Effect of Aftershocks on Earthquake Hazard Estimation: An Example from the North Anatolian Fault Zone (original) (raw)
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Bulletin of the Seismological Society of America
Most probabilistic seismic-hazard analysis procedures require that at least three seismic source parameters be known, namely the mean seismic activity rate λ, the Gutenberg-Richter b-value, and the area-characteristic (seismogenic source) maximum possible earthquake magnitude m max . In almost all currently used seismic-hazard assessment procedures that utilize these three parameters, it is explicitly assumed that all three remain constant over time and space. However, closer examination of most earthquake catalogs has indicated that significant spatial and temporal variations existed in the seismic activity rate λ, as well as in the Gutenberg-Richter b-value. In this study, the maximum likelihood estimation of these earthquake hazard parameters considers the incompleteness of the catalogs, the uncertainty in the earthquake magnitude determination, as well as the uncertainty associated with the applied earthquake-occurrence models. The uncertainty in the earthquake-occurrence models is introduced by assuming that both the mean seismic activity rate λ and the Gutenberg-Richter b-value are random variables, each described by the gamma distribution. This approach results in the extension of the classic frequency-magnitude Gutenberg-Richter relation and the Poisson distribution of the number of earthquakes with their compounded counterparts . The proposed procedure was applied in the estimation of the seismicity parameters in an area that had experienced the strongest and most devastating earthquake in contemporary South African history, namely the 29 September 1969 M w 6.3 Ceres-Tulbagh event. In this example, it was shown that the introduction of uncertainty in the earthquake-occurrence model reduced the mean return periods, leading to an increase of the estimated seismic hazard. Additionally, this study confirmed that accounting for magnitude uncertainties had the opposite effect, that is, it brought about increases in the return periods, or, equivalently, a reduction of the estimated seismic hazard.
Bulletin of the Seismological Society of America, 1992
Most probabilistic seismic-hazard analysis procedures require that at least three seismic source parameters be known, namely the mean seismic activity rate λ, the Gutenberg-Richter b-value, and the area-characteristic (seismogenic source) maximum possible earthquake magnitude m max. In almost all currently used seismic-hazard assessment procedures that utilize these three parameters, it is explicitly assumed that all three remain constant over time and space. However, closer examination of most earthquake catalogs has indicated that significant spatial and temporal variations existed in the seismic activity rate λ, as well as in the Gutenberg-Richter b-value. In this study, the maximum likelihood estimation of these earthquake hazard parameters considers the incompleteness of the catalogs, the uncertainty in the earthquake magnitude determination, as well as the uncertainty associated with the applied earthquake-occurrence models. The uncertainty in the earthquake-occurrence models is introduced by assuming that both the mean seismic activity rate λ and the Gutenberg-Richter b-value are random variables, each described by the gamma distribution. This approach results in the extension of the classic frequency-magnitude Gutenberg-Richter relation and the Poisson distribution of the number of earthquakes with their compounded counterparts (Benjamin, 1968; Campbell, 1982, 1983). The proposed procedure was applied in the estimation of the seismicity parameters in an area that had experienced the strongest and most devastating earthquake in contemporary South African history, namely the 29 September 1969 M w 6.3 Ceres-Tulbagh event. In this example, it was shown that the introduction of uncertainty in the earthquake-occurrence model reduced the mean return periods, leading to an increase of the estimated seismic hazard. Additionally, this study confirmed that accounting for magnitude uncertainties had the opposite effect, that is, it brought about increases in the return periods, or, equivalently, a reduction of the estimated seismic hazard.
