Physics of the Earth and Planetary Interiors 259 (2016) 29–33

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Physics of the Earth and Planetary Interiors

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Letter Splay-fault rupture during the 2014 Mw7.1 Molucca Sea, , earthquake determined from GPS measurements ⇑ Endra Gunawan a, , Munawar Kholil a,b, Irwan Meilano c a Graduate Research on Earthquake and Active , Faculty of Earth Science and Technology, Bandung Institute of Technology, Indonesia b Geospatial Information Agency, Indonesia c Geodesy and Geomatics Engineering, Faculty of Earth Science and Technology, Bandung Institute of Technology, Indonesia article info abstract

Article history: The coseismic slip of the 2014 Molucca Sea, Indonesia, earthquake (MOSEQ) is investigated using GPS Received 4 March 2016 data from continuously monitoring stations. Coseismic models are compared between the main Received in revised form 18 August 2016 fault, with a 25° west-dipping plane, and the 65° west-dipping splay-fault plane. In analyzing this earth- Accepted 26 August 2016 quake with fine faults sized resolution and homogenous fault models, we find that a splay fault ruptured Available online 28 August 2016 during the mainshock. Our finding suggests that the 2014 MOSEQ occurred on an unmapped fault. Although we have limited GPS data available in the region, our results for coseismic slip are sufficient Keywords: to explain the available GPS data. Our estimation suggesting that a maximum coseismic slip of around 2014 Molucca Sea earthquake 36 cm occurred near the hypocenter, with cumulative seismic moment of 4.70 1019 Nm(M 7.1). GPS w Ó Splay-fault 2016 Published by Elsevier B.V. Coseismic slip

1. Introduction ated with the interseismic (Ito et al., 2012; Hanifa et al., 2014; Ohkura et al., 2015), coseismic (Banerjee et al., 2007; Ding et al., The 2014 Molucca Sea, Indonesia, earthquake, hereinafter ter- 2015; Ito et al., 2016), and postseismic (Ardika et al., 2015; med MOSEQ, occurred at 02:31 UTC, 15 November 2014 at a com- Anugrah et al., 2015; Alif et al., 2016) phases. Clear signals from plex plate boundary in eastern Indonesia between North Maluku the GPS data of these three deformation phases have also been Province in the west and North Sulawesi Province in the east. reported in NE Japan (Heki et al., 1997). The Indonesian Agency for Meteorology, Climatology and Geo- One of the underlying motivations of this study is to understand physics (BMKG) reported that tsunami waves of three and nine crustal deformations related to the 2014 MOSEQ. Here, we present centimeters hit Manado and Jailolo Island, respectively, at a dis- an implementation using GPS data to estimate the coseismic slip tance of about 150 km in the SW and SE directions, respectively, distribution of the 2014 MOSEQ. The particular GPS data used for from the epicenter. Although no casualties were reported due to this estimate are static measurements from stations that are part this earthquake, infrastructure and buildings had reported damage of a nationwide GPS network named the Indonesian Continuously in Gorontalo, Minahasa, and West Halmahera. Operating Reference Stations (Ina-CORS). The 2014 MOSEQ took place in a region with active arc-arc col- lision and a subducted plate with an inverted U-shape, having slab- dipping to the west under the active volcanic arcs of Saginhe and to 2. GPS observations and data processing the east under the active volcanic arcs of Halmahera (Hall and Spakman, 2015). The swarm of earthquake activity along the In this study, we use GPS data obtained from Ina-CORS stations Halmahera arcs in November 2015 suggested that this region is located in the region of the 2014 MOSEQ, which are installed and active. Fig. 1 shows the tectonic background of this study, follow- maintained by the Geospatial Information Agency of Indonesia ing Hall (2002). (BIG). These GPS stations are CTER, CBIT, and CTOL. CTER is located Global Positioning System (GPS) data have been widely imple- in Ternate city, North Maluku province, while CBIT is located in mented in the study of Earth science. GPS has shown the capability Bitung city, North Sulawesi province, and CTOL is in Toli-toli city, to capture tectonic processes during an earthquake cycle associ- Central Sulawesi province. Fig. 1 shows location of these GPS sta- tions. The CTER station was constructed on concrete benchmark ⇑ Corresponding author. on top of a roof, while CBIT and CTOL stations were constructed E-mail address: [email protected] (E. Gunawan). on steel and concrete pillars. http://dx.doi.org/10.1016/j.pepi.2016.08.009 0031-9201/Ó 2016 Published by Elsevier B.V. 30 E. Gunawan et al. / Physics of the Earth and Planetary Interiors 259 (2016) 29–33

Fig. 1. Tectonic background of this study. The beach ball indicates the location of the 2014 MOSEQ. Gray dots represent the locations of aftershocks. Red triangles denote the location of the GPS stations used in this study. Inset shows the larger regional setting.

