The Earth s Structure from Travel Times

Transcription

1 from Travel Times Spherically symmetric structure: PREM - Crustal Structure - Upper Mantle structure Phase transitions Anisotropy - Lower Mantle Structure D D - Structure of of the Outer and Inner Core 3-D Structure of of the Mantle from Seismic Tomography - Upper mantle -Mid mantle -Lower Mantle

2 Spherically Symmetric Structure Parameters which can can be be determined for for a reference model -P-wave velocity -S-wave velocity -Density -Attenuation (Q) (Q) --Anisotropic parameters -Bulk modulus K s s --rigidity μ pressure --gravity

3 PREM: velocities and density PREM: Preliminary Reference Earth Model (Dziewonski and and Anderson, 1981)

4 PREM: Attenuation PREM: Preliminary Reference Earth Model (Dziewonski and and Anderson, 1981)

5 Earth s Regions and Fractional Mass

6 The Earth s Crust: Travel Times Continental crust (a) (a) and and oceanic crust (b) (b) with with corresponding travel-time curves

7 The Earth s Crust: Minerals and Velocities Average crustal abundance, density and and seismic velocities of of major crustal minerals.

8 The Earth s Crust: Crustal Types S shields, C Caledonian provinces, V Variscan provinces, R rifts, O orogens

9 The Earth s Crust: Refraction Studies Refraction profiles across North America, (reduction velocity 6km/s) all all the the determination of of lateral velocity variations: PmP PmPMoho reflection Pn PnMoho refraction Pg Pg direct crustal wave wave

10 The Earth s crust: Crustal Types Reflection data data often show show a highly reflective lower crust.this may may indicate fine fine layering or or lamination, some some transition from from crust to to upper mantle. TWT TWT two-way two-way traveltimes

11 The Earth s crust: Crustal Types Recently compiled world-wide crustal thickness (km) (km) indicates cratonic areas and and mountain ranges with with active tectonics. These data data are are important to to correct travel times regionally, i.e. i.e. calculate the the contribution of of crustal thickness to to a teleseismic travel-time perturbation.

12 The Earth s crust: Crustal Types Left: Crust P-velocity profiles for for young (<20 (<20 million year) oceanic basin structures. Right: Crustal P and and S velocities for for oceanic regions older than than million years.

13 The Earth s Upper Mantle: Athenosphere The The high-velocity lid lid above the the low low velocity zone zone (asthenosphere) is is called the the lithosphere. The The upper-mantle velocity structure leads to to complex ray ray paths.

14 Upper Mantle: Phase transitions Upper mantle discontinuities (e.g. (e.g. 410km) are are caused by by phase transitions (left: low low pressure olivine, right: high high pressure β- β- spinel) Various upper mantle seismic models and and experimental results for for minerals and and mineral assemblages.

15 Upper Mantle: Discontinuities Various reflections from from upper mantle discontinuities are are being used used to to investigate the the structural details of of the the transition zones (e.g. (e.g. vertical gradients, thickness of of transition zone, topography of of discontinuities, etc.) etc.)

16 Upper Mantle: Phase transitions The The location of of seismic source within high high velocity anomalies indicates downgoing slab slab structures. Where do do earthquakes seem seem to to happen preferentially?

17 Upper Mantle: Anisotropy Shear wave wave splitting of of the the SKS SKS phase indicates seismic anisotropy in in the the upper mantle. The The alignment of of the the anisotropic symmetry system is is thought to to be be correlated with with tectonic plate plate motion.

18 Lower Mantle: D The The mid-mantle shows little little lateral heterogeneity. The The lowermost mantle (D ) (D ) hast hast strong (possibly >10%) lateral velocity perturbations. The The may may originate in in a thermal boundary layer or or from from subducted lithosphere.

19 Lower Mantle: Diffracted Waves The The lowermost mantle structure can can be be studies using using waves diffracted at at the the core-mantle boundary.

20 The Earth s Core The The Earth s inner inner core core shows considerable anisotropy. Timedependent differential travel times have have led led to to the the speculation that that the the Earth s inner inner core core is is rotating faster than than the the mantle.

21 The Earth s Core: Multiples Multiple reflection ray ray paths PK PK n Pin n Pin the the outer core core and and recording of of PK PK 4 P 4 from from an an underground nuclear explosion.

22 Upper mantle: 3-D structure

23 Mid-mantle: 3-D structure

24 Lower Mantle: 3-D structure

25 Global Cut: 3-D structure

26 Geodynamic Modelling: Subduction Zones Perturbation of of seismic velocity and and density for for a subducting plate plate obtained from from numerical convection modelling including phase transitions.

27 Geodynamic Modelling: Subduction Zones Snapshots through subducting slab slab model and and the the wavefield perturbation due due to to the the slab. slab. The The background model is is PREM.

28 Geodynamic Modelling: Plumes High-resolution numerical study of of plumes and and the the effects of of the the mantle viscosity structure.

29 : Summary The Earth s seismic velocity structure can be be determined from inverting seismic travel times (e.g. using the Wiechert- Herglotz technique for spherically symmetric media). The Earth s radial structure is is dominated by by the core-mantle boundary, the inner-core boundary, the upper-mantle discontinuities (410km and 670km) and the crust-mantle transition (Moho). The 3-D structure of of the Earth s interior can be be determined by by inverting the travel-time perturbations with respect to to a spherically symmetric velocity model (e.g. PREM). The positive and negative velocity perturbations are thought to to represent cold (dense) or or hot (buoyant) regions, respectively. There is is remarkable correlation between fast regions and subductin zones as as well as as slow regions with hot-spot (plume) activity.