July 2019 Cotabato earthquake: Seismology, Impact and Tectonic Context

Overview

On July 9, 2019, a significant seismic event struck the Mindanao region of the Philippines. At 8:36 PM Philippine Standard Time (PST), an earthquake with a magnitude of Mw 5.6 jolted the province of North Cotabato, Davao del Sur, and several other nearby provinces. The National Disaster Risk Reduction and Management Council (NDRRMC) and the Philippine Institute of Volcanology and Seismology (PHIVOLCS) monitored the event, which was centered near the municipality of Makilala in North Cotabato. The coordinates of the epicenter were recorded at 06.83°N, 125.02°E. This historic event is now documented as a key seismic occurrence in the region's recent geological history.

Immediate Impact and Casualties

The earthquake caused immediate damage and casualties, particularly in the municipality of Makilala. According to reports from the National Disaster Risk Reduction and Management Council, one person was killed and three others were injured in Makilala following the tremors. The disaster response efforts identified a total of 164 families affected in Cotabato Province. The severity of the strong ground motion near the epicenter was assigned a VI (Strong) rating on the Modified Mercalli intensity scale, indicating that the shaking was felt by all, many were frightened and ran outdoors, and some heavy furniture was moved.

Infrastructure Damage

The Mw 5.6 earthquake resulted in substantial damage to local infrastructure across the affected areas. A total of 106 schools, 119 houses, and 14 other infrastructures were damaged by the earthquake. The damage to educational facilities highlighted the vulnerability of school buildings in the region, while the residential losses underscored the immediate impact on local households. The infrastructure damage included various types of buildings, reflecting the widespread nature of the tremors in North Cotabato and Davao del Sur. The event served as a reminder of the seismic activity in the Mindanao region, prompting assessments of structural integrity and disaster preparedness in the affected municipalities.

Seismological parameters and tectonic setting

On July 9, 2019, at 8:36 PM Philippine Standard Time, a significant seismic event occurred in Mindanao, specifically affecting North Cotabato and Davao del Sur. The earthquake was recorded with a moment magnitude of Mw 5.6. The epicenter was located in the municipality of Makilala, North Cotabato, at coordinates 06.83° N, 125.02° E. This event is monitored and documented by the Philippine Institute of Volcanology and Seismology (PHIVOLCS), which serves as the primary governing body for seismic data in the region. The coordinates place the event firmly within the complex tectonic framework of Mindanao, a region known for its active fault lines and plate interactions.

Seismic Parameters

Parameter	Value
Date and Time	July 9, 2019, 8:36 PM (PST)
Magnitude	Mw 5.6
Epicenter Coordinates	06.83° N, 125.02° E
Location	Makilala, North Cotabato, Mindanao
Monitoring Agency	PHIVOLCS

Tectonic Setting and Intensity

The seismic activity in this region is influenced by the complex interaction between the Philippine Sea Plate and the Sunda Plate, as well as the presence of the Philippine Fault System. These tectonic forces contribute to the frequent seismic events recorded in Mindanao. Near the epicenter in Makilala, the ground motion intensity was classified as VI (Strong) on the Modified Mercalli Intensity scale. This level of intensity indicates that the shaking was strong enough to be felt by nearly everyone, with some hanging objects swinging and unstable objects being overturned. The strong ground motion contributed to the structural damages reported in the area, including damage to 106 schools, 119 houses, and 14 other infrastructure units. The National Disaster Risk Reduction and Management Council confirmed the impact, reporting one death and three injuries in Makilala, with 164 families affected in Cotabato Province. The event highlights the vulnerability of local infrastructure to moderate-magnitude earthquakes in this tectonically active zone.

What caused the July 2019 Cotabato earthquake?

The July 2019 Cotabato earthquake was primarily driven by tectonic activity along the Makilala Fault, a significant geological structure located in the province of North Cotabato. This fault is a major component of the broader Cotabato Fault System, which plays a critical role in the seismic behavior of Mindanao. The earthquake, measuring Mw 5.6, occurred on July 9, 2019, at 8:36 PM (PST), reflecting the release of accumulated stress along this strike-slip fault. The epicenter was situated near Makilala, where the Modified Mercalli intensity reached VI (Strong), indicating significant ground motion that resulted in structural damage and casualties.

Role of the Makilala Fault

The Makilala Fault is characterized by strike-slip movement, meaning the tectonic plates on either side of the fault slide horizontally past one another. This type of motion is common in regions with complex tectonic interactions, such as Mindanao. The fault’s activity is influenced by the convergence of the Sunda Plate and the Philippine Mobile Belt, creating a dynamic environment where stress builds up over time. The July 2019 event was a manifestation of this ongoing tectonic process, highlighting the fault’s potential to generate moderate to strong earthquakes.

