During September and October 2019, INGEOMINAS (Colombian Institute of Mining and Geology) noted gradual increases in seismic activity near the Arenas crater. Sulfurous odors and phreatic eruptions reported at Nevado del Ruiz prompted an alert level of Yellow on October 1. Chemical analysis of the volcano demonstrated deformative changes and geochemical alterations. Over the next four months a smattering of long-period earthquakes, considered pseudo-Tornillo events, raised worries that the volcano might erupt. In 2010, tilt increased, and on average 890 volcanic tremors were recorded monthly, roughly eight times as many as had been recorded from 2009 to 2010.