0 Introduction

High-speed railway (HSR) is operating with high speed and density in China, but earthquake disasters are extremely harmful to the operating safety of HSR. Therefore, there is an urgent need to establish an effective earthquake emergency response method for the organization and management of HSR safety monitoring, emergency response decision-making, post-event rescue, and recovery operations. At present, the domestic focus on emergency handling of HSR is mainly about the establishment of a traffic safety monitoring system, while the emergency handling of disasters and emergency rescue and recovery work after disasters are rarely studied. The practice of earthquake prevention and disaster reduction in foreign countries in the past decades shows that after a destructive earthquake, emergency rescue is not only an inevitable action to save lives and reduce disasters, but also a manifestation of the government's ability and the comprehensive national strength. Therefore, it is of great significance and has broad application prospects to study earthquake emergency response methods for HSR.

China has the world's largest HSR network. Not only has it formed an "eight vertical and eight horizontal" HSR network, but also it has built an operating HSR mileage accounting for more than 70% of the world's operating mileage. China is also a country prone to earthquakes. Earthquakes may damage the structures of railway lines, bridges, and tunnels, and their vibration excitation can also cause trains to derail or overturn, which seriously affects the safety of HSR operation. Therefore, the stability of the high-speed train system, the safety of operation, the reliability of the structure, and the comfort of riding have become the key technologies to develop for high-speed trains. At present, most foreign scholars focus on the research on the installation and management of HSR facilities, the selection of evacuation routes, emergency management after disaster, and the smooth flow of information. Sisiopiku^[1] and Kong^[2] used traffic simulation modeling for improving emergency preparedness planning. Cui et al.^[3], Perrin et al.^[4], and Liu et al.^[5] made use of risk assessment in design theories of HSR ballastless tracks. Chinese scholars mostly focus on the actual response measures and mechanisms after the HSR earthquakes and conduct research on the optimization and selection of rescue and evacuation channels. Wang et al.^[6] and Xu et al.^[7] researched on emergency plan for HSR earthquake disaster. Sun et al.^[8], Zhang et al.^[9], and Ma et al.^[10] set up the determination of emergency handling range of earthquake disaster prevention system for HSR. Nie et al.^[11], Song et al.^[12], and Deng et al.^[13] studied on a method of continuous prediction of magnitude for HSR earthquake early warning based on support vector machine. For example, Liu et al.^[14] conducted research on key issues such as the composition of the railway earthquake early warning system, the setting plan of monitoring equipment, and the alarm mode based on the investigation of the earthquake hazard of the Beijing-Shanghai HSR and the area along the line. The early warning level of mechanical seismographs is presented, and the MR discrimination standard suitable for P wave detection is proposed. This method requires a long decision-making time, but its effectiveness is high. Early warning is executed if the value of ground motion exceeds a given threshold. It neither distinguishes the phases of P-wave and S-wave, nor does it determine the relevant parameters of the earthquake, and its effectiveness is low. With the development of HSR construction, countries all over the world attach great importance to the seismic monitoring technology of high-speed trains. In Japan, France, Germany, and China's Taiwan region, HSRs are relatively more developed. Fabozzi et al.^[15], Sato et al.^[16], and Song et al.^[17] set up the prediction method of First-Level Earthquake Warning for HSR. Wang et al.^[18] and Zheng et al.^[19] analyzed time delay characteristics of earthquake early warning and emergency disposal for HSR. Zheng et al.^[19], Wang and Zhao^[20], and Yu et al.^[21] utilized the influence of trains on the seismic response of HSR structure under lateral uncertain earthquakes. Earthquake emergency response systems have been established for railway transportation to prevent or reduce the hazards of earthquakes to railway transportation safety. Take the Japanese Shinkansen as an example. Its control range is divided into two types: dynamic control range and fixed control range. To determine the basic parameter of earthquake early warning, the control range is set according to the magnitude and epicenter distance, and the threshold alarm method uses a fixed processing range. In China, with the rapid development of HSR and the substantial improvement of train speed, the threat of earthquake to train operation safety has also increased greatly. It is necessary to accelerate the research and development of HSR disaster prevention system and emergency automatic disposal system including earthquake disaster prevention^[22-28]. Therefore, on the basis of extensive reading and analysis of the existing research results of emergency treatment and dispatching technology at home and abroad, an in-depth research is carried out on the shortcomings of existing research on key technologies such as earthquake emergency treatment methods and emergency control for HSR to provide emergency management and control. Dispatching work not only provides efficient and feasible models and methods, but also decision support and basis for the formulation of HSR earthquake emergency plans. This paper combines China's national conditions and the characteristics of HSR, absorbs and draws on the successful experience and research results of foreign countries, and further studies the laws of earthquake occurrence and seismic wave transmission under different geographical and site conditions, as well as the safety of high-speed trains under earthquake response. Indicators are found to build an earthquake early warning system for the safe operation of HSRs.

