# Urban Seismic Networks: A Worldwide Review
## 2 USN Catalogue
### Technical Characteristics
#### 2.5.1 Sensors  
The type of sensors employed in the network is a key feature of each USN. We classified the sensors adopted by each USN into five different types: (i) velocimeter, (ii) force balance accelerometer, (iii) both velocimeter and accelerometer, (iv) MEMS accelerometer, and (v) low-cost MEMS. The balanced use of velocimeters and accelerometers at different nodes is sometimes adopted to exploit the advantages of both types of sensors; therefore, a dedicated group has been considered to cover this case. For other combinations of sensors, which are far less frequent, the prevalent typology is considered. The distinction between short-period and broad-band sensors is not reported at this stage, though considerations are provided in the text. The most widely employed sensors (in more than 40% of USN) are velocimeters; force balance accelerometers and low-cost MEMS occur less frequently, being encountered in about 20% of the USNs, and the two remaining groups are even less frequent (Figure 7). The main difference between the two MEMS (micro-electro-mechanical systems) groups is the cost for a single device, though this obviously carries a number of implications. This distinction concerns the performance of the sensors as well, in particular the self-noise level, the type of output (usually analog for better sensors), and the stability against environmental conditions. The literature has largely investigated the performance  
Figure 6: (**a**) Histograms for number of nodes, covered area, and network density. (**b**) Scatter plots of node number versus covered area; the coloured shades represent the intensity of observations and the dashed line encloses the area with greatest intensity. (**c**) Scatter plots of network density versus covered area; the blue dashed line and the shaded region indicate the regression line and the 95% confidence interval, respectively. The numbers refer to the IDs in Tables 1 and 2.  
of several models of MEMS sensors [160; 161; 162; 163; 164; 165]. In terms of self noise spectral density, a level in the order of \(10^{-5}\) m \(\cdot\) s\({}^{-2}\)/\(\sqrt{\mathrm{Hz}}\) in the band 1-10 Hz can been considered as a rough boundary between the two groups of MEMS sensors. In the selection of appropriate sites to establish a seismic network, considerations about the background seismic noise play a role. The seismic noise affects the detection capability; thus, sites should be as quiet as possible. A great portion of the noise power, especially at high frequencies, is related to human activities (so-called urban or cultural noise). Inevitably, urban areas suffer strong levels of seismic noise, and these noisy areas are unsuitable to host networks only conceived and designed to locate earthquakes. These conditions represent a relevant limitation for USNs, and are the most usual reason for poorly performing sensors (especially in terms of self-noise and bandwidth), particularly for certain objectives. On the other hand, cheaper sensors allow applications with higher density at equal cost, which could desirable for USNs. These are among the reasons that lower quality devices such as MEMS sensors and even low-cost MEMS can be suitable for seismological applications and employed in a variety of applications in the broad field of seismology, including in USNs [166; 167; 168]. Their use started only about fifteen years ago, and has been increasing sharply over the last decade (Figure 7). During the first decade of their development, USNs were mainly strong-motion networks based essentially on accelerometers (Figure 7). The choice of sensor typology is typically made based on the main objective; in fact, a coupling of the sensor type and main objective occurs very frequently, e.g., velocimeters for earthquake monitoring and site effect assessment, while several other combinations never occur (Figure 8).