earth science- chapter 7 earthquakes

P waves are compressional waves and can travel through gases, liquids, and solids. S waves are shear waves and travel only through solids.

P and S waves

travel fast and are the first or "primary" seismic waves to reach an observer. They are compressional elastic waves that cause the rock to undergo alternating compression and expansion

P waves

Body waves can be of two main types

P waves and S waves

an _______________ ______________ rock will spring back to its original shape and release its stored elastic energy when the force is removed.

elastically deformed

he point on Earth's surface directly above the focus is the

epicenter

During an earthquake, rock moves from a few centimeters to several meters

fact

Flowing metal in the outer core generates Earth's magnetic field.

fact

In 1909, Andrija Mohorovičić discovered that seismic waves passing through the upper mantle travel more rapidly than those passing through the shallower crust. By analyzing earthquake arrival-time data from many different seismographs, he identified the boundary between the crust and the mantle. Today, this boundary is called the Mohorovičić discontinuity, or simply the Moho, in honor of its discoverer.

fact

No S waves arrive beyond _______ degrees

105

Many meteorites are composed mainly of _________ and __________

iron nickel

Most likely, Earth's magnetic field is generated within the _________- core. Metals are good conductors of electricity, and the metals in the outer core are liquid and very mobile. Two types of motion occur in the liquid outer core:

outer core

The overall density of Earth is 5.5 grams per cubic centimeter , but both crust and mantle have average densities less than this value. The density of the crust ranges from to , and the density of the mantle varies from to . Because the mantle and crust account for slightly more than 80 percent of Earth's volume, the core must be very dense to account for the average density of Earth. Calculations show that the density of the core must be to , which is the density of many metals under high pressure.

overall density of earth

A rock that has deformed plastically keeps its new shape when the stress is released and, consequently, does not store the energy used to deform it. Therefore, earthquakes do not occur when rocks deform plastically

plastic deformation

). Two types of surface waves occur simultaneously: an up-and-down rolling motion called side-to-side vibration called

rayleigh waves love waves

Charles Richter devised the Richter scale to express the amount of energy released during an earthquake. Richter magnitude is calculated from the height of the largest earthquake body wave recorded on a specific type of seismograph. The Richter scale is more quantitative than earlier intensity scales, but it is not a precise measure of earthquake energy

richter scale

However, like a rubber band, every ______ has a limit beyond which it cannot deform elastically. Under certain conditions, an elastically deformed rock may suddenly fracture. When large masses of rock in Earth's crust deform elastically and then fracture, vibrations are formed that travel through Earth and are felt as an earthquake.

rock

Rock appears rigid, but if you apply enough stress, rock will deform. When stress is applied to a rock, the rock can deform in one of three ways: (1) elastically, (2) by fracturing, or (3) plastically.

rock fact

the _______________-, located across the San Francisco Bay Area from the Hayward Fault, is estimated to have only a 21% chance of generating a magnitude 7.0 earthquake over the same timeframe

san andreas fault

Much of our knowledge of the composition and structure of the mantle comes from ____________ data

seismic data

Waves that travel through rock are called

seismic waves

This physical "written" record of Earth vibration is called a

seismogram

a device that records seismic waves

seismograph

is the study of earthquakes and the nature of Earth's interior based on evidence from seismic waves.

seismology

Sandy sediment and soil commonly __________ during an earthquake.

settle

Using a global array of seismographs, seismologists can detect direct P and S waves up to 105 degrees from the focus of an earthquake. Between 105 and 140 degrees is a "shadow zone" where no direct P waves arrive at Earth's surface. This shadow zone is caused by a discontinuity, which is the mantle-core boundary. When P waves pass from the mantle into the core, they are refracted, or bent, as shown in Figure 7.27. The refraction deflects the P waves away from the shadow zone.

shallow zone

prediction is based on occurrences of foreshocks, release of radon gas, and changes in the land surface, and the water table

short term prediction

Short-term predictions are forecasts that an earthquake may occur at a specific place and time. Short-term prediction depends on signals that immediately precede an earthquake.

