2. Engineering Geology and Seismology
Lecture#16
CE-312
Engineering Geology and Seismology
Instructor:
Dr Amjad Naseer
Department of Civil Engineering
N-W.F.P University of Engineering and Technology, Peshawar

3. Outlines of the Presentation
Attenuation relationship
Factors influencing the nature of strong motion

4. Attenuation Relationship
Strong-motion attenuation equations are empirical equations that can be used to estimate the values of strong-motion parameters as functions of independent parameters that characterise the earthquake and the site of interest. The name ‘attenuation relationships’ arises from the fact that the equations describe the decay of the strong motion with distance from the source. A more suitable term, especially from the engineering perspective of requiring knowledge about where the motion will be strong rather than where it will have decayed to insignificant levels, would be strong-motion scaling relationships. 4

5. Factors influencing strong motion
In order to derive an attenuation relationship for a given strong-motion parameter, the first step is to decide which independent variables should be included to characterise the earthquake and the recording site. The physical environment that generates that ground motion at a particular location is made up of three components:
the earthquake source,
the source-to-site travel path and
the site itself.
If the generation of strong motion is considered in terms of energy then the fault rupture is the release of energy, the path represents the radiation of energy and the site the influence of local features that may modify the nature of the energy flow. 5

6. Factors influencing strong motion
A) The Earthquake Source
The release of energy at the source is characterised by a measure of the absolute size of the earthquake, i.e. seismic moment or magnitude. The magnitude scales used in most attenuation relationships are Ms and Mw, although some studies use a combination of ML for smaller earthquakes and Ms for larger earthquakes. The moment magnitude Mw is the most physically meaningful measure of earthquake size but the seismic moment is not available for all earthquakes, particularly smaller events. 6

7. Factors influencing strong motion
The influence of the fault mechanism on the nature of the strong motion is a subject of some debate, and it is not included in all attenuation relationships, but those that do include this factor are unanimous that reverse faults produce the strongest motions. Figure 1 shows an example of attenuation relationships that include the influence of the rupture mechanism, which suggest that reverse faults produce stronger motion than strike-slip faults and that this effect is most pronounced at short distances. 7

8. Factors influencing strong motion
B) Distance from the source
The distance from the source to the site needs to reflect how far the waves carrying the seismic energy have travelled when reaching a particular point. The simplest measure of distance would be the horizontal distance from the site to the earthquake epicentre or the slanting distance to the hypocentre. This definition for the distance models the earthquake source as a point and does not take into account the length of the fault rupture, which can be of the order of tens or even hundreds of kilometres. Use of the epicentral distance can therefore result in significant overestimation of the distance between the source of energy release and the site and this will result in unrealistic distance-dependence in the attenuation relationship. 8

9. Factors influencing strong motion9

10. Factors influencing strong motion
C) Soil structure interaction
Presence of large structures, modify the ground motion through interaction with the soil and foundations. This is only included implicitly in attenuation relationships through the exclusion of records from the basement or ground floor of buildings with more than three storeys in some attenuation studies. 10

11. Factors influencing strong motion
D) Topography
Another factor that can have a significant influence is the presence of topographic features such as ridges, basins and valleys. However, no attenuation relationship has included topography explicitly, partly because of the complexity of the effects and the difficulties in defining simple parameters to model them. 11

12. Factors influencing strong motion
E) Surface geology
One feature of the site that does have a very pronounced influence on the nature of the ground motion is the surface geology. The surface geology is accounted for in most recent attenuation studies by the velocity of shear waves over the uppermost 30 m at the site, Vs,30. This may be determined from direct measurements in boreholes or else inferred from empirical relationships between the shear modulus, G, and the SPT blow count, N (e.g. Ohta & Goto, 1978; Imai & Tamouchi, 1982; Lee, 1992). 12

13. Factors influencing strong motion
East-west components of ground velocity from a magnitude 4.3 aftershock of the 1983 Coalinga earthquake in California. The influence of the alluvial layers on amplitude and duration can be appreciated from the records (Reiter, 1990). 13