Seismic b-value anomalies in the Sumatran region: Seismotectonic implications

Accepted Manuscript Seismic b-value Anomalies in the Sumatran Region: Seismotectonic Implications Zhou Gui, Yongliang Bai, Zhenjie Wang, Tongfei Li PII: DOI: Reference:

S1367-9120(19)30019-7 https://doi.org/10.1016/j.jseaes.2019.01.015 JAES 3751

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

29 August 2018 3 January 2019 4 January 2019

Please cite this article as: Gui, Z., Bai, Y., Wang, Z., Li, T., Seismic b-value Anomalies in the Sumatran Region: Seismotectonic Implications, Journal of Asian Earth Sciences (2019), doi: https://doi.org/10.1016/j.jseaes. 2019.01.015

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Seismic b-value Anomalies in the Sumatran Region: Seismotectonic Implications Zhou Gui1, 2, Yongliang Bai1, 2*, Zhenjie Wang1, 2, Tongfei Li3 1. School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China 2. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China 3. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China

*

Corresponding author.

E-mail address: [email protected] (Y. Bai)

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ABSTRACT The spatial variations in the seismic b-value, seismic moment, Bouguer gravity change and fault-plane solution are estimated in the Sumatran region based on homogeneous seismicity data from June 2005 to March 2012 to investigate the seismotectonic characteristics and tectonic activity between the 2004 Sumatran earthquake and the 2012 East Indian Ocean earthquakes. Based on changes in the seismicity rate, this time period is divided into five intervals with different earthquake distributions and faulting styles. The seismic b-values, faulting mechanisms and seismic moment release values are mapped throughout the Sumatran region, and the gravity change every two years between the 2004 event and the 2012 events are determined from satellite (GRACE) gravity data. The b-values vary from 0.45 to 1.59, and an inverse correlation is observed between the b-values and convergence rates. Low b-values are located at depths between 20 km and 40 km in the subduction zone and between 20 km and 35 km in the oceanic plate; the lowest b-values are located at a depth of approximately 28 km in both regions, which indicates a homogeneous fault system between them. The areas with low b-values are associated with megathrust and strike-slip mechanisms, while the area with the lowest b-values is dominated by a megathrust mechanism. Seismically active regions have low b-values associated with a high stress accumulation and high seismic moment release. Low b-value areas are located mainly along the Andaman-Sunda Trench and Andaman-Nicobar-Sumatra forearc. Temporal variations in the seismicity distribution and faulting style are recognized with temporal

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variations in the b-values in each period. Relationships exist between the epicenter of the 2004 event and the location of the maximum Bouguer gravity change in 2012 and between the epicenters of the 2012 events and the location of the maximum Bouguer gravity change in 2004. Furthermore, a zone of continuous 2-year positive gravity change is located near the Nicobar Islands. The 2004 event boost the 2012 events and intraplate seismicity via stress transfer. The seismicity distributions in individual periods within the studied timeframe highlight the active and locked portions of the fault systems.

Keywords Sumatran region; Seismic b-value; Bouguer gravity change; Seismotectonic characteristics; Tectonic activity

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1. Introduction The seismically active Sumatran region is composed of the Sumatran oblique subduction zone (SOSZ), where the denser Indian-Australian plate subducts beneath the more buoyant Eurasian plate (Beck, 1983; Fitch, 1972; Hamilton, 1973, 1979; Mccaffrey, 1992, 2009; Mignan and Woessner, 2012; Moeremans and Singh, 2015), and its adjacent oceanic plate region, which is mainly the diffuse Indo-Australian plate boundary (DIAPB) (Fig. 1). The diffuse plate boundary has been reported to be composed of narrow slivers of reactivated N-S-oriented fractures caused by strike-slip faulting within the northern Wharton Basin (Delescluse and Chamot-Rooke, 2007; Deplus, 2001; Deplus et al., 1998; Petroy and Wiens, 1989; Satriano et al., 2012; Stein et al., 1990). Seismicity from thrust earthquakes and strike-slip earthquakes has been reported in the study area (Nalbant et al., 2005; Newcomb and Mccann, 1987; Zachariasen et al., 1999). Intermediate earthquakes appear as linear clusters in the subduction zone and are associated with the heterogeneity of the incoming plate (Kirby et al., 1996). Four great earthquakes (Mw>8.0) have occurred in the study area since 1976 (https://earthquake.usgs.gov/earthquakes/): the Mw 9.1 Sumatran earthquake in 2004 (Pollitz et al., 2006b), the Mw 8.6 Nias earthquake in 2005 (Meltzner et al., 2012), and the Mw 8.6 and Mw 8.2 East Indian Ocean earthquakes in 2012 (Pollitz et al., 2012). The Sumatran and Nias earthquakes (the first two events) ruptured lengths of ~1500 km and ~400 km along the SOSZ, respectively (Banerjee et al., 2007), and they

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were followed by after-slip processes simultaneous with viscoelastic relaxation (Broerse et al., 2015; Pollitz et al., 2006a). The Mw 8.6 East Indian Ocean earthquake occurred on Apr. 11, 2012, followed two hours later by the Mw 8.2 aftershock ~1.5° south of the epicenter of the mainshock (Wei et al., 2013) (the second two events). These two events occurred approximately 100 km and 300 km, respectively, from the Sunda Trench in the adjacent oceanic plate. The 2004 event increased the seismicity in the adjacent oceanic plate via stress transfer through the Sumatra-Andaman plate boundary (Andrade and Rajendran, 2014) and enhanced the intraplate deformation, promoting the occurrence of the 2012 twin events (Delescluse et al., 2012). Due to the adjacent position of the two sequential earthquakes (Hamilton, 1979; Wiens et al., 2013), it is necessary to determine the seismotectonic characteristics and tectonic activity throughout the entire study area, which encompasses the locations of both sets of earthquakes, and to consider the interaction effects between the subduction zone and its adjacent plate region. Seismicity can indicate seismotectonic characteristics and inter/intraplate tectonic activity. The spatiotemporal changes in seismicity between the two sets of events are clearly expressed in geodetic and geophysical observations. This finding indicates a mutually boosting relationship between the tectonic activity and fault systems, which highlights the importance of simultaneous consideration of the subduction zone and its adjacent plate region. Specifically, the seismic b-value is an empirical value with seismotectonic implications that varies based on the locations and magnitudes of earthquakes; the value of this

