EARTHQUAKES AND THE EARTH'S INTERIOR

Seismology is the study of earthquakes--the photo to the left shows the result of an earthquake--there is a large seismic area in Missouri related to the New Madrid Fault system and a large earthquake is predicted in that region within the next 50-100 years

I. Definition, causes, and morphology of earthquakes

A. Definition of earthquake

vibration of the Earth's surface by sound (shock) waves usually generated by the energy released from rocks rupturing under stress or by friction between moving rock materials at or mostly below Earth's surface
click here for more on definition of earthquake

B. Some major causes of earthquakes

1. Landslides, rockslides, slumping or "caving in" of the Earth's surface, meteor impacts and nuclear explosions

2. Friction (grinding, bumping) between moving rock materials associated with volcanism

3. Fracturing or movement of rock materials along faults or plate boundaries caused by Elastic Rebound are the most disastrous earthquakes form--this can dislodge enormous volumes of rock and unconsolidated materials causing mass wasting to occur such as rockslides and slumping of surface material---liquefaction causes rigid soils to act as a liquid during an earthquake—see pages 348-349 in text

C. Morphology of earthquakes and related terms

1. Focus or hypocenter

· is the exact place of origin of the earthquake disturbance and with few exceptions is located below the surface of the Earth

2. Epicenter

· is the position on the surface of the Earth directly above the focus

click here to see the focus and epicenter -- or see page 337 in text for more examples of focus and epicenter

3. Foreshock(s)

· is (are) the earthquake(s) generated at or very near the focus of the main earthquake disturbance and prior to the main shock--the foreshock(s) is (are) smaller in magnitude than the main shock but may be of substantial magnitude and precede the main earthquake by a short or substantial time interval

4. Aftershock(s)

· is (are) the earthquake(s) generated at or very near the focus of the main earthquake disturbance and following the main shock---the aftershock(s) is (are) smaller in magnitude than the main shock but may be of substantial magnitude and follow the main earthquake by a short or substantial time interval

5. Main shock

· is the main earthquake disturbance generated at the focus

II. Earthquake waves

A. Types of waves

1. Body waves

· waves which travel below the Earth's surface

· P (primary) wave--moves essentially in a straight line path

· S (secondary) wave--moves in a looping motion along its path of movement

click here to see the manner of travel of P and S waves (P wave=spring S wave=rope) or see page 341 in text

2. Surface (L, Love, Long) wave---moves along the surface of the Earth

B. Arrival of earthquake waves at seismic stations

seismic waves move away in all directions (360 degrees-through 3 dimensions) from the focus---the P and S waves which move upwards will reach recording or seismic stations and the earthquake recording devices---the L waves are also recorded

1. Seismograph--an instrument which detects and records seismic waves---as seismic waves shake the foundation on which this instrument is attached, a pen records the waves on a paper attached to a rotating drum

· click here to see the basic parts of a seismograph or page 340 in text

2. Seismogram--the paper on the seismograph with the earthquake wave recordings

· click here to see the seismogram and the recorded seismic waves or see page 342 in text

see more on seismographs and seismograms

C. Tsunami (seismic sea wave)

is a sea wave formed when an epicenter is located near or in the ocean basin
click here to see the generation of a tsunami or see page 350-351 in text
click here to see more on a tsunami

III. Locating the epicenter

A. Travel time (time distance) graph

· using the differences in arrival times of the P and S waves at a seismic station (obtained from information on a seismogram) and a graph which plots the distance body waves travel to the seismic station as a function of time, the distance of an earthquake epicenter from a seismic station can be determined

· click here to see the time travel graph or page 342 in text--as can be seen on the travel time graph, the distance away from the epicenter to the specific seismic station in this example is 3800 kilometers--but the direction from the station is still not known-- click on the first diagram in upper left and see the ground distance to the epicenter using the arrival of P and S waves on the seismogram.

· B. Three point or three arc method

usually the distance from three seismic stations are used to locate the epicenter by triangulation---an arc representing the distance from a seismic station through 360 degrees is constructed on a map---this is done for three seismic stations---the intersection of the three arcs is the location of the earthquake epicenter---the closer the three stations are to the epicenter the more exact the location—the link below, shows seismic stations in Sydney, Australia, San Francisco and Tokyo used to triangulate the location of an epicenter in the Pacific Ocean
click here to see and use of the 3 arc method--- also see page 343 for 3 other cities

IV. Seismic intensity and magnitude

A. Definitions

1. earthquake intensity is a measure of the effects or physical destruction caused by an earthquake disturbance at a particular surface
location

2. earthquake magnitude is a measure of the strength of or energy released by an earthquake disturbance

B. Factors influencing the destructive results (intensity) of an earthquake at a particular surface location

1. total energy released at focus (magnitude)
2. distance of focus to surface location
3. type of rock support in an area--more damage, higher intensity, will occur at surface if support material is a thick soil (greater settling effects) and less damage if solid bedrock
4. building construction
5. density population

C. Mercalli intensity scale (modified)

is a listing of increasing physical destructive results numbered from I to XII
click here to see the Mercalli Scale or see page 344 in text

D. Earthquake magnitude

1. Richter Scale

· describes the strength of the earthquake and is obtained by relating the difference in time arrivals of the P and S waves and wave height or amplitude at a given seismic station

· click here to see the determination of the Richter Number or see page 345 in text

