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Article

The Central Mindoro Fault: An Active Sinistral Fault Within the Translational Boundary Between the Palawan Microcontinental Block and the Philippine Mobile Belt

by
Rolly Rimando
1 and
Jeremy Rimando
2,3,*
1
Department of Science and Technology-Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS), C.P. Garcia Avenue, U.P. Campus, Diliman, Quezon City 1101, Philippines
2
School of Earth, Environment & Society, McMaster University, 1280 Main Street W., Hamilton, ON L8S 4K1, Canada
3
Department of Geography, Geomatics and Environment, University of Toronto Mississauga, 3359 Mississauga Road, Mississauga, ON L5L 1C6, Canada
*
Author to whom correspondence should be addressed.
GeoHazards 2025, 6(1), 6; https://doi.org/10.3390/geohazards6010006
Submission received: 11 December 2024 / Revised: 8 January 2025 / Accepted: 31 January 2025 / Published: 1 February 2025
Browse Figures
Versions Notes

Abstract

:
The NNW-trending Central Mindoro Fault (CMF) is an active oblique left-lateral strike-slip fault as determined from offset morphotectonic features such as spurs and streams. Mapping of the trace and determination of the sinistral strike-slip sense of motion of the CMF is essential not only to the assessment of hazards but also to providing a clearer perspective of its role in accommodating deformation resulting from the NW relative motion between the Philippine Sea Plate and the Sunda Plate. Its sense of motion is also kinematically congruent with the NW-SE translation along a transcurrent zone between the Philippine Mobile Belt and the Palawan Microcontinental Block on the western part of the Philippine archipelago. It is also consistent with the left-lateral motion of other structures within the zone, such as the Verde Passage Fault—another structure believed to be accommodating the NW-SE translation. Mapping of the CMF provides a key constraint in identifying the possible mechanism(s) involved in the dextral strike-slip motion of the 1994 Mindoro Earthquake ground rupture, which is subparallel to the CMF.

1. Introduction

The Central Mindoro Fault (CMF; Figure 1), a prominent structural feature in Mindoro, serves as a boundary between the terranes comprising the western side of Mindoro and the Late Miocene and Pliocene basinal clastic strata to the east [1]. Previous authors refer to the CMF differently. It is referred to as the Mindoro Fault and Wasing fault—a possible southern extension—by JICA-MMAJ [2], as the East Mindoro Fault Zone by Sarewitz and Karig [3], Karig et al. [4], and Rangin et al. [5], and as the Central Mindoro Fault by Sarmiento et al. [6] and the Mines and Geosciences Bureau (MGB) [7]. Karig [1] mentioned it as a probable active fault with a left-lateral sense of strike–slip motion. Rangin et al. [5] also labeled it as a left-lateral strike–slip fault and linked it with another fault with the same sense of motion to its northeast—the Verde Passage Fault (VPF; Figure 1). However, as of yet, there is no study that proves the recency of activity or provides evidence of its kinematics. This uncertainty evidently persisted in other studies, as JICA-MMAJ [2] described the CMF and another fault to the south (Wasing Fault) as normal faults. Wynne et al. [8] also assigned a normal motion along the fault. According to the Mines and Geosciences Bureau (MGB) [7], the CMF is a right-lateral strike–slip fault with a significant normal component. Sarmiento et al. [6] echoed the assessment of the Mines and Geosciences Bureau (MGB) [7] regarding its sense of motion and referred to it as having an onshore active trace of 113.6 km from north to south of the island and an offshore extension of 6.6 km. Despite these claims, no detailed map of the CMF has been made available, nor is there a basis for the assessment of its recent kinematics.
With the exception of a few segments of the Philippine Fault Zone and a number of other active faults, most of the major active faults in the Philippines have yet to be properly mapped in terms of the precise location of active traces, the dominant overall sense of motion, and segmentation. The lack of accurate mapping and determination of the kinematics of active faults is a source of significant uncertainty in terms of earthquake hazards and risks assessment. Rimando and Rimando [11] cited several examples of inaccurate labeling in previous work on the sense of motion of major active faults in northern and central Luzon. The use of more appropriate methods in mapping and determining the recent kinematics of active faults should lead to more informed estimates of the associated magnitude of hazards and the severity of damage from earthquakes and to a better understanding of deformation of regions resulting from plate interactions in the Philippine region. In this paper, we present our mapping of the CMF and the morphotectonic evidence of its sense of motion. Our kinematic analysis will also be useful in elucidating the deformation mechanism(s) involved in the vicinity of the CMF.
This paper also seeks to clarify the CMF’s role in accommodating current upper plate deformation and facilitating recent plate motion and plate boundary translation as part of a complex system of faults in a region believed to be a transcurrent zone. The Philippine Island arc system lies along the NW trajectory of the relative motion between the Philippine Sea Plate (PSP) to the east and the Sunda Plate (SP) to the west (Figure 1 inset). To the east, the PSP subducts northwestward along the Philippine Trench (PT), while the SP subducts southeastward along the Manila–Negros–Sulu–Cotabato Trench System (Figure 1 inset) [12,13,14,15,16,17]). The Manila Trench (MT) swings to the southeast in the vicinity of western Mindoro, while the seismic slab attains an almost vertical dip as it approaches the coast of southwest Mindoro [3]. The trench system loses expression and reappears to the west of Negros as the Negros Trench (NT) and farther south as the Sulu Trench (ST) [12,15]. Whether a collisional front occurs in the gap between the MT and the NT Trench is uncertain. Northwest-verging thrust faults appear to mark the location of the collision front on Panay Island [18]. Shear partitioning across the trenches also occurs through strike–slip faults (mainly the PFZ) and other types of faults and structures [11,12,19,20,21]. The translation of northern Luzon past southern Luzon occurs along an E-W left-lateral transform fault zone between the PT and the East Luzon Trough (ELT; Figure 1 inset) [14,22,23]. How this translation occurs in the Mindoro region as the Palawan Microcontinental Block (PCB) collides with the Philippine Mobile Belt (PMB; Figure 1A) is less clear because of the lack of studies on the nature of the onshore and offshore structures west and south of Mindoro. The possibility that the CMF (Figure 1) is a transcurrent boundary was mentioned by Sarewitz and Karig [3].
The island of Mindoro has been the subject of numerous studies [1,3,4,24,25,26,27,28] because of its importance and role in uncovering a complex tectonic and geologic history that remains poorly understood. Investigating recent deformation through the mapping of active faults provides an opportunity to uncover part of its deformation history.

