Constraints on building susceptibility zone from tephra-lapilli loading through isopach mapping: A case study of the Quaternary (<27.4 - >1.1 ka ), VEI 3–4 eruptions of Sumbing volcano, Central Java, Indonesia

Abstract

This study documents the first record of seven-, Quaternary (<27.4 - >1.1 ka) lapilli fall layers at Sumbing volcano, Central Java, Indonesia. These lapilli layers (from lower towards upper stratigraphic height) are named orange–red lapilli, dark brown lapilli, orange lapilli, red–brown lapilli, blackish brown lapilli, pale red lapilli, and red lapilli. All deposits were dominated by juvenile scoria ($C_{scoria}$ = 54–96%) with various portions of volcanic lithics ($C_{lithics}$ = 4–44%). The products were dispersed across a wide area (400–2149 km² for 1 cm isopach), suggesting sub-Plinian to Plinian intensity, with mean eruption plumes reaching 15–28 km and volcano explosivity index (VEI) of 3–4. The deposition of tephra (lapilli) on the roof provides load pressure, thus threatening buildings from collapse. No significant bulk density (${\uprho }_{bulk}$) variation between each lapilli deposit (690–813 kg/m³ for dry tephra, 856–1007 kg/m³ for wet tephra, and 1208–1422 kg/m³ for saturated tephra). Thus, tephra thickness ($h$) is believed to play the most significant role in controlling load pressure ($P = {\uprho }_{bulk} \times h$), as it varies over three orders of magnitude (< 1–100 cm). Finally, under the assumption that all-nearby settlements followed the national standard for maximum load capacity (MLC; 487 kg/m²), we proposed three susceptible zones (SZs). The estimated radius for SZ I (representing dry tephra condition) is  ~7.7 km and  ~14.6 km for VEI 3 and 4 eruptions, respectively. If tephra becomes wet and/or even saturated with water due to heavy rain (which is likely to occur in tropical regions such as the present case), the estimated radius increases to ~12.7 km and ~15.3 km (VEI 3), and ~17.7 km and ~22.1 km (VEI 4) for SZ II and SZ III, respectively. These areas must be evacuated immediately if VEI 3–4 eruptions were to occur at Sumbing volcano.