# Glossary

The glossary is partly extracted from New Manual of Observatory Practice and some information is taken from Modern Global Seismology.

**Aftershocks**

Earthquakes that follow a large earthquake in a sequence. They are smaller than the mainshock and within 1-2 fault lengths distance from the mainshock fault. Aftershocks can continue over a period of weeks, months, or years, decreasing in frequency with time. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.

**Arrival**

a) The appearance of seismic energy on a seismic record.

b) QuakeML object. The detected phase onset associated to an origin in SeisComP3.

**Arrival time**

The time at which a particular phase of a seismic wave arrives at a station.

**Asthenosphere**

The ductile part of the Earth, just below the brittle lithosphere, in the upper mantle. The lithosphere/asthenosphere reaches down to about 200 km.

**Azimuth**

In general a direction measured clock-wise in degrees against north. In seismology used to measure the direction from a seismic source to a seismic station recording this event.

**Backazimuth**

The direction from the seismic station towards a seismic source, measured in degrees clock-wise against north; sometimes also just called azimuth.

**Benioff zone**

see Wadati-Benioff zone

**Body wave**

see Wave, body

**Body wave magnitude**

see Magnitude, body-wave

**Calibration**

The process of determining the response function (distortion of the input signal) and sensitivity of an instrument or its derived component.

**Circum-Pacific belt**

The zone surrounding the Pacific Ocean that is characterized by frequent and strong earthquakes and many volcanoes as well as high tsunami hazard. Also called the Ring of Fire.

**Coda**

The tail of a seismic signal, usually with exponentially decaying amplitudes, which follow a strong wave arrival. Coda waves are due to scattering and superposition of multi-path arrivals.

**Coda phase**

see Phase, coda

**Coherent**

Seismic signals detected on various seismic sensors of a seismic array or network are said to be coherent if they are related to each other in time, amplitude and/or waveform because they come from the same seismic source.

**Convolution**

A mathematically equivalent operation that describes the action of a linear (mechanical and/or electronic) system on a signal, such as that of a filter on a seismic signal.

**Core**

The innermost part of the Earth. The outer core extends from about 2900 to about 5120 km below the Earth's surface and consists in its main components of a mixture of liquid iron and nickel. The inner core is the central sphere of the Earth with a diameter of 1250 km and consists of solid metal.

**Core-Mantle Boundary(CMB)**

see Gutenberg discontinuity

**Corner frequency**

The frequency at which the curve representing the Fourier amplitude spectrum of a recorded seismic signal abruptly changes its slope. For earthquakes, this frequency is a property of the source and related to fault size, rupture velocity, source duration and stress drop in the source. Also the frequency at which the transfer function / magnification curve of a recording system changes its slope.

**Creep**

Slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation. Also applied to slow movement of landslide masses down a slope because of gravitational forces. Faults that are creeping do not tend to have large earthquakes. This fault condition is commonly referred to as unlocked.

**Crust**

The outermost major layer of the Earth, ranging from about 10 to 70 km in thickness worldwide. The oceanic crust is thinner (about 10 to 15 km) than the continental crust (about 25 to 70 km). The uppermost 15-35 km of the crust is brittle enough to produce earthquakes. The seismogenic crust is separated from the lower crust by the brittle-ductile boundary. The crust is usually characterized by P-wave velocities below 8 km/s (average velocity of about 6 km/s).

**Delay**

The time difference between the arrival time and the end time of the last record achieved plus the half record length. (SeisComP3)

**Depth Phase**

see Phase, pP or Phase, sP

**Detection**

Identification of an arrival of a seismic signal with amplitudes above and/or signal shape (waveform) different from seismic noise.

**Directivity**

An effect of a propagating fault rupture whereby the amplitudes of the generated ground motions depend on the direction of wave propagation with respect to fault orientation and slip direction(radiation pattern). The directivity and thus the radiation pattern is different for P and S waves.

**Epicenter**

Vertical projection of the hypocenter to the surface.

**Event**

a) General term used for a localized disturbance (earthquake, explosion, etc.) which generates seismic waves.

b) QuakeML object. The event is the parent object of several origins. Among these origins a preferred origin and its preferred magnitude is selected to represent the event. An event can be seen as a earthquake folder which contains information about earthquake parameters.

**Fault-plane solution**

Representation of the fault activated in an earthquake and the caused direction of slip on the fault by a circle with two intersecting curves looking like a beach ball. A fault-plane solution is found by the analysis of seismic records at many stations of an earthquake to obtain the radiation pattern. From the radiation pattern the fault parameter and the slip direction are determined using a stereographic projection or its mathematical equivalent. The most common analysis uses the direction of first motion of P wave onsets and yields two possible orientations for the fault rupture and the direction of seismic slip. Another technique is to use the polarization of teleseismic S waves and/or to measure amplitude ratios between different phase types. Further inferences can be made from these data concerning the principal axes of stress in the region of the earthquake. The principal stress axes determined by this method are the compressional axis (also called the P-axis, i.e. the axis of greatest compression, or s1), the tensional axis (also known as the T-axis, i.e., the axis of least compression, or s3), and the intermediate axis (s2).