· after plotting the lag time value of S-P waves and the amplitude value on the respective column plots, a line connecting these two points intersects the center plot at the Richter value--the lag time value would increase and the amplitude value decrease with a function of distance of the station from the epicenter resulting in essentially the same Richter number for all seismic stations--- slight differences in the Richter number for an earthquake may result from one seismic station to another because of extreme distances from the epicenter

2. Magnitudes of energy

· the energy released by the first atomic bombs were equivalent to the energy released by an earthquake with a Richter value of 5

· each increase (or decrease) in Richter number by one equates to a multiple of 32 times more (or less) energy released--a magnitude of 7 is equivalent to 32x32(1024) x the energy released by an earthquake with a 5 magnitude

· usually a magnitude of 4 or less is a minor earthquake while some of the most damaging quakes measure 8.5-9

V. Earthquake categories

based on depth to focus

A. Shallow type

if focus is 0 to 42 miles deep--comprises about 85% of total global energy released by earthquakes

B. Intermediate type

if focus is 42 to 210 miles deep--comprises about 12% of total global energy released by earthquakes

C. Deep type

if focus is 210 miles or greater--comprises about 3% of total global energy released by earthquakes--deepest known focus is 420 miles

VI. Geographical distribution of earthquake epicenters—see page 347 in text

there are nearly 1 million global earthquakes per year with major tremors occurring much less frequent than minor tremors
click here to see the detailed breakdown on magnitudes of earthquakes
click here to see a listing of historical earthquakes or page 354 in text

A. Circum-Pacific region

is an area of many plate boundaries especially of the convergent type where about 80% of global earthquake energy is generated each year--within this region the California-Nevada area averages about 5000 earth tremors per year and accounts for about 90% of the total earthquake energy generated in the US

B. Mediterranean region

a large convergent plate boundary and smaller ones are present across this region and explains the generation of about 15% of global seismic energy each year

C. Ocean ridge areas and intra-continental areas

these two areas constitute other areas of major earthquake activity---the intra-continental areas are not associated with known plate boundaries
click here to see the intracontinental earthquake locations and risk areas in the US
probably the most cited example of an intra-continental earthquake is the New Madrid events (three tremors) of late 1811 and early 1812---these tremors generated along the New Madrid Fault region caused the Mississippi River to flow backwards, steeple bells to ring in Boston, and caused a large lake to form in Tennessee (Real Foot)---a sparse population in that region accounted for a lack of serious damage---the Richter scale had not yet been established but some think the tremors associated with one or more of the main shocks would have registered 8.5 to 9---it is projected that there is a 60% chance for a damaging earthquake within the next 15 years and a 95% chance in the next 50 years for this region

VII. Earthquake prediction, control, and safety rules

A. Earthquake precursors

are omens or events which may be observed prior to an earthquake and can aid in predicting the event

1. Dilatancy

· is the swelling or bulging of the surface rock above an area experiencing a buildup of stress in the rocks--the best example of this phenomenon prior to an earthquake was in the Palmdale California area in the 1970's

· tiltmeters and laser mirrors are used to measure the rate and magnitude of the dilatancy--the use of tiltmeters and laser mirrors to measure Earth surface bulging was treated previously in the discussion of prediction of volcanoes

2. Changing habits of insects or animals prior to an earthquake event

· for some reason some creatures have an innate intuition about an earthquake soon to happen and react to it accordingly--one of the best examples is snakes awaking from hibernation prior to a large earthquake in China

3. Strange glow emitted from rocks prior to earthquake event

· stresses in rock materials may cause a flow of electrons in quartz (piezoelectricity)--some believe the electricity effect can ionize gases causing a glow at the surface

4. Increase of radon gas concentrations in the environment

· prior to some earthquakes higher concentrations of radon gas were detected in stream waters in the area--also prior to some earthquakes higher concentrations of radon at the Earth's surface is believed to have caused interferences in radio signals--stress in the rocks prior to an earthquake could cause the release of more than normal concentrations of radon gas into the environment

B. Earthquake control

some have speculated that lubricating rocks under stress would result in the release of smaller bundles of energy (more but less energetic earthquakes) thereby avoiding the "BIG ONE"-- some have suggested water or other fluids could be added to the suspect area of stress buildup causing a lubrication effect in the stressed area of the rocks

C. Earthquake safety rules

click here to see figure 2 and earthquake safety rules

VIII. Earth's interior as deduced from seismic waves

structure and composition of the interior of the earth has been determined from information obtained from studies of body waves and meteorite composition

A. Body wave paths and velocities in the Earth's interior

1. Homogeneous vs non-homogeneous interior Earth concepts

· the paths of body waves moving through the Earth's interior will be straight if the interior is homogeneous or broken or curved if not homogeneous

· click here to see the idealized path of body waves –however, see in the diagram a curved path if inhomogeneous

2. Specific velocities and paths of body waves

· body waves are not straight therefore the Earth is not homogeneous--the velocities of P and S waves increase with a function of depth changing velocities at the boundary of each minor and major interior Earth section---at the lower mantle and outer core boundary the P wave velocity decreases significantly and the S wave ceases to exist which indicates the presence of a liquid core

· click here to see the specific way body waves travel through the Earth

B. Shadow zones of P and S waves

are areas on the Earth's surface which do not receive P and/or S waves from earthquakes and can be explained by the Earth's interior structure--note the P wave shadow zone lies between 105-140 degrees from the epicenter on both sides of the Earth while that for the S wave includes an area between 105 degrees on both sides from the epicenter
click here to see which shows P and S shadow zones respectively
from the results on velocities, paths, and shadow zones or the manner in which the P and S waves travel within the Earth's interior important information about the Earth's structure can be obtained