SPATIAL RELATION BETWEEN MAIN EARTHQUAKE SLIP AND ITS AFTERSHOCK DISTRIBUTION S. Das and C. Henry1 Department of Earth Sciences, University of Oxford, Oxford, UK Received 16 October 2002; revised 12 May 2003; accepted 26 June 2003; published 27 September 2003. [1] We examine where aftershocks occur relative to the within the main fault area. We identify examples of spatial distribution of the main shock slip using data geometrical and inhomogeneous barriers and sharp and from several recent large earthquakes. No universal re- dull stress concentrations. Rupture in the main shock is lation between high- and low-moment regions and high generally found to nucleate in the region of low slip or at or low aftershock occurrence, or vice versa, is found. We the edge of high-slip regions, the 1996 Biak earthquake generally find that few, and usually the smaller, after- again being the only exception, nucleating in a very high shocks occur in the high-slip regions of the fault, a slip region. Off-fault aftershocks are found for all earth- notable exception to this being the great 1996 Biak, quakes in this study, and they sometimes rupture the Indonesia, subduction zone earthquake. In all cases, nodal plane conjugate to the main shock fault plane. aftershocks occur on favorably oriented planes of weak- INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; ness in regions of increased postseismic stress. Gener- 7215 Seismology: Earthquake parameters; 7230 Seismology: Seismicity ally, they are clustered at both ends of faults, but exam- and seismotectonics; KEYWORDS: aftershocks, earthquake slip, stress ples where aftershocks occur only at one end or where changes, barriers there is no clustering at the ends are found. Aftershock Citation: Das, S., and C. Henry, Spatial relation between main earth- clusters are also found at the edge of unbroken barriers, quake slip and its aftershock distribution, Rev. Geophys., 41(3), 1013, and regions of rapid transition from high to low slip, doi:10.1029/2002RG000119, 2003. 1. INTRODUCTION disproportionately heavily studied, because once one knows where a large earthquake has occurred, arrays of [2] It has now been known for more than a century seismometers can be installed rapidly in the epicentral that aftershocks occur on and around the main shock region to “catch” the aftershocks, and “aftershock chas- region. The first formal study of aftershocks was made by ing” is now a seismic sport! This is very fortunate as Omori [1894] for the 1891 Mino-Owari (Nobi), Japan, aftershocks provide valuable information about the main earthquake. He found that more than 3000 aftershocks shock, among the most important being a good estimate were recorded in the 26-month period following the of the main shock fault area. Richter [1955] was the first earthquake, and his observations led him to postulate to directly link the rupture area of the earthquake to the the law, now called the Omori law, showing that the aftershock distribution based on his study of the after- frequency of aftershocks dies off hyperbolically after the shocks of the Ms 7.6–7.7 21 July 1952 Kern County main earthquake (see Milne and Lee [1939] and Scholz (Arvin-Tehachapi), California, earthquake, for which [1989] for more recent discussions). That is, if N is the portable seismometers were deployed in the epicentral number of aftershocks and t is the time measured from region within hours of the main shock. The earthquake the main shock, then occurred on a fault dipping ϳ60Њ. To quote from Richter [1955], “… the epicenters … are distributed over a c1 N ϭ , roughly rectangular area …. This suggests a rectangular t ϩ c 2 outline in plan for the crustal block displaced in the main where c1 and c2 are constants. event ….” Since then, aftershock areas have been widely [3] Though the cumulative seismic moment for after- used as estimates of the main shock fault area. In fact, shocks is usually negligible (usually only a few percent) even as recently as 1989 and for as large an earthquake compared to the main shock, aftershocks have been as the Mw 8.0 Macquarie Ridge earthquake, the best estimate of the rupture area was from the aftershocks, as the low number and the then still poor azimuthal distri- 1Now at Oxford Computer Consultants, Oxford, U.K. bution of high-quality broadband digital stations were Copyright 2003 by the American Geophysical Union. Reviews of Geophysics, 41, 3 / 1013 2003 8755-1209/03/2003RG000119$15.00 doi:10.1029/2002RG000119 ● 3-1 ● 3-2 ● Das and Henry: AFTERSHOCK DISTRIBUTIONS 41, 3 / REVIEWS OF GEOPHYSICS Figure 1. Aftershock distribution of the 23 December 1972 Mw 6.3 left-lateral strike-slip Managua, Nica- ragua, earthquake (taken from Ward et al. [1974], ᭧Seismological Society of America). The 171 reliably located aftershocks (with at least six P wave arrival times) are