Interseismic coupling, segmentation and mechanical behavior of the Central Chile subduction zone.

International audienceGlobal Positioning System (GPS) measurements carried out in Chile over the last two decades showed that an entire portion of the Nazca-South America subduction zone (38°S ␣ 24°S) was locked over this period of time. The induced accumulation of elastic deformation in the upper-plate was not released until the recent Maule earthquake of 27 February 2010 (Mw 8.8) that ruptured the southern part of this section. Locking or coupling between the two plates varies both with depth and along strike. Here we use our own GPS data (an updated solution of our extended network in central Chile), combined with other published data sets, to quantify the spatial variations of the coupling that prevailed before the Maule earthquake. Using a simple elastic model based on the back-slip assumption, we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation, with no need of a sliver in central Chile. We identify four segments characterized by higher coupling and separated by narrow areas of lower coupling. This segmentation is in good agreement with historical and recent seismicity in Chile. In particular the narrow zones of lower coupling seem to have stopped most large seismic ruptures, including Maule's. These zones are often associated with irregular bathymetric or coastal features (fracture zones or peninsulas). Finally, coseismic and early post-seismic slip distribution of the Maule earthquake, occurring either in previously highly or weakly coupled zones, map a complex distribution of velocity-weakening and velocity-strengthening patches on the subduction interface

Oblique Convergence in the Himalayas of Western Nepal Deduced from Preliminary Results of GPS Measurements

A GPS network consisting of 29 sites was installed in central and western Nepal, with measurements taken in 1995 and partial remeasurements in 1997. Data suggest 15 +/−5 mm/yr of N180° convergence between the Higher Himalayas and India, a result that is consistent with N‐S shortening across the arcuate shape of the Nepalese Himalayas and an oblique underthrusting of the Indian crust below the High Himalayas of western Nepal. A 4 +/−3 mm/year E‐W extension and deviation of the principal shortening axes are inferred east of 83°E, where Quaternary faults (Darma‐Bari Gad fault system and Thakkhola graben) delineate a crustal wedge. This wedge is located on the SE projection of the Karakorum fault and may segment the Himalayan thrust belt. The convergence between the outer belt of western Nepal and India is less than 3 mm/yr, an attenuation consistent with creep on a dislocation locked beneath the Lesser Himalayas. A preliminary model suggests that this N 120°E striking dislocation is affected by a 19 mm/yr thrust component and a 7 mm/yr right lateral component

Prevalence and components of metabolic syndrome in HIV-infected patients at the Tiko Central Clinic and Cottage Hospital in Cameroon

Background: HAART and HIV related metabolic syndrome (MS) is associated with increased cardiovascular risk in aging HIV patients. This study was aimed at comparing the prevalence of MS between HIV-infected patients on HAART and apparently healthy HIV-uninfected individuals and identifying key MS components in these groups of subjects.Methods: This was a hospital-based case-control study. The cases were HIV sero-positive individuals on HAART for at least 6 months and controls were HIV sero-negative individuals.Results: 74/135 (54.8%) participants were females amongst which 53/75 (70.7%) and 21/60 (35%) were in the test and control groups respectively. The prevalence of MS was insignificantly higher in HIV-infected patients on HAART than in control subjects according to the IDF (22.7% versus 20%, p=0.834) and NCEP ATP III criteria (18.7% versus 18.3%, p=1.000) respectively. The most prevalent components of MS in HIV-infected patients on HAART were low HDL-c (100%), abdominal obesity (IDF: 68%, ATP III: 32%), and hypertension (28%). Multivariate analysis of MS components in HIV-infected patients on HAART revealed that hypertension (OR: 15.996, 95% CI: 3.385-75.585; p≤0.001) and high blood glucose (OR: 10.760, 95% CI: 1.642-70.505; p=0.013) were associated with MS. Significantly more HIV-infected females were seen with abdominal obesity than HIV-infected males (86.8% versus 4.5%, p≤0.001).Conclusions: Abdominal obesity is a driving component of MS in HIV-infected patients particularly among females and hypertension is a prevalent and predictor component of MS among HIV patients

GPS-derived interseismic coupling on the subduction and seismic hazards in the Atacama region, Chile