1 Risk Assessment Model of HSR Earthquake Emergency Response

Based on the coupling of the earthquake monitoring system and the disaster monitoring system, the HSR earthquake monitoring system solves the problems of information sharing of the earthquake monitoring system of multiple lines, so that the trains on the adjacent lines or other HSR lines in the earthquake-affected area can take measures early to further improve operational safety. Based on the study of the current situation of earthquake hazards along China's HSRs, this paper constructs the principles of HSR seismic monitoring, monitoring point setting principles, and earthquake disaster emergency response mechanisms. After an HSR earthquake occurs, under the condition of uncertain basic earthquake parameters (e.g., earthquake magnitude, hypocenter location, and time of occurrence), an evaluation index system is constructed for HSR earthquake emergency response, the adaptability of HSR emergency response capabilities evaluation technology is studied, and uncertainty mathematical methods is used to establish an evaluation model for earthquake emergency response. According to the actual needs of HSR earthquake emergency handling capacity, the availability of data acquisition and the computability and other factors, the evaluation index system of HSR earthquake emergency response is given. The calculation method of evaluation index is given, and the corresponding evaluation system is proposed.

Earthquake parameter is a quantitative expression of seismic source characteristics based on seismic data analysis, which includes basic seismic parameters(e.g., epicenter latitude and longitude, focal depth, time of occurrence, earthquake magnitude or seismic energy), seismic mechanism solution and hypocenter dynamics parameters, etc. Therefore, earthquake parameter early warning uses the station's P wave (Primary wave) or S wave (Secondary wave) to determine the magnitude, focal depth, epicenter distance, and other parameters, thus deciding the early warning range and level. The acquisition process of earthquake early warning parameters is a complex process, which involves not only information acquisition and information identification, but also data storage.

1.1 Risk Assessment System of HSR Earthquake Emergency Response

Based on BeiDou Navigation Satellite System and big data technology, and following the design principle of "combination of the static and dynamic, the qualitative and quantitative", the structure of the HSR seismic emergency processing system is generally divided into three levels:

1) The acquisition layer: It collects real-time meteorological parameters, track parameters, environmental parameters, and operating parameters by big data technology.

2) The network layer: It uses BeiDou Navigation Satellite System to accurately locate high-speed trains based on the acquired data, and it realizes real-time data transmission.

3) The application layer: It can obtain information such as driving speed, location, and driving direction, provide navigation, map, and other services. High-speed trains realize active safety control.

After an earthquake, there are many factors that affect the safety of HSR operations, and various factors often interact and restrict each other, forming a complex integrated system. To correctly evaluate the impact of HSR operation under earthquake conditions, it is necessary to eliminate the secondary influencing factors and correctly select the main influencing factors as evaluation indicators. Based on the least square method, this paper establishes a risk definition model of HSR earthquakes, and divides the factors that affect HSR operation safety into four categories (e.g., people, vehicles, tracks, environment, etc.), as shown in Table 1.

1.2 Risk Assessment Model of HSR for Earthquake Emergency Response

Any HSR traffic accident may contain the factors in Table 1, and each type of factor belongs to and only belongs to one of the conditions. Therefore, the risk assessment of HSR earthquake emergency response is the risk assessment of the HSR earthquake emergency response capability in dangerous areas. Based on Table 1, this article builds a risk assessment model for HSR earthquake emergency response. Mainly based on the theory of least squares method, the weight coefficients of various risk assessment indicators are determined, and the risk assessment value of HSR earthquake emergency treatment in dangerous areas can be expressed as a linear combination of the weights and eigenvalues of various influencing factors by Eq. (1):

$ f(\operatorname{risk})=\sum\limits_{i=1}^{\eta_i} \sum\limits_{j=1}^{k_j} w_{i j} \delta_{i j} $

(1)

where f(risk) is the risk evaluation value of the seismic monitoring point of HSR in the dangerous area; w_ij is the weight coefficient of j factor of the i category; δ_ij is the characteristic value of the j factor of the i category at the monitoring point, and $\delta_{i j}=\left\{\begin{array}{l} 1, \text { True } \\ 0, \text { False } \end{array}\right. $; k_j is the number of items of type j factors; η_i is the number of factor affecting the risk evaluation value of earthquake monitoring points.

