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Read the Story. Alaska Peninsula Earthquake Sequences of January 4, 20th-century strain accumulation on the Lesser Antilles megathrust based on coral microatolls The seismic potential of the Lesser Antilles megathrust remains poorly known, despite the potential hazard it poses to numerous island populations and its proximity to the Americas.
As it has not produced any large earthquakes in the instrumental era, the megathrust is often assumed to be aseismic. However, historical records of great earthquakes in the 19th century and earlier, which were most li. While caldera collapses happen infrequently, the collapse-related seismicity damaged nearby structures, and so these events should be included in a complete seismic hazard assessment.
Here, we present an approach to forecast the seismic hazard. One difficulty with automated processing is to screen clipped records. Clipped records in which the amplitude exceeds the dynamic. Fault trace. The intersection of a fault with the ground surface; also, the line commonly plotted on geologic maps to represent a fault. Fault-plane solution. A way of showing the fault and the direction of slip on it from an earthquake, using circles with two intersecting curves that look like beach balls.
Also called a focal-mechanism solution. See also moment tensor. First motion. On a seismogram, the first motion is the direction of ground motion as the P wave arrives at the seismometer. Upward ground motion indicates an expansion in the source region; downward motion indicates a contraction.
Focal depth. The depth of an earthquake hypocenter. The region between the subduction zone and the volcanic chain volcanic arc. Relatively smaller earthquakes that precede the largest earthquake in a series, which is termed the mainshock. Not all mainshocks have foreshocks. The number of times something happens in a certain period of time, such as the ground shaking up and down or back and forth during an earthquake.
The acceleration of gravity 9. When acceleration acts on a physical body, the body experiences the acceleration as a force. The force we are most experienced with is the force of gravity, which causes us to have weight. G is the proportionality constant 6. When there is an earthquake, the forces caused by the shaking can be measured as a percentage of gravity, or percent g.
The science of determining the size and shape of the earth and the precise location of points on its surface. The use of geodesy for measurements. Geologic time scale. A chronological sequence of geologic events usually represented in the form of a chart showing names of various rock layers and indicating the estimated duration of each geologic unit. The study of the planet earth- the materials it is made of, the processes that act on those materials, the products formed, and the history of the planet and its life forms since its origin.
The study of the character and origin of landforms, such as mountains, valleys, etc. The branch of earth science which employs physical measurements and mathematical models to explore and analyze the structure and dynamics of the solid Earth and similar bodies and their fluid envelopes. Referring to the use of scientific methods and engineering principles to acquire, interpret, and apply knowledge of earth materials for solving engineering problems.
A down-dropped block of the earth's crust resulting from extension, or pulling, of the crust. The attraction between two masses, such as the earth and an object on its surface.
Commonly referred to as the acceleration of gravity. Changes in the gravity field can be used to infer information about the structure of the earth's lithosphere and upper mantle. Great circle. The shortest path between two points on the surface of a sphere lies along a great circle. Ground failure. A general reference to landslides, liquefaction, lateral spreads, and any other consequence of shaking that affects the stability of the ground. Ground motion.
The movement of the earth's surface from earthquakes or explosions. Ground motion is produced by waves that are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the earth and along its surface. A mathematical model used to approximate the earth when performing some calculations in seismology. The model is much simpler than the real earth. Harmonic tremor. Continuous rhythmic earthquakes that can be detected by seismographs.
Harmonic tremors often precede or accompany volcanic eruptions. Hertz Hz. A unit of measurement named in honor of Heinrich Hertz, the German physicist who in first produced electromagnetic waves. The past 10, years. It includes most of the time since the end of the most recent ice age.
If slip has occurred on a fault during the Holocene, the fault is commonly considered active. A horst is found together with a graben in an extensional environment.
The graben are the downdropped blocks and the horst are the upthrown blocks that lie next to the graben. The point within the earth where an earthquake rupture starts.
The epicenter is the point directly above it at the surface of the Earth. Also commonly termed the focus. A number written as a Roman numeral describing the severity of an earthquake in terms of its effects on the earth's surface and on humans and their structures.
Several scales exist, but the ones most commonly used in the United States are the Modified Mercalli scale and the Rossi-Forel scale. There are many intensities for an earthquake, depending on where you are, unlike the magnitude, which is one number for each earthquake. Pertains to processes between the earth's crustal plates. Interplate coupling. The ability of a fault between two plates to lock and accumulate stress.
Strong interplate coupling means that the fault is locked and capable of accumulating stress, whereas weak coupling means that the fault is unlocked or only capable of accumulating low stress. Pertains to processes within the plates. Isoseismal line. A contour or line on a map bounding points of equal intensity for a particular earthquake. The general movement patterns and directions of the earth's rocks that produce rock deformation. A movement of surface material down a slope.
Late Quaternary. Refers informally to the past 0. Faults that have slipped during this time are sometimes considered active. Lateral spread or flow. Terms referring to landslides that commonly form on gentle slopes and that have rapid fluid-like flow movement, like water. Least-squares fit. When plotting data points on a graph, the least-squares-fit is the line or curve that comes closest to going through all the points.
