A new insight on the geometry of subducting slabs in northern Luzon, Philippines

Abstract

We used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern Luzon and propose a model for describing the geometry of the subducted slab of the Eurasian plate beneath the northern segment of the Manila Trench. We took into consideration some of the observed bathymetric features (i.e. the bend in the trench line of the Manila Trench at 20°N lat. and the collision and subduction of an extinct mid-oceanic-ridge at 16–17°N lat.) and the intraplate deformation pattern in the North Luzon Ridge region in depicting a model that is consistent with most of the observed features and phenomena in the study area. An earthquake catalog covering the period from 1619 to 1997 was used in studying the distribution of strain energy released, while earthquakes covering the instrumental years (1963–1997) were used to analyze vertical distributions of earthquakes in sections. We refined the location of some of the historical earthquakes using macroseismic data. The inclusion of historical records despite their inaccurate locations could prevent misidentification of some regions as aseismic zones or gaps in seismicity especially along structures with long return periods. A focal mechanism database that covers the period from 1963 to 1997 was used to decipher intraplate deformation pattern and subduction process of the slab. The focal mechanism database consists of first motion solutions from previous authors and CMT solutions from Harvard University. Seismicity, strain energy release, and focal mechanism maps and cross-sections were drawn in order to have a three-dimensional visualization of the geometry and stress regimes of seismogenic zones. A new model of the subducted slab of the Eurasian plate beneath the Manila Trench is proposed. The model suggests the collision and subsequent partial subduction of a buoyant plateau at around 20°N lat. to explain the sharp bend in the trench line, the complicated deformation pattern on the overriding plate fronting the bend and the shallow dip of the subducted slab beneath this zone. A tear in the slab is also inferred to be present as evidenced by the observed gap in strain energy release and the abrupt change in dip from shallow to steep south of 18°N lat. The gap in seismicity and strain energy release (65–300 km depth) at around 17°N lat. may be used to infer the trajectory and location of the subducted extinct mid-oceanic ridge (MOR). The aseismic behavior is probably caused by the subducted ridge, which is still hot and is deforming plastically. This is supported by the heatflow data, which shows high values along the extinct MOR. The subducted part of the MOR may serve as the weakest zone where this tear could be localized. The tear may also explain the cause of the abrupt termination of the eastern chain of volcanoes south of 18°N lat. The above model, which is a refinement of the model introduced by Yang et al. [Tectonophysics 258 (1996) 85], is consistent with the observed seismicity and deformation pattern, observed bathymetric features, spatial distribution and geochemical character of volcanism in northern Luzon.