Boulder, Colorado, Geological Society of America, p. For permission to copy, contact editing geosociety. Introduction to Seismic Monitoring Relevance and Rationale for Monitoring Although earthquakes happen frequently in many parts of the world, any occurrence of a moderate to large event is sudden and unexpected.
However, seismic waves recorded at distances corresponding to depths of around km to km arrive later than expected indicating a zone of low seismic wave velocity. Furthermore, while both the P and S waves travel more slowly, the S waves are attenuated or weakened.
This is interpreted to be a zone that is partially molten, probably one percent or less i. 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.
This zone is called the asthenosphere or "weak sphere. The combination of uppermost mantle and crust above the asthenosphere is called the lithosphere. The lithosphere is free to move glide over the weak asthenosphere. The tectonic plates are, in fact, lithospheric plates.
Theoretical analyses and laboratory experiments show that at these depths pressures ultramafic silicates will change phase atomic packing structure or crystalline structure from the crystalline structure of olivine to tighter packing structures.
A discontinuity at around km depth is particularly distinct. The km discontinuity results from the change of spinel structure to the perovskite crystalline structure which remains stable to the base of the mantle.
Perovskite same chemical formula as olivine is then the most abundant silicate mineral in the Earth. The km discontinuity is thought to represents a major boundary separating a less dense upper mantle from a more dense lower mantle.
Gutenberg explained this as the result of a molten core beginning at a depth of around km. Shear waves could not penetrate this molten layer and P waves would be severely slowed and refracted bent.
This velocity increase is consistent with a change from a molten outer core to a solid inner core. The figure above shows seismic ray paths perpendiculars to seismic wave fronts in the Earth.
What is the Core Made of? That material must be dense: Since the core makes up about one-third of the Earth's mass it must be a material that is common in the solar system. It must account for the observed seismic velocities. It should also be a material with magnetic properties to account for the Earth's magnetic field.
Iron is the obvious candidate. There are several kinds of meteorites that are found on Earth. One class are called differentiated meteorites. They are thought to represent a planetesimal s that was forming with Earth and the other planets.
But the growing planet broke up because of the conflicting gravitational tugs of the Sun and Jupiter.The earth is a noisy place. Seismometers, which measure ground movements to detect earthquakes, volcanic eruptions, and manmade explosives, are constantly recording smaller vibrations caused by ocean waves, rushing rivers, and industrial activity.
Chapter 9 Earthquakes. STUDY. PLAY. earthquake. ground shaking caused by the sudden and rapid movements of one block of rock slipping past another along earth's fractures in earth's crust.
the zone of greatest seismic activity, encompasses the coastal regions of Chile, Central America, Indonesia, Japan and Alaska. When an earthquake occurs the seismic waves (P and S waves) spread out in all directions through the Earth's interior.
Seismic stations located at increasing distances from the earthquake epicenter will record seismic waves that have traveled through increasing depths in the Earth.
Recording the Earth's Seismic Activity. Recording the Earth's Seismic Activity In general, the map that contained all the earthquake distributions mostly had shallow quakes spread around. Deep quakes on the map were a lot less common.
I would estimate that about 85%% of the earthquakes were shallow quakes. Virtual Earthquake shows the recordings of an earthquake's seismic waves detected by instruments far away from the earthquake. The instrument recording the seismic waves is called a seismograph and the recording is a seismogram.
About Latest Earthquakes Version Info Clicking the list icon in the top right corner will load the earthquake list. Clicking the map icon in the top right corner will load the map.
Clicking the options icon in the top right corner lets you change which earthquakes are displayed, and many other map and list options.