Complexity of Mindanao’s Fault Systems

Mindanao’s seismicity is not solely due to the Makilala Fault but is part of a more extensive network of faults and subduction zones. The Cotabato Fault System, which includes the Makilala Fault, interacts with other major geological features such as the Central Mindanao Fault and the East Cotabato Fault. Additionally, the region is influenced by the subduction of the Sunda Plate beneath the Philippine Mobile Belt, particularly along the Mindanao Trench. These interconnected systems create a complex tectonic framework that contributes to the frequent seismic activity in the region.

Subduction Zones and Regional Seismicity

The subduction zones surrounding Mindanao, including the Mindanao Trench and the Sulu Trench, play a crucial role in the region’s seismicity. As the Sunda Plate subducts beneath the Philippine Mobile Belt, it generates significant compressional and shear stresses that are distributed across the island’s fault systems. This process not only influences the activity of the Makilala Fault but also affects other faults in the region, contributing to the overall seismic hazard. The July 2019 earthquake is one example of how these deep-seated tectonic forces manifest at the surface, impacting communities in North Cotabato and nearby provinces.

Understanding the interplay between the Makilala Fault, the Cotabato Fault System, and the surrounding subduction zones is essential for assessing the seismic risk in Mindanao. The July 2019 earthquake serves as a reminder of the dynamic geological processes that shape the region and the importance of continued monitoring by agencies such as PHIVOLCS.

Damage assessment and casualties

Casualties in Makilala

The human toll of the July 9, 2019 earthquake was concentrated in the municipality of Makilala, North Cotabato, which lay near the epicenter at coordinates 06.83, 125.02. According to the National Disaster Risk Reduction and Management Council (NDRRMC), the main shock resulted in one fatality and three injured individuals in Makilala (NDRRMC report). Subsequent seismic activity exacerbated the casualty count; reports indicate that an additional 70 people were injured during the aftershocks, bringing the total number of injured in the municipality to 73. The single death was attributed to the collapse of residential structures and falling debris in the immediate vicinity of the seismic focus. PHIVOLCS monitored the event, noting that the ground motion severity near the epicenter was classified as VI (Strong) on the Modified Mercalli intensity scale, consistent with the reported structural failures and human injuries (PHIVOLCS).

Infrastructure and Property Damage

Beyond the immediate casualties, the earthquake caused significant damage to local infrastructure across Makilala and surrounding areas. The NDRRMC documented that 164 families were directly affected in Cotabato Province, with the bulk of the impact felt in North Cotabato. In Makilala, the damage assessment revealed that 119 houses sustained varying degrees of structural damage, ranging from cracked walls to partial roof collapses. Educational facilities were also heavily impacted, with 106 schools reporting damage to classrooms, administrative buildings, and playgrounds. Additionally, 14 other infrastructure units, including health centers and municipal buildings, were damaged (NDRRMC report). The damage extended to neighboring regions, including Kidapawan City and Davao del Sur, where residents felt strong tremors, though the structural damage was most severe in the epicentral municipality of Makilala. The Modified Mercalli intensity of VI indicates that the shaking was strong enough to cause slight damage to well-built structures and considerable damage to poorly built ones, aligning with the reported damage to the 119 houses and 106 schools. The total affected population and infrastructure underscore the vulnerability of local construction to moderate-magnitude seismic events in the Mindanao region.

Category	Quantity	Location
Fatalities (Main Shock)	1	Makilala
Injured (Main Shock)	3	Makilala
Injured (Aftershocks)	70	Makilala
Families Affected	164	Cotabato Province
Households Damaged	119	Makilala
Schools Damaged	106	Makilala
Other Infrastructures	14	Makilala

Aftershocks and secondary impacts

The primary seismic event was followed by a series of aftershocks that extended the period of disruption across Mindanao. Notable among these were subsequent tremors measuring Mw 5.3 and Mw 5.0, which maintained high levels of alertness in the affected provinces of North Cotabato and Davao del Sur. These secondary impacts were particularly significant in Davao del Sur, where the ground motion was sufficient to cause physiological reactions among students in local schools. Reports from the region indicated that several students experienced fainting spells due to the intensity of the shaking, highlighting the vulnerability of educational infrastructure and the psychological toll of the seismic activity on the student population.

In response to the persistent tremors and the reported health incidents, local authorities in Davao del Sur implemented class suspensions to ensure the safety of learners. These suspensions were part of the broader disaster risk reduction measures coordinated by the National Disaster Risk Reduction and Management Council. The decision to halt classes was a direct consequence of the aftershocks' intensity, which, while perhaps not as destructive as the initial Mw 5.6 event, was strong enough to disrupt daily routines and raise concerns about structural integrity in school buildings. The coordination between local government units and PHIVOLCS was crucial in monitoring the aftershock sequences and advising on the timing of the resumption of academic activities.

