ALBERT

Description: Splay faults, large thrust faults emerging from the plate boundary to the seafloor in subduction zones, are considered to enhance tsunami generation by transferring slip from the very shallow dip of the megathrust onto steeper faults, thus increasing vertical displacement of the seafloor. These structures are predominantly found offshore, and are therefore difficult to detect in seismicity studies, as most seismometer stations are located onshore. The Mw (moment magnitude) 8.8 Maule earthquake on 27 February 2010 affected ~500 km of the central Chilean margin. In response to this event, a network of 30 ocean-bottom seismometers was deployed for a 3 month period north of the main shock where the highest coseismic slip rates were detected, and combined with land station data providing onshore as well as offshore coverage of the northern part of the rupture area. The aftershock seismicity in the northern part of the survey area reveals, for the first time, a well-resolved seismically active splay fault in the submarine forearc. Application of critical taper theory analysis suggests that in the northernmost part of the rupture zone, coseismic slip likely propagated along the splay fault and not the subduction thrust fault, while in the southern part it propagated along the subduction thrust fault and not the splay fault. The possibility of splay faults being activated in some segments of the rupture zone but not others should be considered when modeling slip distributions.

Description: An array of broadband seismometers transecting the Talamanca Range in southern Costa Rica was operated from 2005 until 2007. In combination with data from a short-period network near Quepos in central Costa Rica, this data is analyzed by the receiver function method to image the crustal structure in south-central Costa Rica. Two strong positive signals are seen in the migrated images, interpreted as the Moho (at around 35 km depth) and an intra-crustal discontinuity (15 km depth). A relatively flat crustal and Moho interface underneath the north-east flank of the Talamanca Range can be followed for a lateral distance of about 50 km parallel to the trench, with only slight changes in the overall geometry. Closer to the coast, the topography of the discontinuities shows several features, most notably a deeper Moho underneath the Talamanca Mountain Range and volcanic arc. Under the highest part of the mountain ranges, the Moho reaches a depth of about 50 km, which indicates that the mountain ranges are approximately isostatically compensated. Local deviations from the crustal thickness expected for isostatic equilibrium occur under the active volcanic arc and in south Costa Rica. In the transition region between the active volcanic arc and the Talamanca Range, both the Moho and intracrustal discontinuity appear distorted, possibly related to the southern edge of the active volcanic zone and deformation within the southern part of the Central Costa Rica Deformed Belt. Near the volcanoes Irazu and Turrialba, a shallow converter occurs, correlating with a low-velocity, low-density body seen in tomography and gravimetry. Applying a grid search for the crustal interface depth and vp/vs ratio cannot constrain vp/vs values well, but points to generally low values (〈1.7) in the upper crust. This is consistent with quartz-rich rocks forming the mountain range.

Description: The coupled plate interface of subduction zones—commonly called the seismogenic zone—has been recognized as the origin of fatal earthquakes. A subset of the after-shock series of the great Antofagasta thrust-type event (1995 July 30; Mw = 8.0) has been used to study the extent of the seismogenic zone in northern Chile. To achieve reliable and precise hypocentre locations we applied the concept of the minimum 1-D model, which incorporates iterative simultaneous inversion of velocity and hypocentre parameters. The minimum 1-D model is complemented by station corrections which are influenced by near-surface velocity heterogeneity and by the individual station elevations. By relocating mine blasts, which were not included in the inversion, we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth. A study of the resolution parameters ALE and DSPR documents the importance of offshore stations on location accuracy for offshore events. Based on precisely determined hypo-centres we calculate a depth of 46 km for the lower limit of the seismogenic zone, which is in good agreement with previous studies for this area. For the upper limit we found a depth of 20 km. Our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies; the results are also in agreement with thermal studies for the Antofagasta forearc region.

Description: The structure and seismicity of the subduction zone of central Costa Rica have been investigated with local earthquake tomography down to ca.  50 km depth. Seismic traveltime data sets of three on- and offshore seismic networks were combined for a simultaneous inversion of hypocentre locations, 3-D structure of P-wave velocity and Vp/Vs ratio using about 2000 high-quality events. The seismicity and slab geometry as well as Vp and Vp/Vs show significant lateral variation along the subduction zone corresponding to the changes of the incoming plate which consists of serpentinized oceanic lithosphere in the northwest, a seamount province in the centre and the subducting Cocos Ridge in the southeast of the investigation area. Three prominent features can be identified in the Vp and Vp/Vs tomograms: a high-velocity zone with a perturbation of 4–10 per cent representing the subducting slab, a low-velocity zone (10–20 per cent) in the forearc crust probably caused by deformation, fluid release and hydration and a low-velocity zone below the volcanic arc related to upwelling fluids and magma. Unlike previously suggested, the dip of the subducting slab does not decrease to the south. Instead, an average steepening of the plate interface from 30° to 45° is observed from north to south and a transition from a plane to a step-shaped plate interface. This is connected with a change in the deformation style of the overriding plate where roughly planar, partly conjugated, clusters of seismicity of regionally varying dip are observed. It can be shown that the central Costa Rica Deformation Belt represents a deep crustal transition zone extending from the surface down to 40 km depth. This transition zone indicates the lateral termination of the active part of the volcanic chain and seems to be related to the changing structure of the incoming plate as well.