The main purpose of the ISC is to compile the ISC Bulletin, regarded as the definitive record of the Earth's seismicity. Data is collected from over 130 agencies worldwide and is available online soon after being received. The Reviewed ISC Bulletin is typically available 24 months behind real-time and is manually checked by ISC analysts. With sufficient data events in the Reviewed ISC Bulletin are relocated using the ISC location algorithm (ISCloc).
Mantle seismic velocity tomography is a powerful tool for exploring the Earth's deep structure. Interpreting the velocity anomalies as thermal anomalies allows us a snapshot glimpse of the dynamics of a convecting mantle.
Isotropic seismic velocities independent of direction are a simplifying assumption made in most tomography models. Velocity anisotropy, however, is often interpreted to exist in many areas. These range from the upper crust due to sedimentary layering, to the upper mantle as a result of viscous interaction between the underlying convecting mantle and the rigid lithospheric slabs, down to the core-mantle boundary, where chemical interactions with the molten outer core or a chemically distinct layer of old slabs may introduce anisotropy.
Anisotropy can have many causes. Often, it is a result of alignment of crystal orientation, or possibly alignment of fractures or pockets of melt within a strain field. In general, while isotropic velocities may only give us snapshots of the current thermal and chemical conditions of the mantle, anisotropy can give us a more dynamic picture by giving us additional information about the stress and strain present in the mantle.
A) First iteration SV model inverted from Z component surface waves and L and Z component body wave data. The features on the 2800 km map are an artifact due to poor coverage. B) SAW12D SH model (Li and Romanowicz, 1996) inverted from handpicked T component body and surface waves. C) RMS profiles of SV model (red) and SAW12D (blue). D) Correlation between SV model and SAW12D.
Following the recent work of Bréger et al., 2000 in which it was shown by forward modelling that the structure in D'' can explain a large portion of anomalous PKP traveltimes