Glossary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
[Definitions derived from modern English-language dictionaries, the AGI Glossary of Geology, the Encyclopedia of Geomorphology, Southern California Earthquake Center (SCEC) web pages, and other sources]
age of faulted surficial deposits [database field]
Includes the ages of faulted deposits at the surface. The geologic time terms used for ages (such as Holocene and Pleistocene) are defined elsewhere in the glossary.
Loose sediment composed of clay, silt, sand, gravel, and/or larger rocks. Material is transported from highland areas (hills and mountains) and deposited in low areas by streams.
A low, outspread, relatively flat to gently sloping mass of alluvium that is shaped like an open fan. Commonly deposited by a stream at the place where it issues from a narrow mountain valley upon a plain or broad valley. Other terms that are generally synonymous with alluvial fan are bajada (coalesced alluvial fans) and piedmont slope (broad gently sloping surface mantled by relatively thin alluvium).
California seismic zoning act passed in 1972 in response to the 1971 San Fernando earthquake in order to prevent building across the traces of active faults. More information about the Alquist-Priolo Earthquake Fault Zoning Act and Earthquake Fault Zone maps is available from the California Geologic Survey (http://www.consrv.ca.gov/cgs/rghm/ap/index.htm).
The name of the Army Map Service (AMS) sheet in which the structure is located. If the structure is located in more than one AMS sheet, the name of the sheet in which the majority of the structure is located is listed first, followed by the name(s) of other sheets in which the remainder of the structure is located. These sheets were used as base maps because they cover the entire United States, and are at a convenient regional scale of 1:250,000. Within the conterminous United States, the sheets are 1° x 2° (N-S and W-E); however, in Alaska the sheets are 1° x 3°.
average strike [database field]
The average strike of the surface trace of the structure in degrees is computed from Geographic Information System (GIS) software. For this database we have confined the strike of faults and the trends of fold axes to the northwest and northeast quadrants of the compass.To search for a range of azmuiths, use values between -90 and 90 (°W and °E, respectively).
B
A type of geologic structure characterized by generally subparallel fault-bounded mountains separated by broad alluvium-filled basins. The ranges have been uplifted relative to the valleys along range-front faults and related intrabasin faults.
A thrust fault that does not rupture all the way to the ground surface. Movement along the fault produces uplift in the form of an anticline, but a clear or continuous surface rupture is not recognized. Many Quaternary blind thrust faults are thought to be present in southern California. Two examples of known blind thrust faults are the Elysian Park thrust, which runs underneath downtown Los Angeles, and the Northridge thrust along which the 1994 Northridge earthquake occurred.
C
Narrative comments are provided for many of the database fields that are limited to numerical data or prescribed choices, such as structure number or time of most recent movement.
compiled or modified by [database field]
The year of compilation or update, name, and affiliation of the person(s) responsible for compiling or modifying information in the database record. Names and addresses of these compilers are provided in the list of contributors.
County or counties in which the structure is located. If the structure is in more than one county, the county containing the majority of the structure is listed first, followed by county name(s) for the remainder of the structure. In cases where the features encompass several states, the counties are listed alphabetically by state. Alaska does not have counties, and their boroughs do not cover the entire State. Therefore, in Alaska we use "Education Districts" (Educ. Dist. under the County field) as a geographic division to search for location.
Relatively slow movement along a fault. It is sometimes called "seismic creep" to distinguish it from the slumping of rock or soil on slopes (which is also known as creep). It is sometimes called "aseismic creep", since it does not trigger events greater than microearthquakes. Creep is only known to occur on strike-slip faults.
D
A low-angle to sub-horizontal fault into which more steeply dipping fault(s) commonly merge (sole). The term originally applied to the sole fault of thrust-fault systems in compressional terrains, however, term is now generally associated with normal-fault systems in extensional terrains.
The angle between a geologic surface (for example, a fault plane) and the horizontal. The direction of dip can be thought of as the direction a ball would roll if placed upon a tilted surface. Thus, a ball placed on a north-dipping fault plane would roll northward. The dip of a surface is always perpendicular to the strike of that surface.