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parameter is related to the regional tectonics, the structural heterogeneity of the crust (Bora et al., 2018; Khan and Chakraborty, 2007; Scholz, 1968; Schorlemmer et al., 2003), and the stress distribution (Mousavi, 2016, 2017). In addition, b-values are dependent on the faulting style (Schorlemmer et al., 2005), earthquake depth (Wyss et al., 1997) and thermal gradient (Warren and Latham, 1970). Furthermore, the b-value can be influenced by pure-pressure changes (Mousavi, 2017). High b-values are related to large numbers of weak earthquakes, while low b-values are associated with fewer, stronger earthquakes (Wyss et al., 1997). Gravity change is highly significant for the retrieval of deformation associated with the redistribution of mass in response to the rupture processes of faulting and coseismic stress relaxation, which can continue for years after the occurrence of a mainshock (Han et al., 2006). With the availability of seismic data and satellite gravity data in the study area, the spatial distribution of seismic b-values and their correlations with fault-plane solutions and the seismic moment release are investigated; accordingly, coseismic and postseismic gravity change occurring between the 2004 Sumatran earthquake and the 2012 East Indian Ocean earthquakes are examined. The main goal is to illustrate the following: (a) the seismotectonic characteristics of the Sumatran region based on the correlations between the seismic b-values and the fault-plane solutions and seismic moments, (b) the tectonic activity in the study area between the 2004 Sumatran earthquake and the 2012 East Indian Ocean earthquakes based on gravity change and the distributions of epicenters in separate periods.

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2. Geological setting The SOSZ is categorized as a convergent plate boundary extending northward from the Andaman and Nicobar Islands to Sumatra (Hamilton, 1973) (Fig. 1). The downgoing Indo-Australian plate moves northeastward under the overriding Eurasian plate, resulting in contemporary oblique subduction; the rate of the convergence varies from 63 mm/year in South Sumatra to 52 mm/year in North Sumatra (Prawirodirdjo and Bock, 2004) to 43 mm/year off the Nicobar Islands (Gahalaut et al., 2006) (Fig. 1). The trench-parallel strain accumulation along the SOSZ is taken up by strike-slip faults, particularly the Great Sumatran Fault in the overriding plate (Sieh and Natawidjaja, 2000; Yu et al., 1993). More severe earthquakes are more likely to occur along the subduction thrust faults as well as the rupture areas in the subducting and overriding widened plates (Mccaffrey, 2009). Because of the 10 mm/year difference and the slab-pull force from the Sunda Trench, the Australian plate has detached from the Indian plate (Delescluse and Chamot-Rooke, 2007; Deplus, 2001; Mccaffrey, 2009). The convergence vectors change orientation from almost orthogonal to the trench (southeast off Sumatra) to highly oblique (northwest off the Andaman Islands) (Andrade and Rajendran, 2014). The shear force induced by this change in the convergence direction contributes to subduction thrust faulting and strike-slip faulting (Henstock et al., 2006). The subduction thrust is composed of dip-slip and strike-slip components, while trench-parallel motion occurs in the overriding plate (Mccaffrey, 2009). The slip of the oblique convergence is accommodated along the Sumatran Fault

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as a strike-slip sliver fault and its adjacent sliver plate (Fitch, 1972; Mccaffrey, 1992). Subarya et al. (2006) suggested that the plate movement directions clearly change along the margin from approximately 6°N to 7°N; the location of this direction change represents the boundary between the Andaman segment and the Sumatran segment. The oceanic region studied here is mainly the northern Wharton Basin, which contains a structural fabric of N-S fracture zones and several E-W fossil ridges (Demets et al., 1994). The fossil transform faults are evidenced by offsets in the basement and sediments as well as by the expression of the seafloor (Graindorge et al., 2008). The seismicity and deformation pattern of the northern Wharton Basin result from the complex regional deformation of the DIAPB, the situation adjacent to the Sunda Trench, and the potential reactivation of pre-existing structures (Andrade and Rajendran, 2014). Wiseman and Bürgmann (2012) suggested that the highest strain rates in the Indian Ocean are located in the northern Wharton Basin, which features thermally young, thin oceanic lithosphere that is being sheared along a NNE-SSW-oriented fracture zone. The tectonic activity of the rupture has diffused the deformation zone (Mcguire and Beroza, 2012). The subduction zone was ruptured by the 2004 Sumatran earthquake, and this process initiated around the epicenter near Simeulue Island and spread over a length of 1500 km and a width of 150 km along the trench to the north of the Andaman Islands (Chlieh et al., 2007; Lay et al., 2005) (Tabs. 1 & 2). The maximum slip rate occurred within the segments of the oceanic lithosphere (Delescluse

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et al., 2012). The 2004 Sumatran and 2005 Nias earthquakes were due to the ~N18°E convergence between the Indian and Eurasian plates (Oishi and Sato, 2007; Socquet et al., 2006). The 2012 events were the result of trench-parallel strain accumulated in the SOSZ (Ishii et al., 2013). The ruptures of the 2012 East Indian Ocean earthquakes initiated approximately 400 km offshore to the south-west of northern Sumatra by the reactivation of pre-existing structures (Andrade and Rajendran, 2014) and occurred to the west of the Sumatran megathrust (Hill et al., 2015) (Tabs. 1 & 2). The 2012 earthquakes occurred to the north of the DIAPB (Royer and Gordon, 1997), a seismically active oceanic intraplate area in historical records, and they were caused mostly by strike-slip motion due to compressional stress between the Indian and Australian plates (Andrade and Rajendran, 2014). Fig. 2a shows the earthquake distribution in the Sumatran region. The points depict the epicenters (Mw 1.7 - Mw 7.8) across the study area from Jun. 2005 to Mar. 2012 (Fig. 2a). Earthquakes are evidently clustered within the subduction zone and the adjacent oceanic plate (Fig. 2a).

