We used hypocentral and focal mechanism data in order to characterize the t
ectonic configuration of northern Luzon and propose a model for describing
the geometry of the subducted slab of the Eurasian plate beneath the northe
rn segment of the Manila Trench. We took into consideration some of the obs
erved bathymetric features (i.e. the bend in the trench line of the Manila
Trench at 20 degreesN lat. and the collision and subduction of an extinct m
id-oceanic-ridge at 16-17 degreesN lat.) and the intraplate deformation pat
tern in the North Luzon Ridge region in depicting a model that is consisten
t with most of the observed features and phenomena in the study area. An ea
rthquake 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 histori
cal earthquakes using macroseismic data. The inclusion of historical record
s despite their inaccurate locations could prevent misidentification of som
e regions as aseismic zones or gaps in seismicity especially along structur
es with long return periods. A focal mechanism database that covers the per
iod from 1963 to 1997 was used to decipher intraplate deformation pattern a
nd subduction. process of the slab. The focal mechanism database consists o
f first motion solutions from previous authors and CMT solutions from Harva
rd University. Seismicity, strain energy release, and focal mechanism maps
and cross-sections were drawn in order to have a three-dimensional visualiz
ation 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 degreesN lat. to explain the
sharp bend in the trench line, the complicated deformation pattern on the o
verriding 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 ev
idenced by the observed gap in strain energy release and the abrupt change
in dip from shallow to steep south of 18 degreesN lat. The gap in seismicit
y and strain energy release (65-300 km depth) at around 17 degreesN lat. ma
y be used to infer the trajectory and location of the subducted extinct mid
-oceanic ridge (MOR). The aseismic behavior is probably caused by the subdu
cted ridge, which is still hot and is deforming plastically. This is suppor
ted by the heatflow data, which shows high values along the extinct MOR. Th
e subducted part of the MOR may serve as the weakest zone where this tear c
ould be localized. The tear may also explain the cause of the abrupt termin
ation of the eastern chain of volcanoes south of 18 degreesN lat. The above
model, which is a refinement of the model introduced by Yang et al, [Tecto
nophysics 258 (1996) 85], is consistent with the observed seismicity and de
formation pattern, observed bathymetric features, spatial distribution and
geochemical character of volcanism in northern Luzon. (C) 2001 Elsevier Sci
ence B.V. All rights reserved.