• An earthquake is the vibration, sometimes violent, of the Earth's surface that follows a sudden release of stored energy when a fault ruptures.

• This energy can be generated by a sudden dislocation of segments of the crust, by a volcanic eruption, or by manmade explosions.

Earthquakes define Plate Boundaries

Lisa WaldUSGS Pasadena

U.S. Department of the InteriorU.S. Geological Survey

USGS Earthquake Hazards Program

Plate Tectonics

Major Plates and their Boundaries

• Faults form by brittle failure.

• Most earthquakes occur along active plate boundaries, where plates bump against each other due to mantle convection.

FAULTS• A fault is a fracture in the Earth's crust along which two blocks of the

crust have slipped with respect to each other.

• Faults are divided into three main groups, depending on how they move.

• Normal faults occur in response to pulling or tension; the overlying block moves down the dip of the fault plane.

• Thrust (reverse) faults occur in response to squeezing or compression; the overlying block moves up the dip of the fault plane.

• Strike-slip faults occur in response to either type of stress; the blocks move horizontally past one another.

•

• Specific plate boundaries are associated with particular fault types:

• Convergent: Reverse (thrust)

• Divergent: Normal

• Transform: Strike-slip

THREE TYPES OF PLATE BOUNDARIES

EARTHQUAKE LOCATIONS

• The focus is the location of an earthquake Longitude, Latitude, and depth.

• The epicenter of an earthquake is the point on the Earth's surface directly above the focus.

About 800,000+ quakes (events), occur per year.

About 4,000/yr are felt

About 1-2 “great” (Mb > 7.5) events occur per year

Earthquake Distribution and Plate Tectonics

• Most of the great earthquakes (M>7.5) occur at active plate boundaries:

• 1) active convergent plate boundary (c.p.b.) 85%

• 2) active transform plate boundary (t.p.b.) 8

• 3) recently active c.p.b. 4 • (active less than 20 million yrs ago)

• 4) recently active t.p.b. 2

• 5) active d.p.b. < 1

Seismic Velocities

• VELOCITIES: Vp > Vs > Vl

• VELOCITY increases with depth in earth» P S L

• top of crust 5.5 3.5 2 km/sec

• 30 km 7.5 5.5 4 km/sec

Seismic Velocities

• ___________

• Vp-wave = √ [(4/3 + k)/

• Vs-wave = √ (

shear modulus krigidity

= density in g/cc

EARTHQUAKE SIZE

• Richter or Magnitude Scale • Measures actual energy release - developed in

1930's at Cal Tech• Based on the log 10 A of the p-wave

• Scale -2 to 12. • -CALIBRATION: if A is 1 micron at instrument

100 km from event, then the Magnitude = -2• - there is an increase of 32x as much energy

from one integer to the next. (Therefore, a magnitude 4.1 is about 3x the size of a 4.0!)

EARTHQUAKE SIZE Modified Mercalli intensity scale (MMI)• Measures damage to human (man made) structures.• Intensity depends on: reporting accuracy, population,

development, building codes, and enforcement.• Intensity Scale is I - XII. • Useful for all pre-instrumental events. The few seismographs

operating in the early part of the last century were isolated and uncalibrated. Calibration with explosions occurred from the 1930’s to present.

• The newer Mw scale gives more accurate magnitudes for very large events.

• To assess seismic risk and locations of possible future events, we need to look at pre-instrumental records.

• TO CALCULATE ENERGY RELEASE from MAGNITUDE:

• E = (25.1kJ)(31.6) M

• 1 Mton of TNT = 4.18*1015 Joules, or about a 7.8

• DIFFERENT MAGNITUDE TYPES: • log10 A of P wave = Mb scale -good for 0 - 7 • log10 A of S wave = Ms scale -good for 3 - 7• log10 A of L wave = ML scale -surface waves 3 -

7• For all very large earthquakes occurring since

1973, seismologists currently integrate the area under curve of all waves on the seismogram. This results in the Mw scale, which is more accurate for very large (7.5 and up) events

• Great Chilean Earthquake in May of 1960 is the largest earthquake ever recorded. Its Mw = 9.5.

• Rupture length = 1000 km.

• It occurred where the Nazca Plate under the ocean collides with the South American Plate at 9.7 cm/yr. Currently the world’s fastest rate.

• The largest recorded event in North America was the 1964 Alaska Earthquake,which has a revised magnitude, Mw of 9.2. It ruptured 2000 km of the Alaska coast. The Pacific Plate is colliding with the North American Plate at the coast at about 7 cm/yr.

SEISMIC MOMENT

Seismic Moment, M0 = k*Dav*A

K = rigidity modulusDav = average displacement (slip) of one

side of fault relative to the otherA = area of fault surface that ruptured

Rock k Basalt 2.38*1010 Pa Granite 1.38Sandstone 1.81Lithosphere 3.3Water 0

SEISMIC MOMENT, ENERGY, AND MAGNITUDE

• Energy release can be estimated from the seismic moment:

• E in dyne-cm = M0/20,000

• Mw = 2/3(log10M0 in dyne-cm) - 16

Bigger Faults Make Bigger EarthquakesBigger Faults Make Bigger Earthquakes

1

10

100

1000

5.5 6 6.5 7 7.5Magnitude

Kilo

me

ters

8

Leng

th o

f F

ault

in