Earthquake Return Period Research Articles

Seismic and aseismic slip on the Central Peru megathrust

Slip on a subduction megathrust can be seismic or aseismic, with the two modes of slip complementing each other in time and space to accommodate the long-term plate motions. Although slip is almost purely aseismic at depths greater than about 40 km, heterogeneous surface strain suggests that both modes of slip occur at shallower depths, with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods. Thus, active faults seem to comprise areas that slip mostly during earthquakes, and areas that mostly slip aseismically. The size, location and frequency of earthquakes that a megathrust can generate thus depend on where and when aseismic creep is taking place, and what fraction of the long-term slip rate it accounts for. Here we address this issue by focusing on the central Peru megathrust. We show that the Pisco earthquake, with moment magnitude M(w) = 8.0, ruptured two asperities within a patch that had remained locked in the interseismic period, and triggered aseismic frictional afterslip on two adjacent patches. The most prominent patch of afterslip coincides with the subducting Nazca ridge, an area also characterized by low interseismic coupling, which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past. The seismogenic portion of the megathrust thus appears to be composed of interfingering rate-weakening and rate-strengthening patches. The rate-strengthening patches contribute to a high proportion of aseismic slip, and determine the extent and frequency of large interplate earthquakes. Aseismic slip accounts for as much as 50-70% of the slip budget on the seismogenic portion of the megathrust in central Peru, and the return period of earthquakes with M(w) = 8.0 in the Pisco area is estimated to be 250 years.

Restauración del Tajo de San Pedro en La Alhambra de Granada. Aspectos de cálculo

San Pedro cliff (Figure 1) is a dihedral 65.5 m high, which has progressed to place itself at 23.8 m from the Alhambra wall-palace that is a heritage of the Humankind. Active normal faults surrounding the cliff have created an extension tectonic regime that loosens the ground and activates the slab falls. One of the faces of the dihedral is a major fault. The cliff’s factor of safety subject to the 1000 years return period earthquake is 0.73. The preservation solution proposed is a posttensioned high elastic limit wire mesh and autochthonous vegetation. With this reinforcement, the factor of safety under that earthquake raises to a value around one. A computer simulation has permitted to analyse the environmental impact.

Estimating earthquake hazard parameters from instrumental data for different regions in and around Turkey

We conducted a study of the spatial distributions of seismicity and earthquake hazard parameters for Turkey and the adjacent areas, applying the maximum likelihood method. The procedure allows for the use of either historical or instrumental data, or even a combination of the two. By using this method, we can estimate the earthquake hazard parameters, which include the maximum regional magnitude Mˆ max, the activity rate of seismic events and the well-known bˆ value, which is the slope of the frequency-magnitude Gutenberg-Richter relationship. These three parameters are determined simultaneously using an iterative scheme. The uncertainty in the determination of the magnitudes was also taken into consideration. The return periods ( RP) of earthquakes with a magnitude M ≥ m are also evaluated. The whole examined area is divided into 24 seismic regions based on their seismotectonic regime. The homogeneity of the magnitudes is an essential factor in such studies. In order to achieve homogeneity of the magnitudes, formulas that convert any magnitude to an M S -surface scale are developed. New completeness cutoffs and their corresponding time intervals are also assessed for each of the 24 seismic regions. Each of the obtained parameters is distributed into its respective seismic region, allowing for an analysis of the localized seismicity parameters and a representation of their regional variation on a map. The earthquake hazard level is also calculated as a function of the form Θ = ( Mˆ max, RP 6.0), and a relative hazard scale (defined as the index K) is defined for each seismic region. The investigated regions are then classified into five groups using these parameters. This classification is useful for theoretical and practical reasons and provides a picture of quantitative seismicity. An attempt is then made to relate these values to the local tectonics.