Since Eq. (1) is the monitoring point risk evaluation model for HSR earthquake emergency disposal, the weight coefficient is the measure of the influence of various factors in Table 1 by the risk evaluation value. For the selected HSR dangerous areas, it can be considered that the railway accidents in this area are affected by the factors in Table 1.

Step 1    Define hazardous areas for HSR earthquake emergency response.

Seismic activity in China is highly uneven in spatial distribution, and they tend to concentrate in certain regions. The most obvious manifestation of spatial inhomogeneity is the distribution of earthquakes in zoning, and the seismic activity in each seismic area is uneven. In accordance with the actual spatial distribution of seismic activity in China, an earthquake monitoring system for the safe operation of HSR is constructed. According to the research results of HSR earthquake emergency treatment at home and abroad and related regulations, this article gives the definition of the HSR dangerous area through a comprehensive analysis, as shown in Fig. 1.

Step 2    Calculate the number of railway accidents in earthquake-hazardous areas.

According to Fig. 1, n monitoring points are selected in the dangerous area, and the number of high-speed rail accidents in the dangerous area are calculated. This paper divides the number of accidents by the cumulative traffic volume of the HSR (i.e., the total number of high-speed trains passing the monitoring point during the operation period). To obtain the actual statistical risk evaluation value of the dangerous area, this paper set up Eq. (2):

$ f_p(\text { actual })=\frac{n_1}{t} $

(2)

where f_p(actual) is the actual statistical risk evaluation value of HSR in dangerous area with the unit min; n₁ is the total number of high-speed trains passing the monitoring point during the operating period in the hazardous area; t is the operating period for HSRs in hazardous areas with the unit min; p is the value of the monitoring point in the hazardous area, p=1, 2, …, n.

Step 3    Calculate the risk value of the earthquake-hazardous area.

Corresponding to each monitoring point in the seismic hazard area of HSR, the risk evaluation value of the seismic hazard area of HSR can be obtained according to Eq. (3):

$ f_{\mathrm{p}}(\text { calculated })=\sum\limits_{i=1}^{\eta_i} \sum\limits_{j=1}^{k_j} w_{i j} \delta_{i j} $

(3)

where f_p(calculated) is the risk evaluation value of the seismic hazard area of HSR.

Step 4   Determine the value of the weight coefficient of the risk evaluation index.

Solve the weight coefficient w_ij according to the principle of least square method, i.e., the weight coefficient w_ij in the risk assessment model of the HSR seismic hazard area, make f(w_ij) reach the minimum by Eq. (4):

$f\left(w_{i j}\right)=\sum\limits_{p=1}^n\left[f_{\mathrm{p}}(\text { actual })-\sum\limits_{i=1}^{\eta_i} \sum\limits_{j=1}^{k_j} w_{i j} \delta_{i j}\right]^2 $

(4)

Therefore, according to Eq. (4) to find the partial derivative of f(w_ij) with respect to w_lm and make it equal to 0, there is Eq. (5):

$ \begin{aligned} \frac{\partial f\left(w_{i j}\right)}{\partial w_{l m}} & =-2 \sum\limits_{p=1}^n\left[f_{\mathrm{p}}(\text { actual })-\sum\limits_{i=1}^{\eta_i} \sum\limits_{j=1}^{k_j} w_{i j} \delta_{p i j}\right] \cdot \\ \delta_{p l m} & =0 \end{aligned} $

(5)

where l=1, 2, …, η_i; m=1, 2, …, k_j.

