A comprehensive model of postseismic deformation of the 2004 Sumatra–Andaman earthquake deduced from GPS observations in northern Sumatra

doi:10.1016/j.jseaes.2014.03.016

Journal of Asian Earth Sciences

Volume 88, 1 July 2014, Pages 218-229

https://doi.org/10.1016/j.jseaes.2014.03.016 Get rights and content

Highlights

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Afterslip and Maxwell viscoelastic relaxation explain deformation in northern Sumatra.
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A rheological structure with elastic layer depth of 65 ± 5 km.
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A rheological structure with Maxwell viscosity of 8.0 ± 1 × 10¹⁸ Pa s.
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Our model is applicable to the GPS in northern Sumatra, Thailand, and Andaman–Nicobar.

Abstract

We investigate the postseismic deformation of the 2004 Sumatra–Andaman earthquake (SAE) using 5 years of Global Positioning System (GPS) data located in northern Sumatra. Continuous GPS data from northern Sumatra suggest that the relaxation time in the vertical displacement is longer than horizontal displacements. This implies that there are multiple physical mechanisms that control the postseismic deformation, which refer to afterslip and viscoelastic relaxation. In this study, we introduce an analysis strategy of postseismic deformation to simultaneously calculate multiple mechanisms of afterslip and viscoelastic relaxation. The afterslip inversion results indicate that the distribution of the afterslip and the coseismic slip are compensatory of each other. Also, afterslip has a limited contribution to vertical deformation in northern Sumatra. In our rheology model, we use a gravitational Maxwell viscoelastic response and the result indicates that the elastic layer thickness is 65 ± 5 km and the Maxwell viscosity is 8.0 ± 1.0 × 10¹⁸ Pa s. We find that afterslip plus Maxwell viscoelastic relaxation are appropriate to explain the deformation in northern Sumatra. We also find that our rheology model reproduces the long-term features of the GPS time series in Thailand. Applying our rheology model to the data in Andaman Islands our afterslip estimation is located at the down-dip part of the plate boundary. Finally, we showed that our rheology model is applicable to the GPS datasets of postseismic deformation of the 2004 SAE located in northern Sumatra, Thailand, and Andaman–Nicobar, respectively.

Introduction

After large earthquakes, in many cases significant postseismic deformation is observed (e.g. Feigl and Thatcher, 2006, Wang, 2007). Postseismic deformation is caused by mechanisms such as a viscous flow in the Earth’s upper mantle and/or the lower crust, commonly referred to as viscoelastic relaxation (e.g. Wang et al., 2001, Hu et al., 2004, Bürgmann and Dresen, 2008), and afterslip reflecting the frictional properties of a fault or a plate interface (e.g. Miyazaki et al., 2004, Ozawa et al., 2011). Various studies suggest that multiple mechanisms are responsible for postseismic deformation in many cases (e.g. Pollitz et al., 1998, Sheu and Shieh., 2004, Ryder et al., 2007, Suito and Freymueller, 2009, Wang et al., 2009).

On December 26, 2004, a great megathrust earthquake, the 2004 Sumatra–Andaman earthquake (SAE), occurred in the Sunda subduction zone along the northern Sumatra, Nicobar, and Andaman Islands. This was the first M9 class earthquake recorded by modern global networks of seismic as well as geodetic instruments (Kanamori, 2006). The source fault of this earthquake was as long as ∼1300 km along the Sunda trench (e.g. Lay et al., 2005). The coseismic stress change was large and extensive, causing significant postseismic deformation. Global Positioning System (GPS) data after the 2004 SAE indicate extensive displacements of postseismic deformation, characterized by a trench-ward motion on the continental side of the plate boundary (Fig. 1).

Many studies have investigated postseismic deformation related to the 2004 SAE rupture. In the Andaman Islands, by using GPS data for the first 2 years after the main shock, Paul et al. (2007) concluded that postseismic deformation in this region was dominated by afterslip. Using different GPS data, Gahalaut et al. (2008) came to the same conclusion. Later, using more GPS data for 6 years, Paul et al. (2012) revisited the problem and concluded that a combination of afterslip and viscoelastic relaxation was the major mechanism. Their rheology model consists of a 90 km thick surface elastic layer overlying the upper mantle with a viscosity of 3 × 10¹⁷–10¹⁸ Pa s.

