Northwestern Luzon Research Articles

Mapping and characterization of the liquefaction impacts of the July and October 2022 earthquakes in Northwestern Luzon, Philippines

The 25 July 2022 MW 7.0 and 25 October 2022 MW 6.4 earthquakes caused liquefaction in the lowlands of Northwestern Luzon, Philippines. With the widespread liquefaction occurrence, a methodology was developed for mapping and assessing its impacts, which combined mapping techniques, geomorphological analysis, review of historical liquefaction accounts and empirical equations, utilization of available remote sensing technologies, and application of various geophysical and geotechnical equipment. The majority of liquefaction impacts of the MW 7.0 earthquake occurred along the Abra River delta, while the MW 6.4 earthquake had most of its liquefaction impacts concentrated in river dikes and abandoned channels across the Solsona Basin. Grain size analysis confirmed that the sediments indeed had the potential to liquefy. The soil parameters obtained using the refraction microtremor (ReMi) and screw driving sounding (SDS) tests also showed strong correlations, with liquefaction potential values ranging from low to high. Soil period values were also consistent with the ReMi and SDS results.The results of this study emphasize that: 1) detailed mapping and assessment of areas affected by liquefaction is valuable in aiding researchers and planners in hazard mitigation; and 2) the geophysical and geotechnical approaches employed are suitable and less invasive alternatives in site-specific liquefaction potential assessment.

Expert-driven geothermal play fairway analysis for low to medium temperature hidden systems in northwestern Luzon Island, Philippines

The Philippines is the third largest geothermal power-producing country in the world. However, additional opportunities exist to expand the geothermal portfolio of the country with the exploitation of low- and moderate- temperature resources, as well as assessment and development of hidden geothermal systems (i.e., those with no active surface manifestations). As part of the Geothermal Development Initiative (GEODE) funded through the U.S. Department of State award to the University of Nevada, Reno, a collaboration between the Philippines Renewable Energy Management Bureau (REMB) of the Department of Energy and the University of Nevada in Reno, Great Basin Center for Geothermal Energy occurred from September 2022 to March 2024. As part of this collaboration, an expert-driven geothermal Play Fairway Analysis (PFA) was conducted in the northwestern portion of Luzon Island of the Philippines utilizing existing data sets available to the REMB. The goal of this PFA is to identify areas with greater relative geothermal favorability, determine where the data gaps are, and assist in the discovery of hidden geothermal systems. A geothermal PFA requires the compilation and transformation of datasets into appropriate formats (e.g., tables, shapefiles, and raster files) and evaluation of each dataset to determine how it can be used as a proxy for the key attributes of a geothermal system (e.g., heat and permeability). Once the data are organized, they are weighted for integration into the GIS software to develop a favorability model for heat and permeability. The attribute models are then combined to produce two PFA models: the conventional hydrothermal low- to moderate- temperature geothermal favorability and the hidden systems favorability. The data difference between the conventional geothermal favorability and hidden systems models is that hot springs are not used for the hidden systems model. The conventional favorability model of northwestern Luzon Island indicates areas of higher geothermal favorability in the central and southern sections of the study area. Many of the favorable areas identified in the conventional model already have active leases for geothermal exploration, lending more confidence to the methodology and model, and suggesting that the areas with no current leases and high geothermal favorability could represent future exploration targets. The hidden systems model shows favorable zones in the north, central and southern sectors, and can serve as a tool to decide the areas for exploration of potential hidden geothermal systems, particularly, in the Ilocos Norte, Apayao, and Nueva Viscaya regions.

Two new endemic Begonia species named after historical landmarks of Ilocos Sur, Philippines

Two new species of Begonia endemic to the Philippines are described from the province of Ilocos Sur, Northwestern Luzon: Begonia tiradpassensis (B. sect. Baryandra) from Tirad Pass, and Begonia bessangpassensis (B. sect. Petermannia) from the subsummit of Bessang Pass Natural Monument/Landmark. Based on IUCN Red List list criteria, B. tiradpassensis & B. bessangpassensis are provisionally assessed to be threatened, as they are confined to small populations with anthropogenic disturbance surrounding their distribution area. The discovery of these species highlights the conservation importance of two historical landmarks, Tirad Pass and Bessang Pass, which have left an indelible mark on Philippine history.

