Appendix B: Glossary

abrasion—The mechanical wearing, grinding, scraping, or rubbing away of rock surfaces by friction and impact.

acceleration—Rate of increase in speed of an object.

accelerometer—A seismograph for measuring change in ground speed (motion) with time. Syn. accelerograph.

active fault—A fault along which there is recurrent movement, which is usually indicated by small, periodic displacements or seismic activity.

active tectonics—Tectonic movements that are expected to occur within a future time span of concern to society.

aftershock—Smaller earthquakes following the largest earthquake and in the same general area.

amplitude—Maximum height of a wave crest or depth of a wave trough.

anticline—A fold, convex upward, whose core contains the older rocks.

asperity—Roughness on the fault surface subject to slip. Region of high shear strength on the fault surface.

asthenosphere—The layer or shell of the Earth below the lithosphere, which is weak and in which isostatic adjustments take place, magmas might be generated, and seismic waves are strongly attenuated.

attenuation—The reduction in amplitude of a wave with time or distance.

basalt—A general term for dark-colored igneous rocks, commonly extrusive but locally intrusive (e.g., as dikes), composed chiefly of feldspar and pyroxene. The principal constituent of oceanic crust, which includes gabbro, the coarse-grained equivalent of basalt.

base isolation—A process of foundation construction whereby forces from the ground are not transmitted upward into the building. A transportation bill, which would have included retrofits of bridges identified by the resilience survey, failed to pass or even to be debated by the 2015 Legislature. The Oregonian newspaper did not take the Legislature to task on this issue.

bathymetry—Topography of the sea floor; measuring depths in the sea.

blind fault—A fault that does not break the surface, but can be expressed at the surface as a fold or broad warp.

body wave—A seismic wave that travels through the interior of the Earth.

brittle—1. Said of a rock that fractures at less than three to five percent deformation or strain. 2. In structural engineering, describes a building that is unable to deform extensively without collapsing.

capable fault—A fault along which it is mechanically feasible for sudden slip to occur.

characteristic earthquake—An earthquake with a size and generating mechanism typical for a particular fault source.

colluvial wedge—In cross section, a wedge of coarser-grained material fallen off or washed down from a fault scarp, commonly taken as evidence in a backhoe trench of an earthquake with surface rupture.

colluvium—A general term applied to any loose, heterogeneous, and incoherent mass of soil material and/or rock fragments deposited by rain or slow, continuous downslope creep, usually collecting at the base of gentle slopes or hillsides.

continent—One of the Earth’s major land masses, including both dry land and continental shelves.

continental crust—That type of the Earth’s crust which underlies the continents and the continental shelves, ranging in thickness from about twenty miles up to forty miles under mountain ranges.

core (of Earth)—1. The central part of the Earth below a depth of 1,800 miles. It is thought to be composed mainly of iron and silicates and to be molten on the outside with a solid central part. 2. Sample of rock or sediment obtained by drilling or other means.

creep (along a fault)—Slow slip unaccompanied by earthquakes. Same as fault creep.

cripple wall—Short studs between the mudsill and foundation and the floor joists of the house. Synonym: pony wall

critical facility—A structure that is essential to survive a catastrophe because of its need to direct rescue operations or treat injured people, or because if it were destroyed (such as a dam or nuclear power plant), the effects of that destruction could be catastrophic to society.

crust—The outermost layer or shell of the Earth, defined according to various criteria, including the speed of seismic waves, density and composition; that part of the Earth above the Moho (q.v.) discontinuity.

crystalline rock—An inexact but convenient term designating an intrusive igneous or metamorphic rock as opposed to a sedimentary rock.

density—Mass per unit volume.

deterministic forecast—An estimation of the largest earthquake or most severe ground shaking to be found on a fault, or in a region, the maximum credible (or considered) earthquake, or MCE.

diaphragm—Horizontal element of a building, such as a floor or a roof, that transmits horizontal forces between vertical elements such as walls.

dip—The angle between a layer or fault and a horizontal plane.

dip-slip fault—A fault in which the relative displacement is in the direction of fault dip.

ductile—1. Said of a rock that can sustain, under a given set of conditions, five to ten percent deformation before fracture or faulting. 2. In structural engineering, the ability of a building to bend and sway without collapsing.

earthquake segment—That part of a fault zone or fault zones that has ruptured during individual earthquakes.

elastic limit—The greatest stress that can be developed in a material without permanent deformation remaining when the stress is removed.

epicenter—The point on the Earth’s surface that is directly above the focus (hypocenter) of an earthquake.

epoch—A geologic time unit shorter than a period, e.g., the Pleistocene Epoch.

era—A geologic time unit next in order of length above a period; e.g., the Paleozoic, Mesozoic, and Cenozoic eras.

