Subsections

Stress Changes on the Sunda Megathrust Preceding the $M_{w}$ 8.4 2007 Earthquake

Kelly Grijalva, Roland Bürgmann, and Edwin (Trey) Apel

Introduction

The $M_{w}$ 8.4 September 12, 2007 Sumatra earthquake's occurrence, close in time and space to the 2004 Sumatra-Andaman and 2005 Nias megathrust events, suggests that it could be a triggered earthquake. The 2007 earthquake initiated $\sim$750 km south of the 2005 epicenter, in the southern portion of the historic 1833 rupture zone. Earlier studies of vertical motion, derived from coral growth histories, suggest that interseismic strain accumulating along the 1833 segment had approached levels relieved in the historic earthquake (Natawidjaja et al., 2006). We investigate why the portion of the Sunda subduction zone between the 2005 and 2007 rupture patches, which previously slipped in 1797, did not rerupture or rupture with the 2007 event.

Coulomb Failure Stress Models

We model the coseismic and viscoelastic postseismic deformation from the 2004-2005 earthquake sequence in order to quantify its influence on the hypocentral region of the 2007 earthquake. The elastic coseismic deformation is calculated in a layered spherical geometry using the method of Pollitz (1996) and previously published source parameters (Banerjee et al., 2006; Konca et al., 2007; Zhou et al., 2002). Viscoelastic relaxation is calculated using the method of Pollitz (1992), with a bi-viscous rheology in the asthenosphere that includes an initial short-term viscosity of $5 \times 10^{17}$ Pa s and a long-term viscosity of $1 \times 10^{19}$ Pa s (Pollitz et al., 2006). We use the deformation calculations to model the Coulomb failure stress (CFS) changes along the Sunda megathrust. Previous studies have shown that CFS increases of 1-3 bars are generally sufficient to trigger seismicity and sometimes even a few tenths of a bar appear sufficient to advance (or retard) the occurrence of large earthquakes (e.g. Rydelek and Sacks, 1999; Lin and Stein, 2004). We use a CFS function given by CFS $= \Delta\tau + \mu'\Delta\sigma_n$, which defines CFS as a sum of the change in shear stress $\tau$ and the change in normal stress (clamping is negative) $\sigma_n$, multiplied by an effective coefficient of friction. We assume low frictional fault strength and use an effective coefficient of friction, $\mu' = 0.1$. CFS changes are resolved onto the down-going Sunda slab, with dips increasing at depth, and rake varying along strike.

Although the 2004 event was a $M_{w}$9.2, the $\sim$950 km separation distance prevented the coseismic and postseismic CFS changes at the 2007 hypocenter from being $>$ 0.1 bars. The 2005 earthquake was also too far from the 2007 hypocenter to produce significant CFS changes. The 2004-2005 sequence did produce a slightly negative CFS change in the 1797 rupture patch. Notably, the largest contributor to positive CFS change at the southern portion of the 2007 rupture zone was the $M_{w}$7.9 2000 earthquake. However, the 2000 earthquake still did not produce $>$ 0.1 bars CFS changes at the 2007 hypocenter (Figure 2.17).

Figure 2.17: Total CFS changes resulting from the combination of the 2000, 2004, and 2005 coseismic and postseismic deformation on the Sunda megathrust. The 0.1 bar contour is plotted in black and the earthquake source models overlay the CFS changes.
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Figure 2.18: Beta values showing the relative change in seismicity following the 2004 earthquake in a) 2005, b) 2006, c) 2007 up to the September earthquake. All beta values are based on seismicity between 15-30 km depth.
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Seismicity Rate Changes

We investigated seismicity changes following the 2004 earthquake using the standard beta-statistic approach (Hough, 2005). Beta is defined as $\beta = (N_a - N_e)/\sqrt{variance}$, where $N_a$ is the number of earthquakes occurring after a major event and $N_e$ is the expected number of earthquakes. Beta will be large and positive when there is increased seismic activity. We compare the annual seismicity $M$ $\geq$ 4.5 following the 2004 event with the previous fifteen years of earthquakes. Figure 2.18 shows the beta values for the Sumatra region at the depth range of 15-30 km, which includes the depth of the 2007 earthquake. Based on the high beta values, seismicity increased in the vicinity of the 2007 event during the months following the 2004 and 2005 megathrust earthquakes. In 2005, seismicity also increased near Siberut Island, at the northern end of the 2007 rupture patch. This increased seismicity may be related to aseismic slip on the Sunda megathrust, which could have further stressed the 2007 hypocentral region. During 2006 and 2007, the seismicity level dropped off to average conditions at the 2007 hypocenter location.

Discussion and Future Work

CFS change at the 2007 hypocenter, resulting from the combined 2004-2005 coseismic and postseismic deformation, is likely too small to have triggered the recent earthquake. The slightly negative CFS change in the northern 1797 rupture patch, resulting from the 2004-2005 earthquake sequence, may have delayed the recurrence of the 1797 earthquake. The smaller 2000 earthquake turns out to have had the largest CFS change at the 2007 hypocenter and may help to explain the southern location of the 2007 earthquake. Furthermore, increasing the oblique component to the slab receiver fault geometry, as observed with the larger than average 114$^{\circ}$slip direction for the 2007 earthquake, amplifies this positive CFS change. Investigations of the beta-statistic, for the years following the 2004 earthquake, show that there was increased seismic activity during 2005 on the ends of the 2007 rupture patch. We are starting to analyze geodetic data for the years 2005-2007 in order to assess whether accelerated aseismic slip contributed significantly to the timing and location of the 2007 earthquake.

Acknowledgments

This work is supported by the National Science Foundation grant EAR 0738299.

References

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