Understanding the December 2023 Earthquakes in Eastern Mindanao, Philippines: What Happened and Why It Matters

Abstract

In December 2023, a sequence of earthquakes occurred offshore of eastern Mindanao Island, southern Philippines. The earthquake sequences started with a tsunamigenic earthquake with a magnitude (MW) 7.4, followed by moderate to strong aftershocks (~M5 to ~M6) and a notable MW 6.8 event. The impacts of these events include damage to structures, landslides, liquefaction, sinkhole collapse, and tsunamis. The unusual number of earthquakes in two weeks made us analyze what had happened and what would be the next. Coulomb stress transfer modeling revealed that the second event (MW 6.8), which occurred in less than two (2) days, was triggered by the first event. This occurrence was not unique as we explore past historical events in the area. The possibility of another M>7 earthquake and large aftershocks still remain. Thus, community earthquake and tsunami preparedness should continue in this part of the country.

Full text

Understanding the December 2023 Earthquakes in Eastern Mindanao, Philippines: What Happened and Why It Matters
1
Understanding the December 2023 Earthquakes in
Eastern Mindanao, Philippines: What Happened
and Why It Matters
Deo Carlo E. Llamas, Jeffrey S. Perez, Crystel Jade Legaspi, Jonard Jhon S.
Acid and John Patrick S. Naing
Department of Science and Technology
Philippine Institute of Volcanology and Seismology
_____________________________________________
The PHIVOLCS Open-File Report is an internally reviewed scientific documents on volcano, earthquake, tsunami and other
geologic researches in the Philippines and is published online by the Department of Science and Technology - Philippine
Institute of Volcanology and Seismology (DOST-PHIVOLCS). The frequency of this online publication depends on the
submitted and reviewed research results.
Any part of this online publication may be used and reproduced provided that proper acknowledgement is made.
For inquiries, please contact:
Department of Science and Technology
Philippine Institute of Volcanology and Seismology
PHIVOLCS Bldg., C.P. Garcia Avenue, U.P. Diliman Campus,
Quezon City, Philippines
Tel. Nos.: +632 8426-1468 to 79
Published online: January 2024
PHIVOLCS OPEN-FILE REPORT 24-01

PHIVOLCS Open-file Report No. 24-01
2
Understanding the December 2023 Earthquakes in
Eastern Mindanao, Philippines: What Happened and
Why It Matters
Deo Carlo E. Llamas, Jeffrey S. Perez, Crystel Jade Legaspi, Jonard Jhon
S. Acid and John Patrick S. Naing
Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology (DOST)
ABSTRACT
In December 2023, a sequence of earthquakes
occurred offshore of eastern Mindanao Island,
southern Philippines. The earthquake sequences
started with a tsunamigenic earthquake with a
magnitude (MW) 7.4, followed by moderate to
strong aftershocks (~M5 to ~M6) and a notable
MW 6.8 event. The impacts of these events
include damage to structures, landslides,
liquefaction, sinkhole collapse, and tsunamis. The
unusual number of earthquakes in two weeks
made us analyze what had happened and what
would be the next. Coulomb stress transfer
modeling revealed that the second event (MW 6.8),
which occurred in less than two (2) days, was
triggered by the first event. This occurrence was
not unique as we explore past historical events in
the area. The possibility of another M>7
earthquake and large aftershocks still remain.
Thus, community earthquake and tsunami
preparedness should continue in this part of the
country.
Keywords: Philippine Trench, Philippine Fault,
stress transfer, damaging earthquake
1.0 INTRODUCTION
A magnitude (MW) 7.4 earthquake jolted eastern
Mindanao Island, southern Philippines on 02
December 2023 at 10:37 PM Philippine Standard
Time (PST) (14:37 UTC). Its hypocenter was
located offshore, 08.44°N, 126.59°E - 029 km N
74° E of the municipality of Hinatuan, province of
Surigao Del Sur with a depth of 26 km (DOST-
PHIVOLCS, 2023a) (Figure 1). This earthquake
shook almost the entire island with a maximum
felt intensity of PHIVOLCS Earthquake Intensity
(PEIS) VII (destructive) equivalent to Modified
Mercalli Intensity (MMI) VII (PHIVOLCS,
2023b). Following the MW 7.4 event, a series of
moderate to strong earthquakes (~M5 to ~M6),
including a MW 6.8 event. The hypocenter of this
MW 6.8 event was also located offshore, 08.96°N,
126.91°E - 067 km N 87° E of the municipality of
Cagwait, province of Surigao Del Sur (DOST-
PHIVOLCS, 2023c) (Figure 1).
These earthquakes together with moderate to
strong aftershocks affected more than 170,000
families, with three (3) deaths and 79 injured.
There were more than 8700 damaged houses and
the amount of damage to infrastructures was
approximately 1.1 million Philippine pesos
(~22,000 US$) (NDRRMC, 2023). Geologic
impacts for these events include earthquake-
induced landslides, liquefaction, sinkhole collapse,
and tsunamis that triggered tsunami warnings in
PHIVOLCS Open-File Report No. 24-01
_____________________________________
Citation:
Llamas, D.C.E., Perez, J.S., Legaspi, C.J., Acid, J.J.S. and
Naing, J.P.S., 2024. Understanding the December 2023
Earthquakes in Eastern Mindanao, Philippines: What
Happened and Why It Matters. PHIVOLCS Open-File
Report No. 24-01, 11 pages.

