Fault specific GIS based seismic hazard maps for the Attica Region, Greece

Traditional seismic hazard assessment methods are based on the historical seismic records for the calculation of an annual probability of exceedance for a particular ground motion level. A new fault specific seismic hazard assessment method is presented, in order to address problems related to the incompleteness and the inhomogeneity of the historical records and to obtain higher spatial resolution of hazard. This method is applied to the region of Attica, which is the most densely populated area in Greece, as nearly half of the country’s population lives in Athens and its surrounding suburbs, in Greater Athens Area. The methodology is based on a database of 22 active faults that could cause damage to Attica in case of seismic rupture. This database provides information about the faults slip rates, lengths and expected magnitudes. The final output of this method are four fault specific seismic hazard maps, showing the recurrence of expected intensities that each locality in the map has been shaken at. These maps offer a high spatial resolution, as they consider the surface geology. Despite the fact that almost half of the Attica region lies on the lowest seismic risk zone according to the official seismic hazard zonation of Greece, different localities have repeatedly experienced strong ground motions during the last 15 kyrs. Moreover, the maximum recurrence for each intensity occurs in different localities across Attica. Highest recurrence for intensity VII (151-156 times over 15 kyrs, or up to 96 year return period) is observed in the central part of the Athens basin. The maximum intensity VIII recurrence (114 times over 15 kyrs, or up to 131 year return period) is observed in the western part of Attica, while the maximum intensity IX (73-77/15kyrs, or 195 year return period) and X (25-29/15kyrs, or 517 year return period) recurrences are observed near the South Alkyonides fault system, which dominates the strong ground motions hazard in the western part of the Attica mainland

Localization of Quaternary slip rates in an active rift in 10(5) years: an example from central Greece constrained by U-234-Th-230 coral dates from uplifted paleoshorelines

Mapping, dating, and modeling of paleoshorelines uplifted in the footwall of the 1981 Gulf of Corinth earthquake fault, Greece (Ms 6.9–6.7), are used to assess its slip rate history relative to other normal faults in the area and study strain localization. The 234U-230Th coral ages from Cladocora caespitosa date uplifted shoreface sediments, and paleoshorelines from glacioeustatic sea level highstands at 76, (possibly) 100, 125, 175, 200, 216, 240, and 340 ka. Uplifted Quaternary and Holocene paleoshorelines decrease in elevation toward the western tip of the fault, exhibiting larger tilt angles with age, showing that uplift is due to progressive fault slip. Since 125 ka, uplift rates varied from 0.25 to 0.52 mm/yr over a distance of 5 km away from the fault tip. Tilting was also occurring prior to 125 ka, but uplift rates were lower because the 125 ka paleoshoreline is at 77% of the elevation of the 240 ka paleoshoreline despite being nearly half its age. Comparison of paleoshoreline elevations and sedimentology with the Quaternary sea level curve shows that slip rates increased by a factor of 3.2 ± 0.2 at 175 ± 75 ka, synchronous with cessation of activity on a neighboring normal fault at 382–112 ka. We suggest that the rapid localization of up to 10–15 mm/yr of extension into the narrow gulf (∼30 km wide) resulted from synchronous fault activity on neighboring faults followed by localization rather than sequential faulting, with consequences for the mechanism controlling localization of extension

Occurrence of partial and total coseismic ruptures of segmented normal fault systems: insights from the Central Apennines, Italy

Normal faulting earthquakes rarely rupture the entire extent of active normal faults, and can also jump between neighbouring faults. This confounds attempts to use segmentation models to define the likelihood of future rupture scenarios. We attempt to study this problem comparing the offsets produced in single earthquakes with those produced by multiple earthquakes over longer timescales, together with detailed studies of the structural geology. We study the active normal fault system causative of the Mw 6.3 2009 L’Aquila earthquake in central Italy, comparing the spatial distribution of coseismic offsets, cumulative offsets that have developed since 15 ±3 ka, and the total offsets that have accumulated since the faults initiated at 2-3 Ma. Our findings suggest that: 1) faults within a segmented fault system behave as a single interacting fault segment over time periods including multiple earthquake cycles (e.g. 2-3 Ma or 15±3ka), with single earthquakes causing either partial or total ruptures of the entire system; 2) an along-strike bend causes throw and throw-rates enhancements within the bend throughout the seismic history of the fault system. We discuss the synchronised and geometrically controlled activity rates on these faults in terms of the propensity for floating earthquakes, multi-fault earthquakes, and seismic hazard

Influence of Fault System Geometry and Slip Rates on the Relative Role of Coseismic and Interseismic Stresses on Earthquake Triggering and Recurrence Variability