shown for the period 3 January 1973 to 7 February 1973. The moment magnitude for the main shock is taken from the new catalog of Perez [1999] (available at http://www.ldc.usb.ve/ϳojperez/catalog). The polygons represent the 68% confidence limits in location. Station locations are shown by stars. The white line outlined in black is the main shock fault. The solid lines represent other faults mapped at the surface. not sufficient to constrain the rupture area from the locations in the last 20 or so years, due both to better waves recorded on the seismograms [Das, 1993]. It is quality and more numerous seismograms as well as only since the mid-1990s that the wide deployment of better knowledge of Earth structure, we have come to broadband stations worldwide, particularly in remote see much clearer patterns in the aftershock distributions. locations, finally has allowed the rupture area to be (Comparing Figures 4–9 and 4–10 of Yeats et al. [1997] directly obtained unambiguously by a formal seismo- for the seismicity of southern California for the years gram inversion [Henry et al., 2000; Henry, 2002]. 1932 and 1990, respectively, is very instructive in this [4] Until very good quality seismograms and reliable respect.) From the patterns that have now emerged, we Earth structure became available, the aftershock distri- can identify three distinct regions where aftershocks butions looked like clouds around the main shock rup- occur. First, they occur at the ends of faults; usually, ture area. With improvement in determining earthquake these are the densest clusters, and usually, though not 41, 3 / REVIEWS OF GEOPHYSICS Das and Henry: AFTERSHOCK DISTRIBUTIONS ● 3-3 Figure 3. The main left-lateral strike-slip fault and several similar faults parallel to it in the epicentral region of the 1972 Managua, Nicaragua, earthquake, whose aftershocks are shown in Figure 1 (modified from Figure 8–33 of Yeats et al. [1997], used by permission of Oxford University Press, Inc.) Figure 2. Shear stress change due to an in-plane shear fault in an infinite, homogeneous, elastic medium (taken from Ko- strov and Das [1982]). The shaded areas are regions of stress the fault, shows many complexities in the stress distribu- increase, and the unshaded areas are regions of stress de- tion on the main rupture area [Mikumo and Miyatake, crease. The contour lines are labeled with the stress change as 1979; Madariaga, 1979; Das and Kostrov, 1988; Cochard a percentage of the stress drop on the fault (shown by the bold and Madariaga, 1994, 1996] and increased stress on solid line); the line of no stress change is shown by the dotted unbroken or less slipped regions of the rupture area. lines. Note that in theory one should really plot the quantity Similar stress increases are expected in regions of tran- (shear stress plus ␮ times normal stress), the Coulomb failure sition from higher to lower slip on the main fault, that is, ␮ criterion, being the coefficient of friction, as is often done. regions with large slip gradients. In fact, it is the obser- ␮ Such a figure would depend on the values of and the normal vation that aftershocks do occur over the main fault stress. Note also that such figures would be asymmetric on the rupture area that provides direct proof that earthquake two sides of the fault. The remarkable similarity between faults are indeed heterogeneous and have very nonuni- Figures 2 and 1, which shows a very slight asymmetry of the aftershocks (those on the northern side are shifted slightly to form slip across the fault plane. Last, there is a small the right), may actually imply that the second term in the stress increase in some off-fault locations. Except at the Coulomb criterion is less significant. edges of the fault the stress increase due to the main shock is generally small and cannot usually cause earth- quakes by rupture of virgin rock. Thus, except at these always, they have the largest aftershocks. Next, they places, aftershocks can only occur on favorably oriented occur over the entire fault area. Finally, aftershocks preexisting planes of weakness, and increased stresses in occur off the main fault in a direction perpendicular to markedly different directions will not lead to after- it; these are often the fewest and smallest in magnitude, shocks. Of course, even at the edges of the fault, after- though examples of earthquakes with large off-fault af- shocks will occur on the most optimally oriented preex- tershocks are known. The 1972 Managua, Nicaragua, isting weak planes. That the existence of these ϭ earthquake (Mw 6.3) is a classic example showing all preexisting weak planes is more important than the three types of aftershocks (Figure 1). Clustering of af- stress direction is well known to geologists; Yeats et al. tershocks at the ends of the fault was also clearly seen [1997, p.