The spatial and temporal changes inV p /V s before strong earthquakes in Kamchatka

A method was developed to obtain from a signal station the spatial and temporal distribution ofV_p/V_s ratios before earthquakes of magnitude>6. It was shown thatV_p/V_s values strongly depend upon the relative positions of the stations, the future large earthquake and the foci of the smaller earthquakes used forV_p/V_s determination. The appearance of a zone of anomalousV_p/V_s values with linear dimensions of the order of 100 km was noted at least 4 years before a deep earthquake of magnitude 7. Similar size anomalous zones were detected one year before some magnitude 6 earthquakes.V_p/V_s values decreased by a small but distinct amount during this time. Additionally, local short term minima inV_p/V_s ratios were observed some months before the major event. The epicenters of the large earthquakes were located within the 100 km size zone where the gradients of theV_p/V_s field were largest.