Variations in hazard during earthquake sequences between 1995 and 2018 in western Greece as evaluated by a Bayesian ETAS model

Abstract

Summary Forecasting the spatio-temporal occurrence of events is at the core of Operational Earthquake Forecasting, which is of great interest for risk management, particularly during ongoing seismic sequences. Epidemic type aftershock sequence (ETAS) models are powerful tools to estimate the occurrence of events during earthquake sequences. In this context, a robust seismicity forecasting framework based on Bayesian-inference has been adapted to the Patras and Aegio region in western Greece (one of the most seismically active parts of Mediterranean), and an incremental adaptive algorithm is introduced to train the priors for ETAS model parameters. The seismicity forecasting is capable of accounting for uncertainty in the model parameters as well as variations in the sequence of events that may happen during the forecasting interval. Six seismic sequences between 1995 and 2018 were selected with mainshock moment magnitudes Mw ≥ 6.0. The ETAS model was adapted for each seismic sequence. The number of forecasted events with Mw≥4.5 and their spatial distribution was retrospectively compared with the as-recorded earthquake catalogue, confirming a good agreement between the forecasts and observations. The results show that the adapted model can be employed immediately after a severe mainshock to statistically predict potentially damaging earthquakes during the ongoing seismic sequence. The seismicity forecasts were translated to short-term daily exceedance rates for different thresholds of peak ground acceleration. The results reveal that the seismic hazard increased by up to 33 times in the case of the damaging 1995 Mw 6.5 earthquake in the city of Aegio. However, the results confirmed that in all six studied sequences, the increased seismic hazard decayed rapidly during the two days after the mainshock, and remained relatively high in the following days (roughly ten times the long-term time-independent hazard).

Variations in uniform hazard spectra and disaggregated scenarios during earthquake sequences

“…Table 1 summarises the two sequences that are considered representative samples for this study area. The same lettering scheme as in Azarbakht et al (2022) is used to avoid confusion for readers. Additionally, the spatial distribution of aftershock events, during one month after the mainshock origin time, are shown in Fig.…”

“…The aim of this article is to better understand the potential changes to key outputs of probabilistic seismic hazard assessments (PSHAs), including uniform hazard spectra (UHS) and disaggregated earthquake scenarios. To do this, the increases in the time-independent probabilities obtained from a recent timedependent hazard model, based on Bayesian-inference updating, by Azarbakht et al (2022) are used, to investigate how UHS and disaggregated scenarios can change during a seismic sequence.…”

“…Nevertheless, to our knowledge, there are no examples in the literature on the variation of UHS and the associated dominant scenarios (obtained from seismic disaggregation) during an ongoing sequence. Therefore, in the following, we present examples of these products using a short-term (daily) time-dependent seismic hazard model (Azarbakht et al 2022). The input data and selected study site are discussed in the next section.…”

A decision-making approach for operational earthquake forecasting

“…This city is chosen since it is in one of Europe's highest seismicity regions and it is a testbed of the TURNkey project 1 . Time-dependent seismic hazard analysis for this region is discussed in Azarbakht et al (2021). However, for simplicity, the long-term 𝜆 𝐼𝑀 (𝑖𝑚) = 𝑃𝐺.…”

Improvements to seismicity forecasting based on a Bayesian spatiotemporal ETAS model: Application to 2017-2019 Kermanshah seismic sequence in Western Iran

“…For a given forecasting interval, this leads to spatial distribution of the forecasted events and probability distribution of the number of events. The outcomes of this robust seismicity forecasting are directly applicable in adaptive daily aftershock hazard (e.g., 23,32,53,54 ) and risk assessment procedures (see e.g., 34,36,38 ). This work strives to improve different aspects of the fully probabilistic seismicity forecasting framework proposed in 27 .…”