Observations of seismo-electromagnetic earthquake precursor radiation signatures along Southern Californian fault zones: Evidence of long-distance precursor ultra-low frequency signals observed before a moderate Southern California earthquake episode

Although questioned for a long time, there is accumulating growing evidence for the existence of detectable seismo-electromagnetic phenomena worldwide. California is geologically as well as seismically a unique region for studying these phenomena in depth; and, in particular, the southern California geologic province with a multitude of offshore and inland fault zones with San Diego in its center. At the Naval Ocean Systems Center Low Frequency Noise Laboratory, San Diego CA., we monitor 0.1–10 Hz ultra-low frequency (ULF) and 10–40 Hz extremely low-frequency (ELF) signals using mu-metal loaded multitum search coil sensors, as well as 10–100 kHz very low-frequency signals using large 1 m diameter loop antennas. We have observed precursor seismo-electromagnetic emissions of several earthquake events. In this paper, we report on observations of broadband ULF signals before and during the Upland quake of April 17, 1990 (Ms = 4.6), centered 200 km N 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. Although the exact mechanisms for coupling geologic activity to the ionosphere is not known, we cite a number of hypotheses concerning these mechanisms. Based on this succinct overview, an interpretation of our radio observations of seismic activity is presented and extended to earthquake precursor or predictor studies. We are in the process of expanding this research with the building of more monitoring stations and the improvement of our measurement, data collection, formatting, and data processing capabilities.