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E.A. Roeloffs

Publications and source records attributed to E.A. Roeloffs.

12 recordsLinked to original sources
USGS2012

Hydrologic effects on water level changes associated with episodic fault creep near Parkfield, California

As part of the Parkfield, California, earthquake prediction experiment, water level is monitored in a well 460 m from the main trace of the San Andreas fault on Middle Mountain, in the preparation zone of the anticipated Parkfield earthquake. The well configuration allows water level to be monitored in two fluid reservoirs at depths of 85 and 250 m below land surface. During 1987, water level changes were recorded during 12 of the 18 episodes of accelerated fault creep detected by a creep meter spanning the fault trace 750 m northwest of the well. The creep-related water level changes in the shallow reservoir have durations of less than 1 day, whereas in the deeper reservoir the changes persist for as long as 2 months. These data suggest that the transient nature of the water level changes in the shallow interval is due to vertical flow to the water table and is not evidence that creep events propagate past the well. Phase leads of earth tidal constituents in the water level data from the shallow interval relative to the same constituents in the local volume strain tide support the interpretation of significant flow to the water table at periods of 1 day or less. The form of the water level changes in the deep interval is affected by horizontal flow to the well bore. This effect can be removed from the water level records using a theoretical response curve constrained by the phases of earth tidal constituents in the deep interval relative to the local volume strain tide. For the events where the signal in the shallow interval has been large enough to measure, the sizes of the simultaneous water level changes in the two reservoirs are consistent with the same amounts of volume strain occurring at both depths.

Journal of Geophysical Research Solid Earth↗
USGS2006

Evidence for aseismic deformation rate changes prior to earthquakes

For ten earthquakes in nonmagmatic settings, there are credible published accounts of pre-earthquake deformation-rate changes lasting hundreds of seconds to more than a decade. Although most M > 7.5 earthquakes have occurred without detectable pre-earthquake deformation, the detection threshold for aseismic deformation remains high, in that aseismic slip with moment equivalent to an M5 earthquake would in most (although not all) cases have been missed. Interseismic deformation rates vary without being followed by earthquakes, and a strain-rate change prior to the 1989 Mw6.9 Loma Prieta, California, earthquake is shown to be similar in size to many other rate changes that have occurred since that time. Most examples of pre-earthquake aseismic deformation lasting hundreds of seconds or more probably originate adjacent to, or downdip of, the seismic rupture plane, rather than within the zone that undergoes seismic failure. Copyright ?? 2006 by Annual Reviews. All rights reserved.

Annual Review of Earth and Planetary Sciences↗
USGS2003

Hydrological response to earthquakes in the Haibara well, central Japan - I. Groundwater level changes revealed using state space decomposition of atmospheric pressure, rainfall and tidal responses

For the groundwater level observed at the Haibara well, Shizuoka Prefecture, central Japan, time series analysis using state-space modelling is applied to extract hydrological anomalies related to earthquakes. This method can decompose observed groundwater level time series into five components: atmospheric pressure, tidal, and precipitation responses, observation noise, and residual water level. The decomposed responses to atmospheric pressure and precipitation are independently determined and are consistent with the expected response to surface loading. In the groundwater level at the Haibara well, 28 coseismic changes can be discerned during the period from 1981 April to 1997 December. There is a threshold in the relationship between earthquake magnitude and the well-hypocentre distance, above which earthquakes cause coseismic changes in the residual water level. All of the coseismic water level changes at the Haibara well are decreases, although 33 per cent of the estimated coseismic volumetric strain steps are contraction, which would be expected to cause water level increases. The coseismic changes in groundwater level are more closely proportional to the estimated ground motion than to coseismic volumetric strain steps, suggesting that ground motion due to earthquakes is the major cause of the coseismic water level drops and that the contribution from static strain is rather small. Possible pre- or inter-earthquake water level changes have occurred at the Haibara well and may have been caused by local aseismic crustal deformation.