Understanding the December 2023 Earthquakes in Eastern Mindanao, Philippines: What Happened and Why It Matters
3
Fig. 1. Seismotectonic map and cross-section of the December 2023 earthquake sequence. The map illustrates the first
major shock (MW 7.4) at the deeper part of the subduction interface and the following MW 6.8 at a shallower depth. Here we
plot earthquakes with magnitude greater than 4, highlighting the significant number of sizable aftershocks. Earthquake
location and focal mechanisms are from PHIVOLCS. Topography from NAMRIA and bathymetry from GEBCO.

PHIVOLCS Open-file Report No. 24-01
4
the Philippines and Japan (Perez and Martinez-
Villegas, 2023).
In this study, we attempt to uncover the scientific
narrative behind these events, exploring where
they came from, their historical context, and the
potential dangers they now pose to the affected
areas.
2.0 SEISMOTECTONIC SETTING
Mindanao, located in southern Philippines, is a
seismically active region that has been affected by
moderate to strong earthquakes in the past
(Bautista and Oike, 2000; Perez and Tsutsumi,
2017). It is bounded to the east by the Philippine
Trench, where the Philippine Sea Plate converges
northwestward towards Mindanao at a rate of 10
cm/yr (Cardwell et al., 1980; Argus et al., 2011).
The trench-parallel oblique convergence of the
Sunda Plate and the northwestward movement of
the Philippine Sea Plate is being accommodated
by the Philippine Fault (Fitch, 1972). The
Philippine Fault traverses the entire length of
eastern Mindanao for about 500 km (Tsutsumi and
Perez, 2013; Perez et al., 2015) (Figure 1). These
major tectonic structures, including the Philippine
Trench, have high slip rates (Rangin et al., 1999;
Argus et al., 2011).
3.0 METHODOLOGY
Analysis of spatio-temporal distribution and
characteristics of the earthquake includes the
collection of seismicity data. Earthquake
parameters were sourced from the Philippine
Seismic Network (https://earthquake.phivolcs.
dost.gov.ph/) together with information from the
United States Geological Survey (USGS)
(https://earthquake.usgs.gov/earthquakes), Global
Centroid Moment Tensor (GCMT) Project and
International Seismological Centre (ISC, 2023). A
review of historical (Bautista and Oike, 2000) and
instrumental earthquakes in the area using the
same catalogs was also conducted. The seismicity
data obtained for the December 2023 Surigao del
Sur Earthquakes were plotted in a cross-section
profile using the Generic Mapping Tools (Wessel
et al., 2019).
To determine possible stress triggering mechanism
in the earthquake sequence, a Coulomb stress
transfer model was generated using the Coulomb
3.3 software (Lin and Stein, 2004; Toda et al,
2005).
4.0 EARTHQUAKE ANALYSIS AND
DISCUSSION
4.1 The Initial Shock: Decoding the MW 7.4
Earthquake
The first seismic event, a MW 7.4, reported as a
MW 7.6 by the USGS (USGS, 2023a), happened at
a depth of 25 to 35 km (Figure 1). This earthquake
exhibited a reverse faulting focal mechanism
(DOST-PHIVOLCS, 2023d), indicating that it
resulted from compressional forces. The
combination of its location, focal mechanism, and
substantial magnitude suggests that it originated at
the subduction interface along the Philippine
Trench, where the Philippine Sea Plate pushes and
descends beneath the Philippines. Subduction
zones generate significant stress as tectonic plates
converge, and this accumulated stress is eventually
released, giving rise to powerful earthquakes.
Interestingly, a series of large aftershocks
followed, with magnitudes exceeding 5 and 6,
intensifying the seismic activity in the region.
Figure 1 displays a cross-sectional view of the
earthquake sequence. Basically, we are looking at
the view of the sliced portion of the earth,
exploring what is beneath the surface. We tried to
outline the subduction or megathrust interface
using the plots of the M>4 aftershocks. While we
can roughly sketch a gently sloping subduction
zone from this, it's important to note that the
earthquake depth values may not be well-
constrained. Considering the trench depth on the
seafloor is already about 10 km below sea level,
some adjustments are needed for the earthquake
depths. Nonetheless, we can still outline the
subduction zone, and the cross-section shows that

