Interseismic Strain Accumulation in Metropolitan Los Angeles Distinguished from Oil and Water management using InSAR and GPS
InSAR measurements from 1992 to 2012 are detecting deformation due to oil pumping and groundwater changes throughout metropolitan Los Angeles. This is allowing elastic strain build up on blind thrusts beneath the city to be accurately evaluated using GPS.
Pumping and repressurization of oil fields have generated substantial displacement in metropolitan Los Angeles, causing Beverly Hills, downtown, and Whittier to subside at 3-10 mm/yr and Santa Fe Springs and La Mirada to rise at 5-9 mm/yr.
Displacements of the Santa Ana and San Gabriel Valley aquifers accumulate in response to sustained changes in groundwater over periods of either drought or heavy precipitation. Santa Ana aquifer has subsided nearly 0.1 m in response to lowering of the groundwater level by about 25 m over the past 18 years.
Anthropogenic Vs. Tectonic Motion.
We are assessing horizontal motions due to changes groundwater using an empirical relationship established on the basis of seasonal oscillations of Santa Ana aquifer. Anthropogenic horizontal motion is estimated to be proportional to the directional gradient in vertical motion inferred with InSAR. We are finding this rough approximation to be quite useful for evaluating deviations of GPS positions from a constant velocity. We are also constructing Mogi models of volume change in oil fields to evaluate GPS deviations.
Earthquake Strain Buildup on Blind Thrust Faults.
NNE contraction perpendicular to the big restraining bend in the San Andreas fault is fastest not immediately south of the San Andreas in the San Gabriel Mountains, but instead 50 km south of the fault in northern metropolitan Los Angeles. An elastic model of interseismic strain accumulation fit to GPS data and incorporating a 1D approximation of the rheology of the Los Angeles basin indicates the deep segment of the Puente Hills (–upper Elysian Park) Thrust to be slipping at 9 ±2 mm/yr beneath a locking depth of 12 ±5 km. Please see also our complementary study [Rollins et al., AGU 2015] exploring elastic models with 3D geometry. This 9 mm/yr geodetic slip rate for the Puente Hills Thrust system is faster than the 3-5 mm/yr cumulative geologic slip rate for the upper Elysian Park and Puente Hills Thrust, indicating that M 6.5 and M 7 earthquakes in metropolitan Los Angeles are more frequent than forecast.