On intensity of Earthquake shaking

Magnitude is used as a “rough” measure of the source energy released in an earthquake. All magnitude scales are logarithmic to account for the very wide range (factor of about 1013) of measureable event energies. The intensity scales (various) attempt to describe what is felt locally and at short distances from the earthquake fracture(?). Mercalli (modified) index is essentially subjective, the horizontal peak acceleration is a measureable by strong-motion instruments. Engineering construction codes are based on a “spectral” scale which is somewhat complicated to describe but of that later.

The Modified Mercalli Intensity ScaleWhat sensation do you feel at your locale?

I. Not felt except by a very few under especially favorable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial

collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

MMI Value

Summary Damage

Description Used on

Maps

Description of Shaking

Severity

Full description shortened from Elementary Seismology

I Not mapped Not mapped Not felt.

II Not mapped Not mappedFelt by people sitting or on upper floors of buildings.

III Not mapped Not mapped

Felt by almost all indoors. Hanging objects swing. Vibration like passing of light trucks. May not be recognized as an earthquake..

IV Not mapped Not mapped

Vibration felt like passing of heavy trucks. Stopped cars rock. Hanging objects swing. Windows, dishes, doors rattle. Glasses clink. In the upper range of IV, wooden walls and frames creak.

V Light Pictures Move

Felt outdoors. Sleepers wakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing. Pictures move. Pendulum clocks stop.

VI Moderate Objects Fall

Felt by all. People walk unsteadily. Many frightened. Windows crack. Dishes, glassware, knickknacks, and books fall off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster, adobe buildings, and some poorly built masonry buildings cracked. Trees and bushes shake visibly.

VII StrongNonstructural Damage

Difficult to stand or walk. Noticed by drivers of cars. Furniture broken. Damage to poorly built masonry buildings. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices, unbraced parapets and porches. Some cracks in better masonry buildings. Waves on ponds.

VIII Very StrongModerate Damage

Steering of cars affected. Extensive damage to unreinforced masonry buildings, including partial collapse. Fall of some masonry walls. Twisting, falling of chimneys and monuments. Wood-frame houses moved on foundations if not bolted; loose partition walls thrown out. Tree branches broken.

IX ViolentHeavy Damage

General panic. Damage to masonry buildings ranges from collapse to serious damage unless modern design. Wood-frame structures rack, and, if not bolted, shifted off foundations. Underground pipes broken.

X Very ViolentExtreme Damage

Poorly built structures destroyed with their foundations. Even some well-built wooden structures and bridges heavily damaged and needing replacement. Water thrown on banks of canals, rivers, lakes, etc.

XI

Not mapped because these intensities are typically limited to areas with ground failure.

Rails bent greatly. Underground pipelines completely out of service.

XII

Not mapped because these intensities are typically limited to areas with ground failure.

Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into the air.

Mercalli intensities felt for the 1989 Loma Prieta earthquake (Mw ~7)

On Earthquake Energy/Magnitude

Magnitude / Intensity ComparisonMagnitude and Intensity measure different characteristics of earthquakes. Magnitude measures the energy released at the source of the earthquake. Magnitude is determined from measurements on seismographs. Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment.

Magnitude / Intensity ComparisonThe following table gives intensities that are typically observed at locations near the epicenter of earthquakes of different magnitudes.

Magnitude Typical MaximumModified Mercalli Intensity

1.0 - 3.0 I3.0 - 3.9 II - III4.0 - 4.9 IV - V5.0 - 5.9 VI - VII6.0 - 6.9 VII - IX7.0 and higher VIII or higher

Measuring the Size of the Earthquake (source)Earthquakes range broadly in size. A rock-burst in an Idaho silver mine may involve the fracture of 1 meter of rock; the 1965 Rat Island earthquake in the Aleutian arc involved a 650 kilometer length of the Earth's crust. Earthquakes can be even smaller and even larger. If an earthquake is felt or causes perceptible surface damage, then its intensity of shaking can be subjectively estimated. But many large earthquakes occur in oceanic areas or at great focal depths and are either simply not felt or their felt pattern does not really indicate their true size.

