Earth Sciences 240A Lecture 15

Earthquakes III

Pacific North-West: Washington State

Juan de Fuca Plate subduction zone: Cascadia Subduction Zone

Last devastating quake: 9 p.m. (PST) 26 January 1700

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 (Vancouver Island)

Expected huge earthquake: Van. Is: 5 m SW, and lower

Western Basin and Range

Extension faults

Following change from compression (subduction) to transform on plate edge

Width of Nevada doubled in past 30 my

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: Toronto to Gulf St. Lawrence extent; Paleozoic age

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: Chile 1960

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: Japan 1923

Pacific Plate subducting under island arcs; 10 cm/y rate (v. high)

Sept. 1st M8.2 in Bay

Yokohama 100% burned

Tokoyo 70% burned

10s of thousands killed

One accident: 38,000

Housing density problem

Man-Made Earthquakes

Denver

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

Example Lake Meade/Hoover Dam

Correlation with water height

Hydrologic

Water pressure in porous rock

Capillary connections to lake

Reduction of friction

Geothermal Power Stations

Iceland, New Zealand, California

The Geysers, 90 miles N San Francisco

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