From earthquake resistance structure to earthquake resilience city–urban seismic resilience assessment

Earthquakes pose significant perils to the built environment in urban areas.To avert the calamitous aftermath of earthquakes,it is imperative to construct seismic resilient cities.Due to the intricacy of the concept of urban seismic resilience(USR),its assessment is a large-scale system engineering issue.The assessment of USR should be based on the notion of urban seismic capacity(USC)assessment,which includes casualties,economic loss,and recovery time as criteria.Functionality loss is also included in the assessment of USR in addition to these criteria.The assessment indicator system comprising five dimensions(building and lifeline infrastructure,environment,society,economy,and institution)and 20 indicators has been devised to quantify USR.The analytical hierarchy process(AHP)is utilized to compute the weights of the criteria,dimensions,and indicators in the urban seismic resilience assessment(USRA)indicator system.When the necessary data for a city are obtainable,the seismic resilience of that city can be assessed using this framework.To illustrate the proposed methodology,a moderate-sized city in China was selected as a case study.The assessment results indicate a high level of USR,suggesting that the city possesses strong capabilities to withstand and recover from potential future earthquakes.

Seismic isolation design and resilience improvement of railway station considering the influence of near-fault pulse-like ground motions

To improve the resilience of railway stations,a typical station was selected as the research object,and an isolation design was introduced.Twenty-four groups of near-fault pulse-like ground motions were selected.The seismic resilience of the no-isolation railway stations(NIRS)and the isolation railway stations(IRS)were compared to provide a numerical result of the improvement in resilience.The results show that in the station isolation design,the station's functional requirements and structural characteristics should be considered and the appropriate placement of isolation bearings is under the waiting room.Under the action of a rare earthquake,the repair cost,repair time,rate of harm and death of the IRS were decreased by 8.04 million,18.30 days,6.93×10^(-3)and 1.21×10^(-3),respectively,when compared to the NIRS.The IRS received a seismic resilience grade of three-stars and the NIRS only one-star,indicating that rational isolation design improves the seismic resilience of stations.Thus,for the design of stations close to earthquake faults,it is suggested to utilize appropriate isolation techniques to improve their seismic resilience.

Influence of a non-free field subsurface seismic history on the liquefaction resistance of sandy soils

Seismic-induced liquefaction of sandy soils can fail foundations in the vicinity of buildings.To investigate the effect of a non-free field subsurface seismic history on the ability of saturated sandy soils to resist liquefaction,four shaking events with different accelerations were input to the sandy soils in the non-free-field.The results of the study revealed that:(1)Shallow soils that are not free-field undergo acceleration amplification effects after being subjected to seismic loading.(2)Building overburden pressure reduces the sensitivity of the shallow soils directly below in small and moderate earthquakes,which are more prone to rearranging and forming unstable structures under strong seismic effects.The excess pore pressure response on the load side resembles that of a free site,with the depth range of the liquefaction strength of soils affected by the seismic history,increasing progressively as input seismic intensity increases.(3)After experiencing earthquakes of different intensities,the excess pore pressure directly below the building overburden pressure at 0.1 m and 0.2 m is greater than that at the side.At the same time,the side of the building structure is more prone to liquefaction than the soil directly below it.

A logistic-Lasso-regression-based seismic fragility analysis method for electrical equipment considering structural and seismic parameter uncertainty

Damage to electrical equipment in an earthquake can lead to power outage of power systems.Seismic fragility analysis is a common method to assess the seismic reliability of electrical equipment.To further guarantee the efficiency of analysis,multi-source uncertainties including the structure itself and seismic excitation need to be considered.A method for seismic fragility analysis that reflects structural and seismic parameter uncertainty was developed in this study.The proposed method used a random sampling method based on Latin hypercube sampling(LHS)to account for the structure parameter uncertainty and the group structure characteristics of electrical equipment.Then,logistic Lasso regression(LLR)was used to find the seismic fragility surface based on double ground motion intensity measures(IM).The seismic fragility based on the finite element model of an±1000 kV main transformer(UHVMT)was analyzed using the proposed method.The results show that the seismic fragility function obtained by this method can be used to construct the relationship between the uncertainty parameters and the failure probability.The seismic fragility surface did not only provide the probabilities of seismic damage states under different IMs,but also had better stability than the fragility curve.Furthermore,the sensitivity analysis of the structural parameters revealed that the elastic module of the bushing and the height of the high-voltage bushing may have a greater influence.

