Eastern Mindanao earthquake sequence strikes on subducting, extinct fracture zone

Off the eastern shore of Mindanao Island in the southern Philippines, large magnitude earthquakes result from the highly active Philippine Trench. But the varied fault mechanisms of the recent magnitude 7.4 mainshock and the succeeding shocks are unusual. An extinct oceanic fracture zone in the subducting slab might be the cause.

By Mario Aurelio, Sandra Donna Catugas, Abigail Anicete, Structural Geology and Tectonics Laboratory, University of Philippines National Institute of Geological Sciences, Philippines

Citation: Aurelio, M., Catugas, S.D., Anicete, A., Eastern Mindanao earthquake sequence strikes on subducting, extinct fracture zone, Temblor, http://doi.org/10.32858/temblor.331

This article is also available in Tagalog.

On Dec. 2, 2023, at 10:37 p.m. Manila time (14:37 GMT), a magnitude 7.4 earthquake with a shallow focal depth of 26 kilometers (16 miles) struck offshore, east of the island of Mindanao in the southern Philippines. Despite the strong magnitude, major infrastructure damage has been limited to a few impassable roads due to cracks and landslides, and one bridge closed to traffic. A minor tsunami with wave heights not exceeding 70 centimeters (about 2.2 feet) also occurred. Around 3,900 houses were damaged either partially (3,588) or totally (312), with three reported dead and 48 injured (SitRep No. 3, NDRRMC, 07 December 2023).

Here, we present observations in an attempt to explain the large magnitudes of aftershocks and the unusual co-occurrence of both thrust faulting (compressional) and normal faulting (extensional) events. Such uncommon earthquake characteristics may be associated with the subduction of a more-buoyant-than-usual oceanic lithosphere.

Figure 1a Seismotectonics of the magnitude 7.4 earthquake of Dec. 2, 2023, in the province of Surigao Del Sur, Mindanao (southern Philippines). Result of Coulomb stress change modeling shown, using the magnitude 7.4 mainshock as source fault (thrust mechanism) and the next event (normal fault) as the receiver fault. Focal mechanism solutions of the mainshock (7.4) and 24 succeeding seismic events with magnitudes ranging from magnitude 4.9 to 6.8, and other post -mainshock events of lower magnitudes (small circles) are shown. Locations of cross section Figures 1b and 2 and Coulomb stress transfer cross section Figure 3 are shown. Data: Jarvis et al., 2008 for SRTM topography; Weatherall et al., 2020 for bathymetry; Toda et al., 2011 for CST modeling; PHIVOLCS for earthquake data. GMT (Wessel and Smith, 1995) was used to generate the map. See text for more discussion. Credit: Aurelio, Catugas, Anicete, 2023. — Figure 1a. Seismotectonics of the magnitude 7.4 earthquake of Dec. 2, 2023, in the province of Surigao Del Sur, Mindanao (southern Philippines). Result of Coulomb stress change modeling shown, using the magnitude 7.4 mainshock as source fault (thrust mechanism) and the next event (normal fault) as the receiver fault. Focal mechanism solutions of the mainshock (7.4) and 24 succeeding seismic events with magnitudes ranging from magnitude 4.9 to 6.8, and other post -mainshock events of lower magnitudes (small circles) are shown. Locations of cross section Figures 1b and 2 and Coulomb stress transfer cross section Figure 3 are shown. Data: Jarvis et al., 2008 for SRTM topography; Weatherall et al., 2020 for bathymetry; Toda et al., 2011 for CST modeling; PHIVOLCS for earthquake data. GMT (Wessel and Smith, 1995) was used to generate the map. See text for more discussion. Credit: Aurelio, Catugas, Anicete, 2023