We model Coulomb stress transfer (CST) due to 30 strong earthquakes occurring on normal faults since 1509 CE in Calabria, Italy, including the influence of interseismic loading, and compare the results to existing studies of stress interaction from the Central and Southern Apennines, Italy. The three normal fault systems have different geometries and long‐term slip‐rates. We investigate the extent to which stress transfer can influence the occurrence of future earthquakes and what factors may govern the variability in earthquake recurrence in different fault systems. The Calabrian, Central Apennines, and Southern Apennines fault systems have 91%, 73%, and 70% of faults with mean positive cumulative CST in the time considered; this is due to fewer faults across strike, more across strike stress reductions, and greater along‐strike spacing in the three regions respectively. In regions with close along strike spacing or few faults across strike, such as Calabria and Southern Apennines, the stress loading history is mostly dominated by interseismic loading and most faults are positively stressed before an earthquake occur on them (96% of all faults that ruptured in Calabria; 94% of faults in Southern Apennines), and some of the strongest earthquakes occur on faults with the highest mean cumulative stress of all faults prior to the earthquake. In the Central Apennines, where across strike interactions are the predominant process, 79% of earthquakes occur on faults positively stressed. The results highlight that fault system geometry plays a central role in characterizing the stress evolution associated with earthquake recurrence

Bayesian earthquake dating and seismic hazard assessment using chlorine-36 measurements (BED v1)

Over the past twenty years, analyzing the abundance of the isotope chlorine-36 (36Cl) has emerged as a popular tool for geologic dating. In particular, it has been observed that 36Cl measurements along a fault plane can be used to study the timings of past ground displacements during earthquakes, which in turn can be used to improve existing seismic hazard assessment. This approach requires accurate simulations of 36Cl accumulation for a set of fault-scarp rock samples, which are 5 progressively exhumed during earthquakes, in order to infer displacement histories from 36Cl measurements. While the physical models underlying such simulations have continuously been improved, the inverse problem of recovering displacement histories from 36Cl measurements is still mostly solved on an ad-hoc basis. The current work resolves this situation by providing a MATLAB implementation of a fast, automatic, and flexible Bayesian Markov-chain Monte Carlo algorithm for the inverse problem, and provides a validation of the 36Cl approach to inference of earthquakes from the demise of the Last Glacial 10 Maximum until present. To demonstrate its performance, we apply our algorithm to a synthetic case to verify identifiability, and to the Fiamignano and Frattura faults in the Italian Apennines in order to infer their earthquake displacement histories and to provide seismic hazard assessments. The results suggest high variability in slip rates for both faults, and large displacements on the Fiamignano fault at times when the Colosseum and other ancient buildings in Rome were damaged

Correlation between graben orientation, channel direction change and tectonic loading: The Elysium Province, Mars.

We have investigated the links between regional stress fields, the volcanic centers, rifts, graben and channels in the NW region of the Elysium Province (Fig 1(a) and Fig 1(b)) to determine whether the sequence of stress events occurring during province development can be derived from the morphologies of these features; and thus provide a sequence of development events, which is independent of surface dating techniques. Rift and graben geomorphology was mapped and the neighboring relationships and orientation of individual graben were assessed to determine any spatial clustering or preferred orientation with regional or surface features capable of creating lithospheric flexure or tectonic stress within the study area. Crosscutting analysis determined a time ordered sequence of graben formation and these were related to volcanic centers or regional sources of stress. In addition, mapping showed that different channels share sections with similar shape and orientation, prompting our study of whether these channels, in tandem with the graben, were tectonically influenced during their development. The channel central axes were mapped and compared to identify common sequences of channel direction change. The time sequence of channel direction changes and the time ordered sequence of graben development were then compared. We have demonstrated a correlation between rift and graben direction with channel orientation suggesting a regional stress control from evolving volcanic centers. Overall we derive, for the first time, the temporal pattern of tectonic, volcanic and channel evolution for the northwestern region of this major magmatic province on Mars

Throw-rate variations within linkage zones during the growth of normal faults: case studies from the Western Volcanic Zone, Iceland.

This work investigates how throw-rates vary within fault bends and sites of fault linkage during the process of normal fault growth. In the Western Volcanic Zone, Iceland, through detailed field mapping and field measurements of fault throws, normal faults are mapped and along-strike throw profiles are constructed in order to understand how the throw-rates relate with the local fault geometry along faults at different stages of linkage. The results show that throw-rates increase within linkage zones and propagating fault bends independently from the stage of maturity of the fault bend. This implies that 1) the relationship between the local fault geometry and the along-strike distribution of throw-rate is driven by the deeper part of the fault, where established fault bends start propagating to the surface; 2) faults grow first by linkage and coalescence of separate faults, and then by accumulation of slip on the resultant fault, in agreement with models of fault growth by linkage and coalescence; 3) incipient fault bends can produce uncertainty associated with palaeoseismological results, if fault bends remain unrecognised. Moreover, this work demonstrates that existing models showing increased co-seismic and throw-rates within fault bends and sites of fault linkage found in continental extensional settings are valid in a geodynamic context of a mid-oceanic rifts

