Geo 105, Main points for Earthquakes for Jan. 29
Earthquakes & Earthquake Hazards
- What?
- Earthquakes are rapid slip across a fault
- This generates seismic waves
- Which propagate away from the slipping fault in all directions
- They cause ground shaking in the vicinity of the epicenter
- and, can be recorded by seismographs located far away
- How Big?
- Intensity: a crude measure of earthquake size based on
reports of shaking and/or damage in the epicentral area
For example, maximum intensity of the Sept 2, 1994 Michigan EQ
(magnitude of 3.5) was "V"
- Magnitude: the "best" way to measure earthquake size
Use amplitudes of seismic waves recorded by seismographs.
At the same distance from the epicenter, an increase in the wave
amplitude by a factor of ten translates into an increase in magnitude
by one unit, e.g. from "4" to "5".
- How Many?
- Every year, the various national & international agencies detect and
locate tens of thousands of earthquakes
- But nearly all of these events are small, too small to be felt
- "Gutenberg & Richter Law": For any given region and time interval,
the number of earthquakes increases by about a factor of ten as we decrease the
magnitude by one unit.
- For example, average annual global occurrence:
- Magnitude=8 & larger # is about: 1
- Magnitude=7 & larger # is about: 10
- Magnitude=6 & larger # is about: 100
- Magnitude=5 & larger # is about: 1000
- etc. etc. etc.
- This overall statisticcal "law" can be used to estimate the likelihood of
future earthquakes
- Where?
- Mostly at Plate Boundaries
- Furthermore, Mostly at Subduction Zones
- However, detailed view shows that there can be many active faults around
the main plate boundary fault
- For example, many earthquakes in California that are NOT on the San Andreas fault
- Even more puzzling are the odd places in the middle of plates that have earthquakes
- In the case of the New madrid zone in central USA, we know that a sequence
of great earthquakes occurred in the early 1800's
will a great earthquake happen there again? if so, when?
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