Plate Convergence and Block Motions in 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 Science, Gifu, Japan ∗4Philippine Institute of Volcanology and Seismology (PHIVOLCS), ∗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 (SU) show that WNW motions are dominant due to the conver- gence of the plate (PHS). However, it

was found that elastic deformations caused by a full coupling of the plate interface down to 80 km could explain a maximum of only 29% of the observed sta- 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 is one of the block boundaries. Although it was not possible to determine the coupling ratio at the , 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 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 and the Sunda plate (SU) are 1990 earthquake, the 1965 Taal eruption, and the subducting eastward along the Manila, Negros and Cota- 1981 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 (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 , 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. These values are significantly smaller than the ve- locity difference (5.2 mm/year) between at DAVA in OHP and at SMDT in our campaign, which are located in a Fig. 4. Velocity vectors with respect to the Sunda plate. short distance. Therefore, we combined the OHP velocity Open arrows denote calculated velocities assuming 100% of plate coupling along the upper surface of the subducting with our station velocity without any corrections. Philippine Sea Plate down to a depth of 80 km. Blue and Figure 4 shows obtained station velocities with respect red arrows are velocities observed by the Science and Tech- to the SU. The velocity distribution shows a dominance nology Research Partnership for Sustainable Development of W–NW motion at 3–8 cm/year, which can be attributed Program (SATREPS) and Ocean Hemisphere Project (OHP) to the convergence of the PHS from the east. Northward campaigns, respectively. Error ellipse of 3-sigma are also shown for the observed velocities.

Journal of Disaster Research Vol.10 No.1, 2015 61 Ohkura, T. et al.

(b) 36˚ (a)

10˚

35˚

9˚

34˚ 20 [km] 8˚ 8080 80

60 40 60

40 7˚ 33˚

5cm 5cm 0 [km] 100 6˚ 20 [km] 0 [km] 100 123˚ 124˚ 125˚ 126˚ 127˚ 128˚ 32˚ 131˚ 132˚ 133˚ 134˚ 135˚ (c) (d) 80 80 60 60 40 40 20 20 velocity (mm/yr) velocity (mm/yr) 0 0 0 0 -20 -20 -40 -40 -60 -60 depth (km) -80 depth (km) -80 350 300 250 200 150 100 50 0 250 200 150 100 50 0 distance (km) distance (km) Fig. 5. Comparison of decay in velocity vectors with distance from the trench in Mindanao and SW Japan. (a) Observed velocity vectors with respect to the Sunda plate from 2010 to 2013 and isobaths of the upper boundary of the subducting Philippine Sea Plate (PHS) in Mindanao. (b) Observed velocity vectors with respect to Amur Plate from 2006 to 2009 and isobaths of the upper boundary of the subducting PHS in SW Japan. (c) Profiles of velocities and depth of the upper boundary of the PHS in the rectangle of (a). (d) Profiles of velocities and depth of the upper boundary of the PHS in the rectangle of (b).

vectors in the forearc region can be attributed to oblique 3. Discussions subduction of the PHS [13]. Moreover, we found that the spatial decay of station ve- In order to estimate the effect of coupling of the sub- locity with normal distance from the trench is very char- duction interface on crustal deformation in Mindanao Is- acteristic in Mindanao. This point is obvious in a com- land, we configured the surface of the subducting PHS parison of the spatial decay in Mindanao with that in SW for calculation. In this configuration, we used the Slab Japan (Fig. 5). Both subduction zones have similarities Model created by the United States Geological Survey in their tectonic setting such as oblique subduction of the (USGS) [14] as a reference and extrapolated to the south ◦ PHS and a slip parallel to the plate margin resulting in up to 5.7 N to create 196 triangle elements by the Gmsh transcurrent movements on a strike slip fault [13]. Be- mesh generator [15] to compose a subduction interface cause the subduction angle of the PHS in Mindanao is with a length of 500 km down to a depth of 80 km. Then, about 45◦ and is steeper than that in SW Japan, we expect we calculated the elastic deformation of Mindanao Island a faster decay of velocity in Mindanao than that in SW caused by the coupling at the subduction interface of the Japan. However, almost the same gradient of velocities PHS assuming relative motion of the PHS to the SU de- was noted in both subduction zones. Furthermore, even fined in MORVEL [16]. In the calculation, Green’s func- at the station 300 km from the Philippine trench, a large tion defined by Mead (2007) [17] was used and the rel- amount of station velocity (∼ 5 cm/year) was observed. ative motion velocity is given at the centroid of each tri- This fact suggest that the western edge of Mindanao Is- angle element. As shown in Fig. 4, calculated velocities land is not fixed by the SU and that Mindanao Island is were very small compared with the observed values. For mobile westward by the pushing force caused by subduc- example, the calculated velocity at SMDB, nearest the tion of the PHS. trench, was 2.1 cm/year and only 29% of the observed velocity at 7.3 cm/year even with full locking of the plate interface (100% of the coupling ratio) down to a depth of