Seismic design of variable cross-section damped steel support frame for post equipment

High-voltage electrical post equipment is generally installed on steel supports,which amplifies the seismic inputs and degrades the seismic performance of equipment.This study proposed a variable cross-section damped steel support frame(VCDFS)with viscous dampers to reduce seismic responses of both tall and low-rise electrical equipment.The VCDFS contains a trapezoidal damper layer to generate rocking motions,enabling the diagonal viscous dampers to dissipate seismic inputs.A theoretical model of post equipment with VCDFS is established,and an optimal design procedure is proposed.The analysis shows that the remaining static stiffness ratio λ_(k) is the key parameter that determines the effectiveness of the VCDFS.The VCDFS reduces the average displacement and stress response of a post insulator by 39.4%and 44.6%,respectively,together with a significant decrease in the dynamic amplification factor.Therefore,it is recommended to use the VCDFS instead of the conventional latticed-steel frame in earthquake zones.

Detailed seismic zoning of the East Kazakhstan region in the Republic of Kazakhstan

Kazakhstan is currently drafting new construction regulations that comply with the major provisions of the Eurocodes.Such regulations are created on the basis of seismic zoning maps of various degrees of detail,developed by our Institute of Seismology using a new methodological approach for Kazakhstan.The article is about creating the first normative map of the Detailed Seismic Zoning on a probabilistic foundation for the Republic of Kazakhstan’s East Kazakhstan region.We carried out the probabilistic assessment of seismic hazard using a methodology consistent with the main provisions of Eurocode 8and updated compared with that used in developing maps of Kazakhstan’s General Seismic Zoning and seismic microzoning of Almaty.The most thorough and current data accessible for the area under consideration were combined with contemporary analytical techniques.Updates have been done to not only the databases being used but also the way seismic sources were shown,including active faults now.On a scale of 1:1000000,precise seismic zoning maps of the East Kazakhstan region were created for two probabilities of exceedance:10%and 2%in 50 years in terms of peak ground accelerations and macroseismic intensities.The obtained seismic hazard distribution is generally consistent with the General Seismic Zoning of Kazakhstan’s previous findings.However,because active faults were included and a thoroughly revised catalog was used,there are more pronounced zones of increased danger along the fault in the western part of the region.In the west of the territory,acceleration values also increased due to a more accurate consideration of seismotectonic conditions.Zoning maps are the basis for developing new state building regulations of the Republic of Kazakhstan.

Potential of the Arkhangelsk seismic network for European Arctic monitoring

The Arkhangelsk Seismic Network(ASN)of the N.Laverov Federal Center for Integrated Arctic Research of the Ural Branch of the Russian Academy of Sciences,founded in 2003,includes 10 permanent seismic stations located on the coasts of the White,Barents,and Kara Seas and on the Arctic archipelagos of Novaya Zemlya,Franz Josef Land,and Severnaya Zemlya.The network is registered with the International Federation of Digital Seismograph Networks and the International Seismological Center.We used not only ASN data to process earthquakes but also the waveforms of various international seismic stations.The 13,000 seismic events were registered using ASN data for 2012-2022,and for 5,500 of them,we determined the parameters of the earthquake epicenters from the European Arctic.The spatial distribution of epicenters shows that the ASN monitors not only the main seismically active zones but also weak seismicity on the shelf of the Barents and Kara Seas.The representative magnitude of ASN was ML,rep=3.5.The level of microseismic noise has seasonal variations that affect the registration capabilities of each station included in the ASN and the overall sensitivity of the network as a whole.In summer,the sensitivity of the ASN decreased owing to the increasing microseismic and ambient noises,whereas in winter,the sensitivity of the ASN increased significantly because of the decrease.

