Published Research
Frontal wedge variations and controls of submarine landslides in the Negros-Sulu Trench System, Philippines
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
Frontiers in Earth Science (Marine Geoscience)
https://doi.org/10.3389/feart.2023.1054825
March 2023
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
Frontal wedge characteristics provide clues to the efficiency of the overriding slab for large displacement during megathrust and upper-plate earthquakes, whereas submarine landslides along active margins may trigger or amplify tsunamis. The lack of clear precursors of submarine failures poses difficulty in monitoring and providing real-time alert warning systems. With that, delineating submarine features along active margins, their spatial distribution, and controls provide valuable information in identifying regions susceptible to large submarine landslides and tsunami hazard assessments. In this study, we performed terrain and morphometric analyses on 20 m resolution bathymetry data to map submarine landslides, submarine canyons, and lineaments in the forearc margin of the Negros–Sulu Trench System in the Philippines. Lineaments are distributed mainly along the frontal wedge, where previous seismic surveys revealed that the mapped ridges are morphotectonic expressions of thrusted sediments. The morphological variations of the four frontal wedge segments were attributed to heterogeneous sediment influx, convergence rates, and subduction processes. More than 1,200 submarine landslides and their morphometric parameters were delineated, and exploratory spatial analyses indicate clustering and underlying controls. The tendencies of prolate submarine landslides (high L/W) to significantly cluster along submarine canyons while oblate morphologies (low L/W) along the frontal wedge reflect the different environments and geomorphological conditions to form these contrasting shapes. Ubiquitous small submarine landslides are mainly controlled by submarine canyon systems at relatively shallow depths of <2 km, where high sediment influx from inland sources preconditions instability. Large submarine landslides (>0.5 km3), on the other hand, are significantly most clustered where the Cagayan Ridge seamount collides and subsequently subducts beneath the northernmost frontal wedge. This suggests the dominant role of seamount subduction and related tectonic processes causing slope steepening to mainly induce large submarine landslides. This study unveiled how submarine landslides vary morphologically depending on their spatial, geomorphological, and tectonic controls in the active margin. This new information provides clues in identifying offshore areas susceptible to large submarine landslides that may induce damaging tsunamis in the Negros–Sulu Trench System as well as in other active margins of similar underlying controls.
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
(Figure) Submarine landslides, submarine canyons, and lineaments mapped in the active margin of the NSTS. Rose diagram shows the general northerly trend of the lineaments. Four segments (NT1, NT2, ST1, ST2) were delineated based on the orientation and width variations of the frontal wedge. Squares a–d show the locations of representative submarine features in Figure 5. Red lines are transects of seismic reflection profiles from Schlüter et al. (1996) (Figure 6). The northwest-trending lineaments mapped offshore of Zamboanga Peninsula are inferred to be an extension of the Sindangan–Cotabato–Daguma Lineament (SCDL) in western Mindanao Island (orange lines = onshore faults/lineaments from PHIVOLCS). From Nawanao and others (2023).
