Geology Chapter 11: Earthquakes

Foreshock

A major earthquake may be preceded by a series of smaller earthquakes called foreshocks as strain builds along fault plane

Liquefaction

A result of destruction from seismic vibrations. Liquefaction involves: - Ground shaking draws groundwater up to the surface - Unconsolidated soils become saturated with water and turn into a mobile slurry no longer capable of supporting structures

Earthquake

An earthquake is the vibration of Earth produced by the rapid release of energy: -Energy released radiates in all directions from its source, the hypocenter -Energy is in the form of waves - Sensitive instruments around the world record the event

How does a building's structure, distance from epicenter and nature of the soil and bedrock affect the amount of damage caused by an earthquake?

Buildings resting on unconsolidated soil experience more damage than buildings constructed on solid bedrock. Buildings constructed of blocks and bricks that are not reinforced with steel rods are the most serious safety threats in earthquakes. Being further away from the epicenter means you are more safe from the effects of the seismic waves.

Richter scale

Charles Richter (1935) devised a procedure to measure the size of an earthquake on the basis of ground motion rather than the amount of destruction. Richter Magnitude is determined by Measuring the Maximum Amplitude of the P- or S-wave on the Seismic Wave Pattern. A magnitude 1 difference in the Richter scale indicates a factor of 10 difference in the amplitude of ground motion.

What are the various ways by which future earthquakes may be predicted?

Currently no reliable short-term ways to predict earthquakes. Long-term prediction is calculated with paleoseismology, and looking at seismic gaps along a fault: - Segments where fault is locked - Strain not readily released along these segments and thus builds up - Fault eventually slips within a segment (seismic gap) and enormous strain build-up is released as a major earthquake

Describe the global distribution of earthquakes. Where do most earthquakes occur? Intermediate and deep earthquakes primarily occur along which of the three types of plate boundaries?

Earthquakes seem to be globally distributed along tectonic plate boundaries. Shallow earthquakes occur along all types of plate boundaries. Intermediate and deep earthquakes restricted to convergent plate boundaries. Intermediate and deep (as well as shallow) earthquakes occur in subduction zones. The strongest earthquakes occur mostly over convergent boundaries.

Fault

Fractures along which one rock body slides past another

What are some of the problems associated with using the Mercalli scale to measure the strength of earthquakes?

It is not super accurate because it only measures the amount of destruction and relies on factors such as building practices, which vary considerably. Also, earthquake destruction depends on many factors

Intensity

One measurement of earthquakes; a measure of the degree of earthquake shaking at a given locale based on the amount of damage

Magnitude

One measurement of earthquakes; estimates the amount of energy released at the source of the earthquake

Describe the particle motions for P-waves, S-waves and surface waves.

P waves are push/pull waves: they momentarily push (compress) and pull (stretch) rocks in the direction the wave is traveling. P waves can travel through solids, liquids, and gases. Temporarily change the VOLUME of intervening material. S waves "shake" the particles at right angles to their direction of travel. This is illustrated by fastening one end of a rope and shaking the other end. Temporarily change the SHAPE of the material that transmits them. Only solids transmit S waves. Surface waves either cause everything on earth's surface to move, like waves tossing a ship, and the other type causes earth's materials to move from side to side.

Paleoseimology

Paleoseismology is the study of past seismic activity and is used to predict probabilities of future earthquakes

Know the stages of the Elastic Rebound Theory in explaining earthquakes.

Phase A: Original position of rocks on opposite sides of a fault. This is like a limb stick being held horizontal. Phase B: The movement of tectonic plates causes the rock to bend and store elastic energy. This is equivalent to that stick being bent on both ends. Phase C: once the strength of the rock is exceeded, slippage along fault produces an earthquake. This is equivalent to the stick breaking from stress, shaking up and down on both ends. Phase D: The rocks return to their original shape but in a new location. The stick is now broken into two, stable (unmoving) ends.

Primary (P) waves

Primary (P) waves travel through solid rock at an average velocity of ~5 km/sec • P-waves can travel through: - Solids - Liquids - Gases Push or pull particles of matter in the direction of their travel path. P waves are fastest and arrive first to seismographs

Secondary (S) waves

Secondary (S) waves travel through solid rock at about half the speed of P-waves Also called shear waves because they push material at right angles (90o) to their path of travel Unlike P-waves, shear waves cannot travel through liquids or gases S waves arrive after P waves at seismographs

Seismic gaps

Seismic gaps along a fault: - Segments where fault is locked - Strain not readily released along these segments and thus builds up - Fault eventually slips within a segment (seismic gap) and enormous strain build-up is released as a major earthquake

What are seismic gaps and what do they say about future major earthquakes?

