Overcoming Limitations of Ergodic GMPEs: Properly Separating Earthquake Source and Path Terms

Date: 4/25/2019

Time: 11:00 AM

Room: Pine

Ergodic ground motion prediction equations (GMPEs) are based on an average 1D velocity model and constant scaling. As a result, earthquake path terms, captured by the distance scaling, are the same everywhere, independent of location and azimuth. Standard regression methods for GMPEs include a random effect (constant) for the source term. As a result, if there are differences in the path effects for two earthquakes with equal magnitude, the path differences can be mapped into the random effect for the source term. The inability to separate source and path terms is a limitation of the ergodic framework. Non-ergodic GMPEs solve this problem because 3D velocity models and spatially-varying coefficients allow for differences in path effects for different locations and azimuths which provides for the proper separation of source and path terms.

The non-ergodic approach relies on the use of spatially-varying correlated coefficients to adjust each of the terms in the ergodic GMPE to the non-ergodic condition. The coefficients are typically plotted in map view to show the spatial variability and correlation length of each term. When viewing maps of the spatially-varying coefficient terms; however, there is a tendency to try to interpret the spatial coefficients separately, rather than as correlated sets, leading to strange implications. For example, when the source and geometrical spreading coefficients are plotted on a map, there is a clear inverse correlation between the source and geometrical spreading coefficients that can be interpreted as the ground-motion path effects still being mapped into the source term in the non-ergodic framework. To demonstrate this is not the case, we isolate the source effects by showing ground motion differences between non-ergodic and ergodic models at short distances (<= ~10 km) where path effects are minimal.

Presenting Author: Norman Abrahamson

Authors