2-D Travel Times Analysis of Llaima Volcano, Chile

Arrival-time perturbations of regional and teleseismic events recorded at Llaima Volcano, Chile, is presented here to delineate local lateral variations in geologic and volcano structure. Data for this study was collected on 26 temporary and 5 permanent broadband seismic stations from January-April 2015. While Llaima has been historically a highly active volcano, no local earthquakes were detected during the 4 month deployment. As a result, traditional, local, three-dimensional, body-wave tomography is not an option for analysis. Alternatively, we contoured the relative phase arrivals of regional and teleseismic events recorded on the local 31 station array. If there are no local anomalies below the array, contour maps of relative arrival times should appear as a plane wave. In the case of Llaima, however, the contours consistently showed an arrival time delay anomaly (i.e. low velocity) situated near the volcano edifice. The best fit plane (i.e. plane wave) was then estimated and removed, correcting for the ray incidence angle and azimuth. The final relative travel-times were averaged across all events for each station and interpolated to extract lateral arrival-time variations. We view this as a simple approach to model velocity anomalies in the absence of extensive hypocenter data. Travel-time residual analysis supports a low velocity anomaly at least 10 km in diameter beneath Llaima Volcano. This anomaly likely correlates with a magma body at an unknown depth. There are also slow anomalies at stations LAV and PAX, east and south of the volcano, respectively, near lineaments of the Liquine-Ofqui Fault Zone (LOFZ). The slow anomalies are consistent with initial ambient noise tomography. Our results suggest that a simple travel-time analysis of regional and teleseismic events can be used to extract local lateral variations in structure.