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

ABSTRACT

A seismic tomographic study was done for Nevado del Ruiz Volcano (NRV). More than 1500 high-quality local and regional earthquakes were used for this tomographic study. Three low P wave velocity (low-Vp) and low S wave velocity (low-Vs) zones were found, one low-Vs zone at depths 2-4km located beneath the volcano, a second low-Vp and low-Vs zone at depths 5-10km 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. This 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 (05km) around the active crater. The upper part (02km depth) of the high-Vp and high-Vs zone was characterized by low Vp/Vs ratios (<1.68), while the deeper part (210km) had high Vp/Vs ratios (>1.80). The upper part with low Vp/Vs ratios is correlated with a steam-dominated geothermal system. The high Vp/Vs zone extending from to 2 to 10km depth associated with a high-Vp zone beneath the active vent are interpreted as an intrusive body of magmatic origin which include partial melting zones associated with low-Vs anomalies . A small low-Vp zone was found to the southwest of the volcano. This zone seems to be the source of Long-Period (LP) earthquakes. Based on the data obtained with the tomography in combination with the seismicity, geochemistry, geology and gravimetry, we suggest a model for the seismic activity of NRV. Volcano-tectonic (VT) earthquakes that occurred 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 in separate the VT swarms located to the west of the volcano from the source of Longperiod (LP) earthquakes. The LP earthquakes are located above a high Vp/Vs zone. The LP events seemed to be interacting with the western VT seismicity. (AU)