Ground-Motion Attenuation Model for Small-To-Moderate Shallow Crustal Earthquakes in California and Its Implications on Regionalization of Ground-Motion Prediction Models

Abstract

This paper presents the development of a ground-motion prediction model for small-to-moderate shallow crustal earthquakes (3M5.5, up to 200 km distance) using data from the California ShakeMap systems. Our goal is to provide an empirical model that can be confidently used in the investigation of ground-motion difference between California and other active tectonic regions (such as the Pacific Northwest and British Columbia, Canada) where the bulk of ground-motion data from shallow crustal earthquakes is in the small-to-moderate magnitude range. This attenuation model is developed as a small-magnitude extension of the Chiou and Youngs NGA model (CY2008). We observe, and incorporate into this model, a regional difference in median amplitude between central and southern California earthquakes. The strength of the regional difference diminishes with increasing spectral period. More importantly, it is magnitude dependent and becomes insignificant for M6 earthquakes, as indicated by the large-magnitude California data used in CY2008. Together, these findings have important implications on the practice of utilizing the regional differences observed in small-to-moderate earthquakes to infer the regional differences expected in large earthquakes, including the NGA model applicability in active tectonic regions outside California.

Developments in Ground Motion Predictive Models and Accelerometric Data Archiving in the Broader European Region

“…Bommer et al (2010) showed that GMPEs developed from pan-European datasets and ground-motion models derived from NGA-West1 GMPEs would yield similar ground-motion estimates for moderate-to-large magnitude earthquakes. These authors indicated that the regional differences in ground-motion estimates would be prominent towards smaller magnitude earthquakes, which is a parallel observation with the studies conducted in the other parts of the world (Chiou et al 2010;Atkinson and Morrison 2009). On the other hand, Scasserra et al (2009) emphasized that the use of NGA-West1 GMPEs may over predict the hazard in Italy at large distances because Italian data attenuate faster than the trends depicted in NGA-West1 GMPEs.…”

Investigating Possible Regional Dependence in Strong Ground Motions

“…Median ground motions from southern Californian small earthquakes are up to two times those from northern events of the same size recorded at the same distance at a wide range of periods (Figure 3), which has been related by Chiou et al (2009) to variations in stress drop between the two areas. However, for larger earthquakes, which are the main focus of seismic hazard assessments the clear differences in ground motions between the two regions become negligible.…”

“…Firstly, the importance of reducing 'pseudo-regional differences', such as general site variations between two areas, and, secondly, comparing weak motions in one region to weak motions in another. As shown by, for example, Bommer et al (2007) and Cotton et al (2008) ground motions from small earthquakes scale differently with respect to magnitude and source-to-site distance in small and large events and this effect must be accounted for when examining variations between ground motions in one region and those in another.…”

“…An informative example of regional variation between two areas that are invariably combined when deriving GMPEs is the difference between average ground motions in small earthquakes (M<5.5) between southern and northern (central) California shown by Atkinson & Morrison (2009) and Chiou et al (2009). Median ground motions from southern Californian small earthquakes are up to two times those from northern events of the same size recorded at the same distance at a wide range of periods (Figure 3), which has been related by Chiou et al (2009) to variations in stress drop between the two areas.…”

“…However, for larger earthquakes, which are the main focus of seismic hazard assessments the clear differences in ground motions between the two regions become negligible. This leads Chiou et al (2009) to make this clear statement on the difficulty of using weak motions to examine regional dependency for strong motions (SMM is small-to-moderate magnitude range, NGA is Next Generation Attenuation, CCal is central California and SCal is southern California):…”