Visualization software CiteSpace 5.8.R3 is used to set Node Types as Author, and other settings remain unchanged. The knowledge graphs are shown in Figure 2 and Figure 3, and the top 10 prolific authors are listed in Table 2.

The top 3 authors are Jigang Wei, Changshi Liu and Zi Yan respectively. Among them, Jigang Wei published 90 articles, far ahead of other scholars, and the other scholars published more than 10 articles. The academic research group represented by them promotes the development of China’s emergency logistics. From the perspective of the cooperation among authors and the distribution of research teams, the cooperative relationship between authors in this field is relatively harmonious, with a certain number of research teams. However, the cooperation between authors is limited within the team and lacks of communication between teams. This will directly affect the quality and process of further development of this field, and there is still a large space for development in the future.

Through the time-zone view of the co-occurrence knowledge graph of the core authors, Figure 3 is obtained. We can clearly obtain the founders of the research of emergency logistics in China, such as Changshi Liu, etc., which lays a solid foundation for the development. At the same time, we can find that some up-and-comers in the current research, such as Yanbiao Sun, will be the backbone of the rapid development in the future. In addition, we can easily conclude the active period of emergency logistics research in China, such as the period from 2013 to 2015. However, in recent years, the research activity has slowed down, and it needs to be paid more attention by domestic scholars.

Visualization software Citespace 5.8.R3 is used to set node types as institution, and other settings remain unchanged. The knowledge graph is shown in Figure 4, and the top 10 research institutions in the number of publications are listed in Table 3.

The top 3 research institutions in terms of publication volume are Industrial Economic Research Department of the Development Research Center of The State Council, School of Business Administration of Chongqing Technology and Business University, and School of Management Science and Engineering of Chongqing Technology and Business University, with publication volume of more than 40. Universities and research institutions, as the two major groups of research institutions, are the main body and important position of scientific research innovation, which helps to promote interdisciplinary integration and construction. From the perspective of cooperation between research institutions, most exchanges and cooperation only exist between different universities in the same province or city, or between different colleges in the same university, but there is still a lack of cross-regional cooperation between universities in different provinces and cities. This will make it difficult to form a regional academic community of emergency logistics, and thus restrict the development of this field.

Visualization software CiteSpace 5.8.R3 is used to set node types as keyword, and other settings remain unchanged. The knowledge graph is shown in Figure 5. The data are further processed by Excel, and keywords with the same meaning

are merged, such as “site selection” and “site selection model” are merged into “site selection”, etc. The top 20 keywords in frequency and centrality are obtained, as shown in Table 4 and Table 5.

It can be seen that the keywords with the highest frequency in Table 4 is emergency logistics, followed by emergency supplies, natural disasters, site selection model and emergency, etc. The research mainly focuses on such keywords. Keywords such as demand forecast, two-level planning, emergency distribution and COVID-19 are the research hotspots in recent years. High centrality keywords in Table 5 include emergency, emergency logistics and military logistics, etc. The highest centrality keywords are emergency, which is the source of research in the field of emergency logistics, mainly including emergency logistics, military logistics and emergency management three main subjects. In terms of research methods, it focuses on the use of models and methods such as robust optimization, ant colony algorithm, genetic algorithm and AHP. In terms of research direction, it mainly focuses on emergency supplies, demand forecast, management system, operation mode, evaluation model and supply chain, which reflects the significance of emergency logistics.

On the basis of keyword co-occurrence, this study clustering keywords through LLR algorithm recommended by professor Chaomei Chen, a total of 92 clusters are obtained, in which Q (modularity) = 0.8817, S (silhouette) = 0.9781. The data show that the clustering structure of keywords is significant (Q > 0.3), and the clustering effect is convincing (S > 0.7). In this study, the first 5 large and representative clusters were selected to draw the knowledge graph, as shown in Figure 6. After reading the core literature, the 5 clusters were summarized into 5 research topics, and their expansion was analyzed in detail.

1) Emergency logistics network and information construction

Primary construction stage: Emergency logistics has a low level of informatization and systematicness in China, and lacks a good information platform. Therefore, the construction of information system centered on logistics network collaboration system is one of the keys in the construction of emergency logistics (Wei, 2010) . In the study of urban emergency logistics network, Zhao (2007) focused on the interaction structure of urban emergency management and logistics network, so as to realize the sharing of information resources and emergency supplies. From the perspective of national security strategy, strengthening the informationization construction of emergency logistics is also the top priority to accelerate its construction (Wang, 2009) .

Adjustment and optimization stage: The frequent occurrence of emergencies makes the evaluation of emergency logistics capability gradually put on the agenda (Deng et al., 2010) . Xu et al. (2015) constructed an evaluation index system for the reliability of emergency supply chain from the aspects of information system and network structure. In view of the evaluation of dynamic network topology structure under emergencies, Zhu et al. (2018) proposed an evaluation method for cascade failure resistance of emergency logistics network based on CI-TOPSIS fuzzy multi-criteria group strategy. In addition, taking the May 12 Wenchuan earthquake as the background, Zhou et al. (2020) optimized the design of the traditional initial post-earthquake rescue emergency logistics network in consideration of the facility interruption scenario. However, the outbreak has typical unstructured characteristics, such as fuzzy diffusion boundary and dynamic time-varying situation. Based on this, Liu et al. (2020a) constructed a kind of innovative decision-making framework model for dynamic adjustment and optimization of emergency logistics network, so as to make continuous adjustments according to the actual situation.