Oblique Blind Faulting Underneath the Luzon Volcanic Arc During the 2022 Mw 7.0 Abra Earthquake, the Philippines

AbstractUnknown seismogenic structures lurking beneath convergent margins introduce substantial uncertainty in seismic hazard assessments. In northwestern Luzon, the Mw 7.0 Abra earthquake on 27 July 2022 highlights the seismic activity along an unmapped blind fault underneath the Cordillera Central. By integrating coseismic displacements constrained by radar satellite imagery and Global Navigation Satellite System, we image oblique coseismic slip at 11–22 km depth with peak slip of ∼1 m beneath the Philippine Fault Zone in northern Luzon. The southward propagation of coseismic slip and aftershocks terminated at a distance of 50 km from the northern end of the 1990 Luzon earthquake rupture, leaving a seismicity gap in between. Coulomb stress changes of reaching 2 bars are imparted at the shallow portions of the Vigan‐Aggao and Abra River faults, where the updated 100‐year seismic potential is increased to Mw 7.0–7.7, given the thick seismogenic layer of ∼30 km in northern Luzon.

Review of: "A Research Note on Natural Reclamation Processes that Support Mangrove Biodiversity Spheres: Sedimentation in Three Major River Deltas in Northwestern Luzon Using Aerial Imagery"

Review of: "A Research Note on Natural Reclamation Processes that Support Mangrove Biodiversity Spheres: Sedimentation in Three Major River Deltas in Northwestern Luzon Using Aerial Imagery"

Impacts and causative fault of the 2022 magnitude (Mw) 7.0 Northwestern Luzon earthquake, Philippines

At 00:43 UTC on 27 July 2022, a 15-km deep major earthquake with magnitude (Mw) 7.0 struck Northwestern Luzon, Philippines. The strongest ground shaking felt was at PHIVOLCS Earthquake Intensity Scale (PEIS) VII (destructive), equivalent to Modified Mercalli Intensity (MMI) VII, in Abra and along the coastal areas of Ilocos Sur. More than a thousand landslides, rockfalls and tension cracks were mapped, near the epicentral region, in the northwestern part of the Central Cordillera. Most of the landslides were shallow-seated, many of which were situated along road cuts. Liquefaction manifested as lateral spreads, sand boils, fissures, ground subsidence, and localized swelling was documented along the coastal areas of Ilocos Sur and river channels in Abra and Ilocos Sur. Sea level disturbance was also observed in some coastal areas of Ilocos Sur and La Union. Damages to buildings and infrastructures were documented in areas that experienced PEIS VI (very strong), equivalent to MMI VI and PEIS VII (destructive). Earthquake data, including hypocentral location, aftershock distribution, focal mechanism solutions and strong motion data, and InSAR observation indicate that the earthquake was generated by an almost north-south striking reverse left-lateral oblique fault that is gently dipping to the east. There is no clear indication of a surface rupture based on InSAR analysis and field investigation. The spatial distribution of geologic impacts, such as earthquake-induced landslides and liquefaction, is strongly controlled by the causative fault, the direction of rupture propagation and geology. Peak ground acceleration (PGA) records show a unidirectional rupture propagation and are congruent with the spatial distribution of earthquake impacts. Although earthquake parameters, deformation analysis and field data suggest that the Abra River Fault is the probable causative fault, the derived geometry and kinematics from the seismotectonic analysis challenge our existing understanding of the nature of the Abra River Fault, as well as the other segments of the Philippine Fault. The need to understand these earthquake sources in the country is needed for a better seismic hazard and risk assessment.

