Sensing of seismo-electromagnetic earthquake precursor radiation signatures along Southern California fault zones: Evidence of long distance precursor ULF signals observed before a moderate Southern California earthquake episode

Observations of broadband ULF signals before and after an earthquake are presented. The particular quake in question was the Upland quake of 17 Apr. 1990, which was centered 200 km north of San Diego. The signals were detected with the vertically oriented search coil sensor and not with the horizontally oriented sensors, which suggests a disturbed ionosphere as the most likely source of these signals. The large prequake ULF activity, the rapid decay of ULF activity after the quake, and the absence of any geomagnetic storms indicate a good correlation of the ULF activity with the Upland quake. An interpretation of radio observations of seismic activity is presented and extended to earthquake precursor studies.