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Plate Convergence and Block Motions in Mindanao Island, Philippine As Derived from Campaign GPS Observations

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Plate Convergence and Block Motions in Mindanao Island, Philippine As Derived from Campaign GPS Observations Plate Convergence and Block Motions in Mindanao Island, Philippine as Derived from Campaign GPS Observations Paper: Plate Convergence and Block Motions in Mindanao Island, Philippine as Derived from Campaign GPS Observations Takahiro Ohkura∗1, Takao Tabei∗2, Fumiaki Kimata∗3, Teresito C. Bacolcol∗4, Yasuhiko Nakamura∗5, Artemio C. Luis, Jr.∗4, Alfie Pelicano∗4, Robinson Jorgio∗4, Milo Tabigue∗4, Magdalino Abrahan∗4, Eleazar Jorgio∗4, and Endra Gunawan∗6 ∗1Aso Volcanological Laboratory, Kyoto University 5280 Minami-Aso, Aso, Kumamoto 869-1404, Japan E-mail: [email protected] ∗2Department of Applied Science, Kochi University, Kochi, Japan ∗3Tono Research Institute of Earthquake Science, Gifu, Japan ∗4Philippine Institute of Volcanology and Seismology (PHIVOLCS), Philippines ∗5Graduate School of Integrated Arts and Sciences, Kochi University, Kochi, Japan ∗6Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan [Received August 12, 2014; accepted January 14, 2015] We conducted yearly Global Positioning System (GPS) campaigns in the eastern part of Mindanao from March 2010 to March 2014. The obtained station velocities with respect to the Sunda plate (SU) show that WNW motions are dominant due to the conver- gence of the Philippine Sea plate (PHS). However, it was found that elastic deformations caused by a full Masbate coupling of the plate interface down to 80 km could explain a maximum of only 29% of the observed sta- Leyte tion velocities. In order to interpret the displacement pattern, we applied a rigid block rotation model and determined the Euler vector. As a result, we deter- mined that Mindanao Island could be divided into at least three blocks and that the Philippine fault is one of the block boundaries. Although it was not possible to determine the coupling ratio at the Philippine trench, the dislocation pattern of the Philippine fault showed along-strike variation in Mindanao Island. Keywords: GPS, Philippine fault, Philippine trench, Block motions Fig. 1. Tectonic map of the Philippines and surrounding ar- eas. Convergence rates of each subduction zone, obtained 1. Introduction from GPS station (shown as white circles) velocities, are from Tabei et al. (2008) [1]. The Philippine archipelago is currently wedged be- tween two opposing subduction zones. As shown in Fig. 1, the Eurasian plate and the Sunda plate (SU) are 1990 Luzon earthquake, the 1965 Taal eruption, and the subducting eastward along the Manila, Negros and Cota- 1981 Mayon eruption. In order to mitigate such disasters, bato trenches from the western side of the Philippines Japan’s Science and Technology Research Partnership for whereas the Philippine Sea plate (PHS) is undergoing a Sustainable Development Program (SATREPS) began the westward subduction along the Philippine trench from project “Enhancement of Earthquake and Volcano Moni- the east. Between these trenches, the Philippine fault, a toring and Effective Utilization of Disaster Mitigation In- 1250-km-long, left-lateral strike-slip fault, extends from formation in the Philippines” in 2009. Luzon Island to Mindanao Island almost parallel to the The Philippine fault, regarded as one of the most promi- Philippine archipelago. In the Philippines, major disasters nent fault zones in the world [2], has been highly active caused by earthquakes, tsunamis, and volcano eruptions during the past 150 years with several destructive earth- include the 1976 Moro Gulf earthquake and tsunami, the quakes accompanied by surface rupture in Luzon Island Journal of Disaster Research Vol.10 No.1, 2015 59 Ohkura, T. et al. and in Mindanao Island. Therefore, for disaster mitiga- tion in Mindanao Island, it is important to evaluate the PIMO(IGS) earthquake generation potential of the Philippine fault on NMSM NMLM the island. Conversely, no mega-thrust earthquake (M > 9) in the Philippines has been reported after 1600 in either the NMM earthquake catalogue inferred from intensity informa- NMA tion [3] or the composite PAGER-CAT catalog [4]. How- ever, Ramos et al. (2012) [5], used systematic mapping NMJ of Holocene marine terraces in eastern Mindanao Island NMC to determine that four large earthquakes that caused ter- ∼ race uplift of 2 m occurred during the past 8000 years. NMSF Furthermore, several earthquakes of M ∼ 8 occurred in SMCL trenches around Mindanao Island. Since the distance SMDB from the coast to the trench is shorter than that in the Tohoku district, Japan, tsunami is expected to arrive ear- SMC SMDM lier at the eastern coast of Mindanao than that in Tohoku, SMDT which would leave a shorter evacuation time. Therefore, SMMM SMSK it is also important to assess the earthquake generation SMDS potential of the Philippine trench in Mindanao Island to mitigate disasters caused by tsunamis. In order to