Understanding earthquake disaster in central Himalayas - a perspective of mitigation and hazard prediction

Annually, about 100,000 earthquakes of magnitude more than three hit the earth. As a result, more than 15 million human lives have been lost and damage worth of hundreds of billions of dollars has been inflicted in the recorded history due to these disasters. More than a dozen earthquakes of Ms > 7.5 have occurred in the Himalayan region since 1897.The seismic activity in the Himalayan frontal arc is the result of continued collision between the Indian and Eurasian plates.
 Most of the earthquake generation models currently used for seismic hazard evaluation are based on the assumption of Poisson or other memoryless distribution, i.e. low-magnitude earthquakes follow the Poisson distribution (random event) and large-magnitude events follow the exponential distribution (non-random). The study suggests that the region has low probabilities and large mean return periods for higher-magnitude earthquakes. The earthquake generation process in Nepal Central Himalayas supports the time- and magnitude-predictable model, which is valid for 5.5< Ms <8.6. The analysis suggests that the probability of occurrence of moderate earthquakes (Ms = 5.8-6.5) in the next decade in the Central Himalayan region is very high (0.59-0.91), whereas it is very low (<0.40) for southern Tibet.

PDF interpolation technique for seismic fragility analysis of bridges

This study presents a probability density function (PDF) interpolation technique for the evaluation of seismic fragility curves as a function of the return period. Seismic fragility curves have been developed in terms of various seismic intensities, such as the peak ground acceleration (PGA), peak ground velocity and pseudovelocity spectrum. However, of these seismic intensity measures, the return period of design earthquakes is more useful, as seismic hazard curves are generally represented with the return period of design earthquakes; seismic design codes also require consideration of the return period of the design earthquake spectrum for a specific site. Therefore, this paper focuses on the evaluation of seismic fragility curves as a function of the return period. Seismic fragility curves based on the return period are compared with those based on the PGA as an example bridge. The seismic fragility curves developed in this study make more intuitive sense for the design, retrofit and performance evaluation of bridges.

Recognition of past earthquakes along the Sparta fault (Peloponnesus, southern Greece) during the Holocene, by combining results of different dating techniques

Abstract Sparta fault is an impressive landform, located on the eastern front of Taygetos mountain, southern Greece. Detailed morphotectonic observations on this fault suggest that it should be active at least since Early Quaternary. However, according to the current seismological knowledge, this region is characterized by very low seismicity. The only reported earthquake to have occurred in this area is that of 464 b . c ., a destructive event that devastated the whole city of Sparta. In order to get information on the occurrence of past earthquakes during the Holocene, results of different independent dating works that have performed along the Sparta fault were used. These researchers confirm the existence not only of the 464 b . c . earthquake but also of several more that occurred at ca. 3900 b . c ., 2500 b . c . and 2000 b . c ., 550 a . d . and 1000 a . d . The events that occurred at 2500 and 464 b . c . should correspond to major events of magnitude of the order of 7, which ruptured the entire length of the fault, while these at 3900 b . c ., 2000 b . c ., 550 a . d . and 1000 a . d ., to smaller events of magnitude 6–6.5. The return periods of strong earthquakes along the Sparta fault is estimated to be around 2000 years, but within these periods events of smaller magnitude that ruptured segments of the fault have also occurred.

Seismic Response of Multiple Span Steel Bridges in Central and Southeastern United States. I: As Built

The seismic response of typical multispan simply supported (MSSS) and multispan continuous steel girder bridges in the central and southeastern United States is evaluated. Nonlinear time history analyses are conducted using synthetic ground motion for three cities for 475 and 2,475-year return period earthquakes (10 and 2% probability of exceedance in 50 years). The results indicate that the seismic response for the 475-year return period earthquake would lead to an essentially linear response in typical bridges. However, the seismic response for a 2,475-year return period earthquake resulted in significant demands on nonductile columns, fixed and expansion bearings, and abutments. In particular, pounding between decks in the MSSS bridge would result in significant damage to steel bearings and would lead to the toppling of rocker bearings, which may result in unseating of the bridge deck.

