Peculiarities of Dynamic Slip Nucleation in a Thin Granular Layer

Izvestiya, Physics of the Solid Earth - Abstract—Granular media determine the dynamics of many natural systems including faults in the Earth’s crust. The paper addresses the laboratory...     

On the Prospect of Detecting the Process of Earthquake Preparation in the Spectrum of Seismic Noise: A Laboratory Experiment

The results of laboratory experiments aimed at studying the pattern of the transition process of a model fault into a metastable state are presented. The experiments are conducted on a slider model installed onto a long granite base wherein vibrations are excited. The idea of the experiments is that the mechanical properties of the fault change under the transition into the metastable state. These changes can be detected by the detailed examination of the parameters of microseismic noise. The conducted experiments show that, despite the low Q-factor of the block–fault mechanical system, the spectrum of the recorded oscillations definitely contains the harmonic components corresponding to the eigenmodes of this system. In the model with the interblock contact filled with quartz sand, the fundamental mode of the free oscillations alters most noticeably in the frequency band 1000–1200 Hz, where the clear effect of the spectral peak’s migration towards lower frequencies is observed as the contact approaches the moment of dynamic failure, and the approximately initial value is recovered after the sliding stops. The revealed effect gives hope that the changes in the stress–strain state of the fault zone at the final stage of earthquake preparation can be detected by analyzing the parameters of low-frequency seismic noise. The segment of the record during and after the passage of surface waves from remote earthquakes is perhaps amongst the most favorable for determining the characteristic values of the region under study. These oscillations with a period of a few dozen seconds have significant amplitudes and durations, which promotes the excitation of the resonant vibrations of the blocks.     

Variations in rupture zone stiffness during a seismic cycle

The character of variations in the stiffness of the interblock contact at different stages of the stick-slip motion are experimentally estimated for the first time. Some possible consequences of the observed phenomenon are considered. It is shown that at the final stage of the cycle before the dynamic break the shear stiffness of the contact drastically decreases, which can be detected in nature with the help of seismic methods. This effect can serve as a base for the development of new active and passive methods of monitoring of seismically active regions.     

Some questions of geomechanics of the faults in the continental crust

We present the results of laboratory experiments on studying the formation of different slip modes on the interfaces in a rock massif such as aseismic creep, stick-slip, and periodic slow-slip events. It is shown that the way of releasing the accumulated elastic energy is determined by the mesoscale structure of the gouge rather than by its macroscopic strength characteristics. The evolution of the stress chains which are formed and broken during the displacement on the fracture, as well as the length and number of these chains, completely determines the regularities of the deformation. The role of these load-bearing elements in nature can be played, e.g., by the “contact spots,” which determine the regularities of stress concentration near the interblock boundary. We consider the effects of low-amplitude vibrations on stressed fractures. It is shown that, depending on the mode of deformation, the vibration impact can either reduce or boost the amplitude of separate events and the fraction of energy that is released dynamically. In the conclusion of the paper, we discuss the possibility of using the shear strength of the fault zone as a geomechanical parameter controlling the mode of deformation.     

Determination of the <Emphasis Type="Italic">PT</Emphasis> Conditions that Accompanied a Seismogenic Slip along a Deep Segment of the Marginal Suture of the Siberian Craton

This paper reports data on the structure of the central zone of the seismogenic fault in the originally deep segments of the fault zone. The isotopic analyses have made it possible to estimate the absolute age of the ancient coseismic faults at 673 ± 5 Ma, which is indicative of seismic activation of deep faults in the studied segment of the marginal suture of the Siberian Craton in the Neoproterozoic.     

Seismogenic width of a fault zone

High-precision catalogues containing data on the spatial distribution of earthquake sources associated with several fault zones have been processed. The analysis shows a high degree of spatial localization of seismicity. Most of the events are observed in a narrow zone with a characteristic width of about 100 m. This value is a part of a percent of the fault zone length and corresponds to the upper boundary of estimates concerning the effective fault width, which are made on the basis of data on the characteristic parameters of roughness and waviness of rock discontinuities.     

Parameters determining the portion of energy radiated during dynamic unloading of a section of rock massif

Presented are the main results of laboratory experiments aimed at studing the regularities of transition of the cumulated deformation energy to the kinetic one when a block exhibits stick-slip along model fracture filled with geomaterials with different properties. It was shown that even a small variation of material composition of the fault principal slip zone may result in a significant variation of the portion of seismic energy radiated during dynamic unloading of the enclosing rock massif.     

Precursors of dynamic failure on a tectonic fault

A new method for controlling the dynamic shear stiffness of interblock contact during stick-slip is proposed. This method enables us to reveal changes in the mechanical properties of the contact long before the macroscopic slip will be recorded. In the experiments conducted, the time of precursor manifestation was about 1/3 of the duration of the “seismic cycle.”     

The use of local seismic networks for identifying the seismogenic structure of faults

We consider a method for identifying the segments of localization of deformation processes within fault zones. The method is based on the results of seismic monitoring at a dense seismic network in California. For the first time, by analyzing the seismogenic structure of several fault zones, we highly accurately located the domains involved in active deformation processes. Three-dimensional reconstructions show that these domains are a set of local segments each having a characteristic size of about 100 m, which is commensurable with the scale of the probable anthropogenic impacts on the geological medium.     

The development of logging methods in solving hydrogeological problems at the Aleksandrovka research base

This paper presents the results of the development of a digital logging system and its implementation into everyday practice, at a well of Moscow State University. Complex data interpretation in relation to problems of hydrogeology is discussed. New developments, such as a video well-logging device and a highfrequency induction probe (HFIL) for plastic-cased wells, are used in the system in addition to the obligatory logging types (GL, RL, SP, CL, and temperature measurement).