U54B-01 INVITED
Earthquakes in Fiordland, Southern Chile: Initiation and Development of a Magmatic Process
Several efforts in Chile are being conducted in relation to geophysical monitoring with the objective of disaster mitigation. A long and permanent monitoring effort along the country has been the continuous effort resulting in the recognition and delineation of new seismogenic sources. Here we report on the seismo-volcanic crisis that is currently taking place in the in the region close to the triple junction (Nazca, Antarctica and South America) in southern Chile at around latitude 45°S. On January 22, 2007, an intensity V-VI (MMI) earthquake shook the cities of Puerto Aysén, Puerto Chacabuco and Coyhaique. This magnitude 5 event, was the first of a series of earthquakes that have taken place in the region for nearly a month and a half (until end of February, time when this abstract was written). The closest station to the source area -part of the GEOSCOPE network located in Coyhaique, about 80 km away from the epicenters- reveals seismic activity about 3 hours before the first event. Immediately after the first event, more than 20 events per hour were detected and recorded by this station, rate which decreased with time with the exception of those time intervals following larger events. More than six events with magnitude 5 or more have been recorded. Five seismic stations were installed surrounding the epicentral area between 27 - 29 January and are currently operational. After processing some of the recorded events, a sixth station was installed at the closest possible site of the source of the seismic activity. Preliminary analysis of the recorded seismic activity reveals a concentration of hypocenters - 5 to 10 km depth- along an eight-km NNE-SSW vertical plane crossing the Aysén fiord. Harmonic tremor has also been detected. This seismic activity is interpreted as the result of a magmatic process in progress which will most likely culminate in the generation of a new underwater volcanic edifice. Because the seismic activity fully extends across the Aysén fiord -the only maritime outlet of the cities of Coyhaique, Puerto Aysén and Puerto Chacabuco- and large fisheries plants exist in the area, we will continue monitoring the area with experts of the National Emergency Office, National Geology and Mines Service, and Millennium Nucleus on Seismic Hazards to mitigate any possible disaster.
U54B-02
The importance of earthquake research in the assessment of seismic hazards in Argentina
The history of Argentina has repeated occurrences of damaging crustal earthquakes with examples like the 1944 (Mw 7.0) San Juan earthquake, considered the largest natural disaster. These large earthquakes occur in the continental Andean backarc crust as far as 600 to 800 km east from the Trench. Of high significance is the correlation of this large-sized continental seismicity with the horizontal position of the subducted Nazca plate at about 100-km depth. In addition, lateral variations of the crustal structure are expected since several terranes have been accreted to western South America since the Paleozoic. Given the high seismic potential of this region, understanding of these seismotectonic processes and the crustal structure is essential for the assessment of seismic hazards and the mitigation of their effects. In this presentation we show our work based on an integrated research effort that combines permanent and temporal seismic networks from the Argentinean National Institute for Seismic Disaster Mitigation (INPRES) and IRIS- Passcal arrays. This international collaboration started in 2000 and involves researchers, technicians and students from the University of Arizona (USA), the National University of San Juan (Argentina) and INPRES (Argentina).
U54B-03
Characteristics of Andean earthquakes felt at long distances in Brazil
In the past several decades, more than 40 Andean earthquakes have been felt in Brazilian cities, thousands of kilometers away from the epicenter, especially by oscillations of high-rise buildings. Because of the increase in population and skyscrapers in large cities, such long-distance effects are becoming more frequent. In São Paulo, for example, such macroseismic effects have been observed every 3-5 years on average. We present a study of the characteristics of the earthquakes and cities more susceptible to cause high-rise buildings to oscillate. Most earthquakes occur at intermediate or large depths, such as in the Jujuy and Santiago del Estero regions of northern Argentina. Magnitudes as low as 5.0 mb have caused long distance macroseismic effects. Most affected cities lie in sedimentary basins, such as São Paulo and Manaus, which can amplify the ground motions through basin resonance. In some cases, buildings with the longest axis oriented perpendicular to the largest S amplitudes (SH component) seem to be more susceptible to oscillations. An analysis of the ground motion amplitudes, as recorded by nearby broadband stations, will be presented.