Seasonal variability of satellite-derived primary production in the South China Sea from an absorption-based model

Seasonal patterns of marine net primary production (NPP) are crucial for understanding the marine ecosystem and biogeochemical cycles. Uncoupling of seasonal variations between NPP and phytoplankton chlorophyll-a over different areas has attracted much attention. In this study, following a review of previous studies, monthly climatological NPP data from 2003 to 2020, estimated using the Size-fractioned Phytoplankton Pigment Absorption (aph)-based NPP Model (SABPM), were selected to study the seasonal variability of NPP in the South China Sea (SCS). Results showed the spatial differences of NPP seasonality and its departures from climatology in extreme El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) years. Cluster analysis for climatological monthly data identified significant differences of NPP seasonality in five typical regions. In coastal regions along the northern SCS and off eastern Vietnam, NPP exhibited the most obvious seasonal cycle with maximum (minimum) values in summer (winter), attributable mainly to river discharge and summer upwelling. In regions off northwestern Luzon and coast of southern SCS, NPP showed peaks in winter, which were related to strong mixing and upwelling. In northwestern SCS, NPP was high during May–September in phase with sea surface temperature and the primary controlling factors were found to be shallow nutricline depth and wind-driven mixing. Owing to the deep nutricline depth in the central basin, NPP exhibited little seasonal variability; only a weak signal was observed in spring in phase with photosynthetically active radiation. Local dynamics on regulating the nutrient supply and light availability contribute to these regional differences in NPP seasonality, which could also be affected by extreme climate events. The largest anomalies of the NPP seasonal cycle coincide with 2015/2016 ENSO and super IOD in 2020. During these events, enhanced (weakened) westerly winds caused fall (rise) of SLA and increase (decrease) of NPP in coastal regions along the northern SCS and that off eastern Vietnam. Overall, theaph-based model shows a new perspective to study the spatiotemporal variations of NPP in the SCS.

Luzon predators: Clues from a fossil with bite marks

Luzon Island remained isolated throughout its emergent history with its paleofauna often considered depauperate. However, the terrestrial vertebrate fossil record of the island is still poorly known. A new fossil-bearing paleochannel deposit was discovered in La Union Province, northwestern Luzon, Philippines. This paleochannel is inferred to be of Pleistocene age and has yielded fossils tentatively classified as a bovid (Bubalus sp.), a cervid (Rusa sp.), a suid (Sus sp.), and a large crocodile (Crocodylidae). Interestingly, a proximal fragment of the left tibia of a cervid shows multiple bite marks. This study analyzes the bite marks and modifications on the specimen to infer its possible trace maker and explore its implications in understanding the ancient predatory guild of Luzon. Three sets of modifications were recognized on the specimen, namely the bite marks, the broken proximal end and the dry bone fractures. A crocodilian agent for the bite marks is supported by the bisections, their occurrence in the paleofaunal assemblage and the fluvial setting of the deposit. However, an unknown carnivoran agent interpretation cannot be fully disregarded given the ambiguity of bisected marks and the possibly gnawed proximal end of the specimen. Unfortunately, given the lack of in situ context and apparent overprinting, the extent of the peri- and post-mortem modification remains uncertain which hinders the identification of the predatory agent. Verification of the observations reported in this study would require the systematic collection and detailed taphonomic analysis of additional fossil specimens from the deposit.

Source Model and Characteristics of the 27 July 2022 MW 7.0 Northwestern Luzon Earthquake, Philippines

The geometry and kinematics of the causative fault of the 27 July 2022 moment magnitude (Mw) 7.0 earthquake, which is one of the strongest to hit northern and central Luzon in the past 30 years, were estimated through inverse modeling of line-of-sight interferometric synthetic aperture radar deformation. We modeled rupture along multiple candidate faults based on fit with the pattern of line-of-sight deformation, consistency with focal mechanisms, and compatibility with the known kinematics of the mapped active faults in the region. Our preferred fault model, located west of and parallel to the Abra River Fault (ARF), exhibits localized reverse-slip (average 67° rake) at 15-35 km down-dip. Peak slip occurs at 13-16 km depth, with 95 cm of pure reverse-slip. The existence of a reverse-slip dominated ARF-parallel fault rupture is consistent with a complex shear partitioning model, wherein the NW-SE oblique plate convergence is accommodated not only by the sinistral strike-slip Philippine Fault Zone and the major subduction zones, but also by minor faults in intervening crustal blocks.

