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Analysis of deformation processes resulting from the 2013 Mw 7.2 Bohol earthquake
Abstract
The analysis of pre- and post-earthquake deformation processes provides an idea of plate movement patterns resulting from a strong earthquake. The subject event, the Mw 7.2 earthquake, with its epicenter located 6 kilometers southwest of Sagbayan, Bohol, occurred on October 15, 2013 at 8:12 AM PHT (PHIVOLCS). The main objective of this research is to detect extraneous patterns based on normal movements of active geodetic network stations (AGNS), specifically by considering data prior to and immediately after the event.
This study observed the actual contributions of crustal motion and post-seismic response patterns. Also presented here are the displacements that detail the daily coordinates of AGNS by mapping the spatial movements and plotting the associated temporal variations of the AGNS while comparing these displacements with the elastic strain release expected from this type and magnitude of earthquake. Included in the discussions are the possible contributions of tectonic plate movements surrounding the Philippine islands, as well as the different types of faults characterizing the location. Of particular interest is the Sundaland tectonic block, wherein an AGNS which is PPPC in Puerto Princesa, Palawan is located. The displacements of this particular AGNS are observed to be relatively lower or negligible compared to other AGNS analysed. This could be due to Sundaland being a coherent lithospheric block that is characterized by its low rate of seismicity.
The results obtained from processing AGNS data using Bernese GNSS Software were compared to results obtained from processing campaign data from PHIVOLCS using AUSPOS. The station PILC in Iloilo experienced the highest magnitude of horizontal displacement compared with other AGNS included in this study, due to its proximity to the epicenter of the subject earthquake and its aftershocks. The campaign stations located in Bohol Island have relatively higher magnitudes of horizontal and vertical displacements compared to those of the AGNS. The non-uniform directions of the 2D vectors of AGNS and campaign stations suggest that the tectonic setting of the Philippines is characterized by several micro-blocks.
Key Words: geodynamics, earthquake, post-seismic slip, GNSS
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A devastating earthquake struck Bohol, Philippines, on 15 October 2013. The earthquake originated at 12 km depth from an unmapped reverse fault, which manifested on the surface for several kilometers and with maximum vertical displacement of 3 m. The earthquake resulted in 222 fatalities with damage to infrastructure estimated at USD 52.06 million. Widespread landslides and sinkholes formed in the predominantly limestone region during the earthquake. These remain a significant threat to communities as destabilized hillside slopes, landslide-dammed rivers and incipient sinkholes are still vulnerable to collapse, triggered possibly by aftershocks and heavy rains in the upcoming months of November and December. The most recent fatal temblor originated from a previously unmapped fault, herein referred to as the Inabanga Fault. Like the hidden or previously unmapped faults responsible for the 2012 Negros and 2013 Bohol earthquakes, there may be more unidentified faults that need to be mapped through field and geophysical methods. This is necessary to mitigate the possible damaging effects of future earthquakes in the Philippines.
A dense network of continuous single- and dual-frequency GPS receivers at Taal Volcano, Philippines, reveals four major stages of volcanogenic deformation: deflation, from installation in June 1998 to December 1998; inflation, from January to March 1999; deflation, from April 1999 to February 2000; and inflation, from February to November 2000. The largest displacements occurred during the February-November 2000 period of inflation, which was characterized by ~120 mm of uplift of the center of Volcano Island relative to the northern caldera rim at average rates up to 216 mm/yr. Deformation sources were modeled using a constrained least squares inversion algorithm. The source of 1999 deflation and inflation in 2000 were modeled as contractional and dilatational Mogi point sources centered at 4.2 and 5.2 km depth, respectively, beneath Volcano Island. The locations of the inflationary and deflationary sources are indistinguishable within the 95% confidence estimates. Modeling using a running 4-month time window from June 1999 to March 2001 reveals little evidence for source migration. We suggest that the two periods of inflation observed at Taal result from episodic intrusions of magma into a shallow reservoir centered beneath Volcano Island. Subsequent deflation may result from exsolution of magmatic fluids and/or gases into an overlying, unconfined hydrothermal system.