short term predictions

Surface waves travel more _____________ than body waves.

slowly

Plates move slowly and silently every day. Rocks stretch, bend, and compress, unseen beneath our feet. Geologists understand the mechanisms of earthquakes and know for certain that more destructive quakes will occur along the San Andreas Fault. But no one knows when or where the "Big One" will strike.

slowly and silently

Unlike P waves, S waves move only through ___________. Because molecules in liquids and gases are only weakly bound to one another, they slip past each other and thus cannot transmit a shear wave.

solids

Moment magnitude depends on the __________ of rocks, because a strong rock can store more elastic energy before it fractures than a weak rock can.

strength

s. The San Andreas Fault itself is vertical, but the rocks on opposite sides move horizontally. A fault of this type is called a ____________ fault

strike slip fault

where do many of the strongest recorded earth quakes occur

subduction zones

then radiate from the epicenter and travel along Earth's surface, somewhat like swells on the surface of the sea.

surface waves

. Most earthquakes occur along ____________ plate boundaries.

tectonic

At a depth of about 660 kilometers, seismic wave velocities increase again because pressure is great enough there to produce denser minerals. The zone where the change occurs is called the 660-kilometer discontinuity. The base of the mantle lies at a depth of 2,900 kilometers. Recent research has indicated that the base of the mantle, at the core-mantle boundary, may be so hot that despite the tremendous pressure, rock in this region is partially liquid.

660 kilometer discontinuity

The ___________ Fault last generated an earthquake with estimated 7.0 magnitude in 1868 and is considered by the USGS to be the most likely to generate a magnitude 7.0 or greater earthquake within the next 30 years (31% probability).

Hayward

the other type of body waves, are slower than P waves and thus are the "secondary" waves to reach an observer. Also called shear waves, S waves have a shearing motion that can be illustrated by tying a rope to a wall, holding the opposite end, and giving the rope a sharp up-and-down jerk

S waves

For example, the _______________ in Southern California lies along a tectonic plate boundary that has slipped many times in the past and will certainly again in the future (Figure 7.4). The boundary includes numerous individual faults, some of which are active and some of which are not.

San Andres fault

In the open sea, a tsunami is so flat that it is barely detectable. Typically, the crest may be only 1 to 3 meters high, and successive crests may be more than 100 to 150 kilometers apart. However, a tsunami may travel at 750 kilometers per hour. When the wave approaches the shallow water near shore, the base of the wave drags against the bottom and the water stacks up, increasing the height of the wave. The rising wall of water then flows inland. A tsunami can flood the land for as long as 5 to 10 minutes.

Tsunami

The USGS hosts a large earthquake monitoring network, and information from thousands of deployed seismographs around the world is available as soon as it happens. Basic information provided on the monitoring network includes location, magnitude, and depth of each earthquake. For example, within the hour before this sentence was written, a total of 15 earthquakes were recorded globally; ten were on the island of Hawaii and probably related to the ongoing eruptions there.

USGS

transmits energy from one place to another

a wave

Sumatra-Andaman earthquake was the third-largest seismic event ever recorded and caused the seafloor to move westward about 6 meters and upward about 2 meters. This tremendous displacement of rock initiated a massive tsunami that radiated in all directions, killing an estimated 286,000 people along the Indian Ocean coastlines. Survivors reported that, moments prior to the deadly wave, coastal water retreated, exposing to the air wet mud in ocean bays. Then the wave raced inward, rearing upward as much as 24 meters, as high as a 10-story building, and inundating some areas 30 meters above sea level. Although mortality was highest in Sumatra, people died in coastal areas as far away as Port Elizabeth, South Africa, 8,000 kilometers from the epicenter.

bad tsunami

In many regions, bedrock lies at or near Earth's surface and buildings are anchored directly to the rock. Bedrock vibrates during an earthquake, and buildings may fail if the motion is violent enough. However, most bedrock returns to its original shape when the earthquake is over, so if structures can withstand the shaking, they will survive. Thus, bedrock forms a desirable foundation in earthquake-prone areas.