Today, state of the art seismic systems transmit data from the seismograph via telephone line and satellite directly to a central digital computer. A preliminary location, depth-of-focus, and magnitude can now be obtained within minutes of the onset of an earthquake. The only limiting factor is how long the seismic waves take to travel from the epicenter to the stations - usually less than 10 minutes.

Magnitude

Modern seismographic systems precisely amplify and record ground motion (typically at

periods of between 0.1 and 100 seconds) as a function of time. This amplification and recording as a function of time is the source of instrumental amplitude and arrival-time data on near and distant earthquakes. Although similar seismographs have existed since the 1890's, it was only in the 1930's that Charles F. Richter, a California seismologist, introduced the concept of earthquake magnitude. His original definition held only for California earthquakes occurring within 600 km of a particular type of seismograph (the Woods-Anderson torsion instrument). His basic idea was quite simple: by knowing the distance from a seismograph to an earthquake and observing the maximum signal amplitude recorded on the seismograph, an empirical quantitative ranking of the earthquake's inherent size or strength could be made. Most California earthquakes occur within the top 16 km of the crust; to a first approximation, corrections for variations in earthquake focal depth were, therefore, unnecessary.

Richter's original magnitude scale (ML) was then extended to observations of earthquakes of

any distance and of focal depths ranging between 0 and 700 km. Because earthquakes excite both body waves, which travel into and through the Earth, and surface waves, which are constrained to follow the natural wave guide of the Earth's uppermost layers, two magnitude scales evolved - the mb and MS scales.

The standard body-wave magnitude formula is

mb = log10(A/T) + Q(D,h) ,

where A is the amplitude of ground motion (in microns); T is the corresponding period (in seconds); and Q(D,h) is a correction factor that is a function of distance, D (degrees), between epicenter and station and focal depth, h (in kilometers), of the earthquake. The standard surface-wave formula is

MS = log10 (A/T) + 1.66 log10 (D) + 3.30 .

There are many variations of these formulas that take into account effects of specific geographic regions, so that the final computed magnitude is reasonably consistent with Richter's original definition of ML. Negative magnitude values are permissible.

A rough idea of frequency of occurrence of large earthquakes is given by the following table:

MS Earthquakes

per year ---------- ----------- 8.5 - 8.9 0.3 8.0 - 8.4 1.1 7.5 - 7.9 3.1 7.0 - 7.4 15 6.5 - 6.9 56 6.0 - 6.4 210

This table is based on data for a recent 47 year period. Perhaps the rates of earthquake occurrence are highly variable and some other 47 year period could give quite different results.

The original mb scale utilized compressional body P-wave amplitudes with periods of 4-5 s, but

recent observations are generally of 1 s-period P waves. The MS scale has consistently used

Rayleigh surface waves in the period range from 18 to 22 s.

When initially developed, these magnitude scales were considered to be equivalent; in other words, earthquakes of all sizes were thought to radiate fixed proportions of energy at different periods. But it turns out that larger earthquakes, which have larger rupture surfaces, systematically radiate more long-period energy. Thus, for very large earthquakes, body-wave magnitudes badly underestimate true earthquake size; the maximum body-wave magnitudes are about 6.5 - 6.8. In fact, the surface-wave magnitudes underestimate the size of very large earthquakes; the maximum observed values are about 8.3 - 8.7. Some investigators have suggested that the 100 s mantle Love waves (a type of surface wave) should be used to estimate magnitude of great earthquakes. However, even this approach ignores the fact that damage to structure is often caused by energy at shorter periods. Thus, modern seismologists are increasingly turning to two separate parameters to describe the physical effects of an earthquake: seismic moment and radiated energy.

Fault Geometry and Seismic Moment, MO

The orientation of the fault, direction of fault movement, and size of an earthquake can be described by the fault geometry and seismic moment. These parameters are determined from waveform analysis of the seismograms produced by an earthquake. The differing shapes and directions of motion of the waveforms recorded at different distances and azimuths from the earthquake are used to determine the fault geometry, and the wave amplitudes are used to compute moment. The seismic moment is related to fundamental parameters of the faulting process.