evaluate the earthquake generation poten- tial of the Philippine fault and the Philippine trench in the Fig. 2. Map of GPS campaign stations and the IGS station used in this study. The trace of the Philippine fault in Min- Mindanao area, it is necessary to observe crustal defor- danao Island is also shown as identified by Tsutsumi and mation by Global Positioning System (GPS) observation Perez (2013) [9]. Solid lines indicate baselines used in the in the area and to clarify the slip/locking distribution at GPS data analysis by Bernese 5.0. the trench and at the fault. In Mindanao Island, several GPS campaigns have been conducted. For example, Ran- gin et al. (1999) [6] and Aurelio et al. (2000) [7], used the data of the Geodynamics of South and South-East Asia erly set above the benchmark. (GEODYSSEA) campaign [8] to discuss the block motion In this study, we processed GPS data collected in the across the Philippine fault in Mindanao. In this campaign, 2010–2013 campaign by using Bernese software ver. 5.0 however, only the three sites in Mindanao Island were oc- together with the data from PIMO, an International Global cupied, and only two campaigns were conducted. Tabei et Navigation Satellite System (GNSS) Service (IGS) sta- al. (2008) [1] conducted a GPS campaign under the Ocean tion at Luzon Island, to obtain daily coordinates and dis- Hemisphere Project (OHP) in 1997–2003 in and around placement rates based on the International Terrestrial Ref- Mindanao Island and obtained plate convergence rates in erence Frame 2008 (ITRF2008) system. the plate boundary region around the island. However, the For the calculation procedure, we first selected the site total number of GPS stations was insufficient for reveal- NMMB in the northern part of the study area as the refer- ing the internal deformation in Mindanao Island. There- ence at Mindanao Island because this site had the longest fore, we have started GPS observation in the eastern part observation period in each campaign. We performed of Mindanao Island under the SATREPS project. This baseline analysis between PIMO and NMMB with the paper reports the results of campaign GPS observations coordinate of PIMO tightly constrained in the ITRF2008 conducted from 2010 to 2013. to obtain the station coordinates and velocity at NMMB in the ITRF2008. Next, starting from NMMB, several pairs of sites were set up for the baseline analysis de- 2. Data Acquisition and Result of Analysis pending on the overlap of the observation period (Fig. 2). In this way, daily coordinates of each station were cal- In February 2010, 15 benchmarks for GPS observation culated with respect to NMMB in the ITRF2008. In the were constructed in the eastern part of Mindanao Island calculation, we employed the IGS precise ephemerides, (Fig. 2), and annual campaigns have been conducted since and the wet zenith tropospheric delay was estimated ev- March 2010. In this study, we analyzed the campaign data ery hour at each station. No correction was applied for obtained from March 2010 to March 2013. In each cam- ocean tide loading. The mean of the standard error of the paign, 15 dual-frequency geodetic GPS receivers (Trim- coordinates for all of the stations was 1.0 cm, 0.3 cm, and ble 5700, Trimble NetRS and Leica SR520) were used to 1.0 cm for EW, NS, and UD components, respectively. acquire the data simultaneously at different stations. Each Fig. 3 shows the calculated station coordinates at several observation lasted for three to six days, and the data were stations. By fitting the linear trend to the time series of the collected at 30-s intervals for 24 h. At each observation station coordinates, we determined the station velocity in point, an antenna was attached to a tripod that was prop- the ITRF2008. 60 Journal of Disaster Research Vol.10 No.1, 2015 Plate Convergence and Block Motions in Mindanao Island, Philippine as Derived from Campaign GPS Observations Fig. 3. GPS time series of the International Terrestrial Reference Frame 2008 (ITRF2008) at NMSM, NMLM, SMDT, and SMDB from March 2010 through March 2013. Error bars correspond to three standard deviations of each campaign. We then converted the station velocities in the ITRF2008 to those with respect to the SU by subtract- ing the velocity at each site on the SU predicted by the geophysical model NNR-MORVEL56 [10]. Since our campaign could not cover the western part of Mindanao due to safety concerns, we used velocity data of seven stations in central and western Mindanao obtained by the OHP in 1997–2003 [1]. Although these velocity data were calculated in ITRF2000, the velocity difference at PIMO in ITRF2000 and ITRF2008 was sufficiently small (< 1.0 mm/year). In addition, we calculated the velocity difference between ITRF2000 and ITRF2008 at the location of DAVAby using a transformation parameter between ITRF2000 and ITRF2005 [11] and a transforma- tion parameter between ITRF2005 and ITRF2008 [12]. The obtained velocity differences were −0.1, −1.8and 0.3 mm/year in the EW, NS, and UD components, respec- tively.
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