Average Recurrence Period Research Articles

Contribution for the knowledge of the Alcochete-Pinhal Novo-Setúbal fault (Portugal) using ReMi and H/V Tests

The city of Setúbal is situated 30 km southeast of Lisbon, Portugal, and has experienced significant impacts from various earthquakes, including the one that occurred in 1858. This earthquake serves as an example of a nearby earthquake that caused significant social and structural damage, reaching a intensity of IX-VIII on the Modified Mercalli scale. Despite an average recurrence period of 3000–11000 years, as suggested by several authors, the Alcochete-Pinhal Novo-Setúbal fault (APSF) could potentially cause significant social, structural and economic impacts in future events. Several authors have reported on the APSF with slight variations in its outline, as it traverses multiple urbanized areas. The aim of this study was to locate the APSF in the Setúbal region and, to the best extent possible, determine its orientation and vertical offsets. 24 passive refraction microtremor tests (ReMi) and 145 horizontal-to-vertical (H/V) spectral ratios were employed to compute the variation of transverse wave velocity (VS) with depth and to compute the fundamental frequency (F0) and other natural frequency (F1) of the soils when present. Horizontal sections of VS and the spatial distribution of peak frequencies reveal alignments that can be related to the presence of the APSF fault zone. Considering all the alignments identified in this paper, two areas with distinct orientations for the APSF zone were proposed: one with a NNE-SSW strike and another with a NNW-SSE orientation. The lower values calculated for the fault plane dip (maximum of 5º) in the APSF area suggest a minimal vertical displacement, potentially corresponding to a strike-slip fault.

Recent activity and paleoseismicity of an intraplate extensional fault: the Calamocha fault (Jiloca graben, central Iberian Chain)

The Calamocha fault is an 18-km-long, NNW–SSE striking pure normal fault that moves down the northern sector of the Jiloca graben with respect to the Neogene infill of the Calatayud basin (central Iberian Chain). Its structure and kinematics are characterized by means of detailed geological mapping, morphotectonic analysis and data recording at the outcrop scale. The Calamocha fault represents the inversion of a previous contractional fault zone under the recent tensional stress field (WSW–ENE trending σ3 trajectories). The extensional activity started during the Late Pliocene (ca. 3.8 Ma), accumulating a maximum net slip of 190–230 m (long-term slip rate of 0.05–0.06 mm/a). The palaeoseismological study of three artificial exposures near Calamocha town evidenced recurrent slip during the Late Pleistocene, which proves its active character. Analysis of faulted clastic alluvial units, dated by means of optically stimulated luminescence (OSL), reveals at least eight slip events since 145.9 ± 9.1 ka, the last one being younger than 13.8 ± 0.9 ka. Only a few events represent visible accumulated displacement on the main synthetic rupture surfaces; this allows a rough estimate of the short-term slip rate (during the Late Pleistocene) of about 0.1 mm/a, faster than the long-term rate. The Calamocha fault could potentially produce a characteristic earthquake (in the sense of Schwartz and Coppersmith, J Geophys Res 89:5681–5698, 1984) with moment magnitude Mw ≈ 6.7 ± 0.3 (Mw ≈ 6.9 ± 0.3 in a scenario of activation of the whole Calamocha–Daroca fault zone), average coseismic displacement of 0.5–1.3 m and average recurrence period under 15 ka.

Application of the Asymptotic Extreme Value Probability Theory to Forecasting the Risk of High Floods in the Lower Amur

A characteristic feature of the Amur River is the frequent catastrophic floods, which cause considerable damage to the population and hamper economic activity. Statistical analysis of the maximal annual water levels in the Amur at Khabarovksk gage over the observation period since 1896 to 2016 gave analytical expressions for probability distribution functions of the general populations of water levels over the entire observation period, as well as over periods of originally weak (1896–1960) and current (1961–2016) intense economic development of the Amur basin. It was shown that the risk of high floods had decreased recently by half, while the average intervals of their recurrence had doubled compared with the previous period. The intense economic activity in the Amur basin in the recent decades was shown to have caused a decrease in the most likely water levels by 0.5 m. Overall, the hazard of high floods in the valley of the lower reaches of the Amur have decreased, however, accompanied by an increase in the amplitude of water level marks and its extreme values for individual years, as was manifested in the catastrophic 2013 flood. Calculations show that the average recurrence period of this outstanding flood is 144 years and its probability is 0.7%. The obtained data can be used in the development of a system of bank protection and channel-forming measures in the lower reaches of the Amur in the assessment of the anthropogenic impact on changes in water regime and water and floodplain ecosystems of the river.

