Unusual subduction zones: Case studies in Colombia and Iran

Subduction zones, usually, are plate boundaries with the highest level of seismic activity. However, the distribution of seismicity in these plate margins is not uniform in the frequency and magnitude. Some margins produce frequent earthquakes of only moderate size, some experience great earthquakes separated by periods of little to moderate size activity, while others may have no plate boundary earthquakes during historical time. In the latter it is difficult to ascertain if the subduction zone is strongly coupled or the slab is subducting aseismically. In addition, the distribution of the seismic activity at intermediate depths does not follow a uniform pattern. The abundance of intermediate depth earthquakes show a marked onset at a depth of 30-70 km and then decrease exponentially with increasing depth. However, there are areas at intermediate depths (mostly, deeper than 100 km) that experience a very high rate of stationary seismic activity, the so called “earthquake nests”. The aim of this research is to address the unusual seismicity observed in subduction zones with focus in mainly two areas. First part of the thesis deals with the Bucaramanga nest in Colombia, where Nazca and Caribbean plates converge towards the south American plate and subducting beneath the north Andes block. This area has been marked with complexity in the tectonic settings, lack of volcanoes and high rate of dense seismic activity at intermediate depths. The second part focuses in the Makran subduction zone, in south east of Iran and southern Pakistan, where the oceanic crust of the Oman sea is subducting beneath the Eurasian plate. In this area, the slab interface has been marked with low rate of seismic activity. In order to have an overview of the characteristics and the physical reason behind the nature of the Bucaramanga nest, we first investigated all reported and possible earthquake nests in the world. A general investigation in geometrical, seismological and tectonical aspects of earthquakes nests in Romania (Vrancea), Afghanistan (Hindu Kush), Colombia (Bucaramanga) and a few reported possible nests in Fiji, Ecuador and Chile Argentina border reveal that among all these nests, the smallest and the most active is the Bucaramanga nest. On the other hand, the nature of the Bucaramanga nest is the most disputable compared to the other earthquake nests. We studied the local seismicity obtained from National Network of Colombia from 1993-2001 in and around the Bucaramanga nest. The local data reveals two slabs subducted in the north and south of the Bucaramanga nest. The dip angles of the slabs in the north and south of the Bucaramanga nest are different (about 25±in the north and 50±in the south). The dip angle of the Bucaramanga nest is about 29±, in a good agreement with the northern slab. However, the result we obtained from focal mechanism stress inversion of moderate size earthquakes in the nest, shows that Bucaramanga is experiencing down dip extension, where the plunge of ¾3 is more in agreement with the dip angle southern slab. The fault mechanisms of earthquakes in the Bucaramanga nest show considerable non-double couple components which reveal source complexity in the nest. Also some variation in the mechanism of earthquakes in the nest can be observed based on Harvard catalog. Using a 3D Finite Element Modelling (FEM), with elastic lithosphere and inviscid fluid mantle, we show how collision between the two slabs could concentrate and modify the stress field. The concentrated stress field can explain the high rate of seismic activity in the area of the nest and the modified stress field can answer the variation in the observed focal mechanisms and the source complexity of the earthquakes in the Bucaramanga nest. We therefore suggest that the collision between the subducting slabs is the cause of the Bucaramanga nest. In the second part of thesis, we studied the Makran subduction zone to address the reason for the low rate of seismic activity at the slab interface. In Makran, the oceanic crust of the Oman sea subducts with a very low angle beneath the Eurasian plate. It is bounded to the west by continent- continent collision between Arabia and Eurasia and to the east by continent- continent collision between Indian and Eurasian plates. This subduction zone exhibits different seismic behavior in its fore-arc setting from the west toward the east. The entire eastern Makran has been ruptured throughout history by large earthquakes and currently is experiencing very low seismic activity in its forearc setting. However, the western Makran looks quiet and there is only one known earthquake in historical time, which could have occur in this region. The overriding lithosphere in Makran is marked with segmentation along the Sistan suture zone. The seismicity in Makran shows a steeper slab in the west compared to the east. The absence of seismicity in the fore-arc setting of the western Makran prevents any estimate about the dip angle of slab where it starts bending beneath the overriding lithosphere. However, we used the free air gravity anomaly in the trench area of the western and the eastern Makran to show that the western Makran is experiencing more negative anomaly than the eastern Makran. This indicates steeper dip angle in the western Makran compared to the eastern Makran, where slab bends below the overriding lithosphere. We have also investigated the present state of crustal stress in the overriding lithosphere in Makran by using focal mechanism stress inversion. We expanded the area of investigation to the collision zone between Arabia and Eurasia along Zagros to the west and to the collision zone between Indian and Eurasian plates to the east in order to find out any possible influence of the stress field of the collision zones into the Makran stress field. The results show a clear rotation in the direction of the compressional stress axis from the west toward the east of Makran, where the western Makran is under the influence of Arabian-Eurasian collision zone and the eastern Makran is under the influence of Indian- Eurasian collision zone. Furthermore, to investigate any possible interaction between the Sistan suture zone and the low angle slab, we studied the source of three recent intermediate depth earthquakes at 80, 72 and 58 km depth around the Sistan suture using body wave inversion. The results show no source complexity for the earthquakes at 80 and 72 km depth, but a complex source for the earthquake at 58 km depth. This may indicate that Sistan suture has a thick root and meets the slab around 58 km depth. In order to address the absence of seismic activity in the fore-arc setting of the western Makran, we used the free air gravity anomaly and removed the effect of gravity anomaly perpendicular to the trench. The residual gravity anomaly now is under the influence of shallower structures. We produced Trench Parallel Gravity and Topography Anomaly (TPGA and TPTA, respectively) maps. The TPGA and TPTA maps show that the whole western Makran fore-arc setting is marked with TPGA < -50 mGal and TPTA< -750 m. Based on other studies, the areas in the fore-arc setting with the TPGA< -40 mGal and TPTA < -750 m are prone to the strongest earthquakes. The high coefficient of friction (i.e. strong coupling) in the slab interface can explain this phenomena. The western Makran is separated from the eastern Makran, by a small area of strongly positive TPGA and TPTA. The TPGA and TPTA in the eastern Makran are different from the west, which may confirm heterogeneous frictional properties along the fore-arc setting. We used a 2D elastic-viscoelastic Finite Element (FE) model to show how a high coefficient of friction in the contact zone controls the shear stress build up in the interplate interface. We assigned high coefficient of friction to the area with the most negative TPGA (<-50 mGal) and TPTA (-750 m) in the western Makran. The result shows that it takes several hundreds of years until the shear stress reaches the yield point. This may explain why western Makran is not experiencing seismicity at the present time. Observation of intermediate depth earthquakes with normal mechanism indicates that the eastern Makran is in the mature stages of an earthquake cycle. We used a 2D elasticviscoelastic FE model to study deformation in the overriding plate within a single cycle of earthquake, where the slab interface coupled heterogeneously. We used the rupture zone of the strongest earthquake in the eastern Makran in our model as the area of the strongest coupling in the slab interface. The result shows that the deformation is more pronounced in the overriding lithosphere above the strongly coupled segment. Considering different length for the cycle of earthquakes, we conclude that the shorter the cycle of earthquakes the faster is the rate of deformations, so the rate of deformation in the eastern Makaran should be faster than the western Makran. Since most of the inter plate interface in Makran is onshore, the area of the stronger coupling can be located by continuous geodetic observations in the future