S U M M A R YUsing continuous seismic data from newly available broadband stations in Mexico and Central America we have obtained group and phase velocity maps of fundamental mode Rayleigh wave for the region. These new maps have been calculated for periods between 8 and 60 s from cross-correlations of seismic ambient noise between 100 broadband stations, and stacked for 30 months from 2006 to 2008. The tomographic inversion of the obtained dispersion measurements has been carried out on a 1 • × 1 • grid, resulting in maps with resolution better than 250 km in the well-sampled regions of the model. For short periods (8-16 s) dispersion maps show good correlation with surface structural features. Low-velocity anomalies correlate with sedimentary basins around the Gulf of Mexico and Colorado embayment. High-velocity anomalies at short periods correlate with mountain ranges and regions of thin, extended crust such as the Gulf of California. Both the lowest and highest group velocity anomalies at these periods reach values of up to 15 per cent. For periods between 25 and 40 s, velocity anomalies are related to variations in crustal thickness and temperature. The most prominent low velocity anomaly correlates with thick crust and high mantle temperatures associated with the TransMexican Volcanic Belt and the Central American volcanic arc. The most remarkable features for longer periods (40-60 s) are the differences between high-velocity anomalies beneath the Mayan block, the Gulf of Mexico and the cratonic part of the United States and low-velocity anomalies beneath the Chortis block and northern Mexico. The long wavelength features of our model agree well with previous global and continental scale studies. However, because of the increased station density of the data set used, we are able to obtain reliable dispersion maps for shorter periods, and to image smaller scale features.
In November 2014 a temporary land and marine seismic network was deployed to monitor the drilling of an exploratory well in the Canary Channel (Eastern Canary Islands). This region is characterized by low seismic activity; however, because of the increased awareness of the potential seismic hazard caused by hydrocarbon exploitation activities, the drilling operations were monitored with an unprecedented level of detail for an activity of this kind. According to the reported earthquakes, there was not a measurable increase in seismicity in the vicinity of the well. Overall seismic activity was low, which is consistent with the historical seismicity records. Harmonic tremor, explained here as resonances of the instrument-seafloor system generated by bottom water currents in the area, was commonly detected on the ocean bottom seismometer (OBS) recordings. The marine network data also revealed few dozens of non-seismic short-duration signals per day that appear similar to other events on OBS recorded throughout the world. We suggest that they may be caused by direct perturbations on the OBS, mostly induced by ocean currents in the Canary Channel.
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