Figure 1b Cross section showing the vertical distribution of earthquakes (magnitude 7.4 mainshock and 24 succeeding events with magnitudes between 4.9 to 6.8) in focal mechanism solution plots. Small pale dots indicate historical earthquakes. Approximate trace of the subducting Philippine Sea Plate is shown. Data sources same as in Figure 1a. See text for more discussion. Credit: Aurelio, Catugas, Anicete, 2023. — Figure 1b. Cross section showing the vertical distribution of earthquakes (magnitude 7.4 mainshock and 24 succeeding events with magnitudes between 4.9 to 6.8) in focal mechanism solution plots. Small pale dots indicate historical earthquakes. Approximate trace of the subducting Philippine Sea Plate is shown. Data sources same as in Figure 1a. See text for more discussion. Credit: Aurelio, Catugas, Anicete, 2023

Seismotectonic setting

The major active tectonic structures in eastern Mindanao include the left-moving (sinistral) strike-slip Philippine Fault and its branches that traverse the entire length of eastern Mindanao from the province of Surigao Del Norte to Davao Oriental for about 400 kilometers (Aurelio, 1992; Quebral, 1994) (Figure 1a), and the Philippine Trench to the east, which involves the westward subduction of the oceanic lithosphere of the West Philippine Basin of the Philippine Sea Plate (Cardwell et al., 1980; Aurelio, 2000; Aurelio and Pena, 2010). These two major tectonic features are responsible for generating numerous earthquakes in the region, many of which have been devastating.

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) located the epicenter of the Dec. 2 earthquake at 127.70 degrees E Longitude, 8.46 degrees N Latitude, or about 40 kilometers east of the town of Hinatuan (population approximately 43,000) in the province of Surigao del Sur, and 50 kilometers west of the axis of the Philippine Trench (Figure 1a).

At the latitude where the earthquake occurred, the subducting slab of the Philippine Sea Plate reaches a depth of more than 150 kilometers (Cardwell et al., 1980; Aurelio, 2000) (Figure 1b). From the trench floor, the slab enters the subduction zone at a relatively gentle angle (between 15 and 30 degrees) and steepens to greater than 45 degrees at around 50 kilometers depth. The 26-kilometer focal depth of the mainshock plots at the western end of the gently dipping section of the subducting slab.

Trenchward rupture propagation

A time series plot and cross section suggest that the mainshock was succeeded by lower-magnitude events that propagated generally eastward, toward the shallower sections of the subducting slab (Figures 2 & 3). This observation is consistent with the rupture model released by the U.S. Geological Survey (USGS, 2023). Four days after the mainshock, at least 4,500 seismic events (magnitude range of magnitude 1.3 to 6.8) have been recorded (SitRep No. 3, NDRRMC, 07 December 2023). Of these post-mainshock events, at least 24 have had magnitudes equal to or greater than 5.0. The rest of the events covered a triangular epicentral area about 150 kilometers long and 120 kilometers wide (Figure 1a).

The magnitude 7.4 mainshock shows a compressional focal mechanism solution, consisting of two reverse faults: one striking 9 degrees, dipping 43 degrees and a rake of 109 degrees (NP1), the other at 164 degrees, 50 degrees and 73 degrees respectively (PHIVOLCS Earthquake Data, 2023). The parameters of the west-dipping nodal plane are consistent with those of the plane of the subducting slab, while the east-dipping plane corresponds to a fault plane oriented perpendicularly to it. While the earthquake could have been generated by either of the two nodal planes, the orientation of the distribution of the post-mainshock events suggests that NP2 is the preferred rupturing fault.