The earthquake magnitude is influenced by previous seismicity
Geophysical Research Letters, 2012
Seismic occurrence is characterized by clustering in space, time 3 and magnitude. Correlations between magnitudes of subsequent events have 4 been recently attributed to catalog incompleteness. Here we investigate the 5 effect of catalog completeness on the amplitude of magnitude correlations. 6 The analysis of two California regions with different levels of catalog accu-7 racy and different lower magnitude thresholds indicate that the amplitude 8 of correlations does not depend on catalog incompleteness. Conversely, cor-9 relations are controlled by the probability that two events belong to the same 10 mainshock-aftershock sequence. Numerical simulations of the ETAS model, 11
Bulletin of the Seismological Society of America, 2010
Aseismic deformation is an integral part of the earthquake process and may lead to systematic biases in the estimation of earthquake size, recurrence, and attendant strong ground motions in seismic hazard analyses founded on the geologic description of the locations, lengths, and slip rates of active faults. Observations are reviewed and presented to suggest that large earthquakes systematically rupture to increasingly greater depths below the seismogenic layer and that the portion of slip on faults accommodated by aseismic processes may be inversely related to the length of rupture expected to occur on them. If so, the expected seismic moment per unit area of earthquakes on mapped faults may be systematically overestimated as a function of rupture length when derived from regressions of seismic moment and aftershock area, and estimates of seismic moment rate derived from geologic measures of fault offset might be systematically overestimated as an inverse function of the length of rupture expected to recur on a fault.
Long aftershock sequences within continents and implications for earthquake hazard assessment
Nature, 2009
One of the most powerful features of plate tectonics is that the known plate motions give insight into both the locations and average recurrence interval of future large earthquakes on plate boundaries. Plate tectonics gives no insight, however, into where and when earthquakes will occur within plates, because the interiors of ideal plates should not deform. As a result, within plate interiors, assessments of earthquake hazards rely heavily on the assumption that the locations of small earthquakes shown by the short historical record reflect continuing deformation that will cause future large earthquakes 1 . Here, however, we show that many of these recent earthquakes are probably aftershocks of large earthquakes that occurred hundreds of years ago. We present a simple model predicting that the length of aftershock sequences varies inversely with the rate at which faults are loaded. Aftershock sequences within the slowly deforming continents are predicted to be significantly longer than the decade typically observed at rapidly loaded plate boundaries. These predictions are in accord with observations. So the common practice of treating continental earthquakes as steady-state seismicity overestimates the hazard in presently active areas and underestimates it elsewhere.
Anahtar Kelimeler: Türkiye, deprem tehlikesi, olasılık, beklenen maksimum magnitüd ÖZ Bu çalışmada, Türkiye'nin farklı sismik bölgeleri için geri dönüşüm periyodu, verilen bir zaman aralığındaki olası en büyük magnitüd ile verilen bir zaman aralığı ve magnitüd değeri için depremin oluşma olasılığı gibi deprem hazard parametreleri değerlendirilmiştir. Bu amaçla, deprem hazard parametrelerinin hesabı için gerekli olan değerler Gumbel I asimptotik dağılımı kullanılarak hesaplanmıştır. Çalışmada kullanılan veri KOERI ve ISC kataloglarından derlenmiştir. Sonuçlar, Kuzey Anadolu Fay zonunun orta kısmının gelecekte büyük bir depremin oluşumu için en olası bölge olduğunu göstermektedir. Bu sonuç, magnitüdü M≥7.0 olan bir deprem için en büyük değeri (% 92.3) gösteren olasılık haritası ile güçlü bir şekilde desteklenmektedir. Bu magnitüd değeri için ortalama geri dönüşüm periyodu bu bölgede en düşüktür (39 yıl). Ayrıca, bu bölge için gelecek 100 yıldaki olası en büyük depremin magnitüdü 7.5'i aşmaktadır.
