EARTHQUAKES

Introductory Geosciences I – GEOL 1121

EARTHQUAKES
Written by T. Weiland

*Earthquake definition - a sudden trembling or motion within the earth generated by the release of slowly accumulated stress and strain in the rock.

Sept. 19, 1985 - a 8.1 magnitude earthquake rocks Mexico City killing 7,000 people and causing excessive damage. Although the focus of the earthquake originated over 200 miles to the west along the Pacific Ocean, the seismic waves were transmitted and amplified by the ancient clay-rich lake deposits on which the city is built. A 200 km. segment moved 2 meters in a fraction of a second. Seven and fifteen story buildings collided with each other.

Oct. 17, 1989 - a 7.1 magnitude earthquake caused extensive damage in San Francisco. In 1906, an 8.3 earthquake almost totally destroyed the city.

I. Characteristics of Earthquakes

A. Elastic Rebound Theory - earthquakes are associated with large fracture or faults within the crust or upper mantle. These faults remain locked while the stress slowly accumulates in the rock until suddenly the fault slips and the energy is released. *Earthquakes don't occur below the upper mantle and crust because the elevated temperatures and pressures allow the rocks to deform plastically without storing up stress.

Slip - amount of displacement that adjacent blocks move along the fault

B. Seismic Waves - pulses of energy generated within the earth.

1. P-waves - primary or longitudinal waves which form by the compression and dilations of rock material (slinky-like motion). These are called primary because they are the first to reach the seismic station.

2. S-waves - secondary or shear waves which travel more-slowly and consist of elastic vibrations which are transverse to the direction of travel. These move by a shearing motion in the material; therefore, they will not travel through liquids.

3. Surface Waves - waves that are only transmitted along the surface. These include Rayeigh (orbital) and Love (like S-waves) waves. These are the most damaging waves.

II. Location and Measurement

A. Focus and Epicenters

1. Focus - point at which the rupture occurs. These ruptures are up to 1000 kms. long and travel up to 7200 miles/hr.

2. Epicenter - point on the earth's surface directly above the focus. This is the location that is commonly reported on the news because it can be more easily referenced relative to cities and towns.

B. Method of Location - The difference in the velocities of the P-and S-waves can be used to determine a circular distance to the focus. Information from three separate stations is then combined to determine the precise location. See the diagrams in your notes to learn how this method is applied.

C. Measurement

1. Intensity - Mercalli Intensity Scale - arbitrary scale of earthquake intensity based on human observation and degree of damage.

2. Magnitude

Seismograph - an instrument which magnifies and records the motions of the earth's surface. Usually includes three separate instruments, separate ones to measure vertical, E-W and N-S motion.

Richter Magnitude Scale - based on the amplitude of the seismic waves compensated for the loss of energy according to the distance that they traveled.

*Logarithmic (base ten) scale such that an earthquake of 5 produces seismic waves that are ten times greater in amplitude than an earthquake of 4. Each increase in 1 also has an increase of 30 times the energy.

Destruction begins at 5, with near total destruction at >8. The great earthquakes (>8) occur every 5-10 years.

III. Earthquake Destruction

A. Causes

1. Ground Movement

a. Destruction to Rigid Structures

b. Liquefaction - the sudden loss of strength of a mass of water-saturated sediment. Common in areas of unconsolidated fill.

c. Mass Movement - mudflows, avalanches, and landslides are commonly triggered by earthquakes.

2. Tsunamis (Tidal Waves) - large destructive ocean waves caused by the sudden displacement of seafloor and associated with earthquakes. They have been recorded up to 20 meterts (~60 ft) high and moving at speeds up to 500 mph.

3. Floods and Fires

B. Damage Prevention

1. Construction restrictions and earthquake resistant structures.

2. Prediction - measure strain, groundwater flow, previous released strain (earthquakes), etc.

IV. Earth Structure Based on Seismic Discontinuities

A. MOHO - Mohorovicic Discontinuity - boundary between the mantle and the crust. Detected as a surface along which P-wave velocities increase rapidly (6-8 km/sec.) Peridotite mantle-rock composition results in higher P-wave velocities. See the diagram in your notes.

B. Low Velocity Zone - (Asthenosphere) - at depths between 100 and 250 kilometers, seismic waves show a substantial decrease in velocity. This decrease apparanetly results from mantle rock that is near melting temperatures such that it behaves plastically. As internal earth heat moves upward, it causes this layer to flow and move the overlying brittle layer (lithosphere) in the process of plate tectonics.

C. P-wave Shadow Zone - at angular distances of 0^o to 103^o from the focus, P-waves arrive at the recording station; however, from 103^o to 143^o on both sides, no P-waves are received. It appears that the P-waves encounter a dense interior core which cause a strong bending of the waves due to its higher density.

D. S-wave Shadow Zone - at angular distance of 103^o to 180^o from the focus, S-waves are not transmitted. It is believed that a partially-molten outer core blocks the transmission of S-waves.