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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