Earth Sciences 240A Lecture
15
Earthquakes III
Pacific North-West:
Juan de Fuca
Plate subduction zone: Cascadia Subduction Zone
Last devastating quake:
Japanese records
of tsunami
Evidence along NA
coast
Indian legends
Est. M8.5
7 great quakes in 3500 years; Roughly
500 year intervals
Juan de Fuca
Spreading center close to trench; Plate
warm
‘Sticks’ to underside of NA Plate
Activity occurring puts added
stress on cold top (
Expected huge
earthquake: Van. Is: 5 m SW, and lower
Extension faults
Following change from compression
(subduction) to transform on plate edge
Width
of
Current quakes
1 M6 every 10 years; 1 M7 every 27
years
NA East Coast
St. Lawrence
Charlevoix: Site
of old impact 350 mya
St. Lawrence Rift:
Extensions of oceanic transforms
Some are ‘leaky’
Subduction Earthquake
Distribution
Once slab hot, plastic deformation
Need friction/stress/brittle action
Locations
Near surface; Flexure
of layers (reverse faults)
Interior of slab; Cooler;
Differential movement
Decrease with
depth
Deepest: phase transformation?
Example:
Nazca
Plate 7-8 cm/y ↓; warm slab
May 23, M9.5
1000 km fault rupture (longest)
Huge tsunamai
Accounted for 30-45% all seismic
energy released in 1900-1989 interval
Example:
Pacific Plate subducting under
island arcs; 10 cm/y rate (v. high)
Sept. 1st M8.2 in Bay
Tokoyo
70% burned
10s of thousands killed
One accident:
38,000
Housing density problem
Man-Made Earthquakes
Since 1942; Rocky Mountain Arsenal
Army biological/chemical weapons
site
Waste water
Evaporated until
1961
Pumped underground
(2 miles) later: 9 million gallons/month
1962-65: earthquakes began; M0.7-4.3;
Foci close to holes; 710 events total
“We can’t be responsible!”
Water Reservoirs
Geologic
Weight of water
reactivated faults
Correlation with
water height
Hydrologic
Water pressure in
porous rock
Capillary
connections to lake
Reduction of
friction
Geothermal Power Stations
The
Geysers, 90 miles
Direct use of
groundwater
Subsidence of
ground by 15 ft; Subsidence earthquakes
Bad design but too
late to fix
Intentional Earthquakes
Can locked fault segments be
lubricated?
Early 1970s tests on known faults
Water pumped out = high friction
Water pumped in = low friction
Mid-Term October 15
Section 001
Classroom
50 minutes