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9: Crustal Deformation and Earthquakes - Geosciences

9: Crustal Deformation and Earthquakes - Geosciences


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Learning Objectives

  • Differentiate between stress and strain
  • Identify the three major types of stress
  • Differentiate between brittle, ductile, and elastic deformation
  • Describe the geological map symbol used for strike and dip of strata
  • Name and describe different fold types
  • Differentiate the three major fault types and describe their associated movements
  • Explain how elastic rebound relates to earthquakes
  • Describe different seismic wave types and how they are measured
  • Explain how humans can induce seismicity
  • Describe how seismographs work to record earthquake waves
  • From seismograph records, locate the epicenter of an earthquake
  • Explain the difference between earthquake magnitude and intensity
  • List earthquake factors that determine ground shaking and destruction
  • Identify secondary earthquake hazards
  • Describe notable historical earthquakes

Crustal deformation occurs when applied forces exceed the internal strength of rocks, physically changing their shapes. These forces are called stress, and the physical changes they create are called strain. Forces involved in tectonic processes as well as gravity and igneous pluton emplacement produce strains in rocks that include folds, fractures, and faults. When rock experiences large amounts of shear stress and breaks with rapid, brittle deformation, energy is released in the form of seismic waves, commonly known as an earthquake.

  • 9.1: Stress and Strain
    Stress is the force exerted per unit area and strain is the physical change that results in response to that force. When the applied stress is greater than the internal strength of rock, strain results in the form of deformation of the rock caused by the stress. Strain in rocks can be represented as a change in rock volume and/or rock shape, as well as fracturing the rock. There are three types of stress: tensional, compressional, and shear.
  • 9.2: Deformation
    When rocks are stressed, the resulting strain can be elastic, ductile, or brittle. This change is generally called deformation. Elastic deformation is a strain that is reversible after the stress is released. For example, when you stretch a rubber band, it elastically returns to its original shape after you release it. The type of deformation a rock undergoes depends on pore pressure, strain rate, rock strength, temperature, stress intensity, time, and confining pressure.
  • 9.3: Geological Maps
    Geologic maps are two dimensional (2D) representations of geologic formations and structures at the Earth’s surface, including formations, faults, folds, inclined strata, and rock types. Formations are recognizable rock units. Geologists use geologic maps to represent where geologic formations, faults, folds, and inclined rock units are. Geologic formations are recognizable, mappable rock units.
  • 9.4: Folds
    Geologic folds are layers of rock that are curved or bent by ductile deformation. Folds are most commonly formed by compressional forces at depth, where hotter temperatures and higher confining pressures allow ductile deformation to occur. Folds are described by the orientation of their axes, axial planes, and limbs. There are many types of folds, including symmetrical folds, asymmetrical folds, overturned folds, recumbent folds, and plunging folds.
  • 9.5: Faults
    Faults are the places in the crust where brittle deformation occurs as two blocks of rocks move relative to one another. Normal and reverse faults display vertical, also known as dip-slip, motion. Dip-slip motion consists of relative up-and-down movement along a dipping fault between two blocks, the hanging wall, and footwall. In a dip-slip system, the footwall is below the fault plane and the hanging wall is above the fault plane.
  • 9.6: Earthquake Essentials
    Earthquakes are felt at the surface of the Earth when energy is released by blocks of rock sliding past each other, i.e. faulting has occurred. Seismic energy thus released travels through the Earth in the form of seismic waves. Most earthquakes occur along active plate boundaries. Intraplate earthquakes (not along plate boundaries) occur and are still poorly understood. The USGS Earthquakes Hazards Program has a real-time map showing the most recent earthquakes.
  • 9.7: Measuring Earthquakes
    People feel approximately 1 million earthquakes a year, usually when they are close to the source and the earthquake registers at least moment magnitude 2.5. Major earthquakes of moment magnitude 7.0 and higher are extremely rare. The U. S. Geological Survey (USGS) Earthquakes Hazards Program real-time map shows the location and magnitude of recent earthquakes around the world.
  • 9.8: Earthquake Risk
    Earthquake magnitude is an absolute value that measures pure energy release. Intensity, however, i.e. how much the ground shakes, is determined by several factors. In general, the larger the magnitude, the stronger the shaking and the longer the shaking will last. Shaking is more severe closer to the epicenter. The severity of shaking is influenced by the location of the observer relative to the epicenter, the direction of rupture propagation, and the path of greatest rupture.
  • 9.9: Case Studies
    This section contains multiple case studies of major earthquakes in North America as well as historically important earthquakes in the whole world.

Thumbnail: Epicenter of the 2010 Chile earthquake with a collapsed building in Concepción. (CC-SA-BY; Claudio Núñez).

