Continuous Displacement Occuring Along a Fault Without Generating Earthquakes Istermed

Dangers from Earthquakes and Faults

Charles E. Glass Ph.D., P.E. , in Interpreting Aerial Photographs to Identify Natural Hazards, 2013

5.4.2 Recognizing Reverse Faults

Reverse fault displacements combine vertical and compressional displacements. Reverse faults typically have a wide range of dip angles. Reverse faults with low dips exhibit a sinuous surface expression over hilly to flat terrain. Reverse fault scarps are often difficult to locate precisely due to widespread landslides which cover the fault trace. Surface fracturing is characterized by folding or distortion and displacements along subsidiary faults and fractures within the hanging wall ( Figure 5.12A).

Figure 5.12. (A) Reverse faults display severe damage in the form of landslides over the fault trace caused by the inability of the hanging wall to support the overhang caused by the fault displacement, folds, and compression features within the fractured hanging wall, and compressional block tilting. (B) Thrust fault scarp due to rupture causing the 1968 magnitude 6.9 Meckering WA, Australia earthquake.

Photograph courtesy of Ian Everingham, Peter Gregson, the West Australian newspaper, Alice Snocke, and Wayne and Brenden Peck.

Fracturing and ground instability on the hanging wall of a reverse fault is commonly spread over a wide area (tens of miles in some cases), more than is common with normal-slip and strike-slip faults due mostly to the amount of frictional forces involved, which in turn partition or disperse stresses over greater distances depending generally on rates of movement, the rock types involved, and the presence or absence of fluids.

Figure 5.12B shows a thrust fault rupture resulting in the 1968 magnitude 6.9 Meckering WA, Australia (approximately 130   km east of Perth) earthquake. Note on Figure 5.12B the wide area of damage due to secondary scarps and slumps on the hanging wall of the fault (left on the photograph). The earthquake caused ground rupture of nearly 40   km, with a 2.4   m vertical offset and a 1.5   m horizontal offset. Although the town of Meckering was destroyed during the earthquake, none of its citizens was killed.

The main geomorphic features of reverse-slip faults are listed in Table 5.2.

Table 5.2. Value of Different Image Scales ^a for Recognizing Landforms of Reverse Faults

Geomorphic Feature	Synoptic Scale/Topographic Maps	Intermediate Scale	Large Scale
Scarps	Yes for large scarps	Yes	Yes
Blunting or oversteepening at foot of mountain front	Yes	Yes	Yes
Talus or landslide alignments	Possible/Doubtful	Yes	Yes
Mole-track traces	No	Probably	Yes
Graben or fissure swarms on hanging wall	No	Yes	Yes
Upstream terraces	No	Perhaps if large	Yes
Drag warping of footwall fans, terraces, or sediment	No	Doubtful	Yes
Sinuous traces on flat surfaces	Possible if large/No	Doubtful	Yes
Volcanic features	Yes	Yes	Yes
Mountain front embayment	Yes	Yes	Yes
Canyon ellipticity	Yes	Yes	Yes
Valley ratios	Doubtful/Yes	Yes	Yes

a

Synoptic scale≤1:75,000; 1:25,000≥Intermediate scale≥1:75,000; Large scale≥1:25,000.

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The growth of faults

Andrew Nicol , ... Tom Manzocchi , in Understanding Faults, 2020

6.2.1 Conceptual 'ideal isolated fault' model

Variations in fault displacements over normal and reverse fault surfaces have been documented from seismic reflection and coal-mine datasets ( Rippon et al., 1985; Barnett et al., 1987; Gillespie, 1991; Nicol et al., 1996; Torabi et al., 2019). These data help define displacements on the "ideal isolated" fault which has elliptical displacement contours parallel to the tip-line loop and concentric about a centrally located maximum (Figs 6.2A and 6.4A and B; Barnett et al., 1987; Walsh and Watterson, 1987). As is the case for the shapes of fault surfaces, the geometries and spacings of displacement contours (i.e., displacement gradients) typically depart from the ideal for a range of reasons which are considered below. The displacement contour patterns for the isolated fault and more complex fault surfaces shown in Fig. 6.4 highlight, for example, that the maximum displacements are often not located centrally on the fault surface and, as a consequence, displacement gradients can be variable. Furthermore, faults that terminate along their branch-line (i.e., intersection line) with other faults (i.e. those that are physically linked, or hard-linked, to others) can have displacement contours that are either locally sub-parallel to the branch-line along which the throw is low or zero (Fig. 6.4C), or trend at a high angle to the branch line along which the maximum throw is located (Fig. 6.4D; Yielding, 2016). Despite these caveats, the concept of an 'ideal isolated' fault and its characteristics are valuable for understanding fault geometry, processes and evolution (Barnett et al., 1987; Walsh and Watterson, 1987).

