Induced seismicity around the Tous New Dam (Spain)

SUMMARY Induced seismicity around the Tous New Dam, located 45 km southwest of Valencia, Spain, is studied from a detailed analysis of seismic data from a 16-month period, starting in 1999 January, recorded by the Tous New Dam Seismic Network (TDSN), which comprises a threecomponent station located in the dam and six vertical-component outstations within 25 km of the dam. On the basis of the spatial 2D‐3D and temporal distribution of this seismicity, we have found 24 microearthquakes, of magnitudes 0.4‐3.8, most of which appear correlated in time and space with changes in the water level in the Tous New Dam reservoir. An initial group of earthquakes (all within 12 km or less of the reservoir) occurred over roughly 1 yr, starting in mid-1999 while the water level was at its maximum, continuing during draining in late 1999 and subsequent refilling in early 2000. Thereafter, seismicity was more widespread, reaching up to 40 km from the dam and with the greatest frequency of occurrence coinciding with the rapid draining of the reservoir in mid-2000. The few focal mechanisms able to be found are normal or strike-slip. In general, it is clear that a relationship exists between reservoir water level, which probably modifies the pre-existing tectonic stress field and pore fluid pressures, but no specific mechanism for the induced seismicity has yet been identified.