Summary

Geologic stress, applied force, comes in three types: tension, shear, and compression. Strain is produced by stress and produces three types of deformation: elastic, ductile, and brittle. Geological maps are two-dimensional representations of surface formations which are the surface expression of three-dimensional geologic structures in the subsurface. The map symbol called strike and dip or rock attitude indicates the orientation of rock strata with reference to north-south and horizontal. Folded rock layers are categorized by the orientation of their limbs, fold axes and axial planes. Faults result when stress forces exceed rock integrity and friction, leading to brittle deformation and breakage. The three major fault types are described by the movement of their fault blocks: normal, strike-slip, and reverse.

Earthquakes, or seismic activity, are caused by sudden brittle deformation accompanied by elastic rebound. The release of energy from an earthquake focus is generated as seismic waves. P and S waves travel through the Earth’s interior. When they strike the outer crust, they create surface waves. Human activities, such as mining and nuclear detonations, can also cause seismic activity. Seismographs measure the energy released by an earthquake using a logarithmic scale of magnitude units; the Moment Magnitude Scale has replaced the original Richter Scale. Earthquake intensity is the perceived effects of ground shaking and physical damage. The location of earthquake foci is determined from triangulation readings from multiple seismographs.

Earthquake rays passing through rocks of the Earth’s interior and measured at the seismographs of the worldwide Seismic Network allow 3-D imaging of buried rock masses as seismic tomographs.

Earthquakes are associated with plate tectonics. They usually occur around the active plate boundaries, including zones of subduction, collision, and transform and divergent boundaries. Areas of intraplate earthquakes also occur. The damage caused by earthquakes depends on a number of factors, including magnitude, location and direction, local conditions, building materials, intensity and duration, and resonance. In addition to damage directly caused by ground shaking, secondary earthquake hazards include liquefaction, tsunamis, landslides, seiches, and elevation changes.


Tracking Stress Buildup and Crustal Deformation

The constant plate tectonic motions between the Pacific and North American plates guarantees that the crust in the western US is continually building up stress.

Crustal deformation refers to the changing earth’s surface caused by tectonic forces that are accumulated in the crust and then cause earthquakes.

Overview | Tracking Stress Buildup and Crustal Deformation | Fault Slip Rates and Post-Earthquake Motions

Tracking Stress Buildup

Stressing rate of the crust around California derived from two decades of geodetic measurements. (Public domain.)

The constant plate tectonic motions between the Pacific and North American plates guarantees that the crust in the western US is continually building up stress. The image of crustal velocities provided by extensive GPS coverage reveals where these velocities change rapidly over short distances, demanding that the intervening crustal rock stretch and build up stress over time. Such a map of the stress reveals two main lines where stress is concentrated: The San Andreas fault zone and the Eastern California Shear Zone. These zones have experienced numerous earthquakes over the century and a half that earthquakes have been historically observed.

The mechanism of stress buildup within these fault zones is uncertain. One hypothesis is that the hot rocks below the upper 15-km-thick layer (the upper crust that has the vast majority of continental earthquakes) flows continually in response to periodic earthquakes, forcing the upper crust to bend with this flow. Another hypothesis is that slip of the deeper continuation of faults, steady slip that doesn’t produce earthquakes but still involves motions across the fault, forces the upper crust around the faults to bend and thus concentrate stress. Both hypotheses are the subject of active research. But the fact remains that high stressing rates observed on the surface likely translate to high stressing rates at the depths (

10 km) where earthquakes typically nucleate, so these stressing rates are a guide to the seismic hazard.

Crustal Deformation

Crustal deformation refers to the changing earth’s surface caused by tectonic forces that are accumulated in the crust and then cause earthquakes. So understanding the details of deformation and its effects on faults is important for figuring out which faults are most likely to produce the next earthquake. There are several hypotheses about how this works, but more data is needed to determine which one is the best.

Crustal deformation is a heavily data driven field. To measure the motions of earth’s surface, the USGS employs a variety of methods, including LIDAR, the Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), creepmeters, and alinement arrays. In parts of the U.S. with few or no historically-recorded major earthquakes or where background seismicity is sparse, geodetic data may provide the only insight into present-day seismic hazard. The motions captured by these diverse measurement techniques provide vital information on:

Map depicting crustal deformation instruments deployed in the San Francisco Bay Area. (Public domain.)


