IASP91
IASP91 (also IASPEI model ) is the name of a common reference model for seismic velocities of the earth's interior .
Basics
Seismological earth models show the changes in the propagation speed of seismic waves in the earth's body with depth. The speed distribution is based on the evaluation of the transit times of earthquake waves at different measuring points on earth. The transit time refers to the length of time that a seismic wave needs from its place of origin ( hypocenter ) to a measuring station.
Since the seismic energy has a measuring point on different beam paths - e.g. B. after reflections at different layer boundaries , after passing through different layers or even on a direct route - several different seismic phases can be identified in a seismogram . The transit times of the individual phases are plotted for each station in a transit time diagram depending on the distance between the measurement location and the hypocenter.
From the gradients of the runtime branches and the runtime differences between individual phases, conclusions can be drawn about the speeds and their gradual change with depth.
IASP91
The IASP91 model is a parameterized 1D speed model that represents a summary of global transit time characteristics for an average earth radius of 6371 km. It was published in 1991 by seismologists Brian LN Kennett and E. Robert Engdahl and is considered the standard earth model of the International Association of Seismology and Physics of the Earth's Interior (IASPEI). From 1987 to 1990 the association made great international efforts to create new global run-time tables on the basis of improved technology and a larger number of instruments. These were intended to replace the tables by Harold Jeffreys and Keith Edward Bullen from 1940 , which had been used until then .
Compared to the PREM earth model, which was created a little earlier, IASP91 (with the same radius) shows some differences. Here, for example, the crust is assumed to be somewhat thicker. Furthermore, the depths of the most important discontinuities were changed: According to IASP91, the radius of the inner core of the earth is about 4 km larger, while the core-mantle boundary , which is also called Wiechert-Gutenberg discontinuity after its discoverers Emil Wiechert and Beno Gutenberg , is approx. 2 km higher than with PREM. The mantle discontinuities of the transition zone are for IASP91 at depths of 410 km and 660 km, for PREM, however, at 400 km and 670 km depth.
The global averaging of the transit times as well as the uniformly assumed earth radius lead to inaccuracies compared to locally limited measurement areas. The structures of the earth's mantle and the layers below are generally viewed as almost constant globally, so that here deviations of the reference earth model from reality can be assessed as relatively small. However, great inaccuracies arise in the upper mantle and the earth's crust , since the thicknesses of the lithosphere , the crust and, in particular, of the near-surface sediment layers vary widely across the globe.
In the IASP91 model, an average crust thickness of 35 km was assumed, with a jump in speed at a depth of 20 km. This corresponds to a greatly simplified model of the upper and lower crust , as it is assumed in continental areas. Since the oceanic crust is much thinner at 5–7 km and sediment layers are not taken into account at all, the upper layers must be replaced by a local speed model when using the model.
literature
- Kennett, BLN & Engdahl, ER: Traveltimes for global earthquake location and phase identification. Geophysical Journal International, Vol. 105, pp. 429-465, 1991. (English) doi : 10.1111 / j.1365-246X.1991.tb06724.x
- Kennett, BLN (Editor): IASPEI 1991 Seismological Tables , Research School of Earth Sciences, Australian National University, 1991.