2:45 PM - 3:00 PM
[S04-3-05] An intensity database for earthquakes on the Highveld of South Africa from 1840 to 1950
A database of macroseismic observations from historical earthquakes on the Highveld of South Africa was compiled as a contribution to the seismic microzonation of Johannesburg and the Witwatersrand Basin. The database contains over 3,000 intensity data points (IDPs) that have been assigned from macroseismic observations retrieved from questionnaires, newspaper reports, Observatory log books, Government reports and published seismic compilations. The database includes IDPs from 199 tectonic earthquakes felt in and around Johannesburg between 1840 and 1950. IDPs from over 700 mining-related earth tremors between 1907 and 1950 are also included. Seventeen tectonic events and 28 mining-related tremors have 10 or more IDPs each. Both datasets are comprised of relatively low intensity values mostly determined from human perception of shaking, largely indoors, rather than structural damage. However, 102 and 76 of IDPs from each dataset respectively have intensity values of V MMI-56 or higher. The occasional strong earthquake is consistent with the tectonic setting of the Highveld. The moderate magnitude mining related tremors are shown to increase in frequency, intensity and magnitude as the gold mines progress deeper. This database represents a thorough review of all the available sources of information and correction of transposed digits and other errors in previous compilations. This review also reveals the benefits of using additional contemporary resources from different languages, governments and disciplines in the compilation of IDP datasets. The database led to the development of a seismic history for the Highveld and the gold mines, considering the broader geo-political context of the rise of the independent Boer republics, the “mineral revolution" and the establishment of Johannesburg, and the course of colonial rule in southern Africa.