International audienceThe Atacama region (between 29°S and 25°S) is located in the North-Central area of Chile, a tectonically complex transition area between North and Central Chile. Deformation in Atacama is due mainly to elastic loading on the subduction interface but also to diffuse shortening in the Sierras Pampeanas, Argentina. The seismicity of the subduction is complex in this region: seismic swarms often occur, moderate (Mw ∼ 6) to large (Mw ∼ 7) earthquakes occur repeatedly and finally, megathrust earthquakes of magnitudes significantly larger than 8 occur once in a while, the last one being in 1922—almost a century ago. We use new GPS data we collected in the Atacama region between 2008 and 2012 to complete and densify existing data we acquired since 2004 in North-Central Chile. These new data allow to quantify the motion of the Andean sliver and assess the kinematic coupling on the subduction interface at these latitudes. We find that only 7 per cent of the whole convergence motion is taken up by an eastward rotation of the rigid sliver. A large part of the remaining 93 per cent (approximately 6 cm/yr) gives way to accumulation of elastic deformation in the upper plate, due to locking on the plate interface. This accumulation shows important along-strike and along-dip variations, interpreted in terms of variable coupling which we correlate with seismicity. We identify two areas of low coupling near the 'La Serena' (30°S) and 'Baranquilla' (27.5°S) bays. Both are correlated with the subduction of singular bathymetric features and seem to stop the propagation of large seismic ruptures. These zones are also seismic swarm prone areas, which seem to occur rather on their edges. These low coupling areas separate two seismic segments where coupling is high: the Atacama segment ( ∼100 km long between 29◦S and 28◦S) and the Cha ̃naral segment (∼200 km long between 27◦S and 25◦S). Should they rupture alone, these segments are sufficiently coupled and apparently since long enough, to produce Mw∼8 events. However, a collective failure of both segments could generate a megathrust earthquake of magnitude close to 8.5, similar to the 1819 and 1922 complex events, which produced important tsunamis. Such giant events may occur in the area once a century

Revisiting the North Chile seismic gap segmentation using GPS-derived interseismic coupling

International audienceNo major earthquake occurred in North Chile since the 1877 M w 8.6 subduction earthquake that produced a huge tsunami. However, geodetic measurements conducted over the last decade in this area show that the upper plate is actually deforming, which reveals some degree of locking on the subduction interface. This accumulation of elastic deformation is likely to be released in a future earthquake. Because of the long elapsed time since 1877 and the rapid accumulation of deformation (thought to be 6–7 cm/yr), many consider this area is a mature seismic gap where a major earthquake is due and seismic hazard is high. We present a new Global Positioning System (GPS) velocity field, acquired between 2008 and 2012, that describes in some detail the interseismic deformation between 18°S and 24°S. We invert for coupling distribution on the Nazca-South America subduction interface using elastic modelling. Our measurements require that, at these latitudes, 10 to 12 mm yr−1 (i.e. 15 per cent of the whole convergence rate) are accommodated by the clockwise rotation of an Andean block bounded to the East by the subandean fold-and-thrust belt. This reduces the accumulation rate on the subduction interface to 56 mm yr−1 in this area. Coupling variations on the subduction interface both along-strike and along-dip are described. We find that the North Chile seismic gap is segmented in at least two highly locked segments bounded by narrow areas of weak coupling. This coupling segmentation is consistent with our knowledge of the historical ruptures and of the instrumental seismicity of the region. Intersegment zones (Iquique, Mejillones) correlate with high background seismic rate and local tectonic complexities on the upper or downgoing plates. The rupture of either the Paranal or the Loa segment alone could easily produce a Mw 8.0–8.3 rupture, and we propose that the Loa segment (from 22.5◦S to 20.8◦S) may be the one that ruptured in 1877

Primary Health Centre disaster preparedness after the earthquake in Padang Pariaman, West Sumatra, Indonesia

<p>Abstract</p> <p>Background</p> <p>The West Sumatra earthquake that occurred on September 30, 2009, caused severe damage in some districts, including Padang Pariaman. As Padang Pariaman is an earthquake-prone area, disaster and emergency management is necessary. Due to the limited health facilities, the health services completely rely on Puskesmas (Primary Health Centres, PHCs). This study is aimed at assessing the preparedness of PHCs to response to potential disasters in their surrounding area.</p> <p>Findings</p> <p>Padang Pariaman district was used in a case study setting to assess the readiness and preparedness of the PHCs there to face disasters. Self-administered questionnaire, key informant interview, and direct observation were used to obtain the data on human resources, facilities preparedness, and the procedures. The investigation focused on measuring four aspects, i.e. human resources, facilities preparedness, standard operating procedure (SOP), and policy. Due to the limited co-operation of the head of the PHCs, three PHCs were directly observed as a subsample. The evaluation was performed six months after the impact phase of the earthquake and three months after the PHCs' health staff training on improving the primary health care services. The number and quality of health staff in Padang Pariaman was far below ideal. Fewer than half of the PHCs had emergency facilities and only one considered the need for triage and fire management, whereas the transportation mode was still limited. An SOP and policy for facing disasters were not available in any of the PHCs. Therefore, promoting disaster preparedness, technical provision, including health staff training, is necessary.</p> <p>Conclusions</p> <p>Padang Pariaman district has not yet prepared its PHCs to face disaster, so it is apparent that PHCs' disaster preparedness in Padang Pariaman and also other earthquake-prone areas in Indonesia should be promoted. This should include increasing the number of doctors, providing training for health staff, and developing a comprehensive approach as well as coordination among government, hospitals, PHCs, and NGO's for disaster preparedness.</p

