TEMPORAL VARIATIONS OF THE CODA DECAY-RATE ON KAMCHATKA - ARE THEY REAL AND PRECURSORY

Temporal variations of the decay rate of the coda of local earthquakes are studied for four Kamchatka stations. To suppress any bias caused by the coda Q versus lapse time dependence, I use the deviation of an individual coda decay function from the empirical reference decay func tion, and the analysis is done over a fixed lapse time window. For eac h individual coda record, a decay rate parameter ''alpha'' is determin ed by the following procedure. First, the reference log coda decay fun ction is subtracted from an individual one giving a residual function; second, the slope of this residual versus lapse time is determined ov er the fixed lapse time window giving an alpha estimate. The data used are records of 150-250 Kamchatka earthquakes of 1967-1990 (M-L=3.5-5) obtained by four stations employing three-component 1-s seismographs. The processed data show apparent temporal variation of alpha with hig h statistical significance. Several possibilities are then investigate d of mimicking the genuine temporal variation of alpha by systematic v ariation of other parameters, first of all epicenter wandering and nod al plane rotation. This analysis does not reveal any strong bias and t hus suggests that the observed variation is genuine. Then I show the s ignificance of five apparently precursory anomalies identified retroac tively. Also, I describe briefly the real-time prediction experiment c onducted on Kamchatka for 1982-1990 using coda decay, aimed at the int ermediate-term forecasting of large Benioff zone earthquakes. This exp eriment resulted in the successful forecast of the August 17, 1983, M- W=7.0 event with an accuracy of 2 months in time and 100-200 km in loc ation; magnitude was overestimated by 0.5. Later, a false alarm was al so issued. Thus the experiment confirms the reality of the coda decay rate precursor but also shows that it needs further improvement to bec ome reliable. Physically, the variations of the coda decay rate are as sociated with time-dependent local variations of scatterer density in the lithosphere.