MO = µS‹d› ,

where µ is the shear strength of the faulted rock, S is the area of the fault, and <d> is the average displacement on the fault. Because fault geometry and observer azimuth are a part of the computation, moment is a more consistent measure of earthquake size than is magnitude, and more importantly, moment does not have an intrinsic upper bound. These factors have led to the definition of a new magnitude scale MW, based on seismic moment, where

MW = 2/3 log10(MO) - 10.7 .

The two largest reported moments are 2.5 X 1030 dyn·cm (dyne·centimeters) for the 1960 Chile earthquake (MS 8.5; MW 9.6) and 7.5 X 1029 dyn·cm for the 1964 Alaska earthquake (MS 8.3;

MW 9.2). MS approaches it maximum value at a moment between 1028 and 1029 dyn·cm.

Energy, E

The amount of energy radiated by an earthquake is a measure of the potential for damage to man-made structures. Theoretically, its computation requires summing the energy flux over a broad suite of frequencies generated by an earthquake as it ruptures a fault. Because of instrumental limitations, most estimates of energy have historically relied on the empirical

relationship developed by Beno Gutenberg and Charles Richter:

log10E = 11.8 + 1.5MS

where energy, E, is expressed in ergs. The drawback of this method is that MS is computed

from an bandwidth between approximately 18 to 22 s. It is now known that the energy radiated by an earthquake is concentrated over a different bandwidth and at higher frequencies. With the worldwide deployment of modern digitally recording seismograph with broad bandwidth response, computerized methods are now able to make accurate and explicit estimates of energy on a routine basis for all major earthquakes. A magnitude based on energy radiated by an earthquake, Me, can now be defined,

Me = 2/3 log10E - 2.9.

For every increase in magnitude by 1 unit, the associated seismic energy increases by about 32 times.

Although Mw and Me are both magnitudes, they describe different physical properites of the

earthquake. Mw, computed from low-frequency seismic data, is a measure of the area ruptured

by an earthquake. Me, computed from high frequency seismic data, is a measure of seismic

potential for damage. Consequently, Mw and Me often do not have the same numerical value.

Intensity

The increase in the degree of surface shaking (intensity) for each unit increase of magnitude of a shallow crustal earthquake is unknown. Intensity is based on an earthquake's local accelerations and how long these persist. Intensity and magnitude thus both depend on many variables that include exactly how rock breaks and how energy travels from an earthquake to a receiver. These factors make it difficult for engineers and others who use earthquake intensity and magnitude data to evaluate the error bounds that may exist for their particular applications.

An example of how local soil conditions can greatly influence local intensity is given by catastrophic damage in Mexico City from the 1985, MS 8.1 Mexico earthquake centered some

300 km away. Resonances of the soil-filled basin under parts of Mexico City amplified ground motions for periods of 2 seconds by a factor of 75 times. This shaking led to selective damage to buildings 15 - 25 stories high (same resonant period), resulting in losses to buildings of about $4.0 billion and at least 8,000 fatalities.

The occurrence of an earthquake is a complex physical process. When an earthquake occurs, much of the available local stress is used to power the earthquake fracture growth to produce heat rather that to generate seismic waves. Of an earthquake system's total energy, perhaps 10 percent to less that 1 percent is ultimately radiated as seismic energy. So the degree to which an earthquake lowers the Earth's available potential energy is only fractionally observed as radiated seismic energy.

Determining the Depth of an Earthquake

Earthquakes can occur anywhere between the Earth's surface and about 700 kilometers below the surface. For scientific purposes, this earthquake depth range of 0 - 700 km is divided into

three zones: shallow, intermediate, and deep.

Shallow earthquakes are between 0 and 70 km deep; intermediate earthquakes, 70 - 300 km deep; and deep earthquakes, 300 - 700 km deep. In general, the term "deep-focus earthquakes" is applied to earthquakes deeper than 70 km. All earthquakes deeper than 70 km are localized within great slabs of shallow lithosphere that are sinking into the Earth's mantle.

The evidence for deep-focus earthquakes was discovered in 1922 by H.H. Turner of Oxford, England. Previously, all earthquakes were considered to have shallow focal depths. The existence of deep-focus earthquakes was confirmed in 1931 from studies of the seismograms of several earthquakes, which in turn led to the construction of travel-time curves for intermediate and deep earthquakes.