**Filter(ing)**

A filter attenuates certain frequencies of a (seismic) signal and amplifies others. The process of filtering can be accomplished electronically while recording or numerically in a computer. Filtering also occurs naturally as seismic energy passes through the Earth.

The available and integrated filters in SeisComP3 are Butterworth filter (BW – bandpass, BW_LP – lowpass, BW_HP – highpass), averaging filter (AC), STALTA filter, integration filter (INT), cosine taper (ITAPER), running mean high pass filter (RMHP) and some seismometer response filter: WWSSN_SP, WWSSN_LP, Wood-Anderson (WA) and 5-Second Seismometer (SM5).

**Filter, AC**

A running average filter for a given averaging time window in seconds, e.g. AC(30) computes the average for moving 30seconds time windows. (SeisComP3)

**Filter, BW**

The standard bandpass filter in SeisComP3 is a Butterworth Bandpass filter (BW) realized as a causal recursive IIR (infinite impulse response) filter. An arbitrary bandpass filter can be created for given order and corner frequencies, e.g. BW(3,0.7,2) being a third order bandpass filter with corner frequencies of 0.7 Hz to 2 Hz.

**Filter, BW_HP**

The standard highpass filter in SeisComP3 is a Butterworth highpass filter (BW_HP) realized as a causal recursive IIR (infinite impulse response) filter. An arbitrary highpass filter can be created for a given order and corner frequency, e.g. BW(4,0.8) being a fourth order highpass filter with a corner frequency of 0.8 Hz.

**Filter, BW_LP**

The standard lowpass filter in SeisComP3 is a Butterworth lowpass filter (BW_HP) realized as a causal recursive IIR (infinite impulse response) filter. An arbitrary lowpass filter can be created analog to the highpass for a given order and corner frequency, e.g. BW(4,1) being a fourth order lowpass filter with a corner frequency of 1 Hz.

**Filter, INT**

The standard integration filter in SeisComP3 is realized as a recursive IIR (infinite impulse response) integration filter. An integration filter can be applied for a given time span in seconds, e.g. INT(10) integrates in a moving 10 seconds time window.

**Filter, ITAPER**

A one-sided cosine taper in SeisComP3. The cosine taper is applied to a given time window in seconds, e.g. ITAPER(30) distorts the amplitude of the signal in the first 30 seconds by multiplying with factors between zero and one according to a half cosine function.

**Filter, RMHP**

A highpass filter in SeisComP3 realized as running mean highpass filter. For a given time window in seconds the running mean is subtracted from the single amplitude values. This is equivalent to highpass filtering the data. E.g. RMHP(10) calculates the difference to the moving mean in a 10 seconds time window.

**Filter, SM5**

In SeisComP3 a 5-second seismometer can be simulated. The data format of the waveforms has to be given for applying the simulation (displacement = 0, velocity = 1, acceleration = 2), e.g. SM5(1) is the simulation on velocity data.

**Filter, STALTA**

A STA/LTA filter is the ratio of a short-time average to a long-time average calculated continuously in two consecutive time windows. This method is the basis for many trigger algorithm. The short-time window is for detection of transient signal onsets whereas the long-time window provides information about the actual seismic noise at the station.

In SeisComP3 the STA/LTA can be determined if the time window lengths for both windows are given, e.g. STALTA(2,60) computes the ratio of the average in the 2 seconds time window and the previous 60 seconds time window for moving time windows all over the trace.

**Filter, WA**

The simulation filter of a Wood-Anderson seismometer in SeisComP3. The data format of the waveforms has to be given for applying the simulation filter (displacement = 0, velocity = 1, acceleration = 2), e.g. WA(1) is the simulation on velocity data.

**Filter, WWSSN_LP**

The instrument simulation filter of a World-Wide Standard Seismograph Network (WWSSN) long-period seismometer in SeisComP3. The data format of the waveforms has to be given for applying the simulation filter (displacement = 0, velocity = 1, acceleration = 2), e.g. WWSSNN_LP(1) is the simulation on velocity data.

**Filter, WWSSN_SP**

Analog to the WWSSN_LP, the simulation filter of the short-period seismometer of the WWSSN in SeisComP3. The data format of the waveforms has to be given for applying the simulation filter (displacement = 0, velocity = 1, acceleration = 2), e.g. WWSSNN_SP(1) is the simulation on velocity data.

**First motion**

The first noticeable displacement in a seismogram caused by the arrival of a P wave at the seismometer. Upward motion of the ground at the seismometer indicates a dilatation at the source, downward motion indicates a compression. Due to the presence of seismic noise the proper polarity of the first motion may be difficult to recognize.

**Focal mechanism**

see fault-plane solution

**Foreshocks**

Earthquakes that occur in a series of earthquakes before the largest earthquake, termed the mainshock. Foreshocks may precede the mainshock by seconds to weeks and usually originate at or near the focus of the larger earthquake. Not all mainshocks have foreshocks.