The MDOF equivalent linear system and its applications in seismic analysis and design of framed structures

This paper reviews the applications of the multi degree-of-freedom(MDOF)equivalent linear system in seismic analysis and design of planar steel and reinforced concrete framed structures.An equivalent MDOF linear structure,analogous to the original MDOF nonlinear structure,is constructed,which has the same mass and elastic stiffness as the original structure and modal damping ratios that account for the effects of geometrical and material nonlinearities.The equivalence implies a balance between the viscous damping work of the equivalent linear structure and that of the nonlinearities in the original nonlinear structure.This work balance is established with the aid of a transfer function in the frequency domain.Thus,equivalent modal damping ratios can be explicitly determined in terms of the period and deformation levels of the structure as well as the soil types.Use of these equivalent modal damping ratios can help address a variety of seismic analysis and design problems associated with planar steel and reinforced concrete framed structures in a rational and accurate manner.These include force-based seismic design with the aid of acceleration response spectra characterized by high amounts of damping,improved direct displacement-based seismic design and the development of advanced seismic intensity measures.The equivalent modal damping ratios are also utilized in the context of linear modal analysis for the definition and construction of the MDOF response spectrum.Furthermore,the equivalent modal damping ratios are employed in a seismic retrofit method for steel-framed structures with viscous dampers.Finally,it is demonstrated that modal behavior(or strength reduction)factors can be easily constructed based on these modal damping ratios for a more rational and accurate force-based seismic design,including the determination of inelastic displacement profiles.

Complex of general seismic zoning maps OSR-2017 of Uzbekistan

The quantitative assessment of seismic hazard of Uzbekistan has been examined,and new maps of seismic zoning has been developed.Quasihomogeneous seismological areas and seism-generating zones are considered as seismic sources,based on the analysis of seismotectonic data.The seismological parametrization of seismic sources has been carried out,including determination of parameters of earthquake’s reoccurrence for different power levels,seismic potential and a predominant type motion in the source of occurring earthquakes.The reoccurrence parameters of seismic sources were both determined by directly from Gutenberg-Richter dependence law and using summation and distribution methods.Setting were conducted separately for a sample of strong(M≥5)and weak(M<5)earthquakes.The seismic potential Mmax of seismic active zones was determined by seismological and seismotectonic methods.The predominant type of movement in the faulting for each seismic source is reverse fault.Regional regularities of seismic intensity attenuation with distance of different power levels are determined.The seismic hazard is expressed in points of a macro seismic scale,in velocities and accelerations of ground motions,and it is characterized by calculated seismic intensity with the set probability(P=0.9,P=0.95,P=0.98 and P=0.99)not exceed within 50 years in the constructed maps.In seismic zoning map,the seismic intensity was in average soil conditions.The developed complex of maps considers a number of uncertainties of the input parameters in relation to both incompleteness of initial seismological and seismotectonic data,as well the probabilistic nature of seismic process and seismic intensity.The factors of uncertainty of the input parameters are taken into account by constructing a logic tree.The constructed maps of the general seismic zoning are intended for imple menting antiseismic actions in Uzbekistan.

SeisResoDiff: Seismic resolution enhancement based on a diffusion model

High resolution of post-stack seismic data assists in better interpretation of subsurface structures as well as high accuracy of impedance inversion. Therefore, geophysicists consistently strive to acquire higher resolution seismic images in petroleum exploration. Although there have been successful applications of conventional signal processing and machine learning for post-stack seismic resolution enhancement,there is limited reference to the seismic applications of the recent emergence and rapid development of generative artificial intelligence. Hence, we propose to apply diffusion models, among the most popular generative models, to enhance seismic resolution. Specifically, we apply the classic diffusion model—denoising diffusion probabilistic model(DDPM), conditioned on the seismic data in low resolution, to reconstruct corresponding high-resolution images. Herein the entire scheme is referred to as SeisResoDiff. To provide a comprehensive and clear understanding of SeisResoDiff, we introduce the basic theories of diffusion models and detail the optimization objective's derivation with the aid of diagrams and algorithms. For implementation, we first propose a practical workflow to acquire abundant training data based on the generated pseudo-wells. Subsequently, we apply the trained model to both synthetic and field datasets, evaluating the results in three aspects: the appearance of seismic sections and slices in the time domain, frequency spectra, and comparisons with the synthetic data using real well-logging data at the well locations. The results demonstrate not only effective seismic resolution enhancement,but also additional denoising by the diffusion model. Experimental comparisons indicate that training the model on noisy data, which are more realistic, outperforms training on clean data. The proposed scheme demonstrates superiority over some conventional methods in high-resolution reconstruction and denoising ability, yielding more competitive results compared to our previous research.

