Presented at the 1st Congreso del Instituto de Geociencias de Madrid (September 15-16, 2011)
Many documented examples of substantial water-extraction triggering ground subsidence and/or deformations occur in highly populated areas exposed to strong seismic risks (Mexico City, Jakarta, Teheran, California). Surprisingly, this hydrological geohazard is not considered in quake forecasts in the world, even in such threatening lineaments as the San Andreas fault of California (WGCEP, 2008), in spite of unbiased warnings as those issued by seismologist Leonardo Seeber after the 7.6 Mw event devastating India in 2001: “Widespread water withdrawal by pumping might be a factor in the Gujarat earthquake” (Vu, 2001; and, Leonardo Seeber, personal communication). The only possible exception to this generalized negligence on the risks of human-induced hydroseismicity has been suggested by Garduño-Monroy et al., (2001) in Mexico (Morelia, Michoacán) where a clear association has been described between aquifer overexploitation and potentially earthquake-triggering faults.
A 5.1 Mw earthquake occurred in May 2011 along the reverse/sinistral Alhama de Murcia fault (AMF) in the Lorca region, being the deadliest in Spain for 50 years (IGN, 2011). Even if this fault belongs to an active tectonic zone (Martínez-Díaz, et. al., 2011) seismicity was anomalous in several respects (fig. 1A): 1) the extremely shallow depth of the main shock (1km; USGS, 2011), near the base of the GB, as well as the aftershocks; 2) the migration of the foreshock/aftershocks sequence from the AMF towards the GB; 3) the off-fault location of the shallow aftershocks clustering in the GB; 4) the spatial coincidence between the seismic epicenters (IGN, 2011) and the maximum basin subsidence as described by González and Fernández (2011).
Three apparently unconnected Californian observations make the GDIS hypothesis suggested for Lorca (Fig.2a), even more intriguing on a larger scale such as the most famous fault in the world (the San Andreas fault in California). A) The southern/locked San Andreas Fault segment was the site of the largest earthquake in California in 1857, and presently it still bears the highest probability of a big seism (WGCEP, 2008; Kerr, 2011). In fact, deep tremors and micro-earthquake swarms are scrutinized along the Parkfield/Cholame segment, in case such activity might forewarn a big earthquake (Nadeau et al., 2009; Thomas et al., 2009; Shelly et al., 2011). B) Significant hydrological-induced variations of the stress field along the southern San Andreas fault have been shown to trigger seasonal variations of the seismicity (Parkfield segment; Christiansen, et al., 2007) and extra-loading strains (Salton Sea; Brothers et al., 2011). C) Recent gravity-based satellite findings reveal major groundwater loss by unsustainable agriculture along the Californian Central Valley (bounding the San Andreas Fault), a state-wide environmental crisis widely exposed in the media: “From October 2003 to March 2010, aquifers under the state’s Central Valley were drawn down by 25 million acre-feet, almost enough to fill Lake Mead, the nation’s largest reservoir” (Farmiglietti et al., 2011).