The general direction of dip of the fault surface (for example, E) or a numerical value of dip or range of dip values of the fault surface if known (for example 50°E). General down-dip direction(s) of the structure are defined by compass octants: north (N), west (W), south (S), east (E), northwest (NW), northeast (NE), southwest (SW), southeast (SE), or vertical (V). If separate faults (or parts of a fault) dip in different directions, multiple directions may be listed, beginning with the dip direction that is the most common along the structure. Values of dip may represent near-surface measurements at specific locations or theoretical subsurface dip values (based on cross sections or geophysical modeling).
Slip (movement) that is parallel to the dip of the fault. Normal slip and reverse slip are opposite senses of dip-slip movement and describe movement of the hanging wall block down and up the plane of the fault, respectively. (Compare strike-slip fault movement.)
See right lateral.
E
The early part of the Quaternary era. Early Quaternary refers to the time between 1,600,000 years and 750,000 years ago.
The point on the Earth's surface directly above the (subterranean) point of origin (hypocenter) of an earthquake, typically located by its latitude and longitude.
F
A planar surface that truncates a spur (narrow ridge) as a result of faulting and subsequent erosion. Also know as a triangular facet or triangular spur. These features are commonly regarded as neotectonic features, although the rates and actual processes of their formation are poorly understood.
A fracture or zone of fractures along which there has been displacement of the adjacent blocks relative to one another. There are three major types of faults: normal, reverse, and strike-slip.
For the purposes of this database, we defined four categories of faults
(Classes A-D) based on demonstrable evidence of tectonic movement during
the Quaternary (known or presumed to be associated with large-magnitude
earthquakes). Only Class A and B features are shown on the map and described
in the database; Class C and D features are described separately.
Search
Class A and B features; search Class C and D features
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Definition |
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Geologic evidence demonstrates the existence of a Quaternary fault of tectonic origin, whether the fault is exposed by mapping or inferred from liquefaction or other deformational features. |
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Geologic evidence demonstrates the existence of Quaternary deformation, but either (1) the fault might not extend deeply enough to be a potential source of significant earthquakes, or (2) the currently available geologic evidence is too strong to confidently assign the feature to Class C but not strong enough to assign it to Class A. |
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Geologic evidence is insufficient to demonstrate (1) the existence of tectonic faulting, or (2) Quaternary slip or deformation associated with the feature. |
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Geologic evidence demonstrates that the feature is not a tectonic fault or feature; this category includes features such as joints, landslides, erosional or fluvial scarps, or other landforms resembling fault scarps but of demonstrable non-tectonic origin. |
A commonly used term that is synonymous with the surface trace of a fault.
One of several closely-spaced parallel or subparallel faults in a fault zone.
The three dimensional description of the seismic waves that radiate outward from the focus (hypocenter) of an earthquake. The focal mechanism contains information on the orientation and slip on two perpendicular planes, either of which could represent the fault that ruptured to produce the earthquake. Additional information is needed to select which of the two orientations is correct.
Also known as the hypocenter, the focus of an earthquake is the point on the fault plane where rupture began. This point is defined by latitude, longitude, and depth.
The underlying side of a fault. If you walked on the fault plane, your foot would be on this wall. See also hanging wall.
G
geologic setting [database field]
Generalized description of the geologic setting of the fault in terms of its regional geology and location, amount of total offset, and general age of offset strata.
geomorphic expression [database field]
Generalized description of deformational features at the surface that are related to the fault or fold, such as the size and shape of scarps, offset streams, sag ponds, grabens, shutter ridges, and faceted spurs.
An acronym for geographic information systems, which includes all computer software programs that allow the creation, manipulation, and output of geographically referenced data.
An acronym for Greenwich Mean Time, which is used as the basis of a standard time throughout the world. All time is measured relative to Greenwich Mean Time. The Greenwich Meridian (or prime meridian of the World) is taken as 0° longitude and it is the line from which all other lines of longitude are measured. Local time in most locations of the United States is 5-9 hours earlier than GMT. Seismologists may refer to UTC (Universal Coordinated Time), which is the same as GMT.
An elongate downdropped block bounded by normal faults on its long side.
H
The overlying side of a fault: this is the part that "hangs" above the fault plane. See also footwall.
historic earthquake [database field]
Shows name and date of historical surface deformation. Details of earthquake can be found by clicking the hyperlink.
historical surface deformation [database field]
Time and date, and link to narrative that describes large historical earthquakes that caused surface faulting or other forms of tectonic deformation. In the United States, the vast majority of these earthquakes have occurred in California, Alaska, and Nevada.