**Fourier spectrum**

The relative amplitudes (and phase angles) at different frequencies that are derived from a time series by Fourier analysis.

**Fourier analysis**

The mathematical operation that resolves a time series (for example, a recording of ground motion) into a series of numbers that characterize the relative amplitude and phase components of the signal as a function of frequency.

**Frequency domain**

The transformation of a seismic signal from the time domain (as a seismogram) to the frequency domain is conducted by a Fourier analysis. The signal is represented in the frequency domain by the amplitude and phase components as a function of frequency (see spectrum). The representations of a seismic signal in the time and in the frequency domain are equivalent in a mathematical sense. For some procedures of data analysis the time-domain representation of a seismic record is more suitable while for others the frequency-domain approach is more appropriate and efficient.

**Geometrical spreading**

The component of reduction in wave amplitude due to the radial spreading of seismic energy with increasing distance from a given source.

**Green's function**

A mathematical representation that, in reference to earthquake shaking, is used to represent the ground motion caused by instantaneous slip on a small part of a fault. Green’s functions can be summed over a large fault surface to compute the ground shaking for a large earthquake rupturing a fault of finite size. The fractional fault-slip events that are summed can be records from small earthquakes on the fault or they can be theoretically computed small-earthquake records.

**Gutenberg discontinuity**

The seismic velocity discontinuity marking the core-mantle boundary (CMB) at which the velocity of P waves drops from about 13.7 km/s to about 8.0 km/s and the velocity of S waves drops from about 7.3 km/s to 0 km/s. The CMB reflects the change from the solid mantle material to the fluid outer core.

**Hypocenter**

Coordinates of an earthquake point source. Hypocenters based on P and S wave first arrivals point to the place where the rupture process starts. For large earthquakes the source location determined by P wave first arrivals can differ significantly from the location of maximum energy release.

**Intensity**

A measure of the effects of an earthquake at a particular place at the Earth's surface on humans and (or) structures. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake, the depth of the hypocenter and the local geology at that point. Several scales exist, most of them giving the intensity in 12 degrees, usually written as Roman numerals. Most frequently used are at present the European Macroseismic Scale (EMS-98), and in the United States the Modified Mercalli scale and the Rossi-Forel scale. There are many different intensity values for one earthquake, depending on how far you are away from the epicenter; this is unlike the magnitude value, which is one number for each earthquake as a measure of the amount of seismic wave energy released by it.

**Interplate/intraplate**

Intraplate pertains to processes within the Earth's crustal plates. Interplate pertains to processes between the plates.

**Interplate coupling**

The qualitative ability of a subduction thrust fault to lock and accumulate stress. Strong interplate coupling implies that the fault is locked and capable of accumulation stress whereas weak coupling implies that the fault is unlocked or only capable of accumulating low stress. A fault with weak interplate coupling could be aseismic or could slip by creep.

**Latency**

The time difference between the current time and the arrival time of the record. (SeisComP3)

**Lithosphere**

The outer solid part of the Earth, including crust and uppermost mantle. The lithosphere is about 100 km thick, although its thickness is age-dependent (older lithosphere is thicker). At some locations the lithosphere below the crust is brittle enough to produce earthquakes by faulting, such as within a subducted oceanic plate.

**Love wave**

A major type of surface waves having a horizontal motion that is transverse (or perpendicular) to the direction of propagation. It is named after A. E. H. Love, the English mathematician who discovered it.

**Leaky mode**

A seismic surface wave which is imperfectly trapped, e.g., within a low-velocity layer or a sequence of layers, so that its energy leaks or escapes across a layer boundary causing some attenuation.

**Low-velocity layer/zone**

Any layer in the Earth in which seismic wave velocities are lower than in the layers above and below.

**Magnification curve**

A diagram showing the dependence of amplification, e.g. of the seismic ground motion by a seismograph, as a function of frequency.

**Magnitude**

A number that characterizes the relative size of an earthquake. The magnitude is based on measurement of the maximum motion recorded by a seismograph (sometimes for waves of a particular frequency), corrected for the attenuation with distance. 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). The magnitude scales 1 – 3 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 earthquake sizes but is more difficult to compute than the other types. In principal, all magnitude scales could be cross calibrated to yield the same value for any given earthquake, but this expectation has proven to be only approximately true, thus the magnitude type as well as its value is needed to be specified.

