Velocity structure and a seismic model for Nevado del Ruiz Volcano (Colombia)

A seismic tomographic study was performed for Nevado del Ruiz Volcano (NRV) using more than 1500 high-quality local and regional. Three low P wave velocity (low-VP) and low S wave velocity (low-VS) zones were found; one low-VS zone at depths 2–4 km located beneath the volcano; a second low-VP and low-VS zone at depths 5–10 km located beneath the crater, elongated and dipping to the E–SE; and a third low-VP and low-VS zone at 10 to ∼12 km farther to the east. These three low-VP and low-VS zones are believed to be the location of heat sources. A high-velocity zone for both P and S waves was found at shallow depths (0∼5 km) around the active crater. The upper part (0∼2 km depth) of the high-VP and high-VS zone was characterized by low-VP/VS ratios (<1.68), while the deeper part (2∼10 km) had high-VP/VS ratios (>1.80). The low-VP/VS zone is correlated with a steam-dominated geothermal system. The high-VP/VS is interpreted as an intrusive body of magmatic origin which includes partial melting zones associated with low-VS anomalies. A small low-VP zone in which long-period (LP) earthquakes were clustered was found to the southwest of the volcano. Based on the data obtained with the tomography in combination with seismicity, geochemistry, geology and gravimetry, we suggest a model for the seismic activity of NRV. Volcano-tectonic (VT) earthquakes that occur very often in swarm-like patterns located in several clusters around the volcano seem to be due to changes in stress produced by the passing of fluids and/or gas through many small cracks. A fault and a caldera-like structure separate the VT swarms located to the west of the volcano from the source of LP earthquakes.