# Urban Seismic Networks: A Worldwide Review
## 2 USN Catalogue
### Duration  
The date of starting observation (year, and sometimes month and day) is provided for all the USNs except for a single one. Conversely, the exact date of ending observation is provided only for the fixed-term networks. Certain USNs have been planned as permanent monitoring infrastructure, and remain active at present. For other networks, the authors did not provide any clues as to whether the networks were established as fixed-term or permanent ones. In many of these cases, we have been able to find further references providing information about the activity of the networks. Therefore, for these USNs, the duration reported in Table 2 actually indicates a \"minimum duration\", i.e., the time span between the starting of the network and the last evidence of activity. For the active networks, the duration is considered to be current until 2023.  
The corresponding temporal bar for each of the collected networks is reported in Figure 4. The first urban networks were implemented in the early 1990s, and until the early 2000s there were only a few networks. Subsequently, the number of new operating USNs has steadily increased for about fifteen years. This trend has been sharply increasing since 2016-17, with the relatively lower number of USNs reported in the last few years ascribed to the most recent networks still being unpublished. In addition, Figure 4 reports the frequency density distribution of the duration. About 40% of the USNs have a duration shorter than one year, and 60% last for less than five years. Only approximately the 20% of the networks have been continuously operating for more than ten years.  
The operating time of a network is tightly constrained by its objective(s). Of course, other factors may influence on the duration, most of all the economic resources needed to keep a network working. The duration as a function of the main objective is displayed in Figure 5. For a given objective, the duration may span a wide range of years. Part of this dispersion is due to limits in the assessment of the exact duration, as explained above. However, differences between various objectives are recognizable, even though no universal conclusions can be derived.  
Networks for earthquake monitoring include the widest range of durations, as they comprise both USNs designed for seismic surveillance (i.e., long-term monitoring) and USNs conceived to monitor only a single seismic sequence or seismic swarm. Although the durations cover more than a decade, the networks for site effects are clearly clustered, as is predictable, towards the shortest duration (even less than one year). Conversely, no short-duration networks are targeted to report ground motion intensity. The networks for structural health monitoring have variable duration, and are mostly implemented as permanent infrastructure.  
Figure 4: Temporal bar graph of the collected USNs; the inset displays the frequency density distribution of USN durations.  
Figure 5: Operating times of USNs grouped according to their main objective. The temporal axis is in logarithmic scale and the numbers refer to the IDs in Tables 1 and 2.