Han et al. (2008) detected a postseismic transient signal in the first 2 years after the 2004 SAE using Gravity Recovery and Climate Experiment (GRACE) satellite observations. They estimated the rheological structure of biviscous viscoelastic flow with a transient viscosity of 5 × 10¹⁷ Pa s and a steady state viscosity of 5 × 10¹⁸–10¹⁹ Pa s. Similar results were obtained by Hoechner et al. (2011), who analyzed postseismic deformation using GPS data in the Andaman Islands and geoidal change from GRACE during the first 2 years after the main shock. They reported that the surface elastic layer is 40 km thick, and that Burgers transient rheology in the asthenosphere with a transient Kelvin viscosity of 10¹⁸ Pa s and steady state Maxwell viscosity of 10¹⁹ Pa s reproduce the observation data very well.

Another study reported a postseismic investigation of 2004 SAE using combination data sets of gravity variations and GPS measurements in Thailand (Panet et al., 2010). They concluded that a combination of viscoelastic relaxation and afterslip at the down-dip portion of the rupture is capable of explaining these data very well. Their rheology structure consists of a 60 km thick elastic layer overlying a Burgers viscoelastic asthenosphere with a transient viscosity and a steady state viscosity equal to 4 × 10¹⁷ Pa s and 8 × 10¹⁸ Pa s, respectively. In this model, the upper mantle below 220 km depth has a Maxwell rheology with a viscosity of 8 × 10¹⁸ Pa s.

Using a spherical-earth finite element model, Hu and Wang (2012) reported a short-term postseismic deformation model using ∼1 year GPS displacements in the Andaman Islands and northern Sumatra, and ∼3 year GPS time series in Thailand. The observed deformation is best explained with a model that includes both the afterslip and transient rheology, with transient and steady state viscosities of the mantle wedge of 5 × 10¹⁷ Pa s and 10¹⁹ Pa s.

As we see, previous studies reached different conclusions. Discrepancies are mainly attributed to different assumptions on postseismic deformation mechanisms. Also, previous studies only use a certain part (e.g. the Andaman Islands alone) of observations without investigating postseismic deformation data in northern Sumatra, close to the largest coseismic slip rupture of 2004 SAE (e.g. Subarya et al., 2006, Rhie et al., 2007).

In this study, we tackle the postseismic deformation of the 2004 SAE by taking GPS data in northern Sumatra into account. The analyzed GPS data are obtained from the Aceh GPS Network for Sumatran Fault System (AGNeSS) (Ito et al., 2012). AGNeSS data are important due to the following reasons: (1) The network is located in the near-field of the main slip patch of the 2004 SAE. Significant postseismic deformation is expected near the main rupture of the 2004 SAE; (2) GPS measurements started a few months after the main shock, providing information of early postseismic deformation after the 2004 SAE; (3) Continuous GPS measurements provide a good control on both the horizontal and vertical components; (4) GPS data have never been used in analyzing postseismic deformation comprehensively. Ito et al. (2012) used these data to invert afterslip, in their attempt to estimate slip rate in northern Sumatra. In this study, instead of inverting from a single physical mechanism of afterslip, we take two physical mechanisms of afterslip and viscoelastic relaxation into consideration.

In order to give a clear interpretation of postseismic deformation of 2004 SAE, we include both horizontal and vertical components in our analysis from continuous GPS sites. Vertical GPS data in northern Sumatra provide valuable information to find an optimum rheology model. We also introduce a new strategy of postseismic calculation which takes two physical mechanisms, afterslip and viscoelastic relaxation, into account.

Section snippets

GPS network in northern Sumatra and data analysis

We analyze GPS data in northern Sumatra obtained by AGNeSS operated under the collaboration of Nagoya University, Kochi University, Tohoku University, Institute of Technology Bandung and Syiah Kuala University in northern Sumatra. AGNeSS consists of both continuous and campaign sites. The first continuous site, ACEH, was installed in March 2005, located in Banda Aceh. A detailed explanation of AGNeSS in northern Sumatra is found in a separate publication (Ito et al., 2012). In addition, in this

Characteristics of the displacement data

We show time series of daily solution of the east, north, and vertical components on continuous GPS sites of ACEH and UMLH, and east and north components on the campaign GPS sites of BEUN, JERM, CELA, GEUM, KAWA and MBMG in Fig. 2. For campaign GPS sites we do not use vertical information in this study due to a lack of antenna height records during the campaign GPS observations. The error bars of the daily solution indicate the (one sigma) standard deviation.