The most obvious indication on a seismogram that a large earthquake has a deep focus is the small amplitude, or height, of the recorded surface waves and the uncomplicated character of the P and S waves. Although the surface-wave pattern does generally indicate that an earthquake is either shallow or may have some depth, the most accurate method of determining the focal depth of an earthquake is to read a depth phase recorded on the seismogram. The most characteristic depth phase is pP. This is the P wave that is reflected from the surface of the Earth at a point relatively near the epicenter. At distant seismograph stations, the pP follows the P wave by a time interval that changes slowly with distance but rapidly with depth. This time interval, pP-P (pP minus P), is used to compute depth-of-focus tables. Using the time difference of pP-P as read from the seismogram and the distance between the epicenter and the seismograph station, the depth of the earthquake can be determined from published travel-time curves or depth tables.

Another seismic wave used to determine focal depth is the sP phase - an S wave reflected as a P wave from the Earth's surface at a point near the epicenter. This wave is recorded after the pP by about one-half of the pP-P time interval. The depth of an earthquake can be determined from the sP phase in the same manner as the pP phase by using the appropriate travel-time curves or depth tables for sP.

If the pP and sP waves can be identified on the seismogram, an accurate focal depth can be determined.

IASPEI standard phase listThe following list of seismic phases was approved by the IASPEI Commission on Seismological Observation and Interpretation (CoSOI) and adopted by IASPEI on July 9th 2003.

CRUSTAL PHASESPg At short distances, either an upgoing P wave from a source in the upper crust or a

P wave bottoming in the upper crust. At larger distances also arrivals caused by multiple P-wave reverberations inside the whole crust with a group velocity around 5.8 km/s.

Pb (alt:P*) Either an upgoing P wave from a source in the lower crust or a P wave bottoming in the lower crust

Pn Any P wave bottoming in the uppermost mantle or an upgoing P wave from a source in the uppermost mantle

PnPn Pn free surface reflection PgPg Pg free surface reflection PmP P reflection from the outer side of the Moho PmPN

PmP multiple free surface reflection; N is a positive integer. For example, PmP2 is PmPPmP

PmS P to S reflection from the outer side of the Moho Sg At short distances, either an upgoing S wave from a source in the upper crust or

an S wave bottoming in the upper crust. At larger distances also arrivals caused by superposition of multiple S-wave reverberations and SV to P and/or P to SV conversions inside the whole crust.

Sb (alt:S*) Either an upgoing S wave from a source in the lower crust or an S wave bottoming in the lower crust

Sn Any S wave bottoming in the uppermost mantle or an upgoing S wave from a source in the uppermost mantle

SnSn Sn free surface reflection SgSg Sg free surface reflection SmS S reflection from the outer side of the Moho SmSN

SmS multiple free surface reflection; N is a positive integer. For example, SmS2 is SmSSmS

SmP S to P reflection from the outer side of the Moho Lg A wave group observed at larger regional distances and caused by superposition

of multiple S-wave reverberations and SV to P and/or P to SV conversions inside

the whole crust. The maximum energy travels with a group velocity around 3.5 km/s

Rg Short period crustal Rayleigh wave

MANTLE PHASESP A longitudinal wave, bottoming below the uppermost mantle; also an upgoing

longitudinal wave from a source below the uppermost mantle PP Free surface reflection of P wave leaving a source downwards PS P, leaving a source downwards, reflected as an S at the free surface. At shorter

distances the first leg is represented by a crustal P wave. PPP analogous to PP PPS PP to S converted reflection at the free surface; travel time matches that of PSP PSS PS reflected at the free surface PcP P reflection from the core-mantle boundary (CMB) PcS P to S converted reflection from the CMB PcPN

PcP multiple free surface reflection; N is a positive integer. For example PcP2 is PcPPcP

Pz+P (alt:PzP) P reflection from outer side of a discontinuity at depth z; z may be a positive numerical value in km. For example P660+P is a P reflection from the top

of the 660 km discontinuity. Pz−P P reflection from inner side of discontinuity at depth z. For example, P660−P is a

P reflection from below the 660 km discontinuity, which means it is precursory to PP.

