亚失稳准动态及同震过程变形场时空演化特征——实验与分析

THE SPATIO-TEMPORAL EVOLUTION OF THE FAULT DEFORMATION DURING THE META-INSTABILITY QUASI-DYNAMIC PHASE AND THE COSEISMIC STAGE:A VIEW FROM LABORATORY

断层亚失稳模型指出,在临震亚失稳阶段中各种物理量存在规律性的时空演化特征,控制这些物理参数变化的根本原因是震源的力学过程。为深入观测和分析该过程,文中介绍了一套自主研发的64通道、16位分辨率、4MHz采样频率、可并行连续采集的超动态变形场观测系统(UltraHi DAM),首次实现了在4MHz频率下对应变信号和声发射信号的同步采集。依托该系统对断层失稳变形的全过程,特别是失稳前几s到若干μs的瞬态变形过程,即亚失稳准动态阶段进行了精细、深入的观测,解析了相关的震源力学问题,获得以下认识:1)伴随断层局部卸载而出现的应变局部化加速是进入亚失稳准静态阶段的近场判据;2)亚失稳准动态阶段的应变场特征(应变调整)表现为以应变逐点的逐次加速和往复传递;3)准动态过程中每个子阶段都存在短暂的准备期,其可能有助于临震预测;4)一次断层失稳事件(实验室地震)可以伴随发生多次震源应变高频震荡以及对应的多次声发射事件。

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 metainstability 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 4 MHz 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 4 MHz 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 wavelike 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.