Tracking Stress Buildup and Crustal Deformation

Science Center Objects

The constant plate tectonic motions between the Pacific and North American plates guarantees that the crust in the western US is continually building up stress.

Crustal deformation refers to the changing earth’s surface caused by tectonic forces that are accumulated in the crust and then cause earthquakes.

Overview  |  Tracking Stress Buildup and Crustal Deformation  |  Fault Slip Rates and Post-Earthquake Motions

Ground Movement and Ground Shaking  |  Cone Penetration Testing (CPT)  |  Rock Physics Lab

San Francisco Bay Area Arrays and East Bay Seismic Experiment  |  Neogene Deformation

 

Tracking Stress Buildup

map showing stressing rate of the crust around California

Stressing rate of the crust around California derived from two decades of geodetic measurements. (Public domain.)

The constant plate tectonic motions between the Pacific and North American plates guarantees that the crust in the western US is continually building up stress. The image of crustal velocities provided by extensive GPS coverage reveals where these velocities change rapidly over short distances, demanding that the intervening crustal rock stretch and build up stress over time. Such a map of the stress reveals two main lines where stress is concentrated: The San Andreas fault zone and the Eastern California Shear Zone. These zones have experienced numerous earthquakes over the century and a half that earthquakes have been historically observed.

The mechanism of stress buildup within these fault zones is uncertain. One hypothesis is that the hot rocks below the upper 15-km-thick layer (the upper crust that has the vast majority of continental earthquakes) flows continually in response to periodic earthquakes, forcing the upper crust to bend with this flow. Another hypothesis is that slip of the deeper continuation of faults, steady slip that doesn’t produce earthquakes but still involves motions across the fault, forces the upper crust around the faults to bend and thus concentrate stress. Both hypotheses are the subject of active research. But the fact remains that high stressing rates observed on the surface likely translate to high stressing rates at the depths (~10 km) where earthquakes typically nucleate, so these stressing rates are a guide to the seismic hazard.

Crustal Deformation

Crustal deformation refers to the changing earth’s surface caused by tectonic forces that are accumulated in the crust and then cause earthquakes. So understanding the details of deformation and its effects on faults is important for figuring out which faults are most likely to produce the next earthquake. There are several hypotheses about how this works, but more data is needed to determine which one is the best.

Crustal deformation is a heavily data driven field. To measure the motions of earth’s surface, the USGS employs a variety of methods, including LIDAR, the Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), creepmeters, and alinement arrays. In parts of the U.S. with few or no historically-recorded major earthquakes or where background seismicity is sparse, geodetic data may provide the only insight into present-day seismic hazard. The motions captured by these diverse measurement techniques provide vital information on:

Map depicting crustal deformation instruments deployed in the San Francisco Bay Area.

Map depicting crustal deformation instruments deployed in the San Francisco Bay Area. (Public domain.)

  • The slow ‘background’ tectonic motions between the earth’s plates, thereby constraining the buildup of stress on faults.
  • The offsets across creeping faults such as the Hayward fault, which results in steady motions (typically several millimeters per year) of blocks of crust moving past each other along a common fault boundary.
  • The offsets across a fault during a large earthquake (the coseismic displacements).
  • Rapidly decaying motions that persist for weeks to years after a large earthquake, arising from a combination of continued slip on the fault (‘afterslip’) and possibly its extension into the lower crust and flow of rock in the deeper lower crust and mantle, where the temperature is high enough to permit ductile flow.

Specific problems of interest include:

  • Quantifying fault slip rates, creep rates, and off-fault strain rates as well as the spatial extent of locked and creeping zones.
  • Deformation rates between earthquakes with different recurrence intervals.
  • Comparing models with observed data to assess the accuracy of each.
  • Mapping the distribution of the locked (non-slipping) and freely sliding parts of faults to determine where slip in future earthquakes will occur.
  • Improving earthquake early warning and rapid response using real time high rate GPS data.
  • Investigating the effects of seismic waves travelling across a fault from an earthquake on another fault.