A discussion on corioli force effect and aftershock activity tendency of the <Emphasis Type="Italic">M</Emphasis>=8.1 Kunlun Mountain Pass earthquake on Nov. 14, 2001

Following the theory and definition of the Corioli force in physics, the Corioli force at the site of the M=8.1 Kunlun Mountain Pass earthquake on November 14, 2001, is examined in this paper on the basis of a statistical research on relationship between the Corioli force effect and the maximum aftershock magnitude of 20 earthquakes with M≥7.5 in Chinese mainland, and then the variation tendency of aftershock activity of the M=8.1 earthquake is discussed. The result shows: a) Analyzing the Corioli force effect is an effective method to predict maximum aftershock magnitude of large earthquakes in Chinese mainland. For the sinistral slip fault and the reverse fault with its hanging wall moving toward the right side of the cross-focus meridian plane, their Corioli force pulls the two fault walls apart, decreasing frictional resistance on fault plane during the fault movement and releasing elastic energy of the mainshock fully, so the maximum magnitude of aftershocks would be low. For the dextral slip fault, its Corioli force presses the two walls against each other and increases the frictional resistance on fault plane, prohibiting energy release of the mainshock, so the maximum magnitude of aftershocks would be high. b) The fault of the M=8.1 Kunlun Mountain earthquake on Nov. 14, 2001 is essentially a sinistral strike-slip fault, and the Corioli force pulled the two fault walls apart. Magnitude of the induced stress is about 0.06 MPa. After a comparison analysis, we suggest that the aftershock activity level will not be high in the late period of this earthquake sequence, and the maximum magnitude of the whole aftershocks sequence is estimated to be about 6.0.     

Hysteretic energy prediction method for mainshock-aftershock sequences

Structures located in seismically active regions may be subjected to mainshock-aftershock (MSAS) sequences. Strong aftershocks significantly affect the hysteretic energy demand of structures. The hysteretic energy, E_H,seq, is normalized by mass m and expressed in terms of the equivalent velocity, V_D,seq, to quantitatively investigate aftershock effects on the hysteretic energy of structures. The equivalent velocity, V_D,seq, is computed by analyzing the response time-history of an inelastic single-degree-of-freedom (SDOF) system with a varying vibration period subjected to 309 MSAS sequences. The present study selected two kinds of MSAS sequences, with one aftershock and two aftershocks, respectively. The aftershocks are scaled to maintain different relative intensities. The variation of the equivalent velocity, V_D,seq, is studied for consideration of the ductility values, site conditions, relative intensities, number of aftershocks, hysteretic models, and damping ratios. The MSAS sequence with one aftershock exhibited a 10% to 30% hysteretic energy increase, whereas the MSAS sequence with two aftershocks presented a 20% to 40% hysteretic energy increase. Finally, a hysteretic energy prediction equation is proposed as a function of the vibration period, ductility value, and damping ratio to estimate hysteretic energy for mainshock-aftershock sequences.     

Bayesian estimation of the Modified Omori Law parameters for the Iranian Plateau

The forecasting of large aftershocks is a preliminary and critical step in seismic hazard analysis and seismic risk management. From a statistical point of view, it relies entirely on the estimation of the properties of aftershock sequences using a set of laws with well-defined parameters. Since the frequentist and Bayesian approaches are common tools to assess these parameter values, we compare the two approaches for the Modified Omori Law and a selection of mainshock–aftershock sequences in the Iranian Plateau. There is a general agreement between the two methods, but the Bayesian appears to be more efficient as the number of recorded aftershocks decreases. Taking into account temporal variations of the b-value, the slope of the frequency-size distribution, the probability for the occurrence of strong aftershock, or larger main shock has been calculated in a finite time window using the parameters of the Modified Omori Law observed in the Iranian Plateau.     

Determination of source parameters of strong earthquakes of Kamchatka,the Kurile Islands,and Japan from aftershock sequences

Aftershock sequences of some strong earthquakes of Kamchatka, the Kurile Islands, and Japan are examined. Such source parameters as the length L, along-dip width W, motion on fault D, and stress drop Δσ are determined from the aftershock sequences considered. The values of these parameters were obtained by the formal estimation of linear source parameters (lower bound estimates) and visually (upper bound estimates). The correlation dependences of the obtained parameters on the surface wave (M _S ) and seismic moment (M _W ) magnitudes are calculated.     