Geophysical Journal International↗
USGS2003

Hydrological response to earthquakes in the Haibara well, central Japan - II. Possible mechanism inferred from time-varying hydraulic properties

28 coseismic groundwater level decreases have been observed at the Haibara well, Shizuoka prefecture, central Japan, from 1981 to 1997. These groundwater level changes cannot be explained as the poroelastic response to coseismic static strain. We use the atmospheric pressure and tidal responses of the well, rock properties measured on core samples from the same formation and pumping test results to characterize the hydraulic and mechanical properties of the aquifer. The responses of the Haibara well to the M2 Earth tide constituent and to atmospheric pressure have varied over time. In particular, increasing amplitude and decreasing phase lags were observed after the 1993 pumping test, as well as after earthquakes that caused coseismic water level changes. The tidal response, together with the surface load efficiency derived from the atmospheric pressure response, is used to estimate the mechanical properties of the aquifer. The largest amplitude of the M2 constituent, 2.2 mm, is small enough to imply that pore fluid in this system is approximately twice as compressible as water, possibly due to the presence of a small amount of exsolved gas. Diffusion of a coseismic pressure drop near the well could account for the observed time histories of the water level changes. The time histories of the water level drops are well matched by the decay of a coseismic pressure drop at least 80 m away from the well. Removal of a small amount of gas from the formation in that location might in turn explain the coseismic pressure drops.

Geophysical Journal International↗
USGS1998

Persistent water level changes in a well near Parkfield, California, due to local and distant earthquakes

Coseismic water level rises in the 30-m deep Bourdieu Valley (BV) well near Parkfield, California, have occurred in response to three local and five distant earthquakes. Coseismic changes in static strain cannot explain these water level rises because (1) the well is insensitive to strain at tidal periods; (2) for the distant earthquakes, the expected coseismic static strain is extremely small; and (3) the water level response is of the incorrect sign for the local earthquakes. These water level changes must therefore be caused by seismic waves, but unlike seismic water level oscillations, they are monotonic, persist for days or weeks, and seem to be caused by waves with periods of several seconds rather than long-period surface waves. Other investigators have reported a similar phenomenon in Japan. Certain wells consistently exhibit this type of coseismic water level change, which is always in the same direction, regardless of the earthquake's azimuth or focal mechanism, and approximately proportional to the inverse square of hypocentral distance. To date, the coseismic water level rises in the B V well have never exceeded the seasonal water level maximum, although their sizes are relatively well correlated with earthquake magnitude and distance. The frequency independence of the well's response to barometric pressure in the frequency band 0.1 to 0.7 cpd implies that the aquifer is fairly well confined. High aquifer compressibility, probably due to a gas phase in the pore space, is the most likely reason why the well does not respond to Earth tides. The phase and amplitude relationships between the seasonal water level and precipitation cycles constrain the horizontal hydraulic diffusivity to within a factor of 4.5, bounding hypothetical earthquake-induced changes in aquifer hydraulic properties. Moreover, changes of hydraulic conductivity and/or diffusivity throughout the aquifer would not be expected to change the water level in the same direction at every time of the year. The first 2.5 days of a typical coseismic water level rise could be caused by a small coseismic discharge decrease at a point several tens of meters from the well. Alternatively, the entire coseismic water level signal could represent diffusion of an abrupt coseismic pore pressure increase within several meters of the well, produced by a mechanism akin to that of liquefaction. The coseismic water level changes in the BV well resemble, and may share a mechanism with, coseismic water level, stream discharge, and groundwater temperature changes at other locations where preearthquake changes have also been reported. No preearthquake changes have been observed at the BV well site, however.

Journal of Geophysical Research B: Solid Earth↗
USGS1995

Hydrologic effects associated with the January 17, 1994 Northridge, California, earthquake

This report compiles hydrologic observations in southern California associated with the 1994 Mw = 6.7 Northridge, California earthquake. In southern California, the largest ground water level change was a drop of 52 cm at Crystalaire. Most of the steplike water-level changes recorded following the Northridge earthquake agreed in direction with the sign of the calculated coseismic volume strain field. A spring discharge increase at Southern Pacific Springs was reported to have preceded the earthquake by several days. Outside of southern California, water-level changes were also observed, but are not consistent in sign or size with the static strain field of the earthquake sequence.

California, Nevada, Oregon↗
USGS1987

Earthquakes and groundwater

As part of the U.S Geological Survey prediction experiment at Parkfield along the San Andreas fault in California, a network of water wells is being monitored. this network consists of wells that are situated at seven sites that were drilled by Geological Survey for the express prupose of monitoring water levels. These wells have turned out to be very sensitive volume strain meters. The scientific rationale for the wells as strain meters is explained in some detail below.

Earthquakes & Volcanoes (USGS)↗