Earthquakes - University of Hawaii at Hilo

FOCUS (HYPOCENTER) location of 1st slip on fault, and EPICENTER (surface). Slip
generates seismic waves-propagate in all directions
Seismic Waves
Body Waves: P (primary-compressional) and S (secondary-shear)
Surface Waves: Love (shear) and Rayleigh (“rolling”)
Wave speeds depend on rock type
Difference in ARRIVAL TIME of first P and S waves determine distance to epicenter
3 seismic stations will pinpoint the earthquake location
SIZE of earthquakes:
Magnitude-measure of energy released during earthquake [logrithmic scale]
Richter Magnitude, also called Local Magnitude-surface waves and distance
Moment Magnitude, based on body waves or in the field by:
rock strength, fault slip and area of fault that moves
Intensity: measure of damage done [Modified Mercalli Scale (I-XII)]
Depends on rock type, magnitude of earthquake, distance to epicenter, and building
Hazards from earthquakes:
Ground shaking, landslides, liquifaction, gas/water line breakage (fire and flood),
tsunami, building/roadway collapse
Effect of ground type on amount of ground shaking: Examples from Marina District San
Francisco, Kawaihae harbor, Honolulu
Earthquake Cycle and Elastic Rebound Theory:
Two types of fault motion-CREEP and STICK-SLIP
“Predicting” earthquakes-historic record and pattern of earthquake slip
Tsunami-generated from large landslide, volcanic collapse, or subduction zone earthquake (all
submarine) Long wavelength, fast moving, interact with the seafloor sediment.
Differences in response to local tsunami vs. “foreign” tsunami: time before impact and
Indonesia 12/26/04
Hilo (Chile 1960), (N. Pacific 1946)-wave height up to 55 feet
Cascadia Subduction Zone (WA/OR) 1700 AD
Recognition and effects
Drowned trees
“Orphaned” tsunami in Japan
Sand deposit over bay mud
Earthquakes in Hawaii
Volcano Related (Magma movement)
Landslide Related (giant-hasn’t occurred for many 1000s years)-Moloka`i, Kealakekua,
Oahu examples
Flexural Slip due to weight of islands (M6.7, M 6.0 aftershock)