Geology 5640/6640 Introduction to Seismology 9 Feb 2015 © A.R. Lowry 2015 Last time: Data: Raypaths and Phase Arrivals Phase arrivals are commonly denoted by a shorthand describing their propagation path (e.g. SKS travels first as an S-wave through the mantle, then P-wave through outer core, then S through the mantle). Ray paths are normals to wavefronts and (skeletally) represent propagation paths from a source to receiver. Different phases can be recognized in a seismogram by their differing expressions in vertical and radial/transverse horizontal components, and their relative arrival times (a function of distance to the source). Seismology is the primary tool for investigation of physical properties, structure and processes in the Earth’s interior!

Probe: Earthquakes Many each year strong enough to generate signal at antipodes 10 major (magnitudes 7-8) 32 megaton ~ Largest test 100 large (6-7) 1 megaton 1000 damaging (5-6) 32 kiloton ~ Trinity Lars Stixrude

Detector: seismograph Lars Stixrude

Global Seismic Network USArray Lars Stixrude Seismic networks

To zeroth order, radially homogeneous & isotropic Monotonic and smooth increase with depth except: Core-mantle boundary Smaller discontinuities Near surface Observable: elastic wave velocities and density

Types of Seismic Analysis* Normal modes Relative arrival times (“precursors”) Waveform modeling Impulse response (“receiver functions”) Tomography Anisotropy Noise? (*used for research into Earth’s physical properties and structure… Earthquake source analysis we’ll examine later; industrial & applied seismology is another class!) Generally speaking, the frontiers in Earth Science lie where the Earth deviates from that first-order (radially symmetric, isotropic) picture of Earth structure

Types of Seismic Analysis: Normal modes

What is a free oscillation? 3 rd Harmonic etc. 2 nd Harmonic 1 st Harmonic Fundamental L x Free oscillation = stationary wave  Interference of two counter propagating waves (see e.g. Michel Van Camp

For 2004 Sumatra-Andaman, static (permanent) deformation was recorded to 1000s of km… The static is not instantaneous but propagates at ~Rayleigh- wave velocity. After Park et al, 2005

Seismic normal modes  Periods < 54 min, amplitudes < 1 mm  Observable months after great earthquakes (e.g. Sumatra, Dec 2004) Few hours after the earthquake ( 0 S 20 ) (Duck from Théocrite, © J.-L. & P. Coudray) Michel Van Camp Few minutes after the earthquake Constructive interferences  free oscillations (or stationary waves)

0 S 0 : « balloon » or « breathing » : radial only (20.5 minutes) 0 S 2 : « football » mode (Fundamental, 53.9 minutes) 0 S 3 : (25.7 minutes) Spheroidal normal modes: examples: Animation 0 S 2 from Hein Haak Animation 0 S 0/3 from Lucien Saviot 0 S 29 from: 1_files/Fig1.jpg 0 S 29 : (4.5 minutes)... Rem: 0 S 1 = translation... Michel Van Camp