Interpretation of tidal gravity anomalies in Lanzarote, Canary Islands

Since 1987, gravity tide observations have been conducted on the island of Lanzarote, with different gravimeters at different locations. Very anomalou s values of heat flow data have also been collected in the Timanfaya Nation al Park (southwestern part of the island). Seismic experiments provide us a crustal model of the island, with an average depth of 11.5 km. The gravity tide observations show that, after correction of the indirect oceanic effe ct using global and local ocean charts, the cosine component of the final r esidual vector for station Cueva de los Verdes (northeastern of the island) depends on the ocean model considered. On the other side, the final result s change greatly if we take into account the porosity effects of the lava r ock at this station. Then, the cosine component of the final residual is cl oser to, or greater than, zero. In Timanfaya station (located in the Timanf aya National Park), we observe that both M-2 and O-1 residuals are equally large. Their phases are more consistent with a body tide effect than with a n ocean loading effect. Furthermore, the negative sign of the residuals loo ks consistent with the response of a porous or cavity-filled, local, upper crust under the influence of tidal strain. Therefore, we found a similar ex planation for the existence of negative values of the final residuals at bo th stations. Our study seems to suggests the lack of correlation between he at flow and tidal gravity anomalies. Nevertheless, we must point out again the existing necessity of theoretical studies and highest quality observati ons to answer the different questions regarding the significance of the tid al gravity anomaly-heat flow trends. Also, the effect of tidal strain in a porous or cavity-filled, local, upper crust should be considered. (C), 2001 Elsevier Science Ltd. rill rights reserved.