Geology 101 - Lecture 21

Earthquakes (continued)

I. Types of Faults that earthquakes occur on

Strike-slip fault

Right lateral versus left lateral

Relationship of stress direction and fault type

II. How to identify a fault

Abrupt termination of rock layers that were once laterally continuous
Rocks along fault are crusthed, polished, or striated
Crushed rocks are easier to erode, so linear gullies or valleys may form
Groundwater moves more easily along crushed rock, so springs, seeps, or vegetation may show up
Offset features such as streams, roads
Fault scarp

III. Earthquake Magnitude

Magnitude = Strength of an earthquake
Often measured using the height (or amplitude) of wiggles on the seismogram. Correct for distance from epicenter and rock type.
Measured using a logarithmic scale, so a magnitude 6 quake is 10 times stronger than a magnitude 5 quake
Richter scale = is a measure of earthquake magnitude, but only in southern California! Not used anywhere else in the world.
Moment magnitude
Proportional to the area of fault plane that ruptures times the amount of offset on the fault.
- How do you determine the area of fault plane?
  
  Use distribution of aftershocks.
  Example: Area of fault plane that ruptured during Loma Prieta earthquake can be estimated from the aftershock zone
- So this is why the quake that supposedly shook the Pacific Northwest in 1700 AD was so devastating. The area of the fault that broke was from Vancouver Island in Canada to Cape Mendocino in California. That will create a large magnitude quake!
- Also, this explains why ultimately, subduction zone earthquakes can potentially be much larger than transform margin earthquakes. Transform margin earthquakes occur on a vertical plane. Earthquakes do not occur at great depths, as the temperature gets too high around 15 km or so. So the area of the fault that breaks is constrained by a depth of approximately 15 km.
  
  In contrast, subduction zones have earthquakes occurring on inclined planes. Thus, to reach a depth of 15 km, the length of the inclined fault plane can be much greater than 15 km depending on the angle of inclination. Hence subduction zone earthquakes have the potential of having much higher magnitudes than transform margin earthquakes

III. Modified Mercalli Intensity Scale

Magnitude isn't particularly important to government officials, relief organizations, and other non-scientists, so we also use intensity scales that rank earthquakes by the severity of the damage that they cause

Based on personal reports from witnesses and post-earthquake building inspections.
Ascending order of I through XII.
Greater than Intensity IX indicates ground failure (landsliding and liquefaction)
Generally, area 20 to 50 km from epicenter experiences same amount of ground shaking, but often the destruction varies considerably within this area. Why? (soft sediments versus bedrock, etc.)
Mexico City in 1985: Mexico City is located 400 km from epicenter, but had very serios damage. Mexico city is built on an old lake bed. Soft sediments amplified waves. Reached a natural frequency and buildings collapsed.