Tree-ring reconstruction of past lahar activity at Popocatépetl volcano, Mexico

Abstract

Lahars represent a major threat on the slopes of volcanoes all over the world. In order to realistically assess hazards, knowledge on the occurrence and timing of past lahar activity is of crucial importance. However, archival data on past events is usually scarce or completely missing. Tree-ring records have repeatedly proved to be a reliable data source for the reconstruction of past geomorphic events. However, tree rings have seldom been applied for the identification of past lahars. Therefore, it was the aim of this study: (i) to identify and describe disturbances in tree growth induced by well-documented lahar events and on this basis; and (ii) to recognise older, unknown lahar events with tree-ring analyses. Based on these goals, we collected 140 tree-ring series from 62 trees ( Abies religiosa, Pinus hartwegii and Pinus ayacahuite) standing inside or adjacent to the lahar channel in the Huiloac gorge at Popocatépetl volcano, central Mexico. Most commonly, the known lahar events of 1997 and 2001 resulted in abrupt changes in tree-ring width as well as injuries. The same growth disturbances could be identified in the tree-ring series, indicating that five previously unknown lahar events would have occurred during the 20th century. Popocatépetl is one of the best surveyed volcanoes in the world and past eruptions are precisely noted in archives. As most of these unknown events occurred during periods with no volcanic activity, we believe that they were rainfall-induced rather than related to volcanic activity. In order to assess rainfall intensity threshold values for the triggering of events, the analyses of meteorological data needs to be integrated. In general, the investigated tree species proved to be highly suitable for the reconstruction of mass-movement processes. Therefore, the applied methods can be transferred to other locations where data on past events are missing.