Probabilistic aftershock hazard analysis,two case studies in West and Northwest Iran

Aftershock hazard maps contain the essential information for search and rescue process, and re-occupation after a main-shock. Accordingly, the main purposes of this article are to study the aftershock decay parameters and to estimate the expected high-frequency ground motions (i.e., Peak Ground Acceleration (PGA)) for recent large earthquakes in the Iranian plateau. For this aim, the Ahar-Varzaghan doublet earthquake (August 11, 2012; M _N =6.5, M _N =6.3), and the Ilam (Murmuri) earthquake (August 18, 2014 ; M _N =6.2) have been selected. The earthquake catalogue has been collected based on the Gardner and Knopoff (Bull Seismol Soc Am 64(5), 1363-1367, 1974) temporal and spatial windowing technique. The magnitude of completeness and the seismicity parameters (a,??b) and the modified Omori law parameters (P,??K,??C) have been determined for these two earthquakes in the 14, 30, and 60 days after the mainshocks. Also, the temporal changes of parameters (a,??b,??P,??K,??C) have been studied. The aftershock hazard maps for the probability of exceedance (33%) have been computed in the time periods of 14, 30, and 60 days after the Ahar-Varzaghan and Ilam (Murmuri) earthquakes. For calculating the expected PGA of aftershocks, the regional and global ground motion prediction equations have been utilized. Amplification factor based on the site classes has also been implied in the calculation of PGA. These aftershock hazard maps show an agreement between the PGAs of large aftershocks and the forecasted PGAs. Also, the significant role of b parameter in the Ilam (Murmuri) probabilistic aftershock hazard maps has been investigated.     

New Data on Aftershocks of the December 7, 1988, Spitak Earthquake

The paper addresses the collection and analysis of new data on aftershocks that occurred within 20 days of the main shock of the December 7, 1988, Spitak earthquake, Mw = 6.8. The data were used to improve the location of aftershock hypocenters and magnitudes. Available data concerning this 20-day period were the least reliable in terms of completeness, representativeness, and the accuracy of hypocenter location and, in particular, estimation of energy classes and magnitudes. New data were retrieved from the records and bulletins of the seismic stations of the regional and global networks. Hypocenter parameters were determined by means of the minimization of wave travel-time residuals and subsequent double-difference hypocenter relocation. Digital records of the Obninsk and Arti seismic stations (Δ = 15°–18°) and five more distant stations (Δ = 34°–53°) were used to more accurately estimate the surface-wave magnitude of the main shock and strongest aftershock. The aftershock catalog of the Spitak earthquake was substantially revised. First, the previous hypocenter locations (Aref’ev et al., 1991) were improved using the double-difference method; second, new data were retrieved from the bulletins of Caucasian seismic stations. The minimum magnitude of completeness (M _c = 1.9) of the new catalog for the first 20 days after the main shock (when there were no epicentral observations) is the same as that for the period from December 7, 1988, to December 31, 1989. The new catalog contains information on 2090 aftershocks with magnitude M = 1.9 and more for the period from December 7, 1988, to December 31, 1989. The double-difference method allowed the location of the epicenters of clustered earthquakes to be reliably estimated with a longitude error of no more than 4.6 km, a latitude error of 4 km, and a depth error of 5 km. The new spatial distribution of the aftershock hypocenters is better correlated with the tectonic setting than the old data. The new catalog can be used to assess seismic hazard after strong earthquakes in the region.     

