Medium Earthquakes Research Articles

Natural hazard prevention and the mountain land risk reduction in the Western Carpathians

The Western Carpathians are located out of world main natural hazardous zones. Human casualties are related more to snow avalanches in connection with mountain hiking, some individuals yearly by flooding and rarely by forest fires. Economic lost about 0.1 to 0.2 %, exceptionally up to 0.8 % of the gross domestic product (GDP) proportionally to the Carpathian regions of particular countries. Natural disasters are linked, except of the above mentioned events, to infrequent small and medium scale earthquakes, landslides, and erosion. Records of the most harmful natural events are found in archives since the 16th century. Their systematic study and factor analysis started from the end of the 19th century, and protective measures and organization of impact mitigation developed during the 20th century to minimize the risk.

Wave Field Estimated by Borehole Recordings in the Reclaimed Zone of Tokyo Bay

We estimate the wave field in the reclaimed zone of Tokyo Bay by analyzing the three components of surface and downhole strong-motion accelerograms recorded at two borehole stations during five medium earthquakes. The target period range is between 0.1 and 2.0 sec. First, wave types of observed seismic motions are identified by calculating theoretical amplification of body waves and surface waves and cross-correlation functions between the surface and downhole recordings. Then, the horizontal components of the surface and downhole recordings are separated into S -wave and surface-wave accelerograms. Finally, 2D wave-propagation simulations for basin structures are performed to interpret the recordings. The conclusions are as follows. (1) Main seismic motions are composed of not only S waves, but also Love waves. Surface waves are found to be more dominant than S waves in the period range of more than 1.0 sec. (2) Love waves are more heavily amplified than S waves at the theoretical predominant periods of ground in the soft surficial deposits. (3) The large surface-wave amplification is excited by the surficial deposits, together with a deep basin. (4) In the surficial deposits, quality factors are found to be about 10 around the theoretical predominant periods of ground. (5) The wave-propagation simulations suggest strong heterogeneities near the ground surface and can explain observed seismic motions qualitatively.

Seismic ground motion relationship in southern China based on stochastic finite-fault model

The characteristics of seismic ground motions in southern China are difficult to determine statistically due to a lack of strong ground motion data. In this study, a stochastic finites-fault ground motion model was adopted to simulate the seismic ground motions at bedrock for southern China, based on parameters derived from small and medium earthquakes that have occurred in the region. From these, the response spectra was estimated. A set of ground motion attenuation relationships was then developed based on simulated peak ground motions and response spectral parameters through regression, which would be applicable for use in engineering practice. Through comparison, it was demonstrated that the proposed ground motion relationships are generally consistent with those obtained from other reported ground motion attenuation models for southern China.

Multi-resolution clustering analysis and 3-D visualization of multitudinous synthetic earthquakes

We use modern and novel techniques to study the problems associated with detection and analysis of multitudinous seismic events, which form the background for isolated great earthquakes. This new approach involves multivariate analysis of low and large magnitude events recorded in space over a couple of centuries in time. We propose here the deployment of the clustering scheme both for extracting small local structures and large-scale trends in synthetic data obtained from four numerically simulated models with: uniform properties (U), a Parkfield-type asperity (A), fractal brittle properties (F), and multi-size-heterogeneity fault zone (M). The mutual nearest neighbor (mnn) clustering scheme allows for extraction of multi-resolutional seismic anomalies in both the spatio-temporal and multi-dimensional feature space. We demonstrate that the large earthquakes are correlated with a certain pathway of smaller events. Visualization of the anomalies by using a recently introduced visualization package Amira reveals clearly the spatio-temporal relationships between clusters of small, medium and large earthquakes, indicating significant stress relaxation across the entire fault region. We demonstrate that this mnn scheme can extract distinct clusters of the smallest events, which precede and follow a singularly large magnitude earthquake. These clusters form larger spatio-temporal structures comprising a series of large earthquakes. The link between the large and medium magnitude events is not so clearly understood. Short-ranged correlations are dominated by strong spatio-temporal anomalies, thus reflecting the global seismic properties of the entire fault zone.