Fig. 6.4. Selection of fault surface displacement diagrams for hard- and soft-linked normal faults: hard-linked faults are physically linked with another fault, whereas soft-linked faults transfer displacement without physical linkage at the scale of observation. (A) Displacement contour diagram for a fault constructed using 3D seismic data from the North Sea (modified from Fig. 3 of Barnett et al., 1987). Contours approximate those of the ideal isolated fault. (B) Displacement contour diagram of an equant or penny shaped fault surface from the Gulf of Mexico (Fig. 1 of Nicol et al., 1996). (C) Displacement contour diagram for a fault, imaged in a 3D seismic reflection survey in the Timor Sea. The contoured fault abuts a contemporaneous antithetic fault along a sub-horizontal branch-line (Fig. 10 in Nicol et al., 1996). (D) Displacement contour diagram for a fault abutting a contemporaneous antithetic fault along a sub-vertical branch-line, with a second fault in the hangingwall of the contoured fault surface (tip line indicated by dashed line). Fault from a 3D seismic reflection dataset in the Timor Sea (Fig. 8 in Nicol et al., 1996). (E) Partial displacement contour diagram from the Derbyshire coalfields constructed from coal-seam throws. Displacement low coincides with weak mudstones and seatearth layers (Fig. 6 in Rippon, 1985). (F) Displacement contour diagram from the Derbyshire coalfields constructed from coal-seam throws (Fig. 3 in Rippon, 1985). Propagation of the lower tip-line of this fault is interpreted by Rippon to have been retarded by a thick sandstone bed. Small black filled circles on the contour diagrams show the locations of displacement measurements. Large black filled circles labelled MD show the location of the maximum displacement.

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Tectonic Geomorphology

D. Nash , in Treatise on Geomorphology, 2013

Abstract

Scarps may be produced by all types of fault displacement. Although most fault scarps have a complex initial morphology, a scarp produced by normal faulting of unconsolidated or poorly consolidated frictional materials (i.e., sands and gravels) may have a relatively simple initial morphology consisting of a detachment-limited free face, sloping around 60°, which retreats back, progressively burying its base with raveled debris. Once the free-face burial is complete, the scarp crest and base become more rounded and the scarp reclines provided the scarp remains transport limited and erosional processes are only a function of the gradient. In some situations, scarp age may be determined by morphologic dating.

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Earthquake Seismology

G.C.P. King , M.H. Devès , in Treatise on Geophysics (Second Edition), 2015

4.10.2.2 2D Case: Coulomb Stress on a Plane of Specified Orientation

In a system where the x- and y -axes and fault displacements are horizontal and fault planes are vertical (containing the z direction), stress on a plane at an angle ψ from the x-axis (see Figure 2 ) resulting from a general stress field of any origin is given by:

Figure 2. Map view of Coulomb stress changes caused by a right lateral strike-slip fault in plane strain. (a) Coseismic shear, normal, and Coulomb stress changes for right-lateral fault parallel to the master fault. (b) Shear, normal, and Coulomb stress changes on optimally oriented faults (black and white segments) with respect to a regional stress characterized by a N7°   E (open arrows) compression of 100   bar. The slip distribution on the fault plane is uniform, but it tapers at the fault ends. The effective friction coefficient μ′ is taken to be 0.4.

Redrawn after King GCP, et al. (1994b). Static stress changes and the triggering of earthquakes. Bulletin of the Seismological Society of America 84: 935–953.