Dynamics of Induced Seismicity during the Filling of the Nurek Reservoir

The seismological data in the area of induced seismicity in the region of the Nurek reservoir are analyzed. The analysis is based on the developed database for the earthquakes that occurred from 1955 to 1989 and is aimed at finding the regularities in the variations of the parameters of the transitional seismic regime caused by filling a reservoir. These parameters include the b-value—the slope of the graph of the Gutenberg–Richter frequency–magnitude relationship, the fractal dimension d of the set of the epicenters, and fracture cycle parameter q = αb ? d, where coefficient α determines the ratio between the magnitude and source size M = α log l + β. It is shown that during the filling of a reservoir, these parameters undergo statistically reliable variations: at the initial stages, the b-value increases, the fractal dimension of the set of epicenters decreases, and the fracture cycle parameter q grows and becomes positive in the middle of the time interval of reservoir filling. After a reservoir is filled, these parameters recover their background values. The aftershock sequences of the three strongest earthquakes—before, in the beginning, and in the middle of the reservoir filling period—are studied. It is confirmed that the Omori parameter p for the aftershock sequences during filling is smaller than for the earthquake before filling. Based on the dynamics of the studied parameters, it is conjectured that the relaxation time of the transitional seismic regime after the emergence of induced seismicity is about 10 years.     

Scaling Analysis of Time Distribution between Successive Earthquakes in Aftershock Sequences

The earthquake inter-event time distribution is studied, using catalogs for different recent aftershock sequences. For aftershock sequences following the Modified Omori’s Formula (MOF) it seems clear that the inter-event distribution is a power law. The parameters of this law are defined and they prove to be higher than the calculated value (2–1/p). Based on the analysis of the catalogs, it is determined that the probability densities of the inter-event time distribution collapse into a single master curve when the data is rescaled with instantaneous intensity, R(t; M _th ), defined by MOF. The curve is approximated by a gamma distribution. The collapse of the data provides a clear view of aftershock-occurrence self-similarity.     

Assessment of the Relative Largest Earthquake Hazard Level in the NW Himalaya and its Adjacent Region

In the present study, the level of the largest earthquake hazard is assessed in 28 seismic zones of the NW Himalaya and its vicinity, which is a highly seismically active region of the world. Gumbel’s third asymptotic distribution (hereafter as GIII) is adopted for the evaluation of the largest earthquake magnitudes in these seismic zones. Instead of taking in account any type of M_max, in the present study we consider the ω value which is the largest earthquake magnitude that a region can experience according to the GIII statistics. A function of the form Θ(ω, RP_6.0) is providing in this way a relatively largest earthquake hazard scale defined by the letter K (K index). The return periods for the ω values (earthquake magnitudes) 6 or larger (RP_6.0) are also calculated. According to this index, the investigated seismic zones are classified into five groups and it is shown that seismic zones 3 (Quetta of Pakistan), 11 (Hindukush), 15 (northern Pamirs), and 23 (Kangra, Himachal Pradesh of India) correspond to a “very high” K index which is 6.     

Force reduction factors for steel buildings with welded and post-tensioned connections

The seismic responses of steel buildings with perimeter moment resisting frames (MRF) with welded connections (WC) are estimated and compared to those of similar buildings with semi-rigid post-tensioned connections (PC). The responses are estimated in terms of ductility reduction factors (R _µ,), ductility demands (µ _G ) and force reduction factors (R). Two steel model buildings, which were modeled as complex-3D-MDOF systems, were used in the study. Results indicate that the reduction magnitude of global response parameters is larger than that of local response parameters, contradicting the same reduction implicitly assumed in the static equivalent lateral force procedure, implying that non-conservative design may result. The value of 8 for R, suggested in many codes for ductile steel MRF, and the value of 1 suggested in the well known Newmark and Hall procedure for the ratio of R to µ _G , cannot be justified. The reason for this is that SDOF systems were used to model actual structures, where higher mode effects, energy dissipation and structural overstrength weren’t explicitly considered. The codes should be more transparent regarding the magnitude and the components involved in the force reduction factors. The seismic performance of steel buildings with PC may be superior to that of the buildings with WC, since their force reduction factors are larger and their ductility demands smaller, implying that PC buildings could be designed for smaller lateral seismic forces. The conclusions of this paper are for the particular structural systems and models considered. Much more research is needed to reach more general conclusions.     

