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GEO 1020Geological Processes and Materials
Earthquake Processes and the Solid Earth
Hilmar Bungumhttp://www.norsar.no
What is seismology?
• A tool for investigating the Earth’s interior– Wave propagation– Composition and structure– Dynamics
• Understanding and explaining Earthquakes– As single sources– As ensembles
• Applications– Hazard mitigation– Earthquake engineering– Seismic exploration– etc., etc.
The Science of Earthquakes
• Seismology• Structural Geology• Engineering Seismology• Geotechnical Engineering• Earthquake Engineering• Social Sciences
• Insurance (private and public)• Land-use planning, etc.• Public Authorities
Faults and the generation of earthquakes
• Faults in the Crust• The Formation of Faults• Friction and Stick-Slip• How Faulting Generates Earthquakes• The Amount of Slip on Faults• Deformation without Earthquakes
This is a diagram of the fault break for the 1989 Loma Prieta earthquake in the San Francisco Bay area. The plane of the fault is inclined to the vertical. The fault break is about 30 km long and 7 km high, centeredabout 4 km below the Earth’s surface. This break did not result in surface
faulting.
Are Earthquakes and Faults connected?
Important case studies
• California– 1872 Owens Valley earthquake
• Gilbert (1884)
– 1906 San Francisco earthquake: • Reid (1910, elastic rebound theory)
• New Zealand– 1888 South Island earthquake
• McKay (1890)
• Japan– 1891 Nobi earthquake
• Koto (1893)
Types of seismic waves
P waves (primary; pressure)
S waves (secondary; shear)
R waves (Rayleigh; surface waves)
L waves (Love; surface waves)
Waves radiating from a fault break are of four main types. First to arrive is the P or Primary wave, a back-and-forth motion, that appears as a sharp jolt. It is followed by the S wave (secondary or shear ) that is more of a rolling motion. These are deep waves that travel through the Earth to the surface. The Love and Rayleigh waves travel along the Earth’s
surface.
Range in period of seismic phenomena, seismic waves and
magnitude scales
Amplitude response of some major seismometer systems; basis for magnitude scales
Measuring and Locating Earthquakes
• Seismographs and the Record of an Earthquake• Finding the Epicenter• Defining the Size of an Earthquake
Where and Why do Earthquakes Occur?
• Earthquakes at Plate Boundaries• Earthquake at Continental Rifts and Collision zones• Intraplate Earthquakes• Induced Seismicity
The Storegga subaqueous slide
• Mega slide– Dated to ~8200 ybp– Run-out distance 850 km– Maximum thickness 430 m– Slide scar area 430 km2
– Influenced area 112,500 km2
– Major tsunami effects mapped
• Most likely earthquake triggered
– Which earthquake?– Which fault?
Historical earthquakes, mid Norway
• Earthquakes are plotted together with major fault structures
• Several structures couldhave accomodated earthquakes large enough (M~7) to triggerthe Storegga slide
• In 1988 a M 5.4earthquake occurred in aplace that was quiet both before and after the event
Large historical earthquakes in Stable Continental Regions
No. Earthquake Host Structure Magnitude
1812 New Madrid rift 8.3
1811 New Madrid rift 8.2
1812 New Madrid rift 8.1
2001 Kutch / Gujarat rift 7.9
1819 Kutch / Gujarat rift 7.8
1933 Baffin Bay margin 7.7
1604 Taiwan Straits margin 7.7
1886 South Carolina margin 7.6
1918 Nanai margin 7.4
1929 Grand Banks margin 7.4
1356 Basel rift 7.4
1605 Hainan Island rift 7.3
1906 Exmouth Plateau margin 7.2
1935 Libya margin 7.1
1958 Portugal margin 7.1
1951 So. Tasman rise margin 7.0
Damage from Earthquakes
• Ground Shaking and Displacement• Landslides and Avalanches• Sediment Liquefaction• Fire• Tsunamies• Disease
Rock falls and slides : Maximum distance vs. magnitude
• Rodriguez et al. (1999)
• Solid line: Keefer (1984)
• Lessons:– M~5 affecting ~15 km
– M~7 affecting ~200 km
– M~5 affecting ~100 km2
– M~7 affecting ~25,000 km2
Earthquake Engineering and Zoning
• Zoning• Earthquake Engineering Principles• Example from Bam, December 2003
The initial ground motion is often amplified at a given site. The nature of the rock through which the waves pass may increase the motion, and the nature of the soils at the site has a large influence. Soft soil, such as that in alluvial plains, may amplify the motion several times.
Every object has a fundamental period at which it vibrates if it is set in motion. It cannot vibrate at another period unless it is dragged back and forth. The ground also has a fundamental period. If an object is set in motion by an external force such as ground shaking which is at the fundamental period of the object the result will be ‘resonance’ and the motion of the object will tend to increase.
This shows typical periods for structures. The main determinant of period is building height and proportion; thus, a tall slender object will have a long period and sway back and forth quite slowly. So the Citicorp building in New York will sway gently back and forth every seven seconds.
Ductility is a very important characteristic of building structures because a ductile structure can absorb much more force than a nonductile structure before it fails. Conversely, nonductile structures such as unreinforced masonry or inadequately
reinforced concrete are very dangerous because of the possibility of brittle failure.
Interlude: Seeing Inside the Earth
• The Movement of Seismic Waves through the Earth• The Reflection and Refraction of Wave Energy• Discovering the Crust-Mantle Boundary• Defining the Crust-Mantle Boundary• Defining the Structure of the Mantle• The Core-Mantle Boundary (P-wave Shadow Zone)• The Nature of the Core (S-wave shadow Zone)• Fine-Tuning our Image of the Earth’s Layers• An Integrated View of the Earth
Average travel times for various seismic phases
Travel times vs. distance for a 5-year period using 2.5 million readings