GELOMBANG SEISMIK

Gelombang yang terjadi merupakan getaran dalam suatu medium elastis darikedudukan 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 Primer

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

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

GELOMBANG PRIMER

GELOMBANG SEKUNDER

Seismic waves

Faulting

Pensesaran dan Gelombang

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

Oscillation of body waves

P wave

S wave

P SV V

Direction of wave propagation

Lay and Wallace “Modern Global Seismology” 1995

Primary Waves

Secondary Waves

Movement of EarthMovement of Earth

Rock velocities (km/s)Rock 0 1 2 3 4 5 6 7Alluvium ************ 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

LoveLove

RayleighRayleigh

Oscillation of surface waves

Rayleigh Waves

Typical velocity: ~ 0.9 that of the S wave

Behavior: Causes vertical together with back-and-forth 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

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

• 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

1 2

1 2

sin sinp

v v

Ray parameter

Velocity

Snell’s LawSnell’s Law

wave frontray

Gelombang gempa mengalami refraksi pada saat melalui suatu bidang diskontinuitas

Ray Paths in a Layered Medium

1/1

1/2

1/3

Distance

Time

Seismic stations

Source

Velocity

1 2 3

Reflection and conversion of seismic wave

P P

P

SV

SV

Displacement and stress are continuous at the boundary.

P PSV

Stress is zero at the free surface.

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