GEY 101 - Introductory Geology: Exploring Planet Earth

What is an earthquake

An earthquake is the vibration of Earth produced by the rapid release of energy
- Energy released radiates in all directions from its source, the focus
- Energy is in the form of waves
- Sensitive instruments around the world record the event

Earthquakes and faults

Movements that produce earthquakes are usually associated with large fractures in Earth’s crust called faults
Most of the motion along faults can be explained by the plate tectonics theory

Elastic rebound

Mechanism for earthquakes was first explained by H.F. Reid
Rocks on both sides of an existing fault are deformed by tectonic forces
Rocks bend and store elastic energy
Frictional resistance holding the rocks together is overcome
Earthquake mechanism
- Slippage at the weakest point (the focus) occurs
- Vibrations (earthquakes) occur as the deformed rock “springs back” to its original shape (elastic rebound)
Earthquakes most often occur along existing faults whenever the frictional forces on the fault surfaces are overcome

Foreshocks and aftershocks

Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks
Small earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years

San Andreas is the most studied fault system in the world

Displacement occurs along discrete segments 100 to 200 kilometers long
Some portions exhibit slow, gradual displacement known as fault creep
Other segments regularly slip producing small earthquakes
Still other segments store elastic energy for hundreds of years before rupturing in great earthquakes
Process described as stick-slip motion
Great earthquakes should occur about every 50 to 200 years along these sections

Seismology

The study of earthquake waves, seismology, dates back almost 2000 years to the Chinese
Seismographs, instruments that record seismic waves Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape

Seismographs

More than one type of seismograph is needed to record both vertical and horizontal ground motion
Records obtained are called seismographs

Types of seismic waves

Locating the source of earthquakes

Terms
- Focus - the place within Earth where earthquake waves originate
- Epicenter – location on the surface directly above the focus
Epicenter is located using the difference in velocities of P and S waves
Locating the epicenter of an earthquake
- Three station recordings are needed to locate an epicenter
- Each station determines the time interval between the arrival of the first P wave and the first S wave at their location
- A travel-time graph is used to determine each station’s distance to the epicenter
- A circle with a radius equal to the distance to the epicenter is drawn around each station
- The point where all three circles intersect is the earthquake epicenter

Earthquake belts

About 95 percent of the energy released by earthquakes originates in a few relatively narrow zones that wind around the globe
Major earthquake zones include the Circum-Pacific belt, Mediterranean Sea region to the Himalayan complex, and the oceanic ridge system

Earthquake depths

Earthquakes originate at depths ranging from 5 to nearly 700 kilometers
Earthquake foci arbitrarily classified as shallow (surface to 70 kilometers), intermediate (between 70 and 300 kilometers), and deep (over 300 kilometers)
Definite patterns exist
- Shallow focus occur along the oceanic ridge system
- Almost all deep-focus earthquakes occur in the circum-Pacific belt, particularly in regions situated landward of deep-ocean trenches

Measuring the size of earthquakes

Two measurements that describe the size of an earthquake are
- Intensity – a measure of the degree of earthquake shaking at a given locale based on the amount of damage
- Magnitude – estimates the amount of energy released at the source of the earthquake
Intensity scales
- Modified Mercalli Intensity Scale was developed using California buildings as its standard
- The drawback of intensity scales is that destruction may not be a true measure of the earthquakes actual severity
Magnitude scales
- Richter magnitude - concept introduced by Charles Richter in 1935
- Richter scale
  - Based on the amplitude of the largest seismic wave recorded
  - Accounts for the decrease in wave amplitude with increased distance
  - Largest magnitude recorded on a Wood-Anderson seismograph was 8.9
  - Magnitudes less than 2.0 are not felt by humans
  - Each unit of Richter magnitude increase corresponds to a tenfold increase in wave amplitude and a 32-fold energy increase
- Other magnitude scales
  - Several “Richter-like" magnitude scales have been developed
  - Moment magnitude was developed because none of the “Richter-like” magnitude scales adequately estimates the size of very large earthquakes
  - Derived from the amount of displacement that occurs along a fault

Earthquake destruction

Amount of structural damage attributable to earthquake vibrations depends on
- Intensity and duration of the vibrations
- Nature of the material upon which the structure rests
- Design of the structure
Destruction from seismic vibrations
- Ground shaking
  - Regions within 20 to 50 kilometers of the epicenter will experience about the same intensity of ground shaking
  - However, destruction varies considerably mainly due to the nature of the ground on which the structures are built
- Liquefaction of the ground
- Seiches
  - The rhythmic sloshing of water in lakes, reservoirs, and enclosed basins
  - Waves can weaken reservoir walls and cause destruction
- Tsunamis, or seismic sea waves
  - Destructive waves that are often inappropriately called “tidal waves”
  - Result from vertical displacement along a fault located on the ocean floor or a large undersea landslide triggered by an earthquake
  - In the open ocean height is usually less than 1 meter
  - In shallower coastal waters the water piles up to heights that occasionally exceed 30 meters
  - Can be very destructive
- Landslides and ground subsidence
- Fires

Can earthquakes be predicted?

Short-range predictions
- Goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame
- Research has concentrated on monitoring possible precursors – phenomena that precede a forthcoming earthquake such as measuring uplift, subsidence, and strain in the rocks
- Currently, no reliable method exists for making short-range earthquake predictions
Long-range forecasts
- Give the probability of a certain magnitude earthquake occurring on a time scale of 30 to 100 years, or more
- Based on the premise that earthquakes are repetitive or cyclical
- Are important because they provide information used to
  - Develop the Uniform Building Code
  - Assist in land-use planning