Observed GPS displacements in

Afterslip in northern Sumatra

In our calculation, the total seismic moment released by the afterslip in northern Sumatra for four years between 2005.91 and 2009.87 was 2.45 × 10²¹ N m (M_w 8.2). During 2005.91–2006.90, 2006.90–2007.90, the afterslip moment calculated was 1.12 × 10²¹ N m (M_w 7.9) and 0.52 × 10²¹ N m (M_w 7.7), respectively. This result indicates that the afterslip moment is approximately halved every year.

In Fig. 8 we compare the estimated afterslip distribution during 2005.91–2006.90 with the coseismic slip by Rhie et

Conclusion

We use GPS data located in northern Sumatra consisting of continuous and campaign measurements to analyze the postseismic deformation after the 2004 SAE. All GPS sites experienced southwestward motion during the postseismic period, which is characterized by trench-ward motion on the continental side of the plate boundary. Relaxation time in the vertical displacement is longer than the horizontal displacement, which implies that there are multiple physical mechanisms controlling the postseismic

Acknowledgments

The authors would like to thank Mako Ohzono and anonymous reviewer for the thoughtful comments and constructive suggestions, which help to improve the quality of this manuscript. We also want to thank V.K. Gahalaut and C. Satirapod who kindly provided the GPS data in the Andaman Islands and Thailand. We also thank Fred Pollitz for making the VISCO1D code freely available. The maps and figures were generated using Generic Mapping Tools (GMT) software (Wessel and Smith, 1998).

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A comprehensive model of postseismic deformation of the 2004 Sumatra–Andaman earthquake deduced from GPS observations in northern Sumatra

Highlights

Abstract

Introduction

Section snippets

GPS network in northern Sumatra and data analysis

Characteristics of the displacement data

Afterslip in northern Sumatra

Conclusion

Acknowledgments

Phys. Earth Planet. Inter.

C.R. Geosci.

Likelihood and the Bayes procedure

ITRF2008: an improved solution of the international terrestrial reference frame

J. Geodesy

Coseismic slip distributions of the 26 December 2004 Sumatra–Andaman and 28 March 2005 Nias earthquakes from GPS static offsets

Bull. Seismol. Soc. Am.

Rheology of the lower crust andupper mantle: evidence from rock mechanics, geodesy, and field observations

Annu. Rev. Earth Planet. Sci.

Coseismic slip and afterslip of the great Mw 9.15 Sumatra–Andaman earthquake of 2004

Bull. Seismol. Soc. Am.

Tsunami source of the 2004 Sumatra–Andaman earthquake inferred from tide gauge and satellite data

Bull. Seismol. Soc. Am.

GPS measurements of postseismic deformation in the Andaman–Nicobar region following the giant 2004 Sumatra–Andaman earthquake

J. Geophys. Res.: Solid Earth

Implications of postseismic gravity change following the great 2004 Sumatra–Andaman earthquake from the regional harmonic analysis of GRACE intersatellite tracking data

J. Geophys. Res.: Solid Earth

Slab1. 0: a three-dimensional model of global subduction zone geometries

J. Geophys. Res.: Solid Earth

Investigation on afterslip and steady state and transient rheology based on postseismic deformation and geoid change caused by the Sumatra 2004 earthquake

Geochem. Geophys. Geosyst.

Three-dimensional viscoelastic finite element model for postseismic deformation of the great 1960 Chile earthquake

J. Geophys. Res.

Spherical-Earth finite element model of short-term postseismic deformation following the 2004 Sumatra earthquake

J. Geophys. Res.: Solid Earth

Isolating along-strike variations in the depth extent of shallow creep and fault locking on the northern Great Sumatran Fault

J. Geophys. Res.: Solid Earth

Lessons from the 2004 Sumatra–Andaman earthquake

Philos. Trans. Roy. Soc. A: Math., Phys. Eng. Sci.

The great Sumatra–Andaman earthquake of 26 December 2004

Science

Modelling the global ocean tides: modern insights from FES2004

Ocean Dyn.

Noise in GPS coordinate time series

J. Geophys. Res.: Solid Earth

Space time distribution of afterslip following the 2003 Tokachi-oki earthquake: implications for variations in fault zone frictional properties

Geophys. Res. Lett.

Surface deformation due to shear and tensile faults in a half-space

Bull. Seismol. Soc. Am.

Coseismic and postseismic slip of the 2011 magnitude-9 Tohoku-Oki earthquake

Nature