High-precision Levelling Data Research Articles

Hydro-geotechnical modelling of subsidence in the Como urban area

The urban area of Como (Italy) is particularly susceptible to subsidence. The reason is the particular structure of the subsoil, combined with the anthropic modification of the lakeshore lands linked to the historical evolution of the area. Moreover, this phenomenon exposes the lakefront areas to an increasing risk of flooding. The primary purpose of this study was to develop an effective methodology for the assessment of subsidence in urban areas, to be used as a support in groundwater management. The development of a coupled hydrogeological-geotechnical numerical model for the period 2004–2011 allowed recognizing the shoreline results as most susceptible areas to the phenomenon. Model results were critically evaluated through comparison with time series of PSInSAR and high precision levelling data available in the area. The anthropic perturbations of groundwater flow linked to the construction of lake flooding defenses in 2008–2009, enhanced the subsidence phenomenon in a localized area, pointing out the strong inter-dependence of groundwater circulation, lake level oscillations and geotechnical behavior of soils. Thus, the model revealed the most critical zones and geotechnical units, demonstrating to be a potential powerful tool to predict subsidence scenarios (e.g. the future completion of the floods defense works in the shoreline area).

Recent terrestrial and satellite observations to model deformations at Colli Albani volcano (central Italy)

The Colli Albani volcanic complex (Rome, Italy) has been dominated by episodic eruptions commencing around 561 ka and ending with the most recent activity of the Albano maar phase (<70 ka). Earthquakes of moderate intensity, gas emissions and significant ground deformations are the recent evidences of a residual activity. Former geodetic data from leveling surveys, GPS stations and InSAR observations tracked ongoing significant uplift of the order of few mm/year near the Colli Albani western flank. Different uplift rates were detected by each technique in different time spans, suggesting also the possibility of sporadic recharge of the hydrothermal system. The renewed high precision leveling data from IGMI survey carried out in 1997/1999 and the last leveling survey carried out in 2006 show that the uplift along the route is currently significant at an average rate of ~3 mm/year. Radar interferograms from ALOS satellite show uplift rate of ~6 mm/year, southwest of the central sector of the leveling route. We have undertaken a joint inversion of the various geodetic data (vertical rates from leveling surveys, GPS site velocities and InSAR observations acquired by ALOS satellite) using a nonlinear inversion technique to estimate the parameters of a point-pressure source, possibly capable of explaining the ongoing deformation at Colli Albani volcano.

Recent vertical movements inferred from high-precision leveling data in the Carmel Mountain

Recent vertical movements inferred from high-precision leveling data in the Carmel Mountain

Comment on “Nature of the recent vertical ground movements inferred from high-precision leveling data in an intraplate setting: NE Ardenne, Belgium” by A. Demoulin and A. Collignon

[01] Using yearly leveling surveys performed from 1993 to 1998 in the Ardenne, as well as historic leveling results from 1948 and 1974, Demoulin and Collignon [2000] (hereinafter referred to as D&C) observed that the total vertical ground movement (less than 1 cm) over a 20-to 30year period is barely higher than the yearly displacements measured to a few millimeters per year. They eliminated various sources of errors from their measurements and subsequently interpreted the resulting short-term oscillating crustal displacements as true tectonic motions within the upper crust accompanying the long-term deformation of uplifting areas in intraplate settings. Their justification is that the leveling discontinuities coincide with faults they assume to be active. In their conclusions, D&C suggested that such motions and their variations could be used to monitor potential seismogenic faults and even to foretell earthquakes. [2] Unfortunately, it is our opinion that the data analysis of D&C is inadequate and does not support the subsequent interpretation. Specifically, we interpret the apparent oscillations in the leveling data as the expected expression of the noisy character of leveling differences with amplitudes ranges of 0.5 to 1 cm. If this premise is accepted then there is also no evidence to suggest that the boundary faults in the study area are active. We thus feel that the conclusions of D&C are premature until their observations can be corroborated by improved observations or by other independent measurements. In this note, we present a careful statistical analysis to support our position.

Recent vertical movements from high-precision leveling data in northeast Spain

Historical and modern high-precision leveling data are analyzed to measure recent vertical movements in northeast Spain. The recent vertical movements are deduced by comparing the original height differences (taken from the original logbooks) measured in the field in two surveys carried out by the Instituto Geográfico Nacional in the periods 1871–1922 and 1925–1974. An exhaustive study of the errors involved in the measurements indicates that some of the observed movements are significant. We present the recent vertical movements along eight profiles with significant uplift rates: a southern anomaly of about 1 mm/yr, observed in two independent profiles, associated with the southern part of the Catalan Ranges frontal thrust; one anomaly between Caldetes and Arenys with an average velocity of about 4 mm/yr, which can be related to the seismic activity of this century; two anomalies of about of 2.5 mm/yr in the north and of about 4 mm/yr in the south associated with the NW-SE striking Amer-Brugent fault system, which has been related to a great seismic crisis in the fifteenth century, and one anomaly of about 0.8 mm/yr associated with the Empordà basin dynamics. Other observed recent vertical movements are mainly related to sediment compaction masking possible tectonic deformations. All the northern anomalies are associated with the NW-SE system of faults considered as a “transfer zone” of the NE-SW striking faults which controlled the formation of the Neogene European rifting. The former shows Plio-Quaternary tectonic activity: folds, faults, volcanism, hydrothermalism and historical seismicity. The low seismicity of this century around the Amer-Brugent fault system contrasts with both the high average velocities estimated here and with the fifteenth century seismic crisis. Therefore, if these high uplift rates are tectonic in origin, they may represent an accumulation of elastic strain which would be released in a future large earthquake rather than in continual small ones. According to this hypothesis, a large part of the stress accumulated since the fifteenth century has yet to be released. Further leveling and neotectonic surveys are needed to test this hypothesis.

Geodetic high-precision measurements in active tectonic areas; example: The Rhinegraben

Abstract Geodetic measurements in the Rhinegraben area are discussed. Repeated levellings, together with horizontal control data, torsion-balance measurements, very dense gravity coverage, tiltmeter observations and a high-precision gravity test net were available in order to test present concepts of the Rhinegraben structure and sources of recent crustal movements. The ambiguities of levelling data are stressed, but investigations of high-precision levelling data clearly indicate subsidence of several centimetres in the graben area, whereas multivariate studies of the data available on the graben shoulders do not indicate any significant movement on the shoulders themselves.

Seismic displacement and ground motion near a fault: The Saitama Earthquake of September 21, 1931

The dislocation parameters of the Saitama earthquake (M = 7.0, 36.15°N, 139.24°E) of September 21, 1931, are determined on the basis of first-motion data, aftershock area, and close-in seismograms obtained by a low-magnification long-period seismograph. The earthquake represents a left-lateral strike slip faulting on a plane dipping 80° toward N 196°E with dimensions of 20 km (length) × 10 km (width). The strike of the fault plane is found to be almost parallel to that of the eastern extension of the median tectonic line. A synthetic study suggests that the rupture grows bilaterally at a velocity of 2.3 km/s. The rise time, the final dislocation of the linear ramp dislocation time function, and the particle velocity of the fault dislocation are estimated to be 2 s, 100 cm, and 50 cm/s, respectively. The near-source ground displacements and ground motions derived from the above seismic fault model are consistent with high-precision leveling data and with the field survey that determined the directions of collapse of structures.