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Global seismology: The velocity structure of the Earth Reading: Fowler p126-130, 326-331, 336-348 EPS 122: Lecture 13 – The velocity structure of the Earth Seismic phases Nomenclature EPS 122: Lecture 13 – The velocity structure of the Earth 1 Waves through the Earth P-waves S-waves EPS 122: Lecture 13 – The velocity structure of the Earth Seismic phases Arrival times EPS 122: Lecture 13 – The velocity structure of the Earth 2 Seismic phases Short period arrival times EPS 122: Lecture 13 – The velocity structure of the Earth Seismic phases All arrival times Love and Rayleigh S P EPS 122: Lecture 13 – The velocity structure of the Earth 3 1D velocity model for the Earth Using the arrival times of all these phases at stations around the globe we can calculate a 1D average velocity model for the Earth Uppermost mantle low-velocity zone Transition zone: 410-660 km Earthquakes stop at ~660km Lower mantle VS VP Core-mantle boundary Outer core: must be fluid as VS = 0 Also required for Earth’s magnetic field Inner core solid EPS 122: Lecture 13 – The velocity structure of the Earth Velocity and mineralogy EPS 122: Lecture 13 – The velocity structure of the Earth 4 3D velocity variations? EPS 122: Lecture 13 – The velocity structure of the Earth Mantle convection Features we expect will cause 3D velocity anomalies: • Hot, upwelling molten rock at mid-ocean ridges • Old, cold subducting lithosphere • Hot, upwelling mantle plumes EPS 122: Lecture 13 – The velocity structure of the Earth 5 Imaging 3D velocity variations Tomography Use the variations in traveltimes of seismic arrivals to determine regions of high and low velocity Similar methods used in CAT scan EPS 122: Lecture 13 – The velocity structure of the Earth Borehole tomography Tomography result: velocity variations between the holes Raypaths between holes Low velocities due to a cavity EPS 122: Lecture 13 – The velocity structure of the Earth 6 Whole Earth tomography Use variations in the arrival times of seismic energy If ray has passed through a high-velocity region then phase arrived early If low velocity region phase arrived late Use this information to reconstruct an image of the mantle EPS 122: Lecture 13 – The velocity structure of the Earth Subduction Western pacific EPS 122: Lecture 13 – The velocity structure of the Earth 7 Subduction Western pacific Kuril slab Japan trench Izu slab EPS 122: Lecture 13 – The velocity structure of the Earth Subduction Farallon slab • Originates from a time when there was subduction all along the western US. • We find evidence of this slab extending all the way to the core-mantle boundary EPS 122: Lecture 13 – The velocity structure of the Earth 8 Mid-ocean ridges EPS 122: Lecture 13 – The velocity structure of the Earth Mid-ocean ridges Mid-Atlantic Ridge and Iceland EPS 122: Lecture 13 – The velocity structure of the Earth 9 Hotspots and mantle plumes Strings of volcanic ocean islands recording the motion of the plate Hawaiian chain The formation process EPS 122: Lecture 13 – The velocity structure of the Earth Iceland The plume beneath the Iceland hotspot EPS 122: Lecture 13 – The velocity structure of the Earth 10 Iceland EPS 122: Lecture 13 – The velocity structure of the Earth Azores Canary Cape Verde Montelli et al EPS 122: Lecture 13 – The velocity structure of the Earth 11 Hawaii plume? Montelli et al EPS 122: Lecture 13 – The velocity structure of the Earth African cratons The difference between oceanic and continental crust EPS 122: Lecture 13 – The velocity structure of the Earth 12 Velocity and mineralogy EPS 122: Lecture 13 – The velocity structure of the Earth 13