An integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE-SW to NW-SE trending Rhone-Simplon fault zone and the gently S-dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone-Simplon fault zone, strike-slip earthquakes on steep-dipping faults within the Helvetic nappes are a consequence of regional NW-SE compression and NE-SW extension. To the south, vertical maximum stress and N-S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes andlor continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission-track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene. Terra Nova, 9,[91][92][93][94] 1997
To reduce the field effort required for 2-D and 3-D shallow seismic surveying, we have developed a towed land‐streamer system. It was constructed with self‐orienting gimbal‐mounted geophones housed in heavy (1 kg) cylindrical casings, sturdy seismic cables with reinforced kevlar sheathing, robust waterproof connectors, and a reinforced rubber sheet that helped prevent cable snagging, maintained geophone alignment, and provided additional hold‐down weight for the geophones. Each cable had takeouts for 12 geophones at 1 m or 2 m intervals. By eliminating the need for manual geophone planting and cable laying, acquisition of 2-D profiles with this system proved to be 50–100% faster with 30–40% fewer personnel than conventional procedures. Costs of the land‐streamer system and total weight to be pulled could be minimized by employing nonuniform receiver configurations. Short receiver intervals (e.g., 1 m) at near offsets were necessary for identifying and mapping shallow (<50 m) reflections, whereas larger receiver intervals (e.g., 2 m) at far offsets were sufficient for imaging deeper (>50 m) reflections and estimating velocity‐depth functions. Our land‐streamer system has been tested successfully on a variety of recording surfaces (e.g., meadow, asphalt road, and compact gravel track). The heavy weight of the geophone casings and rubber sheet ensured good geophone‐to‐ground coupling. On the asphalt surface, a greater proportion of high‐frequency (above 300–350 Hz) energy was recorded by the land streamer than by standard baseplate‐mounted geophones. The land‐streamer system is a practical and efficient means for surveying in urbanized areas. Acquisition and processing of 3-D shallow seismic data with the land‐streamer system was simulated by appropriately decimating and reprocessing an existing 3-D shallow seismic data set. Average subsurface coverage of the original data was ∼50 fold, whereas that of the simulated data was ∼5 fold. The effort required to collect the simulated pseudo-3-D data set would have been approximately 7% of that needed for the original field campaign. Application of important data‐dependent processing procedures (e.g., refraction static corrections and velocity analyses) to the simulated data set produced surprisingly good results. Because receiver spacing along simulated cross‐lines (6 m) was double that along in‐lines (3 m), a pattern of high and low amplitudes was observed on cross‐sections and time slices at early traveltimes (⩽50 ms). At greater traveltimes, all major reflections could be identified and mapped on the land‐streamer data set. With this cost‐effective approach to pseudo-3-D seismic data acquisition, it is expected that shallow 3-D seismic reflection surveying will become attractive for a broader range of engineering and environmental applications.
SUMMARY Tomographic inversions of geophysical data generally include an underdetermined component. To compensate for this shortcoming, assumptions or a priori knowledge need to be incorporated in the inversion process. A possible option for a broad class of problems is to restrict the range of values within which the unknown model parameters must lie. Typical examples of such problems include cavity detection or the delineation of isolated ore bodies in the subsurface. In cavity detection, the physical properties of the cavity can be narrowed down to those of air and/or water, and the physical properties of the host rock either are known to within a narrow band of values or can be established from simple experiments. Discrete tomography techniques allow such information to be included as constraints on the inversions. We have developed a discrete tomography method that is based on mixed‐integer linear programming. An important feature of our method is the ability to invert jointly different types of data, for which the key physical properties are only loosely connected or unconnected. Joint inversions reduce the ambiguity in tomographic studies. The performance of our new algorithm is demonstrated on several synthetic data sets. In particular, we show how the complementary nature of seismic and georadar data can be exploited to locate air‐ or water‐filled cavities.
[1] High-resolution seismic reflection surveys across active fault zones are capable of supplying key structural information required for assessments of seismic hazard and risk.We have recorded a 360 m long ultrahigh-resolution seismic reflection profile across the Alpine Fault in New Zealand. The Alpine Fault, a continental transform that juxtaposes major tectonic plates, is capable of generating large (M > 7.8) damaging earthquakes. Our seismic profile across a northern section of the fault targets fault zone structures in Holocene to late Pleistocene sediments and underlying Triassic and Paleozoic basement units from 3.5 to 150 m depth. Since ultrashallow seismic data are strongly influenced by near-surface heterogeneity and source-generated noise, an innovative processing sequence and nonstandard processing parameters are required to produce detailed information on the complex alluvial, glaciofluvial and glaciolacustrine sediments and shallow to steep dipping fault-related features. We present high-quality images of structures and deformation within the fault zone that extend and complement interpretations based on shallow paleoseismic and ground-penetrating radar studies. Our images demonstrate that the Alpine Fault dips 75°-80°to the southeast through the Quaternary sediments, and there is evidence that it continues to dip steeply between the shallow basement units. We interpret characteristic curved basement surfaces on either side of the Alpine Fault and deformation in the footwall as consequences of normal drag generated by the reverse-slip components of displacement on the fault. The fault dip and apparent $35 m vertical offset of the late Pleistocene erosional basement surface across the Alpine Fault yield a provisional dip-slip rate of 2.0 ± 0.6 mm/yr. The more significant dextral-slip rate cannot be determined from our seismic profile.
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