Caribbean3

Seismic WavesAn introduction

Walter D. Mooney,

USGS Menlo Park, CA.

What is an Earthquake?

• Instrumentally recorded (or felt) ground shaking, normally a result of underground movement on a fault

San Francisco 1906 (USGS)Seismogram of the 1906 earthquake recorded in Germany

Seismic waves

Faulting

USGS

Types of Seismic Wave

Three-components of a seismometer record proportional to ground velocity of the P and S waves from a local aftershock of the Killari-Latur EQ, India (1993), at a hypocentral distance of 5.3 km

P. Bormann. 2002. New Manual of Seismological Observatory Practice (NMSOP)

Body Waves

Copyright 2004. L. Braile.

3/42

pv

sv 3s

p

v

v

Bulk modulus = P / (V/ V)

Shear modulusor „rigidity“ = (F/A) / (L/L)

Young´s or „stretch“modulus E = (F/A)/ (L/L) and Poisson ratio = (W/W) / (L/L)

Deformation of material samples for determining elastic moduli

P. Bormann, NMSOP

Note: The incidence angle is 59.5° for the long-period P- wave oscillation and 47.3° for the high-frequency P-wave group.

3-component records at station MOX (top traces) and related plots of particle motion in the horizontal (N - E) plane and two vertical planes (Z - N and Z – E, respectively) of the P- wave onset from seismic event (mining collapse) in Germany (1989; Ml = 5.5; epicentral distance D = 112 km, back-azimuth BAZ = 273°). Left: broadband recording (0.1 – 5 Hz); right: filtered short-period recording (1 – 5 Hz).

4.5 s

1s

P. Bormann (NMSOP)

Particle motion of body waves

Surface Waves

– Form at the free surface

– Amplitude decays exponentially with depth.

Copyright 2004. L. Braile.

January 26, 2001 Gujarat, India Earthquake (Mw7.7)

Recorded in Japan at a distance of 57o (6300 km)

Love Waves

vertical

radial

transverse

Rayleigh Waves

Courtesy J. Mori

Wave Period and Wavelength 　

Velocity 6 km/s

x

t

wavelength　　

period

Space

Time

period 50 sfrequency = 1/period= 0.02 Hz

Velocity = Wavelength / Period

wavelength 300 km

Courtesy J. Mori

Body waves

0.01 to 50 sec 50 m to 500 km

Surface waves 10 to 350 sec 30 to 1000 km

Free Oscillations 350 to 3600 sec 1000 to 10000 km

Static Displacements

-

Period Wavelength

Courtesy J. Mori

Other phases

Digital broadband record of the Seattle Mw = 6,8 earthquake on 28 February 2001 at the station Rüdersdorf (RUE) in Germany (epicentral distance D = 73°). Note the detailed interpretation of secondary phase onsets.

P. Bormann (NMSOP)

Ray theory• Seismic waves can be represented as rays

1

2

1 < 2

Ray Paths in a Layered Medium

1

2

1 > 2

slower

Faster

Faster

Slower

velocity of seismic energy in the layer

sin 1 / 1 = sin 2 / 2 = s1 sin 1 = s2 sin 2

Courtesy J. Mori

1

2

3

Ray Paths in a Layered Medium

1/1

1/2

1/3

Distance

Time

Courtesy J. Mori

Andrija Mohorovicic (1857-1936)

Found seismic discontinuity at 30 km depth in the Kupa Valley (Croatia).

Mohorovicic discontinuity or ‘Moho’　

Boundary between crust and mantle

The Moho

The MohoThe Moho

Copywrite Tasa Graphic Arts

Structure in the Earth results in complicated paths 　Lowrie, 1997, fig 3.69

Bolt, 2004, fig 6.3

USGS

Propagation of Seismic Waves In the Earth; M. Wysession

Courtesy R. Mereu

Courtesy J. Mori

Courtesy J. Mori

Forward Branch

Backward Branch

Courtesy J. Mori

Forward Branch

Backward Branch

Forward Branch

Shadow Zone

Courtesy J. Mori

Forward Branch

Backward Branch

Forward Branch

Shadow Zone

PcP

・ 1912 Gutenberg observed shadow zone 105o to 143o

・ 1939 Jeffreys fixed depth of core at 2898 km (using PcP)

ForwardBranch

BackwardBranch

ForwardBranch

PPcP

PKP

Shadow Zone

Courtesy J. Mori

PcP

Core Reflections

Courtesy J. Mori

P Mantle P

S Mantle S

K Outer core P

I Inner core P

c Reflection from the outer core

i Reflection from the inner core

diff Diffracted arrival

IASP91, Kennett and Engdahl, 1991

Stacked broadband seismograms for shallow earthquakes. Seismic phases are shown in different colors:Blue = verticalGreen = radial horizontalRed = transverse horizontal

P. Bormann. 2002. New Manual of Seismological Observatory

Practice (NMSOP)

Amplitude and Intensity

Seismic waves lose amplitude with distance traveled - attenuation A(t) = A0e -ω0t/2Q

So the amplitude of the waves depends on distance from the earthquake. Therefore unlike magnitude intensity is not a single number.

Normal Modes Normal Modes

Useful for studies of ・ Interior of the Earth ・ Largest earthquakes

l=1 m=1 l=1 m=2 l=1 m=3

Houseman http://earth.leeds.ac.uk/~greg/?Sphar/index.html

Liberty Bell(USA)

Toroidal and Spheroidal Modes

ToroidalSpheroidal

Dahlen and Tromp Fig. 8.5, 8.17

Natural Vibrations of the Earth

Shearer Ch.8.6Shearer Ch.8.6Lay and Wallace, Ch. 4.6Lay and Wallace, Ch. 4.6