Preliminary Reference Earth Model

Preliminary Reference Earth Model
Name	Preliminary Reference Earth Model
Developer	Adam M. Dziewonski, Don L. Anderson
Published	1981
Field	Geophysics, Seismology
Based on	Seismic wave data, Normal mode data

Preliminary Reference Earth Model. The Preliminary Reference Earth Model (PREM) is a one-dimensional, spherically symmetric model of the Earth's interior that defines the average variation of key physical parameters with depth. Developed in the early 1980s, it provides a foundational reference for the planet's density, seismic wave velocities, and other properties. This model synthesizes a vast array of seismological observations to create a standard for comparing regional anomalies and advancing theories of planetary formation.

Definition and Purpose

The Preliminary Reference Earth Model serves as a canonical, radially averaged representation of the Earth's internal structure. Its primary purpose is to establish a standard Earth model against which deviations, such as those detected in seismic tomography, can be measured. It provides essential constraints for research in geodynamics, mineral physics, and studies of the Earth's core. By offering a reference state, it aids in interpreting data from global networks like the Global Seismographic Network.

Development and History

The model was constructed by geophysicists Adam M. Dziewonski and Don L. Anderson and published in 1981 in the journal Physics of the Earth and Planetary Interiors. Its development was enabled by decades of accumulated seismogram data from events like the 1960 Valdivia earthquake and advancements in the analysis of Earth's normal modes. PREM built upon earlier reference models, such as those proposed by Keith Edward Bullen, and incorporated new constraints from studies of the Earth's free oscillations.

Structure and Parameters

PREM divides the Earth into major concentric shells: the crust, mantle, outer core, and inner core. It parameterizes the variation of P-wave and S-wave velocities, density, and the attenuation factor (Q) as functions of radius. Key discontinuities are included, such as the Mohorovičić discontinuity, the transition zone at 410 and 660 kilometers depth, and the core-mantle boundary. The model also specifies parameters for anisotropy in the upper mantle.

Data Sources and Methodology

The model was derived by inverting a large composite dataset of body wave travel times, including those from the International Seismological Centre, and observations of the Earth's normal modes following major earthquakes. The methodology involved solving an inverse problem to find a model that satisfied constraints like the total Earth's mass and moment of inertia, known from astronomical and geodetic studies. Data from deep-focus earthquakes in zones like the Tonga Trench were particularly valuable for probing the lower mantle.

Applications and Significance

PREM is fundamentally used as a starting model for seismic tomography studies that image three-dimensional structures, such as subduction slabs and mantle plumes. It is critical for calculating theoretical seismograms in software like SPECFEM and for interpreting data from projects like EarthScope. The model also provides essential input for simulations in geodynamics and studies of the geodynamo at the Los Alamos National Laboratory.

Limitations and Refinements

The primary limitation of PREM is its assumption of spherical symmetry, which cannot account for lateral heterogeneities like large low-shear-velocity provinces under the Pacific Ocean and Africa. Subsequent models, such as ak135 and iasp91, have refined certain velocity parameters. Ongoing work using data from arrays like USArray and missions like GRACE continues to produce more detailed models, such as LLNL-G3Dv3, which supersede PREM in regional accuracy while relying on its foundational framework.

Category:Geophysics Category:Earth sciences Category:Scientific modeling