We study sourcing in a supply chain with three levels: a manufacturer, tier 1 suppliers, and tier 2 suppliers prone to disruption from, e.g., natural disasters such as earthquakes or floods. The manufacturer may not directly dictate which tier 2 suppliers are used but may influence the sourcing decisions of tier 1 suppliers via contract parameters. The manufacturer's optimal strategy depends critically on the degree of overlap in the supply chain: if tier 1 suppliers share tier 2 suppliers, resulting in a 'diamond-shaped' supply chain, the manufacturer relies less on direct mitigation (procuring excess inventory and multisourcing in tier 1) and more on indirect mitigation (inducing tier 1 suppliers to mitigate disruption risk). We also show that while the manufacturer always prefers less overlap, tier 1 suppliers may prefer a more overlapped supply chain and hence may strategically choose to form a diamond-shaped supply chain. This preference conflict worsens as the manufacturer's profit margin increases, as disruptions become more severe, and as unreliable tier 2 suppliers become more heterogeneous in their probability of disruption; however, penalty contracts-in which the manufacturer penalizes tier 1 suppliers for a failure to deliver ordered units-alleviate this coordination problem.