Figure 2 Time series of the Surigao Del Sur earthquake sequence, starting with the magnitude 7.4 mainshock on Dec. 2, 2023 (10:37 p.m., Manila time) with a thrust mechanism at 26-kilometer focal depth, followed by 24 moderate to large events with magnitudes ranging from magnitude 4.9 to 6.8 within three days. The mainshock was followed by three normal fault events, then by alternating thrust and normal faulting events within 24 hours. A day and a few hours later, a magnitude 6.8 thrust faulting event occurred at about the same depth as the magnitude 7.4 event, and the alternating normal and thrusting faulting events was repeated. See text for more discussion. Credit: Aurelio, Catugas, Anicete. — Figure 2. Time series of the Surigao Del Sur earthquake sequence, starting with the magnitude 7.4 mainshock on Dec. 2, 2023 (10:37 p.m., Manila time) with a thrust mechanism at 26-kilometer focal depth, followed by 24 moderate to large events with magnitudes ranging from magnitude 4.9 to 6.8 within three days. The mainshock was followed by three normal fault events, then by alternating thrust and normal faulting events within 24 hours. A day and a few hours later, a magnitude 6.8 thrust faulting event occurred at about the same depth as the magnitude 7.4 event, and the alternating normal and thrusting faulting events was repeated. See text for more discussion. Credit: Aurelio, Catugas, Anicete, 2023

Earthquake sequence, varied fault mechanisms

Of the 24 events that succeeded the mainshock of magnitude 7.4, three had magnitudes within one order lower (6.8, 6.6, 6.4), and twelve within two orders lower (magnitude range between 5.4 and 6.3). These figures exceed the number of lower-magnitude earthquakes predicted by the Richter-Gutenberg logarithmic relationship between mainshock and aftershocks, suggesting that some of the succeeding seismic events (e.g., the magnitude 6.8 and 6.6 events) constitute an earthquake sequence instead of aftershocks, with possible triggering.

Ten of the 24 events show extensional focal mechanisms, implying that these post-mainshock earthquakes were generated by normal faults. These normal fault events plot in a cluster located east of the mainshock but west of several thrust fault events (Figures 1 and 3).

To test the hypothesis that the first and second sequences of normal faults were triggered by the initial (thrust) mainshock and the second magnitude 6.8 thrust quake, we conducted Coulomb stress change modeling (Toda et al., 2011) to determine stress changes using the mainshock as the source fault, and several post-mainshock events as receiver faults. When a post-mainshock normal fault is used as a receiver fault (fault parameters: 318 degrees, 65 degrees, -104 degrees), the normal fault events plot on a stress increase lobe (Figures 1a and 3), suggesting triggering, but with the opposite fault mechanism (i.e., thrust faulting triggering normal faulting).

A day and a few hours after the magnitude 7.4 event, a magnitude 6.8 earthquake occurred at about the same focal depth but to the east of the first one. The Philippine National Disaster Risk Reduction Management Council, citing reports of PHIVOLCS, considered these as separate seismic events generating their own aftershocks. But our Coulomb stress change model (Figures 1a and 3) indicates that the magnitude 6.8 event plots within the stress increase lobe, suggesting that it could have been triggered by the magnitude 7.4 event as well.

Unusual co-occurrence of thrust and normal faulting events

The nature and characteristics of the seismic events in the December 2023 Surigao earthquake sequence provides some insights on the geometry and structural features of the subducting slab.

The prevailing stress regime in the region is compressional as the Philippine Sea Plate moves westerly with respect to the Philippine archipelago (Aurelio and Almeda, 1999; Rangin et al., 1999). Thus, the coexistence of thrust fault and normal fault mechanism earthquakes in the same subducting slab during a single earthquake sequence is rather unusual.

Furthermore, the clustering of normal fault mechanism earthquakes in between thrust fault mechanism earthquakes (Figure 2) suggests variation of localized stress regimes within the same subducting slab. As a preliminary model, we propose that the rupturing slab involves segments where compression is taking place, but in between is an extensional segment that may either be accommodating intra-compression stress relaxation, or deforming by buckling in a manner that is similar to how the outer bend of a fold deforms by extension. In this scenario, the mainshock could have been generated initially to accommodate accumulated compressive stresses borne by the convergence of the Philippine Sea Plate with the Philippine archipelago by way of thrust faulting. This thrust fault would then have transferred stresses trenchward toward an area of relaxation or a buckle. In that scenario, normal faulting for succeeding earthquakes would be expected. A day later, the magnitude 6.8 event would serve as the second large magnitude thrust fault trigger, transferring stresses toward the same extensional region, thereby creating a new set of normal faulting events (Figures 2 and 3).

