Searching for Earthquake Sources in the Lower Tagus Valley (Portugal): First Results from the ATESTA Project

The area of Lisbon has been struck by destructive earthquakes in the past and with very intense consequences. As of today, two main areas host active faults with concern for the region: offshore with the still unclear source of the famous and catastrophic 1755 earthquake and inland with the Lower Tagus Valley where unknown fault(s) have produced the 1909 and 1531 events with estimated magnitudes ranging from 6 to 7. Those latter events are of particular importance due to their location within an area that is now densely populated. The repetition of such a shock today would have a barely imaginable impact on the population and economy of Portugal. An apparent paradox is that in spite of the high stake and expected impact on the Greater Lisbon area, little is known about the source fault(s) of the 1531 and 1909 earthquakes in terms of location, dimensions, maximum magnitude, slip rate and recurrence period. The ATESTA Project aims at answering those questions by deploying an integrated paleoseismological approach to the Lower Tagus Valley. By combining detailed geomorphological mapping using high-resolution digital eleva- tion models with shallow geophysical imaging (reflection seismics, electrical tomography and ground-penetrating radar), our goal is to identify the continuation of crustal faults at the surface. Paleoseismic trenching is conse- quently used to characterize surface rupture in terms of large recent events. Preliminary results suggest the presence of several fault trace in the Lower Tagus Valley outlined by uplifted ter- races and offset streams and visible in satellite images and the national 10-m-resolution digital elevation model. Those fault traces correspond to structures at depth, as identified by geophysical imaging

Long-term slip history discriminates among occurrence models for seismic hazard assessment

We propose a Bayesian framework for the combination of catalogs of large earthquakes and dated cumulative slip data. It provides a quantitative way of discriminating or ranking the different renewal models. Indeed, once the datasets and priors are chosen, no additional expert opinion is required, and the ranking comes out of the Bayes factors directly. This way, the experience of experts is valorized, but the effects of group dynamics do not directly influence the weighing of the different candidate models

Fault compaction and overpressured faults: results from a 3-D model of a ductile fault zone

A model of a ductile fault zone is incorporated into a forward 3-D earthquake model to better constrain fault-zone hydraulics. The conceptual framework of the model fault zone was chosen such that two distinct parts are recognized. The fault core, characterized by a relatively low permeability, is composed of a coseismic fault surface embedded in a visco-elastic volume that can creep and compact. The fault core is surrounded by, and mostly sealed from, a high permeability damaged zone. The model fault properties correspond explicitly to those of the coseismic fault core. Porosity and pore pressure evolve to account for the viscous compaction of the fault core, while stresses evolve in response to the applied tectonic loading and to shear creep of the fault itself. A small diffusive leakage is allowed in and out of the fault zone. Coseismically, porosity is created to account for frictional dilatancy. We show in the case of a 3-D fault model with no in-plane flow and constant fluid compressibility, pore pressures do not drop to hydrostatic levels after a seismic rupture, leading to an overpressured weak fault. Since pore pressure plays a key role in the fault behaviour, we investigate coseismic hydraulic property changes. In the full 3-D model, pore pressures vary instantaneously by the poroelastic effect during the propagation of the rupture. Once the stress state stabilizes, pore pressures are incrementally redistributed in the failed patch. We show that the significant effect of pressure-dependent fluid compressibility in the no in-plane flow case becomes a secondary effect when the other spatial dimensions are considered because in-plane flow with a near-lithostatically pressured neighbourhood equilibrates at a pressure much higher than hydrostatic levels, forming persistent high-pressure fluid compartments. If the observed faults are not all overpressured and weak, other mechanisms, not included in this model, must be at work in nature, which need to be investigated. Significant leakage perpendicular to the fault strike (in the case of a young fault), or cracks hydraulically linking the fault core to the damaged zone (for a mature fault) are probable mechanisms for keeping the faults strong and might play a significant role in modulating fault pore pressures. Therefore, fault-normal hydraulic properties of fault zones should be a future focus of field and numerical experiment

Optimal representation of our knowledge about seismic sources for PSHA in low deformation areas

