Large Earthquake Triggering, Clustering, and the Synchronization of Faults

Large earthquakes are sometimes observed to trigger other large earthquakes on nearby faults. The magnitudes of the calculated Coulomb stress transfers presumed to cause the triggering are 10⁻²–10⁻³ of the earthquake stress drops. The earthquake stress drops and the triggering delay times are similarly small with respect to the natural recurrence time of the earthquakes. This requires that both faults be simultaneously very close to the ends of their seismic cycles. Paleoseismological data show that for the same regions prior earthquakes have occurred in clusters of ruptures of several faults separated by long quiescent periods. Both observations suggest that synchronization is occurring between faults. Theory and observations indicate that synchronization can occur between nearby faults with positive stress coupling and intrinsic slip velocities within an entrainment threshold. In the south Iceland seismic zone, the central Nevada seismic belt, and the eastern California shear zone, several synchronous clusters that apparently act independently can be recognized. This behavior is the 3D equivalent of the phase locking that results in the seismic cycle of individual faults being dominated by large characteristic earthquakes, and for synchronization of fault segments along a given fault. Rupture patterns of repeated individual earthquakes or earthquake clusters are not identical in either the 2D or 3D cases. The state of this system, which exhibits strong indications of synchrony without exact repetition, may be called fuzzy synchrony

The Rupture Mode of the Shallow Large‐Slip Surge of the Tohoku‐Oki Earthquake

The remarkable feature of the Mw 9.1 Tohoku-Oki earthquake of 2011 was a late, shallow, surge that propagated from about 12 km depth all the way to the trench. This surge had very large slip and was depleted in high-frequency radiation. It was followed by normal faulting in the outer wedge; this requires that the postsurge basal shear stress be close to zero. Explanations for the surge propagating through the velocity-strengthening region by a thermal weakening mechanism fail to produce the near-total stress drop required by the postseismic extension. The surge propagated in a region of material contrast across the plate interface. The combination of a strong nucleation pulse from down dip, velocity strengthening, and the bimaterial effect satisfies the conditions for wrinkle pulse propagation. A wrinkle pulse rupture mode can produce an instability by overcoming velocity strengthening with the normal stress reduction produced by the bimaterial effect. It also leads to total stress drop, satisfying the extension constraint and explaining the prodigious slip amplitude of the surge. Because the surfaces are detached in wrinkle pulse mode, asperity contact is reduced, which explains the dearth of high-frequency radiation during the surge. This behavior appears to be a common feature of many of the greatest subduction earthquakes

On the stress dependence of the earthquake b value

Laboratory experiments have shown that the b value in the size distribution of acoustic emission events decreases linearly with differential stress. There have been a number of observations that indicate that this relation may also hold for earthquakes. Here using a simple frictional strength model for stresses in the continental lithosphere combined with earthquake b values measured as a function of depth in a wide variety of tectonic regions, we verify and calibrate that relation, finding b = 1.23 ± 0.06 − (0.0012 ± 0.0003)(σ1 − σ3), where the stress difference (σ1 − σ3) is in megapascal. For subduction zones, we find that b value correlates linearly with the slab pull force and with the net reduction of plate interface normal force, both of which also indicate a negative linear relation between b value and differential stress

The Brittle‐Ductile Transition Predicted by a Physics‐Based Friction Law

A theory of the brittle‐ductile transition (BDT) is shown to be a direct consequence of a recently developed physics‐based constitutive law for rock friction (Aharonov & Scholz, 2018, https://doi.org/10.1002/2016JB013829), which assumes exponential creep on contacts. The theory was previously tested against experimental data for sliding at low ambient temperature and stress. Here, theoretical interpretation of experimental data at high temperature and stress shows that at some point the real area of contact reaches a maximum value beyond which it becomes fixed. The constitutive law shows that this marks the onset of the BDT, beyond which sliding changes from frictional to an exponential flow law for low‐temperature plasticity. Application to the Earth's crust shows that beyond this point, strength fall linearly with depth until it intersects the power law for bulk flow of the country rock, which marks the lower boundary of the BDT. Modeling, constrained by experimental data for granite, predicts that the BDT starts at a temperature of about 300°C, at a depth of 11–13 km in the continental crust, depending on fault slip rate and temperature gradient. The completion of the BDT is similarly calculated to occur around 475°, at 16–18 km, in agreement with laboratory and field observations. The BDT is thus found to be a region spanning about 175°C with a width of several kilometers.Within the exponential flow region, the structural outcome would be a relatively narrow mylonitized fault zone, which widens into a broader region of shear at the base of the BDT

Slip-length scaling in large earthquakes' Observations and theory and implications for earthquake physics

For twenty years there has been a dilemma in earthquake physics, because the observed scaling law for large earthquakes did not appear to be consistent with the stress-drop invariance of small earthquake scaling. Surprisingly, slip was seen to continue to increase with rupture length L even for events with lengths much longer than the event widths W (the brittle crust down-dip depth), whereas it might have been expected to saturate for lengths much beyond the width. If this implies that the physics of great earthquakes is somehow different from that of their smaller counterparts, this casts serious doubts on predicting the effects of the rare and damaging great events from observations of the more common smaller events. Here we bring together recently compiled observations of very large aspect ratio earthquakes with results of a 3 dimensional dynamic earthquake model to show that slip-length scaling observations are, in fact, consistent with a scale-invariant physics. Further, we discuss the origin of the large earthquake scaling in the model

