Welcome to GLG101E Introduction to Geology Fall 2004 Professor James Tyburczy
Department of Geological Sciences
	Chapter 19 earthquakes

Chapter 19 EARTHQUAKES

See these web sites for more information:

National Earthquake Information Center

Seismology, the study of earthquakes and their waves includes

• 1) earthquake prediction, mechanisms of earthquakes, and earthquake hazard, and
• 2) study of the interior of the earth by examining how seismic waves pass through the earth

Earthquake - an earthquake is a sudden release of seismic energy in the earth's interior. It sends out seismic waves in all directions.

• Epicenter - the point on the earth's surface directly above the earthquake focus
• Focus - the actual point in 3-dimensional space at which the earthquake originates

Elastic rebound theory - Plate tectonic motions continually deform and stress the earth. The plates are continually moving & fault is stuck. The crust starts deforming (stores elastic energy). Then the fault breaks and releases that energy as an earthquake.

Seismic waves

• Body waves
  • P-wave - primary, push-pull (compressional) - fastest, arrive first
  • S-wave - secondary, shear - arrive second, don't pass through liquids
  • Surface waves - L-waves -long, last - slowest, arrive last
• Seismograph - a scientific instrument that measures the ground motion caused by seismic waves
• Seismogram - a record (used to be paper, now on computer) of the passage of a seismic wave at a particular location
• Travel-time curve - a plot of time of arrival of different seismic waves versus distance from the epicenter of the earthquake - used to determine location of earthquakes and earth structure. Determining the epicenter location requires at least 3 seismograms from 3 different stations.

Earthquake Magnitude and Intensity - ways of estimating the size of an earthquake - how much ground shaking or how much energy released

• Richter scale - based on ground motion but number is a measure of the strength of the earthquake at its source. The scale is logarithmic - a 1-unit change in Richter magnitude indicates a factor of 10 increase in ground shaking (which also corresponds to a factor of about 30 change in energy release).
• Newer methods - Moment Magnitude. Based on size of fault plane and amount of slip. Is also a logarithmic scale.
• Mercalli intensity scale - qualitative measure ground shaking at a particular location; used to determine epicenters of historic earthquakes (those that occurred before seismometer networks)

How many earthquakes are there? Globally, a magnitude 8 earthquake occurs every few years, there are about 10-20 magnitude 7 earthquakes every year, several hundred magnitude 6 earthquakes every year, many more of smaller earthquakes.

Where do earthquakes occur?

• 95 % occur on plate boundaries
  • Divergent boundaries - mid-ocean ridges, shallow earthquakes (down to about 20 km ), normal faulting, tensional forces
  • Convergent boundaries - shallow, medium, and deep earthquakes (depths to about 700 km). Compressional forces, reverse and thrust faulting. Locations (foci) of subduction zone earthquakes outline the subducting plate (Benioff zone)
  • Transform boundaries - shallow earthquakes, shear forces, strike slip faulting
• 5 % occur at places not on plate boundaries
  • Hotspots (or intra plate volcanoes), such as Hawaii. Earthquakes associated with magma ascent and volcano formation.
  • Other places - ancient plate boundaries?
    • Example of New Madrid, Missouri: In 1811 and 1812 two very destructive (magnitude 8 or so) earthquakes hit the middle of the North American, very far from any plate boundary. The area continues to be seismically active today

     

  • Destructive effects of earthquakes

  • • Primary effects - Ground motion - intensity and duration of ground shaking and fault rupture - falling buildings
      • Soil type - bedrock versus land fill
      • Building design - building codes, reinforced/unreinforced

      Secondary effects

    • Tsunamis
    • Fire
    • Liquifaction, avalanches & landslide

Earthquake Hazard Maps - maps showing estimated probability of high ground shaking. Areas with the highest probability include western U.S. (plate boundary), Central U.S. near St. Louis (because of 1811, 1812 earthquakes), South Carolina (magnitude 7+ earthquake there in 1866).

Earthquake Hazard and Earthquake Risk

• Seismic hazard - probability of ground shaking exceeding some level (like 1 g) within some time period (like 50 years) at each location
• Seismic Risk - combination of Seismic Hazard with Population Expopsure and Infrastructure Vulnerability. Seismic hazard in Alaska and California is about the same, but Seismic Risk is Alaska is much less because of very low population
• Can't do anything about seismic hazard (except understand it and be aware of it)
• Can do something about Seismic Risk
  • Understand hazard
  • Land use regulations - don't build in dangerous areas
  • Building Codes
  • Emergency reponse
  • Real Time earthquake warnings (?)

Earthquake prediction and hazard

• A good prediction must specify location, magnitude, and time of the impending earthquake (no predictions yet reliably do all three).
• A prediction method must also be able to be understood and used by people other than just the originator (transferability).
• Approaches attempted have been
  • Microearthquakes (precursors or foreshocks)
  • Changes in elevation/tilt ( ground deformation)
  • Changing rock properties (seismic velocity, electrical resistivity, porosity,…) (all likely affected by opening/closing of small cracks in rock around faults)
• Water in wells - water levels, radon gas in wells
• Animal behavior - fails the 'transferability' test

Long Term Forecasts

• Paleoseismology - prehistorical record from earthquake trenching enabling estimating the recurrence interval over a longer time period - used along the San Andreas Fault
• Seismic gap theory ( short segments of active faults may show a time period with a lack of activity suggesting a higher likelihood of an earthquake occurring there)

Intermediate Term Forcasts - Monitoring building of stress on faults might enable prediction on time scale of months (not yet proven)

Very short term earthquake warning - put seismographs very near known faults. Sensors could give 30 seconds or so of warning to major cities.

Earthquake Hazard in Arizona. There are 3 general areas in Arizona where earthquake hazard is modest (but in nearly all of Arizona the hazard is much less than in California)

• Earthquakes along San Andreas Fault in Southern California. Southwest corner of the state and city of Yuma have hazard from these earthquakes
• Northwest corner - from Flagstaff across Grand Canyon to Utah. Many small earthquakes, a few as big as magnitude 6. Caused by tectonic uplift?
• Southeast corner - a few small earthquakes, but in 1887 a magnitude 7 earthquake occurred just across the border in Mexico. It is not a plate boundary, the cause is not well understood.