Geology of Australia and New Zealand, HWS/UC 2007 9. NZ Seismicity and Seismic Hazards
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.
Seismologists detect and study earthquakes with seismographs—instruments that produce seismograms, records of earth vibrations
Seismogram—Record of seismic waves
Three wave types—p (pressure), s (shear), surface
Each “arrives” at a different time, dependent on distance
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.
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!
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
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