Paleoseismic history and slip rate along the Sapanca-Akyazı segment of the 1999 İzmit earthquake rupture (Mw = 7.4) of the North Anatolian Fault (Turkey)

The Sapanca-Akyazı segment (SAS) is located on western part of the North Anatolian Fault (NAF) of Turkey. It was ruptured together with four other segments during the 17th August 1999 İzmit earthquake (Mw = 7.4) which caused ~145-km-long surface rupture in the east Marmara region. We conducted geomorphological investigations and 2D–3D paleoseismic trenching at 3 different sites near the Sakarya River along the SAS to obtain new data for the timing of past earthquakes and slip rate of this section of the NAF. Detailed investigations using Ground Penetrating Radar on the western bank of the Sakarya River reveal 18.5 ± 0.5 m of right-lateral cumulative offset of an alluvial terrace dated as 850 ± 11 years BP using Optically Stimulated Luminescence. The analysis of trench data from the three different sites of the SAS indicates the occurrence of four surface rupturing past earthquakes including the 1999 İzmit earthquake. According to the radiocarbon dating, these paleo-earthquakes can be correlated with the 1719 CE, 1567 CE, and 1037 CE historical earthquakes and suggest an average recurrence period between 273 and 322 years. The total dextral offset, the age of trench units and the terrace deposits together suggest a 22 ± 3 mm/yr slip rate for this portion of the NAF.

Efficacy and Safety of a Large Conjunctival Autograft for Recurrent Pterygium.

PurposeTo evaluate the efficacy and safety of pterygium excision using a large conjunctival autograft for the treatment of recurrent pterygium.MethodsThe medical records of 120 patients (126 eyes) with recurrent pterygium were reviewed. For each affected eye, pterygium excision with a large conjunctival autograft was performed. The graft was harvested from the superior bulbar area and measured more than 8 × 10 mm in size. Only patients who completed at least six months of follow-up were included. Postoperative clinical outcomes, recurrence rate, and complications were analyzed. Patients with any evidence of recurrence after surgery received a subconjunctival bevacizumab injection.ResultsThe average patient age was 56.5 ± 10.2 years, and 45 out of 120 patients were male. The mean study follow-up period was 17.7 ± 17.6 months. Most patients were satisfied with the cosmetic outcome. Postoperative visual acuity improved from 0.69 to 0.75 (p < 0.05). Postoperative refractive astigmatism and corneal astigmatism decreased by 0.55 and 2.73 diopters, respectively (p < 0.05). The postoperative recurrence rate was 4.0%, and the average recurrence period was 7.4 ± 0.6 weeks. A subconjunctival injection of 5 mg bevacizumab was performed in cases of recurrence; no progression of the pterygium was observed following the injection. Postoperative complications included 2 cases of conjunctival graft edema in 2 eyes, 5 donor site scars in 5 eyes, 13 pyogenic granulomas in 13 eyes, and a conjunctival epithelial inclusion cyst in 7 eyes.ConclusionsPterygium excision with a large conjunctival autograft for the treatment of recurrent pterygium produced an excellent cosmetic outcome, a low recurrence rate, and minimal complications. A subconjunctival bevacizumab injection given in cases of recurrence following surgery might be effective in preventing progression of the pterygium.

Quantification of tsunami-induced flows on a Mediterranean carbonate ramp reveals catastrophic evolution

Cool-water carbonates are the dominant limestones in the Mediterranean Basin since the Early Pliocene. Their deposition typically resulted in ramp morphologies due to high rates of resedimentation. Several such fossil carbonate ramps are characterised by a bimodal facies stacking pattern, where background deposition of subaqueous dune and/or tempestite deposits is repeatedly interrupted by anomalously thick sedimentary units, dominated by backset-stratification formed by supercritical flows. A multitude of exceptional triggers (e.g. storms, floods, tsunamis) have been invoked to explain the origin of these supercritical flows, which, in the absence of a quantitative analysis, remains speculative as yet. Here, for the first time, the catastrophic evolution of one such Mediterranean carbonate ramp, on Favignana Island (Italy), is quantified by combining 87Sr/86Sr dating, outcrop-based palaeoflow reconstructions and hydraulic calculations. We demonstrate that rare tsunami-induced flows, occurring on average once every 14 to 35 kyr, lasting a few hours only, deposited the anomalously thick backset-bedded units that form half of the sedimentary record. In between such events, cumulative two years of storm-induced flows deposited the remaining half of the succession by the stacking of subaqueous dunes. The two to four orders of magnitude difference in average recurrence period between the two flow types, and their associated sedimentation rates, emphasises the genetic differences between the two styles of deposition. In terms of sediment transport, the studied carbonate ramp was inactive for at least 99% of the time with gradual progradation during decennial to centennial storm activity. Carbonate ramp evolution attained a catastrophic signature by the contribution of rare tsunamis, producing short-lived, high-energy sediment gravity flows.

