Development of the Palu–Koro Fault in NW Palu Valley, Indonesia
The 220-km-long Palu–Koro Fault, Central Sulawesi, is a major fault with prominent expression in Eastern Indonesia. Many studies about the Palu–Koro Fault have shown its capability of generating large earthquakes, but how the Palu–Koro Fault has evolved remains enigmatic. This study is to investigate the geomorphology of NW Palu Valley based on DEMNAS (Digital Elevation Model of Indonesia) and field observations to understand the development of the Palu–Koro Fault. The study area comprises a high mountain in the west and a valley in the east. There are two major normal faults and a strike–slip fault observed in NW Palu Valley. The western normal fault is a basin-bounding fault, which marks the topographic break between mountain and valley. To the east, another normal fault is observed cutting the old alluvial fans and expressed by planar fault scarps. The strike–slip fault is observed within the basin and crosses the distal part alluvial fans. It is expressed by intra-basin ridges in places which are slightly uplifted from the adjacent surface. The surface rupture of the 2018 Mw 7.5 Palu earthquake in NW Palu Valley also shows left-lateral movement up to 4 m. We consider that the development of the Palu–Koro Fault in NW Palu Valley is characterized by toward-central-basin migration of faulting activity from basin-bounding fault to intra-basin fault.
KeywordsPalu–Koro Fault Active tectonics Tectonic geomorphology
The Palu–Koro Fault is the major active fault in Central Sulawesi with left-lateral movement and a NNW–SSE trend (Katili 1970; Tjia 1978; Hamilton 1979; Bellier et al. 2001; Daryono 2016; Watkinson and Hall 2017). It can be recognized based on its prominent expression spanning from Palu to North Luwu Regency along 220 km. The fault is a major boundary and accommodates the 42 mm year−1 relative motion between the Makassar Block and the North Sula Block (Socquet et al. 2006). It was also interpreted to be connected to the Matano Fault at its southern end (Hamilton 1979; Bellier et al. 2001; Socquet et al. 2006; Daryono 2016; Fig. 1a). The Palu–Koro and Matano faults have been reactivated due to the Mid-Pliocene collision in an E–W direction between the East Sulawesi and the Banggai Sula Blocks in the eastern arm of Sulawesi (Villeneuve et al. 2002; Bellier et al. 2006). Both faults accommodate left-lateral slip transferred from the E–W convergence (Bock et al. 2003; Socquet et al. 2006). To the north, the Palu–Koro Fault continues along offshore and terminates at the west end of the North Sulawesi Subduction (Hamilton 1979; Rangin et al. 1999; Hall and Wilson 2000; Fig. 1a). The convergence rate of the subduction increases toward west from 20 to 54 mm year−1, resulting from a clockwise rotation of North Sulawesi about a pole located at the NE Sulawesi (Silver et al. 1983; Walpersdorf et al. 1998; Stevens et al. 1999; Rangin et al. 1999; Bock et al. 2003).
Seismicity analysis indicates that the Palu–Koro Fault remains active with clustered source locations, and the faulting activity is dominated by strike–slip, normal, and thrust faults earthquakes (Pakpahan et al. 2015). The fault is also known as it has the greatest seismic risk in Eastern Indonesia and is capable of generating large earthquake (Bellier et al. 1998; Walpersdorf et al. 1998; Stevens et al. 1999; Daryono 2016; Watkinson and Hall 2017; Cipta et al. 2017). Indeed, on 28th of September 2018, an Mw 7.5 earthquake occurred and ruptured the fault with up to 7 m left-lateral offset, extending 180 km and traversing Palu City (Bao et al. 2019; Socquet et al. 2019). It was shortly followed by a 4- to 7-m-high tsunami, possibly associated with submarine landslides (Putra et al. 2019; Sassa and Takagawa 2019; Takagi et al. 2019). According to many earthquake catalogs (e.g., United States Geological Survey (USGS); Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG); German Research Centre for Geosciences (GFZ); Global Centroid-Moment-Tensor (GCMT)), the hypocenter was located in Donggala Regency, about 80 km to the north from Palu City and at a depth of 10–15 km.
Although many studies have been undertaken concerning the Palu–Koro Fault, its development remains enigmatic. This study aims to interpret the tectonic geomorphology based on field investigation, including some recent surface rupture and digital elevation data of Indonesia (DEMNAS) along the NW Palu Valley that shows geomorphic evidence of faulting and its relative timing, then we also propose the evolutionary model of Palu–Koro Fault in NW Palu Valley.