[7] $\begin{array}{l}{\sigma }_{11}={\sigma }_{\mathit{xx}}{cos}^2\psi +2{\sigma }_{\mathit{xy}}sin\psi cos\psi +{\sigma }_{\mathit{yy}}{sin}^2\psi \\ {\sigma }_{33}={\sigma }_{\mathit{xx}}{sin}^2\psi -2{\sigma }_{\mathit{xy}}sin\psi cos\psi +{\sigma }_{\mathit{yy}}{cos}^2\psi \\ {\tau }_{13}=\frac{1}{2}\left({\sigma }_{\mathit{yy}}-{\sigma }_{\mathit{xx}}\right)sin2\psi +{\tau }_{\mathit{xy}}cos2\psi \end{array}$

where these relations hold for a left-lateral mechanism (τ ₁₃  = τ ₁₃ ^L ) and similar expressions can be derived for a right-lateral mechanism (as stated in eqn [3]). The Coulomb stress for left-lateral C _f ^L and right-lateral C _f ^R motion on planes orientated at ψ with respect to the x-axis can now be written in the following way:

[8a] $C_{\mathrm{f}}^{\mathrm{L}}={\tau }_{13}^{\mathrm{L}}+{\mu }^{\prime }{\sigma }_{33}$

[8b] $C_{\mathrm{f}}^{\mathrm{R}}={\tau }_{13}^{\mathrm{R}}+{\mu }^{\prime }{\sigma }_{33}$

Equation [8b] is illustrated in Figure 2(a) using a dislocation (earthquake fault) source. An elliptical slip distribution is imposed on the (master) fault in a uniform, stress-free, elastic medium. The contributions of the shear and normal components to the failure condition and the resulting Coulomb stresses for infinitesimal faults parallel to the master fault (commonly referred to as target faults) are shown in separate panels. Such a calculation represents the Coulomb stress on these planes resulting from slip on the master fault (i.e., the figure shows a map of stress changes) and is independent of any knowledge of the prevailing regional stresses or any preexisting stress fields from other events. The signs in the calculation are chosen such that a positive Coulomb stress indicates a tendency for slip in the same right-lateral sense as the fault of interest. Negative Coulomb stresses indicate a reduction of this tendency. It is important to appreciate that because τ ₁₃ changes sign between eqns [8a] and [8b], a negative Coulomb stress for right-lateral fault motion is not the same as a tendency for left-lateral slip.

The distribution of increases and decreases of Coulomb stress shows features common to all subsequent figures. Lobes of increased shear stress appear at the fault ends, corresponding to the stress concentrations that tend to extend the fault. Off-fault lobes also appear on either side of the fault, separated from the fault by a region where the Coulomb stresses have not been increased. If the master fault was infinitesimal in length and thus behaving as a point source, all four lobes would be equal in amplitude to the fault-end lobes at all distances. The normal stress field is similar to the more familiar dilatational field with maxima and minima distributed antisymmetrically across the fault, but we consider only the component of tension normal to the fault. The influence of the normal stress on the Coulomb stress distribution is to reduce the symmetry of the final distribution and to increase the tendency for off-fault failure.

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Remote Sensing and GIScience in Geomorphology

S. Kruse , in Treatise on Geomorphology, 2013

3.5.6.3.1 Faulting

Seismic reflection imaging of strata offset at faults has been a standard method of recovering fault displacement and geometry for many decades. Near-surface capabilities are highlighted here with a high-quality case study of the transpressive Alpine Fault Zone, New Zealand (Kaiser et al., 2011) ( Figure 11). The imaging of a Pleistocene erosional basement–surface horizon provides the means to estimate late Quaternary slip rates on a steeply dipping dominant fault strand (AF in Figure 11). Although only a single fault scarp is visible at the surface, deformation is inferred to extend over a 60-m-wide zone within the fault zone, encompassing two minor fault strands (SF1 and SF2 in Figure 11) that were first imaged in shallow GPR studies (McClymont et al., 2010). The detailed seismic imaging of strata truncations, rotations, and tilting was achieved through dense data coverage and a carefully tailored processing scheme (Kaiser et al., 2011). Of particular importance in processing were corrections for the severe static shifts and strong source-generated noise that typically complicate shallow seismic data. The imaging of complex dipping as well as diffracted and crossing events required careful velocity analyses, dip-moveout corrections, and 3-D migration.

