Earthquake nucleation phases do not determine the event final magnitudes

Abstract:

Understanding earthquake nucleation processes is essential for unraveling rupture physics and can render physical insight into earthquake prediction and early warning operations. One of the key observations is the seismically detectable nucleation phase. The nucleation phases have been proposed for utilization to issue warnings for earthquake hazards, e.g., early determinations of earthquake final magnitudes. Robust observations of nucleation phases over a large range of earthquake magnitudes are of paramount importance to shed light on such problems. However, the topic remains controversial as conclusive observations are yet to be identified. In this study, we observe nucleation phases of ~100 earthquakes of the 2019 Ridgecrest earthquake sequence across the fault system using data from both regional networks and campaign nodal arrays. These earthquakes span a magnitude range from 0.9 to 5.4 with their nucleation phases preceding the P-waves for about 0.5 to 20 seconds. These nucleation phases for each earthquake are almost identical to the P-waves with nearly uniform preceding times at multiple seismographs except their amplitudes are 2 to 100 times smaller on average. In addition, the nucleation phases are present for earthquakes at all depth. We find a diverse pattern of the preceding time and the amplitude ratios between the nucleation phase and the earthquake P-wave. There is a lack of spatial clustering for the two observables and we find no clear scaling relationships between the earthquake magnitude and the amplitude ratio or the preceding time. The ubiquitous nucleation phases suggest that small and large earthquakes in the region are likely modulated by similar nucleation processes that emit precursors immediately before the earthquakes. However, the absence of scaling relationships indicates that earthquake rupture develops stochastically and the scattered observations hamper the predictability of event final magnitudes solely basing on their nucleation phases.