Geology Module 9: Earthquakes and Earth's Interior

Tsunami Travel Times

*New Guinea *Honolulu *South America

How do you locate the epicenter of an earthquake:

- 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% of the energy relased originates in a relatively narrow zone that winds around the globe - Major zones include the Circum-Pacific belt, Mediterranean Sea region, and the oceanic ridge system

Richter scale

- Based on the amplitude of the largest seismic wave recorded - Accounts for the decrease in wave amplitude with increased distance

How would you calculate a magnitude scale?

- Calculated by the average amount of slip on the fault, the area of the fault surface that slipped, and the strength of the faulted rock - Can also be calculated by modeling data from seismograms

How do richter scales work?

- Can be calculated quickly - Magnitudes less than 2.0 are not felt by humans - Each unit of Richter magnitude increase corresponds to a 10-fold increase in wave amplitude and a 32-fold energy increase - Not adequate for describing very large earthquakes

Are important because they provide information used to:

- Develop the Uniform Building Code - Assist in land-use planning

Tsunamis, or seismic sea waves

- In the open ocean wave 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

Whats the intensity scale?

- Modified Mercalli Intensity Scale was developed using California buildings as its standard

Research has concentrated on monitoring possible precursors of major earthquakes:

- Monitor changes in ground elevation - Measure strain in the rocks - Measure changes in groundwater level - Frequency of foreshocks

Tsunami Warning System

- Observations in the Pacific Ocean allow scientists to track tsunamis and issue appropriate warnings to affected areas - Seismic observatories report large earthquakes to the Tsunami Warning Center - A series of deep-water buoys in the Pacific Ocean detect energy released by earthquakes - Tidal gauges measure sea level rise and fall

elastic rebound

- 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

secondary (S) waves

- Shaking motion at right angles to their direction of travel - Travel only through solids - Slower velocity than P waves - Slightly greater amplitude than P waves

The amount of damage to structures depends on:

- The earthquake magnitude - The duration of the vibrations - The nature of the ground beneath the structures - The nature of building materials and construction practices of the region

When 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

Surface Waves

- Travel along Earth's surface - Complex motion - Cause the greatest destruction - Exhibit the greatest amplitude and slowest velocity - Have the greatest periods (time interval between crests)

Fire

- can start when gas and electrical lines are destroyed by an earthquake - Broken water lines make fire control problematic

What is the amount of destruction attributable to an earthquake varies based on:

- magnitude of the earthquake - proximity to the epicenter

Modified Mercalli Intensity Scale

12 points

Earthquakes of Magnitude

5 or greater over a 10 year period

aftershocks

Adjustments that follow a major earthquake often generate smaller earthquakes

short range predictions

Currently no reliable method exists for making short-range earthquake predictions.

tidal waves

Destructive waves that are often inappropriately

long-range forecasts

Give the probability of a certain magnitude earthquake occurring on a time scale of 30 to 100 years, or more

mechanism for earthquakes was first explained by

H.F. Reid

Body Waves vs Surface Waves

Hypocenter, Seismograph #1, #2, #3

Seismic Intensity Map:

Loma Prieta, 1989

fault creep

Some portions exhibit slow, gradual displacement

stick-slip motion

Still other segments store elastic energy for hundreds of years before rupturing in great earthquakes

What is the goal for short-range predictions?

The goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame

Japan Tsunmai

The tsunami generated from the 2011 Tohoku earthquake was 40 meters high and a Pacific-wide event, affecting not only Japan but also the west coast of North America

Seismic Gaps

Tools for forecasting earthquakes

Liquefaction of the ground

Unconsolidated materials saturated with water turn into a mobile fluid

great earthquakes tend to occur:

about every 50 to 200 years along these sections

Earthquakes most often occur

along existing faults whenever the frictional forces on the fault surfaces are overcome

vibration (earthquakes) occur

as the deformed rock "springs back" to its original shape (elastic rebound)

thrust

associated with convergent plate boundaries

normal

associated with divergent plate boundaries

types of seismic waves

body waves, primary waves, secondary waves, and surface waves

Richter magnitude

concept introduced by Charles Richter in 1935

displacement occurs along:

discrete segments 100 to 200 kilometers long

small strike-slip faults are associated with

divergent plate boundaries

magnitude

estimates the amount of energy released at the source of the earthquake

Landslides and ground subsidence

ground shaking causes loose sediments on a slope to slump

seismographs

instruments that record seismic waves

What are the 2 measurements that describe the size of an earthquake are:

intensity and magnitude

What is the drawback of intensity scales?

is that destruction may not be a true measure of the earthquake's actual severity

strike-slip

large faults associated with transform plate boundaries (such as the San Andreas Fault)

intensity

measure of the degree of earthquake shaking at a given locale based on the amount of damage

Plate Tectonic Theory

most of the motion along faults can be explained by

faults

movements that produce earthquakes are usually associated with large fractures in Earth's crust

3 major types of faults

normal thrust strike-slip

foreshocks

often precede a major earthquake by days or, in some cases, by as much as several years

What is the result from vertical displacement along a fault located?

on the ocean floor or a large undersea landslide triggered by an earthquake

What happens when "seeing" seismic waves?

seismic waves reflect and refract as they pass through the different layers of Earth

Amplification of seismic waves

soft sediments amplify the vibrations more than solid bedrock

megathrust fault

subduction zone , the boundary between the subducting and overlying plate

energy

the form of waves

focus (hypocenter)

the place within Earth where earthquake waves originate

What are long-range forecasts based on:

the premise that earthquakes are repetitive or cyclical ex: Using historical records or paleoseismology

earthquake

the vibration of Earth produced by the rapid release of energy

earthquake slippage is at

the weakest point (the focus) occurs

Where is the Epicenter is located where?

using the difference in velocities of P and S waves

Annual Occurrence of Earthquakes with:

various magnitudes