Description: On 27 February 2010 the Mw 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. Shortly after the mainshock a dense network of temporary seismic stations was installed along the whole rupture zone in order to capture the aftershock activity. Here, we present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines. By processing the seismic data between 15 March and 30 September 2010 from stations from IRIS, IPGP, GFZ and University of Liverpool we determined 20,205 hypocentres with magnitudes Mw between 1 and 5.5. Seismic activity occurs in six groups: 1.) Normal faulting outer rise events 2.) A shallow group of plate interface seismicity apparent at 25–35 km depth and 50–120 km distance to the trench with some variations between profiles. Along strike, the aftershocks occur largely within the zone of coseismic slip but extend ~ 50 km further north, and with predominantly shallowly dipping thrust mechanisms. Along dip, the events are either within the zone of coseismic slip, or downdip from it, depending on the coseismic slip model used. 3.) A third band of seismicity is observed further downdip at 40–50 km depth and further inland at 150–160 km trench perpendicular distance, with mostly shallow dipping (~ 28°) thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the coseismic rupture, and presumably above the intersection of the continental Moho with the plate interface. 4.) A deep group of intermediate depth events between 80 and 120 km depth is present north of 36°S. Within the Maule segment, a large portion of events during the inter-seismic phase originated from this depth range. 5.) The magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the Mw 8.8 Maule 2010 earthquake. 6.) Pronounced crustal aftershock activity with mainly normal faulting mechanisms is found in the region of Pichilemu (~ 34.5°S). These crustal events occur in a ~ 30 km wide region with sharp inclined boundaries and oriented oblique to the trench. The best-located events describe a plane dipping to the southwest, consistent with one of the focal planes of the large normal-faulting aftershock (Mw = 6.9) on 11 March 2010.

Description: We have investigated the seismic anisotropy beneath the Central Andean southern Puna plateau by applying shear wave splitting analysis and shear wave splitting tomography to local S waves and teleseismic SKS, SKKS and PKS phases. Overall, a very complex pattern of fast directions throughout the southern Puna plateau region and a circular pattern of fast directions around the region of the giant Cerro Galan ignimbrite complex are observed. In general, teleseismic lag times are much greater than those for local events which are interpreted to reflect a significant amount of sub and inner slab anisotropy. The complex pattern observed from shear wave splitting analysis alone is the result of a complex 3-D anisotropic structure under the southern Puna plateau. Our application of shear wave splitting tomography provides a 3-D model of anisotropy in the southern Puna plateau that shows different patterns depending on the driving mechanism of upper-mantle flow and seismic anisotropy. The trench parallel a -axes in the continental lithosphere above the slab east of 68W may be related to deformation of the overriding continental lithosphere since it is under compressive stresses which are orthogonal to the trench. The more complex pattern below the Cerro Galan ignimbrite complex and above the slab is interpreted to reflect delamination of continental lithosphere and upwelling of hot asthenosphere. The a -axes beneath the Cerro Galan, Cerro Blanco and Carachi Pampa volcanic centres at 100 km depth show some weak evidence for vertically orientated fast directions, which could be due to vertical asthenospheric flow around a delaminated block. Additionally, our splitting tomographic model shows that there is a significant amount of seismic anisotropy beneath the slab. The subslab mantle west of 68W shows roughly trench parallel horizontal a -axes that are probably driven by slab roll back and the relatively small coupling between the Nazca slab and the underlying mantle. In contrast, the subslab region (i.e. depths greater than 200 km) east of 68W shows a circular pattern of a -axes centred on a region with small strength of anisotropy (Cerro Galan and its eastern edge) which suggest the dominant mechanism is a combination of slab roll back and flow driven by an overlying abnormally heated slab or possibly a slab gap. There seems to be some evidence for vertical flow below the slab at depths of 200–400 km driven by the abnormally heated slab or slab gap. This cannot be resolved by the tomographic inversion due to the lack of ray crossings in the subslab mantle.

Description: The amphibian Walvis Ridge Passive-Source Seismic Experiment (WALPASS) have been operated for a period of two years from 2010 to 2012 in the area where the Walvis Ridge intersects the continental margin of northwestern Namibia. The deployment was intended to study the lithospheric and upper mantle structure in the ocean-continent transition area beneath the passive continental margin. The main idea is to find seismic anomalies related to the postulated hotspot track from the continent to the ocean along the Walvis Ridge that links the Etendeka continental flood-basalt province to the Tristan da Cunha hotspot in the mid Atlantic ocean. This could provide clues of the role of plume-lithosphere interaction during the continental break-up. We present here seismic structures of the crustal and mantle lithosphere in this geophysically little studied region using seismic methods including P and S receiver functions and shear wave splitting. The average crustal thickness in the continental Namibia is ~35 km with a relatively low Vp/Vs ratio of 1.7. Underneath the NE extension of the Walvis Ridge the crust is the thickest (45 km) with a high Vp/Vs ratio (〉1.80). The thick crust and high Vp/Vs ratio beneath the Walvis Ridge are consistent with high Vp derived by controlled source seismics, implying a magmatic underplating. A low velocity zone in the mantle is observed at depths of 60-120 km, possibly representing the base of the lithosphere. The P-to-S converted phases at the 410 and 660 km discontinuities arrive 2-3 s earlier, indicating higher upper mantle velocities (+5%). Seismic anisotropy in the mantle derived by the SKS splitting exhibits a pattern of the plume and plate interaction.

