Earthquakes and Earth’s Interior

Earth’s Layers

I. The Crust: the outermost layer of the earth, varies from ~ 5 km of thickness under the oceans to > 60km under some mountain ranges. The composition in most places is a thin layer of sedimentary rock , mostly granite-like (grantic) rocks of the continental crust. The oceanic crust is made up of mostly darker & denser rock similar to basalt (balstic).

II. The Mantle: we have barely penetrated the ground beneath our feet: the deepest drill holes are just 10km – 1/10th of 1% of the earth’s diameter! The solid layers of earth & increasing heat & pressure w/in have prevented direct observation of the its interior. By using what we know about the earth’s upper layer we can infer information ~ its interior. As eq waves travel toward the earth's center, there is a marked boundary between the rocks of the crust & a denser layer; this interface is called the Moho; the layer below the Moho is the earth’s mantle. The mantle, extending to a depth of ~ 2900km, includes most of the earth’s vol. Inferences ~ its compositions:

Scientist have calculated the ave density of earth to be 5.5x the density of H2O – about 2x the density of most rocks on earth’s surface.

Eq waves travel faster in the mantle than the crust. Composition of magma from deep w/in the earth includes a hi proportion of dense, mafic rock.All these observation & more suggest that the mantle is cmpsd of the dense, dark mafic minerals olivine & pyroxene.

III. The Core: the deepest layer & is known as the inner & outer cores & is thought to be a mix of Fe & Ni.The outer core is thought to be liquid b/c S waves can’t pass thru the outer core; the inner core seems to be solid: the higher pressure @ the center of the earth cause the inner portion of the core to form a solid. The solid state is inferred from the speed @ which P waves travel thru is.

IV. EQ Shadow Zone: EQ Shadow Zone: The shadow zone is the area of the earth from angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves. The shadow zone results from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.

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Earthquakes

Causes: an earthquake (eq) is any vibrating, shaking, or rapid motion of the earth’s crust; most occur when stress alonga zone of weakness or a break in the rock known as a fault. When the crust shifts, nrg is suddenly released, radiating in all directions in the form of vibrations. The site underground where the break occurs is the focus; the epicenter is the location on the earth’s surface above the focus. When the vibration reaches the surface we feel them as earthquakes, first @ the epicenter & then @ > distances

I. What Is an Earthquake?A. An earthquake is the vibration of Earth produced by the rapid release of energy B. Focus and Epicenter:

Focus is the point within Earth where the earthquake starts. Epicenter is the location on the surface directly above the focus.

C. Fault: are fractures in Earth where movement has occurred.

Focus, Epicenter, and Fault

Slippage along a Fault

II. Cause of Earthquakes A. Elastic Rebound Hypothesis

Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces.

When the strength of the rock is exceeded, it suddenly breaks, causing the vibrations of an earthquake.

B. Aftershocks and Foreshocks1. An aftershock is a small earthquake that follows the main earthquake. 2. A foreshock is a small earthquake that often precedes a major earthquake.C. Earthquake Waves 1. Seismographs are instruments that record earthquake waves.2. Seismograms are traces of amplified, electronically recorded ground motion made by seismographs.3. Surface waves are seismic waves that travel along Earth’s outer layer

Seismograph

Seismogram

D. Locating Earthquakes: Body Waves : the nrg of an eq radiates in wave away from the focus; the waves can be grouped into 3 categories.

1. P waves: stands for PRIMARY & PUSH-Pull which describes the ground motion as these waves pass. Are push-pull waves that push (compress) and pull (expand) in the direction that the waves travel Travel through solids, liquids, and gases Have the greatest velocity of all earthquake waves The waves are the motion of a spring which is alternatively pulled tighter & then released

2. S waves: Secondary or side-side. They’re similar to waves that travel when you’ve a rope on the ground & move 1 end quickly side-side.

Seismic waves that travel along Earth’s outer layer Shake particles at right angles to the direction that they travel Travel only through solids Waves travel perpendicular to the direction of the travel3. The 3rd category of seismic waves include BOTH P & S waves; these cause the MOST damge as they trave along the

Earth’s surface

A seismogram shows all three types of seismic waves—surface waves, P waves, and S waves

III. Locating an Earthquake A. Earthquake Distance: The epicenter is located using the difference in the arrival times between P and S wave

recordings, which are related to distance.B. Earthquake Direction: Travel-time graphs from three or more seismographs can be used to find the exact

location of an earthquake epicenter.C. Earthquake Zones: About 95 percent of the major earthquakes occur in a few narrow zones.

IV. The Epicenter: a seismograph records the magnitude & the time the seismic waves arrive. Seismologists use the differences in the speed of P & S waves to locate the epicenters of eq. *** the TRAVEL TIME GRAPH in the ESRT shows how long it takes the P&S waves to travel thru the earth; as waves travel outward from the focus, the time delay between the P&S waves increases.