2. Materials and Methods

Numerous morphotectonic features (e.g., pressure ridges, sag ponds, shutter ridges, and offset/deflected streams) that could be used as trace and kinematic indicators were identified along active faults [29,30,31,32,33,34,35,36,37,38,39,40]. Slemmons [41] listed the morphotectonic features associated with active strike–slip, thrust, and normal faults. He also showed the variability in the sense of motion along the different types of faults. Many of the features are formed by simple or pure shear or through a combination of simple and pure shear in releasing or restraining bends or step gaps of fault segments [42,43,44]. Many examples of neotectonic strip maps from around the world demonstrate the use of these characteristic landforms [45,46,47,48]. In the Philippines, the mapping of active faults was greatly aided by the recognition of these features, e.g., [11,49,50,51,52].
For the CMF, the mapping of its active trace and the determination of the sense of motion are carried out using measurements of the displacement of each reference feature, primarily from offset spurs, which are quite abundant. Offsets of streams were also measured and included in the analysis. We used Google Earth Pro to measure the horizontal and vertical offsets from piercing points pricked from each reference feature. Google Earth is quite powerful and convenient to use as it enables the user to zoom in and out of areas in 3D. Its 3D capability comes from the superimposition of satellite images and/or aerial photographs over DEMs from NASA’s SRTM. Google Earth Pro provides the rapid visualization, identification, and measurement of offsets. The method was applied extensively in mapping the complex traces and in determining the overall sense of motion of the Vigan–Aggao Fault in the northern Philippines [11]. Rimando and Rimando [11] addressed accuracy issues in the use of Google Earth to measure horizontal and vertical displacements. Previous studies have demonstrated that only a handful of other methods (e.g., total station surveys, differential global positioning surveys (DGPS), or topographic maps from high-resolution aerial photography) can rival results obtained from using Google Earth [53,54,55,56] and can be more precise than methods that use aerial photography-based topographic maps and 30 m resolution global-coverage DEMs such as the Shuttle Radar Topographic Mission (SRTM) DEM. Possible sources of uncertainty stem from the use of the SRTM DEM by Google Earth. For instance, higher elevation differences between the piercing points on the upthrown and downthrown sides result in larger errors.
The use of vertical separation underestimates the dip–slip component for sites with non-vertical dips. The use of an assumed uniform vertical dip will, thus, cause greater underestimation of the dip–slip component in sites with more gentle dips. Fault dips appear to vary based on the sinuosity of some portions of the trace, which traverses mostly uneven hillside terrains. The varying amounts of erosion (at the upthrown side) and deposition (on the downthrown side) are another source of underestimation or overestimation. Various methods employed in previous studies [57,58,59] use fault dip and slope measurements to estimate the parameters of fault scarps. Wallace [57] developed a nomograph for the determination of the vertical, horizontal, and dip–slip components of displacement from field measurements of the scarp height, fault dip, and slope of the displaced original surface. Caskey’s [58] method only uses the vertical separation and slope angle of the original ground surface, while that of Costa [59] uses the dip of the fault plane and the rake of the slickenlines to estimate the components of displacement. Furthermore, aside from the lack of field outcrops and access, gathering hundreds of field structural and geomorphic data points from a complex fault system could become an arduous and less productive undertaking.