Figure 3 Coulomb stress change model of the magnitude 7.4 Surigao Del Sur earthquake in cross section (see Figure 1a for location), using the mainshock as source fault (thrust mechanism) and the next event (normal fault) as the receiver fault. Focal mechanism solutions of the mainshock (7.4) and 24 succeeding seismic events with magnitudes ranging from 4.9 to 6.8, other post -mainshock events of lower magnitudes and historical earthquakes are shown (small dots). Approximate trace of the subducting Philippine Sea Plate is shown. Data: Jarvis et al., 2008 for SRTM topography; Weatherall et al., 2020 for bathymetry; Toda et al., 2011 for CST modeling; PHIVOLCS for earthquake data. GMT (Wessel and Smith, 1995) was used to generate the section. See text for more discussion. Credit: Aurelio, Catugas, Anicete, 2023. — Figure 3. Coulomb stress change model of the magnitude 7.4 Surigao Del Sur earthquake in cross section (see Figure 1a for location), using the mainshock as source fault (thrust mechanism) and the next event (normal fault) as the receiver fault. Focal mechanism solutions of the mainshock (7.4) and 24 succeeding seismic events with magnitudes ranging from 4.9 to 6.8, other post -mainshock events of lower magnitudes and historical earthquakes are shown (small dots). Approximate trace of the subducting Philippine Sea Plate is shown. Data: Jarvis et al., 2008 for SRTM topography; Weatherall et al., 2020 for bathymetry; Toda et al., 2011 for CST modeling; PHIVOLCS for earthquake data. GMT (Wessel and Smith, 1995) was used to generate the section. See text for more discussion. Credit: Aurelio, Catugas, Anicete, 2023

Subduction of the Mindanao Fracture Zone

At the latitude where the earthquake sequence is occurring, the subduction process involves a section of the West Philippine Basin that contains the east-trending Mindanao Fracture Zone (Taylor and Goldliffe, 2004), which is a relict structure of a transform fault that operated in the early opening stages of the West Philippine Basin around 50 million years ago (Hilde and Lee, 1984). The Mindanao Fracture Zone is characterized by an east-west trending deep submarine valley flanked on both sides by parallel submarine ridges (Figure 1a). These submarine ridges hinder a smooth entry of the subducting slab, which promotes accumulation of elevated compressive stresses within the subduction system. This could have been a major factor in the generation of magnitude 7.4 earthquake in Surigao Del Sur.

Great earthquakes (those greater than magnitude 8.0) have been observed in similar tectonic settings such as in the Peru-Chile Trench where the Nazca Fracture Zone is being subducted, or in the Sunda Trench near Indonesia where the 96 Degree Fracture Zone is subducting. The latter was responsible for the magnitude 9.5 Aceh (Indonesia) earthquake of 2004, one of the deadliest seismic events in the last 100 years (Mueller and Landgrebe, 2012).

In eastern Mindanao, the submarine ridges of the Mindanao Fracture Zone that have already entered the subduction zone past the Philippine Trench continue to slide into the asthenosphere with relative buoyancy, which may explain both the gentle subduction angle in the first 30 kilometers and the difficulty of the plate to dive. Further, the relict normal faults that bounded the former transform ridges may have served as preexisting weak zones that accommodated the normal faulting earthquakes. Such complexities may have contributed to the unusual coexistence of thrust and normal faulting-induced earthquakes of large magnitudes.

Similar scenarios of hindered subduction (hence, the risk of large-magnitude earthquakes) can be found in other subduction systems around the Philippines, such as along the Manila Trench at the latitude where the relict spreading center of the South China Sea is subducting (Pautot et al., 1986; Bautista et al., 2001), or along the East Luzon Trough where the Benham Rise, a large igneous province, is converging with northern Luzon (Lewis and Hayes, 1983; Ringenbach, 1992). Given this setting, these areas are both also vulnerable to large-magnitude seismic events.

