Explosive/Implosive Events Observed in Fault Zone Borehole Seismometers

Geologically, various forms of stress-dependent grainscale, microcrack, and fracture dilatancy had been well observed, and have been proposed to operate within the crust adjacent to seismogenic faults or within fault zones. Fluid driven crack had been shown possible theoretically. However, microeartqhuakes caused by hydraulic fracturing had dimensions of at most a few tens of borehole radii and therefore, unlikely to volcanic earthquakes, these open cracks events are hardly detected. A state-of-the-art 7 level seismometer, named TCDP BHS, was installed from the depth of about 950m to 1300m crossing the large slip fault zone associated with the 1999 Chi-Chi earthquake. The seismometers were installed over the depth range of hanging wall and footwall with the depth interval of about 50-60m. In addition to the observation of micro earthquakes, we observed the events showing the distinct P-wave without S-wave. These distinct P-wave only events had been observed continuously through time. For the five months of observational period, more than 30 events were detected. These events can be classified into different group according to their similarities in P-wave. Group A shows the distinct upward motion, while the Group B shows the distinct downward motion. The events in the same group are almost identical in P-waves, but with slightly difference in pulse width. It suggests the events in the same group have similar mechanisms, but with different source dimension and stress drop. The characteristics of the events from waveform observations suggest these events are repeatable from different locations. The analysis for seismic radiation pattern for double-couple mechanism from the incident angles of the observed events show distinct amplitude ratio of P/SV and P/SH. While a moment tensor modeling for an explosion/implosive source (isotropic components only) gives the synthetics with distinct P-wave without S-wave and with satisfactory explanation to the observed waveforms. It suggests that these events might be resulted from a explosive/implosive mechanism within the fault zone. Whether or not this associated mechanism plays a role to present the status of the stress transition after a large earthquake is still under exploring. If these distinct events are involved with the fluid associated mechanism, the observation of these events will provide the hint to the involvement of fluid in earthquake nucleation.