Multiple severe storms revealed by coral boulders at Pasuquin, northwestern Luzon, Philippines

Over 30 meter-sized coral boulders are scattered 45–140 m away from the edge and above high tide on a Holocene reef flat at Pasuquin, northwestern Luzon, Philippines. The boulders are overturned or tilted as indicated by the framework fossil corals in them, but have the same lithology as those along the reef edge and thus were likely broken off from there. The dimensions of boulders larger than 3 m were calculated from 3D models constructed by photogrammetry. Their volumes range from 10 to 53 m3. Assuming 2.1 g/cm3 for wet density, weights of boulders would range from 21 to 110 metric tons. Boulders of such size and weight can't be moved by normal waves, and thus must have been dislodged by extreme wave events (EWEs). Small and well-preserved corals found on the surface of seven boulders were collected for 230Th dating to reconstruct the timing of displacement. The ages of corals are 1781.6 ± 1.9, 1903.4 ± 2.7, 1945.8 ± 1.2, 1956.9 ± 1.2, 1956.75 ± 0.99, 1978.1 ± 1.5 and 2002.78 ± 0.88 CE, respectively. These ages are considered to constrain the timing of boulder displacement from the reef edge. We propose that typhoon-induced EWEs were responsible for the displacement of these boulders at Pasuquin.

Notes on the genus Amorphophallus Blume ex Decne. (Araceae) of northwestern Luzon, Philippines, including a new species

A botanical exploration conducted by the Northwestern University – Biodiversity Research Unit and other collaborating institutions in the Ilocos Region, Northwestern Luzon, Philippines, led to the documentation of five Amorphophallus species in the area. Here, we provide new distributional records for A. longispathaceus and A. urceolatus, an expanded description and new conservation status for A. adamsensis, and describe new color forms of A. paeoniifolius. Moreover, the new species Amorphophallus flammeus is described and compared to its closest relatives.

Inventory of Rubiaceae species in Mt. Pao Range, Ilocos Norte, Northwestern Luzon, Philippines

Abstract. Batuyong MAR, Calaramo MA, Alejandro GJD. 2021. Inventory of Rubiaceae species in Mt. Pao Range, Ilocos Norte, Northwestern Luzon, Philippines. Biodiversitas 22: 3604-3612. Botanical assessments in the Philippines are mostly conducted on declared Protected Areas (PAs). However, many remaining potentially interesting and largely unexplored floristic sites are not considered PA, one of which is the Mt. Pao Range in the Municipality of Adams, located in Ilocos Norte, Philippines. Among the flowering plant family, Rubiaceae is one with the largest number of endemic species and genera in the Philippines Thus, floristic surveys of Rubiaceae species were conducted, and collections were made from September 2019 to March 2021. A total of 39 species belonging to 13 tribes and 24 genera of the family were recorded. These constitute 7.09% of the total Philippine Rubiaceae. Of these species, 18 are Philippine endemics while two are considered narrow endemic to Mt. Pao. Two endemic genera were also found in the area Antherostele and Kanapia. According to the Updated National List of Threatened Philippine Plants, their conservation status in the Department of Environment and Natural Resources (DENR) Administrative Order No. 2017-11 found one species assessed as Critically Endangered (Antherostele callophylla). This study contributes to the data on the distribution and occurrence of Philippine Rubiaceae. Moreover, it also highlights the area's importance with the presence of threatened and endemic species as an area of concern for protection and conservation.

