RECENT VERTICAL MOVEMENTS FROM HIGH-PRECISION LEVELING DATA IN NORTHEAST SPAIN

Historical and modem high-precision leveling data are analyzed to meas ure recent vertical movements in northeast Spain. The recent vertical movements are deduced by comparing the original height differences (ta ken from the original logbooks) measured in the field in two surveys c arried out by the Institute Geografico Nacional in the periods 1871-19 22 and 1925-1974. An exhaustive study of the errors involved in the me asurements indicates that some of the observed movements are significa nt. We present the recent vertical movements along eight profiles with significant uplift rates: a southern anomaly of about 1 mm/yr, observ ed in two independent profiles, associated with the southern part of t he Catalan Ranges frontal thrust; one anomaly between Caldetes and Are nys 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 m m/yr in the noah 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 abo ut 0.8 mm/yr associated with the Emporda basin dynamics. Other observe d 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 tecton ic 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 estimate d here and with the fifteenth century seismic crisis, Therefore, if th ese high uplift rates are tectonic in origin, they may represent an ac cumulation of elastic strain which would be released in a future large earthquake rather than in continual small ones. According to this hyp othesis, a large part of the stress accumulated since the fifteenth ce ntury has yet to be released. Further leveling and neotectonic surveys are needed to test this hypothesis.