Deformation behaviour of the low-rate active Balanegra Fault Zone from high-precision levelling (Betic Cordillera, SE Spain)

The main segment of the active Balanegra Fault Zone (SE Spain) is monitored through high-precision levelling profiles in order to determine its short-term activity. It belongs to the NW–SE oriented normal and normal–oblique widespread fault set of the Betic Cordillera related to the Eurasian–African plate boundary, and constitutes the possible source of four moderate-size earthquakes (Mw 4.5–6.7) over the last five centuries at a minimum ∼100-year recurrence interval. Successive measurements since 2006 along two levelling profiles determined varying temporal behaviour. A general increase in vertical throw occurred between May 2007 and September 2010, within a remarkable low seismic activity and deformation build-up period, followed by a subordinate regional relaxation stage probably associated to nearby low magnitude earthquakes. In the longest northern profile deformation of up to 1.88 mm and rates reaching 0.78 mm/year with a mean rate of 0.14 mm/year are observed. The shortest southern profile, which is close to the area where the main earthquakes of the 1993–1994 series nucleated, records similar interseismic deformation behaviour although with lower deformation than the northern profile. The Balanegra Fault Zone illustrates complex behaviour in the period comprised between major earthquakes, exemplifying the temporal variability of deformation.