**Magnitude, local (ML)**

Magnitude scale introduced by Richter in the early 1930s (Richter, 1935) to have a common scale for the strength of earthquakes. The basic observation is the systematic decay of the logarithm of the maximum amplitudes with increasing distance for different earthquakes described by:

ML = log A_{max}- log A_{0}

with A_{0}as amplitude of a reference event. For the reference event ML = 0 the formula can be rewritten to

ML = log A__{max}- 2.48 + 2.76 log ∆

with ∆ being the distance of the station to the earthquake location. ML is a magnitude scale for recordings of earthquakes smaller than ML 7 at regional stations. It is usually a measure of the regional-distance S-wave on horizontal component records. The original formula is only valid for records from a Wood-Anderson torsion seismometer with a natural period of 0.8 s and shallow earthquakes in California. Therefore calibration functions for other regions and wider depth ranges are necessary. A Wood-Anderson seismometer has to be simulated. For amplitudes measurements on the vertical component records additional correction factors has to be applied. ML saturates at magnitudes around 7 because the maximum amplitudes of larger earthquakes occur at longer periods than the bandpass of 0.1 s and 3 s for the magnitude calculation.

In SeisComP3 a modified local magnitude Mlv is determined by simulation of a Wood-Anderson instrument and then measuring the amplitude in a 150 s time window on the vertical component of station with distances smaller than 8°.

**Magnitude, body-wave (mb)**

Magnitude developed for teleseismic body waves. mb is defined on the amplitude of the first few cycles of the P-wave, typically a time window of 20 s - 30 s. Only the first few cycles are used to minimize the effects of radiation pattern and depth phases, which result in complicate waveform signatures. The general formula is

with A as the displacement amplitude in micrometers, T as the dominant period of the signal in seconds, Q as a correction term for depth and distance. mb is usually determined at periods around 1s in adaptation to the use of the World-Wide Standard Seismograph Network (WWSSN) short-period stations. A scatter in the order of +/- 0.3 for the station magnitudes is usual. Typically, mb is determined for stations with distances larger than 5° to have a distinct direct P-wave phase. A correction term for the distance has to be determined empirically, which is quite complicate for distances smaller than 20°. This reflects the complexity of the body waves that traverse only in the upper mantle. mb saturates at about magnitude 5.5 to 6.0 because the maximum amplitudes of larger earthquakes occur at lower frequencies than the frequency range between 0.7 Hz - 2 Hz used for the magnitude calculation.

In SeisComP3 mb is determined by simulation of a WWSSN short-period seismometer and then measuring the amplitudes in a 30 s time window at stations in the distance range of 5° to 105°.

**Magnitude, broadband body-wave (mB)**

mB is a magnitude based on body waves like mb, but the amplitude is measured in a broad frequency range and longer time windows. Instead of amplitude measurements on displacement data together with the dominant period, the maximum velocity amplitude Vmax is taken directly from velocity-proportional records with V = 2πA/T. The time window for the measurement can be determined by the duration of the high-frequency (1-3 Hz) radiation (Bormann & Saul, 2008). This time window usually contains the phases P, pP, sP, PcP, but not PP. According to the long time window and broad frequency range used for amplitude measurements mB saturates not like mb.

In SeisComP3 a static time window of 90 s is actually taken for amplitude measurements at stations in the distance range of 5° to 105°.

**Magnitude, surface wave (Ms)**

Ms is a magnitude scale based on teleseismic surface waves. Ms is based on measurements of the maximum horizontal true ground motion displacement amplitudes

in the total seismogram at periods around 20s. For shallow earthquakes the dominant long-period signals are the surface waves. The period of 20 s corresponds to the Airy phase, a local minimum in the group velocity dispersion curve of Rayleigh surface waves. The Moscow-Praque equation for surface wave magnitude is given by

Here the maximum ground particle velocity (A/T)max is used instead of the AHmax to allow a broader spectrum of dominant periods. This formula is valid for distances of 2° to 160° and source depth smaller than 50 km.

**Magnitude, JMA (M_JMA)**

M(JMA) is a magnitude similar to the Ms, but the formula is calibrated for instruments with 5 s period at local distances. The data set for the calibration was gained by the Japan Meteorological Agency (JMA).

This equation is valid for local (< 2000 km) and shallow (< 60 km) earthquakes. For deeper earthquakes additional correction functions have to be applied (Katsumata, 1996).

**Magnitude, moment (Mw)**

The moment magnitude is a magnitude scale related to the seismic moment M

_{0}and thus to the released seismic energy. To obtain the Mw the seismic moment is first determined, e.g. by a moment tensor inversion. Then the Mw is gained by the following relationship between seismic moment and the moment magnitude:

This equation is analog to the relation between M_{s}and M_{0}.

**Magnitude, broadband P-wave moment (Mwp)**

The Mwp is a rapid estimate of the moment magnitude based on the first-arrival P waves on broadband seismograph records. The displacement seismograms of the P wave portion are considered as source time function approximation. The seismic moment is estimated for each station by integrating the displacement records. The combination of multiple records results in an estimation of the moment magnitude without correction for the source mechanism (Tsuboi et al., 1995).

**Mainshock**

The largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks.

**Mantle**

The part of the Earth's interior between the core and the crust.

**Microearthquake**

An earthquake that is not perceptible by man and can be recorded by seismographs only. Typically, a microearthquake has a magnitude of 2 or less on the Richter scale.

