[1] Clusters of recurring, nearly identical microearthquakes have been recorded near Loma Prieta, California. Such events make possible direct measurement of temporal changes in the attenuation operator t* along fixed paths under what are tantamount to laboratory conditions. Analysis of almost 10 years of P wave data following the 18 October 1989 Loma Prieta, California, main shock (M=7.0) reveals a large-amplitude t* increase within the first 3 weeks of the main shock. A gradual t* recovery toward its level prior to the main shock is seen in the years ensuing. The results point to changes in porosity and fluid saturation taking place in parts of the main shock source volume, stemming from coseismic crack opening and postseismic relaxation processes such as postseismic fluid flow into the cracks, crack healing, and rock compaction.