Bulletin of the Seismological Society of America, Vol. 94, No. 1, pp. 88–98, February 2004 A Common Origin for Aftershocks, Foreshocks, and Multiplets by Karen R. Felzer,* Rachel E. Abercrombie, and Go¨ran Ekstro¨m Abstract We demonstrate that the statistics of earthquake data in the global Cen- troid Moment Tensor (CMT) and National Earthquake Information Center (NEIC) catalogs and local California Council of the National Seismic System (CNSS) catalog are consistent with the idea that a single physical triggering mechanism is responsible for the occurrence of aftershocks, foreshocks, and multiplets. Specifically, we test the hypothesis that tectonic earthquakes usually show clustering only as a result of an initial earthquake triggering subsequent ones and that the magnitude of each trig- gered earthquake is entirely independent of the magnitude of the triggering earth- quake. Therefore a certain percentage of the time, as determined by the Gutenberg– Richter magnitude–frequency relationship, an earthquake should by chance be larger than or comparable in size to the earthquake that triggered it. This hypothesis predicts that the number of times foreshocks or multiplets are observed should be a fixed fraction of the number of aftershock observations. We find that this is indeed the case in the global CMT and NEIC catalogs; the average ratios between foreshock, aftershock, and multiplet rates are consistent with what would be predicted by the We give special attention to the Solomon .1 ס Gutenberg–Richter relationship with b Islands, where it has been claimed that unique fault structures lead to unusually high numbers of multiplets. We use Monte Carlo trials to demonstrate that the Solomon Islands multiplets may be explained simply by a high regional aftershock rate and earthquake density. We also verify our foreshock results from the more complete recordings of small earthquakes available in the California catalog and find that foreshock rates for a wide range of foreshock and mainshock magnitudes can be ס projected from aftershock rates using the Gutenberg–Richter relationship with b 1 and the relationship that the number of earthquakes triggered varies with triggering earthquake magnitude M as c10␣M, where c is a productivity constant and ␣ is equal to 1. Finally, we test an alternative model that proposes that foreshocks do not trigger their mainshocks but are instead triggered by the mainshock nucleation phase. In this model, the nucleation phase varies with mainshock magnitude, so we would expect mainshock magnitude to be correlated with the magnitude, number, or spatial extent of the foreshocks. We find no evidence for any of these correlations. Introduction The term “aftershocks” is generally used to refer to a multiplets and foreshocks is different than that for after- cluster of smaller earthquakes following a larger one, which shocks, with multiplets being especially prevalent only is referred to as the “mainshock.” Clusters of earthquakes of where seismic zones contain large, simple asperities, as has comparable size are referred to as “multiplets” (or “dou- been suggested by Lay and Kanamori (1980), and fore- blets,” for pairs of events), and clustered smaller earthquakes shocks occurring because they are triggered by the nuclea- preceding a larger one are called “foreshocks.” There is gen- tion phase of the upcoming mainshock (Ohnaka 1993; eral agreement that aftershocks are triggered by stress Dodge et al., 1995; Hurukawa 1998). changes (of some sort) induced by the mainshock’s rupture. Other researchers argue that the magnitude of a trig- Some researchers argue, however, that the physics behind gered earthquake is independent of the magnitude of the earthquake that triggered it, and thus what we see as after- shocks, multiplets, and foreshocks actually represent a single *Present address: Department of Earth and Space Sciences, 3806 Geol- ogy Building, University of California, Los Angeles, Box 951567, Los triggering process. In this model, when an earthquake oc- Angeles, California 90095-1567. curs, it may trigger slip on small fault patches surrounding 88 A Common Origin for Aftershocks, Foreshocks, and Multiplets 89 the hypocenters of future earthquakes. The number of hy- are supported by the data, testing the first two predictions pocenters triggered, n, is given by with the global data set and the second two predictions with the California earthquake catalog. We also perform addi- c10␣M , (1) tional tests to see whether it is possible to disprove the idea ס n that a single triggering mechanism is all that is necessary to where M is the magnitude of the triggering earthquake, c is explain the observed earthquake statistics. a productivity constant, and ␣ is a parameter that controls the relative number of aftershocks triggered as a function of Predictions of the Single-Mode Triggering Model mainshock magnitude. In this model, each triggered earth- quake then grows to a magnitude randomly chosen from We