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9. NZ Seismicity
and Seismic Hazards
Geology of Australia and New
Zealand, HWS/UC 2007
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.
B
A
C
Determining Focus—
triangulation from three
(or more)seismographs
Epicenter (surface)
vs. Focus
Major Plate Boundaries (i.e. FAULTS)
Richter Scale Magnitude: A logarithmic scale based that is based on the energy
released.
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!
II: Felt by a few people
Number
NZ Quakes Since 15-Sep-06
400
350
300
250
200
150
100
50
0
1
2
3
4
5
Magnitude
•NZ experiences about 10,000-15,000
quakes a year,
•100-150 are big enough to be felt.
http://www.geonet.org.nz/recent_quakes.html
6
7
8
Focus > 40 km
Modified from Anderson and Webb,
1994, NZ Seismicity
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
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