teknik-gempa-04.ppt

7/30/2019 Teknik-Gempa-04.ppt

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Gelombang Gempa

Teknik GempaDIII Teknik Sipil

Jurusan Pendidikan Teknik Sipil dan Perencanaan

FT UNY


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pendahuluan

Gempa bumi merupakan goncangan pada tanah yang disebabkanoleh gerakan acak batuan di sepanjang daerah keruntuhan di mukabumi yang dinamakan dengan fault.

Proses gempa bumi dapat dibandingkan dengan lenturan sebuah

batang/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 lain

daerah kegagalan. Kehancuran pada daerah kegagalan bisa jadimelepaskan 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 w aves)

Gelombang Badan

(body wave)

Gelombang

Permukaan(sur face wave)

Gelombang Primer

(pr imary wave

P wave)

Gelombang Love

(Love wave

L wave)

Gelombang Sekunder

(secondary wave

S wave)

Gelombang Rayleigh

(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 as

the wave is propagating

(along the ray path); and

therefore, perpendicular tothe wavefront.

VP ~ 5 7 km/s in

typical Earths crust;

>~ 8 km/s in Earths

mantle and core; ~1.5

km/s in water; ~0.3

km/s in air.

P motion travels fastest in

materials, so the P-wave is the

first-arriving energy on a

seismogram. Generally smaller

and higher frequency than the S

and Surface-waves. P waves in

a liquid or gas are pressurewaves, including sound waves.

S,

Shear,

Secondary,

Transverse

Alternating transverse

motions (perpendicular to

the direction of

propagation, and the ray

path); commonly

approximately polarized

such that particle motion is

in vertical or horizontal

planes.

VS ~ 3 4 km/s in

typical Earths crust;

>~ 4.5 km/s in

Earths mantle;

~ 2.5-3.0 km/s in

(solid) inner core.

S-waves do not travel through

fluids, so do not exist in Earths

outer core (inferred to be

primarily liquid iron) or in air or

water or molten rock

(magma). S waves travel slower

than P waves in a solid and,

therefore, arrive after the P

wave.


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Wave Type

(and names)

Particle Motion Typical Velocity Other Characteristics

L,

Love, Surfacewaves, Long

waves

Transverse horizontal

motion, perpendicular to

the direction of

propagation and generally

parallel to the Earths

surface.

VL ~ 2.0 - 4.4 km/s in

the Earth depending

on frequency of the

propagating wave,

and therefore the

depth of penetration of

the waves. In general,

the Love waves travel

slightly faster than the

Rayleigh waves.

Love waves exist because of

the Earths surface. They are

largest at the surface and

decrease in amplitude with

depth. Love waves are

dispersive, that is, the wave

velocity is dependent on

frequency, generally with low

frequencies propagating at

higher velocity. Depth ofpenetration of the Love waves is

also dependent on frequency,

with lower frequencies

penetrating to greater depth.

R,

Rayleigh, Surface

waves, Longwaves, Ground roll

Motion is both in the

direction of propagation

and perpendicular (in avertical plane),

and phased so that the

motion is generally

elliptical either prograde

or retrograde.

VR ~ 2.0 - 4.2 km/s in

the Earth depending

on frequency of thepropagating wave,

and therefore the

depth of penetration of

the waves.

Rayleigh waves are also

dispersive and the amplitudes

generally decrease with depth inthe Earth. Appearance and

particle motion are similar to

water waves. Depth of

penetration of the Rayleigh

waves is also dependent on

frequency, with lower

frequencies 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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teknik-gempa-04.ppt

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