Geologically, the El Centro earthquake of 1940 on the Mercalli intensity scale. It was the first major earthquake to be recorded by a strong-motion seismograph located next to a fault rupture. It was intensely studied by structural engineers and assumed to be typical until the Northridge earthquake of 1994. In this region, the geology is dominated by the transition of the tectonic plate boundary from rift to transverse fault. The southernmost strands of the San Andreas Fault connect the northernmost extensions of the East Pacific Rise. Consequently, the region is subject to earthquakes, and the crust is being stretched, resulting in a sinking of the terrain over time.