EarthquakesChapter 16

What is an earthquake?

An earthquake is the vibration of Earth produced by the rapid release of energy

• Energy radiates in all directions from its source, the focus

• Energy moves like waves • Seismographs record the event

Earthquakes and faults • Earthquakes are associated with faults• Motion along faults can be explained by

plate tectonics

Elastic rebound• Mechanism for EQ’s explained by H. Reid

– Rocks on sides of fault are deformed by tectonic forces

– Rocks bend and store elastic energy

– Frictional resistance holding the rocks together is overcome by tectonic forces

Elastic rebound• Earthquake mechanism

– Slips starts at the weakest point (the focus) occurs

– Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound)

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Elastic Rebound Movie in Mapview

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Reid elastic rebound cartoon

Aftershocks

The change in stress that follows a mainshock creates smaller earthquakes called aftershocks

The aftershocks“illuminate” the that ruptured in the mainshock

Red dots show location ofaftershocks formed by 3earthquakes in Missouriand Tennessee in 1811/1812

Normal Fault Quake - NevadaStrike Slip Fault Quake - Japan

Strike Slip Fault Quake - California

San Andreas: An active earthquake zone

San Andreas is the most studied fault system in the world Displacement occurs along discrete segments 100 to 200 kilometers long

• Most segments slip every 100-200 years producing large earthquakes

• Some portions exhibit slow, gradual displacement known as fault creep

Fence offset by the 1906 San Francisco earthquake

Seismology

Seismometers - instruments that record seismic waves

• Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape

A seismograph designed to record vertical ground motion

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Motion of a seismograph

Types of seismic waves • Surface waves

– Complex motion, great destruction – High amplitude and low velocity – Longest periods (interval between

crests) – Termed long, or L waves

Types of seismic waves• Body waves

– Travel through Earth’s interior – Two types based on mode of travel – Primary (P) waves

– Push-pull motion– Travel thru solids, liquids & gases

– Secondary (S) waves– Moves at right angles to their

direction of travel – Travels only through solids

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P-Wave Motion

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S-Wave Motion

Locating the source of earthquakes

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

Earthquake focus and epicenter

Locating the epicenter of an earthquake• Three seismographs 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 then determines each station’s distance to the epicenter

Graph used to find distance to epicenter

Locating the epicenter of an earthquake• A circle with radius equal to distance to the

epicenter is drawn around each station • The point where all three circles intersect is

the earthquake epicenter

Epicenter located using three seismographs

Earthquake belts• 95% of energy released by earthquakes

originates in narrow zones that wind around the Earth

• These zones mark of edges of tectonic plates

Locations of earthquakesfrom 1980 to 1990

Depths of Earthquakes• Earthquakes originate at depths ranging

from 5 to nearly 700 kilometers • Definite patterns exist

– Shallow focus occur along oceanic ridges – Deep earthquakes occur in western

Pacific east of oceanic trenches

Earthquake in subduction zones

Measuring the size of earthquakes

Two measurements describe the size of an earthquake

• Intensity – a measure of earthquake shaking at a given location based on amount of damage

• Magnitude – estimates the amount of energy released by the earthquake

Intensity scales • Modified Mercalli Intensity Scale was

developed using California buildings as its standard

• Drawback is that destruction may not be true measure of earthquakes actual severity

Magnitude scales • Richter magnitude - concept introduced

by Charles Richter in 1935 • Richter scale

– Based on amplitude of largest seismic wave recorded

– Each unit of Richter magnitude corresponds to 10X increase in wave amplitude and 32X increase in energy

Magnitudes scales• Moment magnitude was developed

because Richter magnitude does not closely estimate the size of very large earthquakes

– Derived from the amount of displacement that occurs along a fault and the area of the fault that slips

Earthquake destruction

Amount of structural damage depends on • Intensity and duration of vibrations • Nature of the material upon which the

structure rests (hard rock good, soft bad) • Design of the structure

Tsunamis, or seismic sea waves • Destructive waves called “tidal waves” • Result from “push” of fault block or

undersea landslide on water• In open ocean height is > 1 meter • In shallow coast water wave can be > 30

meters • Very destructive

Formation of a tsunami

Tsunami 1960, Hilo Hawaii

Tsunami Model, Japan Earthquake

Tsunami Model, Alaska Quake

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

Earthquakes cannot be predicted

Short-range predictions • Currently, no method exists for making

short-range earthquake predictions

Long-range forecasts • Calculates probability of a certain

magnitude earthquake occurring over a given time period

End of Chapter 16