Use the following website to observe the most recent seismicity, http://ds.iris.edu/seismon/
Click on the 30 days tab. A table will appear. Use that table and the map to answer the following questions. You can sort the data by simply click on the arrows on the header line of the table
- Distribution and magnitude of recent earthquakes
2. - Where did the 5 largest earthquakes by magnitude occur during the past month? Are they related to plate boundaries? Which type? You can find that out by comparing the map of each earthquake by clicking on the LOCATION tab. A map of the area will appear with the distribution of the strongest earthquakes located in the area. Make sure that on the right side menu the tab “show plate boundary” is on.
Fill the table below, ranking the earthquakes from the strongest to the least strong.
LOCATION Date Magnitude Depth (km) Tectonic setting/ Type of plate boundary Rank
1
2
3
4
5
- To have an appreciation of the energy associated with the strongest earthquake in this set, go to: http://earthalabama.com/energy.htm Enter the magnitude and then click on the tab “compute”.
What is the energy in Joules traveling by seismic waves? __
Look at the table “calculated seismic energy”. If earthquake energy could be harnessed, for how long could that earthquake power the US? For how many years could that power the average USA household? ___
Part 3: Earthquake Intensity and Secondary Earthquake Hazards case studies
Introduction
The main hazard related to an earthquake is the ground shaking, however, earthquakes poses several other hazards that can have longer lasting and more devastating effects on the natural environment and human interests. These are called secondary seismic hazards:
• Landslides and soil liquefaction
• Damage to structures and infrastructures; buildings can collapse, trapping people inside and burying streets in rubble. Damaged roads, rails, bridges can disrupt or cut off entire communities and hinder rescue efforts
• Fires and loss of lifelines from damage to utilities lines
• Tsunami, which present the greatest hazard at coastline communities.
• Permanent ground deformation
• Loss of life and societal disruption. They can severely hinder economy and societal frameworks.
The size of these secondary hazards depends on the earthquake strength and duration, on the local geological conditions and on the type of structures and society preparedness.
In the following exercise, you will evaluate the differences between measurements of intensity and magnitude and their relationship to secondary seismic hazards for case studies of historical earthquakes.
Read the material for each case study and answer the questions. For an in depth study click on the links to access the main source of info. Based on the description, estimate the intensity of the earthquake main shake using the Modified Mercalli Intensity scale on next page.
Mercalli Modified Intensity Scale
CASE STUDY 1: December 16, 1811: New Madrid Fault estimated Richter Magnitude: 8.0
Historical documents from eyewitness report that at the onset of the earthquake the ground rose and fell—bending the trees until their branches intertwined and opening deep cracks in the ground. Landslides swept down the steeper bluffs and hillsides; large areas of land were uplifted; and still larger areas sank and were covered with water that emerged through fissures. Huge waves on the Mississippi River overwhelmed many boats and washed others high on the shore. High banks caved and collapsed into the river; sand bars, and points of islands gave way; whole islands disappeared. Local uplifts of the ground and water waves moving upstream gave the illusion that the river was flowing upstream. Ponds of water also were agitated noticeably. Surface rupturing did not occur, however. The region most seriously affected was characterized by raised or sunken lands, fissures, sinks, sand blows, and large landslides that covered an area of 78,000–129,000 square kilometers, extending from Cairo, Illinois, to Memphis, Tennessee, and from Crowley’s Ridge to Chickasaw Bluffs, Tennessee.
Although the motion during the first shock was violent at New Madrid, Missouri, it was not as heavy and destructive as that caused by two aftershocks about 6 hours later. Only one life was lost in falling buildings at New Madrid, but chimneys toppled and log cabins were thrown down as far distant as Cincinnati, Ohio; St. Louis, Missouri; and in many places in Kentucky, Missouri, and Tennessee. In Lake County uplift the Mississippi River valley was upwarped in several topography bulges. Other areas subsided by as much as 5 m, although 1.5–2.5 m was more common in Arkansas.
https://www.usgs.gov/natural-hazards/earthquake-hazards/science/1811-1812-new-madrid-missouri-earthquakes?qt-science_center_objects=0#qt-science_center_objects
- By reading the text above and comparing the testimonials with the degree of the Mercalli Scale you will be able to estimate the intensity. Use the highest level of damage for your estimate. What was the intensity of the New Madrid Earthquake? Provide the answer and list the evidence you based your answer on.
- Which of the items you described in your answer is related to a secondary hazard?
CASE STUDY 2: April 18, 1906; San Francisco. Estimated Richter Magnitude: 7.8
The earthquake damaged buildings and structures in all parts of the city and county of San Francisco, although over much of the area, the damage was moderate in amount and character. Most chimneys toppled or were badly broken. The business district was built on ground filled in over Yerba Buena cove. Pavements were buckled, arched, and fissured; brick and frame houses were damaged extensively or destroyed; sewers and water mains were broken; and streetcar tracks were bent into wavelike forms.
On or near the San Andreas fault, buildings were destroyed and trees were knocked to the ground. The surface of the ground was torn and heaved into furrow-like ridges. Roads crossing the fault line were impassable, and pipelines were broken, shutting off the water supply to the city. The fires that ignited soon after the onset of the earthquake quickly raged through the city because of the lack of water to control them. They destroyed a large part of San Fran- cisco.
Dislocation of fences and roads indicated the amount of ground movement between 3–4.5 m. In Mendocino County, a fence and a row of trees were displaced almost 5 m. Vertical dis- placement of as much as 0.9 m was observed in Sonoma County. Vertical displacement was not detected toward the south end of the fault. http://www.sfmuseum.org/1906/ew.html
- What was the intensity of the 1906 San Francisco earthquake? List the evidence you based your answer on.
- Which of the items you described in your answer above, is related to a secondary hazard?
- What are the strengths and weaknesses of the Modified Mercalli Scale when studying historical earthquakes?
- Examine the map of Seismic Hazard for the two zones expressed as percentage of the ground acceleration. Look at the map at https://pubs.usgs.gov/sim/3325/pdf/SIM3325_sheet1.pdf
Is the hazard comparable? Why is the hazard area so different in size?
Part 4: BUILDING DESIGN AND GROUND RESONANCE
Resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others. The resonant frequency of any given system is the frequency at which the maximum-amplitude oscillation occurs. All buildings have a natural resonance, which is the number of seconds it takes for the building to naturally vibrate back and forth. The taller the building, the longer its resonance period will be. In this section of the exercise you will learn about building resonance and about engineering solutions to minimize the damage from oscillations. Watch the videos and answer these questions
Ground and building resonance frequencies
https://www.iris.edu/hq/inclass/animation/building_resonance_the_resonant_frequency_of_different_seismic_waves
- Which type of ground has the higher frequency, hard rocks or softer sediment? __
If the period of ground motion matches the natural resonance of a building what will happen?
Design of safer buildings
http://www.iris.edu/hq/inclass/video/build_a_better_wall_demo_why_buildings_fail.
- After watching the video complete the drawing of this frame for a 4 story building with structures that will make it more resistant during an earthquake.