3. Results

The active traces of the CMF (Figure 2A) were delineated by following offset morphotectonic features visible in the 3D images of Google Earth Pro. Figure 2B–M show some of the offset spurs and streams that were used to delineate the active traces. All these examples indicate the left-lateral sense of horizontal displacement. Piercing points (red dots in Figure 2B–M) along an offset reference line (e.g., spur axis or drainage line) were picked from the upthrown and downthrown sides of the trace. Piercing points are primarily used to determine vertical separation. The lateral component of slip is estimated from the horizontal displacement of a reference line, as measured along the strike of the fault trace. As shown below, a more complete and accurate assessment of the sense of motion is derived from measurements of the horizontal and vertical displacements.
The N25°W-oriented CMF zone is composed of multiple traces and can be traced for about 136.5 km from the southwestern part of Puerto Galera in Oriental Mindoro to south of San Roque in Bulalacao, Oriental Mindoro (Figure 1A and Figure 2A). Segmentation (see Figure 2A) was assessed in terms of structural, geological, and geometric criteria, as defined by Knuepfer [60] and DePolo et al. [61]. The branching of faults and intersections with other faults, changes in the fault orientation, stepovers, and gaps were the primary criteria used to differentiate the segments. Based on these factors, 3 major geometric segments along the CMF were identified, namely, segment IA at the northern part of the CMF (Figure 2A), segment II at the southern part of the CMF (Figure 2A), and segment IIIA at the southwestern part of the CMF (Figure 2A).
The horizontal and vertical displacements and V/H ratios are presented in Table S1. The V/H values from the measurement sites along the segments are also presented as plots of V/H versus distance (Figure 3).
To determine the sense of displacement of the entire CMF, each of the major and minor segments are characterized in terms of the key parameters shown in Table 1. Minor segments are also described in terms of their location in relation to the major segments. The mean and maximum V/H values for each segment are useful, not only as indicators of the type of faulting, but also in terms of evaluating structural context, especially of the minor segments. The sense of horizontal displacement at each measurement site within the segments are also indicated and are useful in evaluating the overall sense of horizontal displacement and the type of faulting of each segment and of the entire CMF.
The northernmost segment, segment IA (Figure 2A and Figure 3), is the longest segment (80.25 km) of the CMF. All of the displaced references along segment IA show left-lateral sense of horizontal displacement (Table 1; Supplementary Table S1). Examples of left-laterally offset spurs and streams along the segment are shown in Figure 2B–D. The overwhelming majority of V/H ratios are less than 0.5 (Figure 3 and Table S1). Several exceptions occur near the southern and middle parts of the segment (Figure 3), where V/H ratios are greater or about equal to 0.5. These coincide with or occur near bends along the trace (Figure 2). The northern part of the trace is relatively straight, and the displacements are characterized by lower V/H ratios. The mean V/H for segment I is 0.28 and the maximum V/H is 0.89.
Segment IA is separated from another segment to the south (segment II) by a gap between these left-stepping segments (Figure 2A and Figure 3). Compared with segment I, the 60.4 km long segment II has relatively lower V/H ratios (Figure 3 and Table S1). This is reflected in the lower mean (0.17) and maximum (0.46) V/H values compared to those for segment I (Table 1). Relatively high V/H ratios occur along the southern part and at the northernmost part of the segment (Figure 2A and Figure 3). All horizontal components of displacement along segment II have a sinistral sense of motion. Some of the left-laterally displaced features along the segment are in Figure 2E–G.
The 57 km segment IIIA is located on the southwest part of the CMF zone (Figure 2A and Figure 3). The segment has substantially more displacement measurement points (Figure 3 and Supplementary Table S1), with high V/H ratios, than both segments IA and II. The mean and maximum V/H values are 0.23 and 0.58, respectively, which are higher than those seen in segment II (Table 1). Only two measurement points, however, have V/H ratios of more than 0.5 (Figure 3; Table S1). This is a considerably lower number than seen for segment IA. One coincides near its southern terminus and the other with a bend located mid-way along the segment. Points with relatively high V/H ratios that are close to 0.5 are found along the southern portion of the segment, near the northern terminus, and along segment bends (Figure 3). All measurement points along segment II have a sinistral sense of horizontal displacement (Supplementary Table S1). Examples of the left-laterally displaced spurs and drainage lines are shown in Figure 2H–J.
More minor segments include segment IB (25.6 km; Figure 2A and Figure 3), segment IC (5.8 km; Figure 2A and Figure 3), and the segments in the gap region between segment IIIA and segment II (8.1 km long “Seg. IIIB” in Figure 2A and Figure 3; 7.2 km long ‘Seg. IIIC’ in Figure 2A and Figure 3) and between segment IIIA and segment IA (12.4 km long ‘Seg. IIID’ in Figure 2A and Figure 3). Examples of offset features along segments IIIB, IIIC, and IIID are shown in Figure 2K,L,M, respectively. Segment IB is a 25.6 km long segment to the west of the northern portion of segment and extends farther northwest near Puerto Galera (Figure 2). All displacement measurement points along segment IB have V/H ratios well below 0.5 and display left-lateral horizontal slip. Segments IIIB, IIIC, IIID, and IIIE are characterized by relatively high V/H ratios, with each having measurement points with V/H ratios above 0.5 (Figure 3). The mean V/H values of these segments are considerably higher than those for segments IA, II, and IIIA (Table 1). The maximum V/H values of these segments are relatively higher than those of segments IB, II, and IIIA (Table 1). Segment IIIC has the highest maximum V/H value among all segments.

