Testing a hybrid Coulomb/statistical model of aftershock occurrence

Lead Research Organisation: University of Ulster
Department Name: Sch of Environmental Sciences

Abstract

The Sichuan earthquake in mid-May killed more than 87,000 persons and caused damage on the order of fifty billion pounds. Much of the damage, and further loss of life, resulted from aftershocks - smaller earthquakes triggered by the occurrence of the M7.9 mainshock. Research over the past 15 years or so has increased our understanding of aftershocks and particularly of the controls on their locations. The most important realisation is that large earthquakes induce stress changes in the surrounding crust which can be calculated within a few hours of the mainshock; areas of stress increase correlate strongly with the occurrence of aftershocks. (These 'Coulomb' stresses are computed by resolving the tensorial stress perturbation into shear and normal components on planes of interest; increased shear stress and decrease normal stress encourage failure.) Maps of such stress changes can be used to inform emergency response so that, for example, evacuation shelters are sited in areas where stress has decreased and hence aftershocks are not expected. More useful information for emergency services would be an estimation of the probability of an earthquake above a particular magnitude affecting any given location. Calculating such probabilities is not straight-forward, however, as the only model that directly relates stress and probability changes is based on laboratory friction experiments and relies on a number of assumptions that may not be realistic as well as the determination of a number of poorly constrained parameters. To date, this model has only been subjected to one systematic test and the results were inconclusive. An alternative approach to estimating aftershock probabilities is purely statistical, based on two key observations: the Gutenberg-Richter relation which describes the number-size distribution of earthquakes and the Omori law for decrease in aftershock numbers with time. In a recent test on a single aftershock sequence, the abilities of 7 statistical and 4 stress-based models to forecast probabilities were rigorously tested. Surprisingly, the statistical models fared best, despite their lack of the essential physics that controls the spatial distribution of aftershocks. The reason for this result is open to interpretation, but the stress-based models may have suffered because of the failure of the unphysical assumptions in the friction law or because the required parameters were not estimated correctly. Alternatively, because in this model the expected rate of aftershocks depends on the magnitude of the stress change, there may have been problems with the calculated stress field due to un-modelled small scale heterogeneity in the earthquake slip distribution. The aim of this project is to develop a combined stress/statistical model for aftershock occurrence and rigorously test it against statistical and stress-based models as well as several simple reference models. This new model will retain the important spatial controls that result from the stress perturbation but will circumvent the difficulties associated with the rate-state approach. If successful, we will have a new method for calculating aftershock probabilities.

Publications

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Steacy S (2014) A new hybrid Coulomb/statistical model for forecasting aftershock rates in Geophysical Journal International

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Steacy S (2014) Stress triggering and the Canterbury earthquake sequence in Geophysical Journal International

 
Description The aim of the project was to develop and test a new method for estimating aftershock rates. This new model - which combines physical Coulomb stress modelling with statistical rate forecasting using the STEP model - has been tested on the Canterbury NZ earthquake sequence. The main result is that the new model does a better job of forecasting rates than either STEP on its own or the more traditional Coulomb rate-state model.
Exploitation Route As above, this work could be of use to emergency services and it was also one model considered by an expert elicitation panel on future Canterbury seismic hazard in November 2011. Immediately following large earthquakes, it's important to know the likely locations and rates of the aftershocks which will follow. This method may provide a better estimation of aftershock hazard then existing techniques and hence be of importance to emergency services - for example deciding where to locate emergency shelters.
Sectors Environment,Other

 
Description Expert elicitation panel
Geographic Reach Australia 
Policy Influence Type Membership of a guidance committee
Impact Building codes strengthened in Canterbury following work of expert elicitation panel. Hence buildings less likely to collapse in future earthquakes and public safer. However, cost to meet new codes is non trivial.
 
Description FP7
Amount £143,557 (GBP)
Funding ID 282862 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 09/2011 
End 12/2014
 
Description Operational earthquake forecasting 
Organisation GNS Science
Country New Zealand 
Sector Public 
PI Contribution Lead on stress interaction aspects of work
Collaborator Contribution Lead on statistical aspects of work
Impact The two GJI papers published in 2013, other papers in preparation, numerous conference presentations, my participation in an expert elicitation panel on Canterbury seismic hazard, my appointment as a visiting scientist at GNS
Start Year 2010