John Makario Londoño

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

Abstract 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- V P ) and low S wave velocity (low- V S ) zones were found; one low- V S zone at depths 2–4 km located beneath the volcano; a second low- V P and low- V S zone at depths 5–10 km located beneath the crater, elongated and dipping to the E–SE; and a third low- V P and low- V S zone at 10 to ∼12 km farther to the east. These three low- V P and low- V S 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- V P and high- V S zone was characterized by low- V P / V S ratios ( V P / V S ratios (>1.80). The low- V P / V S zone is correlated with a steam-dominated geothermal system. The high- V P / V S is interpreted as an intrusive body of magmatic origin which includes partial melting zones associated with low- V S anomalies. A small low- V P 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.

Temporal change in coda Q at Nevado Del Ruiz Volcano, Colombia

Abstract An analysis of coda Q ( Q c ), intrinsic Q ( Q i ), scattering Q ( Q s ) and total Q ( Q t ), was made at Nevado del Ruiz Volcano (NRV) for local earthquakes recorded during 1992–1994. Coda Q has been calculated for the first time at Nevado del Ruiz Volcano (NRV) using the single-scattering model proposed by Aki and Chouet (1975). Estimations of Q c were made for five frequency bands centred at 1.5, 3.0, 6.0, 12.0 and 24.0 Hz. The estimated Q c varies from around 60 at 1.5 Hz to around 980 at 24 Hz, depending on the frequency with Q c = 30 ± 1.2 F 1.03±0.001 . in general, Q c values at NRV are rather similar to those obtained in other volcanic areas in the world. A steady trend in Q c -1 was found from January 1992 to January 1994. This variation is coincident with the stability in the volcanic activity since the last phreato-magmatic eruption on 1 September 1989. In the middle of April 1994, a new activity started at NRV and a small volcanic crisis occurred. Continuous increasing was observed in Q c -1 around two months before the crisis. This suggests a possibility that the temporal variation of Q c can be useful for monitoring the volcanic activity at NRV. Q i and Q s were separated from Q t by using a multiple-scattering model called “Multiple Lapse Time Window Analysis”, MLTWA (Fehler et al., 1992). Estimations for Q i , Q s and Q t were made for four frequency bands, 3.0, 6.0, 12.0 and 24.0 Hz. Q i ; values were found to be near to those of Q c for frequencies higher than 6.0 Hz. For low frequencies Q s values were near to Q c values. This can suggest that the intrinsic absorption could be the predominant attenuation phenomenon in the NRV area at frequencies higher than 6 Hz and the scattering phenomenon is predominant for low frequencies; therefore the change in Q c at high frequencies is considered to reflect variations in the inner conditions of the volcano, such as changes in temperature, movement of fluids, and so on.