Microtremor analyses at Teide volcano (Canary Islands, Spain):: Assessment of natural frequencies of vibration using time-dependent horizontal-to-vertical spectral ratios

We use time-dependent horizontal-to-vertical spectral ratios (HVSR) of microtremors to determine the dominant frequencies of vibration of the geological structures beneath several recording sites in the vicinity of Teide volcano (Canary Islands, Spain). In the microtremors, the time-dependent HVSRs (ratiograms) are a useful tool to discriminate between the presence of real dominant frequencies linked to resonances of the subsurface structure and the spurious appearance of peaks due to local transients. We verified that the results are repeatable, in the sense that microtremors recorded at the same site but at different times yield a very similar HVSR function. Two types of results are found: (1) sites where there is no resonance of the propagating microtremors, and therefore no value of a dominant frequency can be assessed; and (2) sites where a stationary peak in the HVSR is found and a dominant frequency related to resonance of the shallow structure can be estimated. These resonant frequencies show substantial spatial variations even for nearby sites, which reflects the complexity of the shallow velocity structure in the Las Canadas area. Large dominant frequencies occur near the caldera walls and also at a few locations that coincide with the intersections of the inferred rims of the three calderas forming Las Canadas. Small dominant frequencies also occur near the caldera rim, and may be due to discontinuities in the caldera wall and/or to local velocity anomalies. Intermediate frequencies are mostly found in the eastern part of the caldera, where a tentative profile of the basement depth has been obtained. Intermediate frequencies have also been measured south of Ucanca and south of Montana Blanca. In view of the present results, we conclude that the use of ratiograms constitutes an improvement of the HVSR method and provides an appropriate tool to investigate the shallow velocity structure of a volcanic region.