4. Discussion

4.1. Along-Trace Variation in Slip and Sense of Motion Along the Central Mindoro Fault

The displacement plots (Figure 3) provided a convenient means of examining the variation in V/H values with distance. These include a horizontal grid line used for visualizing departure from the value V/H = 1. The horizontal grid separates fault types, with lateral horizontal (L) components greater than vertical (V) components (strike–slip faults) shown as distinct from those with vertical components greater than the lateral horizontal components (reverse and normal faults). A value of V/H = 1 is equivalent to a vertical fault with a 45º pitch, where V = L, as shown in Figure 4. This is consistent with Angelier’s (1994) classification of faults, which is based on fault dip and fault plane lineation pitch (Figure 4). At the extreme ends of the pitch and dip spectrums are the pure versions of the types of faults. As the dip becomes gentler, V becomes a less accurate proxy for the slope-parallel component of displacement. Given the same pitch on a 45° dipping fault plane, V is ~30% less than the dip–slip component of displacement. Based on the outcrop pattern of CMF’s trace, the fault dip is quite steep, and so the error should be much smaller than this. Moreover, the effect of the variability of dip and of other sources of error become less significant for larger horizontal components (i.e., for lower V/H values) of displacement associated with strike–slip faulting. An arbitrary horizontal grid line equivalent to 0.5 V/H is also drawn as a reference with which to compare the V/H values. Considering how the sources of uncertainty may underestimate the dip–slip component, the dominance of sites being considerably less than 0.5 V/H should be a good indicator of strike–slip faulting.

4.2. CMF as a Transcurrent Structure

Our determination of the CMF’s sense of motion is also consistent with Sarewitz and Karig’s [3] assessment of it as a transcurrent strike–slip fault. Another major structure which appears to facilitate translation between the PCB and the PMB is the Verde Passage Fault (VPF; Figure 1 and Figure 5). Sinistral strike–slip-type earthquakes are attributed to the VPF [63]. Deeper earthquakes beneath the VPF that have thrust mechanisms are attributed to a subducting slab [63]. From the Verde Passage between Batangas and Mindoro, several works connect the VPF eastward to the Sibuyan Fault, crossing the central Philippines, e.g., [64]. Bischke et al. [65] proposed what could be a similar structure that branches out from the main trace of the PFZ. Other works also connect VPF westward to the MT, e.g., [66]. The Lubang Fault (LF) appears to branch out from the main trace of the Verde Passage Fault near Verde Island (Figure 1 and Figure 5) [10]. Like the CMF, its sinistral strike–slip motion is kinematically congruent with compression associated with the northwestward motion of the PSP. It also probably belongs to the same transcurrent zone that accommodates translation of the SP past the PMB and the PSP [66].
The transfer of the major part of the motion between the PSP and the SP in the Mindoro region could also involve other active faults (mainly strike–slip). This is to be expected in translational plate boundary region where translation between the PCB and the PMB occurs (Figure 4). Chen et al. [63] noted the presence in southwestern Mindoro of southeasterly structures revealed by seismic events having sinistral strike–slip focal mechanisms. Other active faults in western Mindoro (and in other parts of the Philippines), however, are not yet fully mapped and understood in terms of their nature and kinematics. Initial results by Rimando and Rimando (In Preparation) indicate the presence of strike–slip faults which appear to be onland extensions of the SE-oriented terminus of the Manila Trench in SW Mindoro.

