Earthquakes and faults - Putting Down Roots in Earthquake Country
Earthquakes occur all the time all over the world, both along plate edges and along Earthquakes can also occur far from the edges of plates, along faults. Faults are categorized into three general groups based on the sense of slip or Descriptions of the three types of faults that cause earthquakes. Earthquake Physics and Fault-System Science: The destructive force of modeling of the relationships between stress changes and earthquake rates is.
Models that incorporate these features produce event distributions in which the large events fail to be self-similar It, too, seems likely to produce scaling violations both in dynamic behavior and in the geometry of fault systems see Section 5.
The existence of relevant length and time scales does not, per se, invalidate dynamical scaling theories; it may merely limit their ranges of validity. The picture may change appreciably if one considers large arrays of coupled faults and, especially, if one includes the mechanism for creation of new faults as a part of the dynamical system. It is possible that this global system, in some as yet poorly understood average sense, may come closer to a pure form of self-organized criticality.
Chaos and Predictability The theoretical issue of earthquake predictability as distinct from the practical issue of how to predict specific earthquakes remains a central, unresolved issue.
The wide range of event sizes described by the Gutenberg-Richter law, the obvious irregularities in intervals between large events, the fact that chaotic behavior occurs commonly in very simple earthquake-like models, and many other clues, all argue in favor of chaos and thus for an intrinsic limit to predictability.
The interesting question is what bearing this theoretical limit might have on the kinds of earthquake prediction that are discussed elsewhere in this report.
If one could measure all the stresses and strains in the neighborhood of a fault with great accuracy, and if one knew with confidence the physical laws that govern the motion of such systems, then the intrinsic time limit for predictability might be some small multiple of the average interval between characteristic large events on the fault.
Most of the seismic energy is released in the large events; thus, it seems reasonable to suppose that the system suffers most of its memory loss during those events as well. If this supposition were correct, earthquake prediction on a time scale of months or years—intermediate-term prediction of the sort described in Section 2.
The difficulty, of course, is that one cannot measure the state of a fault and its surroundings with great accuracy, and one still knows very little about the underlying physical laws.
If these gaps in knowledge could be filled, then predicting earthquakes a few years into the future might be no Page Share Cite Suggested Citation: However, the geological information needed for earthquake prediction is far more complex than the atmospheric information required for weather prediction, and almost all of it is hidden far beneath the surface of the Earth.
Thus, the practical limit for predictability may have little to do with the theory of deterministic chaos, but may be fixed simply by the sheer mass of information that is unavailable. Progress Toward Realism Two general goals of research in this field are to understand 1 how rheological properties of the fault-zone material interact with rupture propagation and fault-zone heterogeneity to control earthquake history and event complexity, and 2 to what extent scientists can use this knowledge to predict, if not individual earthquakes, then at least the probabilities of seismic hazards and the engineering consequences of likely seismic events.
Finding the answers is an ambitious and difficult task, but there are reasons for optimism. The speeds and capacities of computers continue to grow exponentially; they are now at a point where numerical simulations can be carried out on scales that were hardly imagined just a decade ago. At the same time, the sensitivity and precision of observational techniques are providing new ways to test those simulations.
There exists, at present, a substantial theoretical and computational effort in the United States and elsewhere devoted to developing increasingly realistic models of earthquake faults. Given a situation in which such a wide variety of physical ingredients of a problem remain unconstrained by experiment or direct observation, numerical experiments to show which of these ingredients are relevant to the phenomena may be crucial.
Consider, for example, the assumptions about friction laws that are at the core of every fault model. For slow slip, the rate- and state-dependent law discussed in Section 4. On the other hand, for fast slip of the kind that occurs in large events, there is little direct information. It seems likely that dynamic friction in those cases is determined by the behavior of internal degrees of freedom such as fault gouge, pore fluids, and the like.
Laboratory experiments on multicomponent lubricated interfaces may provide some insight, but the solution to this problem may have to rely on comparisons between real and simulated earthquakes. There are suggestions that a friction law with enhanced velocity-weakening behavior i. This conjecture needs to be tested. Other uncertainties in this category include the geometric structure of faults, the ways in which constitutive properties vary as functions of depth or position along a fault, the statistical distribution of heterogeneities on fault surfaces, and the parameters that govern the interactions between neighboring faults during seismic events.
An equally serious issue is whether small-scale physical phenomena are relevant to large-scale behavior. A truly complete description of an earthquake would involve length and time scales ranging from the microscopic ones at which the dynamics of fracture and friction are determined to the hundreds of kilometers over which large events occur.
Numerical simulations, especially three-dimensional ones, would be entirely infeasible if they were required to resolve such a huge range of scales. There are, however, examples in other scientific areas where this is precisely what occurs. Any direct numerical simulation that fails to resolve this microscopic length scale produces qualitatively incorrect results. There are indications that similar effects occur in some hydrodynamic problems, perhaps even in turbulence At present, it is not known whether any such sensitivities occur in earthquake problems, but there are possibilities.
It is possible that many features of this small-scale behavior are imprinted in important ways on the subsequent large-scale events, but it is also possible that only one or two parameters pertaining to nucleation—perhaps the location and initial stress drop plus the surrounding stress and strain fields, of course —have to be specified in order to predict accurately what happens next. Similarly, if the solidification analogy is a guide, then the small-scale, high-frequency behavior of the constitutive laws might be relevant to pulse propagation, interactions between rupture fronts and heterogeneities, and mechanisms of rupture arrest.