Also, the axis of the Mindanao Fracture Zone is continuous with the Lianga Fault branch of the Philippine Fault (Figure 1a). In the Coulomb stress transfer model (Figures 1a and 3), the magnitude 7.4 earthquake has transferred stresses to the west, implying the promotion of failure on the Lianga Fault, which may translate into a future earthquake.

Particularly because of this recent large-magnitude earthquake sequence, the population in eastern Mindanao should be reminded to be alert against any strong earthquakes in the future that may be triggered by this magnitude 7.4 event — or any other earthquakes. Additionally, communities built on areas of similar tectonic setting, such as those in the vicinity of the Manila Trench, which includes Metropolitan Manila and environs (population approximately 25 million), must be aware of their vulnerability to large-magnitude earthquakes. The general public is reminded to adhere to guidelines issued by government authorities on what to do during and after an earthquake (e.g., PHIVOLCS Earthquake Preparedness Guide, 2009). For infrastructure, it is strongly advised that the earthquake design requirements of the National Structural Code of the Philippines (2015) are strictly followed and enforced.

Science editor: Dr. Alka Tripathy-Lang, Ph.D.

References

Aurelio, M.A. (1992). Tectonique du segment central de la faille Philippine: etude structurale, cinématique et evolution géodynamique. Thèse de doctorat de l’Université de Paris 6,
Université Pierre et Marie Curie, Académie de Paris, T 92-22, France, 500 p.

Aurelio, M.A. (2000). Tectonics of the Philippines revisited. J. Geol. Soc. Phil., 55, 1/2, 119-185.

Aurelio, M.A. and Peña, R.E., Editors (2010). Geology of the Philippines, 2nd Edition – Vol. 1: Tectonics and Stratigraphy. 532 pages. Published by the Mines and Geosciences Bureau, Department of Environment and Natural Resources. Quezon City, Philippines

Aurelio, M.A. and Almeda R.L., 1999. Active deformation and stress state in and around the Philippines: present-day crustal motion from GEODYSSEA. In: Prog. and Abs. GPS 99 – The International Symposium on GPS. Tsukuba, Japan, 18-22 October 1999.

Bautista, B. C., Bautista, M. L. P., Oike, K., Wu, F. T., & Punongbayan, R. S. (2001). A new insight on the geometry of subducting slabs in Northern Luzon, Philippines. Tectonophysics, 339(3–4), 279–310. https://doi.org/10.1016/S0040-1951(01)00120-2

Cardwell, R.K., Isacks, B.L., and Karig, D.E. (1980). The spatial distribution of earthquakes, focal mechanism solutions and subducted lithosphere in the Philippine and northeastern Indonesian Islands. In: Hayes, D.E. (ed.) The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 1. Am. Geophys. Union Monograph, 23, 1-35.

Hilde, T.W.C. and C.S. Lee (1984). Origin and evolution of the West Philippine Basin: A new interpretation, Tectonophysics, 102, 85-104

Jarvis, A., H.I. Reuter, A. Nelson, E. Guevara (2008). Hole-filled SRTM for the globe Version 4, available from the CGIAR-CSI SRTM 90m Database (http://srtm.csi.cgiar.org).

Lewis, S.D. and Hayes, D.E., 1983. The tectonics of northward propagating subduction along Eastern Luzon, Philippine Islands. In: D.E. HAYES, Ed., Tectonic and Geologic Evolution of Southeast Asian Seas and Islands: Part 2. A.G.U. Monograph 27, 57-78.

