S43C-01 INVITED
Cooperative Seismological Studies in the Andes
Since the early nineties a number of passive and active seismic studies have been carried out in the central and southern Andes in a joint effort of Geman, Chilean, Argentian and Bolivian institutions. Applied techniques are high resolution studies of the seismicity and structure. The at that time new seismic techniques of receiver function analysis has been applied in a large scale. Important new results have been obtained like images and maps of the upper plate Moho (even in the volcanic arc, where the existence of the Moho was questioned) and images of the subducting crust down to more than 100 km (where the connection with petrological phase transformations and seismicity was established). A zone of partial melt was found in the upper crust of the high plateau. The ascent path of melt from the subducting slab to the surface has been mapped with seismic tomography. Observational results from the central and southern Andes are compared.
S43C-02 INVITED
Deep Earthquake Mechanics Inferred From Fault-Plane Orientations in Central South America
To place constraints on the physical mechanisms of deep earthquakes, we analyze the rupture properties of >30 intraslab earthquakes with MW ≥ 5.7 in central South America. For each earthquake, we combine a directivity analysis with mapping of the slip distribution to estimate the rupture vector and identify the fault plane. We can distinguish the fault plane of the focal mechanism for ~1/3 of these earthquakes. At intermediate depths, we test whether earthquakes result from dehydration embrittlement reactivating the steep, trenchward- dipping faults of the outer rise. After accounting for the angle of subduction, the outer-rise faults would be approximately vertical. This prediction disagrees with our identified fault planes between 100-300 km depth, which are all subhorizontal. Subhorizontal faults are consistent with only one of the two failure planes expected from the slab stress field, suggesting that the slab fabric or an isobaric rupture process may also influence fault- plane orientations. The occurrence of exclusively subhorizontal faults at intermediate depths agrees with previous studies in the Tonga-Kermadec and Middle America subduction zones. The similarity in results between the three subduction zones despite large differences in temperature, lithospheric thickness, subduction velocity, and subduction angle suggests that the earthquake-generating mechanism is controlled by pressure rather than temperature or other tectonic parameters. Deeper than 300 km, earthquakes slip along both subhorizontal and subvertical fault planes, in agreement with the ambient stress field.
S43C-03
Relocations of instrumentally recorded earthquakes in the Andean region (1918-present)
We have created a comprehensive and self-consistent digital catalog of instrumental seismicity of the Andean region with uniformly computer-determined hypocenters and magnitudes reduced to a common scale. Earthquakes have been relocated by a teleseismic location method that uses recent accurate travel time tables, depth phases, and incorporates corrections for ellipticity, bounce-point bathymetry or topography, and near- station velocity structure. Phase arrival time data for earthquakes after 1963 were obtained in digital form from the International Seismological Centre (ISC). For earthquakes prior to 1964, phase data were obtained by scanning the printed International Seismological Summary (ISS) bulletins and applying optical character recognition. We approached the problem of assigning magnitudes by first combining existing magnitude catalogs into a single multi-valued one. Second we assigned a single magnitude to each event according to a hierarchical scheme and depending on availability. Finally we used these assigned magnitudes to determine the completeness thresholds of the catalog as a function of time. For the early instrumental period (1920's-1963) the catalog is complete down to a magnitude of 6.5, and for the recent period the resulting catalog is complete down to magnitude 5.5. Finally we have analyzed in detail the largest earthquakes in the region and their aftershock sequences, providing estimates of their rupture zones.
S43C-04 INVITED
The Aysen (Southern Chile) 2007 Seismic Swarm: Volcanic or Tectonic Origin?
