By Dr. Lawrence Zamoras
A landslide occurred in the hilly area of Sitio Siguinan, Barangay Sangali, Zamboanga City, which is geographically located at N 7°5’ 1.73”; E 122°11’ 17.03” (Figure 1). This incidence reportedly occurred on February 10, 2021 during nighttime. According to residents, during the incident, which occurred at night, there were neither rains nor earthquakes. There were also no intense impact sounds accompanying the landslide. However, this landslide incident was not reported immediately to the Mines and Geosciences Bureau R-9, which has been tasked to assess geohazard concerns in Region 9.
The discussions over this incident were heard on the radio program on February 12, 2021. Residents relayed this incident to a PHIVOLCS personnel Engineer Allan Labayog, who discussed the incident with the City Disaster Risk Reduction Office. On the same day the MGB geologists composed of El Jedidiah O. Villacampa, Paul Asthor D. Yecyec, Luke M. Donggon and Dr. Lawrence R. Zamoras, visited the area to assess the landslide incident.
1. MANIFESTATIONS AT THE LANDSLIDE CROWN
The landslide crown occurs at the upper part of the slope at an elevation of about 110 meters above sea level (Figure 2). The main scarp is oriented generally east-west dipping roughly vertical with about 5 meters of vertical displacement (Figure 3). As per anecdotal accounts, the tension crack below the crown was a deep open crack, about 30 feet deep, right after the landslide but was later filled with earth materials or the crack closed as the slumping progressed. The sharp tilting of the trees and the exposures of fresh rock materials show that the incident is very recent. The ground material is described as poorly consolidated mixture of pebble, sand and loamy soil. It is, however, observed that the middle portion of the slumping block, just below the crown does not display noticeable tension cracks.
2. MANIFESTATIONS AT THE TOE OF THE LANDSLIDE
At the base of the slope located in the south portion of the area can be observed a widespread crumbling of the ground (Figures 4-6). The incident shows a ground that disintegrates into blocks of irregular shapes and sizes. These blocks are either pushed upward or sideward with pronounced rotation causing the toppling of trees such as coconuts, mangoes, jackfruits, banana, mahogany and gemelina trees, which is one of those that fell into the roof of a house. In between the ground blocks are open and deep gaps that are widespread around the toe section of the landslide. There are reportedly three residential houses situated right at the landslide’s forefront, which the residents flee after the incident. One residential house located right in front of the crumbling ground survived the destruction and is now being removed for transfer to another location.
3. TENSION CRACKS ALONG THE SIDES OF THE SLUMP
The ground cracks along the landslide sides occur as linear fissures oriented N30°W, as observed at Station 03 (Figure 7). These cracks are about half a meter wide and at least a meter deep. Around the mid-section of the slope, the cracks do not show significant vertical displacement. However, we move uphill tracing the fissure, vertical displacement gradually become apparent and at Station 04, the downward slippage is already 1.5 meter as of the time of the visit (Figure 8).
4. POSSIBLE LANDSLIDE MODEL
The landslide’s main tension crack was traced using a GPS from east to west, which defines a curve as shown in Figure 9. Its crown has a typical arcuate shape of about 80 meters wide and 140 meters long which generally faces southeast. The crown shows a vertical scarp with a maximum slippage or displacement of 5 meters as observed in Station 05. Toward the downslope, the slippage gradually decreased to 1.5 meters at Station 04 and then to zero at Station 03. In the central section of the slide, minimal to no tension cracks have been noted. However, the worst ground destruction occurs at the toe of the landslide at Stations 10 to 13.
The possible model for this landslide in Sitio Siguinan is a Rotational Landslide. This model is characterized by a curved slide plane underneath a slumping ground. The detaching block tends to slide in a rotational manner (Figure 10). Maximum vertical slippage would be observed along with the landslide crown, while maximum lateral displacement occurs at the toe of the landslide. Minimal cracks may be observed within the middle part of the rotationally slumping block, but the worst destruction would occur at the toe of the landslide.
