During the late 1970s and early 1980s, John Bell of NBMG studied the subsidence of Las Vegas Valley caused by groundwater extraction. Don Helm, a noted expert in the field of subsidence prediction, joined John and NBMG in late 1989. Removal of solid material from depth dominates the first, along with the subsequent bridging of overlying strata that weaken and eventually collapse into the excavation. Gradual compression of small pore spaces over a large region in response to interstitial fluid removal dominates the second cause, along with rearrangement of the particulate structure of sedimentary material that slowly reaches a sturdier, more compact configuration. Long Beach, Mexico City, and parts of the San Joaquin Valley have subsided as much as 9 meters due to fluid withdrawal.
Land subsidence is the surface expression of cumulative deformation at depth.
NBMG has focused on two aspects of subsidence due to fluid withdrawal. NBMG has been monitoring the subsidence of Las Vegas Valley and analyzing the data for several years. It has become increasingly involved in investigating the complex physical processes that cause subsidence and the related fissures that slowly crack the land surface. NBMG is now recognized as a world leader in understanding these processes and in advancing the state of the science in predicting subsidence and fissuring due to fluid withdrawal. Not only did John Bell and Don Helm of NBMG present papers at the symposium but seven NBMG-supported students and former students at the University of Nevada also attended, and four of them presented papers.