Introduction to Seismology

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Presentation transcript:

Introduction to Seismology Geology 5640/6640 Introduction to Seismology 13 Feb 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 give radial velocity/density structure (and perturbations can image laterally) Precursors are early arrivals with different reflection points Waveforms use “all” of the information content in waves Receiver functions difference the arrival times of transmitted and converted phase arrivals © A.R. Lowry 2015

Negative Velocity Gradients (faster over slower): called “lithosphere/asthenosphere boundary (LAB)”; “mid- lithosphere discontinuity (MLD)”… Interpreted as partial melt, water, metasomatized layer? (Levander & Miller, G3, 2012)

(Levander & Miller, G3 2012)

An alternative estimate of crustal thickness… Perhaps just as interesting is variation in VP/VS ratio of the crust.

Types of Seismic Analysis: Tomography

This looks more complicated than it is; and that’s my point. Frederick Simons

This looks simpler than it is; and that’s my point. Frederick Simons

X-Ray attenuation tomography Projections from all angles: X-ray intensity Reconstructed image: X-ray attenuation constants Frederick Simons

Frederick Simons

Seismic wavespeed tomography Projections from all angles: Waveforms and arrival times Reconstructed image: Wavespeed variations Frederick Simons

Seismic tomography Near surface Old oceans fast Young oceans slow Cratons (old parts of continents) fast Core-mantle boundary Past subduction fast African and Pacific anomalies slow Cause? Ritsema Frederick Simons 11

Recent imaging with EarthScope data reveals a relict strand of Farallon slab still attached to lithosphere that docked during accretion of Siletzia…

Types of Seismic Analysis: Anisotropy

Anisotropy Central Pacific West East Seismic wave velocity depends on direction of: • Propagation (P- & S-waves) • Polarization (S-waves) Explanation (for the mantle): Elastic anisotropy of olivine Alignment of olivine crystals VP anomaly (km s-1) South PROPAGATION DIRECTION Christensen & Salisbury (1979) JGR Conrad & Lithgow-Bertelloni (2002) Science 14

Polarization anisotropy aka shear-wave splitting Upper mantle xenolith Calcite: CaCO3 15

Angle between backazimuth and seismic fast axis Shear-wave splitting If backazimuth coincides with a seismic symmetry axis, the shear wave is not split (Null case) Angle between backazimuth and seismic fast axis Delay time Anisotropic layer Incoming SKS phase Invert the splitting by grid-searching for combination of fast axis and delay time which best removes the splitting 16

Shear-wave splitting Remove splitting: Radial Transverse 1. Minimum Energy on Transverse: Remove transverse Energy 2. Rotation-Correlation: Searching for maximum correlation 3. Eigenvalue criteria: Searching for most linear particle motion 17

Polarization anisotropy Most of shallow Pacific mantle: Horizontally polarized shear wave faster than vertically polarized shear wave VSH>VSV If origin of anisotropy is related to plate motion, might expect VSH-VSV to increase systematically westward. It doesn’t! Ekstrom and Dziewonski (1999) Nature 18

These images actually used ambient noise tomo: Vertical Horizontal Moschetti, Ritzwoller et al. in Nature Lin, Ritzwoller et al., Nature Geoscience

Types of Seismic Analysis: Ambient Noise

Ambient Noise (= freedom from the “tyranny of earthquakes”…) Seismic Interferometry uses the cross-correlation of signals at two sites to create a “virtual source-receiver pair”… Averaging over time periods of weeks to months. Shapiro et al., Science 2005

Lin et al., Geophys. J. Int., 2008

Noisin’ around Yellowstone! Derek Schutt