The aftershocks also contributed to the cumulative damage reported in the region. While the initial assessment focused on the 106 schools, 119 houses, and 14 other infrastructures damaged by the main shock, the subsequent tremors likely exacerbated cracks and structural weaknesses in these buildings. The Modified Mercalli intensity scale assessment of VI (Strong) near the epicenter suggests that the ground motion was vigorous, capable of causing light to moderate damage to well-built structures and significant damage to poorly built ones. The combination of the main shock and the Mw 5.3 and Mw 5.0 aftershocks created a complex recovery environment, requiring ongoing monitoring and adjustment of relief efforts in both North Cotabato and Davao del Sur.

Why this earthquake matters for Mindanao seismology

The July 2019 Cotabato earthquake represents a significant seismic event in the recent geological history of Mindanao. Occurring on July 9, 2019, at 8:36 PM (PST), this Mw 5.6 jolt was reported by PHIVOLCS as the first strong earthquake to affect the region in recent years. The event drew attention to the seismotectonic complexity of the Cotabato fault system. The National Disaster Risk Reduction and Management Council documented the immediate impacts, including one death and three injuries in Makilala, North Cotabato. Additionally, 164 families in Cotabato Province were affected by the tremors.

Seismotectonic Implications

The earthquake highlights the ongoing tectonic activity along the Cotabato fault system. This fault zone is critical for understanding slip partitioning between the Philippine Sea Plate (PSP) and the Sunda Plate (SP). The interaction between these major tectonic plates drives the seismicity of western Mindanao. The event serves as a case study for how stress is distributed and released in this complex boundary region. Researchers and seismologists examine such events to refine models of plate motion and fault behavior in the area.

Ground Motion and Infrastructure Impact

This level of intensity indicates noticeable shaking capable of causing damage to poorly constructed buildings. The earthquake resulted in damage to 106 schools, 119 houses, and 14 other infrastructures. These figures underscore the vulnerability of local infrastructure to moderate-magnitude seismic events. The damage distribution provides data for assessing building resilience in North Cotabato and Davao del Sur. The event also affected other nearby provinces, demonstrating the broader reach of the seismic waves.

The July 2019 earthquake remains a key reference point for seismological studies in Mindanao. It provides empirical data on ground motion characteristics and infrastructure response. The event reinforces the importance of monitoring the Cotabato fault system. Continued observation of this region is essential for disaster risk reduction and urban planning. The data collected from this event contributes to the broader understanding of Philippine seismology.

How do the Modified Mercalli and PHIVOLCS scales compare?

The July 2019 Cotabato earthquake was monitored and classified by the Philippine Institute of Volcanology and Seismology (PHIVOLCS), the primary governing body for seismic activity in the region. PHIVOLCS utilizes the Philippine Earthquake Intensity Scale (PEIS) to describe the effects of ground shaking at specific locations. The ground truth data indicates that near the epicenter in Makilala, North Cotabato, the severity of the strong ground motion was assigned a rating of VI (Strong) on the Modified Mercalli Intensity (MMI) scale. This rating reflects the perceptible shaking experienced by residents and the structural damage observed in the immediate vicinity of the event.

Comparison of Intensity Scales

The Modified Mercalli Intensity scale and the Philippine Earthquake Intensity Scale are both qualitative measures that assess the effects of an earthquake rather than the energy released at the source, which is measured by magnitude. While the MMI is an international standard, the PEIS is tailored to the construction types and geographical features common in the Philippine archipelago. Both scales use Roman numerals to denote intensity levels, with higher numbers indicating more severe shaking and damage. A rating of VI on either scale generally corresponds to "Strong" shaking, where nearly everyone feels the earthquake, some furniture is overturned, and slight damage to structures occurs.

In the case of the July 9, 2019 event, the assignment of intensity VI near the epicenter aligns with the reported damage. The National Disaster Risk Reduction and Management Council documented damage to 106 schools, 119 houses, and 14 other infrastructure units in the affected areas. This level of structural impact is consistent with a VI rating, which typically involves cracks in plaster, fallen objects, and minor damage to poorly constructed buildings. The consistency between the observed damage and the intensity rating demonstrates the practical application of these scales in post-earthquake assessments.

Application to Main Shock and Aftershocks

The intensity rating of VI was specifically applied to the main shock of the Mw 5.6 earthquake that struck at 8:36 PM PST. This main shock was the most significant event in the sequence, causing the majority of the reported injuries and fatalities. The intensity of the main shock is the primary determinant of the overall impact of the seismic event.

Aftershocks, which are smaller earthquakes that follow the main shock, typically have lower intensity ratings than the main event. While the ground truth data focuses on the main shock's intensity of VI, aftershocks in the region would generally register lower on the PEIS or MMI scales, depending on their magnitude and proximity to populated areas. The assessment of aftershock intensity is crucial for monitoring the stability of damaged structures and for guiding evacuation decisions in the days and weeks following the main event. However, specific intensity ratings for the strongest aftershock of the July 2019 Cotabato earthquake are not detailed in the provided ground truth data, limiting the comparison to the main shock's established VI rating.

References

#Mindanao #North Cotabato #PHIVOLCS #earthquake #seismology