Comparison of seismic actions and structural design requirements in Chinese code GB 50011 and international standard ISO 3010

This paper presents a comparison between the Chinese Code GB50011-2001 and the International Standard ISO3010:2001(E), emphasizing the similarities and differences related to design requirements, seismic actions and analytical approaches. Similarities include: earthquake return period, conceptual design, site classification, structural strength and ductility requirements, deformation limits, response spectra, seismic analysis procedures, isolation and energy dissipation, and nonstructural elements. Differences exist in the following areas: seismic levels, earthquake loading, mode damping factors and structural control.

Deprem Magnitüdleri İçin Tekrarlanma Yıllarının Elde Edilmesi : Marmara Bölgesi Örneği

In this study, statistical analysis have been done for 447 earthquake occurrences data, between north (39.50-41.50) and east (26.00-32.50) coordinates in the Marmara Region from 1900 to 2000. It has been found that the probability density and distribution functions of magnitude random variable, and then the return period of earthquakes of various magnitudes have been found.

MICROZONATION OF THE EXPECTED SEISMIC SITE EFFECTS ACROSS PORT VILA, VANUATU

The city of Port Vila, Vanuatu, is located in one of the most active seismic regions on earth. Earthquakes are felt frequently and, due to very rapid plate convergence rates, return period of large earthquakes (M>6) in the New Hebrides Benioff zone can be less than 10 years. Even though Port Vila does not lie on an identified seismic fault zone, strong motions by nearby earthquakes have to be expected due to the city's geographical location close to the plate boundary of New Hebrides convergence zone. An accurate estimation of the seismic ground motion across the city is of prime importance for urban developments and mitigation of earthquake risk. Following many examples of monitored strong earthquakes in the current century, it is evident that the local site effects may have a dominant contribution to the intensity of damage and destruction. In this study we focussed on the first stage of associating site effects and seismic hazard by preparing a microzonation map for Port Vila. The seismic microzonation of the city has been carried out to provide a detailed map of the zones that exhibit site effects in terms of resonance frequencies and approximated amplification of the ground shaking. Having in mind that these data will be used in improving building design to sustain strong ground motions, our analysis is limited to the frequency band of 1–10 Hz, corresponding to the expected resonance of different types of buildings in Port Vila. The Nakamura technique has been used to estimate site amplification effects from single station noise recordings. Interestingly, excluding one site located on an old dump zone, the amplification factors at about the 100 sites surveyed in Port Vila remain below 3 with an average well below 2 in the 1 to 10 Hz frequency band. These results suggest that there is no significant Vs velocity change in consequently layered material and that the uppermost sedimentary layers in the surveyed down town area are relatively thin. These observations are in agreement with the mapping of limestone terraces throughout Port Vila area. However, both the surface geology and results from seismic zonation indicate a thicker (up to several tens of meters) sedimentary cover around the Bauerfield airport and in the Mele terrace zone. Low resonance frequencies (around and below 1 Hz) and amplification factor of the order of 5 were observed over this large area, immediately outside Port Vila. Any building development in this area should take these results into account.

Quantification of vertical movements in the eastern Betics (Spain) by comparing levelling data