62 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. 6. Obtained horizontal velocities divided in two directions: normal to the Philippine fault and parallel to the fault. (a) Fault-normal components are plotted along A–B and C–B with distance. In the A–B profile, the location of the Philippine faults is indicated by a dotted line. (b) Fault-parallel components are plotted along A–B and C–B with distance. Open arrows and circles represent velocities of stations located to the east of the Philippine fault.

80 km. This result also suggests that the western edge of Because elastic deformation caused by the coupling ef- Mindanao Island is not fixed by the SU and that Mindanao fect between the subducting PHS and an overriding plate Island is mobile westward by the pushing force caused by cannot account for the observed velocity field in Min- subduction of the PHS. Therefore, we introduced a rigid danao Island, we checked whether the observed motion block rotation model to interpret the displacement pattern could be explained by rigid block rotations by estimating of Mindanao. the Euler vector of a block from the observed station ve- A rigid block motion is also suggested by changes in locities with respect to the SU. At first, all of the vectors the velocity direction across the Philippine fault. Each were used to calculate these values. From the obtained station velocity was divided into two directions; parallel vector, we then calculated the horizontal velocities at all to the Philippine fault and normal to the fault (Fig. 6). Al- stations. The station with the largest discrepancy between though no significant change in the fault-normal compo- the observed and calculated vectors was removed to make nent was detected across the fault, we recognized a clear the next estimation of the Euler vector. And we iterated offset of 1–3 cm/year at the location of the Philippine fault this procedure to obtain a best-fit model, which was de- in the fault parallel component. It is highly likely that the termined through minimization of the misfit between the Philippine fault is a boundary of the block motion. observed and calculated velocity vectors. This misfit was

Journal of Disaster Research Vol.10 No.1, 2015 63 Ohkura, T. et al.

2013.03

P 23.3

h

i l i p p 2012.03 i NMSM n 4.2 e

F Mean (mm/yr) a

u

l t 2011.03

Displacement Scale 0 1020 30 40 50 mm 2010.03 042 6 8 10 km NMLM Map Scale

Fig. 8. Annual displacement of NMLM relative to NMSM, which are located 10 km apart in the fault-normal direc- tion at the northern part of the Mindanao Island. The av- eraged velocity of NMLM relative to NMSM is annotated with a 2.45-sigma error ellipse and decomposed into fault normal and fault parallel components to be 4.2 mm/year and 23.3 mm/year, respectively.