According to Eq. (4), this paper uses the principle of least squares method through the risk assessment model of HSR seismic hazard areas, a linear equation system with unknown w_ij is obtained by Eq. (6):

$ \sum\limits_{p=1}^n \sum\limits_{i=1}^{\eta_i} \sum\limits_{j=1}^{k_j} w_{i j} \delta_{p i j} \delta_{p l m}=\sum\limits_{p=1}^n f_p(\text { actual }) \delta_{p l m} $

(6)

Solving Eq. (6) can obtain the value of w_ij in the risk assessment model of the seismic hazard area of HSR.

Step 5    Evaluate the accuracy test value of the model.

According to the principle of least squares estimation, the risk assessment model of HSR earthquake emergency treatment can use correlation coefficients to test the accuracy. It can use correlation coefficients to test the rationality of the evaluation model. In this paper, the correlation coefficient f (correlation) of the HSR seismic hazard area between f_p(calculated) and f_p(actual) is used to test the risk assessment model. The functional relationship is Eq. (7):

$ f(\text { correlation })=\sqrt{\frac{\sum\limits_{p=1}^n\left(f_{\mathrm{p}}(\text { calculated })-f(\overline{\text { calculated }})\right)^2}{\sum\limits_{p=1}^n\left(f_{\mathrm{p}}(\text { actual })-f(\overline{\text { acutal }})\right)^2}} $

(7)

According to Eq. (7), the risk assessment model of HSR earthquake emergency response is used to determine the accuracy of the risk assessment model of HSR seismic emergency response. If the calculated value of f(correlation) is larger, it means that the accuracy of the risk assessment model for HSR earthquake emergency response is higher.

The basic goal of HSR earthquake emergency response capabilities is, on the one hand, to ensure that the safety and relevance of people, tracks, vehicles and the environment in the transportation system can be dynamically sensed at any time; and on the other hand, is to ensure that HSR accidents and property damage are minimized. In the event of HSR earthquake, in order to realize the rational dispatch of emergency rescue resources, a comprehensive analysis of emergency rescue efficiency, rescue material consumption, and optimal rescue time is carried out. Maximizing rescue efficiency and minimizing material consumption are the optimization goals. Therefore, this paper constructs the HSR seismic emergency response capability analysis technology system and related models, comprehensively evaluates the HSR emergency preparedness capability, and establishes the HSR seismic emergency response capability analysis model.

2 Emergency Response System for HSR Earthquake

When the intensity of an earthquake reaches a set threshold, the emergency response system will issue an alarm. The indicators of earthquake intensity include intensity and ground motion acceleration. Magnitude is a measure of the size of an earthquake, reflecting the difference in energy released by different earthquakes. It is measured by the vibration amplitude of ground motion recorded by a seismograph. Intensity refers to the degree of damage caused by the impact of the earthquake on the ground at different locations. It is divided into 5 levels, as shown in Table 2. Based on the analysis of the hazards of HSR earthquake disasters and the actual situation of HSR, this paper studies the inherent relationship of HSR earthquake emergency treatment. On the basis of analyzing the influencing factors of earthquake emergency treatment, the optimal target of earthquake emergency treatment is defined, a large number of basic data investigation and statistical analysis are conducted, the mathematical relationship between earthquake emergency treatment target and influencing factors are found out, and the earthquake emergency response of HSR is calculated. The processing situation tracking system and its processing flow is shown in Fig. 2.

Based on Fig. 2, the HSR earthquake emergency response process is set up as follows:

Step 1    Obtain parameter information.The ground motion information of the location is located by the device of the collection module.

Step 2    The computer obtains the latest ground motion parameter information according to the current collection. It calculates seismic parameters and quickly estimates the affected area.

Step 3    Read the preliminarily set railway seismic intensity threshold data.

Step 4    Determine whether the earthquake intensity threshold is exceeded.

Step 5    If the seismic intensity information of the line does not exceed the safety threshold, return to Step 1; if the control module recognizes that the seismic intensity exceeds the safety threshold, the control module activates the safety pre-warning device and enters Step 6.

Step 6    The safety pre-warning device is activated, and the pre-warning starts.

Step 7    When receiving the alarm signal of the early warning device, the spatial coordinate information built in the seismic information acquisition module is immediately obtained, and the affected line is located.

Step 8    The control module starts the car control mode, and the module drives the relay to control the opening and closing of the traction power.

Step 9    Determine whether the relay is successfully disconnected and whether the train is under control. If the train is not under control, return to Step 5 and issue an alarm; if the train is under effective control, enter Step 10 to exit the entire process.

