Source parameters of four strong earthquakes in Bulgaria and Portugal at the beginning of the 20th century

Using original seismograph records and bulletin data we re-determined the origin time, location, seismic moment (M-0) and magnitudes (M-S and M-w) for four earthquakes in the beginning of the 20th century. These are two strong earthquakes April 4, 1904 near Krupnik, Bulgaria (M-w = 6.8, M-S = 7.2 respectively), the April 23 1909 earthquake near Benavente, Portugal (M-S = 6.3), and the June 14, 1913 earthquake near Gorna Orjahovitza, Bulgaria (M-S = 6.3). Twenty-nine traces from original records have been analysed, a large number of original station bulletins have been consulted and a consistent methodology for analysing these early 20th century instrumental information is presented. In spite of a thorough effort in re-assembling and quality control of the original data, large inaccuracies remain in the improved instrumental epicentre locations and origin times. The seismic moment estimates we obtained (2.3 10(18) less than or equal to M-0 less than or equal to 3.9 10(19) Nm) are the first ever determined for these events. The magnitude estimates (6.3 less than or equal to M-S less than or equal to 7.2 and 6.2 less than or equal to M-w less than or equal to 7.0) are robust and systematically lower than most of previous estimates for all earthquakes (Gutenberg and Richter, 1954; Christoskov and Grigorova, 1968; Karnik, 1969). For the largest Krupnik event our estimates agree with those of Abe and Noguchi (1983b) and Pacheco and Sykes (1992). The studied earthquakes all occur in moderately seismic active regions, therefore our results may have significant consequences for hazard estimates in those regions.