This essential reference for graduate students and researchers provides a unified treatment of earthquakes and faulting as two aspects of brittle tectonics at different timescales. The intimate connection between the two is manifested in their scaling laws and populations, which evolve from fracture growth and interactions between fractures. The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws - producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events. The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.

The Mechanics of Earthquakes and Faulting

Background Human-induced earthquakes have become an important topic of political and scientific discussion, owing to the concern that these events may be responsible for widespread damage and an overall increase in seismicity. It has long been known that impoundment of reservoirs, surface and underground mining, withdrawal of fluids and gas from the subsurface, and injection of fluids into underground formations are capable of inducing earthquakes. In particular, earthquakes caused by injection have become a focal point, as new drilling and well-completion technologies enable the extraction of oil and gas from previously unproductive formations. Earthquakes with magnitude (M) ≥ 3 in the U.S. midcontinent, 1967–2012. After decades of a steady earthquake rate (average of 21 events/year), activity increased starting in 2001 and peaked at 188 earthquakes in 2011. Human-induced earthquakes are suspected to be partially responsible for the increase. Advances Microearthquakes (that is, those with magnitudes below 2) are routinely produced as part of the hydraulic fracturing (or “fracking”) process used to stimulate the production of oil, but the process as currently practiced appears to pose a low risk of inducing destructive earthquakes. More than 100,000 wells have been subjected to fracking in recent years, and the largest induced earthquake was magnitude 3.6, which is too small to pose a serious risk. Yet, wastewater disposal by injection into deep wells poses a higher risk, because this practice can induce larger earthquakes. For example, several of the largest earthquakes in the U.S. midcontinent in 2011 and 2012 may have been triggered by nearby disposal wells. The largest of these was a magnitude 5.6 event in central Oklahoma that destroyed 14 homes and injured two people. The mechanism responsible for inducing these events appears to be the well-understood process of weakening a preexisting fault by elevating the fluid pressure. However, only a small fraction of the more than 30,000 wastewater disposal wells appears to be problematic—typically those that dispose of very large volumes of water and/or communicate pressure perturbations directly into basement faults. Outlook Injection-induced earthquakes, such as those that struck in 2011, clearly contribute to the seismic hazard. Quantifying their contribution presents difficult challenges that will require new research into the physics of induced earthquakes and the potential for inducing large-magnitude events. The petroleum industry needs clear requirements for operation, regulators must have a solid scientific basis for those requirements, and the public needs assurance that the regulations are sufficient and are being followed. The current regulatory frameworks for wastewater disposal wells were designed to protect potable water sources from contamination and do not address seismic safety. One consequence is that both the quantity and timeliness of information on injection volumes and pressures reported to regulatory agencies are far from ideal for managing earthquake risk from injection activities. In addition, seismic monitoring capabilities in many of the areas in which wastewater injection activities have increased are not capable of detecting small earthquake activity that may presage larger seismic events.

http://science.sciencemag.org/content/sci/341/6142/1225942.full.pdf

Injection-Induced Earthquakes

Deep drilling and induced seismicity experiments at several locations worldwide indicate that, in general, the brittle crust in intraplate regions is critically stressed, pore pressures are close to hydrostatic, and in situ bulk permeability is ∼10 −17 to 10 −16 m 2 . This high permeability, three or four orders of magnitude higher than that measured on core samples, appears to be maintained by critically stressed faults and greatly facilitates fluid movement through the brittle crust. We demonstrate that such high permeabilities can maintain approximately hydrostatic fluid pressures at depths comparable to the thickness of the seismogenic crust. This leads to the counterintuitive result that faulting keeps intraplate crust inherently strong by preventing pore pressures greater than hydrostatic from persisting at depth.