In order to study large systems on finite computers, investigators frequently study two-dimensional models, often accounting for deformations in the crustal plane perpendicular to the fault in models of transverse faults and omitting or drastically oversimplifying variations in the fault plane i.
What is the relationship between earthquake and faults
How relevant is the third dimension? It is hard to see how the dynamics of large events, especially rupture arrest and pulse propagation, can be studied sensibly without full three-dimensional analyses. The issues of how to make progress toward realism are theoretical as well as computational. There is an emerging realization among theorists working on earthquake dynamics, and in solid mechanics more generally, that the problems with which they are dealing are far more difficult mathematically than they had originally supposed.
One of the reasons that small-scale features can control large-scale behavior, as mentioned above, is that these features enter the mathematical statement of the problem as singular perturbations.
Looking for other ways to read this?
For example, the surface tension in the solidification problem and the viscosity in certain shock-front problems enter the equations of motion as coefficients of the highest derivative of the dependent variable.
As such, they completely change the answer to questions as basic as whether or not physically acceptable solutions exist and how many parameters or boundary conditions are needed to determine them. A related difficulty that is emerging, especially in problems involving elasticity, is that the equations of motion are often expressed most accurately as singular integral equations.
Except for a few famous cases due largely to Muskhelishvili 17such equations are not analytically solvable. There are not even good methods for determining the existence of solutions, nor are there reliable numerical algorithms for finding solutions when they do exist.
In general, the ability to resolve the uncertainties regarding connections between model ingredients and physical phenomena will depend on advances in both mathematics and computer science. These problems are solvable, but they are indeed difficult. There are strongly different conceptions of fault systems, all of which may have merit for some purposes Faults can be modeled as smooth Euclidean surfaces of displacement discontinuity in an otherwise continuous medium; fault systems can be represented as fractal arrays of surfaces; fault segments can be regarded as merely the deforming borders between blocks of a large-scale granular material transmitting stress in a force-chain mode.
Representing the crust as a fault system is especially useful on the interseismic time scales relevant to fault interac- Page Share Cite Suggested Citation: Fault-system dynamics involves highly nonlinear interactions among a number of mechanical, thermal, and chemical processes—fault friction and rupture, poroelasticity and fluid flow, viscous coupling, et cetera— and sorting out how these different processes govern the cycle of stress accumulation, transfer, and release is a major research goal.
Moreover, progress on the problem of seismicity as a cooperative behavior within a network of active faults has the potential to deliver huge practical benefits in the form of improved earthquake forecasting.
The latter consideration sets a direction for the long-term research program in earthquake science. With sufficient knowledge of the rheologic properties of the lithosphere and the necessary computational resources, it should be possible to set up simulations of mantle convection that reproduce plate tectonics from first principles, including the localization of deformation into plate boundary zones.
However, the nonlinearity of the rheology and its sensitivity to pressure, temperature, and composition especially the minor but critical constituent of water make this a difficult problem Tough computational issues are also posed by the wide range of spatial scales that must be represented in numerical models.
Types of Faults Normal faults are the cracks where one block of rock is sliding downward and away from another block of rock.
Earthquakes and Faults / Earthquakes / Science Topics / Learning / Home - GNS Science
These faults usually occur in areas where a plate is very slowly splitting apart or where two plates are pulling away from each other. A normal fault is defined by the hanging wall moving down relative to the footwall, which is moving up. Figure 2 - A normal fault.
The 'footwall' is on the 'upthrown' side of the fault, moving upwards. The 'hanging wall' is on the 'downthrown' side of the fault, moving downwards. Reverse faults are cracks formed where one plate is pushing into another plate. They also occur where a plate is folding up because it's being compressed by another plate pushing against it.
At these faults, one block of rock is sliding underneath another block or one block is being pushed up over the other. A reverse fault is defined by the hanging wall moving up relative to the footwall, which is moving down.
Figure 3 - A reverse fault. When you push sideways hard enough to overcome this friction, your fingers move suddenly, releasing energy in the form of sound waves that set the air vibrating and travel from your hand to your ear, where you hear the snap.
The same process goes on in an earthquake. Stresses in the earth's outer layer push the sides of the fault together. The friction across the surface of the fault holds the rocks together so they do not slip immediately when pushed sideways. Eventually enough stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.
Just as you snap your fingers with the whole area of your fingertip and thumb, earthquakes happen over an area of the fault, called the rupture surface. However, unlike your fingers, the whole fault plane does not slip at once.
The rupture begins at a point on the fault plane called the hypocenter, a point usually deep down on the fault. The epicenter is the point on the surface directly above the hypocenter. The rupture keeps spreading until something stops it exactly how this happens is a hot research topic in seismology.
Aftershocks Part of living with earthquakes is living with aftershocks. Earthquakes come in clusters.
In any earthquake cluster, the largest one is called the mainshock; anything before it is a foreshock, and anything after it is an aftershock. Aftershocks are earthquakes that usually occur near the mainshock.
The stress on the mainshock's fault changes during the mainshock and most of the aftershocks occur on the same fault. Sometimes the change in stress is great enough to trigger aftershocks on nearby faults as well. An earthquake large enough to cause damage will probably produce several felt aftershocks within the first hour. The rate of aftershocks dies off quickly.
The day after the mainshock has about half the aftershocks of the first day. Ten days after the mainshock there are only a tenth the number of aftershocks. An earthquake will be called an aftershock as long as the rate of earthquakes is higher than it was before the mainshock.