Geological history controlling the debris avalanches of pyroclastic fall deposits induced by the 2009 Padang earthquake, Indonesia: The sequential influences of pumice fall, weathering, and slope undercut

Highlights

• The 2009 Padang earthquake (Mw 7.6) induced debris avalanches of pumice fall deposits.

• A total of 159 landslides were identified in the areas with VIII seismic intensity in MMI.

• The pumice grains were strongly weathered at the base of the beds to form halloysite.

• The sliding surfaces were made in the weak halloysite-rich zone.

Abstract

Rapid-moving landslides occurred in many locations during the 2009 Padang earthquake, Sumatra. We interpreted satellite images, performed field surveys, in situ dynamic cone penetration tests, and laboratory analyses for mineralogy and physical and mechanical properties. We found that landslides occurred at 159 locations in an area of 64 km², and these areas had pumice fall deposits overlying paleosols which were heavily weathered debris flow deposits. These landslides in the investigated area occurred in areas with pumice fall deposits thicker than 350 cm; this depth was probably vital in inducing mechanical instability. The areas had sliding surfaces at the base of the pumice fall deposits, where pumice grains were mixed with the underlying paleosol and had been heavily weathered into halloysite-rich clayey materials by interaction with the percolating water from the ground surface. After the deposition of this pumice fall with mantle bedding, the beds were undercut by subsequent river erosion that loosen their downslope lateral support. This geologic history of pumice fall, weathering, and undercutting is not confined to the affected area but it is common to many volcanic areas, creating the opportunity to predict areas susceptible to earthquake-induced catastrophic debris avalanches.

Introduction

Among the most hazardous types of landslides in volcanic areas are debris avalanches and debris flows of pyroclastic fall deposits induced by rainstorms and earthquakes (Moon and Churchman, 2016). These landslides move very rapidly and have a long runout distance (Hungr et al., 2014). Rain-induced landslides of pyroclastic fall deposits typically occur in Italy (Forte et al., 2019; Santo et al., 2018; Vingiani et al., 2015) and Japan (Towhata et al., 2021; Wang et al., 2019). Earthquake-induced landslides of pyroclastic fall deposits, which are the focus of this paper, have occurred in Japan (Chigira, 1982; Chigira, 2014; Chigira et al., 2014, Chigira et al., 2019; Ishihara and Hsu, 1986; Kawamura et al., 2019; Sugimoto et al., 2012; Tanaka, 1985; Yoshida and Chigira, 2012), El Salvador (Crosta et al., 2005; Evans and Bent, 2004; Ishihara and Hsu, 1986; Jibson et al., 2004) and Indonesia (Wang et al., 2011). Because pyroclastic fall deposits cover wide areas near their origin and experience the same weathering history after deposition, a single rainstorm or an earthquake could trigger clusters of such landslides and devastate wide areas. Chigira and Suzuki (2016) summarized the features of earthquake-induced landslides of pyroclastic fall deposits and pointed out the following: they occur on gentle slopes, which are cataclinal or dip slopes because of the mantle bedding nature of pyroclastic fall deposits; the susceptible slopes are generally undercut; and the sliding surfaces are made of a halloysite-rich layer. Moon and Churchman (2016) also summarized the importance of halloysite in earthquake-induced and rain-induced landslides in volcanic areas. In volcanic regions, however, almost all areas are covered by pyroclastics, and halloysite is a common weathering product of volcanic materials (Aomine, 1966; Lowe, 1986), the research of this nature would contribute to hazard mapping methodology for the earthquake-induced landslides of pyroclastic fall deposits.

The 2009 Padang earthquake occurred 60 km west-northwest of Padang, Sumatra Island, Indonesia, at 5:16 pm on 30 September (local time), with a magnitude of Mw 7.6 (McCloskey et al., 2010) (Fig. 1). The seismic intensity recorded in MMI was VIII in Pariaman (30 km ENE of the epicenter) and Bukittingi (73 km NE) and VII in Padang (60 km ESE) (USGS website, 23 December 2013 access). According to an analysis by the USGS (as cited in EERI, 2009), one strong ground motion record was obtained in Padang, involving approximately 20 s of strong shaking with a peak ground acceleration of 0.3 g. The hypocenter was 81 km deep along the boundary between the Australian and Sunda plates. Sumatra Island is on the Sunda plate, under which the Australian plate is obliquely subducting NNE (McCloskey et al., 2010). The dip slip is accommodated on the subduction interface, and the strike-slip component is accommodated largely by the Sumatran fault. The last historical earthquake that occurred at the plate boundary off of Padang was a 8.7 Mw event in 1797, before the 2009 Padang earthquake occurred in the oceanic slab of the Australian plate (McCloskey et al., 2010). The Sumatran fault, which trends NW-SE through Bukittingi, has caused many earthquakes. Recent earthquakes had Mw 6.4 and 6.3 in 2007, with epicenters near Lake Singkarak (EERI, 2007).

The 2009 Padang earthquake did not cause a tsunami but affected an area with a population of approximately 1.2 million in West Sumatra province of Sumatra which Padang is the capital city; the tremor caused severe damages to buildings in low-lying areas in the coastal zone of downtown Padang and Pariaman, and induced many landslides in hilly areas in Tandikat. The induced landslides were dominated by debris avalanches of pyroclastic fall deposits (Wang et al., 2011): Numerous landslides occurred on valley-side slopes and completely buried valley-bottom villages. There were 1195 total fatalities from this earthquake (EERI, 2009), and more than half of the fatalities were from landslides (Bothara et al., 2010). The landslides destroyed three villages, and there were 130 fatalities in Tandikat village alone (Fig. 2A), which was documented on a monument in the village. The landslides were too fast to allow people to escape (Daily News 5 Oct, 2009). Wang et al. (2011) conducted a field survey and performed geotechnical testing of pumice they found in landslide sites, but the landslide distribution and geological structures and basic causes of the landslides were not clarified. In addition to the debris avalanches described above, the 2009 Padang earthquake induced rockfalls on the walls of the Maninjau caldera, but they are beyond the scope of this paper and do not affect the conclusions of this paper.

Padang, which is 50 km southeast of the 2009 landslide area of Tandikat, receives 4010 mm of annual precipitation, and the average temperature is 27 °C, with a dry season from April to September and a rainy season from October to March (Japan Meteorological Agency, 2021, accessed 6 June 2019). Wang et al. (2011) reported an eyewitness account that described the rain started at 2:00 pm on 30 September and continuing until the earthquake at 5:16 pm and inferred that the rainfall intensity was 20–40 mm/h, which suggests that the total amount was potentially 60 to 130 mm just before the earthquake.