How faulting keeps the crust strong

This paper provides a review of hydromechanical (HM) couplings in fractured rock, with special emphasis on HM interactions as a result of, or directly connected with human activities. In the early 1960s, the coupling between hydraulic and mechanical processes in fractured rock started to receive wide attention. A series of events including dam failures, landslides, and injection-induced earthquakes were believed to result from HM interaction. Moreover, the advent of the computer technology in the 1970s made possible the integration of nonlinear processes such as stress–permeability coupling and rock mass failure into coupled HM analysis. Coupled HM analysis is currently being applied to many geological engineering practices. One key parameter in such analyses is a good estimate of the relationship between stress and permeability. Based on available laboratory and field data, it was found that the permeability of fractured rock masses tends to be most sensitive to stress changes at shallow depth (low stress) and in areas of low in-situ permeability. In highly permeable, fractured rock sections, fluid flow may take place in clusters of connected fractures which are locked open as a result of previous shear dislocation or partial cementation of hard mineral filling. Such locked-open fractures tend to be relatively insensitive to stress and may therefore be conductive at great depths. Because of the great variability of HM properties in fractured rock, and the difficulties in using laboratory data for deriving in-situ material properties, the HM properties of fractured rock masses are best characterized in situ.

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The role of hydromechanical coupling in fractured rock engineering

The character of the hydrological changes that follow major earthquakes has been investigated and found to be dependent on the style of faulting. The most significant response is found to accompany major normal fault earthquakes. Increases in spring and river discharges peak a few days after the earthquake, and typically, excess flow is sustained for a period of 6–12 months. In contrast, hydrological changes accompanying pure reverse fault earthquakes are either undetected or indicate lowering of well levels and spring flows. Strike-slip and oblique-slip fault movements are associated with a mixture of responses but appear to release no more than 10% of the water volume of the same sized normal fault event. For two major normal fault earthquakes in the western United States (those of Hebgen Lake on August 17, 1959, and Borah Peak on October 28, 1983), there is sufficient river flow information to allow the magnitude and extent of the postseismic discharge to be quantified. The discharge has been converted to a rainfall equivalent, which is found to exceed 100 mm close to the fault and to remain above 10 mm at distances greater than 50 km. The total volume of water released in these earthquakes was around 0.3 km3 (Borah Peak) and 0.5 km3 (Hebgen Lake) Qualitative information on other major normal fault earthquakes, in both the western United States and Italy, indicates that the size, duration, and range of their hydrological signatures have been similar. The magnitude and distribution of the water discharge for these events are compared with deformation models calibrated using seismic and geodetic information. The quantity of water released over a time period of 6–12 months suggests that crustal volume strain to a depth of at least 5 km is involved. The rise and decay times of the discharge are shown to be critically dependent on crack widths, and it is concluded that the dominant cracks have a high aspect ratio and cannot be much wider than 0.03 mm. Using the estimated depth to which water is mobilized, the modeled crack size, and the measured volumes of water expelled, it is concluded that even at distances of 50 km from the earthquake epicenters, cracks must be separated by no more than 10 or 20 m. In regions of highest discharge nearer the earthquake epicenters, separations of 1 or 2 m are required. These results suggest that water-filled cracks are ubiquitous throughout the brittle continental crust and that these cracks open and close throughout the earthquake cycle. The existence of tectonically induced fluid flows on the scale that we demonstrate has major implications for our understanding of the mechanical and chemical behavior of crustal rocks.

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Hydrological signatures of earthquake strain

The temporal distribution of induced seismicity following the filling of large reservoirs shows two types of response. At some reservoirs, seismicity begins almost immediately following the first filling of the reservoir. At others, pronounced increases in seismicity are not observed until a number of seasonal filling cycles have passed. These differences in response may correspond to two fundamental mechanisms by which a reservoir can modify the strength of the crust—one related to rapid increases in elastic stress due to the load of the reservoir and the other to the more gradual diffusion of water from the reservoir to hypocentral depths. Decreased strength can arise from changes in either elastic stress (decreased normal stress or increased shear stress) or from decreased effective normal stress due to increased pore pressure. Pore pressure at hypocentral depths can rise rapidly, from a coupled elastic response due to compaction of pore space, or more slowly, with the diffusion of water from the surface.

Two types of reservoir-induced seismicity

The 1976 and 1984 Gazli, USSR, earthquakes—were they induced?

