Slide1 : A man walks his bike past a leaning building destroyed by the 1995 earthquake in Kobe, Japan. Fig. 7-CO, p.146
Slide2 : Damage after a 7.9 magnitude earthquake struck Ahmedabad, India, 2001. Fig. 7-1, p.147
Anatomy of an earthquake : Anatomy of an earthquake When you apply enough stress to a rock, it can deform in one of three ways: elastically, fracture and plastic deformation.
At first the rock deforms elastically. If the stress is removed, the rock springs back to its original size and shape (rubber band example), releasing the stored elastic energy. Fig. 7-4, p.149
fracture : Fig. 7-2, p.148 fracture Every rock has a limit beyond which is cannot deform elastically. It may suddenly fracture, releasing the elastic energy and springing back to its original shape. The resultant rapid motion creates vibrations that travel through the Earth and are felt as an earthquake.
Slide5 : Under other conditions rock can exceed its elastic limit and continue to deform like putty. This is called plastic deformation, and the rock deformed this way will keep its new shape when the stress is released (so does not store the energy used to deform it, and earthquakes do not occur). Fig. 7-3, p.148
Slide6 : A road is built across an old fault (the San Andreas Fault).
Plates move 1-16cm/yr. Friction prevents continual slippage (next slide). Fig. 7-4a, p.149
Slide7 : So rock stretches or compresses elastically and potential energy builds up. When the stress is so great, rock snaps loose or fractures; the ground rises and falls and undulates back and forth (earthquake). Fig. 7-4b, p.149
Slide8 : An earthquake is a sudden motion or trembling of the Earth caused by the abrupt release of energy stored in the rocks. Fig. 7-4c, p.149
Slide9 : Now a weakness in rock has formed, so other earthquakes commonly occur along the same faults. To right is a portion of the San Andreas Fault; it is part of the boundary between the Pacific plate (left) and North-American plate. Fig. 7-5, p.149
Slide10 : Earthquake Waves:
Seismology is the study of EQ’s and the nature of the Earth’s interior on evidence of seismic waves (waves that travel through rock, produced by EQ’s and explosions).
Focus: initial rupture point
Epicenter: point on Earth’s surface directly above the focus. Fig. 7-6, p.150
Slide11 : Earthquakes produce several types of seismic waves:
Body Waves: travel through the Earth’s interior and carry some of the energy from the focus to the surface
Surface Waves: radiate from epicenter along the Earth’s surface (like when you throw a rock in a calm lake).
Slide12 : Body Waves: two main types are 'P' and 'S' waves.
Model of a 'P' (primary and compressional) wave. The spring moves parallel to the direction of wave propagation, causing compression and expansion of rock. Move fast (4-7 km/sec in crust) and travel through air, liquid and solid material. Fig. 7-7, p.150
Slide13 : 'S' (shear or secondary) waves only travel through solid material, at 3-4 km/sec in the crust; arrive after 'P' waves. Example to right illustrates that 'S' waves move parallel to rope, but individual particles move at right angles to rope length. Fig. 7-8, p.151
Slide14 : Surface waves, which radiate from the epicenter along Earth’s surface, travel slower than body waves. They produce an up and down and rolling motion, and a side to side vibration (so Earth’s surface rolls like ocean waves and writhes from side to side). Fig. 7-9, p.151
Slide15 : Seismograph: a device that records seismic waves; it creates a seismogram, which is a record of Earth’s vibration. Fig. 7-10, p.151
Slide16 : Seismogram that recorded north-south ground movements during the 1989 Loma Prieta earthquake. Fig. 7-11, p.152
Measurement of Earthquake Strength : Measurement of Earthquake Strength 1. Mercalli Scale: measures intensity of an EQ based on structural damage; the more buildings destroyed, the more intense the EQ. This scales does not measure the energy released during an EQ.
2. Richter Scale (Charles Richter, 1935): expresses the amount of energy released during an EQ; it measure the largest EQ body wave recorded on a seismograph. What is a drawback of this method?
