Київський національний університет імені Тараса Шевченка
Геологічний факультет
Кафедра геології нафти і газу
Конспект лекцій
до
НАВЧАЛЬНОЇ ДИСЦИПЛІНИ
“Earth's Deep Underground Hydrosphere”
(ГЛИБИННА ГІДРОСФЕРА ЗЕМЛІ)
Лекція № 10
”Relationship between hydrogeology and seismic activity”.
для магістрів I курсу геологічного факультету
(спеціальності: геологія нафти і газу, гідрогеологія)
Київ-2013
Конспект лекцій навчальної дисципліни
“Earth's Deep Underground Hydrosphere”
для студентів геологічного факультету
Упорядник: доцент І.М. Байсарович
-2-
Theme 5. “Relationship between hydrogeology and seismic activity”.
Лекція 10
Program:
Induced seismicity;
Earthquake-induced hydrologic phenomena;
Streamflow and springs;
Fluid pressures at seismogenic depths.
1. Induced seismicity
The principle of effective stress and results of laboratory failure experiments with fluid-saturated samples combine to suggest that high pore-fluid pressures should reduce the amount of work required for tectonic deformation. Many natural mechanisms have been suggested for maintaining elevated pore-fluid pressures (e.g., Hanshaw and Zen, 1965; Neuzil, 1995), but pore-fluid pressures can also be affected by human intervention, most directly by the injection or withdrawal of fluids through wells.
The Rocky Mountain arsenal is an example of seismicity directly induced by injection of fluid at depth. The Rocky Mountain arsenal example was essentially an uncontrolled experiment. Injection of liquid waste at a depth of 3.6 km just northeast of Denver, Colorado, unexpectedly generated earthquakes as large as Richter M 5.5 that were felt in the Denver and Boulder metropolitan areas. Shortly after a relationship between the timing of waste injection and the earthquakes was disclosed to the general public by geologist David Evans in November, 1965, the waste-injection program was discontinued. In contrast, the Rangely, Colorado, example was a controlled experiment. Earthquakes were intentionally induced by fluid injection at >1.7 km depth under conditions such that the actual fluid pressures and stress state were known and the earthquake hypocenters could be precisely located.
-3-
From studies of injection-induced seismicity only, where subsurface fluid pressure changes are relatively large, one might conclude that the Mohr-Coulomb failure criteria and effective stress law are adequate models of earthquake occurrence. But many earthquakes are also induced by the filling of surface reservoirs, or by interannual or shorter-term fluctuations in reservoir levels that translate to pressure changes as small as 0.1 to 1.0 bars (e.g., Roeloffs, 1988). For example, most shallow (<10 km) earthquakes in the Aswan, Egypt, area are clearly related to short-term changes in the water level behind the Aswan High Dam (Awad and Mizoue, 1995). Much of the surface reservoir-induced seismicity cannot readily be explained in terms of Mohr-Coulomb failure due to reduced effective stress.
The Rocky Mountain arsenal. In 1962-1966 the U.S. Army injected approximately 6.25x108 liters of waste fluid from munitions production into fractured Precambrian gneiss beneath the Denver basin. The unanticipated result was a swarm of over 1,500 recorded seismic events in 1962-1967 (Figure 5.4).
The three largest earthquakes (Richter M ≥ 5) actually took place in 1967, after waste injection had ceased. The 1962-1967 earthquake epicenters were consistently located in a rectangular, northwest-trending, 10-km by 3-km area roughly centered on the injection well, and the earthquake hypocenters were located approximately between the injection depth of 3.6 km and 7 km depth.
The persistence of the earthquake swarm after injection had ceased was attributed to the continued outward migration of the pressure front (Healy and others, 1968). That is, fluid pressures at some distance from the well continued to increase significantly for some time after injection ceased, despite pressure declines in the more immediate vicinity of the well.
-4-
Figure 5.4 Time series of (a) waste-fluid injection and (b) seismicity at the Rocky mountain arsenal, 1962-1972. From Hsieh and Bredehoeft (1981).
Hsieh and Bredehoeft (1981) later developed quantitative models of the fluid pressure history in the injection zone, using the distribution of earthquake hypocenters to define the dimensions of the “reservoir” and the long-term (9-year) pressure decline in the injection well to define its hydraulic properties. They modeled the system behavior and they concluded that the earthquakes were likely triggered by a rather small pressure buildup of 3.2 MPa (32 bars). That is, earthquakes were apparently confined to areas where the pressure buildup relative to preinjection conditions exceeded 32 bars. Migration of earthquakes epicenters away from the well could be related to the outward migration of this critical pressure increase.
-5-
Prior to waste injection, fluid pressures in the injection zone were probably below the hydrostatic value computed relative to the land surface. The initial pressure in the injection zone is not known, but the best estimate of ~270bars at 3.6-km depth translates to a water level ~920 m below the land surface, and even after the “critical” pressure increase of 32 bars the medium was apparently failing under subhydrostatic fluid-pressure conditions (a pressure gradient of ~83 bars/km relative to the land surface, versus the “normal” hydrostatic gradient of ~100 bars/km). One implication of the Hsieh and Bredehoeft (1981) analysis, then, is that the gneissic rocks beneath the Rocky Mountain arsenal were already very close to failure prior to waste injection so the earthquakes there might eventually have occurred spontaneously. The existence of critically stresses faults (faults near incipient failure) has since been documented in a wide variety of tectonic settings (e.g., Zoback and Healy, 1984). Thus, enhanced pore pressure at depth might be expected to generate earthquakes in many areas.