Short and long-term effects of GPS measured crustal deformation rates along the South-Central Andes

In this study we present the contemporary crustal deformation field along the central and southern Andes (17-42\deg S) estimated from four Global Positioning System (GPS) campaigns conducted in 1994-97. We find that the majority of the observed crustal deformation field is relatively homogenous: roughly parallel to the plate convergence direction with decreasing velocities away from the trench. We attribute this type of deformation pattern to the inter-seismic phase of an earthquake deformation cycle caused by 100\% locking of the thrust interface between the subducting Nazca and the overriding South American plates. We have also detected a strong post-seismic deformation signal in the vicinity of the 1995 M$_w$8.0 Antofagasta (22-26\deg S) and 1960 M$_w$9.5 Chile (38-43\deg S) earthquakes. This type of deformation can be described as short-term in nature compared to geological timescales. The above conclusions are based on the results of the 3-D Andean Elastic Dislocation Model (AEDM). By subtracting the AEDM predicted deformation rates from the observations we obtained a residual velocity field, that highlights the post-seismic, as well as more long-term deformation effects. For example, we find a strong evidence for the continuing crustal shortening across the back-arc, reaching its maximum (4 mm/yr) in the very north of our study area. In addition, between latitudes 29-34\deg S, there is an indication of E-W oriented extension within the fore-arc, in accordance with the recent geologic findings for the N-S oriented normal faulting

Lo único que no sabemos es cuándo

El 12 de febrero de 1835,durante su visita a la ciudad portuaria de Valdivia, Charles Darwin fue testigo del terremoto de Concepción, de magnitud 8,5, que describió así: se ha notado el más violento terremoto que se recuerda aquí (...) Duró dos minutos (...) La Tierra, emblema mismo de la solidez, ha temblado bajo nuestros pies como una costra muy delgada puesta encima de un fluido". No deja de resultar irónico y triste a la vez que,cuando el mundo celebra el bicentenario del gran genio, la misma zona donde se encontraba hace 175 años haya sufrido una devastación de dimensiones semejantes. Aunque, precisamente por el hecho de que en esta zona no haya tenido lugar ningún seísmo importante desde la visita de Darwin, los científicos no se han extrañado de que haya ocurrido el terremoto del pasado 27 de febrero

Prolonged post-seismic deformation of the 1960 great Chile earthquake and implications for mantle rheology

Contemporary crustal deformation of the southern Andean margin shows an interesting feature: While nearly all coastal GPS sites move landward, consistent with interseismic deformation near a locked subduction fault, sites 300-400 km landward of the rupture region of the M-w 9.5 1960 Chile earthquake are moving in the opposite direction. We attribute the seaward motion of these inland sites to a prolonged crustal deformation due to mantle stress relaxation following the 1960 great earthquake. In order to reproduce the observed seaward motion using a three-dimensional finite element model we need to incorporate a mantle viscosity of about 3 x 10(19) Pa s. The possibility that the seaward motion is caused by a silent slip event on the plate interface at large depths cannot be completely excluded, and our analysis provides a working model for future field tests

Geodetic evidence for continuing tectonic activity of the Carboneras fault (SE Spain)

The Carboneras fault zone (CFZ) is a prominent onshore offshore strike slip fault that forms part of the Eastern Betic Shear Zone (EBSZ), located in SE Spain. In this work, we show for the first time, the continuing tectonic ac- tivity of the CFZ and quantify its geodetic slip-rates using continuous and campaign GPS observations conducted during the last decade. We find that the left-lateral motion dominates the kinematics of the CFZ, with a strike slip rate of 1.3 ± 0.2 mm/yr along the N48° direction. The shortening component is significantly lower and poorly constrained. Recent onshore and offshore paleoseismic and geomorphic results across the CFZ suggest a mini- mum Late Pleistocene to present-day strike slip rate of 1.1 mm/yr. Considering the similarity of the geologic and geodetic slip rates measured at different points along the fault, the northern segment of the CFZ must have been slipping approximately at a constant rate during the Quaternary. Regarding the eastern Alpujarras fault zone corridor (AFZ), located to the north of the CFZ, our GPS measurements corroborate that this zone is ac- tive and exhibits a right-lateral motion. These opposite type strike slip motion across the AFZ and CFZ is a result of a push-type force due to Nubia and Eurasia plate convergence, which, in turn, causes the westward escape of the block bounded by these two fault zones

Three-dimensional viscoelastic finite element model for postseismic deformation of the great 1960 Chile earthquake

We develop a three-dimensional viscoelastic finite element model to study postseismic deformation associated with the 1960 great Chile earthquake. GPS observations 35 years after the earthquake show that, while all coastal sites are moving landward, a group of inland sites 200 - 400 km from the trench are moving seaward and that coastal velocities in the 1960 rupture area are distinctly smaller than those to the north. We explain these observations in terms of mantle stress relaxation. The earthquake stretches the upper plate to move seaward, but elastic stresses coseismically induced in the upper mantle resist this motion. Stress relaxation allows seaward motion to take place in the inland area for several decades following the earthquake. With a viscosity of 2.5 × 1019 Pa s for the continental upper mantle, the model well explains the GPS observations. Numerical tests suggest that the continental mantle viscosity value is reasonably well constrained. The model shows the prolonged postseismic seaward motion of the inland area to be a unique feature of earthquakes with very long rupture along strike and large coseismic fault slip. For short rupture and small coseismic slip, the motion will stop very quickly after the earthquake, explaining why this phenomenon is not more commonly observed. With an oceanic mantle viscosity of 1020 Pa s, the model also provides an explanation for tide-gauge constrained postseismic uplift 200 km from the trench that had previously been explained using a model of prolonged afterslip of a deep segment of the Chile subduction fault

