Earthquakes

           Vibrations of the Earth caused by the sudden release of energy, usually as a result of displacement of rock along faults. An earthquake is always followed by a series of smaller earthquakes (aftershocks) which represent further adjustments of rock along the fault. Approximately 13 million people have died as the result of earthquakes during the past 4,000 years. There are currently no reliable methods for predicting when earthquakes will occur.

        Earthquakes are caused by the sudden release of energy along faults.

• Elastic-Rebound Hypothesis - The idea that strain builds up in rock until the elastic limit (strength) of the rock is exceeded. The rock then ruptures (fails) at a point, snapping back toward an unstrained position. releasing the elastic energy as seismic waves radiating outward from the break. The greater the stored strain, the greater the release of energy.
• Seismic and Volcanic Activity - The coincidence of many active volcanic belts with major belts of earthquake activity indicates that volcanoes and earthquakes may have a common cause. Plate interactions commonly cause both earthquakes and volcanoes.

        Seismology is the study of earthquakes, and is a subdiscipline within geophysics.

    Earliest instruments designed to detect an earthquake did not provide any written record of an earthquake.

        Instruments which detect, measure and record seismic waves. On old instruments, movement is recorded on a rotating
paper or magnetic tape strip (seismograms), but modern instruments record data electronically into a computer. Two basic types:

1. Horizontal movement recorder - Need two recorders at right angles to each other (N-S and E-W).
2. Vertical movement recorder.

• Focus (hypocenter) - Point within the Earth from which energy is released. The size of the earthquake source cannot be accurately determined. Deepest earthquakes are at the bottoms of active subduction zones.
• Epicenter - The point on the Earth's surface directly above the focus. Given a geographic location.

        About 90% of all earthquakes have depths < 100 km. Earthquakes can be grouped into three categories based on the depth of their foci:

1. Shallow focus - Foci are less than 70 km depth. Most destructive earthquakes.
2. Intermediate focus - Foci are between 70 and 300 km depth.
3. Deep focus - Foci are greater than 300 km.

        Patterns of increasing earthquake focal depths found at convergent plate margins. The earthquake foci mark the descent of the subducting lithospheric plate.

Most earthquakes (about 95%) occur in seismic belts along the margins of tectonic plates. Enormous pressure is built up at plate boundaries, particularly convergent and transform boundaries. The pressure is ultimately released as earthquakes.
 

• Frequency - More than 150,000 earthquakes strong enough to be felt by someone are recorded annually worldwide. An additional 900,000 earthquakes occur annually that are too small to be felt or recorded as separate events.
• Earthquake Distribution - Occur at convergent (primarily) and transform plate boundaries. May occur in clusters as plates shift position.

• Seismic Belts - There are two major earthquake belts:

1. Circum-Pacific Belt - About 80% of all earthquakes occur in this belt, including the world's most devastating in terms of life and property loss.
2. Mediterranean-Asiatic Belt - Accounts for about 15% of all earthquakes.
• Intraplate and Spreading Ridge Only about 5% of all earthquakes. Most are not very strong, but some intraplate earthquakes are among the largest known.

        Some of the energy released by an earthquake travels through the Earth. The speed of a seismic wave depends on the density and elasticity of the materials through which they travel. There are two general types of seismic earth waves:
 

• Body Waves - Speed decreases with increasing density of rock and increases with increasing rock elasticity. Rock elasticity increases faster than density with depth. Two kinds of body waves:

1. Primary (P) Waves - Compressional waves that vibrate parallel to the direction of wave movement. n-avel through solid, liquid and gas. Fastest seismic wave.
2. Secondary (S) Waves (also known as Shear Waves) - Vibrate perpendicular to the direction of wave movement. Only travel through solids (liquids have no shear strength).

• Surface Waves - Travel along the outer layer of Earth. Rolling, shaking motion causes most of an earthquake's damage. Slowest seismic waves. Two major types:

1. Rayleigh (R) Waves - Behave like water waves with elliptical motion of material in wave. Generally slower than Love waves.
2. Love (L) Waves - Involve shear motion in a horizontal plane. Most destructive kind of seismic wave.

