Jökull


Jökull - 01.12.1982, Page 119

Jökull - 01.12.1982, Page 119
data and methods In this study we have followed three lines of approach. We have searched through written re- cords about the earthquakes, interviewed inhabit- ants of the earthquake zone, and studied aerial photographs. Historical accounts mention surface breakage as- sociated with a few earthquakes, but the descrip- tions are generally short, and only exceptionally suffice to locate faults in the field. Most accounts prior to 1900 were compiled by Thoroddsen (1899 and 1905), who mentions faulting in association with earthquakes in the years 1294, 1308, 1339, 1391, 1630, 1784, and 1896. The 1896faultingisdescribed in considerable detail, but in most of the other cases faultingis only mentioned briefly. In addition, fairly thorough descriptions of fáulting are available in newspaper reports of the earthquake of 1912. Interviewing the inhabitants turned out to be a very effective way ofsearching for earthquake fract- ures. Farmers generally know their property well, and could guide us to structures that fitted our descriptions, even though in some instances they did not realize that these were related to past earth- quakes. Studying aerial photographs is the most fruitful method. Many of the larger fractures and fissures show up on low and intermediate altitude photo- graphs available at the Icelandic Geodetic Survey. After a systematic inspection of the photographs, the likely localities were checked in the field. When a fracture had been located, one usually found it to be a part of a fracture system that could be traced over some distances. The most prominent systems were then mapped by students using compass and tape measure. MAIN CHARACTERISTICS OF THE FRACTURES The appearance of the fractures depends strongly on the nature of the surfaccmaterial where they are exposed. Most of the area studied here is covered by lavas, moraines and glaciofluvial deposits. The fractures are most conspicuous where lava is cover- ed by a thin layer of soil and vegetation. The faults become obscured where the loose material is very thick. Surface conditions also determine how well the fractures are preserved. It is, for example, a hopeless task to trace the faults in the sand filled aa lavas that cover large partsofthe Land district. Wet marshes also tend to obliterate the fault traces. Fig. 3. Major faults and fracture systems of the earthquake of May 6th 1912. Mynd 3. Helstu sprungur og sprungukerfi frájarbskjálft- anum 6. maí 1912. In spite of their variable appearance, there are a few features characteristic of most of the faults, that make them identifiable in the field. Cases of doubt JÖKULL 32. ÁR 115
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