Gelombang Seismik

GELOMBANG SEISMIK

elombang yang terjadi merupakan getaran dalam suatu medium elastis kedudukan normalnya contoh : gelombang gempa

GELOMBANG GEMPA (SEISMIK)  Getaran

gempa yang menjalar di dalam dan permukaan bumi dengan cara longitudinal dan transversal

Jenis-jenis Gelombang Gempa  Gelombang

Tubuh (Body Wave)

Gelombang seismik yang menjalar di dalam tubuh bumi Contohnya : - Gelombang P - Gelombang S

GELOMBANG Gelombang Primer PRIMER 

Gerak partikel yang merambat di dalam suatu medium adalah searah dengan penjalaran gelombang

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Gerak partikel yang merambat di dalam suatu medium adalah tegak lurus dengan penjalaran gelombang

GELOMBANG SEKUNDER

Pensesaran dan Gelombang

Faulting

Seismic waves

Pensesaran (Faulting) di pusat gempa menghasilkan gelombang gempa (seismic waves)

Oscillation of body waves

P wave

Direction of wave propagation

S wave

VP V > S Lay and Wallace “Modern Global Seismology” 1995

Primary Waves

Secondary Waves

Movement of Earth

Rock velocities (km/s) Rock 0 1 2 3 4 5 6 7 Alluvium ************ Sand ********* Mud ****** Shale ************** ***************************** ************************ *********************** Sandstone ************************************* ****************************** ************************** ******************************** ********************** Limestone ********************************** ************************************** *************************** ********************************* ******************************************** Dolomite ************************************** ********************** Anhydrite ********************** Gypsum **************** Salt ************** Granite ******************** Gabbro *********** Adapted from Sheriff and Geldart, 1995

Jenis-jenis Gelombang Gempa (2)  Gelombang

Permukan (Surface Wave)

Gelombang seismik yang menjalar di permukaan bumi Contohnya : - Gelombang Rayleigh - Gelombang Love

Oscillation of surface waves Love

Rayleigh

Rayleigh Waves 

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Typical velocity: ~ 0.9 that of the S wave Behavior: Causes vertical together with back-andforth horizontal motion. Motion is similar to that of being in a boat in the ocean when a swell moves past. Arrival: They usually arrive last on a seismogram.

Love Waves 

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Typical velocity: Depends on earth structure (dispersive), but less than velocity of S waves. Behavior: Causes shearing motion (horizontal) similar to S waves. Arrival: They usually arrive after the S wave and before the Rayleigh wave.

Locating an Earthquake’s Epicenter Seismic wave behavior

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P waves arrive first, then S waves, then L and R After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter (D).

Refraction of seismic wave ray

wave front

Velocity

Snell’s Law

sin θ1 sin θ 2 = = p Ray parameter v1 v2

Gelombang gempa mengalami refraksi pada saat melalui suatu bidang diskontinuitas

Ray Paths in a Layered Medium Time 1/α

Seismic stations

Source Velocity

1/α 1/α

3

2

1

Distance

α1 < α 2 < α 3

Reflection and conversion of seismic wave

P SV

P

P SV Displacement and stress are continuous at the boundary.

- Conversion between P and SV - No SH from P - SV

P

SV

Stress is zero at the free surface.

P