Palu Valley was formed due to transtensional tectonic associated with the Palu–Koro Fault (Bellier et al. 2001, 2006). The valley is bounded by high mountains trending N–S in the east and the west, reaching an elevation of 2.3 km (Fig. 1b). The high mountains dominantly consist of metamorphic complexes and granitic rocks (Sukamto et al. 1973). Bellier et al. (1999) noted two distinct alluvial fan units within the western part of Palu Valley with abandonment ages of ~ 11 kya and ~ 120 kya, respectively, for young and old alluvial fan units, or during the last two humid periods after termination of global glacial stages. The western mountain is characterized by a highly linear scarp with geomorphic features in its central part, majorly showing normal faulting evidence, while the eastern mountain is relatively more eroded and segmented (Watkinson and Hall 2017; Fig. 1b). To the south, the Palu–Koro Fault traverses to a narrow valley, and the fault segments are linked by a NW–SE trending releasing bend (Bellier et al. 2001; Watkinson and Hall 2017). The movement of the fault is mostly indicated by left-lateral stream offsets, observed in the northern and southern part of the fault (Bellier et al. 1998, 2001; Watkinson and Hall 2017).
Bellier et al. (2001) estimated that the Holocene slip rate of the Palu–Koro Fault is 35 ± 8 mm year−1 based on cumulative stream offset and age of young alluvial fan from 10Be cosmogenic dating. GPS measurement from Walpersdorf et al. (1998) suggests a left-lateral slip rate of 34 mm year−1 with 4 mm year−1 in normal component, and Stevens et al. (1999) estimated a slip rate of 38 ± 8 mm year−1 with a locking depth of 2–8 km. Socquet et al. (2006) also calculated the total motion of 40 mm year−1 with four parallel strands, which are locked at a depth of 0–5 km. These geodetic measurements from many studies have confirmed the fast slip rate of the Palu–Koro Fault.
Paleoseismology work from Bellier et al. (1998) revealed that there were three Mw 6.8–8 earthquakes during the last 2000 years with a recurrence interval of about 700 years. If each earthquake had 10 m lateral displacement, the total slip should total 30 m for 2000 years. This total displacement is less than the predicted cumulative slip of 54–86 m for 2000 years if the slip rate is 35 ± 8 mm year−1. Thus Bellier et al. (2001) argued a creeping mechanism for the Palu–Koro Fault. Alternatively, Watkinson and Hall (2017) proposed that a cross-basin strike–slip fault system, which is possibly covered by fluvial deposits and lacks clear geomorphic expression, accommodates the lateral slip deficit. Daryono (2016) also observed that the recent activity of the fault in the northern Palu Valley is expressed by fresh morphology in the middle of the valley, and the western sidewall fault in the northern part of the fault is inactive.
Prior to 2018, the Palu–Koro Fault was characterized by low-level seismicity (McCaffrey and Sutardjo 1982; Bellier et al. 2001). Pakpahan et al. (2015) proposed that seismicity in the Palu Valley is clustered because of the segmentation of the fault and some minor faults. Katili (1970) mentioned that three destructive earthquakes in 1905, 1907, and 1934 were related to the Palu–Koro Fault activity. Based on USGS Catalog, significant earthquakes occurred close to the Palu–Koro Fault in 1968 (Mw 6.7), 1998 (Mw 6.7 and 6), 2005 (Mw 6.3), 2012 (Mw 6.3), and 2018 (Mw 7.5 and 6.1). The 2018 Mw 7.5 Palu earthquake had a strike–slip solution and caused damage to Palu City, Donggala Regency, and Sigi Regency. An Mw 6.1 foreshock occurred before the Mw 7.5 earthquake, followed by several aftershock earthquakes. The surface rupture was sharp and straight along Palu Valley with the rupture velocity of 4.3–5.2 km/s, thus the event has been classified as a supershear earthquake (Bao et al. 2019; Socquet et al. 2019).
Publicly available digital elevation data of Indonesia (DEMNAS) from Geospatial Information Agency of Indonesia (BIG) are used in this study. DEMNAS is available for the whole Indonesian region with 0.27 arc-second or about 8.3 m spatial resolution. Interpretation of DEMNAS is made to investigate the geomorphology of the western mountain and alluvial fans. This study is also based on field investigation to examine the relationship of geomorphic features, including the surface ruptures associated with the 2018 Palu earthquake. The results from DEMNAS and field observations are combined to produce an evolutionary model of the Palu–Koro Fault.