Understanding the December 2023 Earthquakes in Eastern Mindanao, Philippines: What Happened and Why It Matters
5
the MW 7.4 earthquake happened in the deeper part
of this subduction interface.
The aftermath of the MW 7.4 earthquake and its
subsequent aftershocks revealed some geologic
impacts. Among these were documented
occurrences of liquefaction and earthquake-
induced landslides. Notably, the coastal areas of
Surigao del Sur province experienced a minor
tsunami. Structures faced damage due to
successive ground shaking, emphasizing the
profound effect of the earthquake sequence
(PHIVOLCS Quick Response Team, 2023).
4.2 Two Separate Events: Dual Rupture?
Two days later, on December 4, 2023, a separate
MW 6.8, reported by USGS as MW 6.9 earthquake
(USGS, 2023b) occurred approximately 70
kilometers northeast of the initial epicenter,
sharing a similar reverse faulting focal mechanism
(DOST-PHIVOLCS, 2023d). This subsequent
event, which occurred at a shallower depth, shifted
the seismic activity northward and produced a
distinct earthquake cluster (Figure 2). These two
(2) events that are days apart imposed the
question: how are they linked to each other? It is
very probable that the MW 7.4 and MW 6.8
earthquakes may have ruptured different sections
along the Philippine Trench.
To take a closer look at the sequence, we
employed a technique called Coulomb stress
transfer modeling, and it reveals insights into how
the second earthquake was triggered. This
technique revolves around the concept that areas
with stress increase (depicted as red in Figure 3)
are areas where faults are promoted to failure -
they are likely to host earthquakes. In contrast,
those with stress decrease (shown in blue) are
areas where faults are inhibited from failure - they
are less likely to generate earthquakes (Stein,
1999). Based on the model, the MW 7.4 event
increased the stress by 0.4 to 0.5 bar at the location
of the subsequent MW 6.8 earthquake, initiating a
triggering effect (Figure 3). This stress transfer
mechanism essentially means that the stress from
the first quake influenced the occurrence of the
second one. This emphasizes the interconnected
nature of seismic events in subduction zones.
Stein and Toda (2023) performed similar
calculations, in which they explore the stress
increase to surrounding faults by the initial shock.
Their models predominantly indicate that failure is
likely to be promoted, potentially triggering
earthquakes, in regions both north and south of the
MW 7.4 epicenter along the Philippine Trench.
Consequently, the anticipation is for aftershocks,
possibly even another mainshock, to occur in these
specific areas. Furthermore, the possibility of
triggering shallower earthquakes along the trench
is concerning. This is unwelcome news, as it
suggests the potential for tsunami occurrence.
4.3 Echoes from the Past: Historical Seismicity
Historical and instrumental records dating back to
1921 show a pattern of significant earthquakes
(ranging from M7.1 to M8.3) along the Philippine
Trench east of Mindanao (Figure 4). Some of
these earthquakes were confirmed to be
tsunamigenic, which include the 1921 M 7.5, 1924
M 8.3, 1929 M 7.2, 1952 M 7.6, and the 1992 M
7.1 earthquakes (Bautista et al., 2012).
Back in 1992, two major earthquakes, 26 minutes
apart, (M7.1 and M7.2) shook the eastern coast of
Mindanao, roughly 130 kilometers south of the
recent MW 7.4 earthquake (Figure 4). The coast of
Caraga and Manay experienced a tsunami with a
run-up height of 3 to 6 meters as reported by
Daligdig and Tungol (1992). According to a study
by Besana and others (2004), these two
earthquakes also broke apart different sections,
influenced by a geometrical complexity along the
Philippine Trench.
The ruptured area of the recent events lies between
the 1952 MS 7.6, to the north, and the 1989 MW
7.5, to the south (Figure 4). The last similar event
in the area occurred in 1924, almost a century ago,
raising concerns about the accumulated stress
along the trench and its link to the recent seismic
activity. If we assume the accuracy of the 1924
earthquake's location and consider a convergence