Comparison of precision levelling data carried out at different times is a technique that allows the quantification of recent vertical movements. We used the precision levelling data from the Instituto Geográfico Nacional of Spain to improve our understanding of the recent tectonic evolution of some areas in the southeastern Iberian Peninsula (eastern Betics), where significant deformation has occurred during the last century. The resulting movements are presented in recent vertical movement profiles. Profiles with significant movements are discussed, taking into account the geologic and seismic features of each zone. These profiles are: Alicante–Albacete, Alicante–Almerı́a, Larva (near Jaén)–Almerı́a and Almerı́a–El Palo (near Málaga). The results obtained point to the existence of important tectonic deformation rates near the main tectonic structures crossing the profiles. The main tectonic anomalies expressed in constant rates of vertical velocity are: a 2 mm/year step near the ENE–WSW-trending Valldigna–Jumilla fault zone (between Alicante and Albacete); a 0.9 mm/year step located in the NNE–SSW-trending Cocón–Terreros fault zone (between Murcia and Almerı́a); a subsidence of about 1.5 mm/year of the Almerı́a Basin zone that can be related to NW–SE and E–W faults; a step of 1 mm/year related to the ENE–WSW-trending Guadahortuna fault (between Granada and Jaén); a 1 mm/year step associated with the ENE–WSW-trending Cádiz–Alicante fault zone (near the Granada area); and a 1.4 mm/year step related to the tectonic activity of the NW–SE- and NE–SW-trending system of faults that affect the Iberian coast of the Alborán Sea (Almerı́a and Granada zone). The anomalies related to the ENE–WSW- to E–W-trending structures are associated with reverse faults, whereas the anomalies related to the faults oriented in a NE–SW or NW–SE direction are associated with strike-slip faults with a normal component of vertical movement. These movements are consistent with the present-day Betics stress tensor determined with different methodologies, such as the focal mechanism analysis and microstructural and geomorphological studies, that indicate the existence of a maximum horizontal compression in a NNW–SSE direction together with a perpendicular extension. If we consider the rates of vertical deformation to be constant in time and if we consider that all the accumulated stress results in big earthquakes ( M≅6.0) with no ductile deformation, an estimate of the minimum return periods for major earthquakes can be made. The return periods, which range between 500 and 1200 years, depending on the zone, should be regarded as new information in the assessment of seismic hazard. These values are in agreement with those deduced from the seismic catalogues of the areas where there is sufficient information.

Probability of a great earthquake to recur in the Tokai district, Japan: reevaluation based on newly-developed paleoseismology, plate tectonics, tsunami study, micro-seismicity and geodetic measurements

In light of newly-acquired geophysical information about earthquake generation in the Tokai area, Central Japan, where occurrence of a great earthquake of magnitude 8 or so has recently been feared, probabilities of earthquake occurrence in the near future are reevaluated. Much of the data used for evaluation here relies on recently-developed paleoseismology, tsunami study and GPS geodesy. The new Weibull distribution analysis of recurrence tendency of great earthquakes in the Tokai-Nankai zone indicates that the mean return period of great earthquakes there is estimated as 109 yr with a standard deviation amounting to 33 yr. These values do not differ much from those of previous studies (Rikitake, 1976, 1986; Utsu, 1984). Taking the newly-determined velocities of the motion of Philippine Sea plate at various portions of the Tokai-Nankai zone into account, the ultimate displacements to rupture at the plate boundary are obtained. A Weibull distribution analysis results in the mean ultimate displacement amounting to 4.70 m with a standard deviation estimated as 0.86 m. A return period amounting to 117 yr is obtained at the Suruga Bay portion by dividing the mean ultimate displacement by the relative plate velocity. With the aid of the fault models as determined from the tsunami studies, the increases in the cumulative seismic slips associated with the great earthquakes are examined at various portions of the zone. It appears that a slip-predictable model can better be applied to the occurrence mode of great earthquakes in the zone than a time-predictable model. The crustal strain accumulating over the Tokai area as estimated from the newly-developed geodetic work including the GPS observations is compared to the ultimate strain presumed by the above two models. The probabilities for a great earthquake to recur in the Tokai district are then estimated with the aid of the Weibull analysis parameters obtained for the four cases discussed in the above. All the probabilities evaluated for the four cases take on values ranging 35–45 percent for a ten-year period following the year 2000.

Seismic hazard in Fennoscandia: A regionalized study

A seismic hazard evaluation has been conducted for Fennoscandia, including Norway, Sweden, Finland, Denmark, areas of Russia, and adjacent sea areas. Several subregions were considered. Data were taken from a Fennoscandian earthquake catalog comprising both macroseismic and instrumental observations. The analysis was based on a maximum likelihood approach using incomplete historical and complete instrumental data, different detection thresholds, and assessment of earthquake magnitude uncertainty. The output for the selected subregions consists of the probabilities that a given magnitude will not be exceeded in a given interval of time and mean return periods for earthquakes of magnitudes exceeding a given value M L . The evaluated parameters are the maximum regional magnitude, Gutenberg-Richter b-value, and annual activity rate, all with standard errors. The obtained parameters correspond well to the characteristic seismicity features of inspected regions. However, discrepancies between the data used and the data model behind the applied method were observed: both the patterns of earthquake occurrence and the reporting of historical events tend to deviate from the model. Differences in output can be expected according to the decisions to deal with these discrepancies.

