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Pore-Fluid Migration and the Timing of the 2005 M8.7 Nias Earthquake

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Pore-Fluid Migration and the Timing of the 2005 M8.7 Nias Earthquake Pore-fl uid migration and the timing of the 2005 M8.7 Nias earthquake Kristin L.H. Hughes1,*, Timothy Masterlark1,*, and Walter D. Mooney2,* 1DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF ALABAMA, 201 7TH AVENUE, TUSCALOOSA, ALABAMA 35487, USA 2U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025, USA ABSTRACT Two great earthquakes have occurred recently along the Sunda Trench, the 2004 M9.2 Sumatra-Andaman earthquake and the 2005 M8.7 Nias earthquake. These earthquakes ruptured over 1600 km of adjacent crust within 3 mo of each other. We quantitatively present poroelastic deformation analyses suggesting that postseismic fl uid fl ow and recovery induced by the Sumatra-Andaman earthquake advanced the tim- ing of the Nias earthquake. Simple back-slip simulations indicate that the megapascal (MPa)–scale pore-pressure recovery is equivalent to 7 yr of interseismic Coulomb stress accumulation near the Nias earthquake hypocenter, implying that pore-pressure recovery of the Sumatra- Andaman earthquake advanced the timing of the Nias earthquake by ~7 yr. That is, in the absence of postseismic pore-pressure recovery, we predict that the Nias earthquake would have occurred in 2011 instead of 2005. LITHOSPHERE; v. 3; no. 2; p. 170–172. doi: 10.1130/L109.1 The M9.2 Sumatra-Andaman earthquake and subsequent great tsu- 18°N nami of 26 December 2004 ruptured over 1200 km of crust, lasted ~8 min, and killed over 250,000 people in 12 countries surrounding the Indian Ocean (Ammon et al., 2005; Bilek, 2007; Vigny et al., 2005). 16°N RangoonRangoon Three months later, on 28 March 2005, a M8.7 earthquake centered off the coast of Nias Island just west of northern Sumatra ruptured over N 400 km of crust, killed over 1300 people, and caused a minor tsunami 14°N (Fig. 1) (Ammon et al., 2005; Banerjee et al., 2007). Here, we present poroelastic deformation analyses that suggest postseismic fl uid fl ow and AIAI recovery induced by the Sumatra-Andaman earthquake advanced the 12°N timing of the later M8.7 Nias earthquake. AndamanAndaman We constructed fi nite-element models (FEMs) to simulate the SeaSea co seismic stress and pore (fl uid) pressure fi elds of the Sumatra-Andaman WSFW 10°N BPBP S F PhuketPhuket Figure 1. Seismotectonic setting of the Sumatra-Andaman subduction r 8°N y zone (adapted from Hughes et al., 2010). Harvard centroid moment tensor 9 / NINI 5 m (CMT) focal mechanisms are given for the Sumatra-Andaman earthquake 59 and Nias earthquake. Aftershock epicenters (orange dots), spanning 26 mm/yrm SundaS December 2004 through 28 March 2005 and transparent orange area, 6°N u BandaBanda AcehAceh n illuminate the surface projection of the Sumatra-Andaman earthquake d rupture (http://neic.usgs.gov). The rupture initiated on the southeast por- a SPSP tion of the fault and propagated ~1200 km northward. The blue transpar- GSFG 4°N 9.2 S ent area represents the surface projection of the Nias earthquake rup- F ture (http://neic.usgs.gov). The sharply truncated aftershock distribution, shown with a northeast-trending dashed line (red) that bisects Simeulue r y Island, marks the boundary between rupture of the Sumatra-Andaman 1 / TrenchT 8.7 2°N 6 re earthquake and subsequent Nias earthquake and represents the seismic m n SISI c 61 h barrier between the two earthquakes. Black triangles are nearfi eld global mm/yrm NiasNias positioning system sites (Gahalaut et al., 2006; Subarya et al., 2006). The IAPIAP tectonic confi guration is modifi ed from Bird (2003) and overlies a shaded 0° relief image of global relief data (http://www.ngdc.noaa.gov). Abbre- viations: AI—Andaman Islands, BP—Burma plate, IAP—Indo-Australian 90°E92°E 94°E 96°E 98°E 100°E plate, NI—Nicobar Islands, SI—Simeulue Island, GSF—Great Sumatran fault, SP—Sunda plate, and WSF—West Sumatra fault. 0200 400 *E-mails: [email protected]; [email protected]; [email protected]. km 170 For permission to copy, contact [email protected] | |Volume © 2011 3 Geological | Number Society2 | LITHOSPHERE of America Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/3/2/170/3038802/170.pdf by guest on 01 October 2021 Pore-fl uid migration and the timing of the 2005 M8.7 Nias earthquake | SHORT RESEARCH M9.2 92 d M8.7 0.0 Point A Point B –1.0 Figure 2. Graph shows change in P (MPa) P pore pressure for point A (forearc), –2.0 point B (near Nias earthquake hypocenter), and interseismic Interseismic strain accumulation for the sub- duction zone. Map shows coseis- –3.0 mic poroelastic deformation. Seis- Dec 2004 Jan 2005 Feb 2005 Mar 2005 Apr 2005 mic barrier is red line between the Sumatra-Andaman earthquake (north) and Nias earthquake N (south) ruptures. For point B, Indo-Australian the upper forearc and volcanic plate arc have been cut away to view underlying subducting oceanic crust. For point A, the