Fig. 7. Estimation of the Euler vector with data uncertainty of 5 mm/year. Upper left: “A” denotes the location of the ernmost stations were also excluded. An additional block Euler Pole. Upper right: Location and error ellipse of the boundary could exist between these sites and the central Euler Pole. Obtained angular velocity is also shown. Lower: part of Mindanao Island, or the effect of elastic deforma- Solid and open arrows denote observed horizontal velocities tions could have caused a westward decrease of the station and velocities calculated assuming solid block rotations, re- velocity in the region. The Philippine archipelago, being spectively. The 12 stations marked by arrows were judged to be located in a single block; all of the stations located to the wedged between two active subduction zones, contains east of the Philippine fault were excluded. many rigid or elastic tectonic blocks. For future work in Mindanao, it is necessary to introduce rigid block motion and elastic deformation. Moreover, it is necessary to esti- mate plate coupling along the trenches, block movement parameterized as the reduced chi-squared statistic, which and slip-locking distribution along the Philippine fault si- is the total chi-square divided by the number of degrees of multaneously. For such purpose, however, much denser freedom. The calculations were iterated until the differ- GPS observations are required. ences between the calculated and the observed velocities An interesting displacement pattern was observed at all stations become smaller than 5.3 mm/year because across the Philippine fault. Fig. 8 shows yearly displace- this value is the estimation error of observed station ve- ment of NMLM relative to NMSM, which are 10 km apart locities. in the fault normal direction in the northern part of the In the estimation, we ignored the effect of elastic de- Mindanao Island. Across the Philippine fault, left lat- formation in Mindanao Island even though the northwest- eral motion of ∼ 23 mm/year was detected between the ward decrease of station velocities in Fig. 5(c) can be at- two stations. The observed displacement rate across this tributed to the elastic deformation. The best fit model segment of the Philippine fault was as large as the creep in Fig. 7 shows that 12 stations located to the west of rate in Leyte Island (12–26 mm/year) detected by InSAR the Philippine fault are in a single block, and all of the analysis and field observation [19] or by GPS observa- stations located to the east of the fault are excluded. tion [20] and in Masbate Island (23 mm/year) by GPS Fig. 7 shows the obtained Euler vector as shown “A” (Lat.: observation [21]. This part belongs to the Surigao seg- −51.91 ± 1.03◦, Lon.: −22.30 ± 4.04◦, angular velocity: ment, at the northernmost 100 km of the Philippine fault 1.7535 ± 0.4851◦/my). This result suggests that the east- in Mindanao [22], where a ground-rupturing M7.2 earth- ern and western sides of the Philippine fault can be clearly quake occurred in 1879 [3]. The recurrence interval of identified as different blocks. From this result and the off- such an event estimated by paleoseismic trenching sur- set of the fault-parallel component of the station veloci- vey is 300–1000 years [22]. In this segment, left lateral ties across the fault, as shown in Fig. 6, it can be con- motion was accommodated by such events and creep-like cluded that the Philippine fault is a block boundary. These dislocation. A denser GPS observation in this segment features are similar to the block rotation in Luzon Island is recommended to obtain a better resolution of the fault detected by analysis of earthquake slip vectors and GPS- locking depth. derived horizontal velocities [18]. In contrast, this creep-like movement was not observed In the above estimation, the westernmost and the south- in the southernmost part of the Philippine fault in Min-

