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, although 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

Participatory capacities and vulnerabilities assessment: Towards the realisation of community-based early warning system for deep-seated landslides

Existing frameworks of community-based early warning systems (CBEWS) lack focus on its actual implementation. Furthermore, they do not describe how a top-down early warning system (EWS) can be transformed into a CBEWS. Hence, to support the establishment of a community-based early warning system for deep-seated landslides (CBEWS-L), this study aimed to assess the capacities and vulnerabilities of five barangays in the Philippines. The CBEWS-L to be established is envisioned to be information and communication technology (ICT)-based. An ICT4D perspective was therefore taken in conducting this participatory study. Stakeholder mapping, focus group discussions and key informant interviews were used to gather data. Validation was also done through triangulation and post-analysis consultations. The results showed that there are varying sets of capacities and vulnerabilities existing in each community. Moreover, ICT capacities are lacking and are outnumbered by vulnerabilities. Yet, site-specific action points to enhance capacities and resolve vulnerabilities were determined. Still, overall strategies were not uncovered. Furthermore, compounding the capacities and vulnerabilities in each site are indirect factors which, if examined thoroughly, may lead to more complex socio-political issues. It is therefore recommended that in establishing a CBEWS, a comprehensive community risk assessment is first conducted to exhaust all possible action points that can be used in formulating site-specific strategies. Moreover, development of technological solutions must be modular to allow flexibility in accommodating complex community capacities and vulnerabilities

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

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.Instituto Volcanológico de Canarias, EspañaEnvironmental Research Division, Instituto Tecnológico y de Energías Renovables, EspañaAgencia Insular de Energía de Tenerife, EspañaÁrea de Geología Marina, Instituto Geológico y Minero de España, EspañaInstitute of Agriculture and Environment, Massey University, AustraliaPhilippine Institute of Volcanology and Seismology, FilipinasGeochemical Research Center, The University of Tokyo, JapónEarth & Environmental Sciences, Wesleyan University, Estados UnidosPeer reviewe

(Table 1) Summary of maximum clast size and bed thickness of ODP Leg 126 samples

Subaerial debris flows, with water contents ranging from as little as 10 wt% up to no more than about 25 wt% (Pierson, 1986; Pierson and Costa, 1987), are non-Newtonian fluids that move as fairly coherent masses with yield strength (owing to bulk densities and viscosity that are much greater than those of clear water), which enables them to suspend and transport large clasts. Their flow behavior is thought to be predominantly laminar, although the relative importance of laminar and turbulent flow has not been established and is debatable. They leave deposits (debrites) that are characteristically poorly sorted with large clasts in their middle portions and commonly protruding from their tops. Although generally ungraded or normally graded in their upper portions, many have centimeter- to decimeter-thick inversely graded basal zones (Arguden and Rodolfo, 1990, doi:10.1130/0016-7606(1990)1022.3.CO;2)