fi rst few Forearc/volcanic arc kilometers have been stripped off to see into the forearc. PointPoint A Poroelastic deformation Sunda trench Seismic barrier Subducting PointPoint B Pore-pressure change oceanic crust 0 150 300 1.0 MPa –1.0 MPa km earthquake, transient postseismic recovery of stress and pore pressure, and val between the two earthquakes. This increase in pore pressure near the interseismic stress accumulation along the plate boundary (Hughes et al., Nias earthquake hypocenter was due to pore fl uids migrating both down- 2010). FEMs are uniquely capable of simulating a subduction zone as dip and updip, as well as laterally along the strike of the slab within the a three-dimensional problem domain partitioned to represent poroelastic oceanic crust due to coseismic pore-pressure gradients. continental and oceanic crust and elastic mantle components. This 2.0 MPa pore-pressure recovery is two orders of magnitude The Sumatra-Andaman earthquake is still the largest earthquake for greater than the Coulomb stress triggering threshold required for fric- which coseismic deformation was recorded by global positioning system tional slip, i.e., 104 Pa (Stein, 1999). Furthermore, these changes in Cou- (GPS) data. We use the near-fi eld GPS data, recorded for northern Sumatra lomb stress near the Nias earthquake hypocenter due to this pore-pres- and the Andaman and Nicobar Islands, to determine the slip distribution sure recovery were signifi cantly greater than changes attributed to either of the Sumatra-Andaman earthquake with FEM-generated Green’s func- afterslip (McCloskey et al., 2005; Chlieh et al., 2007; Prawirodirdjo et tions and linear inverse methods (Masterlark and Hughes, 2008; Hughes al., 2010) or postseismic viscoelastic relaxation (Pollitz et al., 2006). et al., 2010). It is the stress and pore-fl uid pressure fi elds generated by this Simple back-slip simulations (Savage, 1983) using the FEMs suggest coseismic slip distribution that drive the postseismic poroelastic defor- that the 2.0 MPa pore-pressure recovery is equivalent to 7 yr of inter- mation as excess pore pressure recovers to equilibrium via Darcian fl ow. seismic accumulation of Coulomb stress (0.22 MPa) near the Nias earth- Δσ Δσ Δσ Δ Changes in Coulomb stress—defi ned as c = s + ƒ( n + P), where quake hypocenter—a result that suggests pore-pressure recovery of the σ σ σ c is Coulomb stress, s is shear stress, ƒ is friction, n is normal stress, Sumatra-Andaman earthquake advanced the timing of the Nias earth- and P is pore pressure (Wang, 2000)—quantify the change in tendency for quake by ~7 yr. Therefore, instead of occurring in 2011, the Nias earth- slip to occur along a fault. The Coulomb stress changes introduced by the quake occurred in 2005 due to pore-pressure recovery. The results of this Sumatra-Andaman earthquake, and thus the frictional stability of nearby study indicate that the analysis of pore-pressure recovery is signifi cant in faults, evolved in response to pore-pressure recovery. addressing earthquake triggering at subduction zones worldwide. The coseismic deformation due to the Sumatra-Andaman earthquake and the rheologic confi guration of the three-dimensional FEM produce ACKNOWLEDGMENTS two transient fl ow regimes having two different time constants (Fig. 2). The fi rst fl ow regime is shallow, within a few kilometers depth, and dis- We thank two anonymous reviewers for insightful comments and sug- sipates within 30 d of the Sumatra-Andaman earthquake. The second fl ow gestions. This work is supported in part by the National Aeronautics and regime is deep, within the subducting oceanic crust of the downgoing slab, Space Administration (NASA) under award NNX060F10G, National and it persists for several months after the Sumatra-Andaman earthquake Science Foundation (NSF) Geophysics grant EAR-0911466, and the W. (Fig. 2). The Sumatra-Andaman earthquake initially induced a negative Gary Hooks Endowed Geology Fund. Academic licensing and techni- megapascal (MPa)–scale pore-pressure change near the Nias earthquake cal support for Abaqus software is provided by Simulia Inc., Dassault hypocenter, which then increased (recovered) during the 3 mo time inter- Systèmes. LITHOSPHERE | Volume 3 | Number 2 | www.gsapubs.org 171 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/3/2/170/3038802/170.pdf by guest on 01 October 2021 HUGHES ET AL. REFERENCES CITED Pollitz, F.F., Banerjee, P., Bürgmann, R., Hashimoto, M., and Choosakul, N., 2006, Stress changes along the Sunda Trench following the 26 December 2004 Sumatra-Anda- man and 28 March 2005 Nias earthquakes: Geophysical Research Letters, v. 33, Ammon, C.J., Ji, C., Thio, H.K., Robinson, D., Ni, S., Hjorleifsdottir, V., Kanamori, H., Lay, T., doi:10.1029/2005GL024558. Das, S., Helmberger, D., Ichinose, G., Polet, J., and Wald, D., 2005, Rupture process Prawirodirdjo, L., McCaffrey, R., Chadwell, C.D., Bock, Y., and Subarya, C., 2010, Geo- of the 2004 Sumatra-Andaman earthquake: Science, v.
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