A Long-term Forecast of Shallow Seismicity Based on the Global Strain Rate Map

Abstract

The Global Strain Rate Map (GSRM) of Kreemer et al. (2003) was the main result of Project II-8 of the International Lithosphere Program. The GSRM is a numerical velocity gradient tensor field model for the entire Earth's surface that describes the spatial variations of horizontal strain rate tensor components, rotation rates, and velocities. The model consists of 25 rigid spherical plates and ∼25,000 0.6° by 0.5° deformable grid areas within the diffuse plate boundary zones ( e.g. , western North America, central Asia, Alpine-Himalaya belt). The model provides an estimate of the horizontal strain rates in diffuse plate boundary zones as well as the motions of the spherical caps. This is one of the first successful models of its kind that includes the kinematics of plate boundary zones in the description of global plate kinematics. The vast majority of the data used to obtain the GSRM comes from horizontal velocity measurements obtained using Global Positioning System (GPS) measurements. The latest model version of May 2004 ( i.e. , GSRM version 1.2) includes 5,170 velocities for 4,214 sites worldwide (Holt et al. 2005). Most geodetic velocities are measured within plate boundary zones. The observed velocities are obtained from 86 different (mostly published) studies. The model includes additional constraints on the style (not magnitude) of the strain rate tensor inferred from moment tensors of shallow earthquakes. In addition, geologic strain rates in central Asia inferred from Quaternary faulting data are fit simultaneously with the geodetic velocities to improve the model there. See Kreemer et al. (2000, 2003) for more details. It was always a goal of the GSRM project to support long-term forecasts of seismicity based on tectonic deformation. Two recent developments make this especially timely. First, the Collaboratory for the Study of Earthquake Predictability (CSEP; Jordan et al. 2007) is accepting global …