64 Journal of Disaster Research Vol.10 No.1, 2015 Plate Convergence and Block Motions in Mindanao Island, Philippine as Derived from Campaign GPS Observations danao. As shown in Figs. 4 and 6, the difference of the References: fault parallel component between SMMM and SMCP is [1] T. Tabei, F. Kimata, and T. Ohkura, “Estimation of plate convergent rates in the Philippines-eastern deformation zone from about 5 mm/year, which cannot be explained by a block GPS results,” Abstract of Japan Geoscience Union meeting 2008, rotation model. In this area, there is no historical earth- T229-006, 2008. [2] C. R. Allen, “Circum-Pacific faulting in the Philippines-Taiwan Re- quake caused ground rupturing [3]. Moreover, paleoseis- gion,” J. Geophys. Res., Vol.67, pp. 4795-4812, 1962. mic trenching survey revealed that 500–1300 years have [3] M. L. P. Bautista and K. Oike, “Estimation of the magnitudes and passed since the latest surface-rupturing event and that the epicenters of Philippine historical earthquakes,” Tectonophysics, Vol.317, pp. 137-169, 2000. estimated recurrence interval of such events is longer than [4] T. I. Allen, K. D. Marano, P. S. Earle, and D. J. Wald, “PAGER- 1000 years longer that in the Surigao segment [22]. Thus, CAT: A composite earthquake catalog for calibrating global fatality we could conclude that along-strike variation of the dis- models,” Seismol. Res. Lett., Vol.80, pp. 57-62, 2009. [5]N.T.Ramos,H.Tsutsumi,J.S.Perez,andP.P.BermasJr.,“Up- location pattern of the Philippine fault is evident in Min- lifted marine terraces in Davao Oriental Province, Mindanao Is- danao Island. land, Philippines and their implications for large prehistoric off- shore earthquakes along the Philippine trench,” Journal of Asian Earth Sciences, Vol.45, pp. 114-125, 2012. [6] C. Rangin, X. Le Pichon, S. Mazzotti, M. Pubellier, N. Chamot- Rooke, M. Aurelio, A. Walpersdorf, and R. Quebral, “Plate con- 4. Conclusions vergence measured by GPS across the Sundaland / PhilippineSea Plate deformed boundary: the Philippines and eastern Indonesia,” In order to assess the earthquake generation potential Geophys. J., Vol.139, pp. 296-316, 1999. [7] M. A. Aurelio, “Shear partitioning in the Philippines: Constraints of the Philippine fault and the Philippine trench in the from Philippine Fault and global positioning system data,” Island Mindanao region, we conducted GPS campaigns annu- Arc, Vol.9, pp. 584-597, 2000. ally in the eastern part of Mindanao from March 2010 to [8] W. Simons, B. Ambrosius, R. Noomen, D. Angermann, P. Wilson, M. Becker, E. Reinhart, A. Walpersdorf, and C. Vigny, “Observing March 2014. The obtained station velocities with respect plate motions in SE Asia: Geodetic results of the GEODYSSEA to the SU show that WNW motions are dominant due to project,” Geophys. Res. Lett., Vol.26, No.14, pp. 2081-2084, 1999. [9] H. Tsutsumi and J. S. Perez, “Large-scale active fault map of the the convergence of the PHS, but their spatial decay with Philippine fault based on aerial photograph interpretation,” Active distance from the trench is not significant compared with Fault Research, Vol.39, pp. 29-37, 2013. that in SW Japan. And it is found that elastic deforma- [10] D. F. Argus, R. G. Gordon, and C. DeMets, “Geologically current motion of 56 plates relative to the no-net-rotation reference frame,” tions caused by full coupling of the plate interface down Geochemistry, Geophysics, Geosystems, 2011. to 80 km can explain a maximum of 29% of the observed [11] Z. Altamimi, X. Collilieux, J. Legrand, B. Garayt, and C. Boucher, “ITRF2005: A new release of the Int. Terrestrial Reference Frame station velocities. In order to interpret the displacement based on time series of station positions and Earth Orientation Pa- pattern, we applied a rigid block rotation model and de- rameters,” J. Geophys, Res., Vol.112, pp. B09401, 2007. termined the Euler vector. As a result, we found that Min- [12] Z. Altamimi, X. Collilieux, and L. Metivier, “ITRF2008: an im- proved solution of the international terrestrial reference frame,” J. danao Island could be divided into at least three blocks Geodesy, Vol.85, pp. 457-473, 2011. and that the Philippine fault was one of the block bound- [13] T. J. Fitch, “Plate convergence, transcurrent faults, and internal de- formation adjacent to Southeast Asia and the western Pacific,” J. aries. Geophys. Res., Vol.77, No.23, pp. 