PHIVOLCS Open-file Report No. 24-01
6
rate of ~10 cm/yr (or a subduction rate of ~3.6
cm/yr) between tectonic plates (Argus et al.,
2011), a significant accumulated slip along that
part of the Philippine Trench becomes evident,
potentially explaining the occurrence of the recent
major earthquake and its intense aftershocks.
Fig.2.Temporal plots of the 02 December 2023 magnitude (MW) 7.4 and the 04 December 2023 MW 6.8 earthquakes. The maps depict
different timeframes and show the migration of the seismicity from the epicentral area of the MW 7.4 to the epicentral area of the MW 6.8.

Understanding the December 2023 Earthquakes in Eastern Mindanao, Philippines: What Happened and Why It Matters
7
Fig. 3. Calculated Coulomb stress changes imparted by the MW 7.4 offshore Surigao del Sur Earthquake to the hypocentral
area of the MW 6.8 earthquake. The model shows a stress increase (red areas) in the location of the MW 6.8 earthquake.
The source fault is from the finite fault model of USGS. The stress calculations were computed using the north dipping nodal
plane (strike, dip, rake= 162, 34, 65; model depth= 10 km - adjusted depth) as the receiver fault (DOST-PHIVOLCS,
2023d).

PHIVOLCS Open-file Report No. 24-01
8
Fig. 4. Historical seismicity (M≥6) of offshore Eastern Mindanao and the focal mechanisms of the 2023 MW 7.4 and MW 6.8 offshore Surigao
del Sur Earthquakes. Historical and Instrumental earthquake locations are from Bautista and Oike (2000) and ISC (2023), respectively. Focal
mechanisms of the background seismicity are from USGS and GCMT. Black arrow indicates plate motion (Argus et al., 2011).