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
Alec Benjamin G. Ramirez, Noelynna T. Ramos, Lyndon P. Nawanao Jr., Robelyn Z. Mangahas Flores,
Ishmael C. Narag, Toshitaka Baba, Naotaka Chikasada, and Kenji Satake
Frontiers in Earth Science (Marine Geoscience)
https://doi.org/10.3389/feart.2022.1067002
November 2022
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
Tsunamis have been known to result from a wide range of phenomena, such as earthquakes, volcanic eruptions, submarine mass failures, and meteorite impacts. Of earthquake-generated tsunamis, those arising from strike-slip mechanisms are less common, with the 1994 Mindoro tsunami in the Philippines among the few known examples. The 1994 Mindoro tsunami followed a Mw 7.1 earthquake along the right-lateral Aglubang River Fault. The tsunami affected the coasts surrounding the Verde Island Passage, one of the Philippines’ insular seas located between the islands of Luzon and Mindoro, and east of the West Philippine Sea margin. A total of 78 lives were lost due to the earthquake and tsunami, with 41 being directly attributed to the tsunami alone. Despite the close spatial and temporal association between the 1994 Mindoro earthquake and tsunami, previous numerical modeling suggests the need for other contributing mechanisms for the 1994 tsunami. In this study, we conducted submarine geomorphological mapping of the South Pass within the Verde Island Passage, with particular focus on identifying possible submarine mass failures. Identification of submarine features were based on Red Relief Image Map (RIMM), Topographic Position Index (topographic position index)-based landform classification, and profile and plan curvatures derived from high-resolution bathymetry data. Among the important submarine features mapped include the San Andres submarine mass failure (SASMF). The San Andres submarine mass failure has an estimated volume of 0.0483 km3 and is located within the Malaylay Submarine Canyon System in the Verde Island Passage, ∼1 km offshore of San Andres in Baco, Oriental Mindoro. We also explored two tsunami models (EQ-only and EQ+SMF) for the 1994 Mindoro tsunami using JAGURS. The source mechanisms for both models included an earthquake component based on the Mw 7.1 earthquake, while the EQ+SMF also included an additional submarine mass failure component based on the mapped San Andres submarine mass failure. Modeled wave heights from the EQ-only model drastically underestimates the observed wave heights for the 1994 Mindoro tsunami. In contrast, the EQ+SMF model tsunami wave height estimates were closer to the observed data. As such, we propose an earthquake-triggered, submarine mass failure source mechanism for the 1994 Mindoro tsunami.
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
(Figure) Three-dimensional view of the offshore Aglubang River Fault and the Malaylay Submarine Canyon System (MSCS). The red dots show the location of depth comparisons with the thickness of eroded material in meters. KnC = Kanan Submarine Canyon; KlC = Kaliwa Submarine Canyon. From Ramirez and others (2022).
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
Regina Martha G. Lumongsod, Noelynna T. Ramos, and Carla B. Dimalanta
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
Karstification is influenced by numerous factors, such as climate, geomorphology, and geology. Karstified areas are faced with numerous problems such as ground subsidence and groundwater vulnerability. In rapidly urbanizing areas like Cebu in the Philippines, it is imperative to know the karstification potential, especially if karst landforms are already present. We evaluated the karstification potential of central Cebu using Raster Overlay Analysis (ROA) in a geographic information system (GIS) platform. ROA uses multiple maps which represent different factors affecting karstification, such as precipitation, temperature, vegetation, elevation, slope, drainage, lineaments, and geology. These factors were then correlated with the karstification potential map using the Cell Statistics tool. ROA reveals that 17% of central Cebu has very low karstification potential, while < 65% has low to moderate karstification potential and 18% has high to very high karstification potential. High karstification potential is generally associated with high precipitation (> 2100 mm/yr), low temperature (< 24.5 °C), very dense vegetation, high elevation (> 787 masl), gentle slope (< 10°), very high lineament density (> 0.45), very low drainage density (0), and older limestone bedrock. Areas with moderate to very high karstification potential coincide with cave locations. Cell Statistics (Range) reveals that the karstification potential map is strongly correlated with precipitation and temperature, and weakly correlated with drainage density and geology. While assumed to greatly affect karstification, the weak correlation of local geology with the karstification potential is attributed to lithological heterogeneities in central Cebu. Information on an area’s karstification is beneficial to effective land use planning and reduction of karst-related hazards.
