New constraints on extensional tectonics and seismic hazard in northern Attica, Greece: the case of the Milesi Fault

Northern Attica in Greece is characterized by a set of north dipping, subparallel normal faults. These faults were considered to have low tectonic activity, based on historical earthquake reports, instrumental seismicity, and slip rate estimates. This study presents new data for one of these faults, the Milesi Fault. We run GIS based geomorphological analyses on fault offset distribution, field mapping of post-glacial fault scarps, and ground penetrating radar profiling to image hanging-wall deformation. The first palaeoseismological trenching in this part of Greece allowed obtaining direct data on slip rates and palaeoearthquakes. The trenching revealed downthrown and buried palaeosols, which were dated by radiocarbon. The results of our investigations show that the slip rates are higher than previously thought and that at least four palaeoearthquakes with magnitudes of around M6.2 occurred during the last 4,000-6,000 years. We calculate an average recurrence interval of 1,000-1,500 years and a maximum throw rate of ~0.4-0.45 mm/a. Based on the new geological earthquake data we developed a seismic hazard scenario, which also incorporates geological site effects. Intensities up to IX must be expected for Northern Attica and the south- eastern part of Evia. Earthquake environmental effects like liquefaction and mass movements are also likely to occur. This scenario is in contrast to the official Greek seismic hazard zonation that is based on historical records and assigns different hazard zones for municipalities that will experience the same intensity by earthquakes on the Milesi Fault. We show that the seismic hazard is likely underestimated in our study area and emphasize the need to incorporate geological information in such assessments.This is the author accepted manuscript. The final version is available from Oxford University Press via http://dx.doi.org/10.1093/gji/ggv44

Visualising the seismic landscape

The "seismic landscape" of an earthquake-prone area is the result of progressive tectonic activity penetrating the earths' surface and considers all aspects which resulted from ground deforming seismic activity during the late Quaternary. Palaeoseismology studies earthquake geological effects composing seismic landscapes in order to understand seismic activity far back into history aiming to estimate future earthquake hazards in particular regions. In order to calculate the seismic hazard potential palaeoseismologists use empirical scaling laws between earthquake magnitudes and surface faulting parameters intending to extent the history of slip on a fault. This is commonly done by identifying earthquake recurrence intervals and maximum credible magnitudes. Therefore, reliable identification of earthquake geological effects, accurate measurements, critical but transparent and reproducible analysis, and high-performance geospatial visualisation techniques covering both, developments in space and time, are crucial attributes that provide new insights to earthquake hazards and improve assessments. Here, three innovative methodologies, addressing classical palaeoseismic trenching and sea-level indicator of uncertain archival trustworthiness, complying with above mentioned attributes are presented. The approaches combine continuously new findings from palaeoseismological research with technical innovations in order to reveal precious information that fill gaps in knowledge about the seismic activity of different areas.Firstly, obtaining a more objective palaeoseismic trench log and a 3D view of the sedimentary architecture within the trench walls is the aim of a presented multiparametric workflow. It combines conventional techniques with applications of remote sensing and geophysical GPR measurements. Its usage is highly beneficial for identifying stratigraphic units and measuring representative layer thicknesses and displacements. Furthermore, the multispectral datasets are stored allowing unbiased input for future (re-)investigations.Secondly, tidal notches are a generally accepted sea-level marker and pose a morphological feature on which is focused in more detail. However, considering tidal notches in rifting regions for palaeoseismic studies is controversially discussed since appearing displacements from present-day sea-level yield in unrealistic information about earthquake history when applied to common scaling laws. Therefore, two different workflows aiming to detect actual palaeostrandlines in a 3-dimensional manner and visualising tidal notch sequence development through time by numerical modelling are presented. Consequential implications for palaeoseismological studies are the possibility to identify remnants of tidal notch morphologies caused by significant overwriting of pre-existing notch generations. Hence, newly identified palaeo-sea-levels become available for reconstructing particular sea-level histories. Furthermore, the presented numerical modelling algorithm considers a dynamic late-Holocene sea-level history and distinctly points out how rapid coseismic coastal displacements and gradual sea-level changes interplay. Its application reduces doubts in using tidal notches in palaeoseismological studies. The methodologies presented contribute to quantifying palaeoseismological archives in space and time. In particular, hanging-wall sedimentary architecture and tidal notches pose such archives that can be accessed, addressed, analysed, and correlated to other earthquake environmental effects. Fusing high-quality data and innovative workflows improves objectivity in assessment and makes palaeoseismological interpretations robust. Therefore, elaborative but objective, reproducible, clear, reliable, and innovative methodologies providing valuable inputs to scaling laws significantly improve the evaluation of a seismic landscape and thus, decisively contribute to seismic hazard assessment