Earthquake Notes

Earthquake Notes How Earthquakes Occur • • Elastic Rebound o Rocks store energy elastically o When stored stress exceeds the strength of the friction on the fault OR the strength of the rock… snap. o The longer between quakes, the more energy is stored. The deformation can be measured as a way to predict the size of the next quake. Earthquakes and plates o Diverging: small, shallow o Subduction zone: large, shallow to deep o Suture zone: large, shallow to medium depth o Transform: small to large (large are rare), shallow Basic Earthquake Terms • • Focus: where the fault ruptures, where the earthquake actually begins. o Usually underground o Rupture can propagate along the surface – size of quake depends on the amount of rupture Epicenter: point on Earth’s surface above the focus, where surface waves begin Earthquake Waves • • Body waves: run through interior of Earth o P (primary) wave – compressional, fast o S (secondary) wave – transverse, slower Surface waves: ripples at surface of Earth, much slower Finding the Epicenter • • Use difference in travel time between P and S wave to find distance to epicenter Use several stations to triangulate the one place that is the right distance from each station Measuring Earthquakes • Magnitude v. intensity o Magnitude: amount of energy released in the earthquake ▪ Moment magnitude - depends on the area of the surface that ruptures and the average displacement ▪ Estimated by amplitude of seismogram o What happened to Richter? ▪ Richter magnitude estimated energy from amplitude of the seismogram ▪ Could not distinguish between very large magnitudes ▪ Required use of a specific seismometer o Intensity depends on ▪ distance from epicenter ▪ geological substrate Earthquake Hazards o o o o o Ground motion - Pakistan 2004 ▪ depends on type of rock (SF, 1906) Liquefaction ▪ Water-soaked sediment; during earthquake, sediment sinks ▪ SF 1906, Loma Prieta highway collapse, Niigata, Japan, apartments sink Landslides - Anchorage, 1964 Fire - SF 1906, Loma Prieta Tsunamis ▪ displacement on ocean floor ▪ can cause water to retreat before wave hits ▪ long wavelength, wave height jow in open water ▪ Indonesia Dec. 26, 2004: M9.1 earthquake, 1300 km rupture ▪ arrival time of tsunami immediate in Indonesia, 4 hours in Sri Lanka, 9 hours in Africa ▪ wave height up to 25 meters Regions of Earthquake Hazard • • • • California Alaska New Madrid, Missouri Hawaii Earthquake Safety before, during and after earthquakes Earthquake Safety and Buildings • • Unsafe structures: o Unreinforced masonry (brick) o Pre-1954 houses not bolted to foundation o Insufficient bracing of cripple wall o Porch failures o Concrete frame structures o “Soft-story” structures - missing a wall on the first floor Safe structures o Wood frame o Steel frame o Shear walls o Cross-braced o Base isolators Earthquake prediction • • Precursors o Ground tilt o Seismic-wave velocities o Electrical resistivity o Well water levels o Radon Seismic Gaps Areas along fault with lower-than-expected frequency of earthquakes. May indicate the fault is locked up and preparing to fail. Parkfield Experiment o Parkfield (near Cholame Valley) had earthquakes about every 20 years for decades, all about M6. o Earthquake expected 1988 o An earthquake finally happened 9/28/2004 o Using data from quake to examine precursors o • Public Policy and Earthquakes • Zoning for hazard o Alquist-Priolo Act, 1972 o Set out “earthquake fault zones: 100 cities, 36 counties o Prohibited building on active rupture o Required identification and retrofit of at-risk buildings