Step 10   The train is under control and exits the process.

This paper applies seismic detection technology to make full use of HSR communication network resources to accurately and timely detect the occurrence of earthquakes. HSR management system control the operation of high-speed trains according to the analyzed degree of earthquake damage. To provide rescue and recovery of HSR as soon as possible after earthquakes, HSRs take on effective decision-making strategy. In order to minimize the disaster loss caused by the earthquake, and prevent the occurrence of secondary disasters, a series of management methods and rules are set up such as emergency parking, resumption of driving, emergency rescue, and safety assessment after the earthquake. For example, on the one hand, on the basis of defining the seismic hazard levels of HSRs, construct an HSR seismic response capability analysis technology system, establish an HSR seismic emergency response capability analysis model, and make an overall assessment of the HSR emergency capability; on the other hand, on the basis of constructing an HSR seismic emergency response plan, an optimized decision-making model for HSR seismic emergency response based on time-varying and reliability is established.

3 HSR Emergency Response Model Based on Emergency Events

The needs of HSR earthquake disaster emergency treatment is analyzed, and a reliability model is put forward for identifying dynamic processing demand information in the case of information confusion and uncertainty of multiple different processing demand information sources. From the perspective of risk management, the risks of emergency treatment in earthquake-stricken areas of HSRs are comprehensively analyzed, and a dynamic model is established that identifies the urgency of earthquake disaster treatment which is needed to determine the priority of emergency treatment.

Step 1    Define the scope of earthquake emergency response for HSR.

The alarm modes for safe operation of HSRs can be roughly divided into two types. One is the alarm mode based on a mechanical acceleration alarm machine (corresponding to the early warning of ground motion value). When the seismograph detects that the horizontal acceleration of ground motion exceeds the alarm threshold, there is automatic alarm. The other is based on the alarm mode of the electronic seismograph with P-wave monitoring (corresponding to the warning of seismic ground motion parameter). According to the detected P-wave, the distance from the epicenter to the railway line, the magnitude and other parameters of the earthquake epicenter are estimated. The magnitude and epicenter distance are used to determine whether the train needs to be controlled, and how much of the range of traffic should be controlled. Therefore, corresponding to different levels of earthquakes, the damage radius (the range that trains need to be controlled) is different. If the second alarm mode is adopted, the damage radius R needs to be obtained according to the magnitude M. That is, the distance from the epicenter of the train needs to be controlled. Systematic research is carried out by a combination of theoretical analysis and numerical statistics. In order to obtain scientific and reasonable monitoring results of HSR operation situation, the operation situation monitoring results are defined into five levels. This paper defines the interval levels by fuzzy language.

According to Fig. 3, the quantified function is used to define the HSR operation situation monitoring radius interval, and the monitoring radius relationship is as shown in Eq. (8).

$R=x \cdot \operatorname{tg} \theta $

(8)

where R is the radius of earthquake warning area with km as the unit; x is the depth of the source from the surface with km as the unit; θ is the epicenter angle.

The earthquake warning angle is mainly defined according to the magnitude and focal depth. The value of the earthquake warning angle for different depths and undesired magnitudes is different. The recommended values in this paper are shown in Table 3.

Step 2 Establish a grading model of seismic hazards for HSRs.

According to the degree of earthquake damage, the HSR earthquake emergency events are classified and studied. Based on the establishment of the radiation analysis model of the emergency event in the HSR operation range, the impact range of the emergency events of different levels of damage on the HSR is determined, and hazard classification model is established for the HSR earthquake. The scale of earthquake emergency treatment is defined as shown in Fig. 4.

Based on Fig. 4, according to fuzzy language processing criteria, a hierarchical interval linguistic fuzzy number set and the interval function relationship of the HSR earthquake emergency response system are constructed, as shown in Table 4.

Step 3 Research on earthquake emergency treatment plan for HSR.

The earthquake-affected field and the site conditions along HSR are analyzed, the train control area is determined according to the location of the alarm point (Fig. 5). The attenuation relationship of different sites, the range of the affected field, and the potential impact of the earthquake on the train and the line are determined. The emergency control range of the train is given, and the specific emergency response range plan is formulated according to the control range.

The HSR earthquake emergency treatment plan can be constructed from the above three situations: entering the dangerous area, in the dangerous area, and leaving the dangerous area. The emergency treatment plans for HSR earthquake disasters are constructed in this paper.

