Nucleation of earthquakes and its implication to precursors

The recent argument about nucleation phase of earthquakes reminds us to completely study the concept of earthquake nucleation. The original meaning of nucleation includes concentration, nucleation and initiation of the eruptive processes. Thus, it is needed to discuss how to exactly translate the word 'nucleation' into Chinese in different fields. The basic concept of earthquake nucleation refers to microcrack concentration in rock. It causes local weakening and instability of the rock. The narrow sense of nucleation theory of friction constitutive is significant in friction of fault surfaces, but should not abuse everywhere unconditionally. In terms of thermodynamics, nucleation actually means the variety processes of multiple state parameters of rock. The nucleation is a project that covers multiple courses. In this paper, the studies of damage theory, fracture, earthquake rupture dynamics and constitutive of friction and their implication to earthquake nucleation are remarked. The recently developments are introduced, including the influence of tectonic on the earthquake nucleation process, the method of measuring medium anisotropy, especially shear wave splitting led by concentration and orientation of microcracks. and the experimental study of remote sensing of infrared and microwave radiation related to the nucleation, etc. This paper also discusses the characteristic of large earthquake nucleation, and the implication of above studies to precursors of Strong earthquakes.

Using the match-and-locate method to characterize foreshocks of the July 2019 M_(W)6.4 Ridgecrest,California earthquake

The July 2019 M_(W)6.4 Ridgecrest,California earthquake and its distinct foreshocks were well recorded by local and regional stations,providing a great opportunity to characterize its foreshocks and investigate the nucleation mechanisms of the mainshock.In this study,we utilized the match-and-locate(M&L)method to build a high-precision foreshock catalog for this M_(W)6.4 earthquake.Compared with the sequential location methods(matched-filter+cross-correlation-based hypoDD),our new catalog contains more events with higher location accuracy.The M_(W)6.4 mainshock was preceded by 40 foreshocks within~2 h(on July 4,2019 from 15:35:29 to 17:32:52,UTC).Their spatiotemporal distribution revealed a complex seismogenic structure consisting of multiple fault strands,which were connected as a throughgoing fault by later foreshocks and eventually accommodated the 2019 M_(W)6.4 mainshock.To better understand the nucleation mechanism,we determined the rupture dimension of the largest M_(L)4.0 foreshock by calculating its initial rupture and centroid points using the M&L method.By estimating Coulomb stress change we suggested that the majority of foreshocks following the M_(L)4.0 event and M_(W)6.4 mainshock occurred within regions of increasing Coulomb stress,indicating that they were triggered by stress transfer.The nucleation process before the M_(L)4.0 event remains unclear due to the insufficient sampling rate of waveforms and small magnitude of events.Thus,our study demonstrates that the M&L method has superior detection and location ability,showing potential for studies that require high-precision location(e.g.,earthquake nucleation).

The seismogenic structures and migration characteristics of the 2021 Yangbi M6.4 Earthquake sequence in Yunnan,China

We constructed a more complete earthquake catalog in the 2021 Yangbi M6.4 focal area by re-scanning the continuous waveforms integrated with deep learning and template matching techniques,to explore the seismogenic structures of the Yangbi mainshock and its nucleation process.The new catalog has three times the number of earthquakes than the CENC catalog,and the magnitude completeness has dropped from 1.1 to 0.5.The distribution of earthquakes indicates a broom-shaped structure consisting of several oblique secondary faults and a strike-slip main fault which strikes to 315°with 80°dipping to NE.The earthquakes extend along the fault strike about 27 km in width and 2-13 km at depth and have noticeable variations on seismicity in the mainshock’s north and south.Compared with the north,the south has denser and higher magnitude aftershocks and also has a seismic gap probably weakened by the fluid at the depth range of about 5-6 km.The foreshocks were mainly active in the 8-kilometer-long fault zone south of the mainshock,which show a steady drop in b-values over time and a migration pattern toward the epicenter of two steep jumps,stagnation,and then acceleration which finally triggered the mainshock.While in the north,seldom foreshock occurred,and the aftershocks were delayed triggered 3 hours after the mainshock,and sparsely scattered shallow at depth and small in magnitude.To summarize,the northern part of the Yangbi seismogenic fault is thought to be relatively locked,whereas the southern part has a weakening zone and promotes pre-slip.The nucleation mechanism of the mainshock and its onset at the junction of the locked and pre-slip zones may be a combination of pre-slip and cascade triggering.