Foreshock occurrence before large earthquakes

Abstract

Abstract. Rates of foreshock occurrence involving shallow M -> 6 and M _> 7 mainshocks and M _> 5 foreshocks were measured in two worldwide catalogs over -20-year intervals. The overall rates observed are similar to ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span. The observed worldwide rates were compared to a generic model of earthquake clustering based on patterns of small and moderate aftershocks in California. The aftershock model was extended to the case of moderate foreshocks preceding large mainshocks. Overall, the observed worldwide foreshock rates exceed the extended California generic model by a factor of-2. Significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region, with the rate before thrust events higher and the rate before strike-slip events lower than the worldwide average. Among the thrust events, a large majority, composed of events located in shallow subduction zones, had a high foreshock rate, while a minority, located in continental thrust belts, had a low rate. These differences may explain why previous surveys have found low foreshock rates among thrust events in California (especially southern California), while the worldwide observations suggests the opposite: California, lacking an active subduction zone in most of its territory, and including a region of mountain-building thrusts in the south, reflects the low rate apparently typical for continental thrusts, while the worldwide observations, dominated by shallow subduction zone events, are foreshock-rich. If this is so, then the California generic model may significantly underestimate the conditional probability for a very large (M _> 8) earthquake following a potential (M _> 7) foreshock in Cascadia. The magnitude differences among the identified foreshock-mainshock pairs in the Harvard catalog are consistent with a uniform distribution over the range of observation. IntroductionShort-term earthquake clustering, including the occurrence of foreshocks and aftershocks, is a widely observed phenomenon in shallow crustal seismicity. Nearly half of all earthquakes (most of them aftershocks) are included in short-term clusters [Reasenberg, 1985;Davis and Frohlich, 1991;Ogata et al., 1995]. Moderate or strong earthquakes are occasionally followed by stronger shocks nearby within a few days. This clustering makes possible short-term probabilistic earthquake forecasts after any earthquake based on triggered stochastic models [Kagan and Knopoff, 1987] in which the occurrence times of the triggered events are Poissonian (with varying rate), while their rate, magnitude distribution, and geographic distribution are empirically determined from the clustering behavior observed in the historic seismicity. After a moderate earthquake in an urbanized area, the increased probability of an imminent, larger earthquake is of great concern. For example, after a moderate (M = 5-6) earthquake in e...

Strong foreshock signal preceding the L'Aquila (Italy) earthquake (<i>M</i><sub>w</sub> 6.3) of 6 April 2009

“…However, they precede only some mainshocks and not others. It seems that the incidence of foreshocks decreases with increasing depth of mainshock and possibly depends on the faulting type and orientation (Ohnaka, 1992;Abercrombie and Mori, 1996;Maeda, 1996;Reasenberg, 1999). Although these difficulties The cloud of aftershocks determines the seismogenic area activated.…”

Is Earthquake Triggering Driven by Small Earthquakes?

“…The number of aftershocks n M triggered by a mainshock of magnitude M has been proposed to scale with M as [Utsu, 1969;Kagan and Knopoff, 1987;Kagan, 1991;Reasenberg, 1985;1999;Singh and Suarez, 1988;Ogata, 1988;Reasenberg and Jones, 1989;Yamanaka and Shimazaki, 1990;Davis and Frohlich, 1991;Molchan and Dmitrieva, 1992;Hainzl et al, 2000;Drakatos et al, 2001;Felzer et al, 2002] n M ∼ 10 αM .…”

“…The α-exponent is an important parameter of earthquake interaction that is used in many stochastic models of seismicity or prediction algorithms [Kagan and Knopoff, 1987;Kagan, 1991;Reasenberg, 1985;1999;Ogata, 1988;Reasenberg and Jones, 1989;Console and Murru, 2001;Felzer et al, 2002]. This parameter controls the nature of the seismic activity, that is, the relative role of small compared to large earthquakes.…”

Comment on ‘Testing earthquake prediction methods: “The West Pacific short-term forecast of earthquakes with magnitude MwHRV ≥5.8”’ by V.G. Kossobokov

“…However, some large earthquakes which would be recognized by a majority of seismologists as mainshocks are unquestionably preceded by foreshocks. Reasenberg (1999a;b) determines that between 8% and 17% of M ≥ 6.0 earthquakes in the Harvard catalog have foreshocks. These clear mainshocks should be forecasted by our method if the time delay is greater than one day.…”