Aftershocks Preferentially Occur in Previously Active Areas

Morgan T. Page, & Nicholas J. van der Elst

Published August 13, 2021, SCEC Contribution #11381, 2021 SCEC Annual Meeting Poster #256

The clearest statistical signal in aftershock locations is that most aftershocks occur close to their mainshocks. To first order, aftershocks are triggered at distances following a power-law decay in distance (Felzer and Brodsky, 2006). While individual sequences tend to show clustered aftershock distributions, it is hard to predict the spatial clustering in advance, and the forecast distribution is typically treated as azimuthally isotropic. This distance decay kernel is often used in Epidemic-type Aftershock Sequence (ETAS) modeling. The assumption of spatially isotropic triggering kernels can impact the estimation of ETAS parameters themselves, such as biasing the magnitude-productivity term alpha and assigning too much weight to secondary rather than primary (direct) triggering. Here we show that aftershock locations, at all mainshock-aftershock distances, preferentially occur in areas of previous seismicity. Accounting for the power-law decay in distance, about half of aftershocks occur proportionally to background rate, while the remaining are distributed randomly. On a broader scale, we see only a weak correlation between total aftershock productivity and background rate in California. Areas of low background rate can have both very low and very high aftershock productivity, while areas of high background rate rarely have low aftershock productivity.

Key Words
aftershocks, triggering, aftershock locations, clustering

Citation
Page, M. T., & van der Elst, N. J. (2021, 08). Aftershocks Preferentially Occur in Previously Active Areas. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Forecasting and Predictability (EFP)