Quiz #5
What are the 5 settings that are earthquake-prone? In which setting are magnitude 9 earthquakes possible? In which three of these settings are maximum earthquake magnitudes around magnitude 8? Which of these settings has earthquake foci deeper than about 60 km?
1. Trench-arc -> shallow to deep -> biggest ones around 9-9+(the biggest earthquakes) 2. Large strike-slip fault -> shallow -> biggest ones around 8 3. Mid-ocean ridges -> shallow -> biggest ones around 7.5 4. Continental mountains -> biggest ones around 8 5. Intraplate -> don't occur very often -> biggest ones around 8 Foci is deeper than about 60km in mid-ocean ridges
What is a seismograph and how does it work?
A seismograph is an instrument for measuring seismic waves. Can measure displacements as small as one hundred millionth of a centimeter. A seismograph is securely mounted to the surface of the Earth and has a hanging ball that has a pen attached to it. It marks the waves down on a rolling piece of paper.
What is a seismograph recording or seismogram? Why is there a delay between the arrival of a P-wave and the arrival of an S-wave from the same earthquake? How is this delay used to determine the distance between an earthquake epicenter and a seismograph station? Seismograph recordings from how many stations are required to determine the distance between a particular station and the focus?
A seismograph recording (or seismogram) is a recording of Earth movement from a seismogram. There is a delay between the arrival of a P-waves and the arrival of an S-wave because they take different patterns when they travel and they travel through the earth in different manners. The distance between the arrival of the first P-wave and the arrival of the first S-wave tells you the distance the seismograph is away from the earthquake. Only 1 seismograph is needed to determine the distance between a station and the focus.
What is a tsunami? What is the approximate amplitude and wavelength of a tsunami as it traverses the open ocean? What changes when it approaches land? How fast does a tsunami travel? Is this faster or slower than each of the 3 types of seismic waves?
A tsunami is the uplift of the seafloor during thrust faulting produces a surge of water that moves outward as a long sea wave, or tsunamiis. They are only a few centimeters high in the deep ocean but can increase to many meters high close to shore. The approximate wavelength of a tsunami starts as hundreds of kilometers and then decreases to tens of kilometers. A tsunami travels 500 m/h. This is ____ than P-waves, _______ than S-waves, and _____ than surface waves
What is an earthquake? Do earthquakes occur on faults that are generally locked or unlocked? What is the "earthquake cycle"? What happens during an earthquake?
An earthquake occurs when there is a release of energy as a result of movements within the earth's crust. Earthquakes occur on faults that are locked. When a fault is unlocked, there will be no large earthquakes there. The earthquake cycle = most time bending slowly, rapid breaking (earthquake, seconds to 2 minutes), repeat. They are long cycles (hundreds to several hundreds of years) with large earthquakes. During an earthquake, rock underground suddenly breaks along a fault. This release of energy releases seismic waves that make the ground shake.
How is the direction of first movement recorded on a number of seismograph recordings used to determine the sense of first movement on a fault and the orientation of the fault? Can you determine both of these uniquely from seismograms or can you only narrow them down to a number of possibilities? If the latter is correct, then how many possibilities? How many stations do you need to narrow down the sense of first movement as well as you can (1, 2, 3, many more than 3)?
If the first movement recorded is down, then the fault moved towards the epicenter. However, if the first movement is up, then it is being pushed away from the epicenter. You can determine both of these uniquely from seismograms (MAYBE??)
What is meant by the term "normal modes"? Describe the type of motion that characterizes a particular normal mode. Roughly how much deformation takes place when the earth vibrates in a normal mode? Roughly how fast does the earth vibrate in a normal mode?
Normal modes = when an earthquake occurs, the earth vibrates as a whole. There are 3 types 1. Balloon mode - gets bigger, smaller, bigger 2. Football mode - circular, oblong, circular 3. Twisting - twists back and forth The vibrations change the earth's shape by about a millimeter after a large earthquake. Depending on the mode, 1 vibration takes about 30 minutes
Which of the earthquake-prone settings is represented by the San Andreas? the West Coast of Washington/Oregon? the Sumatra area (which produced the Great Sumatran Earthquake of 2004)? northeastern Japan (which produced the Great Tohoku Earthquake of 2011)? Are the series of New Madrid earthquakes from the 1810's represented by any of these settings? Is there significant seismic risk in the Missouri/Arkansas area? Is there any seismic risk in the northeastern part of the U.S.?
