Seismology is the scientific study of earthquakes and the generation and propagation of elastic waves through the Earth or other planetary bodies. It also includes studies of earthquake environmental effects such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, glacial, fluvial, oceanic microseism, atmospheric, and artificial processes such as explosions and human activities. A related field that uses geology to infer information regarding past earthquakes is paleoseismology. A recording of Earth motion as a function of time, created by a seismograph is called a seismogram. A seismologist is a scientist works in basic or applied seismology.
Geophysics is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists, who usually study geophysics, physics, or one of the Earth sciences at the graduate level, complete investigations across a wide range of scientific disciplines. The term geophysics classically refers to solid earth applications: Earth's shape; its gravitational, magnetic fields, and electromagnetic fields ; its internal structure and composition; its dynamics and their surface expression in plate tectonics, the generation of magmas, volcanism and rock formation. However, modern geophysics organizations and pure scientists use a broader definition that includes the water cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial physics; and analogous problems associated with the Moon and other planets.
A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake, volcanic eruption, magma movement, a large landslide and a large man-made explosion that produces low-frequency acoustic energy. Seismic waves are studied by seismologists, who record the waves using seismometers, hydrophones, or accelerometers. Seismic waves are distinguished from seismic noise, which is persistent low-amplitude vibration arising from a variety of natural and anthropogenic sources.
A seismometer is an instrument that responds to ground displacement and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The output of such a device—formerly recorded on paper or film, now recorded and processed digitally—is a seismogram. Such data is used to locate and characterize earthquakes, and to study the internal structure of Earth.
Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth using seismic waves. The properties of seismic waves are modified by the material through which they travel. By comparing the differences in seismic waves recorded at different locations, it is possible to create a model of the subsurface structure. Most commonly, these seismic waves are generated by earthquakes or man-made sources such as explosions. Different types of waves, including P-,S-, Rayleigh, and Love waves can be used for tomographic images, though each comes with their own benefits and downsides and are used depending on the geologic setting, seismometer coverage, distance from nearby earthquakes, and required resolution. The model created by tomographic imaging is almost always a seismic velocity model, and features within this model may be interpreted as structural, thermal, or compositional variations. Geoscientists apply seismic tomography to a wide variety of settings in which the subsurface structure is of interest, ranging in scale from whole-Earth structure to the upper few meters below the surface.
Reflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite or Tovex blast, a specialized air gun or a seismic vibrator. Reflection seismology is similar to sonar and echolocation.
The Mendocino Triple Junction (MTJ) is the point where the Gorda plate, the North American plate, and the Pacific plate meet, in the Pacific Ocean near Cape Mendocino in northern California. This triple junction is the location of a change in the broad tectonic plate motions which dominate the west coast of North America, linking convergence of the northern Cascadia subduction zone and translation of the southern San Andreas Fault system. This region can be characterized by transform fault movement, the San Andreas also by transform strike slip movement, and the Cascadia subduction zone by a convergent plate boundary subduction movement. The Gorda plate is subducting, towards N50ºE, under the North American plate at 2.5–3 cm/yr, and is simultaneously converging obliquely against the Pacific plate at a rate of 5 cm/yr in the direction N115ºE. The accommodation of this plate configuration results in a transform boundary along the Mendocino Fracture Zone, and a divergent boundary at the Gorda Ridge. This area is tectonically active historically and today. The Cascadia subduction zone is capable of producing megathrust earthquakes on the order of MW 9.0.
The EarthScope project (2003-2018) was an National Science Foundation (NSF) funded Earth science program using geological and geophysical techniques to explore the structure and evolution of the North American continent and to understand the processes controlling earthquakes and volcanoes. The project had three components: USArray, the Plate Boundary Observatory, and the San Andreas Fault Observatory at Depth. Organizations associated with the project included UNAVCO, the Incorporated Research Institutions for Seismology (IRIS), Stanford University, the United States Geological Survey (USGS) and National Aeronautics and Space Administration (NASA). Several international organizations also contributed to the initiative. EarthScope data are publicly accessible.
Passive seismic is the detection of natural low frequency earth movements, usually with the purpose of discerning geological structure and locate underground oil, gas, or other resources. Usually the data listening is done in multiple measurement points that are separated by several hundred meters, over periods of several hours to several days, using portable seismometers. The conclusions about the geological structure are based on the spectral analysis or on the mathematical reconstruction of the propagation and possible sources of the observed seismic waves. If the latter is planned, data are usually acquired in multiple points simultaneously, using so called synchronized lines. Reliability of the time reverse modelling can be further increased using results of reflection seismology about the distribution of the sound speed in the underground volume.