A reweighted damped singular spectrum analysis method for robust seismic noise suppression

(Multichannel)Singular spectrum analysis is considered as one of the most effective methods for seismic incoherent noise suppression.It utilizes the low-rank feature of seismic signal and regards the noise suppression as a low-rank reconstruction problem.However,in some cases the seismic geophones receive some erratic disturbances and the amplitudes are dramatically larger than other receivers.The presence of this kind of noise,called erratic noise,makes singular spectrum analysis(SSA)reconstruction unstable and has undesirable effects on the final results.We robustify the low-rank reconstruction of seismic data by a reweighted damped SSA(RD-SSA)method.It incorporates the damped SSA,an improved version of SSA,into a reweighted framework.The damping operator is used to weaken the artificial disturbance introduced by the low-rank projection of both erratic and random noise.The central idea of the RD-SSA method is to iteratively approximate the observed data with the quadratic norm for the first iteration and the Tukeys bisquare norm for the rest iterations.The RD-SSA method can suppress seismic incoherent noise and keep the reconstruction process robust to the erratic disturbance.The feasibility of RD-SSA is validated via both synthetic and field data examples.

Resilience-incorporated seismic risk assessment of precast concrete frames with“dry”connections

A resilience-incorporated risk assessment framework is proposed and demonstrated in this study to manifest the advantageous seismic resilience of precast concrete frame(PCF)structures with“dry”connections in terms of their low damage and rapid recovery.The framework integrates various uncertainties in the seismic hazard,fragility,capacity,demand,loss functions,and post-earthquake recovery.In this study,the PCF structures are distinguished from ordinary reinforced concrete frame(RCF)structures by characterizing multiple limit states for the PCF based on its unique damage mechanisms.Accordingly,probabilistic story-wise pushover analyses are performed to yield story-wise capacities for the predefined limit states.In the seismic resilience analysis,a step-wise recovery model is proposed to idealize the functionality recovery process,with separate considerations of the repair and non-repair events.The recovery model leverages the economic loss and downtime to delineate the stochastic post-earthquake recovery curves for the resilience loss estimation.As such,contingencies in the probabilistic post-earthquake repairs are incorporated and the empirical judgments on the recovery parameters are largely circumvented.The proposed framework is demonstrated through a comparative study between two“dry”connected PCFs and one RCF designed as alternative structural systems for a prototype building.The results from the risk quantification indicate that the PCFs show reduced loss hazards and lower expected losses relative to the RCF.Particularly,the PCF equipped with energy dissipation devices at the“dry”connections largely reduces the expected economic loss,downtime,and resilience loss by 29%,56%,and 60%,respectively,compared to the RCF.

The 2023 Turkey earthquake doublet: Earthquake relocation, seismic tomography, and stress field inversion

On February 6,2023,two earthquakes with magnitudes of M_(W) 7.8 and M_(W) 7.5 struck southeastern Turkey,causing significant casualties and economic losses.These seismic events occurred along the East Anatolian Fault Zone,a convergent boundary between the Arabian Plate and the Anatolian Subplate.In this study,we analyze the M_(W) 7.8 and M_(W) 7.5 earthquakes by comparing their aftershock relocations,tomographic images,and stress field inversions.The earthquakes were localized in the upper crust and exhibited steep dip angles.Furthermore,the aftershocks occurred either close to the boundaries of low and high P-wave velocity anomaly zones or within the low P-wave velocity anomaly zones.The East Anatolia Fault,associated with the M_(W) 7.8 earthquake,and the SürgüFault,related to the M_(W) 7.5 earthquake,predominantly experienced shear stress.However,their western sections experienced a combination of strike-slip and tensile stresses in addition to shear stress.The ruptures of the M_(W) 7.8 and M_(W) 7.5 earthquakes appear to have bridged a seismic gap that had seen sparse seismicity over the past 200 years prior to the 2023 Turkey earthquake sequence.

On electro-acoustic characteristics of a marine broadband sparker for seismic exploration

The construction of major marine infrastructure projects and the exploration and development of deep-sea mineral resources require fine imaging of seabed strata and structures.The highresolution marine seismic exploration based on a high broadband sparker source is an important approach to reveal seabed stratum and reservoir structure,and identify geohazard.To optimize the performance of sparker seismic source,we investigated the electro-acoustic characteristics of spark discharge under conditions of different charging voltages and electrode numbers.Results show that the sound source level increased with the increase of the charging voltage,whereas the main frequency decreased when the charging voltage increases.In addition,it was found that the charging capacitance had more obvious influence on the main frequency than the sound source level did.Although the load energy decreased with increasing electrode number,the sound source level still increased but the main frequency decreased.Meanwhile,the primary to bubble(P/B)ratio increased with the increase of the electrode number.To gain a deeper insight into the electro-acoustic characteristics,we investigate the relationship between sound source level and power peak,from which a good correlation was observed.A more practical statistical analysis on the rise rate of current was processed,and a perfect logarithmic function was derived.Furthermore,we found that the main frequency was most possibly subjected to the electrical energy,especially the charging energy per electrode.The results indicate that the charging energy per electrode less than 10 J could increase the main frequency to above 300 Hz.At last,the main frequency could be reduced to 20 Hz when the charging energy of a single-electrode discharge was enhanced to over 4 kJ.This study shall be helpful in developing a sparker seismic source and improving the performance for marine engineering exploration and geohazard assessment.