If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the left block moves toward you and the right block moves away.
See also right-lateral. Structures that are important or critical for a community to function, such as roadways, pipelines, powerlines, sewers, communications, and port facilities. A process by which water-saturated sediment temporarily loses strength and acts as a fluid, like when you wiggle your toes in the wet sand near the water at the beach. This effect can be caused by earthquake shaking. The description of rock composition what it is made of and texture. The outer solid part of the earth, including the crust and uppermost mantle.
The lithosphere is about km thick, although its thickness is age dependent older lithosphere is thicker. The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate. Locked fault. A fault that is not slipping because frictional resistance on the fault is greater than the shear stress across the fault it is stuck. Such faults may store strain for extended periods that is eventually released in an earthquake when frictional resistance is overcome.
Love wave. A surface wave having a horizontal motion that is transverse or perpendicular to the direction the wave is traveling. An abbreviation for one million years ago Megannum. Magnetic polarity reversal. A change of the earth's magnetic field to the opposite polarity. This has occurred at irregular intervals during geologic time. Polarity reversals can be preserved in sequences of magnetized rocks and compared with standard polarity-change time scales to estimate geologic ages of the rocks.
Rocks created along the oceanic spreading ridges commonly preserve this pattern of polarity reversals as they cool, and this pattern can be used to determine the rate of ocean ridge spreading. The reversal patterns recorded in the rocks are termed sea-floor magnetic lineaments. A number that characterizes the relative size of an earthquake. Magnitude is based on measurement of the maximum motion recorded by a seismograph.
Several scales have been defined, but the most commonly used are 1 local magnitude ML , commonly referred to as "Richter magnitude", 2 surface-wave magnitude Ms , 3 body-wave magnitude Mb , and 4 moment magnitude Mw. Scales have limited range and applicability and do not satisfactorily measure the size of the largest earthquakes.
The moment magnitude Mw scale, based on the concept of seismic moment, is uniformly applicable to all sizes of earthquakes but is more difficult to compute than the other types. All magnitude scales should yield approximately the same value for any given earthquake. The largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks.
The part of the earth's interior between the metallic outer core and the crust. The identification of separate individual areas having different potentials for hazardous earthquake effects. The boundary between the crust and the mantle in the earth.
This is a depth where seismic waves change velocity and there is also a change in chemical composition. Also termed the Mohorovicic' discontinuity after the Croatian seismologist Andrija Mohorovicic' who discovered it. The boundary is between 25 and 60 km deep beneath the continents and between 5 and 8 km deep beneath the ocean floor.
Moment tensor. A mathematical representation of the movement on a fault during an earthquake, comprising of nine generalized couples, or nine sets of two vectors. The tensor depends of the source strength and fault orientation. It is often represented with "beach balls" just like the focal mechanism or fault plane solution. Natural frequency. The frequency at which a particular object or system vibrates when pushed by a single force or impulse, and not influenced by other external forces or by damping.
If you hold a slinky by one end and let it hang down and then give it one push up from the bottom, the rate of up-and-down motion is its natural frequency. Normal stress. That stress component perpendicular to a given plane. If you lean against a door after you close it, you are applying normal stress to the door.
Normal stress can either be compressional or tensional. Oceanic spreading ridge. The fracture zone along the ocean bottom where molten mantle material comes to the surface, thus creating new crust. This fracture can be seen beneath the ocean as a line of ridges that form as molten rock reaches the ocean bottom and solidifies.
Oceanic trench. A linear depression of the sea floor caused by the subduction of one plate under another. P wave. A P wave, or compressional wave, is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving.
Earthquakes recorded geologically, most of them unknown from human descriptions or seismograms. Geologic records of past earthquakes can include faulted layers of sediment and rock, injections of liquefied sand, landslides, abruptly raised or lowered shorelines, and tsunami deposits. Pedogenic means pertaining to processes that add, transfer, transform, or remove soil constituents. The time interval required for one full cycle of a wave. Plate tectonics. The theory supported by a wide range of evidence that considers the earth's crust and upper mantle to be composed of several large, thin, relatively rigid plates that move relative to one another.
Slip on faults that define the plate boundaries commonly results in earthquakes. Several styles of faults bound the plates, including thrust faults along which plate material is subducted or consumed in the mantle, oceanic spreading ridges along which new crustal material is produced, and transform faults that accommodate horizontal slip strike slip between adjoining plates.
The time period between about 10, years before present and about 1,, years before present. As a descriptive term applied to rocks or faults, it marks the period of rock formation or the time of most recent fault slip, respectively. Faults of Pleistocene age may be considered active though their activity rates are commonly lower than younger faults. Poisson distribution. A probability distribution that characterizes discrete events occurring independently of one another in time. Information about real-time earthquakes, online catalog search of archives, seismicity maps and statistics.
US faults, information about seismic hazards in various areas, tools for seismic design values. The science of earthquakes, FAQ, glossary, links to earthquake topics, Earthquakes for kids.
Global, national and regional networks recording earthquakes and crustal movements, maps, station information, real-time seismic waveforms. Research projects in the Earthquake Hazards Program, external research support, science center activities, software.
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