# **2.3. Earthquake Hazards and Impact on WWTPs**

Direct earthquake impacts on WWTPs are largely due to ground shaking and ground failures, with individual components or stages of the wastewater treatment process responding in different ways. For example, non‐structural elements, such as scrapers, aerators, and mechanical equipment are largely be affected by excessive shaking. Whereas, ground deformation (in the form of fault rupture, landslides, and liquefaction causing settlement or lateral spreading), lead to breakages, lifting, and deformations of structural elements, such as tanks, pipes, or treatment pond bunds. The actual shaking properties required to initiate such processes vary significantly spatially – even at very local scales. This ensures predicting damage due to both shaking and ground deformation on a wider WWTP facilities remains a challenge.

Given the myriad of data that is required to carry out a detailed analysis at each WWTP site, we instead adopt two readily available measures to provide a proxy for estimating asset damage. These are: Peak Ground Acceleration (PGA) and Liquefaction Susceptibility. PGA is an important ground motion parameter used in earthquake engineering applications and design as structural damage is often closely correlated. Liquefaction susceptibility is a measure of how susceptible local ground conditions would be to liquefy given sufficient ground shaking is experienced. Combined these parameters can give a proxy for a combined likelihood of a WWTP facility experiencing damage in an earthquake via the two predominant causes: shaking and ground deformation.

As with the flooding exposure analysis above, PGA and Liquefaction Susceptibility parameters are assigned to each WWTP through a simple spatial intersection. These datasets are described in the following section.

# **2.3.1. Hazard Datasets**

PGA hazard maps are for return periods of 250‐, 475‐, 950‐, 1500‐, and 2475‐years (UNISDR, 2015). As these were created at the global scale, we note that they may underestimate the hazard in local areas for fault lines not captured in historical catalogues. As a result, rather than relying on absolute values of horizontal acceleration, we instead group PGA into five discrete categories: Very High **VH** (>0.65g), High **H** (0.34g ‐ 0.65g), Medium **M** (0.18g ‐ 0.34g), Low **L** (0.092g – 0.18g), and Very Low **VL** (<0.092g). It should be noted that these labels are qualitative, and the actual impacts do not necessarily correlate with the description given the great variability in local conditions and design standards.

Similar categories are assumed for liquefaction susceptibility: VH, H, M, L, VL. These are based on a readily available dataset created for use at the global scale (Zorn and Koks, 2019) using a calibrated geospatial prediction model (Zhu, Baise and Thompson, 2017). This approach defines liquefaction susceptibility based on a range of parameters such as: distance to rivers, depth to groundwater, annual precipitation, distance to coast, and 30‐m averaged shear wave velocity ( _Vs30_ ). As with PGA, site specifics such as soil drainage, the use of infill, and _Vs30_ (amongst others) are overlooked in the method. Regardless, this method has shown promising predictive capacity and is 8