An enhanced Landsat image of the northeastern Tadjik Depression in the Soviet Union was combined with field data to produce a 1:250,000 geologic map of the Vakhsh fold-and-thrust belt and adjacent autochthon. Mapping of the relatively continuous geologic structure in this region first required recognizing marker beds and contacts at or near the resolution of the Landsat sensor and then defining mappable units. Satellite image mapping was complemented by field study, which was necessary in this recently deformed area, both to verify the image mapping and to obtain more accurate structural dips. Deformation of the Cretaceous and Tertiary strata of the fold-and-thrust belt is, in the western half of the mapped area, conspicuously absent north of a buried basement fault that marks the hinge zone of the late Mesozoic passive margin. In the eastern half of the mapped area, however, the major thrusts (displacing rocks northward) have moved coherent sheets over the block fault. These sheets now lie flat atop the autochthon to the north. Tectonically, these observations indicate that (1) the crustal structure inherited from the Mesozoic extensional phase has strongly influenced the late Cenozoic pattern of deformation, (2) the thrust sheets that are displaced over the paleo-hinge zone actively load the basement rocks to the north, and (3) the development of the fold-and-thrust belt (which is markedly asymmetric) has included the progressive overlapping of thrusts. The later thrusts apparently formed internal to the older thrusts and subsequently overrode them.

Structure of the Vakhsh fold-and-thrust belt, Tadjik SSR: Geologic mapping on a Landsat image base

By evaluating seismological data from the belt of shallow intraplate seismicity known as the Gissar-Kokshal seismic zone in Soviet Central Asia in conjunction with recent data on the shallow crustal structure and longer-term geologic deformation, we show that certain size-classes of earthquakes are characteristic of adjacent (and in some places overlapping) geologic/tectonic settings; we call these settings seismic domains. They are both geologically and seismologically distinct; i.e., they contain different rocks and structures and have different frequency-magnitude relationships. For example, in terms of number of events, Gissar-Kokshal seismicity is dominated by the microearthquakes that have occurred within the sedimentary rocks of the Vakhsh fold/thrust belt. These are all shallow and have occurred mostly within the frontal portions of the thrust sheets. More rarely, moderate-size (M < 5.5) earthquakes have ruptured the thrust ramps. On the other hand, the major strain release in this region has accompanied the large (5.5 < M < 8.1) shallow earthquakes on basement faults in the southern Gissar range adjacent to the fold/thrust belt. Within these ranges there is relatively little microearthquake activity. That dichotomy indicates that a significantly different seismic hazard is associated with each of these areas, but the seismological distinction is obscured along the front of the fold/thrust belt, where the domain of small shocks in the sedimentary rocks overlaps the domain of large earthquakes on basement faults. To explain the localization of the large basement events along the southern flank of the fold/thrust belt adjacent to the fold/thrust belt, we hypothesize a mechanism of thrust sheet loading. Thrust loads are greatest in this region, since thrust sheets have overridden large basement block uplifts beneath the Illiac and Peter-the-First range faults.

Seismic domains within the Gissar‐Kokshal Seismic Zone, soviet central Asia

Although relatively little is known about basin formation north of the Himalayas, recent stratigraphic studies of several sedimentary basins (Tarim, Dzhungaria, Tsaidam) have demonstrated a common pattern of subsidence in post-Triassic times1. All these basins show rapid subsidence concurrent with the onset of Himalayan orogenesis, presumably reflecting both tectonic loading and increased sediment supply. Before that time, however, subsidence curves have the characteristic ‘concave-up’ shape of rifted-margin sedimentary environments2. Although data from these basins are not sufficiently detailed to justify a more thorough investigation, the Tadjik Depression, a sedimentary basin comprising the southwestern portion of the Soviet republic of Tadjikistan (Fig. 1), contains a continuous and relatively well documented record of post-Jurassic sedimentation, analysis of which suggests deposition in a rifted-margin environment, on crust thinned by ∼50%.

William Leith

Papers

Two types of reservoir-induced seismicity

The 1976 and 1984 Gazli, USSR, earthquakes—were they induced?

Structure of the Vakhsh fold-and-thrust belt, Tadjik SSR: Geologic mapping on a Landsat image base

Seismic domains within the Gissar‐Kokshal Seismic Zone, soviet central Asia

A mid-Mesozoic extension across Central Asia?