By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40–60° from the horizontal and extended several hundred kilometers into the Earth.
Do kasnih 1920-ih, seizmolozi su počeli identificirati nekoliko istaknutih zona potresa paralelnih jarugama, koje su tipično bile nagnute 40-60° na horizontalnu ravninu i pružale se nekoliko stotina kilometara u Zemlju.