Understanding the December 2023 Earthquakes in Eastern Mindanao, Philippines: What Happened and Why It Matters
9
5.0 SUMMARY: WHAT’S NEXT IN
EASTERN MINDANAO?
Significant aftershocks that may still be felt and
may cause damage to weakened structures may
still occur in the area and it may last for the next
several months. As a result of our study and Stein
and Toda (2023), we cannot rule out the
possibility of another major earthquake (M>7)
along the Philippine Trench, especially in areas
with increased stress. Historical and instrumental
records show that these earthquakes (some are
tsunamigenic) are regular occurrences, often
followed by large aftershocks. Therefore,
Mindanao, especially the coastal communities is at
risk of ground shaking and potential tsunamis
caused by sudden movements beneath the ocean.
Community earthquake and tsunami preparedness
(Perez and Martinez-Villegas) should always be a
priority and must continue. Preparedness,
mitigation, and response efforts must be enhanced
to significantly reduce the risk from these hazards.
Note:
Data Source:
Earthquake parameters for the recent events were
sourced from the Philippine Seismic Network
(https://earthquake.phivolcs.dost.gov.ph/) of the
DOST-PHIVOLCS.
Reviewers:
Dr. Teresito C. Bacolcol
Dr. Ma. Mylene M. Villegas
Ms. Kathleen L. Papiona
REFERENCES
Argus D. F., Gordon R. G., DeMets, C. (2011).
Geologically current motion of 56 plates
relative to the no-net-rotation reference frame.
Geochemistry, Geophysics and Geosystem, 12
(11) Q11001. doi:10.1029/2011GC003751.
Aurelio, M., Catugas, S.D., Anicete, A. (2023).
Eastern Mindanao earthquake sequence strikes
on subducting, extinct fracture zone. Temblor,
http://doi.org/10.32858/temblor.331
Bautista, M.L.P., Bautista, B.C., Salcedo, J.C. and
Narag, I.C. (2012). Philippine tsunamis and
seiches, 1589-2012. Department of Science
and Technology, Philippine Institute of
Volcanology and Seismology.
Bautista, M. L., & Oike, K. (2000). Estimation of
the magnitudes and epicenters of Philippine
historical earthquakes. Tectonophysics 317 (1-
2), 137–169. https://doi.org/10.1016/S0040-
1951(99)00272-3
Besana, G.M., Tanioka, Y., Ando, M., Mirabueno,
M.H., Manahan, J., De Ocampo, J. Perez, J.
and Bautista, B. (2004). The May 17, 1992
earthquakes in southeastern Philippines.
Geophysical Research Letters, 31 (24).
https://doi.org/10.1029/2004gl020917.
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. American
Geophysical Union Monograph, 23, 1-35.
Daligdig, J.A. and Tungol, N.M. (1992). The May
17, 1992 Mindanao Earthquake, PHIVOLCS
Annual Report.
Di Giacomo, D., Engdahl, E. R. and Storchak, D.
A. (2018). The ISC-GEM Earthquake
Catalogue (1904–2014): status after the
Extension Project, Earth Syst. Sci. Data, 10,
1877-1899, doi: 10.5194/essd-10-1877-2018.
Department of Science and Technology –
Philippine Institute of Volcanology and
Seismology (DOST-PHIVOLCS) (2023a).
Earthquake information No.: 3. Available at:
https://earthquake.phivolcs.dost.gov.ph/2023_

