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 anomalous values of heat flow data have also been collected in the Timanfaya National 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 effect using global and local ocean charts, the cosine component of the final residual vector for station Cueva de los Verdes (northeastern of the island) depends on the ocean model considered. On the other side, the final results change greatly if we take into account the porosity effects of the lava rock at this station. Then, the cosine component of the final residual is closer to, or greater than, zero. In Timanfaya station (located in the Timanfaya 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 an ocean loading effect. Furthermore, the negative sign of the residuals looks consistent with the response of a porous or cavity-filled, local, upper crust under the influence of tidal strain. Therefore, we found a similar explanation for the existence of negative values of the final residuals at both stations. Our study seems to suggests the lack of correlation between heat flow and tidal gravity anomalies. Nevertheless, we must point out again the existing necessity of theoretical studies and highest quality observations to answer the different questions regarding the significance of the tidal 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.