Since the risk of earthquakes in Los Angeles is greater than for any other city in the United States, determining which faults are moving and how they move is an essential step in assessing earthquake hazards. After the Whittier Narrows and Northridge earthquakes revealed that blind thrust faults threaten metropolitan Los Angeles, an array of 250 continuously-recording GPS stations (Southern California Integrated GPS Network - SCIGN http://www.scign.org) was deployed to detect and monitor the motion associated with the movement of the blind thrusts (inclined faults that do not reach the surface) and the surface faults. Here we augment the GPS measurements with interferometric synthetic aperture radar (InSAR) imagery and find that the Palos Verdes peninsula is moving towards the San Gabriel mountains at a rate of 4.4 mm/yr + 0.8 (%95 confidence) (0.2 in/yr). This motion can only be explained by slip on the thrust faults that lie beneath the greater metropolitan Los Angeles region.

We also find widespread ground water and oil pumping across Los Angeles produces measurable surface motion that is, in some locations, larger than the expected signal from slip on the blind thrust faults. SCIGN was able to detect and measure these human-induced surface motions that may have gone unnoticed for a number of years and potentially biasing geodetic seismic hazard assessments for Los Angeles. While SCIGN was developed to study how the Earth's surface deforms between and during large earthquakes, it has become a valuable resource.

The following interactive maps classified GPS sites by their proximity to InSAR observed human-induced surface motion or sites that have seasonal motion in their time-series. Red regions on the maps are where the ground surface has been sinking for five or more years. Blue patches are where the ground surface has been rising for 5 or more years. This classification system allows users to discriminate sites that contain primarily tectonic signal from those that have a combination of tectonic and human-induced motions. For more information see: Bawden, G.W., et al., Nature, 412, 812-815 (2001) [Summary][Online article][Printable article (1Mb)] Disclaimer: The appearance of the names or logos of collaborating organizations are provided for information only and do not constitute endorsement, express or implied, by the U.S. Geological Survey (USGS) or the U.S. Government of the referenced organizations, their suitability, content, products, or services, whether they are governmental, educational, or commercial.

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