Earthquakes

Earthquake

An earthquake is the vibration of Earth produced by the rapid release of elastic energy accumulated in rocks. Earthquakes occur when:

• Elastic energy exceeds rock strength and the rock breaks forming a fault.

• Elastic energy accumulated in the rock exceeds the friction that holds rock along an existing fault line.

Earthquakes and Faults

• Fault: large fracture in earth’s crust caused by plate motion. Faults are often, but not always associated with plate boundaries.

• Scarp: vertical offset caused by faulting ( in this diagram it’s actually the mountain range).

Earthquakes and Faults

• The earthquake begins at the focus, which is the initial point of rupture along the fault, at depth.

• The epicenter is the location on the earth’s surface directly above the focus.

• Vibrational energy radiates from the focus in all directions, in the form of waves.

Earthquake energy

• Interior forces (from heat, etc) push tectonic plates.

• at plate boundaries, frictional forces hold plates stationary. While rocks along plate boundary are “stuck”, elastic energy is stored and causes elastic deformation.

Energy release – Elastic rebound

• Frictional resistance holding the rocks together is overcome.

• Resulting movement releases stored energy as rocks return to original shape.

Energy Storage and Release

Released energy is in the form of waves, which cause movement on the earth’s surface and interior

Energy storage and release

• Energy release may also result in fractures in the earth’s crust.

Seismology: Study of Wave Energy in the Earth

• Types of seismic waves• Body waves travel entirely through

earth’s interior– Primary (P) waves

» Push-pull (compress and expand) » Travel through solids, liquids, and gases

– Secondary (S) waves» Slower velocity than P waves » Slightly greater amplitude than P waves» Travel though solids only

Seismology

• Types of seismic waves (cont’d)• Surface waves

– Travel along surface of Earth – Cause greatest destruction – Waves exhibit greatest amplitude and

slowest velocity

Seismic Wave Motion

Earthquake Waves

Seismograph

• Instrument used to record surface and body waves passing through the earth

• More than one type of seismograph is needed to record both vertical and horizontal ground motion

• Records obtained are called seismograms

Seismographs

Figure 14.6

Figure 14.8

Figure 14.9

How Big

• Modern measurement scales (Richter, moment magnitude) measure the amount of energy released by a quake.

• This value is the same no matter how far away from the epicenter the measurement is taken

How Big

An earthquake occurs in Pakistan. Pakistanis report a magnitude of 6.2. Scientists in California would probably measure a magnitude of:

a. Greater than 6.2b. 6.2c. Less than 6.2

Magnitude Scales• measure energy released (objective,

rather than intensity (subjective)• Logarithmic type scale – the energy

released by the earthquake increases by a factor of about 30 for each increment on the scale (i.e. a magnitude 6 earthqake releases roughly 30 times as much energy as a magnitude-5 earthquake.

Measurement of Energy Released during an Earthquake

Richter Magnitude (ML)• introduced by Charles

Richter in 1935• Based on the amplitude of

the largest seismic wave recorded during earthquake

Moment Magnitude (Mm)• Measures the amount of

movement and surface area of a fault that moved during earthquake

• More accurate than ML, especially for very large earthquakes

The table at the right shows the amount of energy released (in terms of TNT needed) for Richter Scale measurements (corrected to account for saturation)

Most earthquakes occur at tectonic plate boundaries

Transform boundaries, e.g. San Andreas Fault Zone

• Strike-slip fault; movement along fault is mainly horizontal

• Fault creep – small, slow movements along fault

• Stick-slip movement – fault moves in a series of jolts with no movement in between. Significant energy buildup possible, resulting in is large-magnitude damaging earthquake

Figure 14.21

Convergent boundaries – one plate sliding under another

• Benioff zone – upper part of sinking plate, where it scrapes past opposing plate, causing earthquake activity along the down-plunging contact zone

• Pacific NW evidence• India-Pakistan border,

2005

Tsunami• When an earthquake occurs

beneath the sea, the sea floor rises and falls, due to rupture and elastic rebound.

• Resulting water displacement forms a fast-moving wave.

• In the deep water of the open ocean, tsunami are barely detectable.

• In shallow water near shore, the wave speed decreases as it drags against the bottom.

• The water “stacks up”, causing a large wall of water to make landfall.

December 2004 Sumatra-Andaman Tsunami

• This shows a portion of the convergent boundary between the Eurasian plate and the Indo-Australian plate. The latter is moving northward, pushing against the former

• Green star shows epicenter of earthquake that caused tsunami.

• Red arrows show plate motion• Red dots show earthquakes >

magnitude 5.0, from 1965 to 2004.

Sea floor moves due to slippage; radiating energy results in tsunami

Tsunami

Often the water retreats before the wave hits

Figure 14.17

Earthquakes along divergent boundaries

Friction along sliding blocks (transform faults)

Earthquakes in plate interiors

• 1811-12, New Madrid, MO, site of aborted divergent boundary

• This area is still seeing active movement along the fault.

Table 14.2

Table 14.3

Figure 14.21