Latest earthquake in the Philippines reveals tectonic complexity

The 6.6-magnitude earthquake that struck Luzon, Philippines on July 24 shows the benefits of citizen science and serves as an alarm bell.

Written by Mario Aurelio, Director, National Institute of Geosciences, University of the Philippines, Mahar Laghmai, Executive Director, Resilience Institute of the University of the Philippines – National Operational Assessment of Hazard Center (nababaha), John Agustin Escudero, Laboratory of Structural and Tectonic Geology at the National Institute of Geosciences, University of the Philippines, Sandra Catugas, Laboratory of Structural and Tectonic Geology at the National Institute of Geosciences, University of the Philippines

Citation: Aurelio, M., Lagmay, M., Escudero, J.A., and Catugas, S., 2021, Latest Philippine earthquake revealing tectonic complexity, Temblor, http://doi.org/10.32858/temblor.191

A 6.6-magnitude earthquake struck Luzon Island, the Philippines, at 4:47 a.m. local time on Saturday, July 24 (data from the Philippine Institute of Volcanology and Seismology, or PHIVOLCS). The epicenter was located off the coast of the Calatagan Peninsula in Batangas Province, about 100 kilometers southwest of the capital, Manila. About 10 minutes later, an aftershock of magnitude 5.7 occurred at roughly the same location. As expected, aftershocks continued to shake the area. Damage was minor, although the area is currently under siege due to heavy rains from the monsoons and cyclones, which cause flooding and landslides.

Calatagan Beach, Batangas, Philippines, near where the earthquake occurred on July 24, 2021. Credit: What’s on My Mind, via Flickr, CC BY 2.0

The Calatagan earthquake is not the first in this region and it won’t be the last. The tectonic complexity of the region where the July 24 earthquake occurred explains the high frequency of seismic events in this region. Given that the region is home to densely populated urban centers, the coexistence of several earthquake-generating systems (subduction zones, slip faults, and extension systems) and nearby active volcanic centers highlights the importance of minimizing each individual phenomenon, as well as the interaction and interaction among them.

complex tectonic setting

The Philippines and the surrounding area include not only six active subduction zones, but also other seismically active structures, including slip faults, expansion systems and active volcanic centers such as the nearby Taal volcano. Because of the complex environment, earthquakes often mask each other, making earthquake sources difficult to identify.

Figure 1 – Tectonic map of the Philippines showing the Philippine Mobile Belt (shade), active subduction zones and active internal faults. Credit: Quoted from Aurelio et al. , (2013)

These trenches, along with several active faults within the Philippine Mobile Belt, help accommodate the oblique convergence between the Philippine Sea Plate and the eastern margin of the Eurasian Plate at a rate ranging from about 3.6 cm per year in Taiwan to about 8.2 cm per year in the southern Philippines ( For example, Sino, 1977). Subduction along the Manila Trench may be at 5.0 cm/yr (eg, Rangin et al., 1999).

The slip slab along the Manila Trench at the latitude where the Calatagan earthquake fell sharply—almost vertically—as evidenced by the corresponding Wadatti-Benioff tracing (Fig. 2). The depth at which the earthquake occurred, as reported by several seismographs (including PHIVOLCS, USGS-NEIC and GEOFON) ranges from 112 to 130 kilometers. Focal mechanism solutions computed by these observatories consistently indicate that the earthquake occurred on one of a pair of roughly parallel thrust faults that struck northwest. The direction of this northwest fault pair corresponds to the geometry of the subduction along the Manila Trench at this latitude.

