Abstract
According to the steady state of fault and energy balance, we provided a new idea to observe the precursors for a stressed fault. The meta-instability (or sub-instability) state of a fault is defined as the transition phase from peak stress to critical stress of fast instability (earthquake generation) during a full period of slow loading and fast unloading. The accumulative deformation energy begins to release in this stage. Identifying its deformation before fast instability would be beneficial to obtain premonitory information, and to evaluate the seismic risks of tectonic regions. In this study, we emphasized to analyze deformation process of the meta-instable stage with stain tensor data from a straight precut fault in granite at a slow loading rate, and observed the tempo-spatial features during the full deformation process of the fault. Two types of tectonic zones and instabilities occur on the stick-slip fault. The low- and high-value segments in the volume strain component appear along the fault strike with a load increment. The former first weakens and then becomes initial energy release segments; the latter forms strong stress-interlocking areas and finally turns into the initial region of fast instability. And there are two stages in the entire instable process of the fault: the initial stage is associated with the release of the low volume strain segments, which means fault pre-slips, slow earthquakes or weak earthquakes. The second one characterizes a strong earthquake through the release of high volume strain parts. The rupture acceleration in the first stage promotes the generation of the second. Moreover, fault instability contains two types of strain adjustments along the fault: the front-like strain change along the transition segments from low- to high- strain portions with volume strain release, and the compressive strain pulse of fault instability after the volume strain release extends to a certain range with loading increment. In laboratory experiments, the front-type strain occurs about 12 seconds before fast fault instability; the compressive pulse initiates within less than 0.1 second, and then the fault turns quickly into a dynamic strain adjustment, which appears quasi-synchronously between different measurement points, and, finally, an earthquake is generated.
Highlights
Since the Brace and Byerlee’s proposal [Byerlee, 1967, 1978; Brace, Byerlee, 1966, 1970] describing shallow earth quake mechanism with stick-slip phenomenon and Diete rich’s description [Dieterich, 1978, 1979] of the sliding law with rate- and state- dependent friction constitute, the sliding friction feature of a simple fault has become one of the most important fields to extensively study earthquake mecha nism, including theoretical analysis, laboratory experiment and field observation
Moore demonstrated that the serpentine con taining talc is a factor to induce high-speed non-earthquake creep of San Andreas Fault [Moore, Rymer, 2007]; Rice [Rice, 2006] systematically studied its absolute and relative weakening process, the expansion effect resulted from fault instability, the influences from pore fluid to earthquake nucleation and dynamic earthquake rupture
Fault instability occurs after the time when the area and the amplitude of the volume strain drop develop to a certain value
Summary
Since the Brace and Byerlee’s proposal [Byerlee, 1967, 1978; Brace, Byerlee, 1966, 1970] describing shallow earth quake mechanism with stick-slip phenomenon and Diete rich’s description [Dieterich, 1978, 1979] of the sliding law with rate- and state- dependent friction constitute, the sliding friction feature of a simple fault has become one of the most important fields to extensively study earthquake mecha nism, including theoretical analysis, laboratory experiment and field observation. Fault instability occurs after the time when the area and the amplitude of the volume strain drop develop to a certain value. (2) The final instability occurs at Points 14 and 9, which is mainly associated with the energy release of the low volume strain areas within a small scale, but Points 4, 3 , 2, 6, and 11 are still keep rising in the high volume strain areas. The former promotes the latter that is related to the entire fault instability. Another is a dynamic strain adjustment about 0.1 s ahead of the entire instability. (4) The variation distribution of volume strain in different segments is distinguishable: the change content in the middle fault is minimum (30 με) in the upper segment
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