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Geology of Australia and New Zealand, HWS/UC
2007
9. NZ Seismicity and Seismic
Hazards
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What is an earthquake?
ELASTIC REBOUND THEORY
Ground vibrations, “seismic waves,” produced by the sudden release of stored strain energy as crust breaks or slips along faults.
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Seismologists detect and study earthquakes with seismographs—instruments that produce seismograms, records of earth vibrations
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Seismogram—Record of seismic waves
Three wave types—p (pressure), s (shear), surface
Each “arrives” at a different time, dependent on distance
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The p and s waves start together… but p travels faster than s, so the farther they go, the greater the p-s time lag.
Distant seismographs record greater lags than near ones—allowing calculation of distance to the rupture.
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A B
C
Determining Focus—triangulation from three (or more)seismographs
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Epicenter (surface) vs. Focus
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Major Plate Boundaries (i.e. FAULTS)
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Richter Scale Magnitude: A logarithmic scale based that is based on the energy released.
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Magnitude vs. Intensity
• Quakes have a single magnitude, corresponding to the energy released. The greater the length of break on a fault, the greater the energy released.
• Intensity varies from the epicenter outward, though not necessarily in a simple way!
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II: Felt by a few people
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http://www.geonet.org.nz/recent_quakes.html
•NZ experiences about 10,000-15,000 quakes a year,
•100-150 are big enough to be felt.
NZ Quakes Since 15-Sep-06
0
50100
150200
250
300350
400
1 2 3 4 5 6 7 8
Magnitude
Number
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Focus > 40 km
Modified from Anderson and Webb, 1994, NZ Seismicity
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Surface Rupture Part of the 1987 magnitude 6.6 Edgecumbe earthquake surface rupture passing through a road producing significant off-set either side of the rupture zone. (Photo by L. Homer). Source: /www.earthscape.org
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Rail Lines 1987 magnitude 6.6 Edgecumbe earthquake, twisted railway lines along a section of Bay of Plenty line, near Edgecumbe. (Photo by L. Homer). Source: /www.earthscape.org