Figure 2 – Earthquake cluster July 24, 2021, magnitude 6.6 earthquake in Calatagan, Batangas, Philippines. Among the active slip faults are the West/Northwest Linear Lubang-Verde Corridor Fault, and the North/Northwest Aglobang River Strike Fault on Mindoro Island. To the northeast of the Calatagan earthquake epicenter, the Macolod Corridor is a northeastern structural depression that hosts several active volcanic centers including Taal Volcano. Credit: Aurelio, Lagmay, Escudero and Catugas

Coulomb stress transfer

Between 4 and 8 April 2017, a series of medium-sized and shallow-focus earthquakes, including a magnitude 5.9 (8 km depth) hit the Mabini Peninsula, located about 20 km east of the Calatagan Peninsula. On August 11, a larger earthquake of magnitude 6.3 (177 km) struck the same area. Modeling of stress transfer at Coulomb for the April 8 event indicates that this earthquake sequence transferred a significant amount of stress toward Taal volcano (Fig. 3), which could have contributed to the initiation of a major eruption three years later in January 2020, and slight success. Volcanic events in recent weeks (including early July 2021).

Figure 3 – Coulomb stress transfer model of the April 8, 2017, 5.9 magnitude earthquake in Mabini, Batangas, Philippines. Credit: Aurelio, Lagmay, Escudero and Catugas, using Coulomb 3.3 from Toda et al. (2011)

Citizen science

The Calatagan earthquake of magnitude 6.6 on the Richter scale and aftershocks were detected by low-cost, privately owned seismometers from as far north as Luzon in Baguio (about 300 km away), to the southern part of the Philippines at Davao, Mindanao (Fig. 4 ) (about 900 km). The records generated by the citizen scientists were consistent with the findings of the government-owned and operated Philippine Seismic Network operated by PHIVOLCS.

Figure 4 – Citizen science seismic charts with the location of the foci of the July 24th magnitude 6.6 Calatagan earthquake as shown in the Shakenet Mobile app. Credit: Aurelio, Lagmay, Escudero and Catugas

The low-cost seismometers, developed by Raspberry Shake, have been tested and tested in the laboratory (Anthony et al., 2019) and in the field (Manconi et al., 2018; Winter et al., 2021; Holmgren, 2021), demonstrating their potential On increasing the intensity of the government earthquake network.

Raspberry Shake seismometers have even been used to monitor the Earth’s calm (Lecocq et al., 2020) in different cities of Luzon Island due to the Covid-19 lockdown (Fig. 5). Citizen science seismometers have tremendous value in earthquake education and research (Bent, 2018; Subedi, 2020), considering that earthquakes pose a serious threat to Filipino communities.

Figure 5 – Ground movement subsides due to the Covid-19 lockdown in the Philippine National Capital Region and adjacent provinces. Credit: Aurelio, Lagmay, Escudero, and Catugas using the method of Lecocq et al (2020)

wake up call

The Calatagan earthquake is another wake-up call in the recent series of devastating earthquakes that have plagued the Philippines (Table 1). In 2019, a series of 4 shallow earthquakes measuring 5.9 to -6.6 on the Richter scale devastated North Cotabato, killing 23 people (with 11 missing) and injuring 563 (NDRRMC, 2019) Earlier in the year, a 6.1 earthquake struck degree (centered in Zambales) central Luzon, killing 18, missing 3 and injuring 256 (NDRRMC, 2019). People felt a tremor from the Zambales earthquake 100 km away in Manila. In 2017, fortunately the earthquake sequence that hit Batangas province in April and August did not result in any fatalities, rather injuring a few and causing 18 million pesos worth of property damage. Like the Batangas earthquakes of 2017, the Calatagan earthquake of 2021 in Batangas did not result in casualties. This might not be the case the next time you hit a big hit.

Prevent, mitigate and prepare

For a country with a long history of devastating earthquakes, the Calatagan earthquake once again highlights the need to prepare against the constant threat of natural hazards. The growing number of seismographers and enthusiasts using low-cost seismographs in the country is most welcome.

The National Institute of Geosciences and the University of the Philippines College of Science have embarked on a program to amplify the efforts of citizen scientists consistent with the guidelines of the Sendai Framework for Disaster Risk Reduction for the Whole of Society and a Science-Based Approach to Disaster Risk Reduction. The program includes promoting education and research on earthquakes and hopefully will increase awareness of disasters in the population and increase understanding of tectonics and earthquakes in the Philippines.

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