1. A lineup on a number of traces that indicates the arrival of new seismic energy, denoted by a systematic phase or amplitude change on a seismic record; an arrival. May indicate a reflection, refraction, diffraction, or other type of wavefront. The distinguishing features of various types of events are discussed in Sheriff and Geldart (1995, 145–146). 2. A command entered by a user, such as a mouse click or a typed command. 3. Evidence in the stratigraphic column of something happening within a very short period of time, such as a volcanic eruption that produces a layer of volcanic ash or tephra, a meteor impact that involves a rare element, etc.; an event bed or event horizon. Events during deposition may be caused by storm waves, flooding, sediment gravity flows (e.g., slumps, turbidite flows), volcanic eruptions, etc. Nondepositional events may be caused by erosion or depositional hiatus (such as sediment bypassing). Events related to sea-level variations, such as condensed sections (q.v.), are sometimes included. Events should be thin and have at least reasonable geographical extent to be useful in age dating. Especially distinctive events are called marker beds (q.v.).