eustatic—Pertaining to worldwide changes of sea level that affect all the oceans, largely caused in the Quaternary by additions of water to, or removal of water from, the continental icecaps.

fault—A fracture or a zone of fractures along which there has been displacement of the sides relative to one another parallel to the fracture.

feldspar—An abundant rock-forming mineral constituting 60 percent of the Earth’s crust.

first motion—On a seismogram, the direction of motion at the beginning of the arrival of a P wave. By convention, upward motion indicates a compression of the ground; downward motion, a dilation.

focal depth—The depth of the focus below the surface of the Earth.

focus—The place at which rupture commences.

footwall—The underlying side of a dipping fault.

forecast (of an earthquake)—A specific area or fault is identified as having a higher statistical probability of an earthquake of specified magnitude range in a time window of months or years.

foreshocks—Smaller earthquakes preceding the largest earthquake of a series concentrated in a restricted crustal volume.

frequency—Number of waves per unit time; unit is Hertz, or one cycle (one complete wave) per second.

free face—Exposed surface of a scarp resulting from faulting; may be modified by erosion.

friction—The resistance to motion of a body sliding past another body along a surface of contact; may generate heat.

g—Acceleration due to the gravitational attraction of the Earth, a rate of 32 feet (9.8 meters) per second, per second, which is 1 g.

geodesy—The science concerned with the determination of the size and shape of the Earth and the precise location of points on its surface.

geomorphology—The science that treats the general configuration of the Earth’s surface; specifically the study of the classification, description, nature, origin, and development of present landforms and their relationships to underlying structures, and of the history of geologic changes as recorded by these surface features.

geothermal gradient—Increase of temperature in the Earth with depth.

GPS—Global Positioning System, in which surveying is accomplished by determining the position with respect to the orbital positions of several NAVSTAR satellites. Repeated surveying of ground stations can reveal tectonic deformation of the Earth’s crust.

graben—A crustal block of rock, generally long and narrow, that has dropped down along boundary faults relative to adjacent rocks.

granite—A deep-seated rock in which quartz constitutes 10 to 50 percent of the light-colored mineral components and in which feldspar is the other light-colored component. Broadly applied, any completely crystalline, quartz-bearing rock found at depth in the Earth’s crust.

Gutenberg-Richter recurrence relationship—The observed relationship that, for large areas and long time periods, numbers of earthquakes of different magnitudes occur systematically with the relationship M = a – bN, where M is magnitude, N is the number of events per unit area per unit time, and a and b are constants representing, respectively, the overall level of seismicity and the ratio of small to large events. Does not apply to large magnitudes.

hanging wall—The overlying side of a dipping fault.

hazard—1. Danger; a feature such as an earthquake or volcano that is dangerous. Equivalent to “peril” in insurance. 2. In insurance, something that increases the danger.

Holocene—The past ten thousand years; an epoch of the Quaternary. For the Alquist-Priolo Act, the Holocene started eleven thousand years ago.

indemnity—Insurance against, or repayment for, loss or damage.

inertia—The tendency of matter to remain at rest or continue in a fixed direction unless acted upon by an outside force.

intensity (of earthquakes)—A measure of ground shaking, obtained from the damage done to structures built by humans, changes in the Earth’s surface, and reports about what people felt or observed.

Intensity magnitude (Mi) – magnitude of an earthquake that occurred in the pre-seismograph era based on reported intensities.

isoseismal—Contour lines drawn to separate one level of seismic intensity from another.

isostasy—That condition of equilibrium, analogous to floating, of the units of the lithosphere above the asthenosphere.

Lahar—Catastrophic mudflow on the flank of a volcano that may reach as far as one hundred kilometers from the volcano when confined to a valley.

Lateral spread—A displacement of non-liquefiable material on a slope that may be as low as 0.1 degrees, overlying a liquefied layer of large areal extent.

Law of Large Numbers—The larger the number of insurance contracts a company writes, the more likely the actual results will follow the predicted results based on an infinite number of contracts.

left-lateral fault—A strike-slip fault on which the displacement of the far block is to the left when viewed from the near side.

liquefaction—The act or process of transforming any substance into a liquid.

lithosphere—A layer of strength relative to the underlying asthenosphere for deformation at geologic rates. It includes the crust and part of the upper mantle and is up to sixty miles (one hundred kilometers) in thickness.

load—The forces acting on a building. The weight of the building is its dead load. Weight of contents, or snow on the roof, etc., are live loads.

magma—Naturally occurring molten rock material, generated within the Earth and capable of intrusion and extrusion as lava, from which igneous rocks such as volcanoes are thought to have been derived through solidification and related processes.

magnitude (of earthquakes)—A measure of earthquake size, determined by taking the common logarithm (base 10) of the largest ground motion recorded during the arrival of a seismic wave type and applying a standard correction for distance to the epicenter.

mantle—The zone of the Earth below the crust and above the core, which is divided into the upper mantle and the lower mantle, composed principally of peridotite.

meizoseismal region—The area of strong shaking and significant damage in an earthquake.

mid-ocean ridge—A long linear elevated volcanic structure formed by the symmetrical spreading of two lithospheric plates away from the ridge crests.

mitigate—To moderate or to make milder or less severe.

modulus of elasticity—The ratio of stress to its corresponding strain under given conditions of load, for materials that deform elastically.

