Models

Learning About Things We Cannot See

Much of geophysics is learning about things that we cannot see. We cannot see the layers of Earth, but we know they are there because of the way seismic waves travel through the planet. Similarly, we cannot see the fault underneath the ground, but we can infer that it is there through our observations at the surface of Earth. One way to understand what is going on inside the Earth is to make models. These models are based on physics and mathematics. The goal is to find a model that best fits the observations.

Some examples of geophysical models are given in Figures 1-3. In Figure 1, seismic hazard maps are shown. These maps rely on knowledge of fault motion determined from GPS measurements of crustal deformation. Figure 2 shows a global model that uses GPS data to determine deformation zones. Figure 3 shows model for the Tokachi-Oki earthquake, both in terms of the deformation from the earthquake and one month of afterslip.

Figure 1.Seismic hazard maps are based on models of shaking and earthquake likelihood. (Credit: USGS)

Seismic hazard maps are based on models of shaking and earthquake likelihood. (Credit: USGS)

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Figure 2.Second Invariant of Strain. Credit: Global Strain Rate Project.

Second Invariant of Strain. Credit: Global Strain Rate Project.

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Figure 3.Left: Model of slip from the 2003 Tokachi-Oki earthquake derived from GPS data. Right: Model of afterslip. Note that the two slip regions are complementary. Credit: Larson and Miyazaki, 2007; Miyazaki et al., 2004.

Left: Model of slip from the 2003 Tokachi-Oki earthquake derived from GPS data. Right: Model of afterslip. Note that the two slip regions are complementary. Credit: Larson and Miyazaki, 2007; Miyazaki et al., 2004.

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