Recent vertical movements from high-precision leveling data in northeast Spain

Abstract

Historical and modern high-precision leveling data are analyzed to measure recent vertical movements in northeast Spain. The recent vertical movements are deduced by comparing the original height differences (taken from the original logbooks) measured in the field in two surveys carried out by the Instituto Geográfico Nacional in the periods 1871–1922 and 1925–1974. An exhaustive study of the errors involved in the measurements indicates that some of the observed movements are significant. We present the recent vertical movements along eight profiles with significant uplift rates: a southern anomaly of about 1 mm/yr, observed in two independent profiles, associated with the southern part of the Catalan Ranges frontal thrust; one anomaly between Caldetes and Arenys with an average velocity of about 4 mm/yr, which can be related to the seismic activity of this century; two anomalies of about of 2.5 mm/yr in the north and of about 4 mm/yr in the south associated with the NW-SE striking Amer-Brugent fault system, which has been related to a great seismic crisis in the fifteenth century, and one anomaly of about 0.8 mm/yr associated with the Empordà basin dynamics. Other observed recent vertical movements are mainly related to sediment compaction masking possible tectonic deformations. All the northern anomalies are associated with the NW-SE system of faults considered as a “transfer zone” of the NE-SW striking faults which controlled the formation of the Neogene European rifting. The former shows Plio-Quaternary tectonic activity: folds, faults, volcanism, hydrothermalism and historical seismicity. The low seismicity of this century around the Amer-Brugent fault system contrasts with both the high average velocities estimated here and with the fifteenth century seismic crisis. Therefore, if these high uplift rates are tectonic in origin, they may represent an accumulation of elastic strain which would be released in a future large earthquake rather than in continual small ones. According to this hypothesis, a large part of the stress accumulated since the fifteenth century has yet to be released. Further leveling and neotectonic surveys are needed to test this hypothesis.