The most recent epoch of the Quaternary period of geologic time. The Holocene was preceded by the Pleistocene epoch. For this report, the Holocene is considered to span the time interval from 10,000 years ago to the present.
An elongate uplifted block bounded by faults on its long side.
Also known as the focus, the hypocenter of an earthquake is the point on the fault plane where rupture began. This point is defined by latitude, longitude, and depth.
I
A measure of the level of earthquake shaking at a specific location. The dominant intensity system used in the U.S. is the Modified Mercalli intensity (MMI) scale. The magnitude of an earthquake is related to the total energy released by the event; an earthquake has only a single magnitude value. The shaking at the earth's surface produced by an earthquake decreases with distance form the epicenter and, therefore, an earthquake can have many intensities.
A contour map showing the distribution of intensities for an earthquake. Isoseismal lines divide areas of equal intensity from one another on an isoseismal map.
isoseisms (or isoseismal lines) 
Lines connecting points at which the intensity for an earthquake is the same.
J
K
An abbreviation for kilo-annum, which is the Latin term for "thousand years" ago. This abbreviation is commonly used when referring to the age of a geologic unit or time of an event with respect to the present time (for example, the event occurred between 2 ka and 4 ka). Note that "ago" is implied by ka, and need not be stated (contrast k.y.).
An abbreviation for kilo-years or "thousand years." This abbreviation is commonly used to designate an interval of time in the past (for example, the event occurred 10 k.y. after deposition of the alluvium).
L
The latter part of the Quaternary era. Late Quaternary refers to the time between 130,000 years ago and the present (contrast ka).
lateral fault or lateral-slip fault
A fault that slips in such a way that the two sides move laterally with respect one another. These terms are generally synonymous to strike-slip faults. See left lateral or right lateral.
The angular distance (in degrees) north or south from the equator of a point on the earth's surface, measured on the meridian (north-south line) to the point (compare longitude).
Horizontal displacement along a fault such that, in plan view, the side opposite the viewer appears to have moved to the left. This term is also known as sinistral.
Length data is end-to-end length of the Quaternary-age fault or fold axis as measured along a straight line from the most distal ends of the surface trace. Cumulative trace length, which is the sum of the lengths of the actual map traces, is provided in the comments when that value is greatly different from the end-to-end length.
A linear topographic feature, or an alignment of topographic features or other surficial features that may reflect control by the underlying geology. Some lineaments are defined by alignments of vegetation, patterns in drainage systems, subtle color changes visible on aerial photographs, or cultural features such as fence lines or power lines. Some lineaments are associated with faults.
The transformation of loose sediment or soil into a fluid state as a result of increasing the pressure of the fluid in between the grains due to strong ground shaking. Liquefaction typically occurs in poorly consolidated, water-saturated sediment. Liquefaction can cause significant earthquake-related damage because structures located on ground that liquefies can collapse or sink into the ground
The angular distance (in degrees) east or west on the earth's surface, measured by the angle contained between the meridian (north-south line) of that place and the prime meridian (at Greenwich, England) (compare latitude). See also GMT.
M
An abbreviation for mega-annum, which is the Latin term for "millions of years." This abbreviation is commonly used when referring to the age of a geologic unit with respect to the present time; for example, the age of the faulted rock is 10 Ma.
A general term for a measure of the total size of an earthquake (contrast with intensity). The size of an individual earthquake can be measured by the strength of the shaking or the duration of the shaking, both of which are directly related to the energy that is released by the earthquake. In modern seismology, the magnitude is determined from seismographic records of an earthquake. Commonly used magnitude measurements include ML, MS, mb, and Mw (see specific entries below.)
The magnitude of an earthquake determined by measuring the maximum amplitude of the P-wave on a seismogram of the event. P-waves, or primary waves, are compressional waves that have the highest velocity of all waves generated by earthquakes.
The largest earthquake in a series of earthquakes that cluster, both geographically and in time. To be definitively called a mainshock, it should generally be at least half a magnitude unit larger than the next largest earthquake in the series. Otherwise, the series of earthquakes may be more accurately characterized as an earthquake swarm.
