ESTIMATION OF RISK PROBABILITY FOR GRAVITY-DRIVEN PYROCLASTIC FLOWS AT VOLCAN-COLIMA, MEXICO

Mapped pyroclastic flow terminations at Colima volcano were used to determine energy lines. We assumed straight energy lines; initial flow velocities of zero and flow movement starting from the volcano summit. Helm coefficients (H/L) of the flows plotted on a histogram cluster in two distinct modes. One corresponds to large pyroclastic flows (pumice flows and block-and-ash flows) for which Helm coefficients range from 0.22 to 0.28. This group has a mean value of 0.24 and a standard deviation of 0.021. The other mode corresponds to small block-and-ash avalanches which have Helm coefficients that range from 0.33 to 0.38, a mean value of 0.35 and a standard deviation of 0.025. No flow terminations yield Helm coefficients in the range from 0.28 to 0.33. This break probably separates fluidized pyroclastic hows from less mobile hot rock avalanches. Plots of Helm coefficients on arithmetic probability paper are approximate probability functions for the two types of flows. Heim coefficients calculated for straight lines that connect population centers with the volcano summit can be used with this type of graph to estimate the probability that either type of pyroclastic flow would reach the site. We used this technique to determine risk probabilities for various localities around Colima volcano. These calculations indicate that Laguna Verde, Yerbabuena, Cofradia-El Fresnal, El Naranjal, Atenguillo, La Becerrera, Montitlan and San Antonio have a probability ranging from 99 to 6% of being covered by large pyroclastic flows. Laguna Verde and Yerbabuena are the sites with the highest probability of being reached by small block-and-ash avalanches. The depression situated south-southwest of Colima volcano is an area with a very high probability of being affected by the pyroclastic phenomena considered above. The small avalanche produced by dome collapse of Colima on April 16, 1991 traveled along the barranca El Cordoban toward the area of the highest probability on our map.