Pz+S (alt:PzS) P to S converted reflection from outer side of discontinuity at depth z Pz−S P to S converted reflection from inner side of discontinuity at depth z PScS P (leaving a source downwards) to ScS reflection at the free surface Pdif (old:Pdiff) P diffracted along the CMB in the mantle S A shear wave, bottoming below the uppermost mantle; also an upgoing shear

wave from a source below the uppermost mantle SS Free surface reflection of an S wave leaving a source downwards SP S, leaving source downwards, reflected as P at the free surface. At shorter

distances the second leg is represented by a crustal P wave. SSS analogous to SS SSP SS to P converted reflection at the free surface; travel time matches that of SPS SPP SP reflected at the free surface ScS S reflection from the CMB ScP S to P converted reflection from the CMB ScSN

ScS multiple free surface reflection; N is a positive integer. For example ScS2 is ScSScS

Sz+S (alt:SzS) S reflection from outer side of a discontinuity at depth z; z may be a positive numerical value in km. For example S660+S is an S reflection from the top of the 660 km discontinuity.

Sz−S S reflection from inner side of discontinuity at depth z. For example, S660−S is an S reflection from below the 660 km discontinuity, which means it is precursory to SS.

Sz+P (alt:SzP) S to P converted reflection from outer side of discontinuity at depth z Sz−P S to P converted reflection from inner side of discontinuity at depth z ScSP ScS to P reflection at the free surface Sdif (old:Sdiff) S diffracted along the CMB in the mantle

CORE PHASESPKP (alt:P') unspecified P wave bottoming in the core PKPab (old:PKP2) P wave bottoming in the upper outer core; ab indicates the

retrograde branch of the PKP caustic PKPbc (old:PKP1) P wave bottoming in the lower outer core; bc indicates the

prograde branch of the PKP caustic PKPdf (alt:PKIKP) P wave bottoming in the inner core PKPpre (old:PKhKP) a precursor to PKPdf due to scattering near or at the CMB

PKPdif P wave diffracted at the inner core boundary (ICB) in the outer core PKS Unspecified P wave bottoming in the core and converting to S at the CMB PKSab PKS bottoming in the upper outer core PKSbc PKS bottoming in the lower outer core PKSdf PKS bottoming in the inner core P'P' (alt:PKPPKP) Free surface reflection of PKP P'N (alt:PKPN) PKP reflected at the free surface N−1 times; N is a positive integer.

For example P'3 is P'P'P' Pz−P' PKP reflected from inner side of a discontinuity at depth z outside the core,

which means it is precursory to P'P'; z may be a positive numerical value in km P'S' (alt:PKPSKS) PKP to SKS converted reflection at the free surface; other

examples are P'PKS, P'SKP PS' (alt:PSKS) P (leaving a source downwards) to SKS reflection at the free

surface PKKP Unspecified P wave reflected once from the inner side of the CMB PKKPab PKKP bottoming in the upper outer core PKKPbc PKKP bottoming in the lower outer core PKKPdf PKKP bottoming in the inner core PNKP P wave reflected N−1 times from inner side of the CMB; N is a positive integer PKKPpre

a precursor to PKKP due to scattering near the CMB

PKiKP P wave reflected from the inner core boundary (ICB) PKNIKP

P wave reflected N−1 times from the inner side of the ICB

PKJKP P wave traversing the outer core as P and the inner core as S PKKS P wave reflected once from inner side of the CMB and converted to S at the

CMB PKKSab PKKS bottoming in the upper outer core PKKSbc PKKS bottoming in the lower outer core PKKSdf PKKS bottoming in the inner core PcPP' (alt:PcPPKP) PcP to PKP reflection at the free surface; other examples are

PcPS', PcSP', PcSS', PcPSKP, PcSSKP SKS (alt:S') unspecified S wave traversing the core as P SKSac SKS bottoming in the outer core SKSdf (alt:SKIKS) SKS bottoming in the inner core SPdifKS

(alt:SKPdifS) SKS wave with a segment of mantle side Pdif at the source and/or the receiver side of the raypath

