Tidal triggering evidence of intermediate depth earthquakes in the Vrancea zone (Romania)
Abstract. Tidal triggering evidence of intermediate earthquakes in the Vrancea region (Romania) is investigated. The Vrancea seismic zone is located in the bend region of the South-Eastern Carpathians (45°–46° N, 25.5°–27.5° E) and is known as one of the most active seismic zones in Europe. We selected earthquakes occurred between 1981 and 2005 from the RomPlus catalog provided by the Institute of Earth Physics of Bucharest with Mω≥2.5 and focal depths between 60 and 300 Km.
We assigned a tidal component phase angle for each event which is computed from the earthquake occurrence time. Then the phase angle distribution was obtained by stacking every angle value in a 360 degree coordinate. Main lunar and solar semidiurnal tidal components M2 and S2 are considered. The phase angle distributions are tested by Permutation test which are introduced for the first time to a tidal triggering study. We compared results with classical Schuster's test. Both tests produce one value, pp for Permutation test and ps for Schuster's test, which represent the significance level for rejecting the null hypothesis that earthquakes occur randomly irrespective of tidal activities M2 and S2 phase distribution are random for the complete data set. However, when we set up a one year window and slide it by 30 d step, significant correlations were found in some windows. As a result of the sliding window, data set systematic temporal patterns related to the decrease of the pp and ps values seem to precede the occurrence of larger earthquakes.