bedrock and earthquakes

The earthquakes occur mostly along the upper part of the sinking plate, where it scrapes past the opposing plate. Additional earthquakes occur within the downward-moving slab as it is bent and extended upon being pulled into the mantle (Figure 7.19). Collectively, these earthquakes form the __________--- zone, named after Hugo Benioff, the geologist who first recognized it. Many of the world's strongest earthquakes occur in subduction zones, including all five of the largest recorded earthquakes.

benioff zone

travel through Earth's interior and carry some of the energy from the focus to the surface

body waves

An earthquake produces two main types of seismic waves:

body waves and surface waves

Modern seismographs use a _________- fixed to a pendulum that is suspended in a magnetic field. When an earthquake causes the pendulum to move within the magnetic field, it creates an electric current that is amplified and recorded by a computer.

coil

is a sudden motion or trembling of Earth caused by the abrupt release of energy that is stored in rocks.

earthquake

Earthquakes occur either when the elastic energy accumulated in rock exceeds the friction that holds rock along a fault, or when the elastic energy exceeds the strength of the rock and the rock breaks.

earthquakes occur

Earth's rotation causes internal stress to build up because the lithosphere slows down at a different speed than other parts of Earth's interior. As a result of differences in the rate of slowdown, every few decades Earth has a cluster of large earthquakes to relieve the stress that has accumulated.

earths rotation

On both the moment magnitude and the Richter scales, the energy of the quake increases by a factor of about 30 for each whole number increment on the scale. Thus, a magnitude-6 earthquake releases roughly 30 times more energy than a magnitude-5 earthquake.

fact

When a rock fracture ruptures during an earthquake and rock on the two sides of the fracture move relative to one another, the fracture becomes a __________

fault

Along some portions of the fault zone, rocks slip past one another at a continuous, snail-like pace referred to as _____________. This movement occurs without violent and destructive earthquakes because the rocks slip continuously and slowly.

fault creep

The initial rupture point, where abrupt movement creates an earthquake, typically lies below the surface at a point called the

focus

are small earthquakes that precede a large quake by a few seconds to a few weeks. The cause of foreshocks can be explained by a simple analogy. If you try to break a stick by bending it slowly, you may hear a few small cracking sounds just before the final snap. Foreshocks are not a reliable tool for short-term prediction because they do not precede all earthquakes. Of those selected for study, foreshocks preceded only about half of the earthquakes. In addition, some swarms of small shocks thought to be foreshocks were not followed by a large quake.

foreshocks

______________- prevents the plates from slipping past one another continuously.

friction

The seismologist then plots a circle around each of the seismic station locations on a map. The radius of each circle is the linear distance to the epicenter from that station. The one point on the map where the three (or more) circles intersect is the location of the earthquake's epicenter.

how to find earthquakes epicenter

Most earthquake fatalities and injuries occur when falling structures crush people. Structural damage, injury, and death depend on the magnitude of the quake, its proximity to population centers, rock and soil types, topography, and the quality of construction in the region.

injuries

Many faults develop as part of a ___________ group of faults that together release much of the strain that accumulates in a region. For example, thousands of separate normal faults exist today in the North American Basin and Range province, introduced in Chapter 6. Many of these faults form the geologic boundary separating each mountain range from its adjacent basin.

larger group

A familiar example of liquefaction occurs when you tap your foot on saturated sand at the beach; the forces produced by your tapping cause compression waves to travel into the sand, liquefying it and producing a slurry of sand and water that flows over your toes. When soils liquefy on a hillside due to earthquake shaking, the resulting slurry flows downslope, carrying structures along with it. Landslides are common effects of earthquakes and are discussed further in

liquefaction

the soil loses its shear strength—its resistance to shear—and behaves like a fluid in a process called

liquefaction

Long-term earthquake prediction is based on the observation that most earthquakes occur on preexisting faults at tectonic plate boundaries.