Firstly, when the ground motion acceleration that has reached the alarm threshold is monitored, a control command to stop power supply and brake the train to stop is issued to the traction power supply system.

Secondly, when an earthquake occurs, the station-level equipment provides earthquake warning information and earthquake parameters to the station integrated information system for the station to refer to when taking emergency plans.

Thirdly, after the earthquake information is transmitted to the operation dispatch center in real time, the dispatch center quickly starts the control plan for post-earthquake inspections and resumption of train operation according to the maximum earthquake parameters.

Finally, the operation dispatch center transmits the obtained earthquake information to the integrated maintenance and the rescue system for quick rescue and quick repair reference.

4 Case Analysis

This paper selects the Beijing-Hong Kong HSR as a case to analyze the earthquake emergency response of HSRs. This paper uses traffic accidents on the Beijing-Hong Kong HSR from December 2019 to December 2020 as sample data, and uses risk evaluation index statistics to evaluate the condition in Beijing, Hebei, Shandong, Henan, Anhui, Hubei, Jiangxi, Guangdong, Hong Kong, etc. The above nine provinces are risk monitoring areas, as shown in Fig. 6.

(1) Acquisition of seismic parameters for HSR.

On the one hand, it obtains the ground motion data, calculates the seismic parameters based on the obtained data, generates the seismic graph, and calculates the seismic intensity of the data acquisition point. On the other hand, the system reads the preset seismic intensity threshold, and compares the calculated seismic intensity with the threshold value. If the earthquake intensity exceeds the threshold value, the alarm process will start working. According to the data stored in the device, the location of the earthquake point will be obtained, and the control mode will be activated. If the high-speed train is under control, the warning process will end. Otherwise, it continues with the alarm process.

(2) The monitoring scope of earthquake disasters.

The geographical distribution of earthquakes is controlled by geological structures. It has certain laws by the most obvious being zoning. The world's earthquakes are mainly distributed in the following areas. One is the Pacific rim seismic belt, which is the most seismically active zone in the world, with 80% of the world's earthquakes concentrated in this zone; the other is the Eurasian seismic belt, where 15% of the world's earthquakes happen. China is located in the southeast of the Eurasian plate. Affected by the Pacific rim seismic belt and the Eurasian seismic belt, it is a country with many earthquakes. Therefore, according to the geographical distribution characteristics of national earthquakes, the seismic area of Beijing-Hong Kong HSR is shown in Table 5.

(3) Risk assessment model for emergency response of HSRs.

HSR seismic monitoring information identification research mainly focuses on the following aspects. The first aspect is information acquisition, which includes monitoring information and seismic bureau information verification, single-point identification and multi-point joint identification, filtering and analysis of interference information; the second aspect is information understanding, which includes geology along the HSR with different structures, the early warning information, and the accurate determination of the release range of early warning information; the third aspect is information recognition, which includes improving the accuracy and speed of recognition. The information collection and risk assessment of the Beijing-Hong Kong HSR is carried out, as is shown in Fig. 6, and the assessment results are shown in Table 5.

5 Conclusions

Earthquake is the most threatening natural disaster for the safe operation of high-speed trains. Currently, earthquake prediction technology is still immature, thus the development of earthquake early warning technology is critically important to reduce or avoid earthquake damage to HSR. For HSR, earthquake early warning is to set up seismographs near the potential seismic source or along the railway. When an earthquake occurs and reaches the alarm level, the speed difference between electromagnetic waves and seismic waves or the speed difference between P waves and S waves is used to raise the alarm to the moving trains before the destructive seismic wave arrives. It forces the trains to slow down or stop for avoiding accidents.

On the basis of considering the efficiency, cost, fairness, and dynamics of earthquake emergency treatment of HSR, the corresponding optimization model of earthquake emergency disposal and dispatching is proposed to provide important feasible methods for HSR earthquake emergency treatment. On the basis of in-depth study of earthquake emergency response control methods, the theoretical system of earthquake emergency response for HSR has been improved. Emergency support capabilities have been improved, and strong technical support has been provided. This paper also correspondingly gives the method for determining the threshold definition index. It determines the entire process of earthquake warning, and completes the corresponding physical procedures. It connects to the user level, improves the warning procedures, and has certain applicability.