Figure 11. High-resolution 3-D land-based seismic reflection imaging of the extent of deformation structures within the shallow expression of active faults in the transpressive Alpine Fault Zone, New Zealand. (a) Location map. (b) Local geology. The authors image a steeply dipping dominant fault strand (AF) that significantly offsets the late Pleistocene erosional basement surface. (c) Seismic reflection image. (d) Interpreted seismic reflection image. Although only a single fault scarp was visible at the surface, deformation was inferred to extend over a 60-m-wide zone within the fault zone, encompassing two minor fault strands (SF1 and SF2) that were first imaged in shallow GPR studies (McClymont et al., 2009). Modified from Kaiser, A.E., Horstmeyer, H., Green, A.G., Campbell, F.M., Langridge, R.M., McClymont, A.F., 2011. Detailed images of the shallow Alpine Fault Zone, New Zealand, determined from narrow-azimuth 3D seismic reflection data. Geophysics 76, B19–B32, with permission from AGU and A.E. Kaiser (personal communication).

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Tsunamis in the Global Ocean

Gerassimos Papadopoulos , in Tsunamis in the European-Mediterranean Region, 2016

1.3 Tsunami sources and generation mechanisms

Earthquake activity is the most frequent cause of tsunamis. During the occurrence of a strong earthquake, the coseismic fault displacement at the sea bottom pushes upward the seawater column. Then, the displaced water mass collapses due to gravity and the tsunami is generated as a gravity wave that propagates outward from its source. The fault displacement and the dimensions of the fault segment involved in the tsunami generation determine the tsunami size at the source. The complexity of the seismic rupture is also important. In fact, the initial size of the tsunami wave may vary with the homogeneous or heterogeneous rupture along the fault plane ( Geist and Dmowska, 1999) as well as with the friction pattern during the rupture (Bilek and Lay, 1999).

However, the concurrence of several geophysical factors is needed for the generation of a tsunami, such as shallow earthquake focus (focal depth less than 100 km) and large magnitude (usually no less than about 6.5) of the causative earthquake. Also, the focal mechanism plays an important role. In fact, the dip-slip type of seismic faulting (normal or reverse) favors tsunami generation since it involves a significant vertical component in the coseismic fault displacement. The largest earthquakes and tsunamis occur in active zones of lithospheric subduction where the earthquake focal mechanism is predominantly reverse faulting (Figure 1.2). Strike-slip ruptures, where the horizontal component of fault motion dominates, does not exclude but disfavor tsunami generation. For example, in the Aegean area, the large earthquake (magnitude M = 7.5) that ruptured in the area of Cyclades Islands, South Aegean Sea, on July 9, 1956 was associated with submarine normal faulting, which is believed to have favored the production of a large tsunami with amplitude in the near-field domain of up to about 15 m (Papadopoulos and Pavlides, 1992; Beisel et al., 2009; Okal et al., 2009; see more details in Chapter 4). On the other hand, in the North Aegean Sea where the strike-slip component dominates in the seismic ruptures, none of the large earthquakes of February 20, 1968 (M = 7.1), December 19, 1981 (M = 7.2), January 18, 1982 (M = 7.0), and May 24, 2014 (M = 6.9) caused tsunamis.

Figure 1.2. Tsunami generation due to coseismic seabed dislocation (Takahashi, 2006).

Several mechanisms can be recognized in the tsunami generation during volcanic eruptions. They may include volcanic earthquakes, caldera or cone collapse, pyroclastic flows, and more (Latter, 1981). For the large Minoan tsunami caused by the Late Bronze Age eruption of Thera (Santorini) volcano by the end of the seventeenth century BC, two main mechanisms were proposed and tested by numerical simulations: caldera forming collapse of the volcanic cone and massive pyroclastic flows rolling down the volcanic cone seaward (Minoura et al., 2000; Pareschi et al., 2006a; Novikova et al., 2011). The second mechanism can be considered as a particular type of volcanic landslide. A more conventional case of volcanic landslide is the one where the volcanic activity triggers the landslide of unstable masses of volcanic and/or other rocks. This happened with the volcanic activity in Stromboli volcano, Aeolian Islands, Italy, on December 30, 2002. The local tsunami produced had a height of ca. 9 m and caused some damage only to outdoor and indoor property (Tinti et al., 2005c). However, coastal or submarine landslide in nonvolcanic areas is also a well-known mechanism for tsunami generation. Such landslides may be due to seismic activity or only to the gravity force. It is assumed that in all these mechanisms the seawater is abruptly displaced and then collapses creating gravity sea wave.

