noted in the historical record, or were selected from the instrumental record on the basis of location and magnitude. {{Harvtxt|Jackson|Kagan|2006|p=S399}} and {{Harvtxt|Kagan|1997|pp=211–212, 213}} argue that the selection parameters can bias the statistics, and that sequences of four or six quakes, with different recurrence intervals, are also plausible.}} Extrapolation from the 1966 event led to a prediction of an earthquake around 1988, or before 1993 at the latest (at the 95% confidence interval).<ref>{{Harvnb|Bakun|Lindh|1985|p=621}}.</ref> The appeal of such a method is that the prediction is derived entirely from the ''trend'', which supposedly accounts for the unknown and possibly unknowable earthquake physics and fault parameters. However, in the Parkfield case the predicted earthquake did not occur until 2004, a decade late. This seriously undercuts the claim that earthquakes at Parkfield are quasi-periodic, and suggests the individual events differ sufficiently in other respects to question whether they have distinct characteristics in common.<ref>{{Harvnb|Jackson|Kagan|2006|p=S408}} say the claim of quasi-periodicity is "baseless".</ref>

The failure of the [[#Parkfield|Parkfield prediction]] has raised doubt as to the validity of the characteristic earthquake model itself.<ref name=":10">{{Harvnb|Jackson|Kagan|2006}}.</ref> Some studies have questioned the various assumptions, including the key one that earthquakes are constrained within segments, and suggested that the "characteristic earthquakes" may be an artifact of selection bias and the shortness of seismological records (relative to earthquake cycles).<ref>{{Harvnb|Kagan|Jackson|1991|pp=21, 420}}; {{Harvnb|Stein|Friedrich|Newman|2005}}; {{Harvnb|Jackson|Kagan|2006}}; {{Harvnb|Tiampo|Shcherbakov|2012|loc=§2.2}}, and references there; {{Harvnb|Kagan|Jackson|Geller|2012}}; {{Harvnb|Main|1999}}.</ref> Other studies have considered whether other factors need to be considered, such as the age of the fault.{{efn|1=Young faults are expected to have complex, irregular surfaces, which impede slippage. In time these rough spots are ground off, changing the mechanical characteristics of the fault.<ref>{{Harvnb|Cowan|Nicol|Tonkin|1996}}; {{Harvnb|Stein|Newman|2004|p=185}}.</ref>}} Whether earthquake ruptures are more generally constrained within a segment (as is often seen), or break past segment boundaries (also seen), has a direct bearing on the degree of earthquake hazard: earthquakes are larger where multiple segments break, but in relieving more strain they will happen less often.<ref>{{Harvnb|Stein|Newman|2004}}.</ref>