The Rudbar-Tarom earthquake of 20 June 1990 in NW Persia: Preliminary field and seismological observations, and its tectonic significance

Abstract The Rudbar-Tarom earthquake of 20 June 1990 in NW Persia ( M S 7.7, m b 6.4, M W 7.3) killed approximately 40,000 people, left more than 500,000 homeless, destroyed three cities (Rudbar, Manjil, and Lowshan) and 700 villages, and slightly damaged another 300 villages, in the densely populated area of the western Alborz mountains, southwest of the Caspian Sea. Both faulting and folding associated with the earthquake were observed in the epicentral area. Co-seismic surface faulting was associated with at least three main discontinuous, complex fault segments with a total length of more than 80 km. These three main fault segments are arranged in a right-stepping en-echelon pattern and are separated by gaps in the observed surface ruptures. Each segment has a strike of 95° to 120°, with oblique left-lateral and reverse motion on faults that are subvertical or have steep dips to the S or SSW. Maximum surface displacements were 60 cm horizontal (left lateral) and 95 cm vertical (south side up). Bedding-plane slip in a reverse or thrust sense was also observed at the surface following the earthquake and may have occurred during co-seismic folding. Seismic body waves of the mainshock confirm the left-lateral nature of the faulting and show that slip occurred in three main subevents that may correspond to the fault segments seen at the surface. The focal mechanism obtained from broadband body waves is almost pure strike slip, with little evidence of the vertical component of motion seen at the surface. There were numerous aftershocks, one of which had a thrust faulting mechanism, different from that of the mainshock. The complex system of surface faults on which the Rudbar-Tarom earthquake occurred had not previously been identified as active. The lack of obvious Quaternary scarps on the faults that moved in 1990 suggests that return periods for earthquakes of this type are relatively long; but this is difficult to quantify, as erosion rates are high. Although the Persian seismic network has insufficient coverage to locate small earthquakes with enough precision to identify active faults, there is much modern and historical seismicity in the area, and abundant geomorphological evidence of rapid active tectonic processes. Previous studies of focal mechanisms and active faulting had emphasized the reverse faulting and folding that are undoubtedly the main processes that build the Alborz mountain belt. The 1990 earthquake provided the first evidence that the belt also undergoes left-lateral shear. On the southern edge of the Alborz mountains the slip direction in earthquakes is approximately NE and occurs on oblique faults. We suspect that in the high Alborz, where the amount of shortening and the topography are greater, this oblique convergence between Persia and the southern Caspian Sea is partitioned into essentially pure reverse motion and left-lateral strike-slip.

Probabilistic assessment of earthquake hazard in Vietnam based on seismotectonic regionalization

The theory of extreme values is applied to the earthquake source zones of Vietnam, using the first and third Gumbel's asymptotic distributions. Data are taken from the earthquake catalogues of Vietnam for the period 1903–1988. For each source zone and for the two main parts of the territory, the characteristics of earthquake activity are estimated. Also the mean return periods for earthquakes of different magnitudes and the probability of their occurrences for a set of return periods are calculated. The highest value of the maximum earthquake magnitude ( M max = 7.0) is predicted for the Ma river source zone and for the whole northern part of Vietnam, within a return period of 123 years. For North Vietnam, the occurrence of earthquakes with magnitudes not less than 6.0 within 50 years is predicted with 90% probability, while for South Vietnam the occurrence of earthquakes with magnitudes exceeding 5.0 is predicted for the same time interval with about 80% probability. From the results it follows that the third distribution fits the observation much better than the first one.

