Progress in the study of earthquake physics has been slower than progress in seismic data collection. There are quite a few researchers in the field of earthquake physics who are themselves engineers (such as Kostrov, Freund and Rice).
Limiting rupture velocities during spontaneous mode-II sliding
If the seismic fault is indeed adequately modeled by dynamic sliding due to friction between two solids, our experiments can be used to address many open seismological questions. This mechanism is called the Burridge-Andrews mechanism, which was first proposed by Burridge and later by Andrews based on theoretical and numerical studies (Burridge 1973; Andrews 1976; Andrews 1985).
The crack-like versus the pulse-like nature of rupture
As described by Ben-Zion (Ben-Zion 2001), the interaction between slip and normal stress allows fractures to grow in a pulse-like mode under shear stress conditions that are low compared to the nominal frictional strength of the interface. In these two groups of experiments we check the possibility of pulse-like ruptures, motivated by recent theoretical and numerical works predicting such a phenomenon.
Directionality of rupture and rupture velocity in inhomogeneous fault systems
Introduction
Recently, two types of modification of the original block-slide model (direct shear) have been introduced. In fracture mechanics, there are two models of the fracture tip: the singular model and the cohesive zone model (Freund 1990; Broberg 1999).
Experimental Set-up
- Photoelastic Fault Model
- The Triggering Mechanism
- Diagnostics
The trigger can be either a sudden increase in the load or a sudden decrease in the fault strength. It is therefore plausible to assume that triggering is usually the result of the local decrease of the fault strength.
Preliminary Results
Breakdowns are seen along the circular shear wave front, and the breakdown velocity V is close to the Rayleigh wave velocity of the CR material as determined from the breakdown tip history. The fracture velocity is greater than the CS shear wave velocity of the material and close to the CP longitudinal wave velocity.
Conclusions and Discussions
The results described in Figures 1.6–1.8 are only indicative of the wide range of behaviors that will be described in the following chapters. In this chapter, we present the experimental discovery of the phenomenon of supershear rupture and its visualization. We also investigate the parameter space that governs the physics of the subRayleigh to supershear transition of dynamic cracks along incoherent (frictional) interfaces.
Introduction
However, the multitude of independently collected evidence warrants further investigations of the mechanics of supershear fracture propagation. The theoretical results of the cohesive zone rupture model finally predict that earthquake ruptures can propagate either at Rayleigh wave speed or supershear speed greater than 2CS. The experimental confirmation of the possibility of supershear (intersonic) fracture followed many years after the first theoretical predictions.
Experimental Design
The far-field tectonic strain is simulated by uniaxial compression exerted at the top and bottom of the system using a hydraulic press (B). As described in Chapter 1, a unique aspect of the experimental design is related to the choice of the rupture triggering mechanism that has not been addressed by previous laboratory earthquake experiments. The dynamic fracture is nucleated in the center of the simulated fault by producing a local pressure pulse in a small area of the interface (Figure 2.1C).
Experimental Results
- Purely SubRayleigh and Supershear Earthquake Ruptures
- The Experimental Visualization of the SubRayleigh to Supershear Earthquake Rupture Transition Earthquake Rupture Transition
In these two cases, the rupture velocities are very close to the Rayleigh wave velocity of the material. An interesting observation is that the rupture velocity is always the Rayleigh wave velocity of the material. The formation of the Mach cone is due to the fact that the rupture is propagating faster than the shear wave velocity of the material.
Theoretical Model for the SubRayleigh to Supershear Transition
- Uniaxial Loading Condition
- Biaxial Loading Condition
Thus, in this relation we are able to estimate the transition length of the supershear rupture L even if we have not taken any snapshots at or before the time instance of the transition. In Figure 2.6 we show the pressure dependence of the transition length L from a set of experiments corresponding to the same inclination angle of 25°. As shown in Figure 2.6, this modified relation agrees well with the experimental data presented in this paper for appropriate choices of micromechanics contact model parameters.
Application to Real Earthquakes
This result is consistent with the transition length of the Kunlunshan earthquake (Bouchon and Vallee 2003). In the next section, we will provide another method to estimate the transition length corresponding to a real earthquake. If the tectonic stress is well below the static fault strength (ie, large s), then the transition length becomes too large for earthquake rupture to achieve supershear.
Conclusions and Discussions
Another reason for the rarity of supershear fractures is due to the quality of data and the nature of the fault. The break in the opposite direction can propagate at a supershear velocity close to the slower P-wave velocity of the system. This type of directionality of the earthquake rupture process is primarily due to the contrast of the material.
Introduction
The recent large earthquakes (1999 Izmit and Düzce) and the seismic migration history along the North Anatolian fault may represent a unique field example of the effect of the material contrast across the fault. This presumably occurs by adding or transferring stress to the failure segment adjacent to the tips of a segment that has recently failed. The stress distribution is very non-uniform as it occurs in addition to the long-term stress renewal and the pre-existing stress inhomogeneities.
Two Types of Ruptures along Inhomogeneous Faults
Consistent with Weertman's 1980 analysis (Weertman 1980), the first type corresponds to fracture growth in the direction of slip of the lower wave velocity material of the system. The second type of self-sustained fracture corresponds to growth in the opposite direction of slip in the lower wave velocity material of the bimaterial system. Such ruptures are generated for sufficiently high values of the friction coefficient ( Ranjith and Rice 2001 ) and are less unstable than the “+GR” ruptures described above ( Cochard and Rice 2000 ).
