4/11/2023 0 Comments Seismac wavesCompression and shear waves travel through the planet at different speeds. Seismographs (seismometers and associated recording systems) detect and measure these waves. Shear waves move rocks perpendicular to the direction the waves are moving. Compression waves alternately compress and release rocks in the direction the waves are moving (similar to the air compression we hear as sound). That movement releases energy, and two types of seismic waves radiate outward from the earthquake through Earth’s interior and along its surface. When an earthquake occurs, rocks at a fault line slip or break, and two sections of Earth’s crust physically move relative to one another. Other Expeditions Highlighting WHOI Research.Expedition to Stellwagen Bank National Marine Sanctuary.What’s the difference between climate and weather?.Why is pressure different in the ocean?.On a seismogram from an earthquake, the P-wave is the first signal to arrive, followed by the slower S-wave, then the surface-waves (Allaby (2008): National Geographic Visual Encyclopedia of Earth). Most seismometers are based on the principle of inertia: a suspended mass tends to remain still when the ground moves. Seismologists can locate the epicenter of an earthquake by comparing travel-times of the different seismic waves, which are recorded by a seismometer on a seismogram. Because of their low frequency, long duration, and large amplitude, they can be the most destructive type of seismic wave. Surface waves are analogous to water waves and propagate on the ground surface. These waves typically follow P-waves during an earthquake and displace the ground perpendicular to the direction of propagation. S-waves cannot pass through air or liquids. S-waves are shear waves which are transverse in nature, particle motion is at right angles to the direction of wave travel. Particles are displaced along the direction of wave propagation. They travel at great speeds (around 6-7 km/s) through any type of material and the first to reach the surface of the Earth. P-waves are longitudinal shock- or pressure waves, the first waves produced by an earthquake. Body waves are further divided into compressional (primary or P-) waves and shear (secondary or S-) waves. The motion of the particles in the rocks that the waves travel through is parallel to the direction of the wave. Body waves consist of successive contractions and expansions, just like sound waves in air. Body waves propagate within the body of the Earth, while surface waves move along its surface. The propagation velocity of seismic waves depends on wave type – body and surface waves can be distinguished – and the properties of the rock: the denser the rock, the faster waves travel. Below the crust, the denser mantle reaches into depths of 2890 km. The continental crust is quite complex in structure and is composed from many different kinds of rocks. The crust is the brittle outermost layer varying in thickness from 25 to 60 km on continents, compared to 4 to 6 km for oceanic crust. Different seismic body waves pass through the earth interior. Thus we can obtain information on the different layers of the earth by comparing the travel-time and geometry of seismic waves. When seismic waves pass through rocks of different densities they are refracted at the layer boundaries, just like rays of light are bent in a glass prism, and their velocity changes. What we know about the earthʼs internal structure originates mostly from the study of seismic waves. The last ones occur along subduction zones, where colder oceanic crust descends underneath another tectonic plate, and can be traces to depths of up to 700 km. The rocks usually break underground at the hypocenter and: the earth shakes! Waves spread from the epicenter, the point on the surface above the hypocenter.Įarthquakes are subdivided according to the depth of their hypocenter into shallow-focus (upper 70 km of the crust), mid-focus or intermediate-depth (70-300 km) and deep-focus earthquakes. ![]() Earthquakes occur when two tectonic plates move suddenly against each other. ![]() These waves can travel long distances and cause the ground to shake, and sometimes are extremely destructive near the epicenter – the scientific name for the point on the earth surface above the hypocenter. Seismic waves are intense vibrations which spread out from the initial point of rupture, called hypocenter, like ripples on a pond, but are much more complicated. Poster-Presentation “Seismic waves” at CMCK
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