Description: The project SFB574 “Volatiles and Fluids in Subduction Zones” aims to better understand the processes and quantities involved in the exchange of fluids in convergent margins. The current phase concentrates on the accretionary margin of south-central Chile, a region that was affected after our data mining phase in 2008 by the rupture of the great 2010 Maule Earthquake (Mw 8.8). Within the project, an “amphibious” network of 15 ocean bottom seismometers and 22 land stations was operated from April to October 2008 along 350 km from the outer-rise to the magmatic arc. Additional readings from 11 permanent stations of the Chilean Seismological Service were included in the database improving the coverage to the north and south. One of the goals of the project is to gain a detailed image of the structure of the crust and upper mantle and the seismogenic zone by analyzing precise local earthquake locations and combined passive and active source seismic tomographic images. To achieve precise earthquake locations and to serve as an initial model for local earthquake tomography, we derived a P- and S-wave minimum-1D model using a very high-quality subset of 260 events with ~4000 P-wave and ~2000 S-wave arrivals. Most of the ~1500 earthquakes recorded over the sixmonth period were originated within the subducting slab down to ~140 km depth, with a higher concentration beneath the main cordillera, at depths of 80-100 km. We observe for the first time with a local network a double Benioff zone in this area. Fewer events were generated at the outer-rise, at depths of ~20-40 km, closely following the NE-SW trend of the oceanic plate faulting. The upper-plate seismicity occurred mainly beneath the main cordillera, and within the margin backstop offshore and SW of Pichilemu, where high activity was clustered in a ~1200 km2 area, between 10-30 km depth. The sparse interplate microseismicity nucleated from ~45 up to ~10 km depth and the upper limit is in good agreement with 2D refraction velocity modeling of the shallow part of the margin. The lack of seismicity supports the models reporting nearly complete interplate locking before the great earthquake. From the relocated catalogue, a subset of events has been selected for the 3D inversion of seismic velocities. We review the first tomographic results to present valuable insights into the structure and stress distribution in the Maule region before the 2010 earthquake.

Description: Chile is one of the most seismically active countries in the world with a M〉8 earthquake occurring approximately every ten years. The recent 27th February 2010 (Mw=8.8) earthquake released the energy accumulated in the Concepción-Constitución seismic gap, contained since the last 1835 earthquake. The hypocenter was located at the subduction interface of the Nazca plate under the South American plate, near the coast of Talcahuano, at a depth of 30 km. The rupture area extended approximately for 450 km long, from the Arauco Peninsula northwards to Pichilemu. Hundreds of aftershocks have taken place in the rupture area, the largest of which was the Pichilemu M=6.9 earthquake on 11th March. A volcano-surrounding network was installed in Llaima and Villarrica volcanoes in Southern Chile in November, 2009. Each network comprises one Broad Band station, one “Ocean” Bottom Station and three Short-Period stations arranged in a circular disposition. The seismological data recording at the two volcanoes is still ongoing until April 2011. Data examples and first results from the analysis of the data recorded from November, 2009 until April, 2010 are presented. The mainshock, foreshocks and aftershocks were retrieved, and are being separated from the local seismicity in the records. During the same time frame, volcanic events are identified to characterize the volcano seismicity at Llaima and Villarrica volcanoes, to correlate this activity with degassing emissions (SO2) measured with mini-DOAS. Any influence of the Maule earthquake on the volcanoes behaviour is also investigated based on the seismic records.

Description: On 2010 March 11, a sequence of large, shallow continental crust earthquakes shook central Chile. Two normal faulting events with magnitudes around M w 7.0 and M w 6.9 occurred just 15 min apart, located near the town of Pichilemu. These kinds of large intraplate, inland crustal earthquakes are rare above the Chilean subduction zone, and it is important to better understand their relationship with the 2010 February 27, M w 8.8, Maule earthquake, which ruptured the adjacent megathrust plate boundary. We present a broad seismological analysis of these earthquakes by using both teleseismic and regional data. We compute seismic moment tensors for both events via a W-phase inversion, and test sensitivities to various inversion parameters in order to assess the stability of the solutions. The first event, at 14 hr 39 min GMT, is well constrained, displaying a fault plane with strike of N145°E, and a preferred dip angle of 55°SW, consistent with the trend of aftershock locations and other published results. Teleseismic finite-fault inversions for this event show a large slip zone along the southern part of the fault, correlating well with the reported spatial density of aftershocks. The second earthquake (14 hr 55 min GMT) appears to have ruptured a fault branching southward from the previous ruptured fault, within the hanging wall of the first event. Modelling seismograms at regional to teleseismic distances ( 〉 10°) is quite challenging because the observed seismic wave fields of both events overlap, increasing apparent complexity for the second earthquake. We perform both point- and extended-source inversions at regional and teleseismic distances, assessing model sensitivities resulting from variations in fault orientation, dimension, and hypocentre location. Results show that the focal mechanism for the second event features a steeper dip angle and a strike rotated slightly clockwise with respect to the previous event. This kind of geological fault configuration, with secondary rupture in the hanging wall of a large normal fault, is commonly observed in extensional geological regimes. We propose that both earthquakes form part of a typical normal fault diverging splay, where the secondary fault connects to the main fault at depth. To ascertain more information on the spatial and temporal details of slip for both events, we gathered near-fault seismological and geodetic data. Through forward modelling of near-fault synthetic seismograms we build a kinematic k –2 earthquake source model with spatially distributed slip on the fault that, to first-order, explains both coseismic static displacement GPS vectors and short-period seismometer observations at the closest sites. As expected, the results for the first event agree with the focal mechanism derived from teleseismic modelling, with a magnitude M w 6.97. Similarly, near-fault modelling for the second event suggests rupture along a normal fault, M w 6.90, characterized by a steeper dip angle (dip = 74°) and a strike clockwise rotated (strike = 155°) with respect to the previous event.

Description: On the central Chilean continental margin off Valparaiso a temporary marine seismological network on the incoming Nazca Plate was supplemented by the Central Chilean Network and a number of temporary landstations. Simultaneous inversion of the complete marine earthquake data set yielded hypocenter distributions from minimum 1-D modeling. These show a considerable seismic activity within the 15 km - 60 km depth range with a maximum number of events between 20 km and 40 km. The distribution shows three areas of increased activity: the O'Higgins Seamount Group on the oceanic plate, the San Antonio Canyon and the northeastern portion of the Valparaiso Basin. There is a marked difference in the distance between the trench and the onset of seismic activity between the San Antonio Canyon area and the Valparaiso Basin area.

Description: T cells from patients with systemic lupus erythematosus (SLE) display a number of abnormalities, including increased early signaling events following engagement of the TCR. Signaling lymphocytic activation molecule family cell surface receptors and the X-chromosome–defined signaling lymphocytic activation molecule-associated protein (SAP) adaptor are important in the development of several immunocyte lineages and modulating the immune response. We present evidence that SAP protein levels are decreased in T cells and in their main subsets isolated from 32 women and three men with SLE, independent of disease activity. In SLE T cells, SAP protein is also subject to increased degradation by caspase-3. Forced expression of SAP in SLE T cells normalized IL-2 production, calcium (Ca 2+ ) responses, and tyrosine phosphorylation of a number of proteins. Exposure of normal T cells to SLE serum IgG, known to contain anti-CD3/TCR Abs, resulted in SAP downregulation. We conclude that SLE T cells display reduced levels of the adaptor protein SAP, probably as a result of continuous T cell activation and degradation by caspase-3. Restoration of SAP levels in SLE T cells corrects the overexcitable lupus T cell phenotype.