The middle part of the Quaternary era. Middle Quaternary refers to the time between 750,000 and 130,000 years ago.
A numerical calculation that defines the strength (magnitude) of an earthquake based on seismograms from stations within 600 km; also commonly known as the Richter magnitude. As initially defined by Charles Richter, ML represented the largest deflection of the needle on a standard seismograph at a distance of 100 km from the epicenter of a shallow earthquake that was recorded in southern California.
Modified Mercalli intensity (MMI) scale
An earthquake intensity scale, originally developed by Italian seismologist Giuseppi Mercalli in 1902, which was later modified in 1931 to reflect conditions in the United States. The scale describes the effects of an earthquake in twelve categories, from I (not felt by people) to XII (total damage) (see intensity).
most recent prehistoric deformation [database field]
Defines one of the four time categories in which the most recent prehistoric surface-rupturing or surface-deforming earthquake occurred based on geologically recognizable evidence of faulting, folding, or liquefaction. The categories are (1) latest Quaternary (<15 ka), (2) late Quaternary (<130 ka), (3) late and middle Quaternary (<750 ka), and (4) Quaternary (<1.6 Ma). This field documents prehistoric events if there has been historical surface faulting or folding.
The magnitude of an earthquake determined from surface waves on a seismogram from a teleseismic earthquake (one located more than 20° away). Surface waves are seismic waves that travel over the surface of the Earth versus than those that travel through the Earth, such as P-waves and S-waves. MS magnitudes are measured from surface waves that have a period of about 20 seconds.
The magnitude calculated from an earthquake's total energy (seismic moment). The seismic moment is a function of the amount of slip on a fault, the area of the fault that slips, and the average strength of the rocks that are faulted. Because MW is directly related to the energy released by an earthquake, it is a uniform means of measuring earthquake magnitude and has become the standard measure of earthquake magnitude in modern seismology.
An abbreviation for for "millions of years." This abbreviation is commonly used when referring an interval of time; for example, the Quaternary period lasted 1.6 m.y.
N
Name of structure (fault, fold, liquefaction feature, or fault section, where appropriate). The earliest reference generally is given preference, except in cases where a more commonly accepted name is widely used in the recent literature. The following comments section typically contains source of the name, other names and the references in which they were used, the geographic limits of the structure as shown in this compilation, and geographic limits from other studies that are different from those shown in this compilation. Minor changes to the original name may be made for reasons of clarity or consistency, where appropriate.
A geologic time period corresponding to the later part of the Tertiary Period (1.6-26 Ma); includes the Miocene (26-5 Ma) and Pliocene (5-1.6 Ma) epochs.
The study of post Miocene (<5 Ma) structures and structural history of the Earth's crust. Commonly considered to be the study of deformation related to the current stress regime.
A fault characterized by predominantly vertical displacement in which the hanging wall moves downward with respect to the footwall of the fault. If the fault surface is exposed, the footwall is the side onto which water would drip. Generally, this type of fault is a sign of tectonic extension. Diagram of a normal fault (put url for html, single diagram for fault terminology)
The structure (fault, fold, or liquefaction feature) is assigned a unique number. All faults referred to by number are shown in brackets (i.e., [2015]). If sectioned, the fault's number will be followed by a letter (a, b, c, etc) with "a" assigned to the most northern or western section of a fault (e.g., fault 207 has three sections: 207a, 207b, 207c).
O
oblique slip (also oblique fault)
Describes fault motion or fault that has a combination of lateral and vertical slip.
P
A geologic time period corresponding to the early part of the Tertiary era (65-26 Ma); includes Paleocene, Eocene, and Oligocene epochs.
The geologic study of prehistoric earthquakes.
paleoseismology studies [database field]
Includes a synopsis of detailed site-specific paleoseismological studies, typically those involving exploratory trenching. Sites of geomorphic or geophysical studies or detailed geologic mapping (without trenching) are not included. Study sites are identified by fault number, section letter, and site number (e.g., 601c-3).
A term used primarily in the southwestern United States to describe a dry, vegetation-free, flat area at the lowest part of an undrained desert basin, underlain by stratified clay, silt, or sand, and commonly by soluble salts. Playa surfaces are occasionally covered by shallow lakes in the wettest parts of the year.
physiographic province [database field]
Identifies the physiographic province in which the structure is located as defined for the conterminous United States by Fenneman and Johnson (1946, Physical divisions of the United States, U.S. Geological Survey) and for Alaska by Wahrhaftig (1965, U.S. Geological Survey Professional Paper 482).