4.3. Possible Deformation Mechanism(s) Involving Active Faults in the CMF Region

Aside from providing suitable structural and tectonic context for the CMF, the determination of the left-lateral strike–slip motion along the CMF also provides a vital constraint in coming up with mechanism(s) to explain the kinematics of the adjacent NNW-oriented ground rupture of the 15 November 1994 MW 7.1 Mindoro Earthquake [67]. The ground rupture, which is subparallel to the CMF, involved oblique dextral strike–slip faulting along the ARF (Figure 1, Figure 5 and Figure 6), which is opposite to the sense of motion of the CMF. The mapped onland trace of the 1994 Mindoro earthquake ground rupture is about 35 km long, but more likely extended farther offshore, probably up to where it meets the LF (Figure 1), as suggested by offshore location of the epicenter (Figure 1) and the occurrence of a tsunami. The subsurface rupture may have extended southward along the trend of the ARF near the CMF. The inferred total length of the ground rupture was about 40 km, with a possible subsurface rupture length of about 48 km. These are within expected values for a MW 7.1 earthquake based on scaling relationships between magnitude and rupture lengths from worldwide earthquake datasets [68]. Aerial and ground field surveys of the ground rupture were carried out right after the earthquake to determine the sense of motion from offset man-made and natural reference features (Figure 6A). A plot of the along-length variation in horizontal and vertical displacements (Figure 6B) indicates an overall dextral strike–slip sense of motion, which is consistent with the earthquake focal mechanism (Figure 1) at a depth of 7–12 km [67]. Vertical displacement is significant only locally, near the southern terminus, and in areas of localized transtension and transpression.
Dextral strike–slip faulting along the ARF is neither directly linked with the NW motion of the PSP-SP, nor a direct participatory structure as a transcurrent strike–slip fault. It is subparallel to the CMF but exhibits dextral strike–slip motion, which is not congruent with a NW relative PSP-SP trajectory. The kinematic paradox that the ARF presents may be resolved by an alternate mechanism(s), through which a dextral strike slip motion along the fault occurs. One possible mechanism is a consequence of the motion of the oblique sinistral left-lateral strike–slip CMF and/or the sinistral VPF. Dextral movement along the ARF may occur through a relative rigid body translation of its western and/or eastern block as a result of movement along the bounding faults. The southern terminus of the 1994 earthquake ground rupture is near the southern end of segment I (Figure 1 and Figure 5). The relatively large obliquity from the rupture trace of segment I’s slip vector, which is greater than that of segment II’s slip vector, is favorable to oblique dextral strike–slip faulting along the ARF. A possible contributory mechanism involves clockwise block rotation, which is also a consequence of the sinistral slip along the major bounding faults (VPF and CMF; Figure 5). The rotation of blocks is demonstrated and is widespread in many regions where the kinematics of major bounding faults are established [69,70,71]. However, independent paleomagnetic, geodetic, and GPS studies, which are beyond the scope of this paper, are needed to test the applicability of a rotation model.
Geohazards 06 00006 g006
Figure 6. (A) An aerial shot of right-laterally displaced rice paddy dikes near the northernmost onshore trace. Most of the horizontal displacements varied from 0 to 4 m with minor vertical components. Sag ponds and mole tracks visible in the photo are among the morphotectonic features marking the location of the ground rupture. (B) The vertical and horizontal displacement variation along the trace of the ground rupture (left of figure). Horizontal displacement is clearly dominant along most of the length of the trace except locally within a pull-apart basin, a bend, or near the southern terminus. Xs mean indistinct horizontal displacement, while 0s refer to sites having no measurable vertical or horizontal dis-placement. Rightmost is a plot for visualizing the variation in V/H with distance (displacement plot modified from Rimando et al. [67] and PHIVOLCS [72]).
Figure 6. (A) An aerial shot of right-laterally displaced rice paddy dikes near the northernmost onshore trace. Most of the horizontal displacements varied from 0 to 4 m with minor vertical components. Sag ponds and mole tracks visible in the photo are among the morphotectonic features marking the location of the ground rupture. (B) The vertical and horizontal displacement variation along the trace of the ground rupture (left of figure). Horizontal displacement is clearly dominant along most of the length of the trace except locally within a pull-apart basin, a bend, or near the southern terminus. Xs mean indistinct horizontal displacement, while 0s refer to sites having no measurable vertical or horizontal dis-placement. Rightmost is a plot for visualizing the variation in V/H with distance (displacement plot modified from Rimando et al. [67] and PHIVOLCS [72]).
Geohazards 06 00006 g006