5. GEOLOGY AND GEOHAZARD SUSCEPTIBILITY OF THE AREA
Based on the 1963 Geologic Map of the Philippines, the Sitio Siguinan area is underlain by a moderately to poorly consolidated sedimentary sequence consisting interbedded calcareous mudstone, sandstone and limestone belonging to the Curuan Formation. In the geologic map of Yumul, et al., however, the area is identified as part of the limestone formation, so probably its bedrock below the ground is a limestone (Figure 11). Areas underlain by such rocks are susceptible to ground subsidence or landslides.
In the Sitio Siguinan landslide area, the fresh open cuts show exposures of poorly consolidated beige-colored mixture of pebble, sand and loamy soil, with slightly calcareous components, as well as occasional patches of coralline limestone (Figure 12). This may be part of a thick soil horizon. The terrain condition in Sitio Siguinan may have been naturally susceptible to karst subsidence hazard because of its underlying rocks. Upon observing the concrete roads, be it the AH-26 or barangay roads within Barangays Sangali and Victoria, cracks on the concrete are so widespread, probably caused by the underlying rock type.
In the Geohazard Susceptibility Map, the upslope area of Sitio Siguinan is categorized under the Moderate Landslide Susceptibility, while the downslope area is under the Low Landslide Susceptibility (Figure 13). However, upon visiting the landslide site, its topography shows a locally steep terrain, which may have been overlooked in the 1:10,000 Scale Geohazard mapping. Thus, its localized steepness will reclassify the landslide area from Moderate to High Landslide Susceptibility. Furthermore, the subsidence-prone underlying rock renders the area to be under the High Subsidence Susceptibility in addition to its High Landslide Susceptibility condition.
6. GEOLOGIC STRUCTURE
The Zamboanga City area has multiple lineaments that are potential fault structures, as indicated in the Generalized Geologic Map for Western Mindanao (Figure 14). These lineaments were delineated based on linear topographic patterns, which are mostly indicative of fault structures. If plotted in this geologic map, the Sitio Siguinan area is located in the vicinity of, if not along, northeast-trending lineament or fault structure. This possibility of the lineament being a fault structure is further affirmed by the recovery of gougy rock materials from the deep well near the landslide toe (Figure 15). Gougy textures of a rock indicate the crushing of the rock along a fault structure. Therefore, the presence of a fault structure across this landslide-prone and subsidence-prone area may have further compounded the geohazard for the Sitio Siguinan area.
CONCLUSIONS & RECOMMENDATIONS
The Sitio Siguinan area lies on a hilly terrain underlain by a limestone formation with a thick soil horizon on top. Based on the MGB Geohazard Map, the landslide site is situated on a Low and Moderate Landslide Susceptibility area; the crown is located on the Moderate Landslide Susceptibility while the toe is located on a Low Landslide Susceptibility. However, upon visit to the landslide site, the topography shows a locally steep terrain rendering reclassification of its landslide susceptibility from Moderate to High. Furthermore, since a limestone formation underlies the area, it is also susceptible to Karst Subsidence Hazard, adding a High Subsidence Susceptibility to the area’s High Landslide Susceptibility.
The type of landslide occurring in Sitio Siguinan is identified as a Rotational Landslide, based on the maximum vertical slippage in the crown, minimal slippage both sides, minimal fracturing in the middle portion, and extreme ground destruction in the toe with lateral displacement. The landslide has a typical arcuate shape measuring about 80 meters wide and 140 meters long which generally moves toward the southeast direction. As the downslope earth movement along landslide is likely to progress, the following are thus recommended:
- Relocation of residential houses situated within the vicinity of the toe of the landslide. Anywhere within the sliding block is also unsafe for building houses. The downward movement of earth materials in this landslide site has just begun. It has introduced loose earth materials that will gradually descend through gravitational forces which will intensify during heavy and prolonged rainfall.
- Observe for and/or monitor presence and indications of similar mass movement (e.g., landslip, tension cracks) along nearby slopes and report situation to the CDRRMC of Zamboanga City and MGB-9 office.
- Since the Sitio Siguinan area and vicinity are underlain by limestone, observe for and/or monitor any signs of sinkhole/cave collapse and subsidence in the area, and report the incident to the CDRRMC of Zamboanga City and MGB-9 office.
Interpretations and conclusions stated in this report are based on available data and within the bounds of prevailing geoscientific principles and practices.