North of Balaroa, a ridge on the young alluvial fan with a NNW–SSE orientation is elevated about 7 m above adjacent ground (Figs. 2b and 4b). A series of ridges with a similar trend can also be observed further north till reaching the coastline. Transpressional deformation related to the Palu–Koro Fault may be responsible for the formation of the ridges.
We also observed surface deformation associated with the 2018 Palu earthquake. Evidence of the major surface rupture is shown in several locations in the western part of Palu City (STD3-8A, STD3-8B, and STD3-9B), showing a left-lateral displacement up to 4 m along a N 350°E direction (Fig. 3c, d). Further west from the major surface rupture, on STD3-10B, a road was offset 60 cm with several minor strike–slip displacements trending N320°E, which are considered as a minor surface rupture (Fig. 3e). We also observed a subsided beach with a vertical displacement of 1.5 m on STD3-14 (Fig. 3f).
Younger faulting activity is mainly expressed by deformation at the distal parts of young alluvial fans in which has formed N–S oriented ridges (Fig. 9c). A series of ridges are uplifted from adjacent topography, indicating young localized uplift, which is plausible as a result of strike–slip faulting. This deformation gives clear evidence that the recent activity of the Palu–Koro Fault is accommodated by intra-basin faulting. This observation agrees with interpretations from Daryono (2016), which suggests that faulting activity is expressed by fresh morphology within the basin in the north Palu Valley.
Our field survey along NW Palu Valley has identified a major surface rupture with up to 4 m left-lateral offset, associated with the 2018 Palu earthquake. This observation is in agreement with the result from satellite imagery processing that shows 4–7 m displacement (Socquet et al. 2019). The major surface rupture is evidence of the recent activity of intra-basin strike–slip faulting. Additionally, there are also minor surface ruptures associated with the 2018 Palu earthquake. A minor strike–slip surface rupture, close to Kabobena Village, could be an upward splay of the major rupture, or alternatively, there may be a buried synthetic fault that was also ruptured during the 2018 earthquake. Another minor surface rupture with a major normal slip component in Beka Village is interpreted as a result of gravitational collapse since it is parallel to the major surface rupture and has a limited extent.
The migration of faulting activity has been previously proposed by Watkinson and Hall (2017), and they inferred that the cross-basin fault diagonally elongates from the narrow valley, close to the releasing bend, till Palu Bay. However, we consider that the progressive migration of faulting activity from the basin-bounding fault toward the cross-basin fault has also occurred in the NW Palu Valley. The migration of faulting activity toward basin center was also observed in other strike–slip fault systems in the world, such as Haiyuan Fault Zone (Zhang et al. 1989), Enriquillo-Plantain Garden–Walton Fault (Mann et al. 1995), and Kyaukkyan Fault (Crosetto et al. 2018). The migration might be caused by the tendency of strike–slip faults to straighten (Zhang et al. 1989; Wu et al. 2009).
The dynamics of the Palu–Koro Fault are recorded in the geomorphology of NW Palu Valley. The topographic break between the western mountain and Palu Valley indicates a major basin-bounding fault that controlled the deposition of old alluvial fans. Fault scarps along the eastern part of old alluvial fans represent normal faulting activity, which initiated the deposition of young alluvial fans. Intra-basin strike–slip faulting, which deforms distal portions of the young alluvial fans, is expressed by a series of N–S trending ridges. The 2018 earthquake also indicates the intra-basin strike–slip faulting activity in NW Palu Valley. The development of the Palu–Koro Fault in NW Palu Valley is characterized by the eastward progressive migration of faulting activity from the basin-bounding fault to a more easterly normal fault and, most recently, to the intra-basin strike–slip fault.
We thank RC for Geotechnology LIPI and its staff for the support. We thank Eko Yulianto for the supervision and for providing constructive suggestions during research. Hiroyuki Tsutsumi and Zakeria Shnizai are thanked for their suggestions. Ian M. Watkinson and an anonymous reviewer are also thanked for their comments, which significantly improved the manuscript.
AP is the main contributor who designed field investigation, collected the data, interpreted field data and digital elevation model, and wrote the manuscripts. PSP contributed to field data collection and wrote the manuscript. Both authors read and approved the final manuscript.
This research was funded by RC for Geotechnology LIPI.
The authors declare that they have no competing interests.
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