49 - Macroseismology
Section snippets
The Nature of Intensity
Intensity can be defined as a classification of the strength of shaking at any place during an earthquake, in terms of its observed effects. It can be compared to the Beaufort Scale of wind speed, and is not a physical parameter as such, although it is evidently related to physical parameters of strength of shaking. Also, one can think of it as a sort of shorthand for longer descriptions which may be common from one earthquake to the next. Thus, instead of writing that the earthquake was
Macroseismic Surveys
After an earthquake is felt in a region, there are several possibilities as to how to collect the data. The final choice depends on many factors, such as:
- 1.
how strong the earthquake was, and, therefore, how important the data are;
- 2.
whether it was a single shock or a part of a long sequence;
- 3.
if there is already an existing network of earthquake observers in the region, and whether the network is dense enough;
- 4.
the financial situation of the institution that collects the data, etc.
Deriving Macroseismic Parameters
There are two basic approaches to deriving most earthquake parameters from macroseismic data. The first is to draw isoseismals and use the enclosed areas, or the average radii. The second is to base the calculations on the intensity data points themselves, without drawing isoseismals. The advantage of the second approach is that any subjectivity in the isoseismal drawing is entirely circumvented. The disadvantage is that the results may be biased by heterogeneity in the distribution of
Seismic Hazard
Since the late 1970s it has been realised that macroseismic methods are essential for most seismic hazard studies. The corresponding increase of interest in seismic hazard necessitated by expanding nuclear energy programmes throughout the world, has ensured a large resurgence of interest in macroseismology. The principal reason for this is the necessity to expand earthquake catalogues back in time. The instrumental earthquake catalogue available for any region never extends further back in time
Acknowledgements
The authors would like to thank Andrea Tertulliani and Edmund Fiegweil for their assistance in compiling Section 2 and the Appendices. Tibor Zsiros helped in sorting out the correct history of the Kövesligethy papers. We would also like to thank Jim Dewey for his helpful review comments. The contribution of Roger Musson to this chapter is published with the permission of the Director of the British Geological Survey (NERC).
Editor's Note
Due to space limitations, four appendices of this Chapter are placed as computer readable files on the attached Handbook CD, under directory \49Musson2. Please see also, Chapter 47, Historical seismology, by Guidoboni; and Chapter 48, Earthquake history, with several subchapters by various authors.