Properties of the Aftershock Sequences of the 2003 Bingöl, M D = 6.4, (Turkey) Earthquake
Pure and Applied Geophysics, 2008
Aftershock sequences of the magnitude M W =6.4 Bingöl earthquake of 1 May, 2003 (Turkey) are studied to analyze the spatial and temporal variability of seismicity parameters of the b value of the frequency-magnitude distribution and the p value describing the temporal decay rate of aftershocks. The catalog taken from the KOERI contains 516 events and one month’s time interval. The b value is found as 1.49 ± 0.07 with Mc =3.2. Considering the error limits, b value is very close to the maximum b value stated in the literature. This larger value may be caused by the paucity of the larger aftershocks with magnitude M D ≥ 5.0. Also, the aftershock area is divided into four parts in order to detect the differences in b value and the changes illustrate the heterogeneity of the aftershock region. The p value is calculated as 0.86 ± 0.11, relatively small. This small p value may be a result of the slow decay rate of the aftershock activity and the small number of aftershocks. For the fitting of a suitable model and estimation of correct values of decay parameters, the sequence is also modeled as a background seismicty rate model. Constant background activity does not appear to be important during the first month of the Bingöl aftershock sequences and this result is coherent with an average estimation of pre-existing seismicity. The results show that usage of simple modified Omori law is reasonable for the analysis. The spatial variability in b value is between 1.2 and 1.8 and p value varies from 0.6 to 1.2. Although the physical interpretation of the spatial variability of these seismicity parameters is not straightforward, the variation of b and p values can be related to the stress and slip distribution after the mainshock, respectively. The lower b values are observed in the high stress regions and to a certain extent, the largest b values are related to Holocene alluvium. The larger p values are found in some part of the aftershock area although no slip occurred after the main shock and it is interpreted that this situation may be caused by the alluvium structure of the region. These results indicate that the spatial distribution in b and p values are generally related to the rupture mechanism and material properties of an aftershock area.
Soil Dynamics and Earthquake Engineering, 1994
This paper addresses the significance, for engineering decisions, of replacing the Poissonian by one-step memory models in describing the occurrence of large periodic earthquakes on fault segments along the plate boundaries. A one-step memory model with a lognormally distributed return period was chosen for the analysis. The constraint imposed on both probabilistic models is that, for a given magnitude interval, the median of the lognormally distributed return period is equal to the expected value of the exponentially distributed return period of the Poissonian model. The hypothetical geologic setting chosen for the analysis is in southern California. It consists of a set of faults, one of which exhibits strongly periodic behavior for larger events (a hypothetical segment of the San Andreas fault about 350 km long, for example) whose occurrence is modeled by either of the two probabilistic models, and of other faults for which the earthquakes form a Poissonian sequence of events. PSV spectral amplitudes, PSVLN and PSVEx for the model with a lognormally and exponentially distributed return period, were evaluated for five probabilities of exceedance (p = 0.01, 0.1, 0.5, 0-9 and 0.99) at a site on the fault and another site away from the fault. The significance of the difference between the PSV amplitudes calculated by the two models, is measured by the ratio of the two amplitudes (by factor f = PSVLN/PSVEx), and by their difference in terms of the overall uncertainty (by the factor J~ = PSVLN-97 98 M. L Todorovska same time, the factor fl in absolute value can be well beyond one for the most conservative estimates, but, in general, not more than one for the most nonconservative estimates. For sites away from the fault and closer to the Los Angeles metropolitan area, the difference in the predicted spectra by the two occurrence models is insignificant in terms of the defined criterion.
Maximum magnitudes in aftershock sequences in Taiwan
Journal of Asian Earth Sciences, 2013
In this work, Båth's Law, the b-value in Gutenberg-Richter Law (G-R Law) in the form of the 1/b relationship, and both the a-and b-values in the G-R Law were introduced in order to estimate maximum aftershock magnitudes of earthquake sequences in the Taiwan region. The averaged difference of magnitude between the mainshock and the maximum aftershock is 1.20, and is consistent with Båth's Law, however, with a large uncertainty. The large uncertainty implies that the difference may result from a variable controlled by other factors, such as the aftershocks number of an earthquake sequence and magnitude threshold for mainshock. With 1/b, since 86% of the earthquake sequences with a M P 6.0 mainshock follow this relationship, the upper bound of the maximum magnitude can be estimated for an earthquake sequence with a large mainshock. The a-and b-values in the G-R Law was also considered by evaluating maximum aftershock magnitudes. As there are low residuals between the model and the observations, the results suggest that the G-R Law is a good index for maximum aftershock magnitude determinations. In order to evaluate the temporal decays of maximum aftershock magnitudes, modified Omori's Law was introduced. Using the approaches mentioned above, the maximum magnitudes and the temporal evolution of an earthquake sequence could be modeled. Among them, the model of the G-R Law has the best fit with observations for most of earthquake sequences. It shows its feasibility. The results of this work may benefit seismic hazards mitigation in the form of rapid re-evaluations for short-term seismic hazards immediately following devastating earthquakes.