OCEANESS 410 4 Global Seismology William Wilcock LectureLab

OCEAN/ESS 410 4. Global Seismology William Wilcock

Lecture/Lab Learning Goals • Understand the distribution of earthquakes on the Earth and their relationship to plate tectonics (see also Lab 1) • Know what an earthquake is, how earthquake sizes are classified, and the different types of body waves. • Understand how seismic waves propagate through the earth along many different paths and how this constrains the internal structure of the earth. • Be able to identify seismic body wave arrivals for a teleseismic earthquake, interpret a seismic travel time curves, and locate an earthquake using S-wave minus P-wave arrival times and P-wave arrival times LAB

Tectonic Plates

Global Seismograph Network

What is an Earthquake • “An earthquake is a sudden and sometimes catastrophic movement of a part of the Earth's surface. Earthquakes result from the dynamic release of elastic strain energy that radiates seismic waves. Earthquakes typically result from the movement of faults, planar zones of deformation within the Earth's upper crust. The word earthquake is also widely used to indicate the source region itself. ” - Wikipedia • Earthquakes radiate two types of seismic waves body waves that travel through the earth and surface waves that travel over it. There are two types of body waves - P waves and S waves

Body Waves: P-waves Primary Wave: P wave is a compressional (or longitudinal) wave in which rock (particles) vibrates back and forth parallel to the direction of wave propagation. P-waves are the first arriving wave and have high frequencies but their amplitude tends not to be very large

Body Waves: S-waves Secondary Wave: S wave is a slower, transverse wave propagated by shearing motion much like that of a stretched, shaken rope. The rock (particles) vibrate perpendicular to the direction of wave propagation. They tend to have higher amplitudes and lower frequencies than P-waves. Swaves cannot travel through liquids (i. e. , the outer core, the oceans)

Surface Waves Surface waves travel over the surface of the earth. They travel more slowly than body waves but tend to have higher amplitudes and often are the most damaging waves from an earthquake

Surface wave P-wave S-wave aftershock 0 S-P 10 20 Time (min) 30

400 Depth (km) 670 Velocity (km/S) 4 8 12 Velocity Structure of the Earth • Upper mantle P waves 8 -10 km/s; S-waves 4 -6 km/s • Lower mantle P-waves 12 -14 km/s S-waves 6 -7 km/s 2900 • Outer Core P-waves 8 -10 km/s S-waves - Do not progagate 5155 • Inner Core P-waves 11 km/s 6371 S-waves 5 km/s

How do waves propagate through the earth 1. Refraction - Snell’s Law Waves bend back towards the surface when traveling through regions where the velocity increases with depth 2. Interfaces When a seismic P-wave propagates across a sharp boundary a portion of the wave will be reflected as P-wave and a portion will be converted to transmitted and reflected S-waves. The same applies to an S-wave. 1 incoming wave gives rise to 4 outgoing waves.

Seismic Phase Names

Seismic Travel Time Curve

S minus P travel times constrain the Earthquake Distance This Figure is wrong in one respect - the seismograms do not show clearly that the Swaves are much lower frequency than P waves. You will see this in the exercise.

S-minus-P travel-times will constrain the distance from station to earthquake Note the lower frequency for S-waves compared to P-waves Surface waves are lowfrequency and high amplitude arrivals Travel-times are relative to P-wave arrival time

Earthquake Location Exercise In the next lab we are going to be doing an earthquake location exercise which is courtesy of Professor Larry Braile at Purdue University. Professor Braile has developed an impressive array of earth science education activities. His web site is. http: //web. ics. purdue. edu/~braile/