Deposits from Multiple Shallow Discrete Blasts with Variable Explosion Depths: Analog Models

Abstract:

The reconstruction of eruptive processes from ejecta deposits is a vital element of volcano hazard assessment. Large-scale experiments provide an opportunity to test ejecta distribution and volume to energy and depth parameters for discrete blasts. Experiments conducted at the University at Buffalo Geohazards Field Station produced meter-scale craters with ejecta deposits. Repeated chemical explosions occurred in constructed strata of compacted aggregates (e.g. sands, gravels) with variable configurations. The experiments revealed that shallow blasts, low scaled depth (<0.004 m/J^1/3), are required to produce significant extra-crater deposits. Sixteen discrete blasts produced ejecta between 5 -18 m from the blast center (> 2x crater radius). Each experiment had between one and four explosions in the same horizontal positions (pad), with variable charge depth and energy. The extra-crater deposits were collected up to 18 m from the blast center. Ejecta produced by discrete explosions can be divided into three zones: proximal, medial and distal. The proximal deposits form a constructional feature around the crater. Medial ejecta forms a thin continuous blanket and distal ejecta consists of isolated clasts scattered beyond the medial blanket. A strong relationship between the shape of the jet produced by shallow blasts and the distribution of ejecta were reproduced through the multiple experiments. The jet shape is controlled by crater geometry and condition. Therefore, crater geometry and condition influences ejecta distribution. Blasts that interact with an existing crater have more vertically focused jets. The elevation of the crater rim, enhanced by proximal deposits, contributes to future depositional patterns by producing confined jets. Small discrete blasts frequently have >50% of their volume in the proximal zone. These experiments show that deposit distribution is sensitive to surface geometries in addition to the energy and depth of the blast.