U54B-04
Seismogenic sources and crustal structure near the seismological station of Angra dos Reis (ESAR), southeastern region of Brazil
The seismological station of Angra dos Reis (ESAR) is operating since the end of year 2000 located near to the nuclear power plants of Angra dos Reis, in the southern portion of Rio de Janeiro state, southeastern Brazil. The main objective of ESAR is defining the level of seismic activity in the area where those plants are installed, using data from a broadband seismograph that operates in a 100 cps continuous format. Epicentral determination of local seismic events is done through the back azimuth calculated with the P wave amplitude in the three component seismograms, and epicentral distance induced from the S-P time difference. Crustal structure for relating the S-P time and epicentral distance was obtained through a seismic refraction experiment using controlled sources, complemented with a detailed study of crustal structure beneath ESAR using teleseimic data and the receiver function method processed in the time domain. Since the beginning of operation ESAR has detected many local and near regional seismic events with magnitude lower than mb 4.0, most of them below mb 2.0, and not included in the Brazilian Seismic Catalogue. Epicenters of those events are spread in both oceanic and continental areas, in some cases forming clusters of different size, some of them, especially those located in the continent could belong to explosions made in stone-quarry, whereas, clusters and isolated epicenters located in the oceanic area could belong to natural earthquakes. Two of these oceanic clusters, located in the continental platform, may belong to seismogenic sources that are active near ESAR, and need further study to define its nature.
U54B-05
Seismic hazard evaluation in Brazilian Southeastern region using the probabilistic method
Brazilian Southeastern region is located in the eastern portion of South America, constituting a passive border with the Atlantic Ocean. That region is a typical intraplate region where current tectonic processes are insignificant, if compared to existing tectonic activity on the borders of tectonic plates, as in the Andean region. An evaluation of seismic hazard was performed in Southeastern region of Brazil using the probabilistic method, which takes in consideration uncertainties in the dimension, location, and time of occurrence of earthquakes, in a systematic way of analysis. The level of seismicity in that region is low; however, it is significant in the seismicity context of Brazilian territory, including earthquakes with magnitude up to 6.3 mb or probably higher, close to 7.0 mb, as shown in this paper. Seismic hazard curves obtained for the main cities of Southeastern region and for specific points inside this region are alike, and present maximum horizontal ground acceleration level between 0.03, and 0.05 g for an annual probability of 10-4. It is concluded any site with outcrop of basement rock, located within Southeastern region of Brazil, should present a seismic hazard curve similar to the curves that are recommended in this paper
U54B-06
Seismic and Geodetic Monitoring of the Nicoya, Costa Rica, Seismic Gap
The Nicoya segment of the Middle America Trench has been recognized as a mature seismic gap with potential to generate a large earthquake in the near future (it ruptured with large earthquakes in 1853, 1900 and 1950). Low level of background seismicity and fast crustal deformation of the forearc are indicatives of strong coupling along the plate interface. Given its high seismic potential, the available data and especially the fact that the Nicoya peninsula extends over large part of the rupture area, this gap was selected as one of the two sites for a MARGINS-SEIZE experiment. With the goal of documenting the evolution of loading and stress release along this seismic gap, an international effort involving several institutions from Costa Rica, the United States and Japan is being carried out for over a decade in the region. This effort involves the installation of temporary and permanent seismic and geodetic networks. The seismic network includes short period, broad band and strong motion instruments. The seismic monitoring has provided valuable information on the geometry and characteristics of the plate interface. The geodetic network includes temporary and permanent GPS stations as well as surface and borehole tiltmeters. The geodetic networks have helped quantify the extend and degree of coupling. A continuously recording, three- station GPS network on the Nicoya Peninsula, Costa Rica, recorded what we believe is the first slow slip event observed along the plate interface of the Costa Rica subduction zone. We will present results from these monitoring networks. Collaborative international efforts are focused on expanding these seismic and geodetic networks to provide improved resolution of future creep events, to enhanced understanding of the mechanical behavior of the Nicoya subduction segment of the Middle American Trench and possibly capture the next large earthquake and its potential precursor deformation.