**Microseism**

a) In a broader sense: A more or less continuous motion in the Earth in a wide frequency range that is unrelated to any earthquake and caused by a variety of usually uncorrelated (incoherent) natural and artificial (man-made) sources.

b) In a more specific sense: That part of seismic noise that is generated by wave motions on lakes and oceans and their interaction with shores, typically with periods between about 2 to 9 s (the stronger secondary microseisms), and 11 to 18 s (the weaker primary microseisms).

**Moho**

The abbreviation for the Mohorovičić discontinuity.

**Mohorovičić discontinuity**

The discontinuity in seismic velocities that defines the boundary between crust and mantle of the Earth. Named after the Croation seismologist Andrija Mohorovičič (1857-1936) who discovered it. The boundary is between 20 and 60 km deep beneath the continents and between 5 and 10 km deep beneath the ocean floor.

**Network magnitude**

a) The network magnitude is an averaged magnitude value based on several station magnitudes of one event. To stabilize the result a 12.5%-trimmed mean is computed, i.e. the smallest 12.5% of the station magnitude values and the biggest 12.5% are not used for the mean calculation in SeisComP3.

b) QuakeML object.

**Noise (seismic)**

Incoherent natural or artificial perturbations caused by a diversity of agents and distributed sources. One usually differentiates between ambient background noise and instrumental noise. The former is due to natural (ocean waves, wind, rushing waters, animal migration, ice movement, etc.) and/or man-made sources (traffic, machinery, etc.), whereas instrumental (internal) noise may be due to the flicker noise of electronic components and/or even Brownian molecular motions in mechanical components. Digital data acquisition systems may add digitization noise due to their finite discrete resolution (least significant digit). Very sensitive seismic recordings may contain all these different noise components, however, usually their resolution is tuned so that only seismic signals and to a certain degree also the ambient noise are resolved. Disturbing noise can be reduced by selecting recording sites remote from noise sources, installation of seismic sensors underground (e.g., in boreholes, tunnels or abandoned mines) or by suitable filter procedures (improvement of the signal-to-noise ratio).

**Nyquist frequency**

Half of the digital sampling rate. It is the minimum number of counts per second needed to define unambiguously a particular frequency. If the seismic signal contains energy in a frequency range above the Nyquist frequency the signal distortions are called aliasing.

**Onset**

The first appearance of a seismic signal on a record.

**Origin**

a) Location (hypocenter), Time and strength estimation of an earthquake based on seismic phases and amplitudes

b) QuakeML object

**Origin time**

Estimated source time of an event belonging to a certain origin; describes the rupture start time.

Attribute of the QuakeML object Origin, see Origin.

**Phase**

a) A stage in periodic motion, such as wave motion or the motion of an oscillator, measured with respect to a given initial point and expressed in angular measure.

b) A pulse of seismic energy arriving at a definite time, which passed the Earth on a specific path.

c) Attribute of the QuakeML object Arrival, see Arrival.

**Phase, coda**

A detection of a single phase of unknown path found within the coda signal envelope, designated as tx, e.g. Px or Sx.

**Phase, P**

The P phase is the arrival of the direct P wave that traveled through the Earth's crust and mantle observed in epicentral distances up to 100°.

**Phase, Pdiff**

The long-period P-wave energy can be diffracted at the CMB forming at distances larger than 100° the Pdiff phase. The reason for the diffraction is the large reduction of the P-wave velocity at the CMB from about 13.7 km/s to 8 km/s. The amplitude of Pdiff is relatively small. Pdiff is observed at distances where the outer core forms the “core shadow” (see also Phase, PKP).

**Phase, Pg**

Pg is the direct P wave arriving first in local distances less than 100 km. For larger distances Pn arrives first (see Pn for details).

**Phase, Pn**

Pn is the P head wave along the Moho arriving first at local distances larger than 100 km (depending on the crustal thickness). Pn has usually smaller amplitudes than Pg.

**Phase, PcP**

The P wave that is reflected at the CMB forms the PcP. At epicentral distances between 30° and 55° PcP is often recorded as sharp pulse.

**Phase, PKiKP**

A P wave that travels through the Earth's crust and mantle and is reflected at the outer core-inner core boundary. At distances between 100° and 113° PKiKP can be the first arrival if no Pdiff is observed.

**Phase, PKP**

The direct P waves traversing the Earth's crust, mantle and outer core without reflection is called PKP. The outer core is a fluid causing a strong refraction at the CMB into the outer core. The strong refraction of the seismic rays into the core causes a “core shadow” that commences at epicentral distances of around 100° and stretches to around 140°. Only Pdiff can be observed in this distance range. PKP is the first arrival at distances larger than around 143°. At a distance of 144° P waves with several paths through the Earth’s core arrive at the same time (caustic) and form a strong arrival.

**Phase, PP**

PP is a reflected P wave at the Earth's surface traversing the Earth's crust and mantle.

**Phase, pP**

A P wave that has a takeoff angle of greater than 90° at the source and therefore is first reflected at the surface near the epicenter. The pP is a depth phase because at teleseismic distances pP has nearly the same path than the P wave except for the path from hypocenter of the earthquake to the reflection point at the surface.