3. Data 3.1 Earthquake data Earthquake catalogs are necessary for b-value calculation. All the earthquake events used in this study occurred during the period between one month after the 2005 Nias earthquake (Apr. 2005) and one month before the 2012 East Indian Ocean earthquake (Mar. 2012) in the region from 90°E to 100°E and from 0° to 15°N. The

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earthquake data were downloaded from the International Seismological Centre (ISC) catalog

through

the

Incorporated

Research

Institutions

for

Seismology

(http://www.iris.edu/hq/retm) and included 7404 shallow-focus (≤70 km) seismic events with magnitudes ranging from Mw 1.7 to Mw 7.8. Centroid-moment-tensors were

extracted

from

the

Global

Centroid-Moment-Tensor

(CMT)

Project

(http://www.globalcmt.org). For the purpose of investigating the faulting styles of the seismicity in the study area, the strike, dip and rake components of 370 earthquakes (Mw≥4.5) were chosen. 3.2 Bouguer gravity data Since the Gravity Recovery and Climate Experiment (GRACE) satellites were launched in Mar. 2002, multiple data analysis centers have published distinct temporal variation products and solutions at various intervals for measuring the Earth’s gravity field. The GRACE mission aims to obtain accurate global and high-resolution models for both the static and the dynamic components of the Earth’s gravity field, which are expected to provide persistent solutions to monitor the mass changes associated with large earthquakes (Mw>8.0), especially in the case of offshore events, as a supplement to GPS, interferometric synthetic-aperture radar (InSAR) and other geodetic methods (Chen et al., 2015). Postseismic gravity change is more sensitive than coseismic change over longer periods of observation. Earth’s gravity change is recorded continuously by GRACE. This study collected monthly GRACE RL05 gravity solutions (Bettadpur, 2007) spanning from Dec. 2002 to Apr. 2014 (gravity data from 2 years before the

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2004 event to 2 years after the 2012 events were used to calculate the coseismic gravity change). A few months were missing due to the lack of accelerometer measurements.

4. Methodology 4.1 Magnitude of completeness The magnitude of completeness (Mc) constitutes the minimum complete record of earthquakes in the study area, that is, a dataset including earthquake events with the minimum Mc will produce the most accurate b-value results (Maden and Öztürk, 2015; Woessner and Wiemer, 2009). Cumulative frequency magnitudes are the basis for estimating Mc (Habermann, 1983). The Mc can be measured using different estimation methods such as the maximum curvature method (Wiemer and Wyss, 2000) and the entire-magnitude-range (EMR) method (Wiemer, 2001). Because conservative methods such as the EMR and best fit methods will lead to relatively high Mc values, the best combination method (Wiemer and Wyss, 2000), which is widely accepted (Bora et al., 2018; Borgohain et al., 2018), was selected to perform a standard analysis:

Mc95  Mc90  maximum curvature

(1)

where Mc95 and Mc90 represent the 95% and 90% goodness-of-fit values estimated by the maximum likelihood method, respectively, and maximum curvature represents the Mc estimated by the maximum curvature method. The selected Mc should be homogeneous throughout time and space (Mignan and Woessner, 2012; Mousavi, 2016). Based on the data from the beginning of 2004 to the end of 2014

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(17742 events, Mw>1.7), the cumulative number of seismic events and the temporal variation in Mc are estimated (Fig. 3). Then, based on the number and distribution of seismic events versus time in the study area, the sample window size chosen in this study is 300. Due to the construction of seismic stations in the Sumatran region after the 2004 event, the curve of the cumulative number of seismic events increases sharply (Fig. 3a), and the Mc curve fluctuates strongly (Fig. 3b) until Jun. 2005, when all the seismic monitoring stations were installed. Therefore, the data from Jun. 2005 to Mar. 2012, during which the cumulative curve in Fig. 3a presents linear growth and the curve in Fig. 3b shows gentle changes, were selected for the following analysis. 4.2 Seismic b-value calculation The frequency-magnitude relationship was first given by Gutenberg and Richter (1942):

log10 N (M )  a  bM

(2)

where N(M) is the cumulative number of earthquakes with a magnitude larger than the target magnitude M , and a and b are two positive constants, where a is described as seismic activity varying from 2 to 8 and b (seismic b-value) represents the slope of the cumulative number relative to the magnitude trend spanning mainly from 0.5 to 1.5 (Borgohain et al., 2018). In Fig. 2b, the study area is partitioned into 1°×1° square grids and is color coded based on the number of events larger than the Mc (Mw ≥ Mc) in each grid. To maintain the continuity of the calculations, a 0.5° moving window is defined. Grids containing at least 25 events are chosen in this

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study to reduce outliers or areas with insufficient data. In this study, the b-values are calculated by using maximum likelihood estimation (Aki, 1965), expressed as follows:

b where

log10 e [( M )  ( M C  M bin / 2)]

(3)

is the average magnitude of all the events above M C and M bin is the

binning width. The software ‘Zmap’ is used to calculate the seismic b-values (Wiemer and Zúñiga, 1994). The relation diagram between the time and the cumulative number of events in the study area shows a series of variations after the 2004 earthquake. The Mc is estimated to be 4.3 in both the SOSZ (2043 events) and the adjacent oceanic plate (327 events), and the b-values of these two regions are calculated as 1.08±0.03 and 1.08±0.07, respectively (Fig. 3c & Fig. 3d). The uncertainty in the b-values is due to the linear derivation of the cumulative number as a function of the magnitudes in the Sumatran subduction zone and its adjacent oceanic plate (Fig. 3c & Fig. 3d), which can be used to evaluate the reliability of the b-values estimations (Bora et al., 2018). The seismic moment, an earthquake parameter that has tectonic implications (Westaway, 1992), is derived from the moment magnitude by the following standard formulation to illustrate the stress accumulation (Kanamori and Brodsky, 2001):

log M 0  1.5* M w  9.1 where

is the seismic moment and

(4)

is the moment magnitude.

4.3 Bouguer gravity change Due to hydrological and oceanic mass fluxes, gravity signals have climate-related

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characteristics and feature seasonal, interannual, or even longer frequency changes. The 2-year average gravity change is used to remove seasonal signals and to eliminate gravity change contamination resulting from the effects of great earthquakes (Chen et al., 2015; Han et al., 2006). Both the spherical harmonic degree and the order of the gravity field are equal to 40 (spatial resolution of 500 km), and a decorrelation filter (DDK5) is applied to reduce the stripe noise of background values. Gravity change occurs in response to proportional slip vectors resulting from the fault geometry over time and are influenced by the rheology of the crust and postseismic relaxation over the subduction zone (Han et al., 2010). Gravity change is calculated to determine changes in the interior mass density driven by deformation. The postseismic gravity change is greater than the coseismic response and can be used to identify tectonic deformation and fault-locking phenomena in a study area based on viscoelastic relaxation (Han et al., 2016). The coseismic gravity change for the 2004 Sumatran earthquake is computed as the difference between the average field from Dec. 2002 to Nov. 2004 before the earthquake and the average field from Jan. 2005 to Nov. 2006 after the earthquake (Fig. 4a). The postseismic gravity change (two years after the earthquake) is calculated as the difference between the mean field from Jan. 2007 to Dec. 2008 and the average value from Jan. 2005 to Nov. 2006 (Fig. 4b). The gravity change two years after the first postseismic gravity change calculation is calculated as the difference between the mean field from Jan. 2009 to Dec. 2010 and the average value from Jan. 2007 to Nov. 2008 (Fig. 4c). The coseismic gravity change

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associated with the 2012 East Indian Ocean earthquake is calculated as the difference between the average field from Apr. 2010 to Mar. 2012 before the earthquake and the average field from May 2012 to Apr. 2014 after the earthquake (Fig. 4d).