Snow avalanche energy estimation from seismic signal analysis

A method to determine the dissipated seismic energy into the ground by a down going avalanche is presented. Evaluation of the seismic energy is useful for avalanche size classification, model validation, and for characterization and better understanding of the avalanche evolution as it propagates downhill along the changing slope. The method was applied to two different type avalanches that were released artificially on 2004/02/28 and 2005/04/15 at Ryggfonn (Norway) avalanche experimental site, operated by the Norwegian Geotechnical Institute (NGI). The analysed seismic data were recorded by the University of Barcelona seismic instruments consisting of two three-component wide-range seismometers located respectively, in the middle and on the side of the avalanche path. The energy determination requires a priori seismic characterization of the site and the knowledge of the avalanche front speed. In this paper a seismic characterization (surface wave phase velocity and amplitude attenuation factor) of the Ryggfonn site is presented. This characterization will serve for subsequent studies. We attribute the main source of seismic signals for the studied events to basal friction and ploughing occurring at the avalanche front and related to the changing slope in the propagation path, which causes high seismic energy dissipation. A comparative study of the evolution of the dissipated seismic energy with the energy generated by a simple sliding block model of constant mass was performed. The observed differences highlight the importance of ploughing and basal friction and the specific characteristics of the avalanches, such as their length and type. The difference between the calculated total dissipated seismic energy for the two similar size avalanches reflects their different flow type. As expected, the dry/mixed event dissipates a smaller amount of energy (not, vert, similar 1.2 MJ) than the dry/dense event (not, vert, similar 2.8 MJ)

3D GPS velocity field and its implications on the present-day postorogenic deformation of the Western Alps and Pyrenees

We present a new 3D GPS velocity solution for 182 sites for the region encompassing the Western Alps, Pyrenees, and southern France. The velocity field is based on a Precise Point Positioning (PPP) solution, to which we apply a common-mode filter, defined by the 26 longest time series, in order to correct for network-wide biases (reference frame, unmodeled large scale processes, ¿). We show that processing parameters, such as troposphere delay modeling, can lead to systematic velocity variations of 0.1 - 0.5 mm/yr affecting both accuracy and precision, especially for short (< 5 yr) time series. A velocity convergence analysis shows that minimum time-series lengths of ~3 years and ~5.5 years are required to reach a velocity stability of 0.5 mm/yr in the horizontal and vertical components, respectively. On average, horizontal residual velocities show a stability of ~0.2 mm/yr in the Western Alps, Pyrenees, and southern France. The only significant horizontal strain rate signal is in the western Pyrenees with up to 4 x 10-9 yr-1 NNE-SSW extension, whereas no significant strain rates are detected in the Western Alps (< 1 x 10-9 yr-1). In contrast, we identify significant uplift rates up to 2 mm/yr in the Western Alps but not in the Pyrenees (0.1 ± 0.2 mm/yr). A correlation between site elevations and fast uplift rates in the northern part of the Western Alps, in the region of the Wurmian ice cap, suggests that part of this uplift is induced by postglacial rebound. The very slow uplift rates in the southern Western Alps and in the Pyrenees could be accounted for by erosion-induced rebound

Snow avalanche speed determination using seismic methods

We present a new method to determine the average propagation speed of avalanches using seismic techniques. Avalanche propagation speeds can reach 70 m/s and more, depending on a wide range of factors, such as the characteristics of the avalanche track (e.g. topography) and the snowpack properties (e.g. density). Since the damage produced by the avalanche depends primarily on the size and on the speed of the avalanche, the knowledge of the latter is therefore crucial for estimating avalanche induced hazard in inhabited mountain areas. However, our knowledge of this basic physical parameter is limited by the difficulty of conducting various measurements in the harsh winter weather conditions that often accompany this natural phenomenon. The method of avalanche speed determination presented in this paper is based on cross-correlation and time-frequency analysis techniques. The data used in this study come from the Ryggfonn (Norway) avalanche experimental site operated by the Norwegian Geotechnical Institute (NGI), and recorded by an array of 6 geophones buried along the main avalanche path during the 2003-2004 and 2004-2005 winter seasons. Specifically, we examine the speeds of 11 different events, characterized by size and snow type. The results obtained are compared with independent speed estimates from CW-radar and pressure plate measurements. As a result of these comparisons our method was validated and has proved to be successful and robust in all cases. We detected a systematic behaviour in the speed evolution among different types of avalanches. Specifically, we found that whereas dry/mixed type flow events display a complex type of speed evolution in the study area with a gradual acceleration and an abrupt deceleration, the speed of the wet snow avalanches decreases with distance in an approximately linear fashion. This generalization holds for different size events. In terms of time duration and maximum speed of the studied events, dry/mixed type avalanches lasted between 8 to 18 s and reached speeds up to 50 m/s, whereas the duration of wet avalanches ranged between 50 and 80 s and their maximum speeds were 10 m/

Caracterización de avalanchas de nieve con métodos sismológicos.

Snow avalanches associated risk in mountain areas can be very high. The knowledge of these phenomena is very important in order to mitigate that risk. The avalanche research group at the University of Barcelona has been working with the seismic signals of snow avalanches since 1994 at different sites all around Europe. The availability of more than 50 seismic records of snow avalanches made possible the identification of the specific characteristics of the seismic signals generated by avalanches. Classical seismological techniques frequently used to study earthquakes have been imported to perform the seismic signal characterization. However, the recent studies of the group have been focused in determining the physical parameters that can help in the description of the phenomena. In this case, seismological techniques have been used, in addition of the characterization of the seismic signals to obtain information of the avalanche itself. In this paper an overview of the most successful results in this field are presented