**Phase, sP**

Another depth phase. The sP is a S wave with a takeoff angle of greater than 90° at the source that is reflected and converted to P wave at the reflection point at the surface near the epicenter.

**Phase, S**

The S phase is the arrival of the direct S wave that traveled through the Earth's crust and mantle observed in epicentral distances up to 100°.

**Phase, Sg**

Sg is the direct S wave arriving first in local distances less than 100 km. For larger distances Sn arrives first (see Phase, Sn for details).

**Phase, Sn**

Sn is the S head wave along the Moho arriving first at local distances larger than 100 km (depending on the crustal thickness). Sn has usually smaller amplitudes than Sg.

**Pick**

a) Automatic or manual determined phase onset

b) QuakeML object

**Polarity**

In seismology the direction of first motion on a seismogram, either up (compression) or down (dilatation or relaxation).

**Polarization**

The shape and orientation in space of the ground-motion particle trajectory. It differs for different types of seismic waves such as P, S and surface waves and may be ± linear or elliptical, prograde or retrograde. It is also influenced by heterogeneities and anisotropy of the medium in which the seismic waves propagate and depends on their frequency or wavelength, respectively. The polarization of ground motion may be reconstructed by analyzing three-component seismic recordings.

**Preferred magnitude**

The network magnitude representing the strength of an event best. (SeisComP3)

Attribute of the QuakeML object Event, see Event.

**Preferred origin**

The origin representing the location of an event best; generally, the location based on the most picks or reviewed/revised by an operator. (SeisComP3)

Attribute of the QuakeML object Event, see Event.

**QuakeML**

A XML scheme developed as an open standard for seismological meta data exchange (www.quakeml.org).

**Radiation pattern**

Dependence of the amplitudes of seismic P and S waves on the direction and take-off angle under which their seismic rays have left the seismic source. It is controlled by the type of source mechanism, e.g., the orientation of the earthquake fault plane and slip direction in space.

**Rayleigh**

A seismic surface wave causing a retrograde, elliptical motion of a particle at the free surface, with no transverse motion. It is named after Lord Rayleigh (1842-1919), who predicted its existence.

**Ray theory**

Theoretical approach, which treats wave propagation as the propagation of seismic rays. It is an approximation, which yields good results for short wave length (high-frequency approximation) and allows easy calculations of travel times.

**Ray-tracing method**

Computational method of calculating ground-shaking estimates that assumes that the ground motion is composed of multiple arrivals of seismic rays and related energy bundles (Gauss beams) that leave the source and are reflected or refracted at velocity boundaries according to Snell's Law. The amplitudes of reflected and refracted waves at each boundary are recalculated according to the Law of Conservation of Energy.

**Recurrence interval**

The average time span between large earthquakes at a particular site. Also termed 'return period'.

**Reflection**

The energy or wave from a seismic source that has been returned (reflected) from an interface between materials of different elastic properties within the Earth, just as a mirror reflects light.

**Refraction**

The deflection, or bending, of the ray path of a seismic wave caused by its passage from one material to another having different elastic properties.

Bending of a tsunami wave front owing to variations in the water depth along a coastline.

**Relaxation theory**

A concept in which radiated seismic energy is released from stored strain energy during the slip along a fault until the adjacent fault blocks reach a new state of equilibrium.

**Residual**

The difference between the measured and predicted values of some quantity (e.g., theoretical and measured phase arrival time).

Attribute of QuakeML object Arrival, see Arrival.

**Ring of Fire**

The zone of volcanoes and earthquakes surrounding the Pacific Ocean which is called the Circum-Pacific belt; about 90% of the world's earthquakes occur there. The next most seismic region (5 - 6 % of earthquakes) is the Alpide belt.

**Root mean square (RMS)**

A statistical measure of the magnitude of a varying quantity defined as

for the time series with the N elements x_{1}to x_{n}.

**Rupture front**

The instantaneous boundary between the slipping and locked parts of a fault during an earthquake. A rupture propagating in one direction on the fault is referred to as unilateral. A rupture may radiate outward in a circular manner or it may radiate towards the two ends of the fault from an interior point, behavior referred to as bilateral.

**Rupture velocity**

The speed at which a rupture front moves across the surface of the fault during an earthquake.

**Seismic array**

An ordered arrangement of seismometers with central data acquisition specially designed to analyze seismic signal based on coherent phases.

**Seismic gap**

A section of a fault that has produced earthquakes in the past but is now quiet. For some seismic gaps, no earthquakes have been observed historically, but it is believed (based on some other methods, such as plate-motion information, strain measurements or geological observations) that the fault segment is capable of producing earthquakes. A long-term seismic gap may give hint to the most probable location of a strong earthquake in the future.