ICEARRAY is an abbreviation for Icelandic Strong-motion Array. The ICEARRAY network is a seismic array of 14 strong-motion stations located within the South Iceland Seismic Zone. Each station consists of a seismograph situated in a protective housing. The stations are spread across a geographical area of approximately 3 km² in the town of Hveragerdi in south-western Iceland. Most of the units are located in the basements of residential buildings in Hveragerdi town centre, which is approximately 35 km southeast of Iceland's capital, Reykjavík. The ICEARRAY project is supported by the 6th Framework of the European Commission through the Marie Curie International Re-integration Grant, the Iceland Centre for Research and the University of Iceland Earthquake Engineering Research Centre.
Lunar seismology is the study of ground motions of the Moon and the events, typically impacts or moonquakes, that excite them.
In geophysics, geology, civil engineering, and related disciplines, seismic noise is a generic name for a relatively persistent vibration of the ground, due to a multitude of causes, that is often a non-interpretable or unwanted component of signals recorded by seismometers.
Hydrate Ridge is an accretionary thrust clathrate hydrate formation, meaning it has been made of sediment scraped off of subducting oceanic plate. It is approx. 200 m high, and located 100 km offshore of Oregon. At hydrate formations, methane is trapped in crystallized water structures. Such methane transforms into the gaseous phase and seeps into the ocean at this site, which has been a popular location of study since its discovery in 1986. Hydrate Ridge also supports a methane-driven benthic community.
The Apollo 14 Passive Seismic Experiment (PSE) was placed on the lunar surface on February 5, 1971, as part of the Apollo 14 ALSEP package. The PSE was designed to detect vibrations and tilting of the lunar surface and measure changes in gravity at the instrument location. The vibrations are due to internal seismic sources (moonquakes) and external. The primary objective of the experiment was to use these data to determine the internal structure, physical state, and tectonic activity of the Moon. The secondary objectives were to determine the number and mass of meteoroids that strike the Moon and record tidal deformations of the lunar surface.
Southern Hydrate Ridge, located about 90 km offshore Oregon Coast, is an active methane seeps site located on the southern portion of Hydrate Ridge. It extends 25 km in length and 15 km across, trending north-northeast-south-southwest at the depth of approximately 800 m. Southern Hydrate Ridge has been the site of numerous submersible dives with the human occupied Alvin submarine, extensive visits by numerous robotic vehicles including the Canadian ROV ROPOS, Jason , and Tiburon (MBARI), and time-series geophysical studies that document changes in the subsurface distribution of methane. It is also a key site of the National Science Foundations Regional Cabled Array that is part of the Ocean Observatories Initiative (OOI), which includes eight types of cabled instruments streaming live data back to shore 24/7/365 at the speed of light, as well as uncabled instruments.
Marine geophysics is the scientific discipline that employs methods of geophysics to study the world's ocean basins and continental margins, particularly the solid earth beneath the ocean. It shares objectives with marine geology, which uses sedimentological, paleontological, and geochemical methods. Marine geophysical data analyses led to the theories of seafloor spreading and plate tectonics.
Anne Sheehan is a geologist known for her research using seismometer data to examine changes in the Earth's crust and mantle.
Fluvial seismology is the application of seismological methods to understand river processes, such as discharge, erosion, and streambed evolution. Flowing water and the movement of sediments along the streambed generate elastic (seismic) waves that propagate into the surrounding Earth materials. Seismometers can record these signals, which can be analyzed to illuminate different fluvial processes such as turbulent water flow and bedload transport. Seismic methods have been used to observe discharge values that range from single-digits up through tens of thousands of cubic feet per second (cfs).
MERMAID is a marine scientific instrument platform, short for Mobile Earthquake Recorder for Marine Areas by Independent Divers.
Seismic wide-angle reflection and refraction is a technique used in geophysical investigations of Earth's crust and upper mantle. It allows the development of a detailed model of seismic velocities beneath Earth's surface well beyond the reach of exploration boreholes. The velocities can then be used, often in combination with the interpretation of standard seismic reflection data and gravity data, to interpret the geology of the subsurface.
