# Urban Seismic Networks: A Worldwide Review
## 2 USN Catalogue
### Network Geometry

Table 3: Summary of the main features of the various sensor and transmission types applicable for the implementation of a USN. Figure 9: Temporal trends of the types of transmission adopted by the USNs listed in Table 2 and their frequency of occurrence. geometry plays a critical role in the precision and accuracy of the hypocentre locations as well as in estimation of the the magnitude [171; 172]. The relevance of a planned geometry is relatively decreased for networks in which the observations are treated independently on a site-by-site basis (e.g., on-site early warning, assessing site effects, seismic intensity mapping), even though the geometry may affect the later data interpolation or modelling. Finally, other networks, such as those designed for structural health monitoring, are completely unaffected by geometry, as the observations at a given site are never combined with observations from other sites. We have not classified the types of geometry in the USN catalogue. Although the positions of the nodes are sometimes provided, it was not possible to assess a metric to clearly and unambiguously support the grouping of USNs into distinct classes of geometries. However, we provide an overview of the various typologies of geometrical arrangements adopted by the USNs (Figure 10). From the reported examples, it is clear that the transition between regular and irregular geometries may not be sharp enough to define clear boundaries between these classes. In most cases, the choice of the geometry is mainly aimed towards providing uniform coverage of the target urban area. However, geographical boundaries (e.g., coastlines), geological boundaries (the need to sample all soil types), and sometimes even administrative boundaries or permission issues can represent constraints when planning the geometry of a network. For all of these reasons, there are few networks in which the geometry can be deliberately designed. In certain cases, the networks are the results of superpositions of other sub-networks with different geometries, resulting in articulated final arrangements [39]. Figure 10: Examples of network geometries with different degrees of regularity. In the right column, examples of USNs including station arrays: six arrays in Helsinki [134] and five linear arrays in Tokyo [123]. Regular networks forming grids with regular meshes are ideal solutions; however, they are hard to implement [92; 120]. The arrangement sometimes aims to ensure homogeneous coverage with equally spaced nodes, even if through an irregular pattern [67; 88] (Figure 10). In certain cases, part of the network is designed to form specific geometrical patterns, i.e., arrays (Figure 10), with several identical sensors deployed regularly (or almost regularly) at different scales [123; 134; 138]. Arrays enhance the seismic signals, allowing the detection of weaker signals and characterization of the wavefield. Finally, a few USNs include vertical arrays, with the deployment of several sensors in a given location at different depths [37; 46; 48; 49; 134]. Arrays are mostly used for seismological experiments, and mainly fall within the group of \"other\" objectives.