Associating Earthquakes with Faults
Earthquake epicenters, when plotted on a tectonic map, rarely lie along the lineaments of known faults. This results from several factors, e.g. inaccuracy in the earthquake source location, a less accurately defined fault lineament, branching along the fault and dipping of the fault surface away from vertical. Associating the observed earthquakes with known faults is a difficult, but important , step in fixing the earthquake design basis. An earthquake which cannot be associated with any known fault is called a floating earthquake, and has to be treated differently.
Associating earthquakes with faults is a matter of interpretation, accuracy of which depends on the available information on the earthquake source and the geological structures (the faults). From the design point of view the interpretation leading to the worst possible event (from safety point of view) is examined. The following steps are helpful in associating earthquakes with known faults.
The earthquake should be located as accurately as possible (say, epicenter within a kilometer and depth within 5 kms). The earthquake may be considered associated with a known fault if, within the accuracy of the earthquake source location and the definition of the fault lineament, the earthquake source may be considered close enough to the fault.
If earthquake effects are observable, the isoseismals will be elongated almost parallel to the strike of the fault, at least in the area close to the epicenter, if the earthquake was associated with the fault.
Earthquakes occur in Seismic gaps. Supporting evidence in the form of occurrence of other earthquakes along the fault, including aftershocks and foreshocks, may be sought from the earthquake history of the region.
If the earthquake in question is of post instrument period, and has been recorded at a sufficiently large number of stations (say, over a dozen) a fault plane solution may allow determination of the strike and dip parameters of the causative fault.
Fault dimensions should be consistent with the magnitude of the maximum credible earthquake.
It is not uncommon to encounter situations in siting when epicenters in the site region cannot be associated with any known geological structure. In such cases the region around the site requires to be investigated further in detail to find undiscovered faults.
Floating Earthquakes and Earthquake Design Basis
When an earthquake cannot be associated with a known fault it is called a floating earthquake. In fixing the earthquake design basis the approach is to assume that the maximum credible earthquake (MCE) could occur at a point on the fault, with which the MCE is associated, which is closest to the site. In case of the floating earthquake, this is not directly possible because the structure is not known. Hence the floating earthquake is to be treated differently in fixing the earthquake design basis. The approach is on the following lines:
The floating earthquake is assumed to occur on a fault closest to the site, and at a point which is closest to the site. Detailed site investigations are carried out to ensure that no significant fault exists between the assumed location of the floating earthquake potential and the site. In case such investigations have not been conclusive the floating earthquake potential is assumed to occur beneath the site, at some depth necessary for the magnitude of the floating earthquake.
Interpretations from Satellite Imageries.
Application of Landsat imageries and aerial photographs has proved very useful in investigating geological faults at moderate costs, both in terms of time and money. Landsat imageries offer a broad overview of the site at low sunangle elevations permitting observation of the entire area of interest for examining structural patterns, accentuating minor differences in topography and vegetation. They allow identification of lineaments, which are the surface expressions of faults and fractures, and are helpful in determining the extensions of known faults. Landsat imagery contains the response of the Earth's surface to radiation in the 0.5 to 1.1 µm wave length range, i.e. the visible and the infrared, in four distinct bands -0.5-0.6 µm, 0.6 -0.7 µm, 0.7 -0.8 µm and 0.8 - 1.1 µm) in 185 km x79 m strips on the Earth's surface. The spatial resolution of the imagery from the multi-spectral scanner is 79 meters and can be enhanced to 55 meters by computer processing. Color composite images, which are produced from data in more than one band, offer increased interpretability of the image. 20% of each landsat image can be viewed stereoscopically.
Recognition of faults on an imagery picture requires locating discontinuities or changes across the surface in terms of the alignment of features along the strike. Displacements along faults brings into juxtaposition features of different rock types. Differences in their physical or botanical properties, if detected using appropriate type of imageries, enable identification of the fault contact. Variation in lithology, vegetation and ground moisture and topography can be detected on imageries (see below).
Identifiable Features of Landsat Imageries
Variation in lithology, vegetation, ground moisture and topography can be detected on landsat imageries. Rock units having differences in reflectance (color) may readily be distinguished on imagery. The texture and overall pattern of rock masses are usually more useful in distinguishing rock types and boundary faults. The dominant textural component of rock masses from imagery standpoint is the texture associated with the stream (or drainage) pattern. Rock mass structure and hardness are reflected in the pattern and density of the drainage system. Changes in pattern, density or texture of the stream pattern often denote a contact between fault surfaces. Thermal properties of rocks may provide useful contrast when imagery equipment is sensitive to radiation in the appropriate thermal range. For example, thermal inertia may vary between two different rock types having the same color and brightness in day time. A thermal scan at night may show one rock type warmer than the other. Faults occurring within single rock units are usually indistinguishable using rock reflectance or texture alone. Different rock or soil types may support different plant assemblages, or a particular plant assemblages ma exhibit variation in growth characteristics, e.g. vigor, density etc. Variation in vegetation may occur as tone contrast across a fault due to changes in type and density of vegetation. Differences in moisture may give rise to denser growth on one side of the fault. A fault may serve as conduit for fluid flow promoting vegetal growth along the fault and associated fractures. The moisture discontinuity may result in measurable changes in ground temperature (due to evaporation) and changes in reflectance of the soils on opposite sides of a fault. Ponding may also occur along the trace of a fault forming a string of "sag" ponds. Vegetation along fault scraps (steepened land surfaces, which are more definite indicators of faulting) may be younger than that of the either side of the fault. Imagery in the near infra red region is required for investigating variations in vegetation, since plant reflectance in this region is a function of plant health and leaf texture. Plants under stress exhibit a lower reflectance in the infrared compared to healthy ones, even though the reflectance in the green region (due to chlorophyll) may not show much variation. Plant stress may be caused either by lack of moisture or toxic fluids flowing along the fault faracture. Active faults may also form a barrier to the free flow of water through near surface materials. Fault scraps may often be detected in materials of uniform mineralogy, surface texture or moisture conditions, simply because one side of the fault is closer to the sensor.
Applications of Aerial Photographs In investigating Faults
Aerial photographs can show some features of a fault which cannot be found directly by ground studies. They provide a resolution of as much as one meter on the ground surface. Aerial photographs on 1:25,000 or 1:50,000 scales are available for most regions. Photographs of different dates can help in identifying temporal changes in superficial and tectonic features. Aerial photographs allow stereoscopic viewing, thus enabling detailed observations on fault features. Examination of aerial photographs is to be followed by ground truth verification of the observed features.
Ground Truth Verification For Active Geological Faults
Features used to recognize active faults
1. Fault Scarps.
2. Rift valleys.
3. Over steepened base of Mountain Fronts.
4. Faceted or triangular Spurs or Ridges.
5. Shutter Ridges.
6. Offset Streams.
7. Drainage Lines, Gullies or Ravines.
8. Benches.
9. Sags or Sagponds.
10. Closed Depressions, Troughs.
11. Side Hill Ridges.
12. Terraces along Sides of Hills.
13. Saddles.
14. Mole Tracks.
15. Open Fissures.
16. Notches of unusual orientation or position relative to Stratigraphy or Lithological Differences in Resistance to erosion.
17. Terraces upstream from Scarps.
18. Furrows.
19. Groundwater Barriers Marked by Alignment, Vegetation Contrast or Lineaments.
20. Folding or Warping of young Alluvial or Erosion Surfaces with Sedimentation or Erosion.
This page was updated on 12 January, 2011