5. Results The average b-values for the subduction zone and its adjacent oceanic plate are estimated to be 1.08±0.03 and 1.08±0.07, respectively, with an Mc of 4.3 (Fig. 3c & Fig. 3d). Fig. 5 shows the spatial distribution of b-values across both the subduction zone and the adjacent oceanic plate, where the blank area is due to the lack of seismic data. The events feature b-values varying from 0.45 to 1.59 (Fig. 5), due to the active tectonic setting on a regional scale. To probe the various behaviors at different depths, the b-value variations versus the focal depth are estimated in both the subduction zone and the adjacent oceanic plate (Fig. 6). The depths of the subduction zone and its adjacent oceanic plate are divided every 8 km from the surface to 70 km. A moving step (overlapping depth) is defined as 4 km to process the data points continuously (Fig. 6). The characteristics of the seismicity in the subduction zone and its adjacent oceanic plate region are as follows. (1) In the subduction zone: A diagram of the seismicity shows low b-values (0.87 1.15) at depths between 20 km and 35 km (Fig. 6a) followed by a strong increase in values from 0.87 to 1.20 at depths between 28 km and 40 km and a sharp decrease in values at depths between 43 km and 48 km (Fig. 6a). A large portion of the foci are located at depths from 21 km to 42 km (b-values: 0.87 - 1.20), and fewer foci are

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located at depths from 42 km to 70 km (b-values: 1.04 - 1.53), while very few events occur at depths from the surface to 21 km (b-values: 1.00 - 1.48) (Fig. 6a). Most earthquake magnitudes are less than 5.5, and only a few earthquakes have magnitudes greater than 6.5 (Fig. 6c). (2) In the oceanic plate: Only the b-values at depths from 20 km to 40 km are valid due to the lack of seismicity at depths from the surface to 20 km and from 40 km to 70 km (Fig. 6b). Low b-values (0.97 - 1.2) are distributed at depths between 20 km and 35 km (Fig. 6b), and most seismic magnitudes are less than 5.5 (Fig. 6d). (3) The event with the maximum magnitude in the subduction zone occurred two months before and was 0.3 larger in magnitude than the event with the maximum magnitude in the adjacent oceanic plate; both events occurred between 2010 and 2011 (Fig. 6c & Fig. 6d). The characteristics of the seismicity in the two regions share the commonalities: the depths of the hypocenters were primarily 20 km - 40 km, and the magnitudes were primarily between Mw 4.5 and Mw 5.5. The seismicity rate changes are shown in Fig. 7. Four significant seismicity rate changes are detected in Nov. 2005, Nov. 2006, Aug. 2008 and Aug. 2010 in the study area. Based on the observed rate changes in Fig. 7, the time from Jun. 2005 to Mar. 2012 is divided into five periods: Jun. 2005 ~ Oct. 2005, Nov. 2005 ~ Oct. 2006, Nov. 2006 ~ Jul. 2008, Aug. 2008 ~ Jul. 2010 and Aug. 2010 ~ Mar. 2012. The postseismic Bouguer gravity change in every two-year period from the 2004 Sumatran earthquake to the 2012 East Indian Ocean earthquakes is examined for

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several specific characteristics to reveal the crustal deformation in the study area (Fig. 4, Tab. 3). Coseismic positive gravity changes occur to the west of Sumatra and the Nicobar-Andaman Islands, while a negative change is concentrated in the Andaman Sea after the 2004 Sumatran earthquake (Fig. 4a). The maximum positive gravity change point (PA), corresponding to 9.64 μGal, is located around the epicenter of the 2012 mainshock (Fig. 4a, Tab. 3). A positive postseismic gravity change calculated by the difference between Jan. 2007 ~ Dec. 2008 and Jan. 2005 ~ Dec. 2006 lies along the Sunda Trench mainly in the subduction zone from 0°N to 12°N; the maximum value (5.24 μGal) is located to the southwest and west of the Nicobar Islands (Fig. 4b, Tab. 3). In addition, a positive gravity change calculated by the difference between Jan. 2009 ~ Dec. 2010 and Jan. 2007 ~ Dec. 2008 appears in the subduction zone adjacent to the Nicobar Islands from 3°N to 10°N, and the peak change (3.46 μGal) is located near the Nicobar Islands (Fig. 4c, Tab. 3). The Aceh region and Simeulue Island show positive gravity change after the 2012 East Indian Ocean earthquakes (Fig. 4d). The maximum positive gravity change point (PB) corresponding to 7.82 μGal is located around the epicenter of the 2004 event (Fig. 4, Tab. 3). The seismic moment (M0) is estimated by Equation (4), and the spatial distribution of log M 0 is mapped (Fig. 8). The spatial distribution of the seismic moment varies from 14.0 to 16.0 (Fig. 8). A high seismic moment release is shown around the Aceh region between the epicentral zones of the 2004 and 2005 earthquakes along the megathrust fault from 6°N to 8°N and the strike-slip fault from 4°N to 13°N (Fig. 8).

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6. Discussion 6.1 Seismotectonic characteristics To understand the seismotectonic characteristics of the study area, the distribution of seismic b-values (Fig. 5) and their correlations with the faulting style and seismic moment are estimated within the subduction zone and its adjacent oceanic plate. The b-values range from 0.45±0.07 to 1.59±0.12 in the study area (Fig. 5). The b-values in the Sumatran subduction zone were calculated by other researchers based on the seismicity from 1964 to 2007 (Roy et al., 2011), from 2002 to 2004 (Nuannin et al., 2005) and from 2000 to 2010 (Nuannin et al., 2012), and low values are located in the seismically active tectonic zone. Our estimation results also yield low b-values in the Aceh-Nias segment and the Andaman-Nicobar segment (Fig. 5). Andaman-Nicobar segment: Low b-values (0.7 ~ 1.0) are calculated from the Nicobar Islands to the south of the Andaman Islands between 5°N and 12°N in the subduction zone and its adjacent oceanic plate and correspond to high seismic moments (14.9 ~ 15.5) (Fig. 8). Aceh-Nias segment: In the region from Nias to the south of Aceh between the Sumatra Fault and the Sunda Trench, low b-values (0.5 ~ 0.9) are detected along strike-slip faults (Fig. 5). Similarly, low b-values in the range between 0.6 and 0.8 are detected off the Sunda Trench between 2°N and 3.5°N in the oceanic plate corresponding to high seismic moments ranging approximately from 15.22 to 16.04; these values are relatively high for the study area. The intensities of these earthquakes can be measured directly from the seismic moment data, which have stress-related implications.