Seismic behaviour of knee braced frames

This paper describes the development of the concept of knee bracing for improving the seismic performance of steel frames. In a knee braced frame the main diagonal braces are connected to short knee elements which span diagonally across the beam–column joints. The knee elements act sacrificially, reducing the demands on the main structure during a small or medium earthquake, and can be replaced after the event. Results from a series of parametric analyses are presented, which demonstrate the potential benefits of the approach. However, careful design of the knee elements is required to ensure that they are able to absorb energy through repeated large plastic deformations without suffering collapse or instability. To optimise the design, an extensive programme of finite element modelling and experimental testing has been undertaken. It is shown that excellent performance can be achieved using standard hot-rolled sections, modified by the addition of web stiffeners to prevent localised buckling failure. Weakening of the knee elements so that they yield very early in an earthquake has potential benefits, but is shown to be unsafe as it promotes premature failure of the element.

Inversion of 3-D crustal P-wave velocity structure in Ningxia and its neighborhood by using direct, reflected and refracted waves

In this paper, simultaneous inversions for hypocentres and three-dimensional (3-D) crustral P-wave velocity structure of Ningxia and its neighborhood are performed, by using the data of direct P wave, reflected Pm and refracted Pn phases from the Moho discontinuity. The number of arrival-time applied to the inversion is 11 816 total, which are recorded by the Seismic Station Network of Ningxia from 1 107 local natural earthquakes happened since 1970. The LSQR algorithm with damping that has been developed in recent years to solve the large-scale sparse matrix is adopted. At different steps of iterative computing process, weighting is made on the actions of each earthquake at first and on each ray further, to aim at the inhomogeneous features of spatial distribution of earthquake and ray. Numerical analogue and computational results of actual multi-group data, and different iterative control processes, show that under a certain residual standard, there is obvious trade-off on the assignment of travel-time residuals between medium parameters and earthquakes parameters (especially the origin time and depth of earthquake), so the obtained solution is related to the selected initial values of medium parameters to some extent; but there are the basically same variation features on different results in the almost same area. That is the solution of surface layer has a close relation to the strata and landform. In Yinchuan basin there is obvious low velocity feature, and it coincides with the results of a man-made earthquake section crossing the region; tomographic patterns show that there are many low velocity regions in middle-lower crust, and it seems to have some connections between the positions of historical strong earthquakes and the low velocity region or the anomalous variations of velocity gradient.

Dynamic damage and its physical meaning as a seismicity status parameter

The dynamic damage D(t) is a comprehensive index on micro fissure of crust under ground. It presents the failure strength about some region and some period. It describes the mean stress strength or the limited damage degree of crust. Practically the mean micro failure strength, frequency and the concentrating degree of the micro fissure in time-space are presented in this formula. For a limited period, the curve slope will point out the change situation of g value which will increase or decrease. When the b value decreasing, g value will increase. When this situation is happening, the small and medium earthquake are increasing in some continuous Δt i periods. So the dynamic damage degree D(t) will increase with the increasing for both accumulating rate g and distribution magnitude. But when the b value decrease, D(t) will increase.

Regional characteristics of stress field in the southern part of the north-south seismic belt in China and its relation with plate movement

The regional characteristics of stress field in the southern part of the north-south seismic belt (NSB) have been analyzed in detail based on the mechanism solutions of 134 medium and large earthquakes from 1933 to 1991. The results show that the southern part of the NSB is a shallow earthquake zone where most earthquakes are caused by the strike-slip faulting. There is a systematic distribution of the directions ofP- andT-axes in the western and the eastern regions of the southern part of the NSB.P- andT-axes in the western region are in the NE-SW direction and in the NW-SE direction.P- andT-axes in the eastern region are oriented in NW-SE and NE-SW, respectively. The directions ofP-axes in the western and the eastern regions show a pattern of a reversal “V” as a whole. The boundary between the eastern and western regions coincides with that between the Tibetan Plateau and the Yangtze crustal block. Based on a lot of mechanism solutions, the result indicates that the direction ofP-axes roughly shows the consistent distributions from the Himalayan collision zone to the eastern region and from the eastern coast collision zone in Taiwan to the eastern region of southern part of the NSB, respectively. It is suggested that the tectonic force due to relative movement between the Indo-Australian and the Eurasian plates is transmitted from the Himalayan collision zone to the western region of the southern part of NSB, simultaneously, the tectonic force due to the relative movement between the Philippine Sea and the Eurasian plates is transmitted from the eastern region coast in Taiwan to the eastern region of the southern part of NSB, and control the stress field there, respectively.

