The tectonic plates are, in fact, lithospheric plates.īelow the low velocity zone are a couple of seismic discontinuities at which seismic velocities increase. The lithosphere is free to move (glide) over the weak asthenosphere. The combination of uppermost mantle and crust above the asthenosphere is called the lithosphere. The asthenosphere separates the strong, solid rock of the uppermost mantle and crust above from the remainder of the strong, solid mantle below. Alternatively, it may simply represent a zone where the mantle is very close to its melting point for that depth and pressure that it is very "soft." Then this represents a zone of weakness in the upper mantle.This zone is called the asthenosphere or "weak sphere." This is interpreted to be a zone that is partiallymolten, probably one percent or less (i.e., greater than 99 percentsolid). Furthermore, whileboth the P and S waves travel more slowly, the S waves are attenuatedor weakened. However, seismic waves recorded at distances corresponding todepths of around 100 km to 250 km arrive later than expectedindicating a zone of low seismic wave velocity. Seismic velocities tend to gradually increase with depth in themantle due to the increasing pressure, and therefore density, withdepth. Summary: thin, more dense, homogeneous, young Summary: thicker, less dense, heterogeneous, oldĬomposition: mafic igneous rock (basalt & gabbro) with thin layer of sediments on top The Moho beneath the oceans is usually about 7 km below the seafloor (i.e., ocean crust is about 7 km thick).ĭepth to Moho: 20 to 70 km, average 30 to 40 kmĬomposition: felsic, intermediate, and mafic igneous, sedimentary, and metamorphic rocks The depth to the Moho beneath the continents averages around 35 km but ranges from around 20 km to 70 km. This seismic discontinuity is now know as the Moho (much easier than "Mohorovicic seismic discontinuity") It is the boundary between the felsic/mafic crust with seismic velocity around 6 km/sec and the denser ultramafic mantle with seismic velocity around 8 km/sec.
distance curve.Mohorovicic (1909) interpreted this to mean that the seismic waves recordedbeyond 200 km from the earthquake source had passed through a lowerlayer with significantly higher seismic velocity. But beyond 200 km the seismic waves arrive sooner thanexpected, forming a break in the travel time vs. Seismic stations within about 200 km of a continental earthquake(or other seismic disturbance such as a dynamite blast) report traveltimes that increase in a regular fashion with distance from thesource. Sudden jumps in seismic velocities across aboundary are known as seismic discontinuities. When seismic waves pass between geologic layers with contrastingseismic velocities (when any wave passes through media withdistinctly differing velocities) reflections, refraction (bending),and the production of new wave phases (e.g., an S wave produced froma P wave) often result. Partially molten areas may slow down the P waves andattenuate or weaken S waves.
Molten areas within the Earth slow down P waves and stop Swaves because their shearing motion cannot be transmitted through aliquid. Seismic waves move more slowly through a liquid than asolid. Anomalously hot areas slow downseismic waves. Seismic wavestravel more quickly through denser materials and therefore generallytravel more quickly with depth. Seismic velocities depend on the material properties such ascomposition, mineral phase and packing structure, temperature, andpressure of the media through which seismic waves pass. Seismic stations located at increasing distances from the earthquake epicenter will record seismic waves that have traveled through increasing depths in the Earth. When an earthquake occurs the seismic waves (P and S waves) spread out in all directions through the Earth's interior.
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