Kinematic multistation discriminator between local quarry blasts and earthquakes

It has long been known that S waves on seismograms of local explosions are often accompanied by strong low-frequency, low-velocity, Rg surface wave trains, often significantly diminished for earthquakes. We utilize this fact to construct a new formal discriminator between earthquakes and explosions by measuring the S-surface-wave group velocity. The method is based on analysing the velogram; that is, the display of the envelope of ground motion versus group velocity V = R/T, where R is the epicentral distance and T the traveltime. We examine the distribution of seismic energy in time and space using envelopes of records from the Israel Seismic Network (ISN), from which we compute the velograms and observe differences in the velograms of quarry blasts and earthquakes. The data include 143 seismic events occurring in three areas (Galilee, Dead Sea, and Gilad) monitored by the ISN; the magnitude range is M(L) = 1.0-2.8 at distances of 15-310 km. From the velograms we measure the group velocity, V(ms), within the 1-4 km s^-1 range at which the velogram reaches its maximum for each available station. The resulting V(ms)(R) function is closely fitted by the empirical relationship a + b ln R, with a and b coefficients varying from event to event. A simple linear function c = b + 0.33a at a threshold C = 0.69 completely separates (a,b) pairs for the 67 Galilee events, and, for the 76 remaining events, one earthquake and four explosions are wrongly classified. After data validation and application of the Fisher linear discriminator, adapted to the events from Galilee, only two misidentified events remain for the whole data set.