9: Crustal Deformation and Earthquakes - Geosciences

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Knowledge of seismotectonics, active deformation, and the structure of the Earth&rsquos crust is a key for the first order perception and assessment of the seismic hazard, and consequently the seismic risk, of an area. It is moreover essential for understanding the geodynamics and on-going surface processes (i.e., erosion, sedimentation, etc.), and for the exploration and management of georeservoirs for energy and waste. This book aims to reveal comprehensive images of shallow earthquake physics, active tectonics, crustal structures, and deformation, at continental, regional and local scales. The implementation of modern theories, the introduction of innovative methods, the application of cutting-edge computational and mapping tools to longstanding problems in tectonics and seismology, and the presentation of comprehensive data analyses representing various geodynamic regimes are welcome. More specifically, we encourage the submission of papers concerning topics (and combinations of) such as

  • Theoretical concepts
  • Methods of seismological, geological, and geodetic data analyses
  • Stress and strain models for actively deforming regions of the Earth
  • Physics-based constraints of significant seismic sources
  • Induced seismicity and physics of induced earthquakes
  • Mechanisms/models of earthquakes nucleation and the role of crustal fluids
  • Combination of field observations with analyses and geophysical interpretations with the aim of understanding how active continental margins are deformed
  • Advances in the relocation of shallow earthquake sequences and in moment tensor inversion
  • New geodesy and geomatics tools (GNSS, UAV etc.) for the observation and monitoring of our planet
  • Advances in understanding the mechanics of geophysical/geodynamical processes monitoring of ground shaking and displacement during earthquakes, and for contributions to tsunami early warning tracking real-time motion of landslides and the safety of structures
  • Development of new satellite image processing algorithms for smart value-added products and datasets such as DTM/DSM models to study and evaluate terrain and topographic surface risks and processes
  • Local and regional seismic tomography
  • Societal benefits of such studies and their contributions to the resilience of megacities.

Prof. Ioannis Kassaras
Dr. Athanassios Ganas
Dr. Paolo Pace
Guest Editors

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Post-seismic deformation

Using a long-term GPS time series after the earthquake, we can also examine the post-seismic deformation process. As an example, Figure 4 shows the deformation at GPS site LGKW (Langkawi Island, Malaysia), which declined continuously over time after the earthquake. An eastward deformation of more than six centimetres was determined during the 80-day period after the earthquake.

Such post-seismic deformation information from all available GPS sites in the earthquake region can help scientists analyse likely elastic, poroelastic and viscoelastic deformation, and plastic flow of the Earth's crust in the earthquake region, giving a better understanding of crustal relocation and redistribution after the earthquake.

Figure 4. Post-seismic deformation at GPS site LGKW (Langkawi Island, northern Malaysia). (Larger image [GIF 165.2Kb])


Geomorphic analysis reveals active tectonic deformation on the eastern flank of the Pir Panjal Range, Kashmir Valley, India

There are plenty of faults that show evidence that they are active. Most of the valley’s floor is occupied by unconsolidated Karewa deposits, in particular on the south–southwest of the Kashmir Valley. In such situations, geomorphic data can reveal the location of active faults. Accordingly, we tried to identify geomorphic indices in SW of the Kashmir Valley (Veshav, Rambiara, and Romushi drainage basins), which revealed the area to be potentially tectonically active. This active faulting was further substantiated by drainage anomalies and field investigations, which provides evidence for an emergent out-of-sequence NE-dipping active reverse fault (identified first time on ground) named the Balapur Fault (BF). The BF can be traced over at least 40 km along the southwest side of the Kashmir Valley. The existence of the active Balapur Fault and of two other inferred faults north of the Panjal Thrust or Murree Thrust presents a picture of a more complex strain-partitioning regime in the Kashmir Himalayas than is usually visualized.

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Active tectonic blocks and strong earthquakes in the continent of China

The primary pattern of the late Cenozoic to the present tectonic deformation of China is characterized by relative movements and interactions of tectonic blocks. Active tectonic blocks are geological units that have been separated from each other by active tectonic zones. Boundaries between blocks are the highest gradient of differential movement. Most of tectonic activity occurs on boundaries of the blocks. Earthquakes are results of abrupt releases of accumulated strain energy that reaches the threshold of strength of the earth’s crust. Boundaries of tectonic blocks are the locations of most discontinuous deformation and highest gradient of stress accumulation, thus are the most likely places for strain energy accumulation and releases, and in turn, devastating earthquakes. Almost all earthquakes of magnitude greater than 8 and 80%–90% of earthquakes of magnitude over 7 occur along boundaries of active tectonic blocks. This fact indicates that differential movements and interactions of active tectonic blocks are the primary mechanism for the occurrences of devastating earthquakes.


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Acknowledgments

[12] We thank the Institute of Surveying and Mapping, Sichuan Earthquake Administration in organizing all GPS monuments establishments and field measurements. We are greatly grateful to Philip England and an anonymous reviewer for their thoughtful comments. We benefit from discussion with Teruyuki Kato and Jin Honglin. This research was financially supported by State Key Basic Research Development and Programming Project of China (2004CB418403) and Seismic Professional Science Foundation (200708030). Figures were drawn with the GMT software [ Wessel and Bercovici, 1998 ].

Auxiliary material for this article contains a table containing the GPS dataset used in the paper, created in September 2008.

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ggge1396-sup-0001-readme.txtplain text document, 1.5 KB readme.txt
ggge1396-sup-0002-ts01.txtplain text document, 22.6 KB Table S1. GPS site velocities in the ITRF2005 frame.
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