make four predictions using the single-mode trig- the Gutenberg–Richter magnitude–frequency relationship, gering model. The first is that since the model holds that -bm, where N is the number aftershocks and doublets result from the same triggering proמ a ס (which gives that log(N of earthquakes larger than or equal to magnitude m, b is a cess, regional aftershock and doublet rates should vary lin- constant typically close to unity, and a is a constant that early with each other. For example, where aftershock rates depends on the local activity rate (Ishimoto and Iida, 1939; are low doublet rates should also be low, and so on. Gutenberg and Richter, 1944). This single-process triggering The second prediction is that, for the same reason, af- model has been used to build statistical models of seismicity tershock and foreshock rates should vary linearly with each (Vere-Jones, 1966; Kagan and Knopoff, 1981; Ogata, 1988; other. Thus, significant regional variations in the rate of fore- Helmstetter and Sornette, 2002), to analyze earthquake clus- shock occurrence as observed, for example, by Reasenberg tering (Console and Murru, 2001), to estimate the prevalence (1999), may be simply explained by regional variations in of secondary aftershocks (Felzer et al., 2002), to study fore- earthquake-triggering efficiency and should correlate di- shocks (Shaw, 1993; Jones et al., unpublished manuscript), rectly with aftershock rate variability. and to find the average time-dependent risk in California of Our third prediction is that aftershock production should ␣M1 a large earthquake being triggered by recent seismic activity vary with mainshock magnitude, M1,asc10 where c is Therefore, if we count aftershocks .1.0 ס ␣ Reasenberg and Jones, 1989). a constant and) The purpose of this article is to use earthquake statistics, above some set cutoff magnitude M2, the number of after- Ն derived from the global International Seismological Centre shocks larger than or equal to M2, N(m M2), should be (ISC) and National Earthquake Information Center (NEIC) given by catalogs and regional California Council of the National M1 ס Seismic System (CNSS) catalog, to investigate whether after- Ն N(m M2) c10 , (2) shocks, multiplets, and foreshocks represent a single physi- cal processes or multiple ones. In order to perform this in- where M may be either smaller or larger than M . ␣ 2 1 vestigation, it is necessary to set a value for in equation Our fourth prediction is that the number of aftershocks ע and a value for b in the Gutenberg–Richter equation. We (1) having a magnitude M2 DM, where M2 is now a variable follow Reasenberg and Jones (1989), Michael and Jones rather than a fixed value, should vary as (1998), Console and Murru (2001), and Felzer et al. (2002) Mמ b because this allows for the reproduction of b M ס ␣ and set (C10( 12) , (3 ס (DM ע N(M Ba˚th’s law, an empirical relationship that states that the av- 2 erage magnitude difference between a mainshock and its This prediction is arrived .1.0 ס where C is a constant and b largest aftershock is independent of mainshock magnitude at by multiplying together prediction 3, that the number of (Utsu, 1961; Ba˚th, 1965; Tapanos, 1990; Felzer et al., 2002). triggered earthquakes should vary as 10M1, with the speci- b ס ␣ b, or more precisely setting ס ␣ In addition, setting fication that the magnitude of each triggered earthquake is which is the typically observed b-value, is consistent ,1.0 ס chosen randomly from the Gutenberg–Richter relationship, with California aftershock counts, as we will demonstrate which gives that the probability of having an earthquake of מ -later, and means (assuming constant stress drop) that after M2. A similar prediction was 10 ן magnitude M is 10a shock production varies directly with mainshock faulting 2 made and investigated by Michael and Jones (1998). area, which agrees with empirical observations (Yamanaka and Shimazaki, 1990; Jones and Hauksson, unpublished manuscript). Other values for ␣ given in the literature in- Testing the Predictions of the Single-Mode clude 0.5 (Console et al., 2003) and 0.8 (Helmstetter, 2003). Triggering Model In the rest of this article we will refer to the single physical model for earthquake triggering, along with the stipulations Data Processing -and that ␣ and b remain invariant with To test the first two predictions of the single-mode trig 1.0 ס b ס ␣ that time, as the “single-mode triggering model.” gering model, we need to compare different regions with In this article we use the single-mode triggering model aftershock, doublet, and foreshock rates that vary signifi- to make four predictions. We then test if these predictions cantly. This requires use of a global catalog. To keep mag- 90 K. R. Felzer, R. E. Abercrombie, and G. Ekstro¨m nitude measurements as consistent as possible, we use we wish to compare aftershock and doublet rates, the two moment magnitudes from the Harvard Centroid Moment data sets cannot be allowed to overlap.