Earthquake forecasts and their applications following the M7.8 2016 Kaikoura earthquake

David A. Rhoades, Matthew C. Gerstenberger, Annemarie Christophersen, & David S. Harte

Published August 14, 2017, SCEC Contribution #7689, 2017 SCEC Annual Meeting Poster #010 (PDF)

Following the M7.8 Kaikoura earthquake of 14 November 2016, GNS Science, through the GeoNet website, provided public forecasts of future earthquake occurrence, similar to the practice that has evolved during major earthquake sequences in New Zealand since 2010. The forecasts were expressed in three forms: (1) Tables of the number of earthquakes expected during future time periods in different magnitude ranges within a defined aftershock region, with associated uncertainties; (2) Maps of the probability of earthquake shaking exceeding certain Modified Mercalli intensities; and (3) Future earthquake scenarios with associated probabilities.

Omori-law decay of aftershock rates and, after two days, the Short-Term Earthquake Probabilities (STEP) model were initially used to estimate the expected number of earthquakes of magnitude 5.0 and above for time periods of 1, 7 and 30 days. About a week after the mainshock, the 1-day forecasts were discontinued. After five weeks, only monthly and yearly forecasts were produced. The latter were based on a hybrid model, with long-term, medium-term and short-term components derived from well-established models. Separate ETAS forecasts were created in parallel and made available on ftp.gns.cri.nz/pub/davidh/Kaikoura2016/.

The long-term component of the hybrid was provided by a smoothed seismicity model, the medium-term component by two versions of the Every Earthquake a Precursor According to Scale (EEPAS) model, and the short-term component eventually by the STEP and ETAS models. Strain rate information has subsequently been incorporated in the long-term component of a 100-year forecast model used to inform transport infrastructure recovery.
The Kaikoura earthquake presented some particular challenges: (1) Careful communication of changes to the estimated mainshock magnitude, the contributing models, and the method for estimating uncertainties (a switch from the Poisson to the negative binomial distribution); and (2) Amendment of forecasts to allow for increased slow slip on the North Island subduction interface after the Kaikoura earthquake. In the absence of any established models to incorporate slow slip into the forecasts, the probability of a triggered major event was assessed by expert elicitation.

Key Words
Earthquake forecasting, New Zealand

Citation
Rhoades, D. A., Gerstenberger, M. C., Christophersen, A., & Harte, D. S. (2017, 08). Earthquake forecasts and their applications following the M7.8 2016 Kaikoura earthquake. Poster Presentation at 2017 SCEC Annual Meeting.

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