4432-4460, 1972. Although we could not determine the coupling rate at [14] G. P. Hayes, D. J. Wald, and R. L. Johnson, “Slab 1.0: A three- the Philippine trench, we noted an along-strike variation dimensional model of global subduction zone geometries,” J. Geo- phys. Res., Vol.117, pp. B01302, 2012. of the dislocation pattern of the Philippine fault in Min- [15] C. Geuzaine and J. F. Remacle, “Gmsh: a three-dimensional finite danao Island. We detected creep-like dislocation in the element mesh generator with built-in pre- and post-processing fa- northernmost segment of the fault, where the recurrence cilities,” Int. J. Numer. Meth. Engng, Vol.79, pp. 1309-1331, 2009. ∼ M [16] C. DeMets, R. G. Gordon, and D. F. Argus, “Geologically current interval of ground-rupturing earthquakes ( 7) is 300– plate motions,” Geophys. J. Int., Vol.181, No.1, pp. 1-80, 2010. 1000 years. No creep-like dislocation was observed in [17] B. J. Meade, “Algorithms for the calculation of exact displacements, the southernmost segment of the Philippine fault, where a strain, an stresses for triangular dislocation element in a uniform elastic half space,”Comput. Geosci., Vol.33, pp. 1064-1075, 2007. longer recurrence interval was estimated. [18] G. Galgana, M. Hamburger, R. McCaffrey, E. Corpuz, and Q. Chen, “Analysis of Crustal Deformation in Luzon, Philippines us- ing Geodetic Observations and Focal Mechanisms,” Tectonophys., Vol.432, pp. 63-87, 2007. Acknowledgements [19] H. Tsutsumi, Y. Fukushima, J. S. Perez, and J. J. Lienkaemper, We are grateful to Mr. Masahiro Hasegawa and Mr. Toshitaka “Aseismic creeping of the Philippine fault in Leyte Island, Philip- pines, revealed by field observation and InSAR analysis,” Japan Kobayashi for their logistical support. Thanks are due to all who Geoscience Union Meeting, 2013. kindly helped us in our observations in Mindanao. We thank Dr. [20] T. Duquesnoy, E. Barrier, M. Kasser, M. Aurelio, R. Gaulon, R. Hiroyuki Tsutsumi and Mr. Jeffrey S. Perez for providing usage S. Punongbayan, and C. Rangin, “Detection of creep along the of the digital map of the Philippine fault in Mindanao. We also Philippine Fault: first results of geodetic measurements on Leyte island, central Philippine,” Geophys Res. Letters, Vol.21, pp. 975- thank two anonymous reviewers for their helpful comments. We 978, 1998. used the GEONET daily coordinate solutions (F3) of the Geospa- [21] T. Bacolcol, E. Barrier, T. Duquesnoy, A. Aguilar, R. Jorgio, R. de la tial Information Authority of Japan (GSI). Almost of all the fig- Cruz, and M. Lasala, “GPS constraints on Philippine fault slip rate in Masbate Island, central Philippines,” Journal of the Geological ures in this paper were drawn by using Generic Mapping Tools Society of the Philippines, Vol.60, pp. 1-7, 2005. (GMT; Wessel et al. (1998) [23]). This study was supported by [22] J. S. Perez, H. Tsutsumi, M. Cahulogan, D. P. Cabanlit, Ma. I. the JST-JICA project “Enhancement of Earthquake and Volcano T. Abigania, and T. Nakata, “Fault distribution, segmentation and earthquake generation potential of the Philippine fault in eastern Monitoring and Effective Utilization of Disaster Mitigation Infor- Mindanao, Philippines,” J. Disaster Res., Vol.10, No.1, 2014 (this mation in the Philippines.” issue). [23] P. Wessel and W. H. F. Smith, “New, improved version of generic mapping tools released,” Eos Trans. AGU, Vol.79, No.47, p. 579, 1998.

Journal of Disaster Research Vol.10 No.1, 2015 65 Ohkura, T. et al.

Name: Takahiro Ohkura

Affiliation: Professor, Aso Volcanological Laboratory, Ky- oto University

Address: Minami-Aso, Aso, Kumamoto 869-1404, Japan Brief Career: 1991- Joined Kyoto Univ. 2000- AVL, Kyoto Univ. Selected Publications: • T. Ohkura, “Structure of the upper part of the Philippine Sea plate estimated by later phases of upper mantle earthquakes in and around Shikoku, Japan,” Tectonophysics, 2000. • Y. Abe, T. Okura, T. Shibutani, and K. Hirahara, “Along-arc variation in water distribution in the uppermost mantle beneath Kyushu, Japan, as derived from receiver function analyses,” JGR, 2013. Academic Societies & Scientific Organizations: • Seismological Society of Japan (SSJ) • American Geophysical Union (AGU) • Volcanological Society of Japan (VSJ)

*Profiles of co-authors are omitted in this special issue.

66 Journal of Disaster Research Vol.10 No.1, 2015