OBSERVATION OF THE SEISMIC NUCLEATION PHASE IN THE RIDGECREST, CALIFORNIA, EARTHQUAKE SEQUENCE

Near-source observations of five M 3.8-5.2 earthquakes near Ridgecrest , California are consistent with the presence of a seismic nucleation phase. These earthquakes start abruptly, but then slow or stop before rapidly growing again toward their maximum rate of moment release. Dec onvolution of instrument and path effects by empirical Green's functio ns demonstrates that the initial complexity at the start of the earthq uake is a source effect. The rapid growth of the P-wave arrival at the start of the seismic nucleation phase supports the conclusion of Mori and Kanamori [1996] that these earthquakes begin without a magnitude- scaled slow initial phase of the type observed by Iio [1992, 1995].