Geology and hazard implications of the Maraunot notch in the Pinatubo Caldera, Philippines

Abstract

The 1991 Pinatubo eruption left 5-6 km 3 of debris on the volcano slopes, much of which has been mobilized into large lahars in the following rainy seasons. Also during the eruption, collapse, localized in part along preexisting faults, left a caldera 2.5 km in diameter that almost immediately began to accumulate a 1.6×10 8 m 3 lake. By 2001, the water had risen to the fault-controlled Maraunot Notch, the lowest, northwestern portion of the caldera rim comprising the physiographic sill of the Caldera Lake. That year, a narrow artificial canal dug into an old volcanic breccia underlying the outlet channel failed to induce a deliberate lake breakout, but discharge from heavy rains in July 2002 rapidly deepened the notch by 23 m, releasing an estimated 6.5×10 7 m 3 of lake water that bulked up into lahars with a volume well in excess of 1.6×10 8 m 3 . Lakes in other volcanoes have experienced multiple breakouts, providing practical motivation for this study. Fieldwork and high-resolution digital elevation models reveal andesites and ancient lacustrine deposits, strongly fractured and deformed along a segment of the Maraunot Fault, a prominent, steeply dipping, left-lateral fault zone that trends N35°-40°W within and parallel to the notch. Seismicity in 1991 demonstrated that the Maraunot Fault is still active. The fault zone appears to have previously been the erosional locus for a large channel, filled with avalanche or landslide deposits of an earlier eruption that were exhumed by the 2002 breakout floods. The deformed lacustrine sediments, with an uncalibrated 14 C age of 14,760±40 year BP from a single charcoal sample, attest to the existence of an earlier lake, possibly within the Tayawan Caldera, rim remnants of which survive as arcuate escarpments. That lake may well have experienced one or more ancient breakouts as well. The 2002 event greatly reduced the possibility of another such event by scouring away the erodible breccia, leaving less erodible fractured andesites and lacustrine rocks, and by enlarging the outlet channel and its discharge capacity. Several lines of evidence indicate, however, that future lahar-generating lake breakouts at the notch may keep populations of Botolan municipality downstream at risk: (1) a volume of 9.5× 10 7 m 3 of lake water remains perched 0.8 km above sea level; (2) seismicity in 1991 demonstrated that the Maraunot Fault is still active and movements of sufficient magnitude could enlarge the outlet and the discharge through it; (3) more likely, however, with or without earthquake activity, landslides from the steep to overhanging channel walls could block the channel again, and a major rainstorm could then cause a rise in lake level and sudden breakouts; (4) intrusion of a new dome into the bottom of the lake, possibly accompanied by phreatic explosions, could expel large volumes of lahar-generating water.

Influence of substrate tectonic heritage on the evolution of composite volcanoes: Predicting sites of flank eruption, lateral collapse, and erosion

“…This discharge produced erosion of this part of the caldera rim deepening the notch by 23 m, and as a consequence releasing an estimated 6.5 × 10 7 m 3 of lake water. This water bulked up into lahars with a volume well in excess of 1.6 × 10 8 m 3 that were the largest lahars ever to be generated by Pinatubo since its eruption in 1991 (Lagmay et al, 2007). From a geological-structural perspective, the Maraunot Notch is controlled by the presence of the Maraunot Fault, an active, northwest-striking, left-lateral basement fault (Fig.…”

Super Typhoon Bopha and the Mayo River Debris-Flow Disaster, Mindanao, Philippines, December 2012

“…All that is required to generate a debris flow is an abundance of loose rock debris and soil and a sudden large influx of water. They can be triggered by sudden downpours such as commonly delivered by tropical cyclones, by reservoir collapses [13], or by landslides dislodged by earthquakes into streams. Red areas are boulder-rich "true debris flow; orange areas are deposits of more dilute " hyperconcentrated" flows.…”

The December 2012 Mayo River debris flow triggered by Super Typhoon Bopha in Mindanao, Philippines: lessons learned and questions raised

“…They can be triggered by sudden downpours such as those commonly delivered by tropical cyclones, reservoir collapses (Lagmay et al, 2007) or landslides dislodged by earthquakes into streams. Many debris flows (Table 1) are associated with volcanoes (Vallance, 2000;Rodolfo, 2000;Lagmay et al, 2007). Casualties can be light or even non-existent in a poorly populated area, such as Mount St. Helens, or where people are familiar with the hazard, such as with lahars at Mount Pinatubo.…”