Internal Layers of the Earth

Transcription

1 Lecture #4 notes Geology 3950, Spring 2006; CR Stern Seismic waves, earthquake magnitudes and location, and internal earth structure (pages in the 4 th edition and and in the 5 th edition) Earthquakes result from building up of stress along a fault in the brittle lithosphere (see table 1 and figure 1 below) of the earth until the stress becomes large enough to overcome the friction along the fault and it ruptures. The stress results from convection inside the deeper plastic asthenosphere of the earth. This is caused by build up of heat generated by decay of radioactive elements. The pattern of convection focuses stress on the boundaries of large areas of lithosphere called plated. The process of the movement of the lithosphere plates above the convecting asthenosphere, which causes earthquakes along the boundaries of plates, is called plate tectonics. Convection may occur throughout the mantle, but most probably only occurs in the upper part of the mantle asthenosphere. Internal Layers of the Earth Lithologic or compositional layers Crust (20-60 km thick) rock (density of about 2.8 gms/cm 3 ) Mantle rocks (denisity of about 3.4 gms/ cm 3 peridotites) Core iron metal (density of >9 gms/cm 3 ) Rheologic layers Lithosphere ( km thick; crust and upper mantle) brittle rocks they rupture to produce earthquakes Asthenosphere plastic rocks they flow and convect Outer core liquid iron metal Inner core solid iron metal

2 Figure 1 Seismic waves - An earthquake generates many different types of seismic waves, including so-called body waves, which pass though the earth, and surface waves, which move along the earth s surface. The body waves include compression waves, which compress and expand the earth in the direction that they move like sound waves, and shear waves, which shake the earth perpendicular to their direction of motion like waves in water (see figure 2 below).

3 Figure 2 Compression waves travel more rapidly and are called P or Primary waves, and shear waves slightly more slowly and are called S or secondary waves (see figure 3 below). Surface waves travel even slower. Figure 3 These waves provide the basis for determining the magnitude and location of an earthquake, and also provide information about the internal structure of the earth

4 Earthquake magnitude the magnitude of an earthquake is best determined from the size (length or area) of the rupture zone and the amount of displacement along this rupture. As the rupture propagates along the fault at finite velocities from the hypocenter (see figure 4 below), seismic waves are generated and the ground shakes, so the size of the rupture also determines the time period that shaking occurs which is a major factor in the potential damage caused by a quake. Figure 4 The Richter earthquake magnitude scale was based on the amplitude of the first S wave arrival as recorded on a seismograph, taking into account the distance away from the quake determined by the difference in arrival time of the P and S wave (see Figure 3 above). However, in large quakes generated over a period of a few minute, S waves generated as the rupture propagates along the fault interfere with each other, and they reach maximum amplitude independent of how big the quake actually is. Therefore the Richter scale has now been abandoned for a new scale called the Seismic Moment Magnitude (M) scale, which determines magnitude from surface waves and/or mapping the size of the rupture zone and displacement. The size of the rupture zone can always be determined from the location of aftershocks, which always occur inside the area of the original rupture.

5 The size of the rupture zone and the displacement along this zone in some large quakes is given in Table1 below, and the amount of energy of the 1960 earthquake in Chile, the largest ever recorded, and other natural events are compared in figure 5 below. The M scale is an order of magnitude scale, so that each number represents a 10-fold increase in size. Table 1 Figure 5

6 Earth s internal structure shear waves do not travel through liquids, and a shadow zone for such waves indicates that the earth has a liquid core (see figure 6 below). This core has high density and is believed to be molten iron, perhaps with a solid iron core even deeper inside the earth. The mantle is solid rock, but the upper mantle, called the asthenosphere (Table and Figure 1 above) is very hot and plastic and can convect, driving the ridged lithosphere plates, which are approximately 100 km thick, around the surface of the earth. Figure 6 Location of earthquakes, plates and plate boundaries the difference in arrival time of P and S waves tell how far a way an earthquake occurred from a seismic station and the information from only three such stations can locate the quake (see figure 7 below). In practice, many seismic stations send electronic information to the USGS earthquake monitoring center in Golden which then makes public both the location and magnitude of the quake. Figure 7

7 The first global compilation of the location of all earthquakes was made in the late 1960s (see figure 8 below) Figure 8 This shows that more than 90% of all quakes occur along boundaries of large regions without many quakes the so-called plates (see figure 1 in lecture #5). Earthquakes occur where plates are pulling apart from each other, sliding past each other, or colliding with each other. Almost 90% of all quakes occur in the upper 100 km of the earth, the lithosphere, and deeper quakes only occur where plates are colliding and one plate is being subducted into the deep earth below the other (see figure 9 below), such as around the Pacific ocean, below the coast of Chile, Alaska, Japan and Indonesia. These subduction zone earthquakes are among the largest quakes that occur. Figure 9 Although 90% of all earthquakes occur at plate boundaries, another 10% occur within plates, and no area on the earth is completely free of the danger of an earthquake.