A study conducted by a group of researchers led by Dr. Carla B. Dimalanta of the National Institute of Geological Sciences, University of the Philippines Diliman investigates the site of convergence between the Palawan Microcontinental Block and the Philippine Mobile Belt in Central Philippines. Entitled Delineating the Collision Zone Between Palawan and Central Philippines Using Seismic Intensity Data Inversion (2005), the study uses seismic intensity data inversion to produce tomographic images of the subsurface.
The collision between the Palawan Microcontinental Block and the Philippine Mobile Belt as observed in Central Philippines is a significant event in the evolution of the archipelago. According to Dr. Dimalanta, previous studies (Mitchell et al.,1985 and McCabe et al.,1985) report that the collision boundary is located in southwest Mindoro, Buruanga peninsula in Panay and Tablas Island. However, the presence of collision-related metamorphic rocks in the islands of Tablas, Romblon, and Sibuyan (Yumul et al., 2003) implies that the collision site may be extended east of the Romblon group of Islands (See Fig. 4). The group of Dr. Dimalanta tried a new approach to study this ancient collision. They utilized the inversion of seismic intensity data to produce a cross-section of the mantle in the study area. Moreover, the hypocenter data (or the point where ground rupture starts during an earthquake) and seismic reflection profiles (or the reflections of seismic waves in the seismograph which provide information on the internal structure of the earth) offered additional constraints in the interpretation.
Seismic intensity data in central Philippines were collected from the Southeast Asia Association of Seismology and Earthquake Engineering (SEASEE) Bulletin and the Philippine Institute of Volcanology and Seismology (PHIVOLCS). The group of Dr. Dimalanta evaluated tomographic or cross-sectional images of the mantle based on data gathered from 114 earthquakes that occurred from 1913 to 2003. Three-dimensional images of the mantle below the central Philippine region revealed a generally NE-SW trending high attenuation pattern. This pattern usually indicates that subduction, or the thrusting of a tectonic plate under an adjacent plate, has occurred. However, analysis of the hypocenter data indicates that there are no features that would correspond to a subducted slab. Compressional features such as faults and folds are detected in seismic reflection profiles which are more prominent in the vicinity of Tablas island. Researchers, thus, suggest slab tearing and mantle plumes, or the upwelling of hot rocks from the mantle, as possible causes of this attenuation pattern.
For the researchers, the use of mantle tomography is an important contribution to the application of geophysical methods in geoscience research in the Philippines. There are very few tomographic investigations done in the Philippines after the first work done by Dr. Glenda Besana and colleagues. The results of this investigation offer additional insights into understanding the collision event that has affected Central Philippines. Aside from this tomography work, additional geological, geochemical and geophysical investigations are currently being carried out in northwest Panay and Mindoro island.
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Dr. Carla B. Dimalanta is an Associate Professor at the National Institute of Geological Sciences whose field of expertise is geophysics. She is a co-author of the article entitled “Collision, subduction and accretion events in the Philippines: a synthesis” published last 2003 in the ISI journal Island Arc.
This paper won the first Island Arc Award given out by the Island Arc and Blackwell Publishing in 2007.