Today, seismologists calculate 'moment magnitude': measure of amount of movement and surface area of a fault that moved during an EQ. This better reflects the total amount of energy released during an EQ.
EQ strength (cont) : EQ strength (cont) A moment magnitude of 6.5 has the energy about equal to the atomic bomb dropped on Hiroshima at the end of World War II.
On both the moment magnitude and Richter scales the energy of the quake increases by a factor of about 30 for each increment on the scale.
The strength of rock determines the largest possible EQ (strong rock can store more elastic energy). The largest ever recorded on the moment magnitude scale were 8.5 and 8.7, about 900 times greater then energy released by the Hiroshima bomb.
Slide19 : Locating the source of an EQ. Fig. 7-12, p.153
Slide20 : Time-travel curve. Fig. 7-13, p.153
Slide21 : Locating the EQ epicenter by triangulation from three seismic stations that recorded the quake. Fig. 7-14, p.154
Slide22 : Most EQ occur along plate boundaries, where plates diverge, converge or slip past one another. Fig. 7-15, p.154
Slide23 : Earthquake hazard map of the San Andreas Fault zone in California (numbers indicate the probability of an EQ with magnitude andgt;5 in the next 30 years). Fig. 7-16, p.155
Slide24 : EQ at a transform plate boundary: the San Andreas Fault Zone. This is a transform plate boundary between the Pacific and North American plates. The fault is vertical, the rocks move horizontally on opposite sides of the fault; this is called a strike-slip fault. The SAF is a right-lateral strike-slip fault. Fig. 7-17, p.155
Slide25 : The 1906 EQ and fire destroyed most of San Francisco. Fig. 7-18, p.155
Slide26 : The 1994 magnitude 6.6 EQ that struck Northridge near LA caused much building damage. About 10,000 EQ occur every year along the SAF and its associated faults (SAFZ). Fig. 7-19, p.156
Slide27 : Earthquakes at a convergent plate boundary, occur along the upper part of the sinking plate where it scrapes past the opposing plate in a subduction zone. Called the Benioff Zone.
What about a continental-continental convergent boundary? Oceanic-oceanic convergent boundary? Fig. 7-20, p.156
EQ at divergent plate boundaries : EQ at divergent plate boundaries EQ occur frequently at the Mid-Oceanic Ridge system as a result of faults that form as the two plates separate. Would the EQ be deep or shallow along the ridge?
Slide29 : Fig. 7-21, p.157
Slide30 : Fig. 7-22, p.159
Slide31 : Fig. 7-23, p.160
Slide32 : A tsunami can develop from an earthquake. Fig. 7-24, p.161
Slide33 : The sea floor drops, sea level falls with it. Fig. 7-24a, p.161
Slide34 : Water rushes into the low spot and overcompensates, creating a bulge. Fig. 7-24b, p.161
Slide35 : A tsunami develops when part of the sea floor drops during an EQ. Water rushes to fill the low spot, but the intertia of the rushing water forces too much water into the area, creating a bulge in the water surface. The long, shallow waves can build up into destructive giants when they reach shore. May travel at 750 km/hr with wave crests 100-150 km apart. Fig. 7-24c, p.161
Slide36 : Fig. 7-25, p.161
Slide37 : Fig. 7-26, p.162
Slide38 : Fig. 7-27, p.163
Slide39 : Velocities of 'P' waves in the crust and upper mantle. Generally, 'P' wave velocity increases with depth. What is the low velocity zone? Fig. 7-28, p.163
Slide40 : Cross section of Earth showing paths of seismic waves. They bend gradually because of increasing pressure with depth. They bend sharply where they cross major layer boundaries in the Earth’s interior. Note that 'S' waves do not travel through the liquid outer core, so direct 'S' waves are only observed within an arc of 105 degrees of the epicenter.
'P' waves are refracted sharply at the core-mantle boundary, so there is a shadow zone of no direct 'P' waves from 105-140 degrees from the epicenter. Fig. 7-29, p.164
Slide41 : Earth’s magnetic field. The magnetic north pole is offset 11.5 degrees from the geographic pole. Fig. 7-30, p.165
Slide42 : p.167