Seismic gaps are believe to be zones that are storing strain that will be released during a future earthquake. Relevant to long-term earthquake prediction.

Seismograph

Seismographs are instruments that record the movement of the ground in relation to a stationary mass on a rotating drum or magnetic tape A typical seismic observatory measures three components of ground motion: - Vertical up-down motion - Horizontal east-west motion - Horizontal north-south motion

Explain how seismologists locate the epicenter of an earthquake?

Seismologists can measure the time interval between the first arrivals of P- and S-waves to determine how far away the epicenter is to a particular seismograph station (using the time intervals). They can then triangulate the location of the epicenter, which requires three or more seismic stations.

Moment magnitude

Seismologists often prefer a scale that measures the actual energy released by an earthquake - thus, moment magnitude. Moment magnitude depends on: Amount of slip on the fault plane; Area of the fault break; Rigidity or strength of the rock. The total energy of an earthquake can be related to its Richter magnitude: Log10E = A +BM E = total energy in ergs M = Richter magnitude A & B: constants that depend on local geology

Aftershock

Smaller tremors called aftershocks may occur over days to months following the main earthquake as the fault readjusts following slippage

Surface waves

Surface waves are confined to the surface and outer layers of the Earth Travel at speeds slightly less than that of S-waves Some are similar to waves in the ocean Do most of the destruction during an earthquake Surface waves are slowest and arrive last to seismographs

How is the Richter magnitude determined?

The Richter scale magnitude is calculated by measuring the amplitude of the largest seismic wave (usually a P or S wave) recorded on a seismogram. This measured amplitude must be adjusted to account for weakening of the ground motion with distance from the epicenter. A magnitude 1 difference in the Richter scale indicates a factor of 10 difference in the amplitude of ground motion.

What does the moment magnitude of an earthquake measure?

The actual energy released by an earthquake. Moment magnitude depends on: - Amount of slip on the fault plane - Area of the fault break - Rigidity or strength of the rock

How does the focus (hypocenter) of an earthquake differ from the epicenter?

The focus/hypocenter is where the earthquake starts inside the earth, whereas the epicenter is where the earthquake occurs in earth's surface (directly above the epicenter).

Wadati-Benioff zones

The narrow zone of inclined seismic activity that extends from a trench downward into the asthenosphere.

Focus

The point beneath Earth's surface where rock breaks under stress and causes an earthquake; also called the hypocenter

Epicenter

The point on Earth's surface directly above the hypocenter is called the epicenter

Tsunami

Tsunamis are also called seismic sea waves: - Destructive waves that are often inappropriately called "tidal waves" • Tsunamis are generated several ways: - Volcanic eruptions (e.g. Krakatoa in 1883) - Large undersea "landslides" triggered by volcanic eruption or earthquake - Vertical displacement along a fault on the ocean floor Wave Heights of Tsunami Dramatically Increase When Approaching Shallower Coastal Waters.

What causes tsunamis? Describe the changes to tsunami waves as they approach the shoreline in terms of wave velocity, wave height, the spacing between waves, and amount of destruction.

Tsunamis are generated by volcanic eruptions, large undersea "landslides" triggered by volcanic eruption or earthquake, and vertical displacement along a fault on the ocean floor Wave heights increase when approaching shallower coastal waters. Additionally, spacing gets shorter, and waves become highly destructive when hitting coast.

Modified Mercalli Intensity Scale

a 12 point (roman numeral) scale developed to evaluate earthquake intensity based on the amount of damage to various structures

Sketch a seismogram showing the order of arrival of P-waves, S-waves and Surface Waves at a typical seismograph station.

p waves first, s waves second, surface waves last.

Fault creep

slow, gradual displacement that happens smoothly with little seismic activity

Seismogram

the records obtained from seismographs that provide useful information about the nature of seismic waves. Seismograms reveal that two main types of seismic waves are generated by the slippage of a rock mass: body waves and surface waves.

Seismology

the study of earthquakes

Seismic waves

vibrations that travel through Earth carrying the energy released during an earthquake. Body (P and S) waves or surface waves.

Elastic rebound (theory)

• Fault plane initially at rest • As force is applied, the fault is locked and resists movement • Continued stress causes strain to build up in rocks around the fault • The fault finally slips, releasing energy as seismic waves • Fault returns to initial state, but fault blocks are now displaced