A study on the focal parameters of Shidian <Emphasis Type="Italic">M</Emphasis>=5.9 earthquake sequences in 2001

On the basis of about 300 earthquake wave forms observed in the Shidian M _S=5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear wave have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5–5.3, the seismic moments are 10¹⁰–10¹⁶ N·m, the corner frequencies are 0.2–0.8 Hz, radii of the focal rupture are 200–2 500 m and the stress drops are 0.1×10⁵–20×10⁵Pa. Through the statistical analyses of variation of corner frequency f _c and stress drop Δσ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. Contrarily, the average stress drops Δσ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of average corner frequency and stress drops before and after the main shock have index significance to the precursory information before a strong earthquake. The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses.     

Precursor times of abnormal <Emphasis Type="Italic">b</Emphasis>-values prior to mainshocks

Seismic observations exhibit the presence of abnormal b-values prior to numerous earthquakes. The time interval from the appearance of abnormal b-values to the occurrence of mainshock is called the precursor time. There are two kinds of precursor times in use: the first one denoted by T is the time interval from the moment when the b-value starts to increase from the normal one to the abnormal one to the occurrence time of the forthcoming mainshock, and the second one denoted by T _p is the time interval from the moment when the abnormal b-value reaches the peak one to the occurrence time of the forthcoming mainshock. Let T* be the waiting time from the moment when the abnormal b-value returned to the normal one to the occurrence time of the forthcoming mainshock. The precursor time, T (usually in days), has been found to be related to the magnitude, M, of the mainshock expected in a linear form as log(T)?=?q?+?rM where q and r are the coefficient and slope, respectively. In this study, the values of T, T _p , and T* of 45 earthquakes with 3?≤?M?≤?9 occurred in various tectonic regions are compiled from or measured from the temporal variations in b-values given in numerous source materials. The relationships of T and T _p , respectively, versus M are inferred from compiled data. The difference between the values of T and T _p decreases with increasing M. In addition, the plots of T*/T versus M, T* versus T, and T* versus T-T* will be made and related equations between two quantities will be inferred from given data.     

Seismicity study in Pournari reservoir area (W. Greece) 1981–2010

The spatial-temporal evolution of seismicity is examined, during the initial impoundment of Pournari reservoir located on Arachthos River (Western Greece), as well as for the next 30 years. The results show that, despite the relatively moderate-to-high seismicity from west to east, there is no remarkable earthquake in the vicinity before the first reservoir impoundment. Immediately after the impoundment (January 1981), and during the first 4 months, a considerable number of low-magnitude seismic events were recorded in the broader area of the dam. Moreover, two independent major events occurred on March 10, 1981 (M _L ?=?5.6) and April 10, 1981 (M _L ?=?4.7) with focal depths 13 and 10 km, respectively. The detailed analysis of the two corresponding aftershock sequences shows that they present different behaviors (e.g., larger b-value and lower magnitude of the main aftershock) than that of other aftershock sequences in Greece. This seismicity is probably due to triggering, via the water loading mechanism and the undrained response due to a flysch appearance on the reservoir basement. The activation of the thrust fault may be attributed to the bulging of evaporites that characterize the disordered structure of W. Greece, via possible water intake. The detailed processing of the recorded seismicity during the period 1982–2010, in comparison with the variations of Pournari Dam water level, shows an increase of shallow seismicity (h?≤?5 km) in the vicinity of the reservoir up to a 10-km distance—in contrast to the initial period, characterized by a number of deeper events due to the background response change from undrained to drained status.     

Changes in source parameters of foreshocks and aftershocks of the 2001 <Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>=6.0 Yajiang,Sichuan, earthquake

In this paper changes in focal mechanisms, parameters of wave spectra, and stress drops for the M _S=5.0 foreshock and M _S=6.0 mainshock in February 2001 in Yajiang County, Sichuan, and seismicity in epicentral region are studied. Comparison of focal mechanisms for the Yajiang earthquakes with distribution patterns of aftershocks, the nodal plane I, striking in the direction of NEN, of the Yajiang M=5.0 event is chosen as the faulting plane; the nodal plane II, striking in the direction of WNW, of the M=6.0 event as the faulting plane. The strikes of the two faulting planes are nearly perpendicular to each other. The level of stress drops in the epicentral region before the occurrence of the M=6.0 earthquake increases, which is consistent with increase of seismicity in the epicentral region. The rate decay of the Yajiang earthquake sequence, changes in wave spectra for foreshocks and aftershocks, and focal mechanisms are complex.     

