Renato U. Solidum

Origin of high field strength element enrichment in the Sulu Arc, southern Philippines, revisited

The enrichment of high field strength elements (HFSE) in Sulu Arc lavas has been proposed as a product of metasomatism of the mantle wedge. It is postulated that a dacitic melt, derived from melting of subducted Sulu Sea basaltic crust, stabilizes in the mantle wedge amphibole, which later breaks down and releases HFSEs into the source of basaltic arc lavas. New data for primitive, high-K calc-alkalic basalts that contain the highest HFSEs among Sulu Arc lavas and seafloor basalts subducting along the Sulu Trench have contrasting chemical and isotopic characteristics. This makes it unlikely that the source of HFSE enrichment in Sulu Arc lavas is melt derived from the subducted Sulu Sea basaltic crust or amphibole formed during metasomatism of the mantle wedge by such melt. We propose that HFSE enrichment in Sulu Arc lavas results from melting of a geochemically enriched component in the mantle wedge.

Interseismic deformation and moment deficit along the Manila subduction zone and the Philippine Fault system

We examine interseismic coupling of the Manila subduction zone and fault activity in the Luzon area using a block model constrained by GPS data collected from 1998 to 2015. Estimated long-term slip rates along the Manila subduction zone show a gradual southward decrease from 90-100 mm/yr at the northwest tip of Luzon to 65-80 mm/yr at the southern portion of the Manila Trench. We provide two block models (Models A and B) to illustrate possible realizations of coupling along the Manila Trench, which may be used to infer future earthquake rupture scenarios. Model A shows a low coupling ratio of 0.34 offshore western Luzon and continuous creeping on the plate interface at latitudes 18°-19°N. Model B includes the North Luzon Trough Fault and shows prevalent coupling on the plate interface with a coupling ratio of 0.48. Both models fit GPS velocities well though they have significantly different tectonic implications. The accumulated strain along the Manila subduction zone at latitudes 15°-19°N could be balanced by earthquakes with composite magnitudes of Mw 8.8-9.2 assuming recurrence intervals of 500-1000 years. GPS observations are consistent with full locking of the majority of active faults in Luzon to a depth of 20 km. Inferred moments of large inland earthquakes in Luzon fall in the range of Mw 6.9-7.6 assuming a recurrence interval of 100 years.

Volcanic Hazard Communication at Pinatubo from 1991 to 2015

When Pinatubo re-awakened in early 1991, very few people within the vicinity were familiar with volcanic hazards, and even fewer believed that Pinatubo could impact them. Scientists knew more, but were still struggling to answer: How often and how explosively did Pinatubo erupt, and when was its most recent eruption? What precursors could be expected in advance of a very large (VEI ≥ 6) explosive eruption? What was happening beneath Pinatubo that was driving 1991 unrest? To reach an exceptionally diverse audience and to counter widespread scepticism, scientists tried a whole package of communication measures, including simplified alert levels; a “worst case” hazard map; a probability tree; personalized briefings for local and national government officials, military and civil defense officials, nuns, and the news media; use of a IAVCEI video on volcanic hazards on broadcast TV and in briefings; volcanology tutorials for school teachers; talks on the mountain with villagers and anti-government guerrillas; and beer and hotdogs too. Forecasts were just-in-time and generally correct about what areas would be at risk. Overall, pre-eruption communication achieved its goal of getting people out of harm’s way. Three lessons stand out: use simple, multipronged communications, especially video; include worst case scenarios in your warnings, together with estimated probabilities thereof; and be willing, as scientists and decision makers, to recommend evacuations even if uncertainty is still high and there is still a chance of false alarm. For more than a decade after the 1991 eruption, rain-induced lahars threatened even more people and more infrastructure than the eruption itself. Several groups of scientists and engineers worked on the lahar threat, each coming up with slightly different long-term assessments that appeared to the public as bickering or incompetence. Scientists’ credibility was seriously diminished. Decisions of what lahar-mitigation projects to build—including a succession of inadequate ones—were influenced less by science and more by public pressure, pragmatism, back-room politics, and profit. Short-term or immediate lahar warnings were communicated by scientists and by police-manned watch points. The scientific warnings were technically superior but the police warnings had greater credibility, as they were from familiar sources and easily understood. Communication of hazard information at Pinatubo saved many lives, and we are proud and privileged to have been part of preventing a much worse disaster. However, margins of safety were narrow and some deaths that did occur could have been prevented by better communication.

The acid crater lake of Taal Volcano, Philippines: hydrogeochemical and hydroacoustic data related to the 2010–11 volcanic unrest

Abstract Studies of the water chemistry of Taal crater lake and echo-sounding surveys have provided new insights into its chemical and physical dynamics. During the volcano-seismic unrest of April 2010–June 2011, the waters of Taal crater lake showed changes in chemical composition and increases in CO2 emissions associated with the seismic unrest. The chemical and isotopic data show that the lake water has contributions from both seawater and meteoric water and receives injections of deep hydrothermal water and gases during periods of intense volcano-seismic unrest. These inflationary periods may lead to faulting of the impermeable cap rock that usually seals the deeper Taal hydrothermal reservoir in response to degassing and convective movements in the underlying Taal magma chamber.