Possible tilt phenomena observed as water level anomalies along the Los Angeles aqueduct
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Abstract:
Water levels in the Los Angeles Aqueduct in southern California fluctuate in a manner that are not easily attributable to normal aqueduct operations. Simple hydraulics suggests that large scale earth tilt can register as water level anomalies with a sensitivity of about .01 ft/microradian. Two aqueduct anomalies which coincide spatially and temporally with independently observed deformational phenomena are used to explore this suggestion.Keywords:
Aqueduct
Tiltmeter
Hydraulics
Water levels in the Los Angeles Aqueduct in southern California fluctuate in a manner that are not easily attributable to normal aqueduct operations. Simple hydraulics suggests that large scale earth tilt can register as water level anomalies with a sensitivity of about .01 ft/microradian. Two aqueduct anomalies which coincide spatially and temporally with independently observed deformational phenomena are used to explore this suggestion.
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Hydraulics
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We present a detailed study of tsunami-induced tilt at in-land sites, to test the interest and feasibility of such analysis for tsunami detection and modelling. We studied tiltmeter and broadband seismometer records of northern Chile, detecting a clear signature of the tsunamis generated by the 2007 Tocopilla (M = 7.6) and the 2010 Maule (M = 8.8) earthquakes. We find that these records are dominated by the tilt due to the elastic loading of the oceanic floor, with a small effect of the horizontal gravitational attraction. We modelled the Maule tsunami using the seismic source model proposed by Delouis et al. and a bathymetric map, correctly fitting three tide gauge records of the area (Antofagasta, Iquique and Arica). At all the closest stations (7 STS2, 2 long-base tiltmeters), we correctly modelled the first few hours of the tilt signal for the Maule tsunami. The only phase mismatch is for the site that is closer to the ocean. We find a tilt response of 0.005–0.01 μm at 7 km away from the coastline in response to a sea level amplitude change of 10 cm. For the Maule earthquake, we observe a clear tilt signal starting 20 min before the arrival time of the tsunami at the nearest point on the coastline. This capability of tilt or seismic sensors to detect distant tsunamis before they arrive has been successfully tested with a scenario megathrust in the southern Peru-northern Chile seismic gap. However, for large events near the stations, this analysis may no longer be feasible, due to the large amplitude of the long-period seismic signals expected to obscure the loading signal. Inland tilt measurements of tsunamis smooth out short, often unmodelled wavelengths of the sea level perturbation, thus providing robust, large-scale images of the tsunami. Furthermore, tilt measurements are not expected to saturate even for the largest run-ups, nor to suffer from near-coast tsunami damages. Tiltmeters and broadband seismometers are thus valuable instruments for monitoring tsunamis in complement with tide gauge arrays.
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Remarkable crustal deformation has repeatedly been observed in the eastern Izu Peninsula, central Japan, synchronized to the occurrence of extensive earthquake swarms in the past 20 years. Among the instrumentation, a borehole tiltmeter installed at station KWN, which is located within a few kilometers of the swarm region, has faithfully recorded large signals of ground tilt associated with more than 10 swarm sequences in the past 10 years. In addition to coswarm signals it became clear that precursory tilt signals preceded major swarm activities. The amplitudes of the precursory tilt signals are of order of 0.1 μrad, and they precede the onset of major swarms by several hours to half a day. This signal is convincing because the same phenomena appeared at least 7 times and were also detected by other independent observations. We speculate that a dike intrusion at depth causes crustal deformation and only as it reaches depths shallower than ∼10 km does seismic activity begin. The precursory tilt signal can be utilized for practical prediction of the start of swarm activity in the eastern Izu Peninsula.
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An array of 14 biaxial shallow-borehole tiltmeters (at 1O(-7) radian sensitivity) has been installed along 85 kilometers of the San Andreas fault during the past year. Earthquake-related changes in tilt have been simultaneously observed on up to four independent instruments. At earthquake distances greater than 10 earthquake source dimensions, there are few clear indications of tilt change. For the four instruments with the longest records (> 10 months), 26 earthquakes have occurred since July 1973 with at least one instrument closer than 10 source dimensions and 8 earthquakes with more than one instrument within that distance. Precursors in tilt direction have been observed before more than 10 earthquakes or groups of earthquakes, and no similar effect has yet been seen without the occurrence of an earthquake.
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Simple hydraulics of open channel water flow is used to find the water level response to earth tilt along the 200 km long Los Angeles Aqueduct. The water level changes are shown to depend on the length scale of the tilt field. Tilts over lengths greater than 10 km permit the water to assume an essentially equilibrium level throughout the tilt field; change in water depth is therefore directly proportional to the degree of slope change. Tilts occurring on length scales shorter than 10 km require the use of an integral relation between tilt and water depth. The relation is used to model a tilt field corresponding to an anomalous water level event observed on the Los Angeles Aqueduct.
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Tilt and strain meters of the deformation-observation network in Hubei Province all responded to the Mw9. 0 Japan earthquake on March 11, 2011. By analyzing the co-seismic responses, we found that firstly there was essentially a linear correlation between response time and epicentral distance. Secondly, there was some correlation between maximum response amplitude and earthquake magnitude as well as between the duration and earthquake magnitude. Thirdly, the response amplitudes and decay rates were different for different types of instruments. Due to less data-sampling frequency, the deformation instruments, could not display the first motion of P and S waves, but responded mainly to far-field surface waves. Before the earthquake, the NS earth tide component recorded by the cave stainmeter at Yichang was distorted for nearly eight hours. While digital deformation observation did not show complete information about the earthquake source, it still reflected some key features of seismic-wave propagation.
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