Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

Historical Seismograms in Sweden 67More recently, Singh et al., (1984) used Uppsala Wiechert seismograms and stationbulletins to estimate surface-wave magnitudes, Ms. Amplitude readings, AEand AN, of 20-sec surface waves were taken from the Uppsala bulletin and wheneverpossible also checked on Wiechert seismogram copies. It has been shown that Uppsalaamplitude data are complete for Ms 2 6.6 from 1906 to 1952 and for Ms 2 6.3from 1953 to 1966. The increase in the magnitude sensitivity is most likely due toan increase in the number of instruments in Sweden, in the 1950's and 1960's, aswell as to improved communication among seismological observatories.Successful attempts have lately been made to compare seismograms of historicaland more recent earthquakes from the same area, recorded at the same seismographstation, i.e. by the same instrument, in order to extract information on focal processes.For example Bakun and McEvilly (Clark, 1984) compared De Bilt recordsfrom three Parkfield earthquakes which took place in 1922, 1934 and 1966. Theyquickly revealed that the records remarkably resemble each other, indicating thatthe 'same earthquake" is taking place over and over. Following the above procedure,we compared in a similar way, two major shocks that occurred in northern Algeriain 1954 and 1980. Corresponding source parameters are listed in Table 2. Thefollowing steps were performed on the data: (1) The original (i.e. no enlargementshave been introduced) smoked-paper seismograms from both events were digitizedat non-equal time intervals. (2) Correction for the offset of the arm pivot was introduced.(3) The signal was corrected for the finite arm length. (4) Data wereinterpolated to produce equally spaced (0.5 sec) points and smoothed by a movingaveraging over three successive points. In Figure 3, the four steps are demonstratedon real Wiechert records. Comparison of corrected records from both events, amplifiedto the same maximum amplitude level is made in Figure 4. Due to ratherweak recorded P-waves, only segments including S- and SS-arrivals and the initialpart of surface waves are shown.With respect to the proximity of the two hypocenters (Table 2), the propagationpaths to Uppsala are practically the same. Then, the astonishing resemblence ofboth records, as seen in Figure 4, suggests high similarity in corresponding focalprocesses. The available fault-plane solutions (McKenzie, 1972; Deschamps et al.,1982) both show a thrust faulting on planes striking ENE and NE for the 1954 and1980 event, respectively.Table 2. Source Parameters of Earthquakes AnalyzedFocal Date Epicen t ral Focal MArea Coordinates DepthO N O E kmN. AlgeriaOrlkansvilleSept 9, 1954 36.20 1.6') n2) 6.52)N. AlgeriaEl AsnamOct 10, 1980 36.lG 1.43) 43) 7.Z3)1) ISS2, Rothe3, ISC