We analyzed GPS data from each station using GAMIT/GLOBK rienced large coseismic displacements of up to 15 mm, while CBIT software (Herring et al., 2010a,b). During our analysis, we included and CTOL experienced displacements of 6 mm and 3 mm, the International GNSS Service (IGS) stations of BAKO, CNMR, respectively. COCO, CUSV, DARW, DGAR, GUAM, HYDE, IISC, KARR, KAT1, KOUC, PIMO, TNML, TOW2, PNGM, XMIS, YARR, PBRI, ALIC, and NTUS, and 3. Coseismic fault models tie our local network to the ITRF2008 reference frame (Altamimi et al., 2011). We use observed coseismic displacements from the GPS data to Our analysis steps of these GPS data are as follows (Gunawan infer the coseismic slip of the 2014 MOSEQ. Our first model (Model et al., 2016). First, daily position with atmospherically used, 1) is constructed using a strike of 200°. In this model, sub-faults are loose-constraint, prior GPS phase observations; the orbit and sized 10 km 25 km. In addition, the depth on top of the fault earth-orientation parameters were fixed. Second, combination of plane is shallow, at 5 km with a 25° west-dipping fault plane these positions and the covariance with GPS solutions computed (Hall, 2002). Fig. 3 shows a schematic cross-sectional view of the as part of MIT’s processing for the IGS. Then, examination on the fault models used in this study. antenna changes is applied. Third, we analyzed daily solutions We perform the coseismic slip inversion assuming an elastic from GPS data at each GPS station and subtracted the velocity of half-space model (Okada, 1992). In order to reduce the model three days after the 2014 MOSEQ to three days prior the main- parameter, we fixed the rake at 75°. During our coseismic slip anal- shock, using the result as the coseismic displacements associated ysis, we used a priori information regarding spatial variation in with this earthquake at each GPS station. In the second and third fault slip. This information is combined with the observational steps, we mapped the loosely constrained solution onto a well- equation to construct a Bayesian model that includes a hyperpa- constrained reference frame by minimizing the position and veloc- rameter (Gunawan et al., 2014). We describe the inversion algo- ity differences of selected stations with respect to a priori values rithm to solve the coseismic slip distribution by minimizing the defined by the IGb08 realization of the ITRF2008 reference frame. following function Fig. 2 shows the coseismic displacements at CTER, CBIT, and CTOL. We found that the coseismic displacements of each GPS station sðmÞ¼ðd GmÞT E 1ðd GmÞþa2mT Hm ð1Þ directed towards earthquake rupture, with displacements at CTER towards the NW direction while displacements at CBIT and CTOL where d is observed coseismic displacements from GPS data, G is directed towards the NE (Fig. 2). Our results show that CTER expe- Green’s function contains synthetic displacement calculated from a priori fault slip information of 1, m is the model parameter, H is E. Gunawan et al. / Physics of the Earth and Planetary Interiors 259 (2016) 29–33 31

where N is total number of data, P is number of sub-faults, and M is number of model parameter. In analyzing a2, we iterate the process in which minimum ABIC give the optimum value. Fig. 4 shows the best-fit hyperparameter used in the analysis. The result of our geodetic data inversion suggests that a large amount of slip, around 15 cm, occurred near the hypocenter (Fig. 5). By assuming rigidity of 3.2 1010 Nm2, we estimate a seismic moment determined from our slip distribution of 4.05 1019 Nm. Using this slip result, we modeled surface dis- placement, finding that calculated displacements poorly fit the dis- placements observed from GPS data (Fig. 2). The root-mean-square (RMS) of the observed and calculated displacements produced by Model 1 is 0.32 cm. A second model (Model 2) was constructed using top depth of 5 km and a 65° west-dipping splay-fault plane (Fig. 3)(Hall, 2002). Fault strikes for Model 2 are similar to those in Model 1, which is 200°. and sub-faults are sized 10 km 10 km. In our inversion procedure, the hyperparameter of the Model 2 analysis is different from that of Model 1 (Fig. 4). Model 2 predicts a large amount of slip, around 36 cm, located near the hypocenter with cumulative seismic moment of 19 4.70 10 Nm(Mw 7.1) (Fig. 5). In comparison to the coseismic displacements observed from GPS data, the modeled displace- ments of Model 2 produced RMS of 0.19 cm. Table 1 shows the coseismic slip parameters based on analysis using Model 2.