**Seismic moment (M _{0})**

The seismic moment is defined as

M_{0}= μ D A

with μ as rigidity of the rock at the fault, D as averaged displacement on the fault and A as fault surface area. The seismic moment can be related to the released seismic energy ES that is proportional to the stress drop ∆σ:

E_{S}≈ 0.5 ∆σ D A

Rearranging both equations yields to:

M_{0}can be determined by the asymptote of the amplitude spectrum at frequency = 0. A common technique for determination of the seismic moment M_{0}is the moment tensor inversion. Assuming reasonable values for the rigidity of the rock (3-6 x 104 MPa in crust and upper mantle) and the stress drop (2-6 MPa) the seismic moment can be related to the surface wave magnitude Ms by the empirical relationship found by Gutenberg and Richter (1956):

log E_{S}= 11.8 + 1.5 Ms

to

log M_{0}= 1.5 Ms + 16.1 (units in cgs)

**Seismic network**

A group of seismic stations that are deployed as single stations or arrays.

**Seismic ray**

Vector perpendicular to the wave front pointing into the direction of wave propagation and marking behind it the “ray trace”. The propagation of seismic waves can be easily modelled as the propagation of seismic rays following Snell's Law. This assumption is a reasonable approximation for high frequency waves.

**Seismic signal**

A coherent transient waveform radiated from a definite, localized seismic source that is usually considered as an useful signal for the location of the source, the analysis of the source process and/or of the propagation medium (in contrast to noise).

**Seismic source**

A localized area or volume generating coherent, usually transient seismic waveforms, such as an earthquake, explosion, vibrator etc.

**Signal-to-noise ratio**

The comparison between the amplitude of the seismic signal and the amplitude of the noise; abbreviated as SNR.

**Slab**

Usually, the part of the lithospheric plate that is underthrusting in a subduction zone and is consumed by the Earth's mantle is called slab.

**Slab pull**

The force of gravity causing the cooler and denser oceanic slab to sink into the hotter and less dense mantle material. The downdip component of this force leads to downdip extensional stress in the slab and may produce earthquakes within the subducted slab. Slab pull may also contribute to stress on the subduction thrust fault if the fault is locked.

**Slip**

The relative displacement of formerly adjacent points on opposite sides of a fault.

**Slip model**

A kinematic model that describes the amount, distribution, and timing of a slip associated with an earthquake.

**Slip rate**

How fast the two sides of a fault are slipping relative to one another, as derived from seismic records in case of an earthquake or determined, as a long-term average, from geodetic measurements, from offset man-made structures, or from offset geologic features whose age can be estimated. It is measured parallel to the predominant slip direction or estimated from the vertical or horizontal offset of geologic markers.

**Slowness**

The inverse of velocity, given in the unit seconds/degree or s/km; a large slowness corresponds to a low velocity.

**Source depth**

Location of an earthquake below the Earth's surface. Earthquakes can occur between the surface and depths of about 700 km. Usually three classes of earthquakes are seperated according to the depth: Shallow earthquakes occur in the depth range of 0 to 70 km; intermediate earthquakes between 70 and 300km depth; and deep earthquakes between 300 and 700 km depth. Earthquakes at large depths occur much less frequent than shallow earthquakes. Additionally, deep earthquakes excite small surface waves compared to the body waves and relatively simple P and S waveforms with more impulsive onsets. A more reliable way to determine the depth of an earthquake is to identify depth phases (e.g. pP, sP) in the waveforms. At stations with large distance to the epicenter the pP wave follows the direct P wave by a time interval that slighty increase with distance but rapidly with depth. The depth can be derived from this time interval by using traveltime curves.

**Source time function**

The source time function describes the ground motion generated at the fault over time. The function is predicted by a theoretical model.

**Station magnitude**

a) The station magnitude is the magnitude value based on the amplitude measurements of a single station. Due to radiation pattern, site and path effects and the calibration of the station the station magnitudes of one event can scatter significantly.

b) QuakeML object

**Stick-slip**

The rapid displacement that occurs between two sides of a fault when the shear stress on the fault exceeds the frictional stress. Also a jerky, sliding type of motion associated with fault movement in laboratory experiments. It may be a mechanism in shallow earthquakes. Stick -slip displacement on a fault radiates energy in the form of seismic waves.

**Stress drop**

The difference between the stress across a fault before and after an earthquake. A parameter in many models of the earthquake source that affects the level of high-frequency shaking radiated by the earthquake. Commonly stated in units termed bars or megapascals (1 bar equals 1 kg/cm², and 1 megapascal equals 10 bars).

**Takeoff angle**

The angle that a seismic ray makes with a downward vertical axis through the source. Rays with takeoff angles less than 90° are labeled with capital letters like P or S. If the takeoff angle is greater than 90° the ray is upgoing and is labeled with lowercase letters (p or s). Such rays can be reflected at the surface near the epicenter forming a depth phase (see Phase, pP or sP).

**Teleseismic**

Pertaining to a seismic source at distances greater than about 2000 km from the measurement site.

**Theoretical onset**

The point where an arrival is expected to appear on a seismic record, based on the known location and depth of the seismic source and according to a velocity model.