New insights on the seismogenic potential of the Eastern Betic Shear Zone (SE Iberia): Quaternary activity and paleoseismicity of the SW segment of the Carrascoy Fault Zone

AbstractThe Carrascoy Fault (CAF) is one of the main active faults that form part of the Eastern Betic Shear Zone, a 450 km fault system that accommodates most of the convergence between the Eurasian (Iberia) and Nubian plates in the Betic Cordillera, south Spain. Although the CAF represents a major earthquake threat to the nearby City of Murcia, studies on its Quaternary tectonics and seismogenic potential are scarce to date. We present evidence that supports the division of the CAF into two overlapping segments with contrasting tectonic structure, Quaternary activity, and landform control: a SW segment, characterized by a broad fold‐and‐thrust zone similar to the forebergs defined in the Gobi‐Altai region, and a NE segment, characterized by a sharp mountain front controlled by strike‐slip tectonics. We attribute the differentiation into these two segments to the stresses associated with topography, which in turn is a consequence of the shortening component, at the middle Pleistocene, after circa 217.4 ka. For the SW segment we infer the occurrence of 9 to 11, Mw 6.7 paleoearthquakes in the last 30.2 kyr, and a slip rate of 0.37 ± 0.08 m/kyr. We date the occurrence of the last surface rupture event after 2750 B.P., and we estimate an average recurrence period of major events of 3.3 ± 0.7 kyr.

Enhanced palaeoseismic succession at the Concud Fault (Iberian Chain, Spain): new insights for seismic hazard assessment

A new trench excavated at the southern sector of the Concud Fault provided evidence of three palaeoseismic events dated to ca. 21, 18 and 13–3 ka BP, respectively. The two youngest ones had not been detected in previous studies. The results extend the total recorded palaeoseismic succession for the fault: eleven events since ca. 74 ka BP to the present day, with an average recurrence period between 7.1 ± 3.5 and 8.0 ± 3.3 ka; total net accumulated slip of about 20.5 m, with average coseismic slip of 1.9 m. The displacement pattern shows alternating periods of fast slip (up to 0.53 mm/a) and slow slip (0.13 mm/a), resulting in average slip rate of 0.29 mm/a. Using this palaeoseismic information, as well as the potential magnitude previously attributed to the characteristic earthquake at the Concud Fault (M ≈ 6.5–6.6), a simple probabilistic seismic hazard analysis has been performed. The estimated probability of occurrence of the characteristic earthquake within the next 500-year period ranges from 2.3 to 26.1 %, according to distinct hypotheses on the elapsed time derived from the uncertainty about the age of the youngest event.

Regional frequency analysis for mapping drought events in north-central Chile

Summary Droughts are among the most important natural disasters, particularly in the arid and semiarid regions of the world. Proper management of droughts requires knowledge of the expected frequency of specific low magnitude precipitation totals for a variety of durations. Probabilistic approaches have often been used to estimate the average recurrence period of a given drought event. However, probabilistic model fitting by conventional methods, such as product moment or maximum likelihood in areas with low availability of long records often produces highly unreliable estimates. Recognizing the need for adequate estimates of return periods of severe droughts in the arid and semiarid region of Chile, a regional frequency analysis method based on L-moments (RFA-LM) was used for estimating and mapping drought frequency. Some adaptations to the existing procedures for forming homogeneous regions were found necessary. In addition, a new 3-parameter distribution, the Gaucho, which is a special case of the 4-parameter Kappa distribution, was introduced, and the analysis procedure was improved by the developments of two new software tools named L-RAP, to perform the RFA-LM analysis, and L-MAP, to map the resulting drought maps. Eight homogeneous sub-regions were delineated using the Gaucho distribution and used to construct return period maps for drought events with 80% and 40% precipitation of the normal. The study confirms the importance of a sub-regional homogeneity test, and the usefulness of the Gaucho distribution. The RFA-LM showed that droughts with a 40% precipitation of the normal have return periods that range from 4 years at the northern arid boundary of the study area to 22 years at the southern sub-humid boundary. The results demonstrate the need for different thresholds for declaring a drought than those currently in use for drought characterization in north-central Chile.

Geomorphic evidence for Holocene earthquakes in the Olinghouse fault zone, western Nevada

