We used the receiver function technique to deduce crustal thickness beneath the northwestern Andean system, using data from the permanent seismic network of Colombia, combined with some of the IRIS and CTBTO stations in Colombia and Ecuador. The estimation of crustal thickness was made using the primaryPtosconversion and crustal reverberations. The bulk crustalV_P/V_Sratio was constrained using a crustal thickness versusV_P/V_Sstacking method, in addition to estimations using a time to depth conversion technique based on results of a modified Wadati diagram analysis. We observed a wide range of crustal thicknesses, including values around 17 km beneath the Malpelo Island on the Pacific Ocean, 20 to 30 km at the coastal Pacific and Caribbean plains of Colombia, 25 to 40 km beneath the eastern plains and foothills, 35 km beneath the Western Cordillera, 45 km at the Magdalena River intermountain valley, 52 to 58 km under the northern Central Cordillera, and reaching almost 60 km beneath some of the volcanoes of the Southern Cordilleran system of Colombia; crustal thickness can be slightly greater than 60 km beneath the plateau of the Eastern Cordillera. The values ofV_P/V_Sare particularly high for some of the stations on the volcanic centers, reaching values above 1.79, probably related to the addition of mafic materials to the lower crust, and in the plateau of the Eastern Cordillera near Bogota, where we speculate about the possibility of crustal seismic anisotropy associated with shear zones.

The global number of megacities is projected to increase from 33 to 43 by 2030. Megacities are critical for the world’s economy; however, their resource management is particularly challenging. The increase of energy demand, in parallel to population growth and climate change, requires urgent investment in sustainable energies. We examine the megacities of Bogotá, Los Angeles, and Jakarta and reveal that the potential geothermal resource base is enough to cover the residential electricity demand by 1.14, 4.25, 1.84 times, respectively. Geothermal energy, a clean baseload resource independent from weather conditions, could significantly contribute to energy needs, improved air quality, and the decarbonization of the world’s megacities. We conclude that it is critical that governments and public are educated about the benefits of geothermal. Moreover, those energy policies coupled with investment in research and development are needed to ensure geothermal is successfully integrated into the future energy mix.

Nevado del Ruiz volcano (NRV), Columbia, is one of the most dangerous volcanoes in the world and caused the death of 25,000 people in 1985. Using a new algorithm for repeated tomography, we have found a prominent seismic anomaly with high values of the Vp/Vs ratio at depths of 2–5 km below the surface, which is associated with a shallow magma reservoir. The amplitude and shape of this anomaly changed during the current phase of unrest which began in 2010. We interpret these changes as due to the ascent of gas bubbles through magma and to degassing of the reservoir. In 2011–2014, most of this gas escaped through permeable roof rocks, feeding surface fumarole activity and leading to a gradual decrease of the Vp/Vs ratio in the reservoir. This trend was reversed in 2015–2016 due to replenishment of the reservoir by a new batch of volatile-rich magma likely to sustain further volcanic activity. It is argued that the recurring “breathing” of the shallow reservoir is the main cause of current eruptions at NRV.