Geology 5640/6640 Introduction to Seismology 11 Feb 2015 © A.R. Lowry 2015 Last time: Seismology as Investigative Tool Deep-Earth investigations use earthquakes.

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Presentation transcript:

Geology 5640/6640 Introduction to Seismology 11 Feb 2015 © A.R. Lowry 2015 Last time: Seismology as Investigative Tool Deep-Earth investigations use earthquakes as sources, three-component broadband seismometers as receivers, and analysis tools include: Normal modes Precursors Waveforms Receiver Functions Tomography Anisotropy Ambient Noise Normal modes are standing waves (global harmonics) whose frequency-wavelength relations give radial velocity/ density structure (and perturbations can image laterally)

Seismology in the News! Earth has an “inner inner core”… Wang et al. (2015) Nature Geoscience Expressed as “east-west” anisotropy in the innermost core, distinct from north-south directed anisotropy in the outermost inner core.

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

Toroidal normal modes: examples: 1 T 2 (12.6 minutes) 0 T 2 : «twisting» mode (44.2 minutes, observed in 1989 with an extensometer) 0 T 3 (28.4 minutes) Animation from Hein Haak Animation from Lucien Saviot Rem: 0 T 1 = rotation 0 T 0 = not existing Michel Van Camp

PREM is derived from normal modes, but also used for structure

Types of Seismic Analysis: Relative Arrivals (“Precursors”)

SS and SS Precursors: Imaging upper mantle discontinuities and transition zone structure Schmerr and Garnero [2006, JGR] Ed Garnero

Schmerr [2009, thesis] Upper mantle discontinuities: mineral phase boundaries Ed Garnero

Temperature Pressure  Olivine  Wadsleyite  Ringwoodite Perovskite + Ferropericlase colder mantle shallower 410 deeper 660 warmer mantle deeper 410 shallower 660 Olivine phase transitions and temperature Ed Garnero

1296 earthquakes, 1403 stations, 16,500 seismograms Upper mantle beneath S. American & surrounding oceans Ed Garnero

410 topography 660 topography Schmerr and Garnero [2007, Science] Upper mantle discontinuity topography Ed Garnero

Types of Seismic Analysis: Waveform Modeling

Sun D et al. PNAS 2007;104: ©2007 by National Academy of Sciences Waveform modeling of a D” triplication from the Ethiopia/Kenya array from a deep Sandwich Island event

Types of Seismic Analysis: Receiver Functions

Jessie Lawrence

All signals can be represented as sums of sines & cosines Waveform is a source wavelet convolved with Earth structure! “Divide” S-waveform direction sinusoids by P-waveform direction sinusoids–this removes source wavelet and leaves structure!

Consider a seismogram to be a signal from the earthquake source, s(t), that goes through a series of filters: Earth e(t) Instrument i(t) Each has an impulse response. The earth’s impulse response is the Green’s function… Alan Levander U(t) = s(t)  i(t)  e(t) (where  denotes convolution) A receiver function tries to take i(t) and s(t) out of U(t), leaving us with e(t)

(From an EarthScope Transportable Array site in northern Nevada) Ps PpPs PsPs PpSs

(Levander & Miller, G 3, 2012) Negative Velocity Gradients (faster over slower): called “lithosphere/asthenosphere boundary (LAB)”; “mid- lithosphere discontinuity (MLD)”… Interpreted as partial melt, water, metasomatized layer?