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.

A B

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

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

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