Geochemistry of glass shards in the volcanic ash layers of the Izu-Bonin Arc

To examine the processes and histories of arc volcanism and of volcanism associated with backarc rifting. 130 samples containing igneous glass shards were taken from the Plioccne-Quatemai^ succession on the rift Hank (Site 788) and the Quaternary fill in the basin fill of the Sumisu Rift (Sites 790 and 791). These samples were subsequently analyzed at the University of Illinois at Chicago and Shizuoka University. The oxides determined by electron probe do not account for the total weight of the material; differences between summed oxides and 100% arise from the water contents, probably augmented by minor losses thai result from alkali vaporization during analysis. Weight losses in colorless glasses are up to 9%; those in brown glasses (dacitcs to basalts) arc no more than 4.5%; shards from the rift-flank (possibly caused by prolonged proximity to ihc scafloor) generally have higher values than those from the rift-basin fill How much of the lost water is magmatic, and how much is hydrated is uncertain; however, although the shards absorb potassium, calcium, and magnesium during hydration in the deep sea, they do so only to a minor extent that does not significantly alter their major element compositions. Therefore, the electron-probe results are useful in evaluating the magmatism recorded by the shards. Pre- and syn-rift Izu-Bonin volcanism were overwhelmingly dominated by rhyolile explosions, demonstrating that island arcs may experience significant silicic volcanism in addition to the extensive basaltic and basaltic andestic activity, documented in many arcs since the 1970s, that occurs in conjunction with the andesitic volcanism formerly thought to be dominant. Andesitic eruptions also occurred before rifting, but the andesitic component in our samples is minor. All the pre- and syn-rift rhyolites and andesites belong to the low-alkali island-arc tholeiitic suite, and contrast markedly with the alkali products of Holocene volcanism on the northernmost Mariana Arc that have been attributed to nascent rifting. The Quaternary dacites and andesites atop the rift flank and in the rift-basin fill are more potassic than those of Pliocene age, as a result of assimilation from the upper arc crust, or from variations in degrees of partial melting of the source magmas, or from metasomatic fluids. All the glass layers from the rift-flank samples belong to low-K arc-tholeiitic suites. Half of those in the Pliocene succession are exclusively rhyolitic: the others contain minor admixtures of dacite and andesite, or andesite and either basaltic andesite or basalt. In Contrast, the Quaternary (syn-rift) volcaniclastics atop the rift-flank lack basalt and basaltic andesite shards. These youngest sediments of the rift flank show close compositional affinities with five thick layers of coarse, rhyolitic pumice deposits in the basin fill, the two oldest more silicic than the younger ones. The coarse layers, and most thin ash layers that occur in hemipelagites below and intercalated between them, are low-K rhyolites and therefore probably came from sources in the arc. However, several thin rhyolitic ash beds in the hemipelagites are abnormally enriched in potassium and must have been provided by more distal sources, most likely to the west in Japan. Remarkably, the Pliocene-Pleistocene geochemistry of the volcanic front does not appear to have been influenced by the syn-rift basaltic volcanism only a few kilometers away. Rare, thin layers of basaltic ash near the bases of the rift-basin successions are not derived from the arc. They deviate strongly from trends that the arc-derived glasses display on oxide-oxide plots, and show close affinities to the basalts empted all over the Sumisu Rift during rifting. These basalts, and the basaltic ashes in the basal rift-basin fill, arc compositionally similar to those erupted from mature backarc basins elsewhere

Insights on Recent Ground Deformation of Taal Volcano based on continuous GPS Measurements

Six Global Positioning System stations around the active Taal Volcano caldera complex on Volcano Island (Philippines) detected an uplift event between December 2014 and May 2015. The greatest deformation was measured south of Main Crater Lake (MCL). The event preceded swarms of volcano-tectonic earthquakes (VTs) beneath Volcano Island. Along with seismicity, flux rates of CO2 dissolved in the waters of MCL rose gradually, reaching saturation point by March 2015. Taal Lake’s water level similarly increased. According to GPS observation data, Taal Volcano has slowly deflated since June 2015. Likewise, seismicity and water levels decreased to baseline levels by October 2015. Volcano deformation during this period of unrest is most consistent with pressurization of Taal Volcano’s hydrothermal system rather than direct magma processes, in agreement with seismicity and gas monitoring. We modeled the continuous GPS deformation velocities to estimate the location, depth and volume change of the deformation source before and after the episode of unrest. We inverted the deformation velocities using an analytical model that assumes an elastic, homogeneous and isotropic crus

Empirical Correlation between Standard Penetration Resistance (SPT-N) and Shear Wave Velocity (Vs) for Soils in Metro Manila, Philippines

The measurement of the shear wave velocities (Vs) of soils is an important aspect of geotechnical and earthquake engineering, due to its direct relation to the shear modulus (G), which in turn influences the stress–strain behavior of geomaterials. Vs can be directly measured or estimated using a variety of onsite tests or in a laboratory. Methods such as downhole PS logging require boreholes and may not be logistically and economically feasible in all situations. Many researchers have estimated Vs from other geotechnical parameters, such as standard penetration test resistance (SPT-N), by means of empirical correlations. This paper aimed to contribute to this subject by developing an empirical relationship between Vs and SPT-N. Data from twenty sites in Metro Manila were obtained from geotechnical investigation reports. Vs profiles of the same sites were also acquired using the refraction microtremor method. New empirical relationships were developed for all, sandy, and clayey soil types, using a non-linear regression method that is applicable for Metro Manila soils. Statistical evaluation and comparison of the proposed correlations with other previous works suggested the viability of the empirical model