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
Raul Benjamin C. Mendoza, Noelynna T. Ramos, and Carla B. Dimalanta
Journal of Asian Earth Sciences: X
https://doi.org/10.1016/j.jaesx.2022.100097
June 2022
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
The heavily populated Cebu Island is cut by the Central Cebu Fault System (CCFS). While the CCFS has not produced any Mw > 5.0 earthquakes in the past century, recent strong earthquakes in the adjacent islands have brought attention to the seismic hazards in the region. Fault properties such as strike, dip, slip direction, and surface trace length were determined based on literature review, fieldwork, and analysis of geomorphic features. Empirical relations were utilized to estimate down-dip width and magnitude. The gathered data were used to create a three-dimensional model of the four major faults in the CCFS: Balamban Fault, Central Highland Fault, Uling-Masaba Fault, and Lutac-Jaclupan Fault. The 3D model was used to generate peak ground acceleration maps of central Cebu, should an earthquake occur along any of the major faults. Site corrections were made based on the seismic velocity of the upper 30 m of the subsurface. The major faults are estimated to be capable of generating Mw 6.4 to 7.1 earthquakes. Worst-case scenarios in densely populated areas show 0.40 to 0.70 g of PGA, suggesting the potential for severe damage in central Cebu. We explored the advantages of using raster mathematics in a GIS platform for calculating and presenting ground motion. These advantages include rapid calculations for tens of millions of points, reducing the effects of interpolation artifacts in final map products. This study emphasizes the importance of detailed structural, geological, and geomorphological data in modeling seismic hazards. Further investigations on the seismogenesis of the CCFS segments are recommended.
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
(Figure) Major faults of the CCFS delineated on a geological map. TG – Talavera Group, NG – Naga Group. Colored markers represent the seismicity in the area recorded in the ISC, USGS, and PHIVOLCS catalogs. See supplementary material (S1) for details of the rock formations and structural units. From Mendoza and others (2023).
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
Sheinna May D. Claro, Noelynna T. Ramos, Allan Gil S. Fernando, Daisuke Ishimura, and Adam D. Switzer
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
The Philippines' active tectonic setting and vulnerability to tsunami hazards underscore the necessity to understand tsunami sources and their impacts. Several tsunamigenic earthquakes have devastated coastal communities in the past but geological evidence of these infrequent extreme wave events (EWEs) have yet to be described and analyzed in detail. This study documents and establishes evidence of potential tsunami deposits preserved in a mangrove environment in western Mindanao Island, which was inundated by the 1976 Moro Gulf tsunami. The 1976 Moro Gulf tsunami, by far the worst tsunami disaster in the Philippines, is associated with a Mw 8.1 earthquake along the Cotabato subduction zone. The sedimentological characteristics of potential tsunami deposits in three coastal sites bordering Pagadian and Illana Bays in Zamboanga del Sur, Mindanao, southern Philippines were analyzed and compared to describe the preservation of washover deposits in different coastal systems. The potential washover deposits of the 1976 tsunami were identified based on their sedimentary characteristics and features that contrast with the background (pre-tsunami event) sediments. The potential 1976 tsunami deposits are predominantly sand-sized and coarser than the background sediments. They also contain mud rip-up clasts, magnetite lamina, and an erosive base, similar to reported washover deposits elsewhere. The thickness of the washover deposits ranges from 7 to 12 cm. The thickest sand layer was observed in a mangrove swamp, implying that local topography influenced the distribution and preservation of the deposit. A potentially older EWE deposit was also observed in Pagadian City. The background sediments in both the mangrove swamps and coastal plains are mud to fine sand. While scarcity in historical data and rapidly changing environmental conditions pose challenges in studying washover deposits in a tropical setting, this study highlights essential information and lessons for future researchers of washover deposits in the Philippines. Sedimentological data on the potential EWE washover deposits in western Mindanao Island serve as fundamental information in understanding the processes and mechanisms of extreme wave events in this region. This study further establishes geological evidence of the country's worst historical tsunami, the 1976 Moro Gulf tsunami, which contributes to our knowledge of tsunami deposits in tropical settings.
Lyndon P. Nawanao Jr. and Noelynna T. Ramos
(Figure) Sedimentary facies observed in Pagadian City. Washover deposits in Transect 1 are described in detail. The sand II layer in PAG2 is interpreted to be potentially associated with the 1976 Moro Gulf tsunami. Meanwhile, sand II in PAG1 is hypothesized to be associated with a prehistoric EWE. Close-up photos of sand II (red and pink boxes) and burrow (blue box) in PAG3 are also shown. The Munsell color designations are labeled for each layer. The elevation profile was derived from the DTM acquired by the UP Phil-LiDAR 1 Program. Pink circles on the elevation profile show the approximate location where the samples/cores were extracted. The depth was measured from the surface of extraction. From Claro and others (2022).
