Transcript EARTHQUAKES - City University of New York

• 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
Plate Tectonics
USGS Earthquake Hazards Program
Lisa Wald
USGS Pasadena
Major Plates and their Boundaries
U.S. Department of the Interior
U.S. Geological Survey
• 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.)
• 2) active transform plate boundary
(t.p.b.)
85%
8
• 3) recently active
c.p.b.
• (active less than 20 million yrs ago)
4
• 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
»
• top of crust
• 30 km
P
S
L
5.5
7.5
3.5
5.5
2
4
km/sec
km/sec
Seismic Velocities
•
___________
• Vp-wave = √ [(4/3m + k)/r]
_____
• Vs-wave = √ (m/r)
m = shear modulus
k = rigidity
r = 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 modulus
Dav = average displacement (slip) of one
side of fault relative to the other
A = area of fault surface that ruptured
Rock
Basalt
Granite
Sandstone
Lithosphere
Water
k
2.38*1010 Pa
1.38
1.81
3.3
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 Earthquakes
Kilometers
Length of Fault in
1000
100
10
1
5.5
6
6.5
7
Magnitude
7.5
8