Study of return periods of earthquakes in some Selected Indian and adjoining regions

Recurrence intervals for earthquakes of magnitudes from 5.0 to 8.5 have been worked out using Gumbers extreme value theory and compared with those determined by Gutenberg-Richter’s frequency-magnitude relationship for SIX regions, namely, (A) Hindukush, (B) Kashmir and Himachal Pradesh, (C) India-western Nepal border, (0) Nepal-Sikkim border, (E) Northeast India and (F) Andaman and Nicobar Islands. A data sample for the period 1962 to 1976 and another for a longer period have been used for the purpose and limitations of the results obtained are discussed, Gumbel's extreme value theory gives better estimates of the return period of the maximum magnitude earthquakes when data for longer period is taken. The recurrence intervals thus estimated for earthquakes of magnitude 8/6 for the six regions are 22/2, 203/10, 222/11, 160/9, 34/4 and 58/4 years respectively.

Discussion: Engineering applications of new probabilistic seismic ground-motion maps of Canada

Concerning the development of the earthquake design requirements of the National Building Code of Canada (NBCC) starting from 1956, the authors state Uzumeri et al. (1978) have described these and subsequent developments in NBCC seismic loading provisions up to the 1977 edition of the code (p. 671). This paper contains erroneous references to the 1970 edition of the NBCC and its Commentary No. 3, Earthquake loads (Ferahian 1970) as pointed out in my discussion of the paper (Ferahian 1979). For historic authenticity, if for nothing else, it should not have been quoted without simultaneously referencing the discussion. In the development of the 1970 seismic regionalization map (authors' Fig. 1 ), the parameter chosen was A , , , the seismic ground acceleration with an annual probability of exceedance of 1 in 100, corresponding to the earthquake with a 100 year return period. Apart from the reasons given elsewhere (Ferahian 1970, 1979), this earthquake was chosen because the instrumental recording of our earthquakes does not predate the turn of the century. Estimates were made for the A,, and A,, seismic ground accelerations but because of the uncertainties inherent in the study, only the former were given in Commentary No. 3 for some representative cities in Canada. It was believed then that the confidence limits in these estimates were such that even the A , , estimates could be out by a factor of 2 (Ferahian 1970), and the corresponding factor would be much higher for A 500 For the seismic regionalization maps of Canadaadopted for the 1985 NBCC-the authors have used the earthquake with a 10% probability of exceedance in 50 years, which for all practical purposes corresponds to the 500 year return period earthquake. They claim, without giving proof, that their estimates of the peak seismic accelerations and velocities corresponding to

Investigations of earthquake probabilities in the Indian Ocean seismic belts

Gumbel's extreme-value theory is used to estimate the probability of occurrence and average return periods for earthquakes in the Indian Ocean seismic belts. The nature of seismic activity, and annual and 50 year maximum magnitudes of earthquakes are also discussed. The earthquake occurrence model of autocorrelation lends support for the periodicity of the most probable earthquake in these belts. The percentage probability of recurrence of earthquakes of magnitude 8 and above has been estimated for the region mentioned.

Estimated earthquake Probabilities in the northeast India and Andaman-Nicobar islands

Gumbel's extreme values theory has been applied to estimate the probability of occurrence and return periods of the largest earthquakes in the northeast India and Andaman-Nicobar Islands The statistical model of Epstein and Lomnitz (1966) is discussed with reference to the Gumbel's extreme value theory. The mean lines of expected extremes based on 46 years data of yearly extreme values of earthquakes in the three regions are plotted separately and the mean return periods of the largest possible earthquakes with their probability of recurrence are estimated. The most probable largest earthquakes in the Richter scale that may occur in an year and in an interval of 50 years are also estimated and reported.

Earthquake Hazard in New England

The earthquake return periods (the mean interoccurrence time) and the probability of earthquake occurrences for southern New England are calculated from the available seismic data for the period 1725 through 1974. For this region the occurrence of larger earthquakes varied with time and the seismic activity was higher in the period 1725 through 1824 than in the next 100 years (1825 through 1924). This variation introduces large uncertainties into calculations of the earthquake hazard. The estimated return period, based on data covering the time period 1725 through 1974, for earthquakes in the southern New England area of intensity VI or greater is 25 years and for intensity VIII or greater is 130 years.