The earlier of two epochs in the Quaternary period. For the purpose of this compilation, the Pleistocene epoch is considered to have begun about 1.6 million years ago and ended at the beginning of the Holocene epoch, about 10,000 years ago. (put url for html showing time diagram)
Q
The period of geologic time starting about 1.6 million years ago and continuing to the present. It is divided into two epochs: the Pleistocene and the Holocene. Late Quaternary refers to the time between 130,000 years ago and the present day. Pre-Quaternary refers to any time before about1.6 million years ago.
R
recurrence interval [database field]
Time interval between deformation events (faulting, folding, or liquefaction), based on historical data, calendric or calibrated radiocarbon dates. Intervals reported in 14C years based on radiocarbon dates or in k.y. (thousands of years) based on non-numerical methods (such as stratigraphy or geomorphology). Also includes the time period for which this recurrence interval is valid (e.g., 10-130 ka). Other published recurrence intervals, starting with the one that applies to the most recent time interval, are included in "Comments."
Includes complete bibliographic citations for all references for each structure.
In-text citations of references are presented in a standard USGS format. For authors having multiple publications in the same year, a reference-specific number is added at the end (e.g., Collins, 1988 #1243), in lieu of the traditional alphabetic character for authors having multiple publications in the same year (e.g., Collins, 1988a).
reliability of location [database field]
Reliability of location (Good or Poor) refers to the scale of the source map from which the trace of the structure was taken and to the method by which the trace of the structure was mapped. To qualify as a "good" location, either (1) the trace of the structure was shown on a topographic base map at a scale of 1:250,000 or more detailed and was accurately located on the original map using photogrammetry or similar methods, or (2) the trace of the structure was published on a topographic base map at a scale of 1:100,000 or more detailed, but transferred without photogrammetric methods. Traces that do not meet the above standards (less detailed/smaller scale, planimetric base, transfer by inspection, etc.) constitute a "poor" location. Judgments of reliability may not directly relate to line symbols (solid, dashed, dotted) that are used to represent the structure on geologic maps; however, all concealed or inferred faults are considered poorly located.
A fault in which the displacement is predominantly vertical, and the hanging wall moves up with respect to the footwall. The footwall is the side of the fault onto which water would drip if the fault is exposed. If the fault has a dip angle of less than 45 degrees, it is called a thrust fault.
Introduced in 1935 by Charles F. Richter and Beno Gutenberg, the Richter scale is based on a logarithmic expression that quantifies earthquake magnitude&emdash;typically it refers to local magnitude, but for large earthquakes, it commonly refers to surface-wave magnitude. (Large earthquakes are also commonly assigned a moment magnitude, which is based on seismic moment and is a better measure of the energy of an earthquake than are local or surface-wave magnitudes.) Since the Richter scale is logarithmic, very small earthquakes (microearthquakes) can have a negative magnitude. Although the scale has no theoretical upper limit, the practical upper limit, given the strength of materials in the crust, is just below 9 for local or surface-wave magnitudes and just below 10 for moment magnitudes. (See also ML, local magnitude.)
A slip surface that develops in the early stage of shearing (faulting). Such shears are typically arranged en echelon, usually at inclinations of 10°-30° to the direction of relative movement.
Horizontal displacement along a fault such that, in plan view, the side opposite the viewer appears to have moved to the right. This term is also known as dextral.
S
A small body of water (pond) that occupies an enclosed depression or sag formed where recent fault movement has impounded drainage. Most common along strike-slip faults and in normal-fault grabens.
A prominent, fairly linear slope or escarpment. Scarps are often produced by faulting, especially that which involves a significant amount of dip slip. However, scarps can also be caused by stream erosion, wave erosion (lake shorelines) or landsliding. Fault scarps separate adjacent ground surfaces that are at different elevations.
A measure of the strength of an earthquake, computed from the product of the area of fault rupture, the average amount of slip, and the shear modulus (rigidity) of the rocks offset by faulting. The moment can also be calculated from the amplitude spectra of seismic waves.
seismic zone (or seismic belt)
A region of the Earth's crust, generally an elongated region, associated with active seismicity. It may not be associated with a particular subsurface or surface fault.
