The 1966 Kremasta reservoir earthquake sequence

The ten-month (February-November, 1966) earthquake sequence thought to be associated with the filling of the Kremasta reservoir in northern Greece can be divided into two distinct segments. The first, consisting of the main shock (m_b = 5.6, M_s = 5.8) and its aftershocks in the following nine months, indicate normal faulting primarily on a northwest-striking, southwest-dipping, fault plane to a depth of 35-45 km. The second, which began in late October with another large (m_b = 5.8, M_s = 5.3) event, consists of thrust faulting not on the previous fault plane. The Kremasta sequence occurs at the transition between extensional and compressive faulting in a distributed plate boundary region. The main shock occurred near the top of an aftershock zone much larger than the expected fault area, in accord with a model in which the main shock and many of its aftershocks were induced by pore water diffusion along a preexisting feature. Alternatively, the main shock may be background seismicity and the aftershocks are located near the lake due to pore pressure effects. The later thrust aftershocks are less likely to be directly reservoir related.