Abstract
The difficulties of locating earthquakes in the early instrumental period are not always fully appreciated. The networks were sparse, and the instruments themselves were of low gain, often had inappropriate frequency response and recording resolution, and their timing could be unreliable and inaccurate. Additionally, there was only limited knowledge of earth structure and consequent phase identification and propagation. The primitive Zöppritz tables for P and S, with no allowance for the core, did not come into use until 1907, and remained the main model until the adoption of the Jeffreys-Bullen tables in the mid-1930s. It was not until the early 1920s that studies of Hindu Kush earthquakes revealed that earthquake foci could have significant depth. Although many early locations are creditably accurate, others can be improved by use of more modern techniques. Early earthquakes in unusual places often repay closer investigation. Many events after about 1910 are well enough recorded to be re-located by computer techniques, but earlier locations can still be improved by using more recent knowledge and simpler techniques, such as phase re-identification and graphical re-location. One technique that helps with early events is to locate events using the time of the maximum phase of surface waves, which is often well reported. Macroseismic information is also valuable in giving confirmation of earthquake positions or helping to re-assess them, including giving indications of focal depth. For many events in the early instrumental period macroseismic locations are to be preferred to the poorly-controlled instrumental ones. Macroseismic locations can also make useful trial origins for computer re-location. Even more recent events, which appear to be well located, may be grossly in error due to mis-interpretation of phases and inadequate instrumental coverage. A well converging mathematical solution does not always put the earthquake in the right place, and computer location programs may give unrealistically small estimates of error. Examples are given of improvements in locations of particular earthquakes in various parts of the world and in different time periods.
Highlights
Instrumental recording of earthquakes began only in the late nineteenth century
This paper summarises various location techniques and points out some difficulties associated with them that are not always appreciated
Assuming a velocity in the deep earthquake zone 10% higher than normal gave a solution in which the epicentre was moved 60 km (k = 0.9) and the focal depth reduced by 50 km
Summary
Instrumental recording of earthquakes began only in the late nineteenth century. Until knowledge of earthquakes and their dis-. Because of limitations in the design of early instruments, and the lack of knowledge of earth structure, the advent of instrumental recording did not immediately result in a major improvement in earthquake location, but rather. In the early days the locations could be unreliable, and instrumental information was used to refine macroseismic studies rather than replace them. Even at the present time instrumental and macroseismic investigations can be used to supplement each other, and their combined use produces the best results. This paper summarises various location techniques and points out some difficulties associated with them that are not always appreciated
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