Introduction to Seismology Geology 5640/6640 Introduction to Seismology 17 Feb 2017 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 Waveforms use “all” of the information content in waves but are computationally intensive! Receiver functions difference the arrival times of transmitted and converted phase arrivals Tomography uses travel-times for large numbers of cris-crossing rays to image velocity structure © A.R. Lowry 2017

Assignment I is now posted on the course website… Due Monday, Feb 27 at the beginning of class (submit to Xiaofei.Ma@aggiemail.usu.edu) Note: Not all of the relationships you’ll need have been covered in course notes, but if you’ve been reading your text you’ll know where to look!

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 4

Polarization anisotropy aka shear-wave splitting Upper mantle xenolith Calcite: CaCO3 5

Angle between back-azimuth and seismic fast axis Shear-wave splitting If back-azimuth coincides with a seismic symmetry axis, the shear wave is not split (Null case) Angle between back-azimuth 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 6

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 7

Polarization anisotropy Most of shallow Pacific mantle: Horizontally polarized shear wave is faster than the vertically polarized shear wave VSH > VSV If the origin of anisotropy is related to plate motion, might expect VSH – VSV to increase systematically westward. It doesn’t! Ekstrom and Dziewonski (1999) Nature 8

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

Example of deep Earth seismology: 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 outermost inner core.

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