Fault injection based on intentional electromagnetic interference (IEMI) is attracting considerable attention in the field of physical attacks on cryptographic devices due to its non-contact and non-invasive nature. This paper explores the relations between injection intensity and fault occurrence during IEMI-based fault injection. The basic idea in this type of attack is to generate a map of the effect of such fault injection for different frequencies. Based on the maps generated for an evaluation board, we demonstrate how an injected EM wave propagates across the board depending on its intensity and frequency. We also demonstrate in detail the propagation of induced EM waves inside the target module (i.e., a cryptographic LSI chip) and other modules. Through a map generation experiment, we examine the conditions under which transient faults suitable for attacks are generated in the cryptographic module. In addition, we discuss a possible countermeasure against IEMI-based fault injection.