        Seismograms show that P waves always arrive at a recording station first, followed by S, and finally surface waves (L before R).

        Plots of average P- and S-wave travel times against distance from the focus. Allow the distance to the earthquake source to be calculated. The farther the distance between the seismograph station and the earthquake source, the greater the difference in arrival times between the body waves.

        If the Earth were composed of homogeneous material, seismic waves should travel in straight line paths. When there is a Such variations can give clues to the Earth's interior layering.

• Distances less than 11,000 km - Seismic waves have predictable travel schedules. P wave speed = 1.8 x S wave speed. L wave speed = 0.9 x S wave speed.
• Distances greater than 11,000 km - P waves are observed to be delayed and S waves disappear completely. Experiments have shown that P waves are slowed down by liquids, and S waves cannot pass through liquids. Strange behavior of seismic waves could be explained if the Earth's interior contained a liquid layer (outer core).

• Worldwide Standard Seismograph Network - A worldwide seismograph network was begun in 1961. Allows the accurate location of earthquakes around the world.
• Earthquake Intensity - Measure of an earthquake's destructive power (size and strength). Measured by the Modified Mercalli Intensity Scale which qualitatively links earthquake power to its effect on buildings. Somewhat subjective. Intensity depends on:

1. Amount of energy released by the earthquake.
2. Duration of shaking.
3. Distance from the epicenter.
4. Focal depth of earthquake.
5. Type of rock and degree of consolidation.
6. Population density.
7. Type of building construction.
• Earthquake Magnitude - Quantitative measure of the energy released by an earthquake at its source. Measured by the Richter Scale magnitude (each step = 31 times increase in energy = 10 times increase in earthquake wave amplitude). Magnitude 6 earthquake = Megaton nuclear bomb. Largest recorded earthquake was magnitude 8.6 (1964 Alaska earthquake). Most earthquakes have a magnitude of < 2.5. The largest earthquakes (> 8.0 magnitude) occur on average every 5 years. Magnitude determined by:

1. Measurement of size (amplitude) of seismic waves.
2. Distance from epicenter - Magnitude decreases with distance from source.
3. Sensitivity of the seismograph.

        Of the hundreds of thousands of earthquakes that occur every year, only one or two are likely to cause severe results. Destructive effects depend on earthquake magnitude, distance from epicenter, time of day, geology of area, type of building construction, and duration of shaking. The most destructive earthquakes occur during working and school hours in densely populated areas. Earthquake hazards include:

• Ground Shaking - Causes the most damage and loss of life. Buildings on bedrock move as a unit with the ground and suffer the lower damage during an earthquake. Worst damage occurs to buildings on poorly consolidated material or water-saturated sediment. Structural damage depends on:

1. Earthquake magnitude - Determines the intensity and duration of vibrations.
2. Underlying geology - The nature of material supporting the foundation determines how much damage occurs. Poorly consolidated material is subjected to longer shaking and greater S-wave amplitudes. Water-saturated sediments tend to behave like a fluid and flow when shaken (liquefaction).
3. Design of the structure (materials and type of construction). Adobe and mud-walled structures are weakest. Unreinforced brick and concrete buildings have no flexibility and tend to collapse.
• Fire - Often more dangerous that the earthquake itself. Severed gas and water lines can lead to devastating fires. Valves can be installed to cut off lines from breaks.
• Tsunami (seismic sea wave) - Most result from vertical displacement of the ocean floor or from submarine landslides during an earthquake. May also be generated from volcanic explosions (Krakatoa). Can reach speeds up to 800 km/hr and heights up to 65 m. A seismic sea wave warning system was established in 1948 for the Pacific basin after a 1946 tsunami killed 154 in Hawaii.
• Ground failures may include:

1. Landslides - Seldom occur farther than 40-50 km from the epicenter, but can be the major cause of death in some earthquakes.
2. Land Movements - Earthquakes can be accompanied by significant vertical and horizontal movement of the land surface. Fault Creep - Slow periodic movement of the land on opposite sides of a fault. Over time, fault creep can result in significant movement.