Description: Passive continental margins offer the unique opportunity to study the processes involved in continental extension and break up and the role of hot-spot related magmatism. Several geophysical experiments (Seismics, Magnetotellurics and Seismology) were therefore carried out 2011 and 2012 in northern Namibia to image crust and upper mantle at the landfall of the Walvis Ridge. The aim of these studies is to shed light on the present-day structure of this area to understand the dynamics of the breakup of Pangaea and the processes involved. First results of the seismic part of the project show anomalous velocity structures and reflective properties in the mid- and lower crust. The lower crust (onshore) is characterized by an unusual high velocity body which might be associated with magmatic processes of the plume continent interaction. The distribution of the upper mantle wave propagation velocities shows a rather complex, very inhomogeneous pattern. Onshore magnetotelluric (MT) data were acquired at 167 sites in a ~140 km wide and ~260 km long corridor from the Atlantic Ocean through the Kaoko Mobile Belt onto the Congo Craton. The data are generally of excellent quality. A first inspection of magnetotelluric and vertical magnetic transfer functions indicates significant three-dimensional (3-D) structures in the crust and upper mantle, particularly in the Western Kaoko Zone in the vicinity of prominent shear zones. The seismological team operates a passive-source seismic experiment for two years onshore/offshore NW Namibia. The seismic network consists of 28 land-based and 12 ocean-bottom stations covering an area of 400km x 800km. Different seismic methods, such as body wave and surface wave tomography, receiver function, shear wave splitting, etc, will be used to image the seismic anomalies in the upper mantle and to map the thickness of the crust and mantle lithosphere in this ocean-continental transition area. The aim is to find the mantle deformation styles related to the plume-lithosphere interaction along the Walvis Ridge.

Description: A passive seismic array which operated between December 2007 and November 2010 has been deployed in the southern Puna plateau between 25°S to 28°S latitude and 65W to 70W longitude in Argentina and Chile to address fundamental questions on the processes that form, modify and destroy continental lithosphere and control lithospheric dynamics along Andean-type continental margins. This experiment consisted of 75 stations has been conducted by Argentine, Chilean, German and US scientists and was designed to improve our understanding of the evolution of the Central Andean plateau in an area where there is a very little geophysical data available.We present here some preliminary results form P and S receiver functions that enables us to address fundamental questions about the role of crustal and mantle lithospheric delamination in the evolution of plateau regions and the evolution of the continental crust in general. Our aim is to determine the seismic structure and thickness of the continental crust and the lithospheric mantle beneath the southern Puna plateau.We also want to determine the thickness of the mantle transition zone beneath the southern Puna and constrain the geometry and rheology of the subducting slab. As a collaborative project, our network has been designed to use other standard broadband seismological methods besides receiver function involving surface wave tomography, tomographic travel time inversion, and shear wave splitting analysis.

Description: The high elevation of the southern Puna plateau, the widespread melting of its crust, the gap in intermediate depth seismicity and the recent eruptions of ignimbrite complexes can be explained by delamination of the lithospheric mantle beneath it. To test this hypothesis, an array consisting of 73 broad band and short period seismic stations was deployed in the region for a period of 2 years starting in 2007. We inverted the data using the two plane wave approach and obtained 1-D and 3-D Rayleigh wave phase velocities. Our dispersion curve shows that at short periods (〈70 s) the phase velocities are slightly higher than those of the Tibetan plateau and lower than those of the Anatolian plateau. At periods of 100–140 s we observe a low velocity zone that might be remnant hot asthenosphere below a flat slab (7–10 Ma). We estimate the average continental lithosphere thickness for the region to be between 100 and 130 km. Our three-dimensional Rayleigh wave phase velocities show a high velocity anomaly at low frequencies (0.007, 0.008, and 0.009 Hz) slightly to the north of Cerro Galan. This would be consistent with the hypothesis of delamination in which a piece of lithosphere has detached and caused upwelling of hot asthenosphere, which in turn caused widespread alkaline-collision related volcanism. This interpretation is also corroborated by our shear wave velocity model, where a high velocity anomaly beneath the northern edge of Cerro Galan at 130 km depth is interpreted as the delaminated block on top of the subducting Nazca slab.

Description: The Sierra Nevada Mountains in Southern Spain is one of the prominent features in the Western Mediterranean tectonic region. This mountain range with high topography (_ 3400 m) is located in the Central Betic Cordillera surrounded by Neogene-Quaternary sedimentary basins. We deployed 40 seismic broadband stations during one year in a North-South profile, to collect teleseismic events and perform a high-resolution P-to-S and S-to-P receiver function analysis. The spacing between stations, around 2km, allows mapping with high accuracy the variations of the crustal structure and the mantles discontinuities from the coast, through the mountain range to the near basin and test the hypothesis about the lack of crustal root underneath Sierra Nevada Mountains.

Description: We analyse data from seismic stations surrounding the Alboran Sea between Spain and North Africa to constrain variations of the lithosphere–asthenosphere boundary (LAB) in the region. The technique used is the receiver function technique, which uses S-to-P converted teleseismic waves at the LAB below the seismic stations. We confirm previous data suggesting a shallow (60–90 km) LAB beneath the Iberian Peninsula and we observe a similarly shallow LAB beneath the Alboran Sea where the lithosphere becomes progressively thinner towards the east. A deeper LAB (90–100 km) is observed beneath the Betics, the south of Portugal and Morocco. The structure of the LAB in the entire region does not seem to show any indication of subduction related features. We also observe good P receiver function signals from the seismic discontinuities at 410 and 660 km depth which do not indicate any upper-mantle anomaly beneath the entire region. This is in agreement with the sparse seismic activity in the mantle transition zone suggesting the presence of only weak and regionally confined anomalies.