In some cases, being creepy may have its benefits
We live on the surface of Earth’s stiff crust, which is broken by faults. Many of the San Francisco Bay Area’s faults cut through to the earth’s surface: offsetting fences, distorting building foundations, and ripping up roadways. Heck, the San Andreas Fault can be seen from space. Even if the fault surface expressions had gone unnoticed, Bay Area residents are aware of their proximity to some of the earth’s largest faults on account of the earthquakes often rumbling under their feet. Earthquakes release energy stored in the rocks on either side of the fault. If the slip along the fault is quick enough, the release sends waves of elastic energy, called seismic waves. Some of the seismic waves produce intense ground shaking near the source of the earthquake. Although earthquakes can be a hazard for those living nearby, seismic waves from a single earthquake can be recorded in other locations around the world, helping geoscientists study the interior of the Earth.
Between earthquakes, or seismic slip events, most faults are locked since the strain in the rocks remains below a critical threshold. Faults that are exposed to the surface may have another option: aseismic fault slip can occur as the rocks on either side of the fault slowly and steadily continues to deform and slip past each other. This is fault “creep” occurs in the upper crust, driven by deformation in the hotter, more flexible lower crust. Creep is attributed to low frictional strength along the fault, less stress acting perpendicular to the fault, and elevated fluid pressures in the rock (SFSU).
On the Hayward Fault in the East Bay, creep rates are just a few millimeters per year. Over time, this creep is enough to offset curbs and destroy houses. Strain relieved during the Hayward Fault creep may delay the next earthquake; but geologists have yet to accurately predict the creep’s impact on the timing and magnitude of earthquakes. Creep can be temporary, and creep rates can change. Furthermore, the near-surface creeping section of a fault is generally dwarfed in size by the locked area. A recent article about creep on the Hayward Fault reflects on the perspective that creep may reduce the risk of larger earthquakes, but there are so many faults in this tectonically active area that the threat of a big earthquake is ever-present.
Want to learn more?
Recent New Yorker article by Jeremy Miller on the Hayward Fault
- Creep: a simple review by San Francisco State University’s Fault Creep Monitoring Project
- Earthquakes: a simple review by Professor Nelson at Tulane University