**Time domain**

A seismic record is usually presented in the time domain, i.e., as a display of varying amplitudes of (filtered) ground motion as a function of time (in contrast to the equivalent representation in the frequency domain) (see also Fourier analysis).

**Transfer function**

The transfer function of a seismic sensor-recorder system (or of the Earth medium through which seismic waves propagate) describes the frequency-dependent amplification, damping and phase distortion of seismic signals by a specific sensor-recorder (or medium). The modulus (real term = absolute value) of the transfer function is termed the frequency response function or magnification curve, e.g. of a seismograph.

**Travel time**

The time required for a wave traveling from its source to a point of observation.

**Travel-time curve**

A graph of arrival times, commonly of direct as well as multiply reflected and converted P or S waves, recorded at different points as a function of distance from the seismic source. Seismic velocities within the Earth can be computed from the slopes of the resulting curves.

**Wadati-Benioff zone**

A dipping planar (flat) zone of earthquakes that is produced by the interaction of a downgoing oceanic crustal plate with a continental plate. These earthquakes can be produced by slip along the subduction thrust fault (thrust interface between the continental and the oceanic plate) or by slip on faults within the downgoing plate as a result of bending and extension as the plate is pulled into the mantle. Slip may also initiate between adjacent segments of downgoing plates. Wadati-Benioff zones are usually well developed along the trenches of the Circum-Pacific belt, dipping towards the continents.

**Wave, body**

A seismic wave that propagates through the interior of the Earth, as opposed to surface waves that propagate near the Earth's surface. P and S waves, which shake the ground in different ways, are examples.

**Wave, P**

P (primary) waves are compressional waves involving volumetric variations in the media. The sense of particle motion is linear and parallel to the propagation direction. P waves are body waves that traverse the interior of a body/Earth and can propagate in fluids.

The general nomenclature for P waves: At local and regional distances a special nomenclature is used to describe the travel path of the first P and S arrivals. Pg, Pb/P* and Pn phases are separated. Pg is the direct P wave arriving first in distances less than around 100 km. Pn is the head wave along the Moho arriving first at larger distances than 100 km (depending on the crustal thickness). Pn has usually smaller amplitudes than Pg. Pb or P* is the rarely observed head wave travelling along the midcrustal velocity discontinuity. The general nomenclature of P waves entitles reflections at the topside of boundaries with lowercase letters (m – Moho reflection; c – CMB reflection; i – inner core-outer core boundary reflection), e.g. PmP is a reflected P wave at the Moho. Reflections at the bottomside of boundaries get no additional letter, e.g. PP is a reflected P wave at the Earth's surface. Refracted rays get capital letters (K – through the outer core; I – through the inner core), e.g. PKIKP is a P wave traversing the crust/mantle, the outer core, the inner core, again the outer core and again the mantle/crust.

**Wave, S**

S (secondary) waves are shear waves without any volumetric variation in the media. The sense of particle motion is perpendicular to the propagation direction. S waves are body waves that traverse the interior of a body but can not propagate in fluids.

Analog to the P arrivals Sg, Sb/S^* and Sn arrivals are distinguished in local and regional distances. The general nomenclature of S waves is analog to the P waves. The reflections at the topside of boundaries have lowercase letters (m – Moho reflection; c – CMB reflection), e.g. SmS is a reflected S wave at the Moho. Reflections at the bottomside of boundaries get no additional letter, e.g. SS is a reflected S wave at the Earth's surface. Refracted rays get capital letters (J – through the inner core), e.g. SKJKS is a S wave traversing the crust/mantle, the outer core as a P wave, the inner core as a S wave, again the outer core as a P wave and again the mantle/crust as S wave. S waves can not travel through the outer core because the outer core consists of a fluid.

**Wave, Surface**

Surface waves are seismic waves observed only at the free surface of the media. Two types of surface waves are distinguished: Love waves (L) and Rayleigh waves (R). Both result from the interaction of P and S waves near the free surface.

**Waveform (data)**

The complete analog or sufficiently dense sampled digital representation of a continuous wave group (e.g., of a seismic phase) or of a whole wave train (seismogram). Accordingly, waveform data allow to reconstruct and analyze the whole seismic phase or earthquake record both in the time and frequency domain whereas parameter data describe the signal only by a very limited number of more or less representative measurements such as onset time, maximum signal amplitude and related period.

**WaveformID**

Attribute of the QuakeML objects Pick, StationAmplitude and StationMagnitude describing the source of the underlying waveform source. The WaveformID contains information about the NetworkCode, StationCode, LocationCode and ChannelCode

**Wave front**

The surface formed by all elements of a propagating wave, which swing in phase; the wave front is perpendicular to the seismic rays, which are oriented in direction of wave propagation.

**Wavelength**

The distance between successive points of equal amplitude and phase on a wave (for example, crest to crest or trough to trough).

**Weight**

Attribute of the QuakeML objects Arrival and MagnitudeReferences defining the effect of the referenced object (e.g. Pick)