Search

In the last two decades, enhanced computational resources and deployments of dense station networks inspired new kinds of methods that exploit the full amplitude and phase information of the recorded waveform. These waveform-based or data-driven methods do not rely on the identification of the phases in the seismograms and thus typically cope better with lower signal-to-noise ratios than ray-based methods. They are thus especially well-suited for applications in geothermal reservoirs where large numbers of microseismic events may occur quasi-simultaneously. Time-Reverse Imaging (TRI) is one waveform-based method for locating and characterising seismic events. The recorded seismic traces are time-reversed and re-inserted at their respective recording positions for a numerical simulator solving the elasto-dynamic wave equation. The time-reversed wavefield is back propagated using a suitable velocity model until it converges towards the true source locations. Applying suitable imaging conditions, i.e. observing the energy density during the back propagation, results in high-accuracy source images. In recent years, passive seismic datasets have rapidly increased in volume and resolution. This demands the development of automatic tools to fully analyse these datasets. TRI has the potential to completely, efficiently and accurately locate and characterise most seismic events in these large datasets. The increased data volume and larger computing capacities resulted in the generation of high-resolution three-dimensional velocity models and subsequently it became feasible to apply TRI as a routine tool in many applications. Here we report recent advances in investigating the potential of TRI to locate and characterise seismic events are reported. In particular, the influence of the station network configuration, the velocity model and the signal-to-noise ratio on the source-location accuracy is reviewed. The method's ability to locate and characterise seismic events in the context of geothermal reservoirs, which often experiences a large number of small magnitude seismic events, is shown. Recent advances such as the determination of the time-dependent moment tensor using TRI are also reported. The applicability of the method is demonstrated through examples from real case applications.

The Calama-Olacapato-El Toro (COT) fault zone transversally crosses the Andean chain and the Puna Plateau at about 24° S. We describe the field geological structural data collected along this fault zone where it crosses the Puna Plateau (Argentina). The investigated area goes from the Chile-Argentina border to a few km east of the San Antonio del Los Cobres village, along an ideal NW-SE strip that follows the surface trace of the COT fault zone. Available radiometric dating and local stratigraphic controls enabled us to date the main tectonic events. Fault kinematics has been reconstructed through the individuation of striated fault planes and the offset of surface deposits and landforms. A series of main faults have been detected in the area, aligned along the known surface trace of the COT lineament. Our observations suggest that it consists of a series of discrete NW-striking fault segments partially overlapping laterally. The strike range of the various fault segments is in the order of 5-20°. The surface trace is rectilinear suggesting a vertical dip at least in the uppermost crust. Kinematics is characterised by oblique movements with a dominant left-lateral strike-slip component. A vertical component is usually present in the form of relative uplift of the northeastern tectonic block. These faults offset Pliocene or Pleistocene volcanic deposits suggesting a Plio-Quaternary activity of the Puna Plateau segment of the COT fault zone. In some places along the Puna Plateau, open fissures have also been observed along the COT fault zone. These fissures are only partially filled and are not located along or close to slopes. Moreover they affect “hard” rock deposits. Thus, they cannot be interpreted as due to local gravity deformations or “soft” sediment compaction. Taking into account also the low erosion rate of the area, we believe that these fissures might be interpreted as an evidence of prehistoric/historic surface faulting. We carried out also paleoseismic investigations along a trench crossing the main fault trace, which suggest that here faulting occurred through multiple events with surface ruptures. Further westward in the Chilean region, field evidence of Plio-Quaternary motions disappears, and the COT lineament should be older in age there. We will also present other structural data depicting the kinematics and geometry of the secondary faults detected along the COT fault zone, as well as the paleostress tensors reconstructed by numerical inversion of the striated fault plane data