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Earthquakes

Presented by Lind Gee
Berkeley Seismological Laborartory

Quantifying earthquakes
What are we measuring and why?

S(t) = F (earthquake source, Earth, instrument)

• Intensity - First intensity scale devised in 1783 by Domenico Pignataro - 5 levels (slight, moderate, strong, very strong, violent)
• More detailed scale devised by PNC Egen in 1823 - 6 levels, but with more detailed descriptions
• 1846 - use of intesity data to define areas of equal intensity - isoseismals
• Intensity maps became a way to locate an earthquake - and measure the relative size
• Modified Mercalli Scale, first developed in Italy by Giuseppe Mercalli near the end of the 19th century

I = F(size, type of faulting, distance (including depth), geology, topography)
Example from 1906, 1989
Roman, rather than arabic numbers to avoid confusion with magnitude
Comparing past events with present
Modern computer age

But .... not objective

• Location The classic "triangulation" method using the travel time differences between S and P waves
  Absolute travel time are used as well, using a model of the Earth to predict arrival times
• Depth
  Depth phases help us determine the source depth of an earthquake
• Magnitude
  Charles Richter studied the variation of amplitudes with distance - and started the magnitude craze:
• Local magnitude, which was soon followed by
  Surface-wave magnitude
  Body-wave magnitude

These empirical measures of earthquake size tend to saturate as earthquake size increases.

• Moment - An effort of characterize the size of all earthquakes, based on a representation of the earthquake source process. Moment can be simply related to observable parameters
  Mo = u A d, where u is the shear rigidity, A is the area of fault ruptured (which may be estimated from the distribution of aftershocks), and d is the amount of slip during the earthquake.
• Mechanism
  Description of earthquake faulting
• Earth monitoring
  Seismic networks
  Large scale - global
  Regional scale - Northern California
  Small scale - Parkfield, Hayward fault
• GPS networks
  western US velocity field
  uplift in Mammoth Lakes
• Other types of instrumentation
  creep, water level, strain, ....
  SAFOD project
  • A drill hole into the fault!

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updated January 28, 2002

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