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.
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 activeplate 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
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 = √ [(4/3m + 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 k Basalt 2.38*1010 Pa Granite 1.38 Sandstone 1.81 Lithosphere 3.3 Water 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(log10M0in dyne-cm) - 16
Bigger Faults Make Bigger Earthquakes Length of Fault in