4. Discussion

Different fault models yield different coseismic-slip results. The estimated seismic moment from the mainshock in Model 1 of 19 4.05 10 Nm, corresponding to Mw 7.0, is lower than the esti- mated seismic moment in Model 2 of 4.70 1019 Nm, equivalent

to Mw 7.1. Based on the smaller misfit between observed and mod- eled displacements in Model 2 compared to Model 1, we prefer Fig. 2. Coseismic displacements of the 2014 MOSEQ. Blue lines imply the surface Model 2. Moreover, Model 2 produces a similar seismic moment projection of the fault plane. Black arrows indicate the observed displacements at to the seismic moment obtained by seismic analysis using the Glo- each GPS station. Red arrows denote the calculated displacements of (top) Model 1 and (bottom) Model 2. (For interpretation of the references to color in this figure bal Centroid-Moment-Tensor (CMT) catalog (Dziewonski et al., legend, the reader is referred to the web version of this article.) 1981). This suggests that geodetic estimation of seismic moment agrees with the seismological result, which might be a good indica- tion that coseismic displacements measured using GPS data are not significantly biased by early postseismic deformations. Regarding the case of splay-fault rupture, there are only a few reports to date of splay-fault rupture during earthquake occur- rences, such as in the cases of the 1944 Tonankai, Japan, earth-

Fig. 3. Schematic cross-sectional view of the fault planes of Model 1 and Model 2 used in this study (not to scale). matrix of Laplacian smoothing operator, E is measurement error, and a2 is hyperparameter. Model parameter of m can be calculated as

1 m ¼ðGT E 1G þ a2HÞ GT E 1d ð2Þ The hyperparameter, a2, is then determined using Akaike’s Bayesian information criterion (Yabuki and Matsu’ura, 1992).

ABICða2Þ¼ðN þ P MÞ log sðmÞP log a2 þ log jjGT E 1G þ a2Hjj ð3Þ

Fig. 4. Hyperparameter as a function of ABIC. Best-fit parameters are indicated by the black triangles for (top) Model 1 and (bottom) Model 2. 32 E. Gunawan et al. / Physics of the Earth and Planetary Interiors 259 (2016) 29–33

Fig. 5. Coseismic slip distribution of the 2014 MOSEQ calculated using (left) Model 1 and (right) Model 2.

Table 1 Coseismic slip parameters of Model 2.

Long.* (°) Lat.* (°) Top depth (km) Slip (cm) Long.* (°) Lat.* (°) Top depth (km) Slip (cm) 126.8508 2.2937 5 5.27 126.6817 2.3553 23 7.55 126.8200 2.2092 5 8.93 126.6509 2.2707 23 13.42 126.7893 2.1246 5 11.90 126.6201 2.1861 23 18.54 126.7585 2.0400 5 15.12 126.5893 2.1016 23 22.24 126.7277 1.9554 5 17.19 126.5585 2.0170 23 25.50 126.6969 1.8709 5 19.07 126.5278 1.9324 23 29.29 126.6661 1.7863 5 21.09 126.4970 1.8479 23 31.78 126.6353 1.7017 5 22.17 126.4662 1.7633 23 33.66 126.6046 1.6171 5 22.98 126.4354 1.6787 23 35.85 126.5738 1.5326 5 23.87 126.4046 1.5941 23 36.41 126.5430 1.4480 5 23.16 126.3739 1.5096 23 35.68 126.5122 1.3634 5 21.55 126.3431 1.4250 23 33.78 126.4814 1.2789 5 19.38 126.3123 1.3404 23 31.16 126.4507 1.1943 5 16.45 126.2815 1.2558 23 27.67 126.4199 1.1097 5 12.55 126.2507 1.1713 23 23.10 126.3891 1.0251 5 8.88 126.2199 1.0867 23 17.11 126.3583 0.9406 5 4.96 126.1892 1.0021 23 12.48 126.3275 0.8560 5 2.09 126.1584 0.9176 23 8.18 126.2967 0.7714 5 0.00 126.1276 0.8330 23 3.98 126.2660 0.6868 5 0.00 126.0968 0.7484 23 1.44 126.7662 2.3245 14 7.64 126.5971 2.3861 32 5.24 126.7355 2.2399 14 13.26 126.5663 2.3015 32 9.57 126.7047 2.1554 14 18.56 126.5355 2.2169 32 12.41 126.6739 2.0708 14 22.47 126.5048 2.1324 32 14.65 126.6431 1.9862 14 25.63 126.4740 2.0478 32 16.78 126.6123 1.9016 14 29.35 126.4432 1.9632 32 19.42 126.5816 1.8171 14 31.72 126.4124 1.8786 32 21.03 126.5508 1.7325 14 33.51 126.3816 1.7941 32 22.32 126.5200 1.6479 14 35.64 126.3508 1.7095 32 24.05 126.4892 1.5634 14 36.08 126.3201 1.6249 32 24.70 126.4584 1.4788 14 35.10 126.2893 1.5403 32 24.38 126.4276 1.3942 14 33.00 126.2585 1.4558 32 23.20 126.3969 1.3096 14 30.10 126.2277 1.3712 32 21.54 126.3661 1.2251 14 26.21 126.1969 1.2866 32 19.40 126.3353 1.1405 14 21.46 126.1662 1.2021 32 16.55 126.3045 1.0559 14 15.50 126.1354 1.1175 32 12.55 126.2737 0.9713 14 10.31 126.1046 1.0329 32 9.65 126.2430 0.8868 14 5.38 126.0738 0.9483 32 6.83 126.2122 0.8022 14 2.00 126.0430 0.8638 32 3.67 126.1814 0.7176 14 0.00 126.0122 0.7792 32 1.72

