EARTHQUAKES

Figure shows the maximum intensity earthquake which can occur and indicates that most of California is at extreme risk. Not shown is the frequency of large magnitude earthquakes, which for California is higher than anywhere else in the nation. Thus we must be earthquake aware since we are at great risk in southern California.

Tectonism - Forces working to distort the earth's crust. Rocks are deposited in originally horizontal layers. When we see them uplifted frequently they are highly contorted or deformed and no longer in horizontal layers.

Basic Types of Earth Movement

1. Abrupt Movement - Earthquakes accompanied by measurable uplift or depression of the earth's surface. Generally only a few meters, but occurs in a matter of seconds. Alaskan earthquake caused uplift of as much as 10-15 meters in a few seconds.
   1. Vertical Displacement - Upward or downward movement of rock masses. Classic example is Sagami Bay, Japan where all historic earthquakes can be correlated in the cliffs along the bay, because each was accompanied by vertical displacement.
   2. Horizontal Displacement - San Andreas Fault, 1000 km long reaching from offshore north of San Francisco to the Gulf of California. Characterized by horizontal displacement of in excess of 100 km over the last 10-20 million years. Movement from a single earthquake can be as much as 10 meters.
2. Slow Movement - Creep in which the fault moves slowly and continuously over a long period of time. Average rate of movement of the central portion of the San Andreas is .5- 2 cm/yr.

Causes of Earthquakes

Earthquake Waves

1. Body Waves (Figure)- Waves moving through the body of the earth



1. Push-Pull, Primary (P) Waves - Compressional waves moving parallel to the direction of propagation. Can move through solids, liquids or gas.
2. Shake, Secondary (S) Waves - Shear waves traveling or advancing at right angles to the direction of movement. Travel only through solids.
2. Surface (L) Waves - Waves Similar to ripples on a pond

Interpreting Earthquakes

Scales of Earthquake Intensity/Magnitude (Table)

1. Modified Mercalli - Based on personal interviews of victims in the quake area. Has XII degrees of intensity.

2. Richter Scale - Based on the magnitude of energy released during a quake as measured by a seismograph (Describe how a seismograph works). Richter scale corrects for distance of the recording device from the epicenter. Scale is logarithmic so each increase by 1 represents a ten-fold increase in magnitude and actually a 30-fold increase in the amount of energy released. Largest quake ever recorded subject of some debate, but is either Alaskan (1964) at 9.2 or one in South America (1976) which may have been near 9.5.

Recording Earthquakes

What happens:

1. P Wave arrives first, followed by S Wave. P Wave travel times about 2.5 times those of S Wave due to differing path of travel. Travel times vary systematically to a distance of 11,000 km from the focus.
2. Beyond 11,000 km P Waves are delayed several minutes over predicted arrival time and S Waves do not arrive at all. Why?

Locating Earthquakes

1. Difference between arrival times of P and S waves is determined. This gives distance to the epicenter from the seismograph.
2. Three seismographs are triangulated to give actual location of the epicenter (Figure).
3. Once distance to epicenter is known a correction factor is applied to amplitude of largest wave (usually S) to determine magnitude.

Distribution of Earthquakes

• Average about 150,00 quakes a year. About 6,000 are strong enough to be recorded.

• Generally most of the energy is released in the one or two large quakes which occur each year. Due to log scale, energy of all others barely total that of the large quakes.

• Occur in belts coincident with those of active volcanoes. These belts lie along plate boundaries and can be used to outline the plates.

Prediction and Control

1. Prediction
   1. Stress Meters
   2. Tilt Meters
   3. Recording Small Quakes
   4. Changes in Fluid Pressure in Wells
   5. Observing Animal Behavior

2. Control
   1. Evidence indicates that fluid injection can trigger small movements along earthquake faults. But study is still in its infancy.

Structure of the Earth (See Figure from Introduction)

• Crust - Averages 33 km in thickness beneath the continents. Varies from about 20 km to 60 km. Seems to consist of an upper granitic layer, underlain by gabbro (?). onclusion based on increase in the velocity of P waves. Beneath oceans crust only 5 km thick and not layered. Consists entirely of basalt/gabbro.
• Mantle - Separated from crust by Mohorovichic Discontinuity (Moho). This is a zone of abrupt increase in P and S wave velocity. Indicates major change in the nature of the mantle. Mantle thought to consist largely of peridotite with lesser eclogite (metamorphic basalt) and dunite (olivine-rock). Mantle is a solid or near solid with a density of about 3.3 gr/cm³. Extends to a depth of 2900 km.
• Core

1. Outer core - 2,200 thick. Must be liquid due to disappearance of S waves and abrupt slowing of P waves. Probably consists of iron, nickel and some silicon.
2. Inner Core - 1270 km thick. Probably a solid, but not known. Consists of same elements as outer core. Density of core about 15 gr/cm³.