Hcable for Time-Lapse Seismic Monitoring of Marine Carbon Capture and Storage

To ensure project safety and secure public support, an integrated and comprehensive monitoring program is needed within a carbon capture and storage(CCS) project. Monitoring can be done using many well-established techniques from various fields, and the seismic method proves to be the crucial one. This method is widely used to determine the CO_(2) distribution, image the plume development, and quantitatively estimate the concentration. Because both the CO_(2) distribution and the potential migration pathway can be spatially small scale, high resolution for seismic imaging is demanded. However, obtaining a high-resolution image of a subsurface structure in marine settings is difficult. Herein, we introduce the novel Hcable(Harrow-like cable system) technique, which may be applied to offshore CCS monitoring. This technique uses a highfrequency source(the dominant frequency>100 Hz) to generate seismic waves and a combination of a long cable and several short streamers to receive seismic waves. Ultrahigh-frequency seismic images are achieved through the processing of Hcable seismic data. Hcable is then applied in a case study to demonstrate its detailed characterization for small-scale structures. This work reveals that Hcable is a promising tool for timelapse seismic monitoring of oceanic CCS.

Application of 9-component S-wave 3D seismic data to study sedimentary facies and reservoirs in a biogasbearing area:A case study on the Pleistocene Qigequan Formation in Taidong area,Sanhu Depression,Qaidam Basin,NW China

To solve the problems in restoring sedimentary facies and predicting reservoirs in loose gas-bearing sediment,based on seismic sedimentologic analysis of the first 9-component S-wave 3D seismic dataset of China,a fourth-order isochronous stratigraphic framework was set up and then sedimentary facies and reservoirs in the Pleistocene Qigequan Formation in Taidong area of Qaidam Basin were studied by seismic geomorphology and seismic lithology.The study method and thought are as following.Firstly,techniques of phase rotation,frequency decomposition and fusion,and stratal slicing were applied to the 9-component S-wave seismic data to restore sedimentary facies of major marker beds based on sedimentary models reflected by satellite images.Then,techniques of seismic attribute extraction,principal component analysis,and random fitting were applied to calculate the reservoir thickness and physical parameters of a key sandbody,and the results are satisfactory and confirmed by blind testing wells.Study results reveal that the dominant sedimentary facies in the Qigequan Formation within the study area are delta front and shallow lake.The RGB fused slices indicate that there are two cycles with three sets of underwater distributary channel systems in one period.Among them,sandstones in the distributary channels of middle-low Qigequan Formation are thick and broad with superior physical properties,which are favorable reservoirs.The reservoir permeability is also affected by diagenesis.Distributary channel sandstone reservoirs extend further to the west of Sebei-1 gas field,which provides a basis to expand exploration to the western peripheral area.

Optimal intensity measures for longitudinal seismic response of tunnels

To study the ground motion intensity measures(IMs)suitable for the design of seismic performance with a focus on longitudinal resistance in tunnel structures,21 different seismic intensity parameters are selected for nonlinear calculation and analysis of tunnel structures,in order to determine the optimal IM for the longitudinal seismic performance of tunnel structures under different site conditions.An improved nonlinear beam-spring model is developed to calculate the longitudinal seismic response of tunnels.The PQ-Fiber model is used to simulate the longitudinal nonlinear behavior of tunnel structures and the tangential interactions between the tunnel and the soil is realized by load in the form of moment.Five different site types are considered and 21 IMs is evaluated against four criteria:effectiveness,practicality,usefulness,and sufficiency.The results indicate that the optimal IMs are significantly influenced by the site conditions.Specifically,sustained maximum velocity(V_(SM))emerges as the optimal IM for circular tunnels in soft soil conditions(CaseⅠsites),peak ground velocity(V PG)is best suited for CaseⅡsites,sustained maximum acceleration(A_(SM))is ideal for both CaseⅢand CaseⅤsites,and peak ground acceleration(A PG)for CaseⅣsites.As site conditions transition from CaseⅠto CaseⅤ,from soft to hard,the applicability of acceleration-type intensity parameters gradually decreases,while the applicability of velocity-type intensity parameters gradually increases.