4.4. Hazard Implications

Since CMF and its major segments are significantly longer than the ARF, larger earthquakes are to be expected from its movement. Using Wells and Coppersmith’s [68] equation relating earthquake magnitude to surface rupture length, we can estimate the possible earthquake magnitude (MW) for the CMF. Since there are no paleoseismic data from which the seismic activity of CMF and its segments can be inferred, we will assume scenarios in which only the individual segments or the entire CMF could rupture co-seismically to arrive at a seismic hazard assessment of the fault. The co-seismic rupture of CMF’s longer segments (Table 1) IA (80.25 km), II (60.4 km), and IIIA (57 km) yield magnitude estimates of MW 7–7.4. The shorter segment IB (25.6 km), on the other hand, yields magnitude estimates of MW 6.6–6.8. If the entire length of the CMF (136.5) is involved, estimated magnitudes ranging from MW 7.4 to 7.8 might be expected.

5. Conclusions

Our mapping of the active trace and determination of the recent kinematics of the CMF provide a clearer and more complete perspective of its role in accommodating deformation in the Mindoro area and add to the understanding of deformation mechanisms operating in the various regions resulting from the relative motion between the PSP and the SP. The oblique sinistral strike slip-motion of the CMF is kinematically congruent with the stress field associated with the NW-SE relative motion between the PSP and the SP. The NNW-oriented CMF accommodates part of the trench-parallel component of the NW relative motion between these plates. It also belongs to a sinistral transcurrent zone where NW-SE translation between the PSP and the PCB occurs. The other structures with left-lateral strike–slip motion, such as the VPF, more likely belong to this transcurrent zone that accommodates NW-SE translation. The right-lateral strike–slip motion of the 1994 Mindoro Earthquake ground rupture on the east side of the CMF, which is subparallel to the CMF, is neither consistent with the NW trajectory of the PSP-SP motion nor with it as a direct participatory structure in the translation process between the PSP and SP. Its kinematics may be explained by other mechanisms. The dextral strike–slip motion of the ARF could be an accommodation of the oblique sinistral strike–slip motion of the CMF or the result of clockwise rotation of the block between the CMF and the VPF resulting from the sinistral strike–slip motion along these major bounding faults.
Other types of faulting may also be involved in the vicinity of the transcurrent zone to accommodate the NW relative plate motion. It is uncertain how the SE end of the Manila Trench participates in the translation of the PCB and PMB. Although it is commonly perceived to be a thrust fault where subduction initiates, its kinematics and its onland extension(s), if any, should be investigated. If onland extension(s) exist(s) (which appears to be the case based on our initial mapping), then the CMF belongs to a wider transcurrent zone where NW-SE translation of the PCB and the PMB occurs through a number of strike–slip faults. The mapping of other offshore structures south and southwest of Mindoro and west of Panay should also be performed to ascertain the nature of deformation in these areas. The relative plate motion between the PCB and the PSP in Central Visayas is accommodated by shear partitioning, through subduction (along the NT and the PT), strike–slip faulting (e.g., PFZ), thrusting (on the western part of Panay and through active thrust faults in the Negros-Bohol area), and possibly through other types of structure.
Based on the length of the entire fault and its segments, the sinistral strike–slip CMF is capable of generating earthquakes ranging in magnitude from MW 6.6 to MW 7.8. An incorrect assessment of the sense of displacement of the major faults in northern and central Luzon will result not only in the overestimation or underestimation of hazards, but also in a misleading assessment of the deformation mechanism involved and of the contributions of the major structures in accommodating deformation in the region.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/geohazards6010006/s1. Table S1: Central Mindoro Fault Displacement Data.

Author Contributions

Conceptualization, R.R.; methodology, R.R.; software, R.R.; validation, R.R. and J.R.; formal Analysis, R.R. and J.R.; investigation, R.R. and J.R.; resources, R.R. and J.R.; data curation, R.R. and J.R.; writing—original draft preparation, R.R. and J.R.; writing—review and editing, R.R. and J.R.; visualization, R.R.; supervision, R.R.; project administration, R.R.; funding acquisition, R.R. and J.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was conducted while first author was with and partly funded by the Department of Science and Technology—Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS) in accordance with the General Appropriations Act of the Republic of the Philippines. Part of the research was also partly funded by Jeremy M. Rimando.