基于Gutenberg-Richter定律快速估算最大余震震级:以2017年九寨沟MS7.0地震为例

针对九寨沟M_S7.0地震之后不同时间段的余震序列目录,利用推定最大余震震级,给出了实际最大余震震级的估计值。结果表明,推定最大余震震级随主震后时间尺度的延长而趋于稳定,且该值与实际发生的最大余震的震级一致。需要强调的是,就九寨沟地震序列而言,当余震数据较为完备时,采用主震后较短时间段内（1～2天）的余震目录就可以较准确地估算出主震区域内可能发生的最大余震震级。实际上,主震后12h（0.5天）的余震数据已完全可以给出最大余震震级的有效下限。此外,计算中我们采用了里氏震级M_L和面波震级M_S的余震目录,结果显示,2种震级类型目录的估算结果完全一致,表明利用推定最大余震震级估算实际最大余震震级的方法不受震级类型的影响。据此,该最大余震震级快速评估方法可进一步推广应用于我国大陆地区中强震后强余震灾害分析评估中。目前的拟合技术也显示出随着测震技术的不断进步以及余震识别能力的提高,快速评估方法可以在主震后短时间（<1天）内准确地预测可能发生的最大余震震级。     

The Tolud Burst of Seismicity and the Earthquake of November 30, 2012 (<Emphasis Type="Italic">M</Emphasis><Subscript>C</Subscript> = 5.4, <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript> = 4.8) that Accompanied the Start of the 2012‒2013 Tolbachik Eruption

This paper reports a study of the Tolud earthquake sequence; the sequence was a burst of shallow seismicity between November 28 and December 7, 2012; it accompanied the initial phase in the Tolbachik Fissure Eruption of 2012?2013. The largest earthquake (the Tolud earthquake of November 30, 2012, to be referred to as the Tolud Earthquake in what follows, with K_S = 11.3, M_L = 4.9, M_C = 5.4, and M_W = 4.8) is one of the five larger seismic events that have been recorded at depths shallower than 10 km beneath the entire Klyuchevskoi Volcanic Cluster in 1961?2015. It was found that the Tolud earthquake sequence was the foreshock–aftershock process of the Tolud Earthquake. This is one of the larger seismicity episodes ever to have occurred in the volcanic areas of Kamchatka. Data of the Kamchatka seismic stations were used to compute some parameters for the Tolud Earthquake and its largest (M_L = 4.3) aftershock; the parameters include the source parameters and mechanisms, and the moment magnitudes, since no information on these is available at the world seismological data centers. The focal mechanisms for the Tolud Earthquake and for its aftershock are consistent with seismic ruptures at a tension fault in the rift zone. Instrumental data were used to estimate the intensity of shaking due to the Tolud Earthquake. We discuss the sequence of events that was a signature of the time-dependent seismic and volcanic activity that took place in the Tolbachik zone in late November 2012 and terminated in the Tolud burst of seismicity. Based on the current ideas of the tectonics and magma sources for the Tolbachik volcanic zone, we discuss possible causes of these earthquakes.     

Simulation of spatial and temporal properties of aftershocks by means of the fiber bundle model