表層地盤の影響を取り除いた工学的基礎波の統計的スペクトル特性 : 仙台地域のボアホールで観測された多数の中小地震記録を用いた解析

表層地盤の影響を取り除いた工学的基礎波の統計的スペクトル特性 : 仙台地域のボアホールで観測された多数の中小地震記録を用いた解析

実用化したアクティブ・マス・ドライバ・システムの制震効果

A seismic-response-controlled structure was realized in Tokyo in August, 1989 for the first time in the world. Because this structure is a very slender ten-story office building, the Active Mass Driver (AMD) system has been installed to actively suppress vibrations caused by medium earthquakes and frequent winds. By June, 1990, the building had experienced about ten earthquakes, during which measurements were taken of the response of the structure and the control system. These measurements are simulated by a numerical analysis to represent the interaction between the structure and the control system. By comparing the uncontrolled responses calculated without the AMD System, the effectiveness of the AMD system is assessed.

実在建築物の 1 次の固有周期と減衰定数に関する研究

This paper discusses more quantitatively by means of various aspects on the first natural periods and damping coefficients obtained by vibration test, microtremors observations and seismic observations in actual buildings. Some of the significent aspects of the results of this paper are summarized as follows : 1) The experimental values are smaller than structural design values, the average rate of those values of SRC and S buildings are 60% and 80%, respectively. The seismic values are larger than the experimental values, the average rate of those values are about 20% against medium earthquake (70〜110 gal). 2) In the San Fernando Earthquake, the relation between the maximum acceleration on the top floor and the first natural periods (T_B) or the predominant periods (T_P) during the earthquake are obtained as follows, respectively : α_<mzx>=2.0×10^2T^<-0.45>_B…(7), α_<mzx>=2.4×10^2T^<-0.37>_B…(8) 3) The values of the predominant periods and first damping coefficients of actual buildings show a tendency to increase with the acceleration of the earthquake motion. 4) The empirical formura concerning the first natural periods of actual SRC and RC buildings are obtained as follows, respectively : SRC buildings : T(sec)=0.058N_1…(5) RC buildings : T(sec)=0.073(N+03N_0)…(6) in which N_1, N and N_0 represent number of stories of main structure part, above the G. L. and underground part, respectively.

Aseismic design of turbine houses for nuclear power plants

The turbine house does not belong to the safety-related parts of equipment of a nuclear power plant. A special protection against earthquakes is not demanded by the authorities as long as it is proven that safety-related parts of equipment will not be restricted in their function by a collapse of the turbine house. The degree of an aseismic design is largely up to the customer, who has to weigh the risk of costs and availability against the additional costs, that are necessary for the earthquake calculation and for constructive hardening. Low-tuned turbine foundations as a result of helical-spring-support, which are constructed by the KWU exclusively, pose special problems with the aseismic design. This will be discussed in particular in this paper. The spring-supported mass constitutes about a quarter of the building mass. For mechanical reasons the spring elements are chosen in such a way, that the turbine foundation has a natural frequency of about 3 Hz. Thus, it remains within the same frequency range as the turbine house and within that very range which is particularly amplified by an earthquake. It is therefore likely that resonance effects as well as oscillation annulment effects may occur. The standardized calculation methods for conventional buildings without safety function such as DIN 4149 (Germany) or SIA 162 (Switzerland) do not cover the oscillation conduct of such a complicated structure. One receives information about possible relative displacements between the building and the turbine foundation (hammering effect) and about the stresses on the turbine and other components only by dynamic calculation methods such as the time-history or the response spectrum method. Depending on the assumed earthquake (operating basis earthquake) the following constructive hardening measures will be necessary: 1. (1) Small earthquakes — reinforcement of the building structure, the concept of the building structure may be kept unchanged. 2. (2) Medium earthquakes — reinforcement of the building structure, slight alterations of the building concept; installation of damping elements at the spring assemblies; and reinforcement of the turbine foundation without changing the concept. 3. (3) Strong earthquakes — essential alteration of the building concept such as light steel construction or renunciation of the machine hall, additional shear-walls, deeper foundation level of the building; installation of damping elements at the spring assemblies; reinforcement of turbine bearings; and modified design of the turbine foundation.