Usually for a specific geographic area, the seismicity describes the geographic, depth, and magnitude distribution of earthquakes.
The recording made by a seismograph in response to ground motions caused by an earthquake, explosion, or other source. Old records were recorded mechanically on paper, but modern records are recorded digitally. The seismogram's x-axis is usually time, whereas the y-axis records ground amplitude, velocity, or acceleration.
An instrument that detects, magnifies, and records earthquake and other ground motions.
sense of movement [database field]
Sense of movement for a fault is based on the angle of dip of the fault and the relative direction of movement across the fault. Terms used to describe sense of movement include dip-slip, normal, reverse, thrust, strike-slip, dextral (right-lateral), sinistral (left-lateral), and oblique.
A ridge formed by vertical, lateral or oblique displacement on a fault that crosses an area having ridge and valley topography, with the displaced part of the ridge "shutting in" the valley.
See left-lateral.
Trend or bearing of the line marking the intersection of a fault plane (or other planar geologic feature) with a horizontal surface. Strike is always at a right angle to dip.
Slip (movement) that is parallel to the trace of the fault. Two kinds of strike slip occur: right-lateral (also referred to as dextral) and left-lateral (also referred to as sinistral). Also known as lateral-slip fault movement. (Compare dip-slip fault movement.)
The rate of motion obtained when amount of offset is divided by time interval. The common units of measure are mm/yr or m/k.y. (equivalent units). The average slip rate at a point along a fault is commonly determined from geodetic measurements, displacement of manmade features, or from offset geologic features whose age can be estimated or measured. Offset is measured parallel to the predominant slip direction or estimated from the vertical or horizontal separation of geologic features. In special cases, interval slip rates may be calculated if the times and amounts of slip of prehistoric earthquake events have been determined. This type of high-quality data is rather sparse.
slip-rate category [database field]
Defines one of four slip-rate categories as determined by the compiler or based on reported slip rates. The categories include (1) less than 0.2 mm/yr, (2) 0.2 to less than 1 mm/yr, (3) 1 to 5 mm/yr, and (4) greater than 5 mm/yr. "Comments" include a brief description of published slip rates and pertinent documentation or the basis for the compiler's selection. Generally, two types of slip rates are reported. The first type is herein termed a "geologic slip rate" and is typically derived from the age and amount of offset of geologic features. These rates are averages of slip over several to many earthquake cycles. The second type defines an interval or paleoseismic slip rate that is calculated on the basis of known times and amounts of slip for two or more prehistoric earthquakes.
State or states where the structure is located. If the structure is in more than one state, the state containing the majority of the structure is listed first, followed by state name(s) for the remainder of the structure. In cases where the features encompass numerous states, the states are listed alphabetically by state. Although "State" does not apply to Puerto Rico, Guam or other United States Territories, we place these names in the state database field for search purposes.
A fault in which the dominant sense of motion is horizontal, parallel to the strike of the fault . Also known as a lateral-slip fault. Motion is commonly described as left-lateral (sinistral) or right-lateral (dextral).
The breakage of ground along the surface trace of a fault, caused by the intersection of the part of the fault that ruptured in an earthquake with the Earth's surface.
The intersection of a fault with the surface of the Earth. It is sometimes, but not always, expressed at the surface by geomorphic evidence such as scarps, ridges, valleys, saddles, sag ponds, etc. Also called fault line or fault trace.
Contains a concise summary of information that serves as a thumb-nail sketch of what is known about the structure.
T
The earliest of two periods in the Cenozoic era of geologic time. The Tertiary period begins at the end of the Cretaceous period (about 65 million years ago) and ends at the beginning of the Quaternary period (about 1.6 million years ago).
A fault of less than 45° dip in which the hanging wall moves up relative to the footwall.
A poorly understood process which involves slippage on a fault located in the same region as, but not directly associated with, another fault which ruptures in a major earthquake.
U
V
W
X
Y
year of historic deformation [database field]
Shows year that historical deformation occurred on structure. Details of deformation shown under historical surface deformation in fault description. Detail of earthquake can be found by clicking the hyperlink next to the year. Enter four digit year in search form.
Z