Asperities and barriers on the seismogenic zone in North Chile: state-of-the-art after the 2007 Mw 7.7 Tocopilla earthquake inferred by GPS and InSAR data

The Mw 7.7 2007 November 14 earthquake had an epicentre located close to the city of Tocopilla, at the southern end of a known seismic gap in North Chile. Through modelling of Global Positioning System (GPS) and radar interferometry (InSAR) data, we show that this event ruptured the deeper part of the seismogenic interface (30–50 km) and did not reach the surface. The earthquake initiated at the hypocentre and was arrested ~150 km south, beneath the Mejillones Peninsula, an area already identified as an important structural barrier between two segments of the Peru–Chile subduction zone. Our preferred models for the Tocopilla main shock show slip concentrated in two main asperities, consistent with previous inversions of seismological data. Slip appears to have propagated towards relatively shallow depths at its southern extremity, under the Mejillones Peninsula. Our analysis of post-seismic deformation suggests that small but still significant post-seismic slip occurred within the first 10 d after the main shock, and that it was mostly concentrated at the southern end of the rupture. The post-seismic deformation occurring in this period represents ~12–19 per cent of the coseismic deformation, of which ~30–55 per cent has been released aseismically. Post-seismic slip appears to concentrate within regions that exhibit low coseismic slip, suggesting that the afterslip distribution during the first month of the post-seismic interval complements the coseismic slip. The 2007 Tocopilla earthquake released only ~2.5 per cent of the moment deficit accumulated on the interface during the past 130 yr and may be regarded as a possible precursor of a larger subduction earthquake rupturing partially or completely the 500-km-long North Chile seismic gap

Contrasting Decollement and Prism Properties over the Sumatra 2004-2005 Earthquake Rupture Boundary

Styles of subduction zone deformation and earthquake rupture dynamics are strongly linked, jointly influencing hazard potential. Seismic reflection profiles across the trench west of Sumatra, Indonesia, show differences across the boundary between the major 2004 and 2005 plate interface earthquakes, which exhibited contrasting earthquake rupture and tsunami generation. In the southern part of the 2004 rupture, we interpret a negative-polarity sedimentary reflector ~500 meters above the subducting oceanic basement as the seaward extension of the plate interface. This predécollement reflector corresponds to unusual prism structure, morphology, and seismogenic behavior that are absent along the 2005 rupture zone. Although margins like the 2004 rupture zone are globally rare, our results suggest that sediment properties influence earthquake rupture, tsunami hazard, and prism development at subducting plate boundaries

In-situ evidence for dextral active motion at the Arabia-India plate boundary

International audienceThe Arabia-India plate boundary--also called theOwen fracture zone--is perhaps the least-known boundary among large tectonic plates1-6. Although it was identified early on as an example of a transform fault converting the divergent motion along the Carlsberg Ridge to convergent motion in the Himalayas7, its structure and rate of motion remains poorly constrained. Here we present the first direct evidence for active dextral strike-slip motion along this fault, based on seafloor multibeam mapping of the Arabia-India-Somalia triple junction in the northwest Indian Ocean. There is evidence for 12km of apparent strike-slip motion along the mapped segment of the Owen fracture zone, which is terminated to the south by a 50-km-wide pull-apart basin bounded by active faults. By evaluating these new constraints within the context of geodetic models of global plate motions, we determine a robust angular velocity for the Arabian plate relative to the Indian plate that predicts 2-4mmyr−1 dextral motion along the Owen fracture zone. This transformfault was probably initiated around 8 million years ago in response to a regional reorganization of plate velocities and directions8-11, which induced a change in configuration of the triple junction. Infrequent earthquakes of magnitude 7 and greater may occur along the Arabia-India plate boundary, unless deformation is in the formof aseismic creep