Stella Pytharouli

Scale-dependent influence of pre-existing basement shear zones on rift faulting: a case study from NE Brazil

Rifting of continental crust initiates faults that are commonly influenced by pre-existing structures. We document newly identified faults cutting Precambrian units in the interior of the NE Brazilian margin to assess the effects of structural inheritance on both rift geometry and fault architecture. Stratigraphic and structural data indicate that the faults were active in the main phase of rifting of Gondwana. The influence of pre-existing structures on the Mesozoic rift faulting is scale dependent. Regionally, the faults trend parallel to subvertical, crustal-scale Brasiliano (c. 750–540 Ma) shear zones. Mylonitic foliations and broadly distributed low strain in the lower crust indicated by shear-wave splitting controlled the overall orientation and kinematics of the rift faults. However, outcrop observations of the faults show that at scales up to hundreds of metres, mylonitic foliations have little influence on fault architectures. Faults cross-cut shear zones and do not commonly utilize foliation planes as shear fractures. Instead, slip zones and fractures have a range of orientations that form acute angles to the local foliation orientation. This observation explains the range of focal mechanisms associated with seismicity that coincides with ancient shear zones in intra-continental areas.

Brittle structures focused on subtle crustal heterogeneities : implications for flow in fractured rocks

Host rock mechanical heterogeneities influence the spatial distribution of deformation structures and hence predictions of fault architecture and fluid flow. A critical factor, commonly overlooked, is how rock mechanical properties can vary over time, and how this will alter deformation processes and resultant structures. We present field data from an area in the Borborema Province, NE Brazil, that demonstrate how temporal changes in deformation conditions, and consequently processes, exert a primary control on the spatial distribution and geometric attributes of evolving deformation structures. Furthermore, each temporal deformation phase imparted different hydraulic architecture. The earliest flowing structures are localized upon subtle ductile heterogeneities. Following fault formation, both fault core and damage zone were flow conduits. In later stages of faulting pseudotachylyte welding created a low-permeability fault core and annealed high-permeability fractures within the fault damage zone. Modern flow occurs along a zone of later open shear fractures, defined by the mechanical strength contrast between the host rock and annealed fault. This second hydraulically conductive zone extends hundreds of metres from the edge of the annealed fault damage zone, creating a flow zone far wider than would be predicted using traditional fault scaling relationships. Our results demonstrate the importance of understanding successive deformation events for predicting the temporal and spatial evolution of hydraulically active fractures.

Analysis of short and discontinuous tidal data: a case study from the Aegean Sea

Abstract An algorithm, which permits to compute the statistically significant tidal constituents and amplitude, duration and frequency of recurrence of meteorological surges, in the case of short, discontinuous or noisy tidal records, is presented. This algorithm is based on step by step filtering techniques and least squares based spectral analysis in both the frequency and time domain, and hence, it permits to avoid, first, the noise introduced in measurements as a result of interpolations, zero padding or fragmentation, and second, large uncertainties introduced by automated tidal computation routines, mainly designed for high tidal amplitudes and high signal to noise ratio data. Additionally, it is simple to follow by non-specialists in oceanic processes. The efficiency of the proposed algorithm is demonstrated on the basis of comparison of annual and interannual, high accuracy and conventional tidal records from the Aegean. This region is characterised by an astronomical tide of the order of 10 cm, about one order of magnitude smaller than the meteorological tide, and hence represents an extreme tidal environment, with very low-signal-to noise ratio values. Such data permit to evaluate the adequacy of an algorithm to successfully analyse records of short duration or with significant gaps, cases in which conventional algorithms fail. The proposed algorithm is therefore suitable for the analysis of recent, high sampling frequency but short duration records, many of which are available online, and for the identification of small amplitude tectonic, oceanographic signals, useful for climatological studies, satellite altimetry calibration or even to test malfunction of instruments.

TOLERANCE OF A LASER REFLECTORLESS EDM INSTRUMENT

Abstract Results of experiments with a laser reflectorless EDM instrument to test its tolerance are reported. Field distance measurements were simulated with measurements to >50 targets, identified with various surfaces, mostly not planar or normal to the sighting axis of instrument. Measured distances, covering the full nominal range of the instrument used, are characterized by systematic errors in the form of fluctuations with a maximum value up to 140mm at the distance of 20m, attenuating at longer distances. It was also found that the colour of target surfaces controls the maximum range of measurements, and some surface such as common mirror lead to erroneous results. Conclusions of our study are broadly consistent with conclusions of experiments with laser scanners.

Investigation of the parameters controlling the crest settlement of a major earthfill dam based on the threshold correlation analysis

Abstract The factors which control the crest settlements of the Kremasta Dam (Central Greece), one of the highest earthfill dams in Europe, were analyzed on the basis of the threshold correlation. The latter is a new stochastic technique which permits identification of parts of time series which are highly correlated on the basis of the optimization of the correlation coefficient between two parameters in combination with a high-pass filter of gradually increasing width. Based on a unique, >35 years long monitoring record consisting of leveling data, reservoir levels and precipitation levels we formed three time series describing the parameters which may affect the dam geometry (fluctuations of the reservoir level, of the rate of change of the reservoir level and of the rate of rainfall) and a fourth time series describing the dam settlements (settlement index fluctuations). Threshold correlation analysis permitted to identify critical levels for each of the above three parameters influencing the dam settlements. It was found that none of the three parameters alone can explain excessive dam settlements, but if the critical values of the three parameters are at the same time exceeded (i.e. under a conditional Boolean probability), the rate of the settlements exceeds its critical value. This approach may prove useful for other dams, but also in the analysis of data in other fields of science and engineering.

