Rupture Velocity Estimation and Source Parameter Analysis of Micro-Seismic Events at The Geysers, California, USA, Applying A Time-Domain Technique

The study of earthquake rupture complexity plays a crucial role in assessing and mitigating seismic hazards.
Among the earthquake source parameters, the rupture velocity is generally unknown and assumed as a fixed percentage (60%-90%) of the shear wave velocity, although it controls the rupture duration, directivity, amplitude, and frequency content of the radiated wavefield. The rupture velocity is indeed the key parameter that determines the earthquake rupture length and indirectly the stress release, through the seismic moment. The observed large variability of stress drop estimates (over three orders of magnitude) can be partly related to the uncertain estimate of the rupture velocity and its possible heterogeneity or scaling with magnitude.

In this study, we tackle the specific problem of estimating earthquake rupture velocity, together with other characteristic source parameters, using a time-domain technique, applied to a 24-event dataset of micro-seismic events occurring in The Geysers area in California, USA. We propose a methodology that combines P and S half-pulse durations to infer independent estimates of the rupture radius and speed of microearthquakes assuming a circular fracture surface. For this aim a previous technique, that uses the P- and S-wave log-displacement curves as a function of the time along the seismogram has been used to measure attenuation-corrected, average P and S half-durations and plateau levels from a set of recorded earthquake waveforms. Refined estimations of seismic moment, rupture radius, and velocity allowed to determine accurate estimations of the stress release. Results show that rupture velocity is not constant but increases with magnitude in the explored range, reaching in 16 cases supershear speed values. Noteworthy, a self-similar, constant stress drop scaling is obtained only if a variable rupture velocity with magnitude is used for rupture radius and stress release determinations. The possibility of extremely fast ruptures (super-shear) occurring in the considered geothermal area can be attributed to the effect of lubrication and/or pore pressure increase due to massive volumes fluids injected under high pressure in the subsoil, to exploit the energy resources of the geothermal reservoir.

This work paves the way for a deeper comprehension of the physical and geological conditions determining the nucleation, propagation, and arrest of the fracture in crustal rock volumes with different faulting mechanisms, and offers a new and interesting approach for a more complete and accurate earthquake source parameters estimation through a time domain technique.