The forecasting efficiency under different selected regions by Pattern Informatics Method and seismic potential estimation in the North-South Seismic Zone

In 2022,four earthquakes with M_(S)≥6.0 including the Menyuan M_(S)6.9 and Luding M_(S)6.8 earthquakes occurred in the North-South Seismic Zone(NSSZ),which demonstrated high and strong seismicity.Pattern Informatics(PI)method,as an effective long and medium term earthquake forecasting method,has been applied to the strong earthquake forecasting in Chinese mainland and results have shown the positive performance.The earthquake catalog with magnitude above M_(S)3.0 since 1970 provided by China Earthquake Networks Center was employed in this study and the Receiver Operating Characteristic(ROC)method was applied to test the forecasting efficiency of the PI method in each selected region related to the North-South Seismic Zone systematically.Based on this,we selected the area with the best ROC testing result and analyzed the evolution process of the PI hotspot map reflecting the small seismic activity pattern prior to the Menyuan M_(S)6.9 and Luding M_(S)6.8 earthquakes.A“forward”forecast for the area was carried out to assess seismic risk.The study shows the following.1)PI forecasting has higher forecasting efficiency in the selected study region where the difference of seismicity in any place of the region is smaller.2)In areas with smaller differences of seismicity,the activity pattern of small earthquakes prior to the Menyuan M_(S)6.9 and Luding M_(S)6.8 earthquakes can be obtained by analyzing the spatio-temporal evolution process of the PI hotspot map.3)The hotspot evolution in and around the southern Tazang fault in the study area is similar to that prior to the strong earthquakes,which suggests the possible seismic hazard in the future.This study could provide some ideas to the seismic hazard assessment in other regions with high seismicity,such as Japan,Californi,Turkey,and Indonesia.

The seismicity in the middle section of the Altyn Tagh Fault system revealed by a dense nodal seismic array

The left-lateral Altyn Tagh Fault(ATF) system is the northern boundary of the Qinghai-Xizang Plateau, separating the Tarim Basin and the Qaidam Basin. The middle section of ATF has not recorded any large earthquakes since1598 AD, so the potential seismic hazard is unclear. We develope an earthquake catalog using continuous waveform data recorded by the Tarim-Altyn-Qaidam dense nodal seismic array from September 17 to November23, 2021 in the middle section of ATF. With the machine learning-based picker, phase association, location, match and locate workflow, we detecte 233 earthquakes with M_L-1–3, far more than 6 earthquakes in the routine catalog. Combining with focal mechanism solutions and the local fault structure, we find that seismic events are clustered along the ATF with strike-slip focal mechanisms and on the southern secondary faults with thrusting focal mechanisms. This overall seismic activity in the middle section of the ATF might be due to the northeastward transpressional motion of the Qinghai-Xizang Plateau block at the western margin of the Qaidam Basin.

Seismicity of Tehri Area of the Himalayan Region, India

A database for the seismicity of the Tehri region (29.5˚N - 31.5˚N and 77.5˚E - 79.5˚E) from November 1, 1853, to March 31, 1989, has been prepared using a Compatible Personal Computer System. The seismicity database is complete for events with mb ≥ 4.5 only since 1963. It is inferred that the general seismicity of the area is considerably low, which is associated with four main tectonic features identified based on the spatial distribution of events in the area. Earthquakes in the Tehri area is of shallow focus, and maximum seismic activity is confined in the region beyond 60 km east and northwest of Tehri. The cumulative Number of Events as a Function of Time (CNET) for the period from 1963 to 1988 has indicated that precursory swarms do not precede the medium-sized earthquakes of the Tehri area. However, the CNET curves for total events and those with mb ≥ 4.6 have indicated a sharp 2-fold seismicity rate increase from 1986 compared to the preceding period. The October 20, 1991 (IST) earthquake of mb = 6.5 of Uttarkashi is believed to be associated with this seismicity rate change. The continuous increasing trend of the CNET curve before 1986 has been attributed to the detection changes.