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The variations in the b-values with depth in the study area are estimated separately in the subduction zone and its adjacent oceanic plate (Fig. 6). The low b-values and the high seismic moment release values in the subduction zone and in the adjacent oceanic plate are situated at depths from 20 km to 40 km and from 20 km to 35 km, respectively. Most tectonic activity occurs between 20 km and 40 km (Fig. 6). The Moho is indicated at a depth between 16 km and 30 km (Macpherson et al., 2012), where the estimated b-values are low (0.85 ~ 1.2) in both the subduction zone and the adjacent oceanic plate, due to their adjacent positions and similar seismotectonic settings within the same fault system. This result indicates a coupled relationship between the depth of the Moho and b-value to investigate the crust-mantle transition, which is similar to the observations in the Indo-Burma ranges of northeast India (Bora et al., 2018). Fig. 5 shows the relation between the spatial distribution of b-values and faulting styles. Thrust earthquakes occur all along the subduction zone or are close to the Sunda Trench in the adjacent oceanic plate, while strike-slip earthquakes are distributed along the strike-slip faults in the subduction zone, pre-existing faults or close to the Andaman Trench in the oceanic plate. Seismicity is clustered around the Ninety East Ridge-Andaman forearc convergent zone (the location is shown in Fig. 5), where the strain is high, and this area exhibits a combination of megathrust and strike-slip mechanisms (Lange et al., 2010). The seismicity zone associated with thrust mechanisms features b-values between 0.6 and 1.0, while the strike-slip

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earthquake region features b-values from 0.5 to 1.0. The area with earthquakes characterized by both thrust and strike-slip components exhibits b-values greater than 0.85. This pattern is similar to the observations of other regions, such as the Middle East (Mousavi, 2017). Thrust events have the lowest b-values, strike-slip events have intermediate b-values, and normal events or combinations of thrust and strike-slip events have the highest b-values (Bora et al., 2018; Schorlemmer et al., 2005). Fig. 5 shows that the convergence rate varies from 39 mm/yr (north of the Andaman Islands) to 45 mm/yr (west of Aceh) and higher to 47 mm/yr (west of Nias) (Socquet et al., 2006) corresponding to b-values from 0.6 to 1.2 and seismic moment release values from 14.6 to 15.6(Fig. 8). In contrast, high rates of convergence correspond to low b-values and high seismic moment release values. Low b-values, which are associated with a high accumulation of stress, correspond to a high seismic moment release (Bora et al., 2018). Therefore, the segments of high stress accumulation in the study area are associated with a high probability of earthquakes. The low b-values reflect a high degree of compressional stress in the region due to the frictional conditions of the heterogeneous structure (Mishra et al., 2007). The seismic b-values can be treated as an indirect stress meter for the seismotectonic setting of the study area (Bora et al., 2018). Thus, the b-values are regarded as indicators of different structures along the subduction zone in different segments (Roy et al., 2011). The seismotectonic characteristics in the Aceh-Nias segment and the Andaman-Nicobar segment are discussed below.

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Due to the stress perturbations induced by the 2004 Sumatran earthquake, the seismic activity changed significantly by increasing along the West Andaman fault, the Sumatra system and the offshore thrust faults west of Sumatra Island (Cattin et al., 2013). Intracontinental strike-slip faults are regarded as weak zones, and form close to discontinuities in strength (Molnar and Dayem, 2010). The deformation of the Andaman-Nicobar forearc basin is affected by the obliquity of convergence, and back thrusting and compression in this area are the dominant effects (Curray, 2005). The geologically rapid gravitational collapse of the Sumatra forearc has induced bending in the subducting plate off Sumatra and has produced the distinctive plateau-like morphology of the forearc, generating anomalous near-trench intraplate earthquakes and a prism morphology (Craig and Copley, 2018). Strike-slip earthquakes that are close to the Sunda Trench and have oblique components are influenced by slab-pull and slab-bending forces at the plate boundary (Abercrombie et al., 2003; Bergman and Solomon, 1980; Christensen and Ruff, 1988; Rajendran et al., 2011). The high compressional stress in the oceanic plate and tensional stress in the subduction zone are related to the locked thrust zone caused by high shear loading, which has obstructed the kinetics of the subduction and descent of the oceanic plate (Renata et al., 1988). 6.2 Tectonic activity Seismicity rate changes are evident in the significant changes in the occurrence rates and the associated magnitude distributions (Habermann, 1983). Based on the four significant seismicity rate changes in Oct. 2005, Oct. 2006, Jul. 2008 and Jul.

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2010(Fig. 7), the seismic activities from Jun. 2005 to Mar. 2012 are classified into five time groups (first: Jun. 2005 ~ Oct. 2005; second: Nov. 2005 ~ Oct. 2006; third: Nov. 2006 ~ Jul. 2008; fourth: Aug. 2008 ~ Jul. 2010; and fifth: Aug. 2010 ~ Mar. 2012). These rate changes indicate regional perturbations in the dynamic stress field due to fluid flow or pore pressure within the crust (Chiarabba et al., 2015; Ishibe et al., 2012). Faulting style changes and seismic distribution changes from Jun. 2005 to Mar. 2012 are evident in the epicenters in the Nicobar-Andaman segment and the Aceh-Nias segment(Fig. 9), and these changes suggest a temporal variation in the b-value. In addition, the spatial variations in the b-value have been estimated for the seismicity from Jun. 2005 to Oct. 2006, from Nov. 2006 to Jul. 2008, from Aug. 2008 to Jul. 2010, and from Aug. 2010 to Mar. 2012(Fig. 9), and the results illustrate locking behavior in distinctive stages (Lange et al., 2010). (1) Nicobar-Andaman segment Only a few earthquakes (Mw≥5.5) occurred on both sides of the Andaman Trench west of Nicobar Island from Jun. 2005 to Oct. 2005(Fig. 9a). The period from Nov. 2005 to Oct. 2006 was the most seismically active time of the five periods, and the epicenters form a linear cluster along the Andaman Trench from 6°N to 8.5°N, along the Andaman forearc from 10°N to 14°N and at approximately 77.5°N adjacent to the West Andaman fault with mainly megathrust mechanisms and low b-values (0.5 ~ 0.8) (Fig. 9a). More megathrust events with strike-slip components and low b-values (0.7 ~ 1.3) occurred from Nov. 2006 to Jul. 2008 from 10°N to 11°N adjacent to the Andaman