Diversity of Rubiaceae in Ilocos Norte, Northwestern Luzon, Philippines: A Preliminary Checklist, Their Distribution, and Conservation Status

The northern Cordillera Mountain Range of Ilocos Norte is an important site for floristic studies in the northwestern part of the Philippines. However, most of its forested areas remain understudied and limited data are available on the structure of its floristic communities. Here, a preliminary checklist of Rubiaceae species in Ilocos Norte is provided with notes on their distribution and conservation status. The field survey was conducted from April 2019 to March 2020 in Kalbario Patapat Natural Park (KPNP), Metropolitan Ilocos Norte Watershed Forest Reserve (MINWFR), and Mt. Lammin. A total of 57 species belonging to 28 genera and 15 tribes were recorded, which constitute 10.36% of the total number of Philippine Rubiaceae species. The genera with the highest number of recorded species are Psychotria, Ixora, and Ophiorrhiza with nine, six, and four species, respectively. Of these species, 27 are Philippine endemics that are widely distributed, and four species are endemic to the province. Among the sites surveyed, the tropical evergreen lowland rain forest of KPNP had the highest number of recorded species. With regard to their conservation status according to the IUCN (International Union for Conservation of Nature) red list category, one was assessed as Endangered. Moreover, based on the updated national list of threatened Philippine plants and their categories identified in the DENR (Department of Environment and Natural Resources) AO (Administrative Order) No. 2017-11, two are listed as Threatened species, one Critically Endangered, and one Vulnerable. According to the IUCN red list categories and criteria (2019), the four endemic species in the province recognized in the study are categorized as follows: one Critically Endangered (Antherostele callophylla), one Endangered (Psychotria tayabensis var. castanea), and two Vulnerable (Antirhea microphylla and Pyrostria triflora).

Overprinting porphyry‐type veinlets on the intrusive rocks and phreatomagmatic breccias in the Southwest prospect, southwestern Sto. Tomas II (Philex), Baguio District, Philippines

AbstractThe Southwest prospect is located at the southwestern periphery of the Sto. Tomas II porphyry copper–gold deposit in the Baguio District, northwestern Luzon, Philippines. The Southwest prospect hosts a copper‐gold mineralization related to a complex of porphyry intrusions, breccia facies, and overlapping porphyry‐type veinlets emplaced within the basement Pugo metavolcanics rocks and conglomerates of the Zigzag Formation. The occurrences of porphyry‐type veinlets and potassic alteration hosted in the complex are thought to be indications of the presence of blind porphyry deposits within the Sto. Tomas II vicinity. The complex is composed of at least four broadly mineralogically similar dioritic intrusive rocks that vary in texture and alteration type and intensity. These intrusions were accompanied with at least five breccia facies that were formed by the explosive brecciation, induced by the magmatic–hydrothermal processes and phreatomagmatic activities during the emplacement of the various intrusions. Hydrothermal alteration assemblages consisting of potassic, chlorite–magnetite, propylitic and sericite–chlorite alteration, and contemporaneous veinlet types were developed on the host rocks. Elevated copper and gold grades correspond to (a) chalcopyrite–bornite assemblage in the potassic alteration in the syn‐mineralization early‐mineralization diorite (EMD) and contemporaneous veinlets and (b) chalcopyrite‐rich mineralization associated with the chalcopyrite–magnetite–chlorite–actinolite±sericite veinlets contemporaneous with the chlorite–magnetite alteration. Erratic remarkable concentrations of gold were also present in the late‐mineralization Late Diorite (LD). High XMg of calcic amphiboles (>0.60) in the intrusive rocks indicate that the magmas have been oxidizing since the early stages of crystallization, while a gap in the composition of Al between the rim and the cores of the calcic amphiboles in the EMD and LD indicate decompression at some point during the crystallization of these intrusive rocks. Fluid inclusion microthermometry suggests the trapping of immiscible fluids that formed the potassic alteration, associated ore mineralization, and sheeted quartz veinlets. The corresponding formation conditions of the shallower and deeper quartz veinlets were estimated at pressures of 50 and 30 MPa and temperatures of 554 and 436°C at depths of 1.9 and 1.1 km. Temperature data from the chlorite indicate that the chalcopyrite‐rich mineralization associated with the chlorite–magnetite alteration was formed at a much lower temperature (ca. 290°C) than the potassic alteration. Evidence from the vein offsetting matrix suggests multiple intrusions within the EMD, despite the K‐Ar ages of the potassic alteration in EMD and hornblende in the LD of about the same age at 3.5 ± 0.3 Ma. The K‐Ar age of the potassic alteration was likely to be thermally reset as a result of the overprinting hydrothermal alteration. The constrained K‐Ar ages also indicate earlier formed intrusive rocks in the Southwest prospect, possibly coeval to the earliest “dark diorite” intrusion in the Sto. Tomas II deposit. In addition, the range of δ34S of sulfide minerals from +1.8‰ to +5.1‰ in the Southwest prospect closely overlaps with the rest of the porphyry copper and epithermal deposits in the Sto. Tomas II deposit and its vicinity. This indicates that the sulfides may have formed from a homogeneous source of the porphyry copper deposits and epithermal deposits in the Sto. Tomas II orebody and its vicinity. The evidence presented in this work proves that the porphyry copper‐type veinlets and the adjacent potassic alteration in the Southwest prospect are formed earlier and at a shallower level in contrast with the other porphyry deposits in the Baguio District.