Data Availability Statement

The original contributions presented in this study are included in the open-access Zenodo repository at https://doi.org/10.5281/zenodo.14386668. Further inquiries can be directed to the corresponding author.

Acknowledgments

A large part of this work was accomplished while the first author was with the Department of Science and Technology—Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS) which provided logistical support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (A) Active tectonic features of the Philippines and (B) The Central Mindoro Fault (CMF) and other structural and tectonic features in the Mindoro region. The epicenter (yellow star) and focal mechanism solution for the 1994 Mindoro earthquake, which occurred along the Aglubang River Fault (ARF), are also shown. The focal mechanism solution after USGS is shown [9]. Traces of active faults, except for the Central Mindoro Fault in Mindoro Island, are from the Philippine Institute of Volcanology and Seismology [10]. CMF—Central Mindoro Fault; VPF—Verde Passage Fault; LF—Lubang Fault; ARF—Aglubang River Fault. The base map is from a near-global 30 m resolution Digital Elevation Model (DEM) (TessaDEM). The inset in (A) shows the active tectonic features of the entire Philippine archipelago (Source: PHIVOLCS [10]). SP—Sunda Plate; PSP—Philippine Sea Plate; MT—Manila Trench; NT—Negros Trench; ST—Sulu Trench; CT—Cotabato Trench; ELT—East Luzon Trough; PT—Philippine Trench; PFZ—Philippine Fault Zone. Also shown are the PMB (Philippine Mobile Belt) and the PCB (Palawan Microcontinental Block). The dashed line marks the approximate location of the PCB boundary.
Figure 1. (A) Active tectonic features of the Philippines and (B) The Central Mindoro Fault (CMF) and other structural and tectonic features in the Mindoro region. The epicenter (yellow star) and focal mechanism solution for the 1994 Mindoro earthquake, which occurred along the Aglubang River Fault (ARF), are also shown. The focal mechanism solution after USGS is shown [9]. Traces of active faults, except for the Central Mindoro Fault in Mindoro Island, are from the Philippine Institute of Volcanology and Seismology [10]. CMF—Central Mindoro Fault; VPF—Verde Passage Fault; LF—Lubang Fault; ARF—Aglubang River Fault. The base map is from a near-global 30 m resolution Digital Elevation Model (DEM) (TessaDEM). The inset in (A) shows the active tectonic features of the entire Philippine archipelago (Source: PHIVOLCS [10]). SP—Sunda Plate; PSP—Philippine Sea Plate; MT—Manila Trench; NT—Negros Trench; ST—Sulu Trench; CT—Cotabato Trench; ELT—East Luzon Trough; PT—Philippine Trench; PFZ—Philippine Fault Zone. Also shown are the PMB (Philippine Mobile Belt) and the PCB (Palawan Microcontinental Block). The dashed line marks the approximate location of the PCB boundary.
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Figure 2. (A) The active traces of the Central Mindoro Fault (CMF) as delineated from offset spurs and drainage lines. Segment color codes shown on the right side of the figure. (B–M) Selected Google Earth perspective views of offset features along the Central Mindoro Fault. Displacements are dominantly horizontal (left-lateral) as with most of the offsets along the segment. Locations of Google Earth perspective views are indicated by black squares in (A). Except for segment Ib, all segments are represented by at least one image of offset features.
Figure 2. (A) The active traces of the Central Mindoro Fault (CMF) as delineated from offset spurs and drainage lines. Segment color codes shown on the right side of the figure. (B–M) Selected Google Earth perspective views of offset features along the Central Mindoro Fault. Displacements are dominantly horizontal (left-lateral) as with most of the offsets along the segment. Locations of Google Earth perspective views are indicated by black squares in (A). Except for segment Ib, all segments are represented by at least one image of offset features.
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Figure 3. Traces of the Central Mindoro Fault (CMF) segments (segment color codes shown on left side of figure; see text below in this section for the segmentation criteria used). Segment names are indicated by combinations of Roman numerals and letters of the English alphabet. Segment names based on location may be assigned later by the Philippine Institute of Volcanology and Seismology (PHIVOLCS). Corresponding offset plots are also shown above or below each segment. The color of the V/H data markers follows the color of segments where the measurement points belong. Oblique left-lateral strike–slip faulting is indicated by the dominance of the horizontal component of displacement relative to the vertical component.
