关键词: 亚失稳准动态阶段, 变形场, 时空演化, 应变准备期, 同震阶段

Abstract: A crucial question in earthquake science is how earthquakes start. Field and experimental observations show a short period exists between the fault reaching peak stress and the coseismic event. Therefore, it is of fundamental significance to capture the spatio-temporal evolution of a fault’s deformation during this premonitory stage. It can help us understand how the rupture of an earthquake initiates and also provide precursory information. Stick-slip events or lab quakes can be produced in controlled conditions to mimic earthquakes in nature. In previous studies, we proposed the fault meta-instability model focusing on depicting this stage(hereinafter referred to as the meta-instability stage)and interpreting the transition from energy/stress accumulation to energy/stress release. We further divided the meta-instability stage into two substages, i.e., the quasi-static phase and a quasi-dynamic phase, corresponding to slow energy release and irreversible energy release elevated rate.
However, how the meta-instability stage can facilitate the final failure remains puzzled. In contrast, the meta-instability stage exhibits slow and mild deformation, while the coseismic stage is fast and violent. In order to bridge these two processes, it is essential to record the complete dynamic process of stick-slip events, including the premonitory and coseismic stage. Thus, the data acquisition system required must feature a high signal-noise ratio, high frequency, continuous recording, and dense instrumentation. In 2016, we developed an ultra-high-speed, multi-channel and continuous recording data acquisition system for deformation measurement(UltraHiDAM). UltraHiDAM has 64 channels, 16-bit resolution, and 4MHz sampling frequency, and can perform parallel continuous data acquisition. It is able to record strain signals and acoustic emissions continuously and synchronously at a high sampling frequency up to 4MHz for as long as a few hours. To our best knowledge, it is the first system that is capable of doing so.
Based on this system, we conducted a series of stick-slip experiments. We recorded the entire deformation process of the laboratory earthquake cycles, including the relatively slow deformation in the quasi-static phase(several seconds before the stress drop), the relative fast deformation in the quasi-dynamic phase(a few microns before the stress drop), and the complete process of the transient coseismic slip. High frequency continuous synchronous sampling allows us to reveal as many details as possible of unstable sliding transient processes, and analyze mechanical problems related to the seismic source.
We report results of stick-slip experiments using saw-cut bare-surface granodiorite samples. The main findings of this paper are summarized as follows: First, the substages can be further recognized based on the local deformation characteristics(Table 2). Second, strain and stress start to localize before the quasi-static phase; such localization’s acceleration indicates the whole fault has entered the quasi-static phase. Third, the strain field during the quasi-dynamic phase is characterized by a wave-like acceleration and reciprocating propagation(Fig. 9). Fourth, there is a short preparation period for each sub-stage of the quasi-dynamic process(Fig. 6). The existence of such preparation periods may help the imminent earthquake prediction. Finally, even for the stick-slip events captured on a simplified plane laboratory fault, the coseismic process can be multiple rupture events, each event has its own AE waveform that is distinguishable in time(Fig. 8).
The implications are that there is indeed precursory information during the different substages before the coseismic event, most of which are associated with the localization and propagation of strain and stress. An earthquake source’s actual mechanical process can be complex in terms of multiple stress drops and ruptures.

Key words: the quasi-dynamic stage of meta-instability, deformation field, spatio and temporal evolution, strain preparation period, coseismic stage