EARTHQUAKE, FORESHOCKS 199 moment rate functions. Journal of Geophysical Research, 104, 863–894. EARTHQUAKE, FORESHOCKS Bridgman, P. W., 1945. Polymorphic transition and geological phe- nomenon. American Journal of Science, 243A,90–97. Mian Liu Dziewonski, A. M., Chou, A.-T., and Woodhouse, J. H., 1981. Department of Geological Sciences, University of Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. Journal of Missouri, Columbia, MO, USA Geophysical Research, 86, 2825–2852. Ekström, G., 1994. Anomalous earthquakes on volcano ring- Synonyms fault structures. Earth and Planetary Science Letters, 128, Preshocks 707–712. Frohlich, C., and Apperson, K. D., 1992. Earthquake focal mecha- nisms, moment tensors and the consistency of seismic activity Definition near plate boundaries. Tectonics, 11, 279–296. Foreshocks. Smaller earthquakes preceding a large earth- Gilbert, J. F., and Dziewonski, A. M., 1975. An application of quake (the mainshock) in the same ruptured area. normal mode theory to the retrieval of structural parameters and source mechanisms from seismic spectra. Philosophical Transaction of Royal Society London, 278A, 187–269. Introduction Isacks, B. L., and Molnar, P., 1971. Distribution of stresses in the Earthquakes typically occur in sequences that may include descending lithosphere from a global survey of focal-mechanism foreshocks, the mainshock (the largest event or events), solutions of mantle earthquakes. Reviews of Geophysics and Space Physics, 9, 103–174. and Earthquake, Aftershocks. Earthquake sequences with- Isacks, B. L., Oliver, J., and Sykes, L. R., 1968. Seismology and the out a clear mainshock are called swarms. newglobal tectonics. Journal of Geophysical Research, 73, Foreshocks are the most obvious premonitory phenom- 5855–5899. enon of earthquakes. They are thought to indicate earth- Kagan, Y. Y., 1991. 3-D rotation of double-couple earthquake quake nucleation and hence may have the potential for sources. Geophysical Journal International, 106, 709–716. short-term earthquake prediction (Scholz, 2002). How- Kanamori, H., Ekström, G., Dziewoński, A. M., Barker, J. S., and Sipkin, S. A., 1993. Seismic radiation by magma injection: an ever, proof of the physical link between foreshores and anomalous seismic event near Tori Shima, Japan. Journal of earthquake nucleation is inconclusive, and earthquake Geophysical Research, 98, 6511–6522. prediction using foreshocks has not been reliable. Kawakatsu, H., 1991. Insignificant isotropic components in the The problem begins with their recognition – foreshocks moment tensor of deep earthquakes. Nature, 351,50–53. are earthquakes that are called foreshocks retrospectively, Kirby, S. H., Stein, S., Okal, E. A., and Rubie, D., 1996. Deep earth- when a large event (the mainshock) followed. To be con- quakes and metastable mantle phase transformations in sidered as foreshocks, these events need to occur within subducting oceanic lithosphere. Reviews of Geophysics and Space Physics, 34, 261–306. certain spatial and temporal windows, and these windows Knopoff, L., and Gilbert, J. F., 1959. Radiation from a strike-slip vary in different studies. Foreshocks are typically located earthquake. Bulletin of the Seismological Society of America, within the mainshock’s source area with a dimension 49, 163–178. roughly the length of the fault rupture, which is in the Li, X., Shao, G., and Ji, C., 2009. Rupture process of the Mw = 8.1 order of 100 km for a magnitude 7.0 earthquake. Tempo- Samoa earthquake constrained by joint inverting teleseismic rally, most foreshocks occur a few days to hours before body, surface waves and local strong motion. Eos Transactions, American Geophysical Union, 90, (52), U21D– the mainshock (Jones and Molnar, 1979). In some cases, 03, [abstract]. seismicity increases a few months or years before the Nettles, M., and Ekström, G., 2009. Ice loss and glacial earthquakes mainshock. Some workers consider such increased seis- in Greenland and Antarctica. Eos Transactions, American Geo- micity foreshocks; others do not (Scholz, 2002). physical Union, 90(52), G52B–01 [abstract]. Depending on the choice of these spatial and temporal Okal, E. A., 1996. Radial modes from the great 1994 Bolivian earth- windows, a global survey shows that up to 70% of large quake: no evidence of an isotropic component to the source. Geophysical Research Letters, 23, 431–434. earthquakes (M 7) may be preceded by foreshocks Stein, S., and Wysession, M. E., 2003. An Introduction to Seismol- (Jones and Molnar, 1979). While the percentage varies ogy, Earthquakes and Earth Structure. Malden: Blackwell, with different studies and in different regions, it is impor- 498 pp. tant to note that not all earthquakes are preceded by Sykes, L. R., 