The Relationship Between Initiation of Landslides and Rainfall Intensity–Duration Thresholds in South-East Queensland, Australia

Rainfall contributes to slope instability when infiltrating water reduces matric suction and elevates pore water pressure beyond critical thresholds. Empirical rainfall intensity–duration (I-D) thresholds define the minimum rainfall conditions necessary to initiate landslides and are widely adopted in regional early warning systems. This study derives I-D thresholds for shallow landslide initiation in South-East Queensland (SEQ), Australia, using quantile regression applied to 104 rainfall-induced shallow landslide events recorded between 1974 and 2018. Thresholds at the 2nd, 10th, 50th, and 90th percentiles were derived over a duration range of 0.3 to 383 h and intensity range of 0.15 to 13.7 mm h−1. The 2nd percentile, adopted as the conservative regional early warning threshold, is expressed as I = 0.719 × D−0.220, where I is rainfall intensity (mm h−1) and D is event duration (h). To facilitate inter-regional comparability, normalised thresholds expressed in terms of mean annual precipitation (MAP) were also derived, yielding a 2nd percentile threshold of IMAP = 6.070 × 10−4 × D−0.207. Both I-D and IMAP -D thresholds fall substantially below existing global benchmarks, reflecting the pronounced susceptibility of SEQ’s deeply weathered residual soils to infiltration-driven failure. Independent validation against real-time tilt sensor and volumetric water content monitoring data from five kinematic failure events recorded at Maleny, Queensland (2016–2020), confirmed that all events plotted above the 2nd percentile threshold, with zero false negatives. The results provide a quantitative, operationally validated framework for regional shallow landslide early warning in subtropical Australia.