characterization of the seismic sources, the definition of the attenuation law and the computation of the probabilistic seismic hazard. Our work is focus on the first two steps. Given that most active faults are not characterized well enough, in low deformation areas, seismic sources are generally defined as areal zones, delimited with finite boundary polygons, within which the geological features of active tectonics and the seismicity are deemed homogeneous. Besides the lack of data (e.g., narrow range of recorded magnitudes), the application of this representation generates different problems: 1) a large sensitivity of resulting hazard maps on the location of zone boundaries, while these boundaries are set by expert decision; 2) the zoning can not represent any variation in faulting mechanism; 3) the seismicity rates are distributed throughout the zones and we lose the location of the determinant information used for their calculation. We propose an exploratory study for an alternative procedure in area source modeling. This method allows to obtain a limit, and its uncertainties, between two zones, separated by two different seismic activity rates. Since we obtain this limit, we can recover the seismic activity rates for both zones.The important features for this developed method is the location and magnitude of the largest earthquakes. Given than the largest events are not recorded by instruments,we decide to use the Bakun and Wentworth method (1997) to better characterize the epicentral region and the magnitude of the instrumental earthquakes. Because of the unusual shape of the isoseismal lines of the 1909 Benavente event, we decided to apply this methodology to this event. The result show that the estimated epicenter (Kárnik, 1969) is within all the confidence-level. Because of the low magnitude estimation, we decide to test the sensibility of this method to the attenuation law. A new law is developed using a compilation of macroseismic reports and will be used to re-estimate the epicentral region and the magnitude of the 1909 Benavente event.The logarithmic trends of intensities with the median distance suggests a logarithmic form for the attenuation law. Then, this law will be used to re-evaluate the estimations of both epicentral region and magnitude of the 1909 Benavente event

PROBABILISTIC MODELLING OF EARTHQUAKE OCCURRENCE: FIRST EXAMPLES OF DATA INTEGRATION WITHIN A BAYESIAN FRAMEWORK

PROBABILISTIC MODELLING OF EARTHQUAKE OCCURRENCE: FIRST EXAMPLES OF DATA INTEGRATION WITHIN A BAYESIAN FRAMEWOR

Interpreting lab creep experiments: from single contacts to interseismic fault models

See Abstract volum

A way to synchronize models with seismic faults for earthquake forecasting: Insights from a simple stochastic model

Numerical models are starting to be used for determining the future behaviour of seismic faults and fault networks. Their final goal would be to forecast future large earthquakes. In order to use them for this task, it is necessary to synchronize each model with the current status of the actual fault or fault network it simulates (just as, for example, meteorologists synchronize their models with the atmosphere by incorporating current atmospheric data in them). However, lithospheric dynamics is largely unobservable: important parameters cannot (or can rarely) be measured in Nature. Earthquakes, though, provide indirect but measurable clues of the stress and strain status in the lithosphere, which should be helpful for the synchronization of the models. The rupture area is one of the measurable parameters of earthquakes. Here we explore how it can be used to at least synchronize fault models between themselves and forecast synthetic earthquakes. Our purpose here is to forecast synthetic earthquakes in a simple but stochastic (random) fault model. By imposing the rupture area of the synthetic earthquakes of this model on other models, the latter become partially synchronized with the first one. We use these partially synchronized models to successfully forecast most of the largest earthquakes generated by the first model. This forecasting strategy outperforms others that only take into account the earthquake series. Our results suggest that probably a good way to synchronize more detailed models with real faults is to force them to reproduce the sequence of previous earthquake ruptures on the faults. This hypothesis could be tested in the future with more detailed models and actual seismic data.Comment: Revised version. Recommended for publication in Tectonophysic

Exploring the linkages between managerial leadership, communication and teamwork in successful event delivery

Recent growth of events has triggered research into the determinants of successful event delivery. Communication is one of the determinants, and the importance of managerial leadership in enabling communication across an event's team is recognised. Empirical research on the attributes of event managers that make them good leaders from the perspective of an event's team is however limited. Through in-depth, semi-structured interviews with employees of an established events company in the UK, this study explores the role of managerial leadership in the success of an event, referring in particular to the enablers and inhibitors of effective communication. The study finds that leadership capacity of managers correlates with their personal and inter-personal competencies. On a personal level, poor motivational and interaction skills reduce the event's team performance. On an inter-personal level, insufficient recognition of the efforts applied by individual team members as well as the entire team serves as an inhibitor

Integrating laboratory creep compaction data with numerical fault models: A Bayesian framework

We developed a robust Bayesian inversion scheme to plan and analyze laboratory creep compaction experiments. We chose a simple creep law that features the main parameters of interest when trying to identify rate-controlling mechanisms from experimental data. By integrating the chosen creep law or an approximation thereof, one can use all the data, either simultaneously or in overlapping subsets, thus making more complete use of the experiment data and propagating statistical variations in the data through to the final rate constants. Despite the nonlinearity of the problem, with this technique one can retrieve accurate estimates of both the stress exponent and the activation energy, even when the porosity time series data are noisy. Whereas adding observation points and/or experiments reduces the uncertainty on all parameters, enlarging the range of temperature or effective stress significantly reduces the covariance between stress exponent and activation energy. We apply this methodology to hydrothermal creep compaction data on quartz to obtain a quantitative, semiempirical law for fault zone compaction in the interseismic period. Incorporating this law into a simple direct rupture model, we find marginal distributions of the time to failure that are robust with respect to errors in the initial fault zone porosity