The earthquakes began in May 1965 and the biggest shocks had occurred later in October and November after the reservoir was full. The epicenters were located at two faults near the dam. As many as 25 shocks occurred a day. The shocks stopped once the reservoir was emptied and no damage was found. After refilling, the shocks decreased and an "equilibrium" was believed to have been reached, one that did not respond to variations in water load. Another large shock occurred several years after filling. By 1971, there were no more seismic shocks around the dam or reservoir. No known detailed geological studies were conducted prior to construction of the dam and several faults are known to exist in the area. The Contra Dam is a concrete slender arch dam with a height of and crest length of . The dam is wide at its base and wide at its crest. The dam structure contains of concrete and the structure itself has a surface area of . The dam is slender in design and its horizontal arches are in an elliptical shape. The horizontal curvature of the dam decreases from the center of the dam towards its abutments and the curvature from the center of the arches increases from the crest down to the foundation. The thickness is constant with the exception of where the dam is received by its abutments; here it is thicker to reduce pressure on the rock. The strong rock can accept stresses of 70 kg/cm2. Vertically, the thickness of the dam increases from the crest down to the foundation and has a slight curve from the center towards the crest. This helps alleviate tensile stress for when the reservoir is not only full but empty. The dam's outlet works consist of two discharge pipes, capable of releasing up to each. One discharges into the original diversion tunnel and another is located at the base of the dam and discharges into the river valley below. The dam creates Lago di Vogorno which has a capacity and surface area of , collected from a catchment area of . The Verzasca River has very irregular seasonal flows which can reach as high as . Lombardi considers it one of his most aesthetically pleasing dams, primarily because of the slenderness of the concrete arch and the "outstanding cleanness of the design." The design also reduced the amount of concrete needed, therefore reducing the cost as well. The dam has two spillways, each located on the structure, near its abutments on either side which have a maximum discharge capacity of . Each spillway contains six fixed-crest weirs that are wide.