Searching for Scaling With Magnitude of Signals in the Early Portion of P Waveforms Recorded in South African Mines

The process of earthquake rupture may follow a cascade process where the eventual event size is determined by the (evolving, generally heterogeneous) stress-strength conditions on the fault. Some studies, however, reported scaling of signals in the early P waveforms with the final event magnitude, which may be related to the nucleation process. In this study we use a large seismic data set of near-source stations to investigate if any such scaling can be observed with a variety of proposed techniques. Umeda (1990) and Ellsworth and Beroza (1995) suggested that P waveforms are characterized by an initial phase of low amplitude followed by strong motion of the main event, and that the time between these two scales with the final magnitude. Iio (1995) measured the time difference between the very first onset and the projected onset if the P wave was a ramp function, and suggested that this scales with the final event size. A second set of methods, following Nakamura (1988), Allen and Kanamori (2003) and Kanamori (2004), look at how the frequency content of the waveform in the first few seconds changes with final event magnitude. Using high sampling rate data recorded within South African mines, we search systematically for the Ellsworth/Beroza-type nucleation, Iio-type nucleation, and Nakamura-type period in the early P waveforms. With hypocentral distances as low as ~400m and low seismic attenuation, any scaling with magnitude or nucleation phases that might exist have a good chance of being recoded at the stations. From an initial study of 64 events, ranging between approximately magnitude -1.5 and 2.5, at 26 stations, we find that candidates for Ellsworth/Beroza-type nucleation phases exists for only ~30% of the waveforms. This phase and the Iio-type phase do not appear to scale with the final event size in the examined data. Measurements of the Nakamura-type period in the first few seconds show overall scaling with the final event magnitude, albeit with a large scatter.