SKP Unspecified S wave traversing the core and then the mantle as P SKPab SKP bottoming in the upper outer core SKPbc SKP bottoming in the lower outer core SKPdf SKP bottoming in the inner core S'S' (alt:SKSSKS) Free surface reflection of SKS S'N SKS reflected at the free surface N−1 times; N is a positive integer S'z−S' SKS reflected from inner side of discontinuity at depth z outside the core,

which means it is precursory to S'S'; z may be a positive numerical value in km S'P' (alt:SKSPKP) SKS to PKP converted reflection at the free surface; other

examples are S'SKP, S'PKS S'P (alt:SKSP) SKS to P reflection at the free surface SKKS Unspecified S wave reflected once from inner side of the CMB SKKSac SKKS bottoming in the outer core SKKSdf SKKS bottoming in the inner core SNKS S wave reflected N−1 times from inner side of the CMB; N is a positive integer SKiKS S wave traversing the outer core as P and reflected from the ICB SKJKS S wave traversing the outer core as P and the inner core as S SKKP S wave traversing the core as P with one reflection from the inner side of the

CMB and then continuing as P in the mantle SKKPab SKKP bottoming in the upper outer core SKKPbc SKKP bottoming in the lower outer core SKKPdf SKKP bottoming in the inner core ScSS' (alt:ScSSKS) ScS to SKS reflection at the free surface; other examples are:

ScPS', ScSP', ScPP', ScSSKP, ScPSKP

NEAR SOURCE SURFACE REFLECTIONS (Depth phases)pPy All P-type onsets (Py) as defined above, which resulted from reflection of an

upgoing P wave at the free surface or an ocean bottom; WARNING: The character y is only a wild card for any seismic phase, which could be generated at the free surface. Examples are: pP, pPKP, pPP, pPcP, etc

sPy All Py resulting from reflection of an upgoing S wave at the free surface or an ocean bottom; For example: sP, sPKP, sPP, sPcP, etc

pSy All S-type onsets (Sy) as defined above, which resulted from reflection of an upgoing P wave at the free surface or an ocean bottom; for example: pS, pSKS, pSS, pScP, etc

sSy All Sy resulting from reflection of an upgoing S wave at the free surface or an ocean bottom; for example: sSn, sSS, sScS, sSdif, etc

pwPy

All Py resulting from reflection of an upgoing P wave at the ocean's free surface

pmPy

All Py resulting from reflection of an upgoing P wave from the inner side of the Moho

SURFACE WAVESL Unspecified long period surface wave

LQ Love wave LR Rayleigh wave G Mantle wave of Love type GN Mantle wave of Love type; N is integer and indicates wave packets traveling

along the minor arcs (odd numbers) or major arc (even numbers) of the great circle

R Mantle wave of Rayleigh type RN Mantle wave of Rayleigh type; N is integer and indicates wave packets traveling

along the minor arcs (odd numbers) or major arc (even numbers) of the great circle

PL Fundamental leaking mode following P onsets generated by coupling of P energy into the waveguide formed by the crust and upper mantle

SPL S wave coupling into the PL waveguide; other examples are SSPL, SSSPL ACOUSTIC PHASESH A hydroacoustic wave from a source in the water, which couples in the ground HPg H phase converted to Pg at the receiver side HSg H phase converted to Sg at the receiver side HRg H phase converted to Rg at the receiver side I An atmospheric sound arrival, which couples in the ground IPg I phase converted to Pg at the receiver side ISg I phase converted to Sg at the receiver side IRg I phase converted to Rg at the receiver side T A tertiary wave. This is an acoustic wave from a source in the solid earth, usually

trapped in a low velocity oceanic water layer called the SOFAR channel (SOund Fixing And Ranging)

TPg T phase converted to Pg at the receiver side TSg T phase converted to Sg at the receiver side TRg T phase converted to Rg at the receiver side AMPLITUDE MEASUREMENT PHASESA Unspecified amplitude measurement AML

Amplitude measurement for local magnitude

AMB

Amplitude measurement for body wave magnitude

AMS Amplitude measurement for surface wave magnitude END Time of visible end of record for duration magnitude UNIDENTIFIED ARRIVALS

x (old:i,e,NULL) unidentified arrival rx (old:i,e,NULL) unidentified regional arrival tx (old:i,e,NULL) unidentified teleseismic arrival Px (old:i,e,NULL,(P),P?) unidentified arrival of P-type Sx (old:i,e,NULL,(S),S?) unidentified arrival of S-type