Seismotectonics

Seismotectonics is a specialized branch of science that focuses on the relationships between earthquakes, fault lines, and the tectonic processes. It is considered an interdisciplinary field that incorporates elements of geology, seismology, and geodesy. The overarching objective of seismotectonics is to capture data and information from earthquakes and use that data and information to analyze the nature and characteristics of the quake’s causal factors and physical characteristics. Scientists use both quantitative and qualitative techniques when analyzing seismotectonic data.

By performing the seismotectonic analysis of earthquake data, researchers can quantify and compare the relative characteristics of tectonic stress in a given region. This process plays an integral role in determining the overall level of seismic risk that exists within a particular geographic area. Standard practices in seismotectonic modeling generally correlate seismic risk with proximity to fault lines, and specifically to the known or estimated slip rates associated with those fault lines.

Background

Plate tectonics is built on the theory that the Earth’s landmasses represent the surface-level features of a network of subterranean structures known as plates, which are in a slow but constant state of motion. These plates shift and move as a result of natural and ongoing processes including radioactivity and the rock cycle. The rock cycle describes the breakdown of old rocks and the production of new rocks by melting, erosion, compaction, deformation, and cooling. As the rock cycle progresses, geographic features such as mountains and volcanoes are formed. Radioactive processes that occur deep within the Earth as part of the rock cycle generate heat, which propels the motion of tectonic plates.

Earthquakes also occur as a result of plate tectonics. They are most likely to strike at or near fault lines, which are regions where two adjacent plates meet. Faults form as a result of the force driving the motion of the underlying tectonic plates. They allow the impacted masses of rock to move relative to each other. That movement typically occurs slowly and is known in geology and seismology as “creep.” When the movement happens quickly or abruptly, an earthquake occurs.

A fault’s slip rate describes the rate at which the two masses of rock meeting at a fault are slipping past one another. Scientists determine slip rates by making precision measurements and observing human-made features for subtle displacements over time. Slip rates can be specifically determined or estimated, depending on the characteristics of the physical area being studied. They are usually measured in units including millimeters per year (mm/yr) or meters per thousand years (m/k.y.).

Seismotectonics draws on practices and techniques more readily associated with other disciplines, including geology, seismology, and geodesy. Geology is an Earth science focused on the processes involved in the Earth’s formation and ongoing evolution and on the Earth’s physical and chemical composition. Seismology is the study of earthquakes and related phenomena, both on Earth and other planetary bodies. Geodesy involves the scientific study and mathematical modeling of the Earth’s shape, gravity, and orientation in space.

Overview

In the most general sense, seismotectonics is the study of earthquakes in tectonic contexts. Seismotectonic theory embraces the idea that earthquakes are an active manifestation of the Earth’s tectonic movements. Scientific practitioners in the discipline have endeavored to create maps of known fault lines and fault zones and the relationships of those faults to plate tectonics.

One of the most valued contributions of seismotectonics relates to the discipline’s tracking and measurement of an earthquake characteristic known as moment release. Moment release describes the amount of energy released during an earthquake. Seismotectonics experts have determined that 95 percent of the moment release caused by seismic events occurs as a result of relatively minor or shallow rock slippages along the boundaries of the plates. Scientists have also used seismotectonic analysis to determine that a large majority (85 percent) of the moment release caused by earthquakes occurs in what are known as subduction zones. A subduction zone is a site where the rocks belonging to one tectonic plate sink below another plate into the Earth’s interior, where they break down under high heat and pressure and are recycled into new rocks.

Seismotectonics engages extensively with the data and information captured during earthquake events, which scientists study and analyze using both quantitative and qualitative methods. Quantitative seismotectonics applies the data obtained by measuring the physical force of an earthquake to the geometrical characteristics associated with the host region to determine the relative level of seismic risk associated with a specific location. Qualitative approaches use techniques including mapping and diagramming, which allows researchers to study the geometric and physical characteristics of the forces unleashed by an earthquake. The directions and patterns these forces take give scientists information about the orientations of the involved fault planes. With this information, they can more accurately predict the nature and severity of future earthquakes at the same site or fault zone.

Another branch of the discipline, known as comparative seismotectonics, looks at the function of earthquakes in the context of the larger geological and tectonic processes. Comparative seismotectonics also considers the ways in which specific tectonic environments tend to generate earthquakes with particular characteristics. In addition, seismotectonics also studies a phenomenon known as aseismic faulting, in which rock masses create fault lines and zones as they move past one another without creating any detectable earthquakes.

The concept of induced seismicity, which describes seismic events caused by human activity, also falls under seismotectonics. Since the mid-twentieth century, scientists have known that certain human activities—especially the creation of artificial underground water reservoirs—can trigger seismic events. Induced seismicity is also associated with fossil fuel resource development and extraction, and has emerged as an area of additional concern as the environmental impacts of the oil and gas industry become an increasingly high-profile issue.

Bibliography

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Yule, J. Doug, Michele L. Cooke, and David D. Oglesby. "Seismotectonics of the San Andreas Fault System in the San Gorgonio Pass Region: A Synthesis." Geosphere, vol. 20, no. 2, 23 Feb. 2024, doi.org/10.1130/GES02710.1. Accessed 13 Nov. 2024.