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Trench in the subduction zone, and fewer earthquakes occurred along the Andaman Trench from 6°N to 8.5°N in this period than in the previous period(Fig. 9b). More strike-slip earthquakes clustered along the Andaman Trench from 6°N to 7.5°N, from 10°N to 11.5°N and from 13.5°N to 15°N in the fourth period than during the previous three periods (Fig. 9c). The earthquakes occurred mostly along the Andaman forearc between 10°N and 15°N and adjacent to the Andaman Trench between 7°N and 8°N in the oceanic plate during the fifth period, and these tremors had smaller magnitudes than the earthquakes that occurred in this segment from Jun. 2005 to Jul. 2010 (Fig. 9d). (2) Aceh-Nias segment Most of the seismicity from Jun. 2005 to Oct. 2005 occurred along the Sunda Trench and clustered around the Aceh region and featured megathrust mechanisms (Fig. 9a). During the period from Nov. 2005 to Oct. 2006, a large amount of seismicity with megathrust mechanisms and low b-values (0.5 ~ 0.9) formed both a linear cluster along the Sumatra forearc from the epicenter of the 2004 event to 0° and a cluster around the Aceh region(Fig. 9a). The distribution and style of the earthquakes from Nov. 2006 to Jul. 2008 were similar to those in the previous period and had low b-values (0.5 ~ 0.8) (Fig. 9b). Fewer and smaller earthquakes were detected from Aug. 2008 to Jul. 2010 (b-value: 0.6 ~ 1.0) (Fig. 9c), but they exhibited a distribution and style similar to those in the previous period. In addition to the continuation of the previous period’s distribution and style characteristics (b-value: 0.7 ~ 1.2), strike-slip

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earthquakes occurred near the position of the 2012 mainshock in the oceanic plate from Aug. 2010 to Mar. 2012 (b-value: 0.7 ~ 1.0)(Fig. 9d). Stress was statically loaded to the adjacent oceanic lithosphere (northern Wharton Basin) by the 2004 earthquake, which enhanced the seismicity rate in the region within one year (Wiseman and Bürgmann, 2012). The strike-slip fault rate is related to the obliquity of the plate convergence, and the strike-slip earthquakes around the Andaman-Nicobar forearc re-energized the rupture process of pre-existing faults (Paul et al., 2014). Left-lateral faulting clusters from 5°N to 9°N, where the Ninety East Ridge impinges on the Andaman Trench, caused the deformation style to differ from north to south (Andrade and Rajendran, 2011; Rajendran et al., 2011). The cluster of earthquakes around the Aceh region is due to the activation of the transcurrent fault after the 2004 event in a region that had previously behaved as a rigid block (Das et al., 2006). The stress in the oceanic plate of the Aceh-Nias segment is released by the normal faults off the Sunda Trench, and the P-axis orientation of the earthquakes is NW-SE (Delescluse et al., 2012), consistent with the pre-2004 style (Andrade and Rajendran, 2014). Seismicity in the arc-trench system occurs mostly along pre-existing weak zones, in the forearc basin and in the accretionary prism (Das et al., 2006). The fault interaction and stress transfer mechanisms are explored in this study by examining the seismicity sequence distributions and their corresponding Coulomb stress changes (Perniola et al., 2004). The seismicity distribution changes responded

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to the stress transfer after the 2004 event, which involved perturbations of the static coseismic stress as well as the postseismic stress transients (Wiseman and Bürgmann, 2012). The stress changes caused by the 2004 earthquake led to the 2012 events and an increase in seismicity along the Sunda Trench (Andrade and Rajendran, 2014). The 2004 event imparted stress to the oceanic lithosphere, leading to a high rate of strike-slip earthquakes (Wiseman and Bürgmann, 2012). Large postseismic deformation and related stress changes occurred in the adjacent oceanic plate, due to the thin and warm lithosphere (Shapiro et al., 2008). Continuous downdip forces from the main subduction zone caused stress-related changes in the intraplate seismicity (Renata et al., 1988). Trench-parallel displacement could have accumulated since the 2004 event through the oblique convergence of the plates (Ishii et al., 2013). The epicenter of the 2004 Sumatran earthquake was located near the position of the maximum gravity change for the 2012 events; similarly, the epicenter of the 2012 mainshock was located near the position of the maximum gravity change for the 2004 event (Fig. 4a,Fig. 4d). The 2004 earthquake prompted the 2012 earthquakes as well as a series of intraplate seismic events attributed to stress transfer along the subduction zone of the Sumatra-Andaman plate boundary (Andrade and Rajendran, 2014; Lay et al., 2011). In other words, the 2012 events were caused by the oblique subduction and the 2004 Sumatran earthquake (Ishii et al., 2013). The continuous 2-year positive gravity change zone was concentrated in the subduction zone and adjacent oceanic plate from 0° to 13°E after the 2004 Sumatran earthquake with attenuations in the

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range and intensity over time, and the maximum positive gravity change shown in Fig. 4b and Fig. 4c are 5.24 μGal and 3.46 μGal (Tab. 3), respectively. A moderate positive correlation is detected between the annual gravity change every two years (Fig. 4b, Fig. 4c) and the occurrence of earthquakes (Fig. 6) in the subduction zone (Mitsui and Yamada, 2017). The positive gravity change around the Nicobar Islands (Fig. 4b, Fig. 4c) is interpreted as seafloor uplift with a negligible postseismic density perturbation (Han et al., 2008). This region seems to be highly seismogenic because of stress accumulation and is at a high risk of earthquakes. Uniform viscous relaxation within the asthenospheric mantle accounts for the time lag between the large 2004 and 2012 earthquakes, and this relationship supports the hypothesized interaction between the megathrust interface of the subduction zone and the adjacent oceanic deformation zone (Delescluse et al., 2012). The maximum stress relaxation time between the 2004 and 2012 earthquakes was reported to be 7 ~ 10 years, during which short-term stress built up as a consequence of viscous relaxation in the asthenospheric mantle (Delescluse et al., 2012).