* Longitude and Latitude at left bottom corner; strike is 200°, dip is 65°, rake is 75°, fault length and width are 10 km 10 km. quake (Baba et al., 2006), 1946 Nankai, Japan, earthquake (Sagiya and Thatcher, 1999; Banerjee et al., 2007). Thus, splay- (Cummins and Kaneda, 2000), and the 2004 -Andaman fault rupture for some earthquake cases is still controversial. earthquake (DeDontney and Rice, 2012). However, other studies For the case of the 2014 Mw 7.1 MOSEQ, our preferred fault- using different methods and data have suggested that there was rupture model, Model 2, suggests that it occurred on a 65° west- no slip along splay faults during these earthquake occurrences dipping, splay-fault plane. This suggests that the 2014 MOSEQ E. Gunawan et al. / Physics of the Earth and Planetary Interiors 259 (2016) 29–33 33 occurred on an unmapped fault. This finding coincides with the 2005 Nias earthquakes from GPS static offsets. Bull. Seismol. Soc. 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Silent fault slip following an interplate thrust hypocenter, with cumulative seismic moment of 4.70 10 Nm earthquake at the Japan Trench. (Mw 7.1). Herring, T.A., King, R.W., McClusky, S.C., 2010a. GAMIT Reference Manual Release 10.4, Report, 1–171. Massachusetts Institute Technology, Cambridge. Herring, T.A., King, R.W., McClusky, S.C., 2010b. GLOBK Reference Manual: Global Acknowledgments Kalman filter VLBI and GPS analysis program, Release 10.4, Report, 1–95. Massachusetts Institute Technology, Cambridge. We thank Jeff Freymueller, anonymous reviewer and the Editor Ito, T., Gunawan, E., Kimata, F., Tabei, T., Simons, M., Meilano, I., Agustan, Ohta, Y., Nurdin, I., Sugiyanto, D., 2012. Isolating along-strike variations in the depth for their thoughtful comments and constructive suggestions, which extent of shallow creep and fault locking on the northern Great Sumatran Fault. help to improve the quality of this manuscript. This research was J. Geophys. Res.: Solid Earth 117 (B6) (1978–2012). partially funded by the Australian Department of Foreign Affairs Ito, T., Gunawan, E., Kimata, F., Tabei, T., Meilano, I., Agustan, Ohta, Y., Ismail, N., Nurdin, I., Sugiyanto, D., 2016. Co-seismic offsets due to two earthquakes and Trade (DFAT) for Graduate Research on Earthquake and Active (Mw6.1) along the Sumatran fault system derived from GNSS measurements. Tectonics at the Bandung Institute of Technology and Indonesia Earth Planets Space 68, 57. http://dx.doi.org/10.1186/s40623-016-0427-z. Endowment Fund for Education (LPDP) No. PRJ-1048/LPDP/2015. Ohkura, T., Tabei, T., Kimata, F., Bacolcol, T.C., Nakamura, Y., Luis, A.C., Pelicano, A., Figures were generated using the Generic Mapping Tool (Wessel Jorgio, R., Tabique, M., Abrahan, M., Jorgio, E., Gunawan, E., 2015. Plate convergence and block motions in Mindanao Island, Philippine as derived and Smith, 1998). from campaign GPS observations. J. Disaster Res. 10 (1), 59–66. Okada, Y., 1992. 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