Based on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three-dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (~4.5 – 7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ~1.75 to ~1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (~5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (<1.80) region to a subducted seamount that might be part of the Carnegie Ridge (CR). To the South, the low Vp region is associated with high Vp/Vs (>1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events (SSE) offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period.

Tephra fallout hazard assessment is commonly undertaken with the development of probabilistic maps that rely on numerical models. Among the steps for map production, the definition of input parameters of the model (including atmospheric conditions), the physical approximations of the numerical simulations, and the probabilities of occurrence of different eruption types in specific time frames are among the most critical sources of uncertainty. In this paper, we present a tephra fallout hazard assessment study for two volcanoes (Cotopaxi and Guagua Pichincha) in Ecuador. We utilize the coupled PLUME-MoM/HYSPLIT models, and we develop a procedure for uncertainty quantification where: i) we quantify the uncertainty on eruptive source parameters and eruption type occurrence through expert elicitation; ii) we implement a new procedure for correlations between the different parameters, and iii) we quantify the uncertainty of the numerical model by testing it with past eruptions and by deriving coefficients of mean model overestimation/underestimation. Probability maps of exceedance, given a deposit thickness threshold, and thickness maps, given a probability of exceedence, are produced for eruption of sub-Plinian and Plinian types, which are then merged into single maps concerning the next eruption. These are described according to the uncertainty distribution of eruption type occurrence probabilities, in terms of their 5th percentile, mean and 95th percentile values. We finally present hazard curves describing exceeding probabilities in 10 sensitive sites within the city of Quito. Additional information includes the areal extent and the people potentially affected by different isolines of tephra accumulation.