Seismology – Summary

Earthquakes -- yearly Number of events per 12000 km2

Earthquakes -- Why? Stresses applied to rocks cause brittle failures

Fault types Normal: Spreading ridges Mid-Atlantic Ridge Thrust: Subduction zones Cascadia Strike-slip: Transform boundaries San Andreas

Beachballs Normal (Mid-Atlantic Ridge)

Beachballs Reverse thrust (Cascadia)

Beachballs Reverse oblique (Aleutians)

Beachballs Strike-slip (San Andreas)

Beachball -- summary Beachball equivalents for various fault types

Earthquake moment tensor Components of the moment tensor – 9 elements

Fault Plane Scale of earthquake: Mo = fault plane area x slip x breaking strength of rock

First motion wave field We see two quadrants of compression and two of dilatations

Earthquake wave types Body waves (travel through volume of Earth): P-waves (sound) S-waves (shear) Surface waves (travel across surface of Earth: Love waves (horizontal) Raleigh waves (vertical and horizontal)

Travel-time curves

P-to-S travel times P-wave velocity through rock is usually about 70% faster than S-wave velocity. The delay between P-wave and S-wave arrivals at a seismograph determines distance from source event.

Locating a seismic event If we can determine the distance of an event from 3 or more seismographs, we can locate its epicentre. ...from Press and Seiver

Benioff Zone Cascadia subduction zone

Earth Structure Schematic of interior structure of the Earth