Mohoroviˇci´c discontinuity—The boundary surface or sharp seismic-velocity discontinuity that separates the Earth’s crust from the underlying mantle, marked by an abrupt change in speed of seismic waves. Syn. Moho.

moment (of earthquakes)— A measure of earthquake size based on the rigidity of the rock times the area of faulting times the amount of slip. Dimensions are dyne-cm or Newton-meters.

moment magnitude (Mw)—Magnitude of an earthquake estimated by using the seismic moment.

moment-resistant frame—Steel frame structures with rigid welded joints, more flexible than shear-wall structures.

mudsill—The lowest board between a house and its foundation.

neotectonics—1. The study of the post-Miocene structures and structural history of the Earth’s crust. 2. The study of recent deformation of the crust, generally Miocene and younger. 3. Tectonic processes now active, taken over the geologic time span during which they have been acting in the presently observed sense, and the resulting structures.

normal fault—A fault in which the hangingwall appears to have moved downward relative to the footwall.

ocean basin—The area of the sea floor between the base of the continental slope, and the mid-ocean ridge.

olivine—An olive-green, grayish-green, or brown mineral, common in basalt and peridotite.

P wave—The primary or fastest wave traveling away from a seismic event through the rock and consisting of a train of compressions and dilations of the material.

paleoseismology—That part of earthquake studies that deals with geological evidence for earthquakes and fault rupture.

paradigm—A pattern, example, or model.

peridotite—Rock composed predominantly of the minerals pyroxene and olivine; the major component of the Earth’s mantle.

peril—The risk, contingency, event, or cause of loss insured against, as in an insurance policy.

period—1. The time interval between successive crests in a wave train; the period is one divided by the frequency of a cyclic event. 2. The fundamental unit of the geological time scale, subdivisions of an era, itself subdivided into epochs. Example: Quaternary Period.

plate—A large, relatively rigid segment of the Earth’s lithosphere that moves in relation to other plates over the deeper interior.

plate tectonics—A theory of global tectonics in which the lithosphere is divided into a number of plates whose pattern of horizontal movement is that of rigid bodies that interact with one another at their boundaries, causing seismic and tectonic activity along these boundaries.

Pleistocene—An epoch of the Quaternary Period, after the Pliocene and before the Holocene.

pony wall—See cripple wall.

precursor—A change in the geological conditions that is a forerunner to earthquake generation on a fault.

prediction (of earthquakes)—The estimation of the time, place, and magnitude of a future earthquake.

premium—An amount payable for an insurance policy.

probability—The number of cases that actually occur divided by the total number of cases possible; the likelihood that an event will take place.

probability of exceedance of a given earthquake size—The odds that the size of a future earthquake will exceed some specified value.

pyroxene—A group of dark, rock-forming silicate minerals.

quartz—Crystalline silica, an important rock-forming mineral.

Quaternary—The second period of the Cenozoic era, following the Tertiary, consisting of the Pleistocene and Holocene epochs.

radiometric—Pertaining to the measurement of geologic time by the study of the disintegration rates of one element or isotope to another.

recurrence interval—The average time interval between earthquakes in a seismic region or along a fault.

reinsurance—A contract in which the insurer becomes protected by obtaining insurance from someone else upon a risk that the first insurer has assumed.

retrofit—Reinforcement or modification of an existing building.

reverse fault—A fault that dips toward the block that has apparently been relatively raised.

rheology—The study of the deformation and flow of matter.

Richter scale—Logarithm to the base 10 of the maximum seismic-wave amplitude, in thousandths of a millimeter, recorded on a Wood-Anderson seismograph at a distance of sixty miles (one hundred kilometers) from the earthquake epicenter. Also called local magnitude, or M1.

right-lateral fault—A strike-slip fault on which the displacement of the far block is to the right when viewed from the near side.

rigidity—The resistance of an elastic body to shear.

risk—The amount of loss, and the chance of loss occurring.