Microfractures: A review

Microfractures are small, high-aspect-ratio cracks in rock that result from application of differential stresses. Although the term has been used to refer to larger features in the petroleum engineering and geophysics literature, in geologic parlance the term refers to fractures visible only under magnification, having lengths of millimeters or less and widths generally less than 0.1 mm. Nevertheless, populations of these structures typically encompass a wide size range and in some cases they form the small-size fraction of fracture arrays that include much larger factures. In geologic settings, microfractures commonly form as Mode I (opening) fractures where the minimum principal stress exceeds the elastic tensile strength creating a narrow opening displacement; in isotropic rocks such fractures mark the plane perpendicular to the least compressive principal stress during fracture growth. These planar or curviplanar openings provide an opportunity for fluids and/or gases to enter the created cavity. Cement deposits or crack closure may trap fluids or gases, leaving mineral precipitates and a track of enclosed fluids and gases. In transmitted light these precipitates frequently manifest as fluid-inclusion planes (FIPs). Cathodoluminescence (CL) images show that many are cement-filled microveins. Microfractures can be used to assess the paleostress history or fluid movement history of a rock body. Also, because sudden opening produces acoustic emissions, microfractures created in the laboratory can be used to assess the rock-failure process. Here we review recent discoveries made using microfractures, including fracture patterns, strain, fracture growth and size-scaling, evolution of stresses around propagating faults (process zones), far-field tectonic stresses, and insights into the state of stress leading to earthquakes

The mechanics of first order splay faulting: The strike-slip case

First order splay faults, as defined here, are secondary faults that form at acute angles symmetrically on either side of a primary fault of the same sense of shear. We show that these faults form when the primary fault becomes critically misaligned with the principal stresses such that splay fault formation, on the optimum plane for faulting, is favored. First order splay faults, in distinction from other splay faults, are secondary only in the temporal sense – they are subsequent but not subordinate, in a tectonic sense, to the primary fault. Here we analyze the case of strike-slip faults, and compare it with data for several continental transform fault systems, where we show that the splay faults form in the most favorable direction: parallel to the plate motion vector. We also discuss and speculate on several outstanding problems with regard to first order splay faults: the placement of them in space, means by which primary faults become misoriented in the stress field, and the mechanics of first order splay fault-primary fault junctions, once formed

Transition regimes for growing crack populations

Numerous observational papers on crack populations in the material and geological sciences suggest that cracks evolve in such a way as to organize in specific patterns. However, very little is known about how and why the self-organization comes about. We use a model of tensilelike cracks with friction in order to study the time and space evolution of normal faults. The premise of this spring-block analog is that one could model crustal deformation for long time scales assuming a brittle layer coupled to a ductile substrate. The long time-scale physics incorporated into the model are slip-weakening friction, strain-hardening rheology for coupling the two layers, and randomly distributed yield strength of the brittle layer. We investigate how the evolution of populations of cracks depends on these three effects, using linear stability analysis to calculate the stable regimes for the friction as well as numerical simulations to model the nonlinear interactions of the cracks. We find that we can scale the problem to reduce the relevant parameters to a single one, the slip weakening. We show that the distribution of lengths of active cracks makes a transition from an exponential at very low strains, where crack nucleation prevails, to a power law at low to intermediate strains, where crack growth prevails, to an exponential distribution of the largest cracks at higher strains, where coalescence dominates. There is evidence of these different length distributions in continental and oceanic normal faults. For continental deformation the strain is low, and the faults have power-law frequency-size distributions. For mid-ocean ridge flanks the strain is greater, up to an order of magnitude higher than the continental strain, and faults have exponential-like frequency-size distributions. No theory has been offered to explain this difference in the distributions of continental and mid-ocean faults. In this paper we argue that they are indicative of different stages of evolution. The former faults are at an early stage of relatively small deformation, while the latter are at a later stage of the evolution. For high strain the faults reach a saturation regime with system size cracks evenly spaced in proportion to the brittle layer thickness. We asymptotically approximate the time space evolution of faults as a long time-scale phenomenon, thereby avoiding modeling the short time-scale earthquakes. We show that this assumption is valid, which implies that the faults that creep and faults with earthquakes display the same time and space evolutions

Microfracturing of rock in compression.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Geology and Geophysics, 1967.Vita.Bibliography: leaves 156-164.Ph.D

The seismic coupling of subduction zones revisited

The nature of seismic coupling for many of the world's subduction zones has been reevaluated. Geodetic estimates of seismic coupling obtained from GPS measurements of upper plate deformation during the interseismic period are summarized. We compared those with new estimates of seismic coupling obtained from seismological data. The results show that with a few notable exceptions the two methods agree to within about 10%. The seismological estimates have been greatly improved over those made 20-30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence, in the intervening years, of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record. The data also, again with a few notable exceptions, support the frictional instability theory of seismic coupling, and in particular, the test of that theory made by Scholz and Campos (1995). Overall, the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value. There is also considerable variation of coupling within individual subduction zones. Earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence, but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation