Gelombang P dan Gelombang S: Bagaimana Gelombang Seismik Bergerak

P-Waves: The First Arrivals

When a fault ruptures, the first seismic energy to leave the Hypocenter (Focus)The actual point within the Earth where an earthquake rupture initiates. Also called the focus. Depth of the hypocenter significantly affects how an earthquake is felt at the surface. travels as P-Wave (Primary Wave)The fastest seismic wave, traveling through both solid rock and liquid at 5-8 km/s. P-waves compress and expand material in the direction of travel, like a slinky. They arrive first at seismograph stations.s — primary waves, so named because they arrive first at distant SeismographAn instrument that detects and records ground motion caused by seismic waves. Modern digital seismographs can detect movements smaller than a nanometer. stations. P-waves are compressional waves: the rock alternately compresses and expands in the same direction as the wave is travelling, like sound waves in air. Because they involve compression and expansion of the rock rather than shearing, P-waves can travel through solids, liquids, and gases alike.

This ability to travel through all materials gives P-waves a crucial role in understanding Earth's interior. When P-waves pass through the liquid outer core, they slow dramatically and are refracted; through the solid inner core, they speed up again. By precisely mapping how P-wave travel times deviate from predictions, seismologists have mapped the detailed structure of Earth's layers, a technique called Seismic TomographyA technique that uses seismic wave travel times to create 3D images of Earth's interior structure, similar to a medical CT scan. Reveals mantle plumes, subducting slabs, and other deep structures..

P-wave speeds in crustal rocks typically range from 5 to 8 kilometres per second, varying with rock type, pressure, and temperature. In the mantle, they reach 8–13 km/s. This speed makes P-waves the first warning of an impending earthquake to arrive at distant monitoring stations — and the foundation of Earthquake Early Warning (EEW)A system that detects an earthquake and sends alerts to people and systems before strong shaking arrives. Can provide seconds to tens of seconds of warning, enough to take protective action. systems.

S-Waves: The Shaking Waves

The second arrival is the S-Wave (Secondary Wave)Seismic waves that move rock perpendicular to the direction of travel, arriving after P-waves. S-waves cannot travel through liquids, which proved the Earth's outer core is liquid. — shear wave or secondary wave. S-waves are transverse waves: the rock moves perpendicular to the direction the wave is travelling, like the motion along a shaken rope. This shearing motion is what causes most of the ground shaking people feel during an earthquake, because the back-and-forth or up-and-down movement S-waves produce is far more disorienting and structurally damaging than the push-pull of P-waves.

S-waves travel at roughly 60 percent of P-wave speed — typically 3–5 km/s in the crust. A critically important property: S-waves cannot propagate through liquids, because liquids cannot sustain shear stress. This S-wave shadow zone beyond about 103–143 degrees from an earthquake was the original evidence that the Earth has a liquid outer core. The distinction is still exploited operationally: the absence of S-waves at distant stations is diagnostic of a liquid layer between source and receiver.

Why P-Waves Travel Faster Than S-Waves

The speed difference between P-Wave (Primary Wave)The fastest seismic wave, traveling through both solid rock and liquid at 5-8 km/s. P-waves compress and expand material in the direction of travel, like a slinky. They arrive first at seismograph stations.s and S-Wave (Secondary Wave)Seismic waves that move rock perpendicular to the direction of travel, arriving after P-waves. S-waves cannot travel through liquids, which proved the Earth's outer core is liquid.s arises from the physics of wave propagation in elastic media. P-wave velocity depends on both the bulk modulus (resistance to compression) and the shear modulus (resistance to shearing), while S-wave velocity depends only on the shear modulus. Because the bulk modulus contribution is always positive, P-waves are always faster than S-waves in the same material.

This speed difference has a tremendously practical consequence. After an earthquake, the P-wave arrives at a monitoring station first, followed some seconds later by the S-wave. The time gap between the two arrivals — the "S minus P" time, or S-P interval — is directly proportional to the distance from the station to the EpicenterThe point on the Earth's surface directly above the hypocenter (focus) where an earthquake originates underground. Often reported as the earthquake's location in news reports.. Measure the S-P interval in seconds, multiply by approximately 8 kilometres, and you have a rough distance estimate. With three or more stations, triangulation precisely locates the earthquake.

How the P-S Time Gap Locates Earthquakes

The elegant method of locating earthquakes using the P-Wave (Primary Wave)The fastest seismic wave, traveling through both solid rock and liquid at 5-8 km/s. P-waves compress and expand material in the direction of travel, like a slinky. They arrive first at seismograph stations.–S-Wave (Secondary Wave)Seismic waves that move rock perpendicular to the direction of travel, arriving after P-waves. S-waves cannot travel through liquids, which proved the Earth's outer core is liquid. time difference is one of the oldest tools in observational seismology. An S-P interval of 10 seconds suggests the Hypocenter (Focus)The actual point within the Earth where an earthquake rupture initiates. Also called the focus. Depth of the hypocenter significantly affects how an earthquake is felt at the surface. is roughly 80 kilometres away. An interval of 30 seconds suggests about 240 kilometres. By plotting a circle of appropriate radius around each station and finding where three or more circles intersect, the earthquake location — and with additional information, its depth — can be pinpointed.

Modern earthquake location methods use exactly this principle, but with data from hundreds of stations processed simultaneously by computer algorithms that minimise the mismatch between observed and predicted arrival times. The Global Seismographic Network (GSN)A worldwide network of 150+ broadband seismograph stations that provides comprehensive monitoring of global earthquake activity. Jointly operated by USGS, NSF, and IRIS. and regional Seismic NetworkA coordinated group of seismograph stations that continuously monitor earthquake activity. The Global Seismographic Network (GSN) includes 150+ stations providing worldwide coverage.s provide continuous streams of SeismogramThe recorded output of a seismograph, showing ground motion as a function of time. Seismologists analyze seismograms to determine earthquake magnitude, depth, and location. data that feed into automated detection and location systems capable of publishing earthquake locations within minutes of the event. The Seismic Alert SystemMexico's SASMEX, one of the world's first public earthquake early warning systems, operational since 1991. Provides up to 60 seconds of warning for Mexico City from coastal earthquakes. in Japan uses the P-wave arrival at the closest stations to predict shaking intensity at distant locations before the slower, more damaging S-waves arrive.

What S-Waves Tell Us About Earth's Interior

Beyond locating earthquakes, the behaviour of S-Wave (Secondary Wave)Seismic waves that move rock perpendicular to the direction of travel, arriving after P-waves. S-waves cannot travel through liquids, which proved the Earth's outer core is liquid.s has been indispensable for mapping Earth's internal structure. The discovery that S-waves cannot pass through a region corresponding to depths of roughly 2,900 to 5,100 kilometres demonstrated the existence of the liquid outer core. The Body WaveSeismic waves that travel through the Earth's interior, including P-waves and S-waves. They travel faster than surface waves and are the first signals to arrive at distant seismograph stations. velocity profile with depth — how fast P and S waves travel at different depths — traces the density and elastic properties of Earth's layers and can be inverted to produce three-dimensional models of mantle structure.

Seismic TomographyA technique that uses seismic wave travel times to create 3D images of Earth's interior structure, similar to a medical CT scan. Reveals mantle plumes, subducting slabs, and other deep structures. uses small variations in P and S travel times from thousands of earthquakes recorded at thousands of stations to build three-dimensional images of the Earth's interior. Slow-velocity zones at depth often indicate hotter, less rigid material — potentially rising mantle plumes or Hotspot (Geology)A location in the mantle where hot rock rises as a plume, creating volcanic activity independent of plate boundaries. Hawaii and Yellowstone are classic examples. tracks. Fast-velocity zones often correspond to subducted oceanic slabs sinking into the mantle. This imaging has revealed that the mantle is far more heterogeneous than the simple layered models of earlier decades.

Feeling the Waves: What You Experience During a Quake

If you are close to a moderate or large earthquake, the P-wave often arrives as a sudden sharp jolt — a brief bang or bump that makes people think something has struck the building. It may be mistaken for a sonic boom or a truck collision. A second or two later, the S-Wave (Secondary Wave)Seismic waves that move rock perpendicular to the direction of travel, arriving after P-waves. S-waves cannot travel through liquids, which proved the Earth's outer core is liquid. arrives with the characteristic rolling, swaying shaking that everyone recognises as an earthquake. This is the motion that can throw people off their feet, topple furniture, and damage structures.

Following the S-wave, Surface WaveSeismic waves that travel along the Earth's surface rather than through its interior. Slower than body waves but typically cause more damage due to their larger amplitude and longer duration.s — Love WaveA type of surface wave that causes horizontal shearing of the ground. Named after mathematician A.E.H. Love, these waves are particularly damaging to building foundations.s and Rayleigh WaveA surface wave that causes the ground to move in an elliptical motion, similar to ocean waves. Named after Lord Rayleigh. Often responsible for the rolling sensation felt during earthquakes.s that travel along the Earth's surface rather than through its interior — arrive with a slower, more sustained rolling motion. For large distant earthquakes, the surface waves may arrive minutes after the body waves and can produce long-duration, low-frequency shaking that is particularly damaging to tall buildings susceptible to Structural ResonanceThe amplification of building motion when earthquake wave frequency matches the building's natural frequency. Low-rise buildings resonate with high-frequency waves; tall buildings with low-frequency.. Recognising these distinct wave types helps explain why earthquake shaking often feels like a quick jolt followed by stronger swaying — you are experiencing first the Body WaveSeismic waves that travel through the Earth's interior, including P-waves and S-waves. They travel faster than surface waves and are the first signals to arrive at distant seismograph stations.s, then the surface waves, in sequence.