long term prediction

Long-term earthquake prediction recognizes that earthquakes have recurred many times along existing faults and will probably occur in these same regions again. For example, in the United States, although some faults exist in plate interiors (as in the New Madrid earthquake zone), the most active faults lie along the West Coast at plate boundaries (Figure 7.20) and within the actively extending Basin and Range Province.

long term prediction

These convecting, spinning liquid conductors are thought to generate Earth's ___________ field

magnetic field

The most recent survey of the magnetic North Pole, conducted in 2017, determined that the Pole is moving north-northwest at about 55 km per year. Figure 7.29 shows the movements of the magnetic North and South Poles over the past few centuries. In addition to causing the magnetic poles to move around, the constantly moving liquid of the outer core at times leads to polarity reversals in the magnetic field itself, so that the North Pole becomes the South Pole and vice versa. These polarity reversals occur over geologic timeframes and are recorded in basalt of the ocean floor as discussed in Chapter 6.

magnetic poles

Before seismographs became widespread, geologists evaluated earthquakes on the ________ which measures the intensity of an earthquake and is based on structural damage. On the Mercalli scale, an earthquake that destroyed many buildings was rated as more intense than one that destroyed only a few.

mercalli scale

Mexico City provides one example of what can happen to clay-rich soils during an earthquake. The city is built on a high plateau ringed by even-higher mountains. European settlers built the modern city on water-soaked, clay-rich, sediment deposited in an ancient former lake. On September 19, 1985, an earthquake with a moment magnitude of 8.1 struck about 500 kilometers west of the city. Seismic waves shook the wet clay beneath the city and reflected back and forth between the bedrock sides and bottom of the basin, just as waves in a bowl of jelly bounce off the side and bottom of the bowl. The reflections amplified the waves, which destroyed more than 500 buildings and killed between 8,000 and 10,000 people. Meanwhile, there was comparatively little damage in Acapulco, a city much closer to the epicenter but built on bedrock.

mexico city earthquake

Most seismologists now use _________ magnitude rather than Richter magnitude, because it more closely reflects the total amount of energy released during an earthquake.

moment

The product of these two values and the shear strength of the faulted rock allow them to calculate the

moment magnitude

Two types of motion occur in the liquid outer core: (1) Because the outer core is much hotter at its base than at its surface, the liquid metal convects. (2) The rising and sinking currents of molten metal are then deflected by Earth's spin.

motion

However, _____ faults can form if the orientation and curvature of the existing faults no longer can most efficiently release stress that builds in the rock.

new

Over the past few decades, short-term prediction has _________ been reliable.

not

The mantle is almost 2,900 kilometers thick and composes about 80 percent of Earth's volume.

the mantel

The Moho lies at a depth ranging from 4 to 70 kilometers.

the moho

oceanic crust is __________ than continental crust, and continental crust is thicker under mountain ranges than it is under plains.

thinner

Geologists use a _________- curve to calculate the distance between an earthquake epicenter and a seismograph. A travel-time curve is constructed using P-wave and S-wave arrival times from multiple stations at different locations for an earthquake with a known epicenter and occurrence time. The resulting graph can then be used to measure the distance between a recording station and an earthquake whose epicenter is unknown by plotting the time delay between the two wave types on the travel-time curve.

travel time curve

Sea waves produced by an earthquake are often called tidal waves, but they have nothing to do with tides. Therefore, geologists call them by their Japanese name,

tsunamis

No major earthquakes have occurred in the central or eastern United States in the past 100 years, and no current lithospheric plate boundaries are known in these regions. However, the largest historical earthquake sequence in the contiguous 48 states occurred near New Madrid, Missouri. In 1811 and 1812, three shocks with estimated moment magnitudes between 7.3 and 7.8 altered the course of the Mississippi River and rang church bells 1,500 kilometers away in Washington, D.C. These earthquakes occurred within a zone of active extensional tectonics called the New Madrid Fault Zone and located in the lower Mississippi River Valley region.

united states earthquakes

Modern geologists know that most earthquakes occur along plate boundaries, where tectonic plates diverge, converge, or slip past one anothe

where earthquakes occur