PHIVOLCS Open-file Report No. 24-01
10
Earthquake_Information/December/2023_120
2_1437_B3F.html (Accessed December 15,
2023).
Department of Science and Technology –
Philippine Institute of Volcanology and
Seismology (DOST-PHIVOLCS) (2023b).
PHIVOLCS Earthquake Intensity Scale
(PEIS). Available at: https://www.phivolcs.
dost.gov.ph/index.php/earthquake/earthquake-
intensity-scale (Accessed December 15,
2023).
Department of Science and Technology –
Philippine Institute of Volcanology and
Seismology (DOST-PHIVOLCS) (2023c).
Earthquake information No.: 2. Available at:
https://earthquake.phivolcs.dost.gov.ph/2023_
Earthquake_Information/December/2023_120
3_1949_B2F.html (Accessed December 15,
2023).
Department of Science and Technology –
Philippine Institute of Volcanology and
Seismology (DOST-PHIVOLCS) (2023d).
PHIVOLCS SWIFT CMT. Seismological
Observation and Earthquake Prediction
Division, DOST-PHIVOLCS, Diliman,
Quezon City. https://swift1.phivolcs.dost.
gov.ph/~pvjica/swift_top.html
Dziewonski, A. M., Chou, T.-A. and Woodhouse
J. H. (1981). Determination of earthquake
source parameters from waveform data for
studies of global and regional seismicity,
Journal of Geophysical. Research, 86, 2825-
2852. doi:10.1029/JB086iB04p02825.
Ekström, G., Nettles, M. and Dziewonski, A. M.
(2012). The global CMT project 2004-2010:
Centroid-moment tensors for 13,017
earthquakes. Physics of the Earth and
Planetarty Interiors, 200-201 (1-9).
doi:10.1016/j.pepi.2012.04.002.
International Seismological Centre (2023). ISC-
GEM Earthquake Catalogue,
https://doi.org/10.31905/d808b825.
National Disaster Risk Reduction and
Management Council (NDRRMC) (2023).
Situational report No. 18 for magnitude 7.4
and 6.8 Earthquakes in Surigao del Sur.
Available at https://monitoring-
dashboard.ndrrmc.gov.ph/page/reports/magnit
ude-mw-74-and-mw-68-earthquakes-in-
surigao-del-sur (Accessed December 15,
2022).
Perez, J. S. and Martinez-Villegas, M. M., 2023,
Two offshore earthquakes in the Philippines:
What do coastal communities need to know?,
Temblor, http://doi.org/10.32858/temblor.329.
Perez, J.S. and Tsutsumi, H., 2017. Tectonic
geomorphology and paleoseismology of the
Surigao segment of the Philippine fault in
northeastern Mindanao Island, Philippines.
Tectonophysics. 699, 244-257.
https://doi:10.1016/j.tecto.2017.02.001
Perez, J. S., Tsutsumi, H., Cahulogan, M. T. C.,
Cabanlit, D. P., Abigania, M. I. T., and
Nakata, T. (2015). Fault distribution,
segmentation and earthquake generation
potential of the Philippine fault in eastern
Mindanao, Philippines. J. Disaster Res. 10 (1),
74–82. doi:10.20965/jdr.2015.p0074
PHIVOLCS Quick Response Team (2023). The 02
December 2023 Offshore Surigao Del Sur Mw
7.4 Earthquake. unpublished report.
Rangin C., Le Pichon, X., Mazzotti, S., Pubellier,
M., Chamot-Rooke, N., Aurelio, M.,
Walpersdorf, A. and Quebral, R. (1999). Plate
convergence measured by GPS across the
Sundaland / Philippine Sea Plate deformed
boundary: the Philippines and eastern
Indonesia. Geophysical Journal International.
Vol.139, Issue 2, pp. 296-316.
https://doi.org/10.1046/j.1365-
246x.1999.00969.x
Stein, R.S. (1999), The role of stress transfer in
earthquake occurrence, Nature 402, 605–609,
https://doi.org/10.1038/45144.

Understanding the December 2023 Earthquakes in Eastern Mindanao, Philippines: What Happened and Why It Matters
11
Stein, R. S. and Toda, S. (2023) Major earthquake
strikes the Philippines, followed by unusually
large aftershocks, Temblor,
http://doi.org/10.32858/temblor.330.
Storchak, D.A., D. Di Giacomo, I. Bondár, E.R.
Engdahl, J. Harris, W.H.K. Lee, A. Villaseñor
and P. Bormann (2013). Public Release of the
ISC-GEM Global Instrumental Earthquake
Catalogue (1900-2009). Seismological
Research. Lett., 84, 5, 810-815, doi:
10.1785/0220130034.
Storchak, D.A., D. Di Giacomo, E.R. Engdahl, J.
Harris, I. Bondár, W.H.K. Lee, P. Bormann
and A. Villaseñor (2015). The ISC-GEM
Global Instrumental Earthquake Catalogue
(1900-2009): Introduction, Phys. Earth Planet.
Int., 239, 48-63, doi:
10.1016/j.pepi.2014.06.009.
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/
Tsutsumi, H., and Perez, J. S. (2013). Large-scale
digital mapping of the Philippine fault zone
based on aerial photograph interpretation.
Active Faults Research 39, 29–37.
doi:10.11462/afr.2013.39_29
USGS (2023). M 7.6 - Mindanao, Philippines.
https://earthquake.usgs.gov/earthquakes/event
page/us7000lff4/executive (Accessed
December 15, 2023).
USGS (2023). M 6.9 - Mindanao, Philippines.
https://earthquake.usgs.gov/earthquakes/event
page/pt23337004/executive (Accessed
December 15, 2023).