A checklist and conservation status of vascular plants in the Limestone forest of Metropolitan Ilocos Norte Watershed Forest Reserve, Northwestern Luzon, Philippines

Abstract. Batuyong MAR, Calaramo MA, Alejandro GJD. 2020. A checklist and conservation status of vascular plants in the Limestone forest of Metropolitan Ilocos Norte Watershed Forest Reserve, Northwestern Luzon, Philippines. Biodiversitas 21: 3969-3981. The Metropolitan Ilocos Norte Watershed Forest Reserve (MINWFR) is among the remaining intact limestone formations and a critical protected area in Northwestern Luzon. There have been few published floristic studies despite its undeniable rich biological importance. Therefore, this paper primarily aims to provide a preliminary checklist of vascular plants in MINWFR and their conservation status. Consequent field visits and surveys were made from April 2019 to March 2020. Results revealed a total of 173 species distributed in 140 genera belonging to 59 families. The most represented families are Rubiaceae, Fabaceae, Orchidaceae, and Malvaceae. This forest supports 49% or 28.65 % local endemics, five of which are narrow endemics, namely Cyanometra warburgii, Pyrostria triflora, Syzygium ilocanum, Thrixspermum nicolasiorum, and Antirhea microphylla. The latter, however, extends up to Ilocos Sur. Based on IUCN criteria and DENR records, a total of 18 species are threatened, one species is recorded as critically endangered, five endangered, nine vulnerable, three other threatened, two near threatened, 55 as least concern, and the rest are not evaluated. Other noteworthy species present in the area are the two dominant endangered species, Podocarpus costalis, and Podocarpus polystachyus. An interesting spinescent Rubiaceae species were recorded and currently under examination to establish its identity. This checklist serves as a basis to effectively manage this vulnerable area surrounded by human-induced disturbances and threats.

A case study of Chlorophyll a response to tropical cyclone Wind Pump considering Kuroshio invasion and air-sea heat exchange

New evidences provided that the tropical cyclone (TC) Linfa in 2015 induced looping path of Kuroshio invasion into the northeastern South China Sea (NESCS) through the northwestern Luzon Strait (LS), based on the in-situ measurements, satellite data and model output data. This TC-enhanced Kuroshio invasion with low nutrients and denser waters suppressed the TC “Wind Pump” induced upwelling and nutrients uptake, and therefore inhibited the Chlorophyll a concentration (Chl-a) increase from surface to ~50 m in the open ocean of the NESCS. The TC-induced Kuroshio invasion promoted the generation of the strong cyclonic eddy to its left side where weak Ekman Pumping Velocity was observed. This enhancing cyclonic eddy then dominated the nutrients uplift and increased the surface and subsurface (0–50 m) Chl-a through eddy pumping rather than Ekman Pumping. The TC-declined anti-cyclonic eddy, which shoaled the Mixed Layer Depth (MLD), benefited to the nutrient uptake through TC-induced upwelling and thereby increased the surface Chl-a and raised the Chl-a Maximum Layer (CML) to ~20 m over the southwestern LS. The temporal Chl-a variations were also influenced by TC intensities and biochemical processes. The air-sea heat budget analysis indicated that, the air-sea heat exchange contributed to nearly 80% of the sea surface cooling (SST cooling) over the northwestern LS with Kuroshio invasion, while eddy-induced upwelling dominated the SST cooling over the western LS, and the wind-driven upwelling (and mixing) controlled the SST cooling over the southwestern LS. These different formations of SST cooling then played important role in Chl-a variations. This study is the first case of TC “Wind Pump” induced Chl-a variations considering air-sea heat exchange, Kuroshio invasion and mesoscale eddies over LS, which would help to evaluate the influence of TCs over the other major heat transport arteries of the world ocean: The Gulf Stream area.

