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Gelombang GempadanInstrumen PencatatnyaTeknik Gempa

Jurusan Pendidikan Teknik Sipil dan PerencanaanFT UNY2010

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Back to 2nd century CE

Zhang Heng, a Chinese scientist

Zhang Hengs Seismoscope

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Introduction to EQ Wave Gempa bumi merupakan goncangan pada tanah yang disebabkan

oleh gerakan acak batuan di sepanjang daerah keruntuhan dimuka bumi yang dinamakan dengan fault.

Proses gempa bumi dapat dibandingkan dengan lenturan sebuahbatang/tongkat hingga mengalami patah. Tegangan yangterakumulasi selama lenturan secara tiba-tiba dilepaskan saattongkat/batang tersebut hancur. Getaran-getaran dihasilkan saattongkat/batang melentur kembali ke keadaan sebelum melentur.

Di bumi, gelombang-gelombang gempa disebabkan oleh lepasnyategangan-tegangan pada baguan secara tiba-tiba pada sisi laindaerah kegagalan. Kehancuran pada daerah kegagalan bisa jadi

melepaskan seluruh atau sebagian tegangannya. Setiap tegangansisa seringkali dilepaskan oleh penyeimbang minor di sepanjangdaerah kegagalan sehingga menyebabkan gempa-gempa kecilyang dinamakan dengan aftershock.

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Gelombang Gempa(seismic waves)

Gelombang Badan

(body wave)

Gelombang

Permukaan(surface wave)

Gelombang Primer(primary wave

P wave)

Gelombang Love(Love wave

L wave)

Gelombang Sekunder(secondary wave

S wave)

Gelombang Rayleig(Rayleigh wave

R wave)

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Wave Type(and names)

Particle Motion Typical Velocity Other Characteristics

P,

Compressional, Primary,Longitudinal

Alternating compressions

(pushes) and dilations(pulls) which are directed

in the same direction asthe wave is propagating(along the ray path); andtherefore, perpendicular tothe wavefront.

VP ~ 5 7 km/s in

typical Earths crust;>~ 8 km/s in Earths

mantle and core; ~1.5km/s in water; ~0.3km/s in air.

P motion travels fastest in

materials, so the P-wave is thefirst-arriving energy on aseismogram. Generally smallerand higher frequency than the Sand Surface-waves. P waves ina liquid or gas are pressurewaves, including sound waves.

S,

Shear,Secondary,Transverse

Alternating transversemotions (perpendicular tothe direction ofpropagation, and the raypath); commonly

approximately polarizedsuch that particle motion isin vertical or horizontalplanes.

VS ~ 3 4 km/s intypical Earths crust;

>~ 4.5 km/s inEarths mantle;~ 2.5-3.0 km/s in

(solid) inner core.

S-waves do not travel throughfluids, so do not exist in Earths

outer core (inferred to beprimarily liquid iron) or in air orwater or molten rock

(magma). S waves travel slowerthan P waves in a solid and,therefore, arrive after the Pwave.

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Wave Type

(and names)

Particle Motion Typical Velocity Other Characteristics

L,

Love, Surface

waves, Longwaves

Transverse horizontalmotion, perpendicular to

the direction ofpropagation and generallyparallel to the Earths

surface.

VL ~ 2.0 - 4.4 km/s inthe Earth depending

on frequency of thepropagating wave,and therefore thedepth of penetration ofthe waves. In general,the Love waves travelslightly faster than theRayleigh waves.

Love waves exist because ofthe Earths surface. They are

largest at the surface anddecrease in amplitude withdepth. Love waves aredispersive, that is, the wavevelocity is dependent onfrequency, generally with lowfrequencies propagating athigher velocity. Depth of

penetration of the Love waves isalso dependent on frequency,with lower frequenciespenetrating to greater depth.

R,

Rayleigh, Surfacewaves, Long

waves, Ground roll

Motion is both in thedirection of propagationand perpendicular (in a

vertical plane),and phased so that themotion is generallyelliptical either progradeor retrograde.

VR ~ 2.0 - 4.2 km/s inthe Earth dependingon frequency of the

propagating wave,and therefore thedepth of penetration ofthe waves.

Rayleigh waves are alsodispersive and the amplitudesgenerally decrease with depth in

the Earth. Appearance andparticle motion are similar towater waves. Depth ofpenetration of the Rayleighwaves is also dependent onfrequency, with lowerfrequencies penetrating to

greater depth.

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P Wave

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S Wave

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L Wave

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R Wave

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Thank You