The earthquake swarm of December 2007 in the Mila region of northeastern Algeria

Abstract

In December 2007, the Mila region of northeastern Algeria experienced thousands of microearthquakes (0.8 ≤ Md ≤ 3.9) recorded by eight temporary stations, in addition to permanent stations. Most of the events were too small to be located precisely, but a set of 122 precisely located events shows an alignment of epicenters, extending mainly in a horizontal band at about 1–2 km depth in a NNW–SSE direction and concentrated in a small area, 3 km southeast of Jebel Akhal, a small rocky hill between the Beni Haroun dam/reservoir and the Oued Athmania reservoir. The reservoirs are connected by pipelines, and a pumping station ensures water can be transferred between them at transient pressures of up to 80 bars. During the pumping in 2007, only 45 % of the transferred water (~600,000 m3 per day) was recovered at the Oued Athmania reservoir, and a large amount of the slightly pressurized water leaked through defective joints in a tunnel that passes through the Jebel. This water penetrated deeply into the soil with the assistance of preexisting fractures, faults, and karsts. Nine days after the first pumping started, a local increase in pore fluid pressures at shallow depths triggered seismicity southeast of Jebel Akhal, where the faults were probably close to failure. The focal mechanisms show a near vertical N–S strike-slip fault plane under regional NW–SE tectonic compression. One of the fault plane solutions is consistent with the NNW–SSE direction along which the seismic events are aligned. Furthermore, a long-term comparison of the seismic activity in the region versus water levels behind the dam and the pumping of water shows that the earthquake swarm was a one-off event related to the pumping operation.