Uncertainty in strain-rate from field 1 measurements of the geometry, rates and kinematics of active normal faults: implications for seismic hazard assessment

Multiple measurements of the geometry, kinematics and rates of slip across the Auletta fault (Campania, Italy) are presented, and we use these to determine: (1) the spatial resolution of field measurements needed to accurately calculate a representative strain-rate; (2) what aspects of the geometry and kinematics would introduce uncertainty with regard to the strain-rate if not measured in the field. We find that the magnitude of the post last-glacial maximum throw across the fault varies along strike. If such variations are unnoticed, different values for a representative strain-rate, hence different results in seismic hazard calculations, would be produced. To demonstrate this, we progressively degrade our dataset, calculating the implied strain-rate at each step. Excluding measurements can alter strain-rate results beyond 1σ uncertainty, thus we urge caution when using only one measurement of slip-rate for calculating hazard. We investigate the effect of approximating the throw profile along the fault with boxcar and triangular distributions and show that this can underestimate or overestimate the strain-rate, with results in the range of 72-237% of our most detailed strain-rate calculation. We discuss how improved understanding of the potential implied errors in strain-rate calculations from field structural data should be implemented in seismic hazard calculations

Spatial migration of temporal earthquake clusters driven by the transfer of differential stress between neighbouring fault/shear-zone structures

Uncertainty concerning the processes responsible for slip-rate fluctuations associated with temporal clustering of surface faulting earthquakes is a fundamental, unresolved issue in tectonics, because strain-rates accommodated by fault/shear-zone structures are the key to understanding the viscosity structure of the crust and seismic hazard. We constrain the timing and amplitude of slip-rate fluctuations that occurred on three active normal faults in central Italy over a time period of 20–30 kyrs, using in situ 36Cl cosmogenic dating of fault planes. We identify five periods of rapid slip on individual faults lasting a few millennia, separated time periods of up to 10 millennia with low or zero slip-rate. The rapid slip pulses migrated across the strike between the faults in two waves from SW to NE. We replicate this migration with a model where rapid slip induces changes in differential stress that drive changes in strain-rate on viscous shear zones that drive slip-rate variability on overlying brittle faults. Earthquakes increase the differential stress and strain-rate on underlying shear zones, which in turn accumulate strain, re-loading stress onto the overlying brittle fault. This positive feedback produces high strainrate episodes containing several large magnitude surface faulting earthquakes (earthquake clusters), but also reduce the differential stress on the viscous portions of neighbouring fault/shear-zones slowing the occurrence of large-magnitude surface faulting earthquakes (earthquake anticlusters). Shear-zones on faults experiencing anticlusters continue to accumulate viscous strain at a lowered rate, and eventually this loads the overlying brittle fault to failure, initiating a period of rapid slip through the positive feedback process described above, and inducing lowered strain-rates onto neighbouring fault/shear-zones. We show that these patterns of differential stress change can replicate the measured earthquake clustering implied by the 36Cl data. The stress changes are related to the fault geometry in terms of distance and azimuth from the slipping structure, implying that (a) strain-rate and viscosity fluctuations for studies of continental rheology, and (b) slip-rates for seismic hazard purposes are to an extent predictable given knowledge of the fault system geometry

Distributed normal faulting in the tip zone of the South Alkyonides Fault System, Gulf of Corinth, constrained using 36Cl exposure dating of Late-Quaternary wave-cut platforms

The geometry, rates and kinematics of active faulting in the region close to the tip of a major crustal-scale normal fault in the Gulf of Corinth, Greece, are investigated using detailed fault mapping and new absolute dating. Fault offsets have been dated using a combination of 234U/230Th coral dates and in situ 36Cl cosmogenic exposure ages for sediments and wave-cut platforms deformed by the faults. Our results show that deformation in the tip zone is distributed across as many as eight faults arranged within ~700 m across strike, each of which deforms deposits and landforms associated with the 125 ka marine terrace of Marine Isotope Stage 5e. Summed throw-rates across strike achieve values as high as 0.3-1.6 mm/yr, values that are comparable to those at the centre of the crustal-scale fault (2-3 mm/yr from Holocene palaeoseismology and 3-4 mm/yr from GPS geodesy). The relatively high deformation rate and distributed deformation in the tip zone are discussed in terms of stress enhancement from rupture of neighbouring crustal-scale faults and in terms of how this should be considered during fault-based seismic hazard assessment