A. Finding the Distance from an EQ epicenter: to calculate proceed as follows:1. Subtract the arrival time of the P wave from the arrival time of the S: use minutes & seconds.2. On the clean edge of a sheet of paper, make 2 marks to show that interval along the travel-time graph.3. Keeping the edge of the paper vertical, slide the marks on the edge of the paper along the P&S curve of travel

time scale until the marks coincide w/both curves4. Follow the marked edge of your paper down to the horizontal axis to find the distance to the epicenter.

This procedure hasn’t yet located the epicenter but has established the distance from the recording station to the epicenter; if a circle w/a radius = to the distance is drawn around the recording station, the epicenter should be located somewhere along that circle. To find the exact location, follow the above procedure to find the distance from the epicenter to 3 seismic recording stations; draw circles around each of these @ the proper stations. The epicenter is the pt. @ which the 3 circles intersect.

V. Origin of the Time of an EQ : a person knows they feel an eq, but may not know when it actually occurred. To find the origin time you need to know the time@ which the 1st recorded @ the station, i.e., the arrival of the 1st P wave & the amt of time it took the waves to get there (TRAVEL TIME); if you know the distance to the epicenter (from above), the travel time can be determined from graph in the ESRT.EG: if the epicenter is 400km away, the travel time for P-waves is 7min; thus, if the eq was recorded @ a station @ 2:25 pm (14:25:00) & the P wave took 7min t get there (travel time), then the eq must’ve actually happened 7 min earlier @ 2:18 pm (14:18:00)

VI. Measuring EarthquakesA. Historically, scientists have used two different types of measurements to describe the size of an earthquake

—intensity and magnitude.B. Intensity: scales such as the Mercali Scale are based upon the reports of people who’ve experienced the e-q &

observed the destruction.

Intensity scales do NOT measure nrg lvls, but Do indicated human impact; b/c of the uncertain nature of the earth’s material thru which the waves travel, it’s often hard t locate the epicenter. Mercalli reports help do this.

C. Richter Scale Based on the amplitude of the largest seismic wave Each unit of Richter magnitude equates to roughly a 32-fold energy increase The increase of 1 unit in the scale means a 10 fold increase in shaking: a mag 6 has 10x as much shaking as a

mag 5 & a 100x more than a mag 4. Does not estimate adequately the size of very large earthquakes

D. Momentum Magnitude Derived from the amount of displacement that occurs along the fault zone Moment magnitude is the most widely used measurement for earthquakes because it is the only

magnitude scale that estimates the energy released by earthquakes. Measures very large earthquakes

VII. Seismic Vibrations: The damage to buildings and other structures from earthquake waves depends on several factors. These factors include the intensity and duration of the vibrations, the nature of the material on which the structure is built, and the design of the structure.

A. Building Design: Factors that determine structural damage. Intensity of the earthquake Unreinforced stone or brick buildings are the most serious safety threats Nature of the material upon which the structure rests The design of the structure

B. Liquefaction Saturated material turns fluid Underground objects may float to surface

VIII. Tsunamis: A tsunami triggered by an earthquake occurs where a slab of the ocean floor is displaced vertically along a fault.

A tsunami also can occur when the vibration of a quake sets an underwater landslide into motion. Tsunami is the Japanese word for “seismic sea wave.”

A. Tsunami Warning System: Large earthquakes are reported to Hawaii from Pacific seismic stations Although tsunamis travel quickly, there is sufficient time to evacuate all but the area closest to the epicenter.

B. Other Dangers 1. Landslides: With many earthquakes, the greatest damage

to structures is from landslides and ground subsidence, or the sinking of the ground triggered by vibrations.2. Fire: In the San Francisco earthquake of 1906, most of the destruction was caused by fires thatIX. Predicting Earthquakes A. Short-Range Predictions: So far, methods for short-range predictions of earthquakes have not been successful.B. Long-Range Forecasts: Scientists don’t yet understand enough about how and where earthquakes will occur to

make accurate long-term predictions. A seismic gap is an area along a fault where there has not been any earthquake activity for a long period of

time.

Earthquakes & Volcanoes

EQ & volcanoes are not random over the earth – most occur in zones of crustal activity, where crustal deformation & mt. building take place. A large # of the world’s volcanoes & seismic events occur around the Pacific Ocean in a region called the Ring of Fire (See ESRT p5).

Japan & western US are on the ring & are damaged frequently by eq & volcanoes.

I. Seismic Hazards: eq cause damage in several ways: Shaking: of buildings & bridges causing landslides, damage, collapse, & death. Damage of roads & railroad tracks from ground upheaval. EQ in the ocean can cause huge wave Tsunamis that, once it hits land, causes major destruction & death. Fire: from broken gas & power lines.

In areas w/high risk of eq, govt have developed building codes for buildings & bridges to be able to w/stand moderate quakes. Fig below shows seismic risk areas of the US.

II Volcanic Hazards: may be from hot lava, hot ash, &/or toxic gases; the lava & ash can bury cities (name 2 ancient cities from Mt.Vesuvius) & the toxic fumes can suffocate people. However, volcanoes can also be helpful: some of the most fertile soil is cmpsd of weathered volcanic material. Also adds to land mass: Hawaiian Island.