Mueller, R.D. and Landgrebe, T.C.W. (2012). The link between great earthquakes and the subduction of oceanics fracture zones. Solid Earth, 3, 447-465, doi:10.5194/se-3-447-2012

National Structural Code of the Philippines (2015). 7th Edition. Published by the Association of Structural Engineers of the Philippines

NDRRMC SitRep No. 3 (07 December 2023). National Disaster Risk Reduction Management Council (NDRRMC) Situational Report No. 3. Situational Report for the Magnitude Mw 7.4 and Mw 6.8 Earthquakes in Surigao Del Sur, Mindanao (2023). Available at: https://ndrrmc.gov.ph/attachments/article/4246/SitRep_No_03_for_the_Magnitude_Mw7_4_and_Mw6_8_Earthquakes_in_Surigao_del_Sur.pdf

Pautot, G., Rangin, C., Briais, A., Tapponier, P., Beuzart, P., Lericolais, G., Mathieus, X., Wu, J., Han, S., Li, H., Lu, Y. and Zhao, J. (1986). Spreading direction in the Central South China Sea. Nature, 321, 6066, 150-154.

Philippine Institute of Volcanology and Seismology Earthquake Data (2023). Available at: https://swift1.phivolcs.dost.gov.ph/swiftdb/

Philippine Institute of Volcanology and Seismology, Earthquake Preparedness Guide (2009). Accessible at: https://drive.google.com/file/d/0B8_KPQhbpktXTGlzM0hHRGV2OGs/view?resourcekey=0–KN65IjUp6rRUh9rNmefzQ

Quebral, Ramon D. (1994). Tectonique du segment meridional de la faille philippine, Mindanao Oriental, Philippines: passage d’une zone de collision à une zone de décrochement. These de doctorat, Université Pierre et Marie Curie, Paris, France.

Rangin, C., Le Pichon, X., Mazzotti, S., Pubellier, M., Chamot-Rooke, N., Aurelio, M., Walpersdorf, A. and Quebral, C. (1999). Plate convergences measured by GPS across the Sundaland/Philippine Sea Plate deformed boundary: the Philippines and eastern Indonesia. Geophys. J. Int., 139, 296-316.

Ringenbach, J.C. (1992). La Faille Philippine et les châines en décrochement associés (centre et nord de Luzon): Evolution cénozoique et cinématique des déformations uaternaires.
Doctoral Dissertation, Université de Nice – Sophia Antipolis, France. 316 p.

Taylor, B. and Goodliffe, A.M. (2004). The West Philippine Basin and the initiation of subduction, revisited. Geophys. Res. Lett., 31, L12602, doi:10.1029/2004GL020136

Toda, S., Stein, R.S., Sevilgen, V. and Lin, J. (2011). Coulomb 3.3 Graphic-rich deformation and stress-change software for earthquake, tectonic, and volcano research and teaching—user guide: U.S. Geological Survey Open-File Report 2011–1060, 63 p., available at https://pubs.usgs.gov/of/2011/1060/

United State Geological Survey, 2023. Finite fault slip model of the December 2, 2023 M7.6 Mindanao, Philippines earthquake. Accessible at:
https://earthquake.usgs.gov/earthquakes/eventpage/us7000lff4/finite-fault

Weatherall, P., Tozer, B., Arndt, J.E., Bazhenova, E., Bringensparr, C., Castro, C.F., Dorschel, B., Ferrini, V., Hehemann, L., Jakobsson, M., Johnson, P., Ketter, T., Mackay, K., Martin, T.V., Mayer, L.A., McMichael-Phillips, J., Mohammad, R., Nitsche, F.O., Sandwell, D.T., Snaith, H., Viquerat, S. (2020). The GEBCO_2020 Grid – a continuous terrain model of the global oceans and land. British Oceanographic Data Centre, National Oceanography Centre, NERC, UK. doi:10.5285/a29c5465-b138-234d-e053-6c86abc040b9

Wessel, P. and Smith, W.H.F., (1995). New version of the Generic Mapping Tools released. EOS Trans. Am. Geophys. Union 76, 329.

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