5. Seismology

William Wilcock

OCEAN/ESS 410

A. Earthquake Seismology

Lecture/Lab Learning Goals

• Understand the distribution of earthquakes on the Earth and their relationship to plate tectonics (see also lab1)

• 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. S-waves cannot travel through liquids (i.e., the outer core, the oceans)

Surface WavesSurface 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

0 10 20 30

P-wave S-wave

S-P

Surface wave

aftershock

Time (min)

4Velocity (km/S)

8 1240

670

2900

5155

Dep

th (

km)

6371

0 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

•Outer Core

P-waves 8-10 km/s

S-waves - Do not progagate

•Inner Core

P-waves 11 km/s

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. InterfacesWhen 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

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

S minus P travel times constrain the Earthquake Distance

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://www.eas.purdue.edu/~braile

B. Reflection Seismology

Lecture/Lab Learning Goals

• Understand what seismic impedance is and how it controls the amplitude of seismic

• Know how seismic reflection data is collected• Be able to explain how reflection data is stacked and

converted into a seismic record section• Be able to interpret reflection profiles collected on mid-

ocean ridges in terms of oceanic crustal structure - LAB

Reflections from Interfaces

When a downgoing P-wave meets an interface, a portion of the wave is reflected.

Amplitudes of Reflections for vertical rays

A0

Reflected Amplitude

Transmitted Amplitude

V1, 1

V2, 2

The amplitude of the reflected and transmitted phase depends on the seismic velocity, V and the density, in each layer.

Larger contrasts in the product of velocity and density (known as impedance) result in large amplitude reflections

Marine Reflection Seismology - Airgun Sources

Reflection data is relatively easy to acquire in the oceans. Seismic sounds (shots) can be generated with arrays compressed air guns (airguns) towed behind the ship

Marine Reflection Seismology - Hydrophone Streamers

The airgun shots are recorded by arrays of hydrophones towed behind the ship in a streamer. The seismic streamers contain 1000’s of hydrophones and can be >10 km long. A modern 3-D seismic ship will tow several (the records is 20) streamers.

Marine Reflection Seismology - Geometry

The streamer records waves reflected from interfaces

Marine Reflection Seismology - DataThe seismic data recorded for a particular shot will look display a geometric effect termed “normal moveout” (NMO) which reflects the increased distance the wave travels for as the source-receiver offset increases

Offset

Tim

e, s

Time

X0

Marine Reflection Seismology - Sorting RecordsThe records are sorted so that they all have the same mid-point (Common Mid-Point - CMP)

Marine Reflection Seismology - Airgun SourcesThe seismic records can be corrected for geometric affects and stacked (summed) to produce a single record for the reflections below each each point

Before Geometric Correction

After Geometric Correction

Stacked (summed)

Marine Reflection Seismology - Filled Wiggle PlotsStacked records are plotted on the same plot with the horizontal axis showing position along the profile. Rather than showing lines for each record the plots often show filled regions for positive (or negative) displacements

Tim

e, s

Position

0

6

3

Dep

th, k

m

Structure of a mid-ocean ridge crust

molten

2A

A reflection profile across the East Pacific Rise

Reflections come from the seafloor, the base of layer 2A (pillow basalts), the axial magma chamber (AMC) and the Moho (M)

Intersecting Record Sections from the East Pacific Rise