San Andreas - Strike-slip The West Coast of Washington/Oregon - trench-arc The Sumatra area - strike slip Northeastern Japan - trench-arc Are the series of New Madrid earthquakes represented by any of these settings? Continental mountains Is there significant seismic risk in the Missouri/Arkansas area? YES! Is there any seismic risk in the northeastern part of the US? Yes
What is a seismic wave? What are the three main types of seismic waves and how does each type cause the rocks they are traveling through to vibrate? What is the approximate velocity of a seismic wave? Approximately how long does it take a seismic wave to get to the other side of the earth? What are the relative velocities with which the different types of waves travel? Which of the waves travel through liquid and which do not?
Seismic waves typically have a velocity of 6-8km/sec Three main types: - P-waves: travels through liquids, solids, and gases. Travel as a series of contractions and expansions pushing and pulling particles in the direction of their path of travel. velocity = 5-8 km/s they take around 9 minutes - S-waves: Travel through solids and gases (NOT LIQUIDS). Travel as shear waves that push material at right angles to their path of travel. velocity = 1-8 km/s they take between 10-30 minutes - Surface waves: Travel across the surface of the Earth. velocity = 4-8 km/s they take between 30-50 minutes
How are seismograms used to determine the location of the focus of an earthquake? What is the minimum number of stations needed to do this?
Seismograms are used to determine the location of an earthquake by looking at where the circles of the disatnces from the stations overlap. 3 stations are needed to determine the location of the focus of an earthquake.
What is the Richter magnitude of an earthquake and how is it determined from a seismograph recording? Seismograph recordings from how many stations are required to determine the Richter magnitude of an earthquake? For an increase in magnitude of one unit, what is the increase in the amount of ground motion? In the amount of energy released? Would you consider a magnitude 4 earthquake a large earthquake? A magnitude 8? What is the largest magnitude earthquake? How many earthquakes of magnitude 8.6 and higher have there been anywhere on earth since 1950? How does moment magnitude differ from Richter magnitude?
The Richter magnitude of an earthquake is the measure of the amount of ground motion generated by an earthquake. It is determined by the amount of ground motion measures with a seismograph at a particular site, with a correction for the distance of the seismograph from the epicenter. Only 1 station is required. For an increase in magnitude of one unit, the ground motion increases by a factor of 10. A mag. 4 earthquake would not be considered large, but a mag. 8 one would be. There have been 4 earthquakes of mag. 8.6. They have been Tohoku (2011), Sumantra (2004), Alaska (1964), and Chile (1960). Moment magnitude differs from Richter magnitude because its the area of rupture on fault X the amount of slip
What are the terms epicenter and focus?
The epicenter is is the point on the earth's surface directly above the focus. The focus is the origin of the earthquake.
Which portions of the San Andreas Fault would be considered the most dangerous with regard to potential to produce a devastating earthquake in the coming few decades? - the portion near Los Angeles? - the portion near San Francisco? - the portion midway between Los Angeles and San Francisco? Why is seismic risk considered high in the Pacific Northwest even though no large earthquakes have occurred there historically? On average, what is the length of time between large earthquakes on the portion of the San Andreas near Los Angeles? How long has it been since the last large earthquake along that portion of the San Andreas?
The portion that would be most devastating is the portion near San Francisco?? The seismic risk is considered high because On average, the earthquakes are 125 years apart?? The seismic risk in the Pacific Northwest is considered high because there are many faults that can produce damaging earthquakes. The oceanic plate is converging underneath the North American plate, and this causes stress to build up. It has been 113 years since the last large earthquake.
How do we know that the outer core is liquid? How do we know that the outer core is a material in which P-waves travel relatively slowly? Why are there seismic shadow zones for P-waves? S-waves? What is refraction? Why do seismic waves travel in curved paths through large portions of the earth? Why do P-waves bend when they cross the core-mantle boundary? What is reflection?
We know that the outer core is liquid because S-waves do not travel though it. This leaves a shadow zone. We know that the P-waves travel relatively slowly because they bend back in and then refract. This leaves a P-waves shadow zone. Refraction is the bending of waves as they enter a material in which they travel faster or slower. - If waves pass into a material in which they travel slower, they bend so that they travel in a direction that is closer to being perpendicular to the interface between the materials. - However, if they pass into a material in which they travel faster, they bend so that they travel in a direction that is less perpendicular to the interface between the materials. Reflection is the bouncing of waves off interfaces (ex. The bouncing or reflection of might waves off a mirror). In the earth, seismic waves reflect off of boundaries characterized by abrupt changes in density.