Interpretations of Reservoir‐Induced Seismicity May Not Always Be Valid: The Case of Seismicity during the Impoundment of the Kremasta Dam (Greece, 1965–1966)

The ‘Kremasta seismic sequence’ in western Greece is one of the most commonly cited examples of Reservoir Induced Seismicity (RIS). Here, we show that this ‘sequence’ is a result of normal tectonic activity and that only some small, unrelated microseismic events are reservoir induced. Shortly after the beginning of the impoundment of the Kremasta Dam in 1965, the then newly established seismic monitoring network in Greece recorded two Ms ≥ 6.0 events and numerous small shocks spread over a 120 km wide region. These were interpreted as a single seismic sequence (namely the Kremasta seismic sequence), and assumed to be reservoir induced. We revisit the epicenter locations of these events and interpret them in the framework of the regional tectonic context and the local hydrogeology. Placing these events into the local context shows that they represent an amalgamation of separate, ordinary (tectonic) seismic sequences. Further, the regional rocks are highly fragmented by small faults and the spatial distribution of seismic events is not consistent with a model of stress transfer from reservoir loading. In addition, it is not likely that events at such long (> 20-30 km) distances from the reservoir could be induced by an initial reservoir load head of 30 m. Whilst the larger magnitude events are tectonic, after impoundment local residents reported an unusual frequency of small microseismic events felt only within 10 km of the dam. We provide evidence that these are a result of the collapse of numerous shallow karstic cavities adjacent and beneath the reservoir due to increased water load (locally 100-150 m depth). This study has significant implications for interpretation of seismic triggering mechanisms in other regions: earthquake occurrence within the proximity of reservoirs during and after impoundment time cannot be assumed to be RIS unless supported by seismological, geological and hydrogeological evidence.

Detection of weak seismic signals in noisy environments from unfiltered, continuous passive seismic recordings

Robust event detection of low signal-to-noise ratio (SNR) events, such as those characterized as induced or triggered seismicity, remains a challenge. The reason is the relatively small magnitude of the events (usually less than 2 or 3 in Richter scale) and the fact that regional permanent seismic networks can only record the strongest events of a microseismic sequence. Monitoring using temporary installed short-period arrays can fill the gap of missed seismicity but the challenge of detecting weak events in long, continuous records is still present. Further, for low SNR recordings, commonly applied detection algorithms generally require pre-filtering of the data based on a priori knowledge of the background noise. Such knowledge is often not available. We present the NpD (Non-parametric Detection) algorithm, an automated algorithm which detects potential events without the requirement for pre-filtering. Events are detected by calculating the energy contained within small individual time segments of a recording and comparing it to the energy contained within a longer surrounding time window. If the excess energy exceeds a given threshold criterion, which is determined dynamically based on the background noise for that window, then an event is detected. For each time window, to characterize background noise the algorithm uses non-parametric statistics to describe the upper bound of the spectral amplitude. Our approach does not require an assumption of normality within the recordings and hence it is applicable to all datasets. We compare our NpD algorithm with the commonly commercially applied STA/LTA algorithm and another highly efficient algorithm based on Power Spectral Density using a challenging microseismic dataset with poor SNR. For event detection, the NpD algorithm significantly outperforms the STA/LTA and PSD algorithms tested, maximizing the number of detected events whilst minimizing the number of false positives.

Deformations of Pournari I dam (Greece) based on long-term geodetic monitoring data

A big number of large earth fill dams worldwide are expected to exceed their design lifespan until 2020. This number is becoming larger when taking into account predictions that dam infrastructure is not expected to withstand future natural disasters, with severe consequences to public safety and significant economic losses. Systematic monitoring and analysis is an important tool that enables the evaluation of the on-going performance of a dam, the validation of laboratory models and the assessment of different engineering designs. Currently, relationships describing the long-term evolution of dam deformations are based on only a few studies and mainly focused on the magnitude of crest settlements and the downstream horizontal deflections of the dam. This study presents the post-construction analysis of the behaviour of one of the largest earth fill dams in Greece, based on long-term monitoring data. The 29 year long dataset consisted of geodetic measurements of vertical and horizontal deformations from the crest and the body of the dam, reservoir level fluctuations, in addition to rainfall records of the dam area. Our analysis shows that the settlement of the crest remains within normal limits while the rate of deformations was stabilised almost seven years after the completion of the dam, which is longer than the period suggested by previous studies. The impoundment of the reservoir had a significant effect on the horizontal deflections of the body of the dam: an upwards and upstream movement in part of the downstream shoulder was detected. We compare our results with those obtained from the analysis of one of the highest earth fill dams in Europe, the Kremasta dam. We suggest that this pattern, though rarely mentioned in the international literature, is neither uncommon nor abnormal (when the horizontal deflections are still within safety limits) and is related to changes in the water pressure within the dam along with the on-going consolidation of the dam clay core.

Evaluation of the condition of slab-paved pedestrian pavements based on analysis of geodetic surveys

The 3D geometric characteristics and the quality of two slab-paved pedestrian pavements of high and poor quality of material and craftsmanship, respectively, surfaced with concrete slabs, were studied using geodetic measurements. A total station and mini reflectors were used to record the coordinates of the corners of all concrete slabs. The residual coordinates between a best-fit plane and the two slab-paved surfaces were then statistically analysed, and the offsets and the 3D tilting of each slab were estimated and compared. It was found that even small (e.g. < 0.5°–1° tilting) deviations from the standard values of the high-quality pedestrian pavement lead to constructions that can be classified as defective, even by a visual inspection. Hence, the proposed methodology permits a quantitative estimation of the geometric characteristics, and could even define specifications for the geometric quality of, at a first step, slab-paved pedestrian pavements.