7. Conclusion The seismic b-value distributions are estimated by seismicity data between the 2004 Sumatran earthquake and the 2012 East Indian Ocean earthquakes (from Jun. 2005 to Mar. 2012) in the Sumatran region. The relationships among b-values, fault-plane solutions, seismic moments and gravity change are analyzed to investigate the seismotectonic characteristics and tectonic activity in the study area.

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The seismic b-values range from 0.45 to 1.59; low values are detected from Nicobar Island to south of the Andaman Islands between 5°N and 12°N in the subduction zone and its adjacent oceanic plate, from Nias to south of Aceh between the Sumatra Fault and the Sunda Trench (b-value: 0.5 ~ 1.1), and off the Sunda Trench between 2°N and 3.5°N in the oceanic plate (b-value: 0.5 ~ 0.7). The variations in b-values with depth show that low values are confined to depths from 20 km to 40 km in the subduction zone and from 20 km to 35 km in the oceanic plate; these results indicate seismically active depths. The lowest b-values appear at a depth of approximately 28 km in both adjacent regions, indicating the existence of a homogeneous fault system. The b-values associated with megathrust and strike-slip earthquakes are low (0.5 ~ 1.0), and the convergence rates and b-values are inversely correlated. Low b-values correspond to a high seismic moment release and therefore indicate stress accumulation in the region. Active tectonic structures and faults are located along the Andaman-Sunda Trench and the Andaman-Nicobar-Sumatra forearc. The changes in the seismicity distribution and faulting style suggest temporal variations in the b-value, while the spatial variations in the b-values during each period (Jun. 2005 ~ Oct. 2006, Nov. 2006 ~ Jul. 2008, Aug. 2008 ~ Jul. 2010, and Aug. 2010 ~ Mar. 2012) can be used to illustrate the tectonic activity in the study area. The epicenter of the 2004 event was located near the area of the maximum Bouguer gravity change in 2012, and the epicenters of the 2012 events were located near the area of the maximum Bouguer gravity change in 2004. Furthermore, the gravity change in every 2-year

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period after the 2004 event shows attenuations in both the intensity and the size. The 2004 Sumatran earthquake boosted the 2012 East Indian Ocean earthquake and other intraplate seismicity via a stress transfer process. The seismicity distribution changes over time in different segments of the study area illustrate the stress changes as well as the active and locked portions of the fault systems.

Acknowledgments The authors would like to thank Dr. Dipok from Diphu Government College and S. Mostafa Mousavi from Stanford University for their support in improving this study. We also thank the reviewers for the valuable comments that improved the manuscript greatly. The solutions for GRACE were obtained from the International Center for Global Earth Model (ICGEM). The seismic data were gathered from the Incorporated Research Institutions for Seismology (IRIS). The calculations of b-values were performed with Zmap (Wiemer and Zúñiga, 1994). This research benefited from financial support from the National Natural Science Foundation of China (Grant Nos. 41506055, 41476042, 41476046, and U170120019) and the Fundamental Research Funds for the Central Universities of China (No. 17CX02003A).

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https://doi.org/10.1029/2005JB003877. Stein, C.A., Cloetingh, S.A.P.L., Wortel, R., 1990. Kinematics and mechanics of the Indian Ocean diffuse plate boundary zone. Proceedings of the Ocean Drilling Program, Scientific Results 116, 261-277. Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J.-P., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., McCaffrey, R., 2006. Plate-boundary deformation associated with the great Sumatra–Andaman earthquake. Nature 440, 46-51. Warren, N.W., Latham, G.V., 1970. An experimental study of thermally induced microfracturing and its relation to volcanic seismicity. Jounal of Geophsical Research 75, 4455-4464. Wei, S., Helmberger, D., Avouac, J.P., 2013. Modeling the 2012 Wharton basin earthquakes off‐ Sumatra: Complete lithospheric failure. Journal of Geophysical Research Solid Earth 118, 3592-3609. Westaway, R., 1992. Seismic moment summation for historical earthquakes in Italy: Tectonic implications. Journal of Geophysical Research Solid Earth 97, 15437–15464. Wiemer, S., 2001. A software package to analyze seismicity : ZMAP. Seism.res.lett 72, 373-382. Wiemer, S., Wyss, M., 2000. Minimum magnitude of completeness in earthquake catalogs: Examples from Alaska, the western United States, and Japan. Bulletin of the Seismological Society of America 90, 859-869. Wiemer, S., Zúñiga, F.R., 1994. ZMAP - A software package to analyze seismicity,

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Tables Table 1. Earthquakes with tectonic settings (Pollitz et al., 2012) Date Region Name

Tectonic Environment

Sense of Slip

2004/12/26

Sumatran

Sunda megathrust

Thrust

2005/03/28

Nias

Sunda megathrust

Thrust

East Indian Ocean

Intraplate

Strike-slip

East Indian Ocean

Intraplate

Strike-slip

(Year/Month/Day)

2012/04/11 (Mw 8.6) 2012/04/11 (Mw 8.2)

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Table 2. Focal mechanism solutions (from the GCMT catalog) Fault plane

Date (Year/Month

Lat

Lon

Depth

Scalar

(°N)

(°E)

(km)

moment

Mw

/Day)

Strike

Dip

Rake

(°)

(°)

(°)

2004/12/26

9.1

3.09

94.26

28.6

3.95E+29

329

8

110

2005/3/28

8.6

1.67

97.07

25.8

1.05E+29

333

8

118

2012/4/11

8.6

2.35

92.82

45.6

9.14E+28

20

76

5

2012/4/11

8.2

0.90

92.31

54.7

2.89E+28

107

83

-177

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Table 3. Co- and postseismic positive gravity change Positive gravity change Date Position Fig.

Difference between Jan. 2005 ~ Dec. Near the epicenter of the 2012

4a

2006 and Dec. 2002 ~ Nov. 2004

Fig.

Difference between Jan. 2007 ~ Dec. Southwest and west of the

4b

2008 and Jan. 2005 ~ Dec. 2006

Fig.

Difference between Jan. 2009 ~ Dec.

4c

2010 and Jan. 2007 ~ Dec. 2008

Fig.