The Aysen seismic swarm began January 23, 2007, with a magnitude 5.2 (USGS) earthquake and, after an apparent decrease in activity, continued with a magnitude 5.6 event on February 26. The swarm is characterized by numerous felt earthquakes of small to moderate magnitude, located at crustal depths beneath the Aysen Canal, a prominent fiord of the Chilean littoral. The region is characterized by the subduction of an active oceanic spreading ridge: the Chile Ridge, the divergent Nazca-Antarctic plate boundary, is currently subducting beneath continental South America along the Chile Trench at approximately 46.5°S, forming a plate triple junction in the vicinity of the Taitao Peninsula, somewhat south and west of the swarm. Also, the Liquine-Ofqui dextral strike- slip fault traverses the Aysen Canal in the vicinity of the swarm. This fault has been interpreted as a 1000 km long dextral intra-arc strike-slip fault zone, consisting of two major strands which extend north from the Chile Margin triple junction. The Liquiñe-Ofqui system is marked by several pull-apart basins along its trace through the area. Seismic activity along the Liquiñe-Ofqui fault zone has been poorly studied to date, largely because teleseismic events clearly related to the fault have been few, and southern hemisphere seismic stations are lacking. However, we deployed a dense temporary broad-band seismic network both onland and on the islands in the Aysen region, which allowed us to capture the initial phases of the swarm on some 20 stations, and to determine the background seismicity patterns in this area for the two years preceding the swarm. The swarm could be caused by several processes: the spatial and depth distribution of the events suggests that they are well correlated with reactivation of the southern end of the Liquiñe-Ofqui fault, as defined by geologic studies and onshore gravity data collected in southern Chile. The swarm may be related to formation of new volcanic center between Volcan Hudson (last erupted 1991) and Volcan Maca. Given uncertainties in the event locations, the 2007 seismic swarm could also result from a combination of tectonic motions on the Liquiñe-Ofqui fault system and magmatic arc activity. The two earthquakes with magnitudes over 5 and the numerous felt earthquake of the swarm clearly indicate that seismic hazard estimations in this previously quiescent region must be re-estimated.
S43C-05
Crustal Structure and Crustal Seismicity in the South-Central Andean Backarc
Crustal seismicity in the Andean backarc between 30°S and 33°S is responsible for the largest damaging earthquakes in Argentina. This region is characterized by the presence of a northeast elongated flat subduction of the Nazca plate at about 100-km depth, a shut off of the volcanic arc and the basement cored uplifts of the Sierras Pampeanas. The overriding South American plate is composed of accreted terranes, sometimes reactivated by Paleozoic-Mesozoic extensional processes. We have used regional broadband data recorded during the CHilean ARgentinean Geophysical Experiment (CHARGE) to characterize the crustal structure and crustal seismicity by modeling regional broadband waveforms. Our study shows differences in the backarc terrane crustal seismic parameters that may have implications for the nucleation of present-day crustal seismicity. The Cuyania terrane has a more mafic composition, thicker crust with possible partial eclogitization at lower crustal levels and exhibits large numbers of earthquakes of higher magnitudes from historic to modern times. In contrast, the Pampia terrane to the east has thinner more quartz-rich crust and is comparatively seismically quiet for moderate-sized events.
S43C-06
Peculiar seismotectonic characteristics of Nazca's subducted slab, in the Andean region: Why do they exist?
The peculiar morphology of Wadati-Benioff Zone (WBZ) beneath Andean region presents controversial seismotectonic characteristics of the subducting Nazca plate beneath South American plate: WBZ with an almost flat behaviour under Central-Northern Peru region, and beneath Central Chile region, intercalated with steeply portion of the slab; the almost completely aseismic portion between 300 and 500 km of depth; the existence of deep earthquakes in South American and their controversial focal mechanism. There are several hypotheses trying to explain a suitable origin for those deep earthquakes that occur in depths between 500 and almost 700 km, where the occurrence of brittle failure is improbable to exist due to the presence of high temperature and pressure conditions at those depths. We propose in this work - based mainly in the spatial distribution of relocated hypocentres, and in the joint interpretation with recent published results related to seismotectonic aspects of Andean region - a top-to- northwest shear of the portion of Nazca subducting plate between 24°S and 01°S, in such an amount that its deepest corresponding extremes, at around 600 km of depth, seem to be presently, under latitudes between 29°S and 06°S, respectively. The proposed northwestern displacement of South American plate may be provoking that shear process of Nazca slab, which should be larger at shallower depths of the slab, and limited to those latitudes below the Andean region. The NW displacement of Nazca slab could explain the existence of flat subduction beneath Central-Northern Peru region as a consequence of a probable northwards migration of the buoyant Nazca ridge after subduction under South American plate in around 15°S. Similar explanation could be used for the flat WBZ beneath Central Chile and the Juan Fernandez ridge. This hypothesis permits to infer for some very deep South American earthquakes shear, planar mechanisms at high pressure, some times as almost horizontal focal mechanism, similar to the mechanism on a sub-horizontal nodal plane determined for the MW 8.3 deep Bolivian event of June 1994.