In the European–Mediterranean region, all the earlier discussed different tsunami generation mechanisms have been recognized. Therefore, it is of value to introduce some terminology with the aim of better describing and distinguishing between such mechanisms. Here I follow the suggestion I made several years ago as regards this particular issue (Papadopoulos, 1993a) (Table 1.2). The term seismic tsunami refers to tsunami generation that results from coseismic fault displacement of the sea floor. An earthquake that generates tsunamis with this mechanism is called a tsunamigenic earthquake (Figure 1.3). However, this term should not be confused with the term tsunami earthquake in the terminology introduced by Kanamori (1972) to characterize the 1992 Nicaragua earthquake source and other earthquakes whose tsunamis were disproportionately large with respect to their size measured either by seismic moment or by magnitude. Any other mechanism of tsunami generation is nonseismic. This term, however, may refer to two alternatives. The first includes an earthquake as only a triggering factor, for example, of a coastal or submarine landslide (Figure 1.4) or of the collapse of a submarine volcano because of the earth shaking. Such a mechanism of tsunami generation is called pseudoseismic since no coseismic fault displacement is involved. However, when the landslide occurs without any seismic triggering then the mechanism of tsunami generation is purely aseismic.

Table 1.2. Classification of tsunamis according to their generation mechanism

Seismic tsunamis: produced by tsunamigenic earthquakes (mechanism: coseismic fault displacement)
Nonseismic tsunamis: without direct involvement of earthquake activity
Aseismic tsunamis: due to landslides, volcanic activity, or other causes without the involvement of earthquake activity	Pseudoseismic tsunamis: due to landslides or volcano collapse due to earthquake activity acting only as a triggering factor

Source: After Papadopoulos (1993a).

Figure 1.3. Schematic diagram of tsunamigenic mechanism from pyroclastic flow at the slope of a volcanic cone with plume entering the water (Novikova et al., 2011, redrafted from Watts and Waythomas, 2003 and Walder et al., 2003).

Figure 1.4. Tsunamigenic landslide mass released either at the shore or at a height, H, above water level accelerates down a steep slope and only decelerates when it reaches the bottom at depth h (Novikova et al., 2011, redrafted from Watts and Waythomas, 2003 and Walder et al., 2003).

For the caldera collapse tsunamigenic mechanism of the Santorini (Thera) LBA eruption, Novikova et al. (2011) considered a dynamic series of landslides.

Large meteorites that may impact the ocean should not be ruled out as possible agents of tsunami generation. It is suggested that this happened with the very large impact-induced tsunami that occurred at Chicxulub, Mexico, at the Cretaceous-Tertiary boundary around 65 million years ago and possibly was associated with the extinction of the dinosaurs. However, such impacts are quite rare. Also, one should not neglect anthropogenic actions that may result in tsunami production, for example, submarine nuclear bomb testing.

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Other Unconventional Petroleum Resources

Caineng Zou , in Unconventional Petroleum Geology (Second Edition), 2017

2.4.3.1.3 Fault Structural Zones near the Faulted Trough Capable of Generating Hydrocarbons are Enrichment Area of Gas in the Faulted Depression

Major basement faults control the distributions of volcanic reservoirs along with the formation of volcanic traps that are usually distributed along the faults. Variation in fault displacement or striking lead to the development of noselike structures in the down-dropped wall and stratigraphic overlapping, unconformities, and buried hill traps in the uplifted wall; inversion activities can also form reversal structures, etc. in the down-dropped wall. Faults are significant pathways for oil and gas migration. Meanwhile, fractures around fault zones can connect pores in volcanic reservoirs, enlarging the effective pore spaces. Fractures are permeable pathways for groundwater and induce the development of secondary dissolved pores and fractures, which improve the reservoir properties. Major basement faults are favorable for oil and gas migration and improve the reservoir properties. Consequently, fault-structure zones control the gas accumulation in faulted depressions and therefore are the enrichment area for natural gas. The gas pools in the Xujiaweizi faulted depression are distributed along major basement faults (the Xuxi, Xuzhong, and Songzhan faults). The Changling 1 gas field is also located near major basement faults.