1967. Mechanisms of earthquakes and nature of foreshocks. faulting on the mid-oceanic ridges. Journal of Geophysical Research, 72, 2131–2153. Vvedenskaya, A. V., 1956. Opredelenie polej smeshchenii pri Characteristics of foreshocks zemletryasenyakh s pomoshchyu teorii dislokatsii. Izv Akad Foreshocks, when they do occur, are typically few in com- Nauk SSSR, Ser Geofiz, 6, 277–284 [in Russian]. parison with aftershocks, sometimes consisting of only Wilson, J. T., 1965. A new class of faults and their bearing on con- – a few or a single event, hence are difficult for quantitative tinental drift. Nature, 207, 343 347. analysis. When considered collectively, they are shown to exhibit a temporal pattern similar to that of aftershocks Cross-references (Jones and Molnar, 1979; Shaw, 1993). Most foreshocks Earthquakes, Source Theory occur within 10 days before the mainshock, and their Earthquake Rupture: Inverse Problem numbers rapidly accelerate till the occurrence of the 200 EARTHQUAKE, FORESHOCKS mainshock. The collective time sequence of foreshocks Can foreshocks be used to predict earthquakes? seems to fit an inverse Omori’s law: As the most obvious precursor of earthquakes, fore- nðtÞ¼atÀq shocks are thought to be useful for short-term prediction of earthquakes, and successful predictions have been where nðtÞ is the number of foreshocks by time t before the reported in numerous cases. The best example is perhaps origin of the mainshock, and a and q are constants, with q the 1975 Haicheng earthquake (magnitude 7.3) in China, close to 1 (Kagan and Knopoff, 1978; Papazachos, 1975). which was predicted largely based on foreshocks, more Thus, the number of foreshocks tends to increase hyper- than 500 of which were recorded within 4 days before bolically as the time approaches the origin of the the mainshock (Jones et al., 1982). However, a recent mainshock. Some studies suggest that foreshocks may investigation (Wang et al., 2006) has revealed that the have lower b-values (see entry Earthquake, Aftershocks) role of foreshocks in this prediction was more psycholog- relative to other earthquakes (Scholz, 2002). ical than scientific: the jolts and damages from increased It is not clear why some earthquakes have foreshocks seismicity in the preceding months stressed the minds of while others do not. Studies of global and regional earth- earthquake workers and the general public, and the inten- quake catalogs have found that neither the time sified foreshocks in the last day before the mainshock sequences nor the largest magnitude of foreshocks cor- prompted some local officials to order an evacuation. In relate to the magnitude of the mainshock (Jones and other places where official orders were not issued, the Molnar, 1979), suggesting either the size of the increased seismicity caused many residents to evacuate mainshock is independent of the earthquake nucleation voluntarily. process, or foreshocks are not part of earthquake nucle- Aside from the problem that many earthquakes do not ation (Abercrombie and Mori, 1996). The incidence of have foreshocks, short-term earthquake prediction using foreshocks is found to decrease with increasing depth foreshocks is challenging in theory and in practice. For of the mainshock (Abercrombie and Mori, 1996). For foreshocks to be a predictor, they must be an essential part earthquakes in California, more foreshocks are associ- of the physical process leading to the mainshock. ated with strike-slip events than with thrust events Although this physical link between foreshocks and (Abercrombie and Mori, 1996). This is opposite to the mainshock is suggested in the nucleation model, its vali- results from a study of global catalogs by Reasenberg dation in natural fault zones has not been conclusive. On (1999), who attribute the low rate of foreshocks for the other hand, if foreshocks are fundamentally the same Californian thrust events to their relatively greater as other earthquakes (Felzer et al., 2004), then foreshocks depths. cannot be predictors of mainshocks. In practice, we have yet to find any reliable ways to recognize foreshocks from background seismicity before the occurrence of the Causes of foreshocks mainshock. The causes of foreshocks remain uncertain. Some studies Nonetheless, there must be some symptoms associated link foreshocks to earthquake nucleation (Scholz, 2002). with the stress buildup and physical property changes in Theoretically, fault ruptures only when slip has occurred and around the fault leading to a major rupture, and over a fault patch of some critical radius. Ohnaka (1992) foreshocks are the most obvious symptoms known to has proposed a theoretical model that attributes the gener- us. Further studies of foreshocks will improve