# Urban Seismic Networks: A Worldwide Review
## 2 USN Catalogue
### Coverage  
With regard to the number of nodes and the area covered by each USN, the catalogue includes a very wide range of combinations; both factors span several orders of magnitude, and their their ratio (i.e., number of nodes per unit area) likewise covers a wide range of density values.  
The number of nodes forming a USN depends on several factors. Usually, these factors result from the combination of the objectives to be accomplished and the area being monitored, although the logistical needs and available financial resources can contribute to quantifying the sensor nodes to be installed, as some networks can be integrated over time, especially permanent ones. Most of the USNs have some tens of nodes, with only sixteen of them having more than a hundred sensors. USNs usually extend between some tens and few thousands of km\({}^{2}\) (see Figure 6). It is worth highlighting that the size of the area covered by a USN often does not appear to be strictly related to the extension of the corresponding urban/metropolitan area where it is located, and might depend on the network's purpose as well. Sometimes the target of an USN is only a portion of a given urban area, e.g., a campus, historic center, or neighborhood, while sometimes it extends to the far outskirts of the urbanized territory, and may include different contiguous urban centers under a single network. In these latter cases, USNs can extend up to 10\({}^{4}\)-10\({}^{5}\) km\({}^{2}\), representing real regional-scale networks. As far as density is concerned, the densest USNs count even more than 100 nodes per km\({}^{2}\), while in the loosest networks a single monitoring site covers a surface greater than 1000 km\({}^{2}\). However, most USNs have densities ranging between 0.01 and 1 nodes/km\({}^{2}\) (Figure 6).  
[PERSON] et al. (2019) [8] classified only those seismic networks characterized by specific combinations of values of area and density as \"urban-scale networks\" (the solid grey contour in Figure 6b,c). Actually, only a few USNs in our collection fall within this definition; these fourteen USNs all fall within a limited part of it. Therefore, the definition of a USN provided by [8] is too restrictive from the point of view of the covered area and too inclusive when it postulates very-high-density networks over relatively large areas. This is due to the very limited collection underlying their observations, as they reported only five USNs implemented in Italy).  
Our catalogue helps to better define USNs in terms of the combination of covered area, number of nodes, and their ratio. Most USNs lie within the red region in Figure 6b, which can be considered the typical combination of covered area and number of nodes for a USN: a few tens of nodes spread over a surface of 10-1000 km\({}^{2}\). Moreover, we found that when considering all the USNs and including all the objectives, the density of the USN decreases linearly on a log-log scale when the covered area increases (see Figure 6c). This scale relationship is very clear, and can be used as a guideline when planning new USNs. This relation may suggest the indicative number of sites necessary to establish a new network over an urban area of a given extension, or conversely, can provide indications of the maximum extension achievable by the network when a given number of sensors is available. Of course, the final plan of a new network must support all its specific targets, which are peculiar and unique for each case. However, exploiting the indications from a vast collection of other applications can represent decisive support as well as a measure of comparison with the state of the art.