S wave—The secondary seismic wave, traveling more slowly than the P wave and consisting of elastic vibrations at right angles to the direction of wave travel.

seafloor spreading—A hypothesis that oceanic crust is being created by convective upwelling of magma along the mid-oceanic ridges or world rift system and by a moving-away of the new material at a rate of a fraction of an inch to five inches per year.

seiche—Standing or propagating water waves generated by seismic waves.

seismic gap—An area in an earthquake-prone region where there is a below-average release of seismic energy.

seismic moment—See moment (of earthquakes).

seismic wave—An elastic wave in the Earth usually generated by an earthquake or explosion.

seismicity—The occurrence of earthquakes in space and time.

seismogenic—Characterized by earthquakes.

seismogram—Record of an earthquake written on a seismograph.

seismograph—An instrument for recording as a function of time the motions of the Earth’s surface that are caused by seismic waves.

seismology—1. The study of earthquakes, including geodesy, geology, and geophysics. 2. The study of earthquakes, and of the structure of the Earth, by both naturally and artificially generated seismic waves.

serpentine—Green, streaky rock formed by the addition of water to peridotite. The California state rock.

shear wall—A wall of a building that has been strengthened to resist horizontal forces.

shoreline angle—The boundary between a freshly cut sea cliff and the marine wave-abraded platform.

slip—The relative displacement of formerly adjacent rock materials on opposite sides of a fault, measured in the fault surface.

slow earthquakes—Earthquakes that rupture at such slow speeds that they produce little or no shaking.

soft story—A section or horizontal division of a building extending from the floor to the ceiling or roof above it characterized by large amounts of open space that reduces its resistance to horizontal forces, such as a ground-floor garage or a ballroom in a hotel.

soil—1. A natural body consisting of layers or horizons of mineral and/or organic constituents of variable thicknesses, which differ from the parent material in their morphological, physical, chemical, and mineralogical properties and their biological characteristics. 2. All unconsolidated materials above bedrock (engineering).

stick slip—A jerky, sliding motion associated with fault movement.

strain—Change in the shape or volume of a body as a result of stress.

stress—Force per unit area.

stress drop—The sudden reduction of stress across a fault during rupture.

strike—The direction of trend taken by a structural surface as it intersects the horizontal.

strike slip—In a fault, the component of movement that is parallel to the strike of the fault.

strike-slip fault—A fault on which the movement is parallel to the strike of the fault.

subduction—The process of one lithospheric plate descending beneath another.

subduction zone—A long, narrow belt in which subduction takes place.

surface-wave magnitude (Ms)—Magnitude of an earthquake estimated from measurements of the amplitude of earthquake waves that follow the Earth’s surface.

surface waves—Seismic waves that follow the Earth’s surface only, with a speed less than that of S waves. There are two types of surface waves—Rayleigh waves and Love waves.

swarm (of earthquakes)—A series of earthquakes in the same locality, no one earthquake being of outstanding size.

syncline—A fold of which the core contains the stratigraphically younger rocks; it is concave upward.

tectonic geomorphology—The study of landforms that result from tectonic processes.

tectonics—A branch of geology dealing with the broad architecture of the outer part of the Earth; that is, the regional assembling of structural or deformational features, a study of their mutual relations, origin, and historical evolution.

tephrochronology—The dating of tephra (pyroclastic material, such as ash) from a volcano.

teleseism—Record of an earthquake that occurs far from the recording seismograph, generally thousands of miles away.

thrust fault—A fault with a dip of forty-five degrees or less over much of its extent, on which the hangingwall appears to have moved upward relative to the footwall.

topography—The general configuration of a land surface or any part of the Earth’s surface, including its relief and the position of its natural and man-made features.

trace—The intersection of a geological surface with another surface, e.g., the trace of bedding on a fault surface, or the trace of a fault or outcrop on the ground surface.

transform fault—A plate boundary that ideally shows pure strike-slip displacement.

trend—A general term for the direction or bearing of the outcrop of a geological feature of any dimension.

trench—1. Long, narrow, arcuate depression on the sea floor which results from the bending of the lithospheric plate as it descends into the mantle at a subduction zone. 2. Shallow excavation, dug by bulldozer, backhoe, or by hand, revealing detailed information about near-surface geological materials.

triple junction—Point where three plates meet.

tsunami—An ocean wave caused by seafloor movements in an earthquake, submarine volcanic eruption, or submarine landslide.

turbidite—A sediment or rock deposited from a turbidity current, a flow of sediment-charged water that is denser than clear water.

ultimate strength—The maximum differential stress that a material can sustain under the conditions of deformation.

underwriting—The writing of one’s signature at the end of an insurance policy, thereby assuming liability in the event of specific loss or damage.

URM—Unreinforced masonry, a type of construction that is not strengthened against horizontal forces from an earthquake.

volcanology—The branch of geology that deals with volcanoes.

wavelength—The distance between two successive crests or troughs of a wave.

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