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Thermochronological constraints on Cenozoic exhumation along the southern Caribbean: The Santa Marta range, northern Colombia

Ana María Patiño , ... Sebastián Echeverri , in Andean Tectonics, 2019

Effect of topographic change

Our 3D thermokinematic reconstruction also constrains the magnitude and age of topographic amplification allowed by the data. First, despite the topographic amplification being subordinate to fault displacement for generating good fits ( Fig. 8E), a greater amount of good models occurs for topographic amplification of 50% (Fh   =   0.5) and 70% (Fh   =   0.3). More importantly, had this amplification occurred, it must have started before 7   Ma (Fig. 8F). The observed bedrock and detrital data do not allow recent (i.e., post late Miocene) relief growth.

A spatial analysis of modeling results is facilitated by visual comparison along a profile of measured bedrock and detrital ages with modeled ages derived from the best-fit model. First, measured bedrock data from all three thermochronometers are fairly well reproduced by the best bedrock model (i.e., that with highest B LogL) throughout a transect perpendicular to our modeled faults (Fig. 8H). A closer look at this comparison shows that data from the northwestern corner are better reproduced than data farther to the SE. We ascribe this result to either (1) having implemented the model with a constant gradient of 70% in vertical displacement (the velocities in the easternmost sector of the range should have been lower, which should result in older cooling ages) or (2) potential kinetic variability in apatite fission-track formation not captured in a model with homogeneous kinetic parameters (Dpar). Second, a comparison of measured detrital ages in the Guatapurí catchment with modeled ages obtained from the best-fit model for detrital data (i.e., that with highest DLogL) illustrate a fair fit of median and mean values for both AHe (Fig. 8J) and AFT ages (Fig. 8K), and a poorer fit of the tails of the measured distribution of AFT ages. We ascribe this latter pattern to the natural variability of single-grain AFT ages, and/or to annealing kinetic variability not captured by the single-kinetic AFT modeled ages.

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Earthquake Seismology

L. Grant , in Treatise on Geophysics, 2007

4.19.2.1.2 Application of geochronology

Geochronology is dating of earth materials, surfaces, and processes. Geochronology is essential for paleoseismology because it constrains dates of paleoearthquakes and average rates of fault displacement. The most useful geochronologic methods for paleoseismic investigations yield high-resolution ages for common, Late Quaternary materials such as soils or buried flora and fauna ( Lettis and Kelson, 2000). Table 1 (from Noller et al. (2000a)) summarizes geochronologic methods for dating Quarternary materials in fault zones. A detailed discussion of this complex topic is beyond the scope of this chapter. A comprehensive compilation and summary of Quaternary geochronologic methods and applications is provided by Noller et al. (2000b). The most commonly used methods are also described by McCalpin (1996) and Yeats et al. (1997). Recent developments in cosmogenic surface exposure dating methods are beginning to be widely applied in paleoseismology to determine the age of offset features for slip-rate measurements (e.g., van der Woerd et al., 2006).

Table 1. Classification of Quaternary geochronologic methods

Sidereal	Isotopic	Radiogenic	Chemical and biologic	Geomorphic	Correlation
Dendrochronology	Radiocarbon	Fission track	Amino-acidracemization	Soil profile development	Stratigraphy
	Uranium series	Thermoluminescence	Rock-varnish cation ratio	Rock-varnish development	Paleomagnetism
Varve chronology	210Pb	Optically stimulated luminescence	Obsidian and tephra hydration	Scarp morphology and landform modification	Tephrochronology
	U–Pb Th–Pb				Paleontology
Historical records		Electron-spin resonance	Soil chemistry	Rate of deformation	Tectites and microtectites
	K–Ar and 39Ar–40Ar		10Beaccumulation in soils	Rate of deposition	Climate correlation
Sclerochronology and growth rings	Cosmogenic isotopes	Infrared stimulated luminescence	Lichenometry	Rock and mineral weathering	Astronomical correlation
					Stable isotopes
				Geomorphic position	Archeology

Source: Adapted from Noller JS, Sowers JM, Colman SM, and Pierce KL (2000a) Introduction to quaternary geochronology. In: Noller JS, Sowers JM, and Lettis WR (eds.) AGU Reference Shelf Series 4: Quaternary Geochronology: Methods and Applications, pp. 1–10. Washington, DC: American Geophysical Union.

