Size of earthquakes

MODIFIED MERCALLI SCALE

Defines the INTENSITY of an earthquake by the amount of damage caused

Characteristics:

• Depends on subjective assessment of the damage and not any measurement with an instrument

• Does not provide an accurate measurement of the strength of the earthquake

(Why?)

• Useful for planners and building officials

RICHTER MAGNITUDE SCALE

Specifies the amplitude of the largest ground motion generated by the earthquake at a seismograph station located 100 km from the epicenter

• The amplitude can be measured directly from a seismogram (more quantitative)

• The scale is logarithmic

• An increase of one magnitude on the Richter scale means approximately 30- fold increase in energy released

Characteristics of Richter Scale

MOMENT MAGNITUDE SCALE

Provides more accurate measure of the total energy released during earthquakes than the Richter scale.

Seismic moment = (amount of slip) x (the area of rupture) x (rock strength)

Bigger earthquakes:

• Cause more slip

• Break more rocks (bigger rupture area)

• Happen in stronger rocks (why?)

Locating earthquake epicenters

or HYPOCENTER

The instant an earthquake wave appears on the seismogram is called the ARRIVAL TIME of the wave

5 10 15 20 25 30 35 40 45 50 55 60

Time (Minutes)

Arrival time Arrival time

Distance between the earthquake epicenter and the recording station

P- wave arrival time

S- wave arrival time

Use TRAVEL – TIME CURVE

The P- and S- wave arrival time difference from at least three different seismographs can be used to locate the epicenter of an earthquake

More on that during lab this week

Surface Waves and Effects of Earthquakes

City of Salinas, after the October 17, 1989, Loma Prieta Earthquake

http://pubs.usgs.gov/dds/dds-29/web_pages/salinas.html

There are two broad groups of seismic waves

Body wavesCan travel through the interior of the earth

Surface wavesTravel along the surface of the earth

SURFACE WAVES

• Not much used in studying the interior of the earth

• Slower than body waves

• Most destructive of the earthquake waves

Two types of surface waves

Love waves (“horizontal” surface waves)

Rayleigh waves (“vertical” surface waves)

http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm

Rayleigh waves Make the surface go up and down like ripples

Love waves Make the ground move sideways like a moving snake

http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm

Effects of earthquakes

• Ground shakes, buildings collapse, structural damage, death and destruction…

• Fire

• Landslides

• Tsunamis (do NOT call them tidal waves)

http://staff.aist.go.jp/kenji.satake/animation.html

2004 Indonesian tsunami,

Tsunami generation

Earthquake prediction

• Monitoring faults for small tremors and foreshocks

• Radon emissions

• Measuring ground tilts

• Animal behavior

• Studying historic earthquake patterns

http://www.usgs.gov/hazards/images/maps/earthquake_lores.jpg