Figure 3. Traces of the Central Mindoro Fault (CMF) segments (segment color codes shown on left side of figure; see text below in this section for the segmentation criteria used). Segment names are indicated by combinations of Roman numerals and letters of the English alphabet. Segment names based on location may be assigned later by the Philippine Institute of Volcanology and Seismology (PHIVOLCS). Corresponding offset plots are also shown above or below each segment. The color of the V/H data markers follows the color of segments where the measurement points belong. Oblique left-lateral strike–slip faulting is indicated by the dominance of the horizontal component of displacement relative to the vertical component.
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Figure 4. The classification of faults based on the variation in V, T, and L with fault dip and fault lineation pitch (Source: Angelier [62]).
Figure 4. The classification of faults based on the variation in V, T, and L with fault dip and fault lineation pitch (Source: Angelier [62]).
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Figure 5. Sinistral strike–slip faulting of the CMF within a transcurrent left-lateral strike–slip zone that accommodates translation between the SP and the PMB. SP—Sunda Plate; PMB—Philippine Mobile Belt; MT—Manila Trench; NT—Negros Trench; CMF—Central Mindoro Fault; VPF—Verde Passage Fault; LF—Lubang Fault; ARF—Aglubang River Fault. Modified from the Philippine Institute of Volcanology and Seismology [10]. Also shown in the upper right corner is a schematic representation of the right-lateral strike–slip motion of the ARF brought about by an oblique sinistral strike–slip motion along the CMF and/or by the clockwise rotation of the block, bounded by the CMF and the VPF.
Figure 5. Sinistral strike–slip faulting of the CMF within a transcurrent left-lateral strike–slip zone that accommodates translation between the SP and the PMB. SP—Sunda Plate; PMB—Philippine Mobile Belt; MT—Manila Trench; NT—Negros Trench; CMF—Central Mindoro Fault; VPF—Verde Passage Fault; LF—Lubang Fault; ARF—Aglubang River Fault. Modified from the Philippine Institute of Volcanology and Seismology [10]. Also shown in the upper right corner is a schematic representation of the right-lateral strike–slip motion of the ARF brought about by an oblique sinistral strike–slip motion along the CMF and/or by the clockwise rotation of the block, bounded by the CMF and the VPF.
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Table 1. The description of the CMF segments based on key attributes related to their kinematics.
Table 1. The description of the CMF segments based on key attributes related to their kinematics.
SegmentSegment TypeLength
(km)		V/HSense of Horizontal Displacement
MeanMax
IAMajor segment80.250.280.89All 94 measurement sites indicate sinistral sense of horizontal displacement
IBMinor segment; adjacent and subparallel to IA25.60.230.36All 16 measurement sites indicate sinistral sense of horizontal displacement
IIMajor segment60.40.170.46All 74 measurement sites indicate sinistral sense of horizontal displacement
IIIAMajor segment570.230.58All 57 measurement sites indicate sinistral sense of horizontal displacement
IIIBMinor segment; occurs within gap between II and IIA8.10.410.6613 measurement sites indicate sinistral sense of horizontal displacement; 3 measurement sites indicate dextral sense of horizontal displacement
IIICMinor segment; occurs within gap between II and IIA7.20.451.0All 9 measurement sites indicate sinistral sense of horizontal displacement
IIIDMinor segment; occurs within gap between IA and IIIA12.40.370.84All 16 measurement sites indicate sinistral sense of horizontal displacement
IIIEMinor segment; adjacent and subparallel to IIIA5.90.370.72All 9 measurement sites indicate sinistral sense of displacement
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Rimando, R.; Rimando, J. The Central Mindoro Fault: An Active Sinistral Fault Within the Translational Boundary Between the Palawan Microcontinental Block and the Philippine Mobile Belt. GeoHazards 2025, 6, 6. https://doi.org/10.3390/geohazards6010006

AMA Style

Rimando R, Rimando J. The Central Mindoro Fault: An Active Sinistral Fault Within the Translational Boundary Between the Palawan Microcontinental Block and the Philippine Mobile Belt. GeoHazards. 2025; 6(1):6. https://doi.org/10.3390/geohazards6010006

Chicago/Turabian Style

Rimando, Rolly, and Jeremy Rimando. 2025. "The Central Mindoro Fault: An Active Sinistral Fault Within the Translational Boundary Between the Palawan Microcontinental Block and the Philippine Mobile Belt" GeoHazards 6, no. 1: 6. https://doi.org/10.3390/geohazards6010006

APA Style

Rimando, R., & Rimando, J. (2025). The Central Mindoro Fault: An Active Sinistral Fault Within the Translational Boundary Between the Palawan Microcontinental Block and the Philippine Mobile Belt. GeoHazards, 6(1), 6. https://doi.org/10.3390/geohazards6010006

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