The western boundary of the Philippine Sea (PH) Plate in the Philippines and eastern Indonesia corresponds to a wide deformation zone that includes the stretched continental margin of Sundaland, the Philippine Mobile Belt (PMB), extending from Luzon to the Molucca Sea, and a mosaic of continental blocks around the PH/Australia/Sunda triple junction. The GPS GEODYSSEA data are used to decipher the present kinematics of this complex area. In the Philippines, the overall scheme is quite simple: two opposing rotations on either side of the left-lateral Philippine Fault, clockwise to the southwest and counterclockwise to the northeast, transfer 55 per cent of the PH/Sundaland convergence from the Manila Trench to the northwest to the Philippine Trench to the southeast. Further south, 80 per cent of the PH/Sunda convergence is absorbed in the double subduction system of the Molucca Sea and less than 20 per cent along both continental margins of northern Borneo. Finally, within the triple junction area between the Sundaland, PH and Australia plates, from Sulawesi to Irian Jaya, preferential subduction of the Celebes Sea induces clockwise rotation of the Sulu block, which is escaping toward the diminishing Celebes Sea oceanic space from the eastward-advancing PH Plate. To the south, we identify an undeformed Banda block that rotates counterclockwise with respect to Australia and clockwise with respect to Sundaland. The kinematics of this block can be defined and enable us to compute the rates of southward subduction of the Banda block within the Flores Trench and of eastward convergence of the Makassar Straits with the Banda block. The analysis made in this paper confirms that this deformation is compatible with the eastward motion of Sundaland with respect to Eurasia determined by the GEODYSSEA programme but is not compatible with the assumption that Sundaland belongs to Eurasia, as was often assumed prior to this study.
A unique GPS velocity field that spans the entire Southeast Asia region is presented. It is based on 10 years (1994–2004) of GPS data at more than 100 sites in Indonesia, Malaysia, Thailand, Myanmar, the Philippines, and Vietnam. The majority of the horizontal velocity vectors have a demonstrated global accuracy of ∼1 mm/yr (at 95% confidence level). The results have been used to (better) characterize the Sundaland block boundaries and to derive a new geokinematic model for the region. The rotation pole of the undeformed core of the Sundaland block is located at 49.0°N–94.2°E, with a clockwise rotation rate of 0.34°/Myr. With respect to both geodetically and geophysically defined Eurasia plate models, Sundaland moves eastward at a velocity of 6 ± 1 to 10 ± 1 mm/yr from south to north, respectively. Contrary to previous studies, Sundaland is shown to move independently with respect to South China, the eastern part of Java, the island of Sulawesi, and the northern tip of Borneo. The Red River fault in South China and Vietnam is still active and accommodates a strike‐slip motion of ∼2 mm/yr. Although Sundaland internal deformation is general very small (less than 7 nanostrain/yr), important accumulation of elastic deformation occurs along its boundaries with fast‐moving neighboring plates. In particular in northern Sumatra and Malaysia, inland‐pointing trench‐perpendicular residual velocities were detected prior to the megathrust earthquake of 26 December 2004. Earlier studies in Sumatra already showed this but underestimated the extent of the deformation zone, which reaches more than 600 km away from the trench. This study shows that only a regional Southeast Asia network spanning thousands of kilometers can provide a reference frame solid enough to analyze intraplate and interplate deformation in detail.
Global Positioning System (GPS) data collected with P-codeless receivers at four regional tracking sites in Japan are processed using GIPSY software with precise ephemerides of GPS satellites. Daily solutions of site coordinates from July 1992 to September 1994 yield relative velocities between tracking sites on the North American plate (NA), the Philippine Sea plate (PH), and the Eurasian plate (EU). The precision of site coordinates with respect to the reference site at Tsukuba (TSU) reaches 5-25 mm for 1, 000 km baselines. A time series analysis of site coordinates leads to the following geodetic results and tectonic implications. Coseismic movement is detected at the Shin totsukawa site (STK) located about 200 km east-north-east of the epicenter of the Southwestern off Hokkaido earthquake in July 12, 1993. The horizontal displacement estimated with GPS is 3 cm west and 1 cm south, which is consistent with that estimated from a fault model. Assuming the velocity of TSU with respect to NA measured by Very Long Baseline Interferometry (VLBI), velocities obtained with GPS are compared to the latest plate motion models. They show good agreement at Chichijima (CCJ) on PH and at Kanoya (KNY) on EU. A small residual velocity at CCJ suggests that the back arcspreading of Izu-Bonin arc at the west of CCJ may not be significant. KNY moves to the south-south-east about 0.5 cm/year faster than the model. This is consistent with an eastward movement of Shanghai on EU revealed by VLBI.
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