        Successful prediction would include time frame, magnitude and location. Remains an elusive goal, although measurement techniques have improved dramatically in the last 50 years:
 

• Seismic Risk Maps - Indicate the likelihood and potential severity of future earthquakes. Based on historical records of past earthquakes and the distribution of known faults. Identify areas of seismic gaps (see below) and earthquake periodicity.
• Earthquake Precursors - Most earthquakes have precursors (short term and long-term changes in the Earth that take place prior to an earthquake). Unfortunately no reliable precursors have been identified:

1. Seismic Gap - A region in an area of seismic activity that has had little earthquake activity for a number of years. Represent areas along a fault that are locked and building up stress. Points out areas which are "overdue" for an earthquake. Seismic gaps are considered to be potential sites for major earthquakes. Long-range predictor.
2. Crustal Tilt and Elevation Changes - Can be caused by increasing pressure in subsurface rocks in the region of the epicenter.
3. Measured by tiltmeters.
4. Changes in Rock Properties (Dilatancy Model) - Rock volume (dilatancy), electrical and magnetic field changes can precede an earthquake. Caused by high pressure producing numerous cracks in rock prior to failure.
5. Changes in Well Water Levels, and Gases - Water levels can rise in response to increasing pressure (decreases pore space in rock). Radon levels increase.
6. Animal Behavior - Animals are more sensitive than humans to changes in the Earth.
7. Radio Waves - Amplitude of ultralow frequency radio waves increased just prior to the 1989 Loma Prieta earthquake.

• Earthquake Prediction Programs - Include monitoring systems along major fault zones as well as laboratory and field studies of the behavior of rocks before, during and after major earthquakes. The U.S., Japan, China and Russia have government-sponsored programs. The Chinese have successfully predicted several earthquakes, but failed to predict the 1976 Tangshan earthquake that killed 242,000.

        Needs much more study. It was accidentally discovered that earthquakes can be triggered by pumping fluids into fault zones. Water acts as a lubricant, allowing faults to move. May be able to reduce stress on major faults by triggering small earthquakes using fluid injection. The best defense currently is good emergency and evacuation plans.

        Study of the travel of seismic waves through the Earth have given information about its interior. Body waves are refracted (bent) and reflected at boundaries between two different kind of materials (discontinuity) and travel more slowly through dense material than through less dense material. From the study of seismic waves, the Earth is divided into three major layers:
 

• Crust - Thin, outer layer of the Earth. Composed of solid rock. Base of the crust is marked by the Mohorovicic Discontinuity (Moho), which is marked by increased body wave speed below the discontinuity and by partial refection of body waves at the boundary. Two basic kinds of crust are indicated by seismic waves:

1. Continental Crust - Averages about 35 km thick, but ranges from 20-90 km. Average composition of granodiorite with low density.
2. Oceanic Crust - Ranges from 5-10 km thick. Composed of mafic rocks (primarily basalt and gabbro).

• Mantle - Thickest layer of the Earth (contains > 80% of the Earth's volume). Increase in body wave speed below the Moho indicates high density, mafic rock (upper mantle is believed to be composed of peridotite). The mantle is essentially solid, but portions of it are capable of slow flow. Bottom of mantle is about 2900 km deep.
• Core - Composed of a liquid outer core (Shadow Zones - S waves disappear when traveling deeper than 2900 km and P waves are slowed down) and a solid inner core (P wave velocity increases again). Believed to be composed primarily of iron mixed with nickel, cobalt, oxygen, sulfur, silica, and carbon. The Earth's magnetic field is caused by the circulation of the outer liquid core, but the exact mechanism is unknown. Core extends to the Earth's center at 6370 km depth.