The rupture processes of any heterogeneous material constitute a complex physical problem. Earthquake aftershocks show temporal and spatial behaviors which are consequence of the heterogeneous stress distribution and multiple rupturing following the main shock. This process is difficult to model deterministically due to the number of parameters and physical conditions, which are largely unknown. In order to shed light on the minimum requirements for the generation of aftershock clusters, in this study, we perform a simulation of the main features of such a complex process by means of a fiber bundle (FB) type model. The FB model has been widely used to analyze the fracture process in heterogeneous materials. It is a simple but powerful tool that allows modeling the main characteristics of a medium such as the brittle shallow crust of the earth. In this work, we incorporate spatial properties, such as the Coulomb stress change pattern, which help simulate observed characteristics of aftershock sequences. In particular, we introduce a parameter (P) that controls the probability of spatial distribution of initial loads. Also, we use a “conservation” parameter (π), which accounts for the load dissipation of the system, and demonstrate its influence on the simulated spatio-temporal patterns. Based on numerical results, we find that P has to be in the range 0.06 < P < 0.30, whilst π needs to be limited by a very narrow range (0.60 < π < 0.66) in order to reproduce aftershocks pattern characteristics which resemble those of observed sequences. This means that the system requires a small difference in the spatial distribution of initial stress, and a very particular fraction of load transfer in order to generate realistic aftershocks.     

A new <Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">w</Emphasis></Subscript> estimation parameter for use in earthquake early warning systems

We propose a method that employs the squared displacement integral (ID2) to estimate earthquake magnitudes in real time for use in earthquake early warning (EEW) systems. Moreover, using τ _c and P _d for comparison, we establish formulas for estimating the moment magnitudes of these three parameters based on the selected aftershocks (4.0 ≤ M _s  ≤ 6.5) of the 2008 Wenchuan earthquake. In this comparison, the proposed ID2 method displays the highest accuracy. Furthermore, we investigate the applicability of the initial parameters to large earthquakes by estimating the magnitude of the Wenchuan M _s 8.0 mainshock using a 3-s time window. Although these three parameters all display problems with saturation, the proposed ID2 parameter is relatively accurate. The evolutionary estimation of ID2 as a function of the time window shows that the estimation equation established with ID2 ^Ref determined from the first 8-s of P wave data can be directly applicable to predicate the magnitudes of 8.0. Therefore, the proposed ID2 parameter provides a robust estimator of earthquake moment magnitudes and can be used for EEW purposes.     

Some Probabilistic and Statistical Properties of the Seismic Regime of Zemmouri (Algeria) Seismoactive Zone

Statistical tests have been used to adjust the Zemmouri seismic data using a distribution function. The Pareto law has been used and the probabilities of various expected earthquakes were computed. A mathematical expression giving the quantiles was established. The extreme values limiting law confirmed the accuracy of the adjustment method. Using the moment magnitude scale, a probabilistic model was made to predict the occurrences of strong earthquakes. The seismic structure has been characterized by the slope of the recurrence plot γ, fractal dimension D, concentration parameter K_sr, Hurst exponents H_r and H_t. The values of D, γ, K_sr, H_r, and H_t diminished many months before the principal seismic shock (M = 6.9) of the studied seismoactive zone has occurred. Three stages of the deformation of the geophysical medium are manifested in the variation of the coefficient G% of the clustering of minor seismic events.     

A novel non-iterative direct displacement-based seismic design procedure for self-centering buckling-restrained braced frame structures

Self-centering buckling-restrained braces (SCBRBs) were proposed recently to minimize residual deformation of the braces induced by yielding or buckling. Although earthquake resilience of structures equipped with the SCBRBs can be well achieved using displacement based designs (DBDs), previously proposed DBD procedures generally involve iterations. In this study, a novel direct displacement-based design method with a non-iterative procedure, named R–C_R DDBD, is proposed and applied to design of steel braced frame structures with SCBRBs. Unlike previously adopted DBD, the yield displacement does not need to be assumed initially in the proposed procedure. Instead, the yield strength and yield displacement are determined directly by the predetermined objective drift (ratio), using the relation of the strength reduction factor (R) and constant-strength inelastic displacement ratio spectra (C_R spectra), i.e. the R–C_R relation. Since the derived R–C_R relation is independent with the peak ground acceleration of the earthquake records when stiffness and strength degradation are not considered, the proposed procedure can be accurate for any seismic level. The R–C_R DDBD is supposed to begin with the knowledge of the seismic excitation level (according to the structure category, site classification and owner’s requirements) and the corresponding target drift; the end of the design is to obtain the cross sections of main frame members and all the bracing parameters. The result of two 7-story buildings designed according to the R–C_R DDBD procedure demonstrates that this procedure can be effective and fairly simple for practical seismic design.