TOMODEC. High resolution seismic tomography of Deception Island (Antarctica), and modelling of seismo-volcanic sources. SEISMIC MODELS

Brief description

In January 2005 a multi-parametric international experiment was conducted that encompassed both Deception Island and its surrounding waters. This experiment used as main platforms the Spanish Oceanographic vessel 'Hesperides', the Spanish Scientific Antarctic base 'Gabriel de Castilla' at Deception Island and four temporary camps deployed on the volcanic island. This experiment allowed us to record active seismic signals on a large network of seismic stations that were deployed both on land and on the seafloor. In addition other geophysical data were acquired, such as: bathymetric high precision multi-beam data, and gravimetric and magnetic profiles. The seismic and bathymetric data have been analyzed but the magnetic and gravimetric data have not. We provide P-wave arrival-time picks and the seismic tomography results in velocity and attenuation. P-waves first arrival. To determine the P-wave first arrivals we used the SAC routine called APK. With this algorithm, the detection of a pick is based upon abrupt changes in the ratio of a short term and long term running average of the signal. Once detected, the pick is subjected to an optional validation phase which attempts to distinguish a true event from cultural noise. Once validated, the pick is further evaluated to determine other characteristics of the event. To assess the performance of the APK routine, we compared the automatically obtained picks with the real traces. Usually we observed that the results for detected traces differed by an average of 0.02 s (standard deviation 0.26 s) with respect to the hand-picked arrivals. This can be considered an acceptable performance of the automatic routine. However, to be sure we do not use any 'falsely triggered' arrivals, and to take in account as many phase arrivals as possible, we visually checked the automatic algorithm result. For this manual task, we used the UPICKER software. This program permitted trace gather plotting, starting 2 s before shot time and with duration of 20 s long. This choice depended on the known interval between shots, which was 2 minutes. We checked the error introduced by filtering the traces in SAC and we found out that it is usually below 1 sample. The final database includes delay times from 70,411 crustal P-waves first arrivals automatically picked and manually checked. On average each station, that we used, recorded almost 1500 shots. Seismic velocity tomography. The seismic tomography in velocity was the first and significant advance of TOMO-DEC experiment and was performed using a well-known and tested inversion method to invert the P-wave travel times. This method applies LSQR inversion algorithm and uses the shortest ray tracing time. To obtain the three-dimensional tomography of Deception Island, we considered two-step approach and we built two grid configurations. Firstly, we applied the method to the study of a larger region, encompassing Deception Island and surroundings. In this case the volume, geometrically represented in a x,y,z Cartesian system, is 53 x 52 x 12 km wide and centered in the middle of Port Foster (latitude -62 degrees 58' and longitude -60 degrees 40'). It is parameterized by a 0.25 km grid-node spacing for the ray tracing and 0.5 km grid-node spacing for the perturbational grid. In a second case, we focused on Port Foster and we reduced the studied area to a sub-region 12 x 14 x 7 km, centered in the middle of the bay (latitude -62 degrees 57.2' and longitude -60 degrees 37.2'). We increased the density of nodes in the parameterization grids, by using a 0.1 km grid for the ray path tracing and 0.2 km for the velocity perturbation. 2-D scattering and intrinsic attenuation model. 2D regional maps of inverse intrinsic (Qi−1), scattering (Qs−1) and total (Qt−1) quality factors for the volcanic environment of Deception Island were obtained on the base of diffusion approximation. These maps were obtained using the above described active seismic data base. Attenuation parameters were estimated by fitting observed energy envelopes to the diffusion model and then, the spatial distribution of the attenuation parameters was obtained by averaging all the single source-receiver couples in 1x1 km cells and using a back-projection method based on a Gaussian-type weighting function 2D intrinsic- and scattering-Q images were produced. The depth at which the information is obtained can be approximately estimated assuming depth equal to the minor axis; in this case, 6 km on average. 3-D coda-waves normalization attenuation model. Another derived result is the high-resolution 3D P-wave attenuation tomography model obtained by using the coda normalization method where energy ratios of 20,293 high-quality waveforms were analyzed in a single-step inversion. We applied the Thurber-modified ray-bending approach in the 3D velocity model described above. Due to observational data associated with incoherent estimates of the ray paths, the final model is restricted to depths of 1-4 km. The inverted area is a region of 20 x 20 km in surface centered in Deception Island. The seismic tomography was obtained using cells of 2 x 2 km in horizontal and 1 km in depth with a maximum depth of 4km b.s.l. In case of the derived data (MODELS), the seismic tomography (in velocity) results are located in the folder 'V models' with two models, 'Dense grid' and 'Small grid'. The associated P-waves onset database is placed in the folder 'P travel time'. For seismic attenuation results the folder is named 'Q models' with two models, the bi-dimensional model separating intrinsic and scattering Q, named 'Qi-Qs' and the 3D total Q model named 'TotalQ-3D'.