Local marine reservoir age variability at Luzon Strait in the South China Sea during the Holocene

Here, we report new estimates of local marine radiocarbon reservoir ages from northwestern Luzon Island in the Philippines, on the east side of the South China Sea. Data for two periods, through the 1940’s and the mid-Holocene, were derived from modern and fossil corals. Our results from Luzon Island show variable ΔR values, from −39 ± 25 to 337 ± 27 14C yr over the past 6000 years and highlight the importance of understanding temporal ΔR changes to obtain accurate radiocarbon dates for mid-Holocene samples. An abrupt shift in ΔR at ∼5.5 ka BP, in the east side of the South China Sea, can be attributed to changes in the intensity of the upwelling, in the eastern Pacific, and to the East Asian Monsoon. Additional reservoir age data from the Pacific is likely to reveal details of oceanographic and climate changes at this time.

Charting the Abra gold trade network of Northwestern Luzon using ethnohistorical archaeology and WorldView-3 satellite imagery

The paper will look at the Abra Network from the Early Historical to Historical Period (10th to Early 20th c) which encompasses areas of Ilocos Sur as well as the whole province of Abra. It is argued that this particular network features the migration by coastal peoples to the interior in order to fully control strategic “choke” points in the gold trade with the Lepanto and Angaqui networks as well as to control jump-off points to the leeward side of the Cordillera Mountains – the Cagayan valley. Analysis was done using data from GIS predictive modelling and WorldView3 satellite remote sensing, in conjunction with archaeological data from 2011 and 2012 field seasons as well as published primary and secondary ethnohistorical data. Several Spanish period maps provided important data on early settlements and trails, after georeferencing through GIS software.

Bulked gold, riverine trade, hiking trails, and WorldView2 satellite remote sensing in Northwestern Luzon: The Angaqui network

This paper is based on a case study of the Angaqui gold trade network in Northwestern Luzon during the Early Historical to Historical Period (10th to early 20th c). Multiple data sources including GIS predictive modelling and remote sensing through high-resolution and multispectral WorldView2 satellite imagery, written primary historical and secondary historical sources, indigenous peoples oral tradition, and results of systematic archaeological survey and excavation were integrated in the research to come close to a wholistic view. Availability of the satellite imagery facilitates a more regional and multi-scalar approach to archaeology of the region. Remote sensing has revealed segments of old trails within the network. The written and oral tradition both complement each other when correlated with available archaeological data. Availability of historical visual documentation also provides a means to reconstructing the gold evanescent market encounter in Northwestern Luzon.

Early Historical gold trade networks in Northwestern Luzon, as reconstructed from ethnohistorical accounts, WorldView2 satellite remote sensing and GIS predictive modelling: The Gasweling case

The paper shows the utility of high resolution multispectral WorldView2 (WV2) satellite imagery in remote sensing archaeological features associated to the gold evanescent market encounter within the Gasweling network in Northwester Luzon. Image enhancement of the features was done through the Brovey transform (Panchal and Thakker, 2015) pan-sharpening technique and data fusion (Kvamme, 2006) of WorldView2 indices. These features include Ijang/Ili or hill fort defenses as well as trail segments. In delineating the area with potential archaeological features, a close reading of conquistador accounts of Juan de Salcedo in 1572 and Alonso Quirante in 1624 was performed. Also geographic information systems (GIS) least coast path predictive modelling as well as visibility analysis was implemented in order to model the evanescent market (Allegre, 1998) encounter that took place at the coastal settlements.