Radiocarbon dating is the most widely used method for dating Holocene and latest Pleistocene earthquakes. The half-life of radioactive ¹⁴C (5730 years) limits the application of radiocarbon dating to organic matter formed from carbon fixed within the last 50   000 to 60   000 years (Trumbore, 2000). The amount of ¹⁴C in atmospheric CO₂ has varied in the past, particularly in the last few centuries due to anthropogenic emissions. To compensate for this variation, radiocarbon ages are calibrated to correspond to calendar ages (absolute ages). Calibrations based on tree rings and glacial varves extend back to the Early Holocene. Calibration curves are not linear. Plateaus in the calibration curves limit the precision of radiocarbon dating (Trumbore, 2000). This problem is acute for the last few centuries. For example, Yeats and Prentice (1996) note that the two largest historic ruptures of the San Andreas Fault in California, which occurred in 1857 and 1906, are indistinguishable using radiocarbon dating.

All methods listed in Table 1 have limitations and uncertainties. Uncertainty in geochronologic methods is a major source of uncertainty in paleoseismic data (Lettis and Kelson, 2000). In addition to uncertainty in calibration and accuracy of analysis, errors may be introduced in the selection, collection, and interpretation of field samples. Therefore, most paleoseismic studies employ multiple methods of dating to reduce uncertainty and cross-check results. Methods such as radiocarbon dating that yield accurate, high-precision ages of common or widely distributed materials are preferred. Recent improvements in geochronology methods and in the statistical treatment of dates have reduced uncertainties in previously published ages (e.g., Sieh et al., 1989; Biasi and Weldon, 1994; Biasi et al., 2002).

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Earthquake Seismology

A. Nur , in Treatise on Geophysics (Second Edition), 2007

4.22.2.1.1 Fault displacement

It is well known that earthquakes are the result of sudden displacement on faults in the Earth's crust. When earthquakes are large enough and shallow enough, that fault displacement may break the Earth's surface, resulting in ground displacement, which may cut through any structures that straddle the fault trace. The fault scarp described beneath the Mycenaean walls is unmistakable evidence that large earthquakes have occurred repeatedly in Mycenae. However, the fact that the wall was built atop the scarp only tells us that the earthquakes that formed the scarp predate the wall and that later earthquakes that activated the fault were insufficient to topple that wall (or the wall was subsequently repaired).

In tells, it is sometimes possible to bracket an earthquake between two successive layers of occupation. If a fault cuts and displaces the remains of one wall, but an undeformed wall has been built over the fault in the next layer, we know that the earthquake happened sometime between the building of the first wall and the building of the second.

An example of this is from the Crusader fortification of Ateret, by the banks of the Jordan River in northern Israel. The foundations of the fortress' walls have been displaced horizontally by motion on a fault that cuts through the walls; thus, the displacement must have occurred after the walls' construction in 1178 (and destruction by Saladin a year later). An early modern Muslim structure built within the ruins is also displaced, but whereas the Crusader walls are displaced by 2.1   m across the fault, the walls of the Muslim structure (dated somewhere during the Ottoman Turkish period of 1517–1917) are displaced by only 0.5   m. Trenches dug across the zone of deformation indicate that the slip occurred during several earthquakes (Ellenblum et al., 1998). By combining trench data with current geodetic measurements to rule out aseismic slip, Ellenblum et al. (1998) were able to determine that this fault is currently locked and that the displacement that has accumulated in the ruins has been accommodated by at least two large earthquakes. Thus, their research has contributed information toward the seismic risk estimates in the region, a region that includes heavily populated areas of Jordan, Israel, and Syria.

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