Characteristics of dense nests of deep and intermediate-depth seismicity

1. 2D and 3D view of seismicity in the Probable nests. Blue events are relocated data by Engdahl and Villasenõr. 2D and 3D view of seismicity in the known nests. Blue events are relocated data by Engdahl and Villasenõr. Location of known (Red circle) and probable (Green circle) nests in the world. Characteristics of dense nests of deep and intermediate-depth seismicity Zoya Zarifi and Jens Havskov , Department of Earth Sciences, Bergen University, Allegt. 41, 5007 Bergen, Norway (zoya.zarifi@geo.uib.no; jens.havskov@geo.uib.no) Focal mechansim of events based on HARVARD CMT solution in the area of the known and probable nests. Summary of observed seismicity in the known nests. Summary of observed seismicity in the probable nests. Abstract The aim of this poster is to review the intermediate and deep nests of earthquakes, which are not related to volcanoes. A nest is defined by stationary activity and not by earthquake swarms which take place occasionally, although each nest may include several swarms. A nest should be active in a more or less continuous mode with the events concentrated in a small volume so the activity in this volume is substantially larger than in the surrounding areas. The three well known intermediate depth nests in the world are the Bucaramanga nest in Colombia at 6.8ºN and 73.1ºW and centered at 160 km depth, Vrancea in Romania at 45.7ºN and 26.5ºE at depths between 70 and 180 km, and the Hindu Kush in Afghanistan. In Hindu Kush the earthquakes tend to occur as clusters of events but the majority are located at 36.5ºN and 71ºE in depth between 170-280 km. Here we have tried to compare the size, tectonic setting and focal mechanisms of the earthquakes to understand what mechanisms are responsible for existence of nests. All these nests are located in old subducted slabs. Among them, the smallest and the most active nest is Bucaramanga, and the deepest nest is Hindu Kush. The focal mechanism of earthquakes in Hindu Kush and Vrancea show reverse faulting, while in Bucaramanga the focal mechanism is variable but the majority of them have reverse mechanisms. Almost all studies agree that the seismicity in Vrancea is the result of progress of slab detachment from its upper part (slab break-off). In Bucaramanga and Hindu Kush, the responsible driving force is not known exactly. Many studies suggest that in Bucaramanga, the driving mechanism is fluid migration or dehydration reactions or a complex concentrated stress field, while in Hindu Kush the possible driving mechanism is distorted slab or collision of two subducting slabs from opposite directions. We also investigated some reported possible nests in the world. We found evidence that there are probably nests in Fiji, in Chile-Argentina border region and in Ecuador. • Possible origin of the known nests • Bucaramanga nest: Dehydration reaction and fluid migration can be a reason for the existence of the nest, but more probable reason can be concentration of a complex stress field in the area of the nest, which may arise from the collision of two or more slabs. • Vrancea nest: Slab detachment, which is propagated from north of Carpathian toward south, and the last fragment of this slab is detaching today in the Vrancea area. This detachment does not imply a passively sinking slab without any mechanical coupling to the overlying crust. Some kind of coupling is necessary to cause strong extension inside the slab, but the non existence of slab pull related to crustal stress field indicates that this coupling is not strong enough to transfer slab pull forces to the overlying crust. • Hindu Kush nest: The distorted subduction or collision of two slabs from opposite direction can be a driving mechanism for existence of the nest. • Data • The global seismicity based on ISC database(1964-2000). • The HARVARD CMT solution (1976-present). • The relocated ISC data by Engdahl and Villasenõr(2002). Goal Investigation of Size, Seismicity, Tectonic, Seismic moment release and Focal mechanisms of known nests (Bucaramanga in Colombia, Vrancea in Romania and Hindu Kush in Afghanistan) to find out their characteristics and using them to investigate some places which are called probable nests (These places are located in Fiji, Ecuador and Chile- Argentina border). • Conclusions • For the known nest, the dip of slabs are vertical or change to near vertical in the vicinity of the nests. • The focal mechanisms of events in the known nests and the probable nest in Fiji show reverse faulting. Also, these nests are located in regions of complex tectonics and perhaps close to distorted features in slabs. • The seismicity in different nests had different reasons. In Vrancea nest the detachment of slab is responsible for existence of nest. In Bucaramanga existence of nest is related to existence of a complex stress field in the area or dehydration reaction. In Hindu Kush a distorted subduction or collision of two slabs from opposite direction might be a reason. In Fiji the distortion of slab in depth may be the reason of seismicity and in Ecuador and Chile-Argentina nest, the reason is almost unknown, but they may be related to existence volcanic in those areas, which is not the case for the rest of nests. • Most of big earthquakes occur in Fiji and after that Vrancea experiences the biggest events. • The Bucaramanga nest is the most active nest if seismicity normalized by volume. • The rate of seismic moment release in all nests excluding nest in Chile- Argentina border is higher than their surrounding area, expressing higher activity inside the nest. • The earthquakes inside the nest have bigger magnitude comparing to surroundings. • It seems that possible nests in Chile- Argentina and Ecuador experience a stress regime different from the other nests, since the focal mechanism of events in these two nests (Normal faulting) are different from the other nests (Thrust faulting). • Tectonic Setting in the nest area • Bucaramanga nest is located in an area which four plates of North Andes block as part of south American plate, Panama block, Caribbean and Nazca plates are converging • Vrancea nest is located in a collision zone between three tectonic units of eastern european platform, Moesian sub plate and inter Alpine sub-plate. • Hindu Kush nest is located in a place of collision between India and Eurasia, apparantly along the Indus sature zone. An other view reveals that Hindu Kush nest is located in a contorted Wadati- Benieff zone. • Fiji probable nest is located in Fiji island arc, the location of deep seismicity in this region shows that Fiji nest located in a contorted feature inside the slab. • Probable nest In Ecuador is located in the slab of South America and close to Cotopaxi volcano. • probable nest in Chile-Argentina border is located in Chile Subduction and close to a volcanic area, which is called Socompa. • Possible origin of the probable nest • Fiji nest: Not known, but existence of nest close to maximum curvature in the location of deep seismicity may show the change in the slab geometry in the deeper part, and produce the stress in the area of the nest. • Ecuador and Chile- Argentina nests: Not known, but both of them are located in the area, close to active volcanoes. So, if this is the case, the stress which concentrated in the vicinity of weak zone, which has generated because of dehydration in depth, can be a reason for the observed seismicity in this area.