Experimental Set-up
The material with lower wave speed at the bottom (Polycarbonate) has a shear wave speed C =960 m/s and a longitudinal wave speed =2,182 m/s. The shear wave velocities are measured directly for each material by following the shear wave fronts through high-speed photography and photoelasticity. Photoelasticity, sensitive to maximum shear stress fields, is perfectly suited to measure shear wave velocities and to investigate shear-dominated fracture processes in brittle, transparent, and birefringent solids (see discussion in Chapter 2).
Experimental Results
- Case-1, GR Rupture and Sub-shear Rupture
- Case-2, GR Rupture and Supershear Rupture
- The Dependence of Transition Length on P
On the one hand, the fault moving westward is the fault propagating in the direction of material sliding with lower wave speed (positive direction). 3.3, is clearly different from the structure of the subRayleigh, westward-moving fault, shown in the lower inset. The one moving west (positive direction) has a velocity VW =+, while the one moving east (opposite direction) is supershear.
Comparison of the Experimental Results to Existing Numerical and Theoretical Studies
The initiation of the "−" mode is always preceded by a completely subRayleigh, crack-like fracture, the speed of which depends on the loading, geometry and properties of the bimaterial. They also appear to primarily favor the triggering of the “+GR” mode in bimaterial systems with low wave speed (Andrews and Ben-Zion 1997). This type of preference has led to the designation of the "positive" direction as the "favored" fracture direction and +C as the "favored" fracture velocity.
Explanation of Earthquake Series on North Anatolian Fault Using the Experimental Results Experimental Results
This explanation is of course only plausible if one assumes again that the material south of the North Anatolian fault (at its western end) is the lower wave speed solid. The remaining faults of the series were "irregular" in the sense that they had dominant eastward growth. As previously detailed, out of the remaining four faults of the series, the Izmit and Düzce events were bilateral with a western branch of the "+GR" type (concordant with the others) and an eastward, supershear branch of the.
Discussions and Conclusions
This would in turn result in a total eastward rupture length that is slightly shorter than the total westward rupture length of the earthquake series. In the positive direction, the rupture tip propagated at a generalized Rayleigh wave speed, while in the negative direction, the rupture tip propagated either sub-Rayleigh or at a velocity very close to the P wave velocity of slower wave speed solids. As an example, we have shown how this theory explains observations of a historical sequence of migration earthquakes along the North Anatolian Fault.
Introduction
Harris and Day considered cases where the width of the low velocity zone (LVD) varied from 200 to 1000 m. Let us first denote the ratio of the shear wave velocity of the host rock to that of the fault core (low velocity zone) by rS. Ben-Zion and Huang used a mesh size of 0.25 m, while the width of the low-velocity zone to mesh size ratio varies from 20 to 1000 in their simulations.
Experimental Set-up
The ratio of the width of the fault core to the width of the primary slip zone ranges from 63.5 to 254. The loading direction is vertical and the fault core is inclined at an angle α to the horizontal axis. In our experiments, we can vary the loading level, the inclination angle and the width of the defect core b.
Experimental Results
- The Effect of Fault Core on Faulting and On Wave Propagation Characteristics Characteristics
- Visualizing Crack-like and Pulse-like Ruptures
- The Birth and Growth of the Slip Pulse
- The Effect of the Fault Core Width b on Faulting
- The Effect of the Far-field Loading P on Rupture Speeds
- The Sub-shear to Supershear Transition and the Birth of an Unstable Pulse Pulse
- Comparison of Experiments with Available Numerical Results
As we can see from the figures, the break in the positive direction (left) always propagates with a speed close to the generalized Rayleigh wave speed of the system. The pulse propagates at a speed close to the generalized Rayleigh wave speed of the system. However, the load greatly affects both the condition and the speed of the fracture, which grows to the right (positive direction).
Conclusions and Discussions
These include the observation of spontaneously generated supershear fractures, of the subRayleigh to supershear fracture transition, directionality in inhomogeneous faults, of the generalized Rayleigh wave speed fractures and of pulse-like fractures. These laboratory observations have been used to explain a number of field observations and are capable of settling several long-standing debates in the field of the physics of natural earthquakes. This experimental setup can also be used to address other issues in the field of earthquake dynamics; some examples will be discussed in the section on future work.
Summary of the Thesis Work
Supershear rupture (velocity faster than the shear wave velocity of the material) was observed to propagate at a velocity close to the longitudinal velocity of the material. Spontaneous ruptures were observed to propagate approximately at generalized Rayleigh wave velocity in the same direction as the lower velocity solid slip. In the opposite direction, we observed either subRayleigh or supershear fracture depending on the loading condition.
Future Work
- Effect of Inhomogeneities of Fault Strength on Faulting (Barrier and Asperity) and Asperity)
- Effect of Fault Steps on Faulting (Dynamic Triggering and Rupture Arrest)
- Direct Measurements of Slip History
- Direct Measurements of Transient Temperature Increase (Heat Production and Flash Heating)
In order to verify the impact of barriers and bumps on rupture, we will use simulated barriers and bumps in a laboratory failure model. IPVM can remotely measure the in-plane velocity of a small optical marker (reflector) attached to the sample surface (see Figure 5.3) at high spatial and temporal resolution. Heat production during earthquakes is a long-standing problem in the geophysical community.
Conclusion
34;Evidence for supershear transition during the 2002 Denali Fault earthquake." Bulletin of the Seismological Society of America (submitted). 34;Observation of near-field earthquakes triggered by dynamic deformation." Bulletin of the Seismological Society of America. 34; Unilateral rupture dominance for a global catalog of large earthquakes.” Bulletin of the Seismological Society of America.