Researchers: M. Shirzaei, University of Arizona and R. Bürgmann, University of California, Berkeley
Written by Linda Rowan
29 June 2018
The San Francisco Bay Area is a populated hub of economic development and susceptible to seismic and hydrologic hazards. Satellite and GNSS observations show that areas underlain by artificial fill and old mud deposits (deposited during the Holocene epoch) are undergoing subsidence at a far higher rate than previously appreciated. Combining the rate of subsidence with the potential sea level rise suggests that much more of the bay area could be inundated with water over the coming decades causing more damage and displacement.
The San Francisco Bay Area is situated in a coastal zone that is susceptible to earthquakes, volcanoes, floods, severe weather and sea level rise. The bay area is heavily populated and a hub for economic development. Concern about sea level rise and coastal inundation due to rising seas or moving faults has been studied for a long time. Tide gauge measurements date back to at least 1850.
Here the authors use global navigation satellite system (GNSS), topographic light detection and ranging (LIDAR) and interferometric synthetic aperture radar (InSAR) satellite observations to determine with better spatial and temporal resolution the rate of subsidence in the bay area. Subsidence is the amount that the land surface is moving down related to compaction of sediments, reduction of water in underground aquifers and sudden ground movements caused by earthquakes and other processes. An accurate accounting of the rate of subsidence is essential for estimating the amount of flooding and other damages that the area might experience.
Hundreds of SAR images from three different satellites, taken from July 2007 to October 2010, are combined to measure the amount of deformation occurring over a large spatial and temporal range. About 30 GNSS sites are used as reference points and tie lines to measure points of motion in the east-west, north-south and vertical directions.
The SAR and GNSS data are combined to create a local land subsidence map for the time period of 2007 to 2010. The subsidence rate is about 2 millimeters per year, however, portions of Treasure Island, San Francisco, the San Francisco Airport and Foster City are subsiding at higher rates of 10 millimeters per year. Much of the high rate of subsidence is due to compaction of sediments, either artificial landfill or old muds. The local map is placed into a continental reference frame by integrating it with high resolution topographic LIDAR to consider the impact of different estimates of sea level rise with the measured subsidence rate. The measured land subsidence rates are then projected into current day and future estimates.
The authors consider the possible amounts of inundation due to projected sea level rise and land subsidence. Over the next 100 years, sea level and land subsidence could cause 125 to 429 square kilometers of land to be inundated with water. This is much more than projections of sea level rise alone and point out the importance of understanding the contribution of land subsidence to future hazards. Even if sea level does not rise as forecast, subsidence alone may cause 45 square kilometers of land to be at risk for flooding. Such inundation could displace 480,000 people and cause $100 billion in damages by 2100. Observing and estimating local land subsidence will help the San Francisco Bay Area plan and prepare for future changes in climate, sea level and land height.
M. Shirzaei, R. Bürgmann, Global climate change and local land subsidence exacerbate inundation risk to the San Francisco Bay Area, Sci. Adv. 4, eaap9234 (2018). doi:10.1126/sciadv.aap9234.
subsidence, compaction, sea level rise
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