Surface Ruptures

Surface rupture is the offset of the ground due to an earthquake that breaches the Earth’s surface. It is one of the most stark visual expressions of an earthquake rupture, as it reveals the permanent deformation that happens when two sides of a fault slip past one another.

Most earthquakes do not cause surface rupture. Some faults are buried, meaning they don’t extend to the Earth’s surface. Even when an earthquake starts on a fault that reaches the surface, the rupture itself may not travel all the way up the fault to cause surface rupture.  

Surface ruptures from normal and reverse (collectively called dip-slip) faults feature vertical offsets, while strike-slip faults produces lateral offsets. Many earthquake surface ruptures are combinations of both vertical and lateral offsets. Structures that span a fault intersecting earth’s surface are likely to suffer great damage during surface ruptures. A winning strategy for humans is to avoid building across, or even near, surface fault expressions. 

Unlike California, Washington and Oregon have no statutes regulating construction on top of faults with surface rupture potential. This is due in part to the relative lack of historic earthquakes that have ruptured the surface in the Pacific Northwest. It is difficult to characterize the hazard. Within the past dozen years, geological studies have revealed several dangerous active faults in the Puget Lowland that have produced significant surface rupture in the past 4,000 years. The search for other active, crustal faults in the Pacific Northwest continues today.