Seismic Waves An introduction Walter D Mooney USGS

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

Faulting Seismic waves 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 P. Bormann, NMSOP Bulk modulus = P / ( V/ V) Shear modulus or „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 Copyright 2004. L. Braile.

Particle motion of body waves 4. 5 s 1 s 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). Note: The incidence angle is 59. 5° for the long-period P- wave oscillation and 47. 3° for the high-frequency P-wave group. P. Bormann (NMSOP)

Surface Waves – Form at the free surface – Amplitude decays exponentially with depth. Copyright 2004. L. Braile.

January 26, 2001 Gujarat, India Earthquake (Mw 7. 7) Rayleigh Waves vertical radial Love Waves transverse Recorded in Japan at a distance of 57 o (6300 km) Courtesy J. Mori

Wave Period and Wavelength　 Velocity 6 km/s Space x wavelength 　　 Time wavelength 300 km t period 50 s frequency = 1/period= 0. 02 Hz period Velocity = Wavelength / Period Courtesy J. Mori

Period Wavelength 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 - 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

Ray Paths in a Layered Medium sin q 1 / a 1 = sin q 2 / a 2 = s 1 sin q 1 = s 2 sin q 2 a = velocity of seismic energy in the layer a 1 a 2 q 1 slower q 2 a 1 < a 2 Faster a 1 a 2 Faster q 1 q 2 Slower a 1 > a 2 Courtesy J. Mori

Ray Paths in a Layered Medium Time 1/a 3 1/a 2 1/a 1 Distance a 1 a 2 a 3 Courtesy J. Mori

The Moho 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 Copywrite Tasa Graphic Arts

Structure in the Earth results in complicated paths　 Lowrie, 1997, fig 3. 69 USGS Bolt, 2004, fig 6. 3

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 Shadow Zone Forward Branch Backward Branch Courtesy J. Mori

Pc. P Backward Branch Forward Branch Shadow Zone PKP Forward Branch Pc. P P Shadow Zone Forward Branch Backward Branch Forward Branch ・ 1912 Gutenberg observed shadow zone 105 o to 143 o ・ 1939 Jeffreys fixed depth of core at 2898 km (using Pc. P) Courtesy J. Mori

Pc. P 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 Diffracted arrival diff IASP 91, Kennett and Engdahl, 1991

Stacked broadband seismograms for shallow earthquakes. Seismic phases are shown in different colors: Blue = vertical Green = radial horizontal Red = 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) = A 0 e -ω0 t/2 Q So the amplitude of the waves depends on distance from the earthquake. Therefore unlike magnitude intensity is not a single number.

Normal Modes Liberty Bell (USA) l=1 m=1 Useful for studies of ・ Interior of the Earth ・ Largest earthquakes l=1 m=2 l=1 m=3 Houseman http: //earth. leeds. ac. uk/~greg/? Sphar/index. html

Toroidal and Spheroidal Modes Toroidal Spheroidal Dahlen and Tromp Fig. 8. 5, 8. 17

Natural Vibrations of the Earth Shearer Ch. 8. 6 Lay and Wallace, Ch. 4. 6