Earthquake resistant design of cohesive earth slopes

As many major slope failures have occurred during medium and strong earthquakes, the assessment of the seismic stability of slopes in earthquake-prone areas is of great concern to engineers. Prior to 1964, little attention was given to the development of rational aseismic design methods for earth slopes. Most design methods were based on a static analysis used in conjunction with an arbitrarily selected lateral force acting on the slope with soil strengths determined by conventional laboratory tests. However, the catastrophic slope failures which occurred during the Alaskan earthquake of 1964, resulted in a considerable reappraisal of such static design methods. In recent years significant progress has been made in developing new methods of laboratory testing to determine dynamic soil properties, improved techniques of analysing the dynamic response of slopes and embankments to earthquakes, and new concepts of aseismic design methods for earth slopes. Although research has yet to provide all the answers, the current state of knowledge at least provides an improved guide to engineering judgement in the assessment of the stability of earth slopes during earthquakes.

Peligro Sísmico en el Bloque de Jalisco, México.

Historically large and very destructive earthquakes as well as tsunamis occurred along the Jalisco Block at the west part of Mexico. Using available historic data and recent seismic studies, we revise and reevaluate the localization of these earthquakes. We conclude that exists high seismic hazard, therefore it is necessary a systematic and continuous study of the seismicity along the area. The north shore of Jalisco present the highest seismic hazard, nevertheless we cannot ignore medium earthquakes (6.9

Post-elastic seismic response of a reinforced concrete frame

The two-phase philosophy of structural seismic design involves the choice of a structural system which will remain elastic under small or medium earthquakes and which will deform inelastically, but not catastrophically, in a major seismic disturbance.

Investigations on earth crust and nature of seismicity of Carpatho-Balkan Province

Earthquakes are grouped according to frequency (epicenters density), magnitude, and focal depth. Seismological, gravimetrical (Mohorovičić and Conrad surfaces), structural-geological data are correlated and superposed on maps and profiles. An earthquakes frequency map shows an orderly distribution of maxima along deep fractures, which divide the Carpatho-Balkan province into a mosaic of individualized, more or less independent, mobile blocks. Three types of earth crust are distinguished according to thickness: Mesian platform type (28-32 km) of reduced thickness, relatively high position of "basaltic" layer, shallow earthquakes (Mesian Platform and Burgas- Strandzha region merging into western Black Sea basin); Carpathian type (44-52 km) of greatest crust thickness, steeply plunging into the mantle under folded structures (Carpathians), or gently increasing under the Rhodopes median massif, shallow to deep earthquakes; Intermediate type (34-44 km) of shallow to medium earthquakes (Ciscarpathian foredeep, Balkans). The variable thickness of earth crust may, in certain cases, be the consequence of Modern (Alpine and later) vertical movements of isolated crust blocks; thus the unexpected thickness under the western Rhodopes (double the crust thickness under the Pannonian massif) would be due to the steady rise of this median massif. — B. V. Brajnikov

Epicentres of Earthquakes, 1913–1932*

THE Seismological Committee of the British Association has since 1911 made itself responsible, in a greater or less measure, for collating the records of earthquakes made by seismological stations all over the world. The work of computing the epicentres was first undertaken by Prof. John Milne at Shide, and on his death was taken over by Prof. H. H. Turner, who gave the International Seismological Summary† (formerly the Bulletin of the B.A. Seismological Committee) the form it now bears. Such determination of epicentres, based on least squares solutions of the recorded times of arrival of P waves at the various stations, now covers both accurately and completely all the medium and large earthquakes recorded in the period 1913-1932 inclusive. It is the purpose of the present note to direct attention to the geographical distribution of these epicentres.