2) Emergency logistics model and system

Emergent or abnormal nature is a significant mark that distinguishes emergency logistics from general logistics, which requires that the emergency logistics supply chain must have reliability (Wang et al., 2005; Zheng, 2009) . In view of this, Ji & Zhu (2007) established an optimization model of emergency logistics resource distribution to respond to the requirements of rapid distribution in emergencies and solve the problem of shortage of anti-disaster materials. A scientific, reasonable and fully functional emergency logistics system is the basic guarantee for responding to emergencies, and also an important work for the Chinese government to comprehensively improve the ability and efficiency of emergency logistics management (Li et al., 2012) . In the rescue of emergency supplies in major emergencies, Zhizhu Lai et al. (2020) established a multi-objective finalization model and a robust optimization model for the location of emergency logistics center, and studied the site selection of emergency logistics center and the transportation of emergency supplies. In addition, emergency logistics is an important support to deal with public health emergencies. Based on blockchain application, Li et al. (2020b) explored and improved emergency logistics under emergencies. Combined with the COVID-19 epidemic, Jiang et al. (2020b) analyzed the problems in China’s emergency supplies support and established a GOVERNMENT-led BCM emergency supply chain system.

3) Emergency management system construction and improvement

Wenchuan earthquake emergency response reflected some deficiencies in China’s emergency response system, such as the lack of emergency logistics coordination management system (Jing & Song, 2009) . It shows that a scientific management system is the basis for the effective organization of emergency logistics. Cheng (2010) pointed out the defects and problems existing in the operation of the current natural disaster emergency logistics management system, and proposed key tasks and measures to improve its operational effectiveness. At the same time, to comprehensively improve the ability to prevent and respond to emergencies, the construction of emergency management system should rely on the support of the emergency industry, and accelerate the formation of a scientific and technological support system for national public safety and emergency management (Liu & Li, 2012) . In the era of extensive application of modern information technology and the Internet, the outbreak of COVID-19 has brought challenges and opportunities to international emergency procurement management, and also provided good experience and practical cases for comprehensive summary of national emergency procurement plans in the future (Meng, 2020) . Through information update from the perspective of grid management, Qu et al. (2022) established a multi-stage time-constrained response-timeliness location model for emergency supplies distribution, which is conducive to the development of actual rescue.

4) Emergency logistics site-path model

After studying the core literature, it is found that the researches of domestic scholars on this topic mostly focus on the construction of site-path model. Huang & Xie (2009) combined ant colony algorithm and radial basis neural network to establish a location decision model of food emergency distribution center. To solve the location-path problem of disaster reduction system, Zeng et al. (2010) established a LRP model aiming at minimizing the total cost. Li et al. (2013) established an optimization model of multi-objective site selection and multimodal transport for post-earthquake emergency supplies distribution, so as to minimize the total time of emergency supplies distribution and the total loss of unmet emergency supplies at the disaster-hit points. In view of the uncertainty of emergency supplies demand and time of transportation vehicles between demand points, Wang et al. (2015a) set up a minimum time and cost minimization of dual stochastic programming model under the certain emergency restrictions, and establishes Sun et al. (2019) establish an emergency facility site-path robust optimization model with the goal of minimizing the sum of the rescue time of material delivery to demand points. From the perspective of COVID-19 prevention and control, Li et al. (2020a) established an emergency response decision-making model including multi-level collaborative storage location and logistics distribution, in order to cope with the uncertainty and dynamic change of demand for emergency supplies in public health emergencies.

5) Emergency supplies support capability

In the early practice of dealing with all kinds of major unexpected public events, China has had certain ability to support emergency supplies, but it still reflects the status quo of the relatively backward construction of emergency logistics system (Wei & Zhang, 2009) . In this regard, Wang & Wang (2012) proposed a coordinated optimization scheduling scheme for multi-period emergency supplies in multi-epidemic areas, and constructed a multi-objective stochastic programming model for a class of emergency logistics network optimization. Wang et al. (2013) constructed a multi-objective emergency supplies point selection model under transport capacity constraints, aiming at the shortest supply time and minimum supply cost. Efficient delivery of relief materials is a key link in emergency rescue of major public emergencies (Ma et al., 2013; Huang et al., 2021) . In view of the robust uncertainty of disaster relief information in the process of emergency supplies distribution, Zhu et al. (2016) constructed a multi-stage and multi-objective robust optimization model for emergency supplies distribution, and proposed a robust control strategy on this basis. Liu et al. (2016) constructed a multi-objective fuzzy LRP optimization model based on opportunity constrained programming, so as to effectively solve the fuzzy location-path problem of post-earthquake emergency supplies distribution. In addition, Zhang & Ni (2020) constructed a dual-objective optimization model of efficiency and equity to achieve a fair and reasonable distribution of relief supplies on the basis of considering that relief supplies can be delivered separately (Table 6).