AbstractMapping and morphologic analysis of young fault scarps in the Olinghouse fault zone of western Nevada yields information about the ages and magnitudes of Holocene-late Pleistocene surface-rupturing earthquakes. Twenty profiles of single-event scarps in the eastern third of the fault zone yield ages that average 4.5 ± 2.0 ka. Ages were determined assuming linear diffusion of scarp material, with the diffusion coefficient as a function of scarp offset determined from profiles of Lake Lahontan high-stand shorelines located nearby. The mid-Holocene age is consistent with the observation that the scarps are cutting alluvial material that buries the 15.4-ka shorelines. The minimum length of faulting during this earthquake was 8 km, and the maximum normal fault offset was about 1 m, suggesting an earthquake of about M 6.7. Several older scarp segments have cumulative normal offsets of 2.5 to 4.8 m and one or two crest bevels above the most recent scarp slope, suggesting that two to three earthquakes have formed these scarps. The average diffusion age of five multiple-event scarp profiles is about 13 ka. If this represents a weighted average of the ages of the earthquakes that formed the scarps, then two or three large earthquakes have occurred since about 21 ka, which gives a speculative average recurrence period of 8 to 16 kyr. Historical accounts and geomorphic evidence of young left-slip offsets of up to 3.6 m (M 7.1) at the western end of the Olinghouse fault zone suggest that it may have ruptured historically. Thus, the potential exists for large earthquake generation all along the 20+ km length of the fault zone.

Earthquake Recurrence Rates in the Central Atlantic United States

Abstract The seaboard area from Rhode Island to Maryland has not experienced an earthquake larger than magnitude 5.1 since European settlement. The rate of occurrence of small earthquakes for the last 200 years suggests that the 500-year earthquake might be about magnitude 5.2. An earthquake comparable to the 1886 Charleston, S. C. earthquake (magnitude 6.7) has an average recurrence period calculated to exceed ten thousand years. The effect of variability in the seismic activity rate and possible incompleteness of the record of activity makes the accuracy of recurrence estimates for this region uncertain.

An Illustration of a Sea Level Control Model from a Subsiding Coastal Fan System: Pleistocene Ogasayama Formation, Central Japan

The sea level control model is a framework to explain coastal alluvial fan processes controlled by relative sea level changes and the resulting stratigraphic sequences. The model generally predicts three types of stratigraphic units, namely: (1) open fan deposits with tripartite organization for each relative sea level rise beyond the previous high stand, and subsequent sea level fall (A-type); (2) fan valley fill deposits, constructed during sea level rise up to the previous high stand (B-type); and (3) dissection-fill deposits consisting predominantly of overflow deposits (C-type). Combination of the model with a eustatic fluctuation curve suggested from an oxygen isotope record, modified for tectonic subsidence, allows a semi-quantitative estimation of numbers and dimensions of respective stratigraphic units. Estimations for the time interval recorded by the Ogasayama Formation, which shows all the depositional units predicted by the model, compare favorably with the field observations. Thicknesses predicted are directly related to the suggested amounts of relative sea level changes, which are mostly around for all types of stratigraphic units. The average recurrence period for A-type bodies is and for B-type bodies , time intervals generally assumed to be in the order of eustatic sea level fluctuations. The general agreement between model and observations in terms of both stratigraphic architecture and semi-quantitative analysis illustrates the direct control of relative sea level fluctuations on the construction of coastal fan deposits. Alternate occurrences of humid and dry climatic conditions associated with the eustatic cycle may be responsible for progradation of Gilbert-type deltas within the fan valleys, which show finer grain size than the coarsest open fan deposits.

Earthquake deformations in the Lisan deposits and seismotectonic implications

Earthquake deformations and induced sedimentary structures preserved in Quaternary sediments include faults, folds, fissures, slumps, sand boils and other effects of liquefaction. Such deformations and structures are well preserved in the Lisan deposits of the Dead Sea. Of most importance are the fold-type deformations known as décollement structures which are present all along the eastern side of the Lisan and seem to decrease gradually westwards to disappear in the middle of the Lisan. These may indicate that palaeoearthquakes originating along the Araba fault have triggered such structures due to shaking of elastoplastic unconsolidated sediments over gentle slopes dipping to the west. Preliminary results from studies on décollement structures preserved in a section representing some 1733 years of continuous deposition in the uppermost? Pleistocene, in the vicinity of Wadi Araba, indicate that: (1) seismic activity has fluctuated with time. Average recurrence period is about 340 ∓ 20yr for earthquakes with magnitudes greater than or equal to 6.5. Earthquakes with magnitude greater than 7 seem to have occurred along the Araba fault. (2) Deduced earthquake magnitudes conform to the frequency-magnitude relationship: log N = 5.24 - 0.68M. (3) The deduced seismic slip rate along the Araba fault seems to be not less than 0.64 5 0.04 cm yr-1.

A Comparison of Estimates of Seismic Risk in the Central United States

Abstract Using the maximum likelihood method and a simple least-squares fit relating magnitude and the number of earthquakes per year in the central Mississippi Valley, Nuttli obtained equations which predict that the average recurrence period for a New Madrid-size earthquake is 810 and 980 years, respectively. Using different variants of Gumbel’s theory of extreme events, the average recurrence period is found to be 1100 years (Type I) to 215,000 years (Type III based on recent data only). Conventional methods, including Gumbel’s Type I theory, which make no allowance for an upper limit on the size of expectable earthquakes may overpredict the hazard from earthquakes which are larger than any which have occurred in the historic past.