Difference between May 2012 ~ Apr. Near the epicenter of the 2004

4d

2014 and Apr. 2010 ~ Mar. 2012

Peak value 9.64 μGal

mainshock 5.24 μGal Nicobar Islands Near the Nicobar Islands

3.46 μGal

7.82 μGal Sumatran earthquake

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Table 4. Seismicity characteristics with the depth and faulting style in the subduction zone (SZ) and the adjacent oceanic plate (OP) Depth Time

Area

40 km ~ 70 0 km ~ 20 km

Faulting style

20 km ~ 40 km km

Jun. 2005 ~

SZ

occasional

most frequent

frequent

Almost entirely

Oct. 2005

OP

few

frequent

almost none

megathrust

Nov. 2005 ~

SZ

occasional

frequent

frequent

Almost entirely

Oct. 2006

OP

almost none

occasional

almost none

megathrust

SZ

occasional

frequent

frequent

mostly

OP

almost none

occasional

almost none

Nov. 2006 ~ megathrust and Jul. 2008

a few strike-slip Aug. 2008 ~

SZ

occasional

frequent

less frequent

megathrust and

Jul. 2010

OP

almost none

occasional

almost none

strike-slip

Aug. 2010 ~

SZ

very few

frequent

occasional

almost entirely

Mar. 2012

OP

almost none

occasional

almost none

45

megathrust

Figures

Fig. 1

46

Fig. 1.

47

Fig. 3

48

Fig. 2.

49

Fig. 3.

50

Fig. 4.

51

Fig. 7.

52

Fig. 8.

53

Fig. 9.

54

Figure captions Fig. 1. Geological setting after Mccaffrey (2009) and Meltzner et al. (2012) (10°S ~20°N, 80°E ~ 110°E). The short yellow line shows the change in the azimuth of plate movement at 6°N to 7°N (Mccaffrey, 2009). The white arrows show the motion directions of the Indo-Australian plate, and their labels show drifting speed (Moeremans and Singh, 2015). The white dotted area shows the diffuse Indo-Australian plate boundary zone (Stein et al., 1990). The great earthquakes (2004 Sumatran earthquake (Mw 9.1), 2005 Nias earthquake (Mw 8.6) and 2012 East Indian Ocean earthquakes (Mw 8.6 and Mw 8.2)) are shown with beachballs representing the focal mechanism solutions, and the seismicity data were obtained from the USGS.

Fig. 2. Epicentral distribution of Mw 1.7 ~ Mw 7.8 (a) and epicentral distribution of Mw 4.3 ~ Mw 7.8 (b) from Jun. 2005 to Mar. 2012 in the Sumatran region. The seismicity data were obtained from the Incorporated Research Institutions for Seismology (IRIS). Square grids (1°×1°) in different colors indicate the number of earthquakes within the grids.

Fig. 3. Cumulative number of seismic events (a) and magnitude of completeness (Mc) (b) as a function of time. Cumulative number versus magnitude in the subduction zone (c) and its adjacent oceanic plate (d). (a) Yellow stars show the great earthquake (the 2004 Sumatran earthquake, the 2005 Nias earthquake and the 2012 East Indian Ocean

55

earthquakes). (b) Standard deviations are shown by dashed lines. (c/d) The straight red line is the best fit line for the relation between the cumulative number and magnitude, and Mc (magnitude of completeness) is the threshold magnitude (Jun. 2005 ~ Mar. 2012).

Fig. 4. Coseismic gravity change for the 2004 Sumatran earthquake (a) and the 2012 East Indian Ocean earthquakes (d); and postseismic gravity change for the 2004 event as the difference between Jan. 2007 ~ Dec. 2008 and Jan. 2005 ~ Dec. 2006 (b) and difference between Jan. 2009 ~ Dec. 2010 and Jan. 2007 ~ Dec. 2008 (c). PA (2°N, 93°E) is the maximum positive gravity change point for the 2004 event. PB (4°N, 96°E) (compressional quadrant) is the maximum positive gravity change point for the 2012 events.

Fig. 5. Spatial variations in b-values with epicenters from Jun. 2005 to Mar. 2012 (Mw≥4.5). The focal mechanisms of the earthquakes (Mw≥5.5) are shown by the beachballs.

Fig. 6. The scatter plots of seismic focal depths (a, b) and magnitudes (c, d) versus time in the subduction zone (a, c) and the adjacent oceanic plate (b, d), with the blue lines indicating the variations in b-values and their errors with depth in the subduction zone (a) and its adjacent oceanic plate (b) from Jun. 2005 to Mar. 2012. Due to the clustering

56

characteristics of the seismicity, earthquakes with colored markers indicate depths from the surface to 20 km, from 20 km to 42 km and from 43 km to 70 km in the subduction zone (a) and from the surface to 20 km, from 20 km to 40 km and from 41 km to 70 km in the adjacent oceanic region (b).

Fig. 7. Times of rate increases and decreases in the Sumatran region from Jun. 2005 to Mar. 2012. Mean times of changes and the standard deviations and determinations of the magnitude distribution of the significant changes are shown in the figure.

Fig. 8. Spatial variations in seismic moment (lgM0 ) from Jun. 2005 to Mar. 2012. The focal mechanism solutions of the great earthquakes (2004 Sumatran earthquake (Mw 9.1), 2005 Nias earthquake (Mw 8.6) and 2012 East Indian Ocean earthquakes (Mw 8.6 and Mw 8.2)) are shown as beachballs, and the seismicity data were obtained from the USGS.

Fig. 9. Spatial distributions and faulting styles of seismicity in the separate periods of Jun. 2005 ~ Oct. 2005 and Nov. 2005 ~ Oct. 2006 (a), Nov. 2006 ~ Jul. 2008 (b), Aug. 2008 ~ Jul. 2010 (c) and Aug. 2010 ~ Mar. 2012 (d). The time from Jun. 2005 to Mar. 2012, with the spatial variations of b-value estimated by the seismicity included in each period marked on the sub-figures, is divided based on the four significant seismicity rate changes with time. The epicenters (Mw≥4.5) are shown as points, and the focal

57

mechanisms of the epicenters (Mw≥5.5) are shown as beachballs. Graphical abstract

Highlights: Spatial variation of seismic b-values is estimated in the Sumatran Region including the Sumatran oblique subduction zone and its adjacent oceanic plate. Spatial variation indicates a heterogeneous fault system in the region. The variations in b-values with depth are detected in the subduction zone and its adjacent oceanic plate respectively. The epicenters of the 2004/2012 events are located around the 2012/2004 maximum Bouguer gravity change zones.

58