A tale of two disasters: biases in risk communication

Theme for 2016: Recognizing and representing eventsRisk communication, where scientists inform policy-makers or the populace of the probability and magnitude of possible disasters, is essential to disaster management – enabling people to make better decisions regarding preventative steps, evacuations, etc. Psychological research, however, has identified multiple biases that can affect people’s interpretation of probabilities and thus risk. For example, availability (Tversky & Kahneman, 1973) is known to confound probability estimates while the descriptionexperience gap (D-E Gap) (Hertwig & Erev, 2009) shows low probability events being over-weighted when described and under-weighted when learnt from laboratory tasks. This paper examines how probability descriptions interact with real world experience of events. Responses from 294 participants across 8 conditions showed that people’s responses, given the same described probabilities and consequences, were altered by their familiarity with the disaster (bushfire vs earthquake) and its salience to them personally. The implications of this for risk communication are discussed.Matthew B. Welsh, Sandy Steacy, Steve H. Begg, Daniel J. Navarr

Spatio-temporal analysis of stress diffusion in a mining-induced seismicity system

Abstract. The spatio-temporal correlation of micro-earthquakes occuring in a mining-induced seismic system (Creighton mine, Ontario, Canada) is investigated. It is shown that, when considering only the after-events correlated to a main event, i.e., not accounting for the uncorrelated regime of ‘background ’ activity, the spatial distribution of these afterevents occuring at t after the main event change with t. This change takes the form of an expanding pattern, characterized by a typical scale Lc(t) varying as Lc(t) ∼ t H, H being estimated to 0.18. This diffusion exponent is found to increase when considering only a subset of the most energetic events as mainshocks. We interpret this result as the indication of a stress (sub-)diffusion mechanism, involving propagation on the heterogeneous fractal fault network. 1

Slip distribution and stress changes associated with the 1999 November 12, Duzce (Turkey) earthquake (M (w)=7.1)

The 1999 November 12 Duzce earthquake (M (w) = 7.1) was apparently the eastward extension of the August 17, Izmit earthquake (M (w) = 7.4). The Duzce event caused heavy damage and fatalities in the cities of Duzce and Bolu. Here a finite-fault inversion method with five discrete time windows is applied to derive the co-seismic slip distribution of the Duzce earthquake. The fault plane is best modelled as a 40 x 20 km(2) plane, with a strike of 262degrees and a dip of 65degrees to the north, and that the majority of slip occurred in two distinct patches on either side of the hypocentre, implying bilateral rupture. The possible triggering of this event by the Izmit earthquake is investigated using Coulomb stress modelling of all large events since 1943 with the inclusion of secular loading. The results show that although the Duzce rupture plane was in a stress shadow prior to the Izmit earthquake, that event caused a significant Coulomb stress load, taking the Duzce fault out of the stress shadow, which probably precipitated failure. A comparison of the mapped Coulomb stress change with the inferred slip shows no correlation between the two. Finally, the stress modelling indicates that the northern branch of the North Anatolian fault zone, beneath the Sea of Marmara towards the city of Istanbul, is presently the most highly loaded segment of the North Anatolian Fault Zone

Near-field propagation of tsunamis from megathrust earthquakes

We investigate controls on tsunami generation and propagation in the near-field of great megathrust earthquakes using a series of numerical simulations of subduction and tsunamigenesis on the Sumatran forearc. The Sunda megathrust here is advanced in its seismic cycle and may be ready for another great earthquake. We calculate the seafloor displacements and tsunami wave heights for about 100 complex earthquake ruptures whose synthesis was informed by reference to geodetic and stress accumulation studies. Remarkably, results show that, for any near-field location: (1) the timing of tsunami inundation is independent of slip-distribution on the earthquake or even of its magnitude, and (2) the maximum wave height is directly proportional to the vertical coseismic displacement experienced at that location. Both observations are explained by the dominance of long wavelength crustal flexure in near-field tsunamigenesis. The results show, for the first time, that a single estimate of vertical coseismic displacement might provide a reliable short-term forecast of the maximum height of tsunami waves

Onto what planes should Coulomb stress perturbations be resolved?

[1] Coulomb stress maps are produced by computing the tensorial stress perturbation due to an earthquake rupture and resolving this tensor onto planes of a particular orientation. It is often assumed that aftershock fault planes are ‘‘optimally oriented’’; in other words, the regional stress and coseismic stress change are used to compute the orientation of planes most likely to fail and the coseismic stress is resolved onto these orientations. This practice assumes that faults capable of sustaining aftershocks exist at all orientations, an assumption contradicted by the observation that aftershock focal mechanisms have strong preferred orientations consistent with mapped structural trends. Here we systematically investigate the best planes onto which stress should be resolved for the Landers, Hector Mine, Loma Prieta, and Northridge earthquakes by quantitatively comparing observed aftershock distributions with stress maps based on optimally oriented planes (two- and three-dimensional), main shock orientation, and regional structural trend. We find that the best model differs between different tectonic regions but that in all cases, models that incorporate the regional stress field tend to produce stress maps that best fit the observed aftershock distributions, although not all such models do so equally well. Our results suggest that when the regional stress field is poorly defined, or in structurally complex areas, the best model may be to fix the strike of the planes upon which the stress is to be resolved to that of the main shock but allow the dip and rake to vary