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

Hist orieal Se is mograms Reeo rded in Venezuela 4653. Description of the Historical Seismograms from VenezuelaThe absence of time marks and test records for instrumental calibration posesa large problem for accurate interpretation of these historical records. Anotherproblem is the interpretation of the seismograms themselves, as shown by the followingexample: In Centen+Grau (1940, p. 308), we find that the horizontal, twocomponentastatic Wiechert seismograph at Cagigal (OCV) recorded the first earthquakeon November 4, 1933, at 04:15 HLV. In the NNESSW direction, the durationof the record was about 1 minute. The earthquake was felt in the Barquisimeto-C dcut a- Merida-San Cristobal- Cucut a- Pamplon a region. Comparing this data tothat from the Earthquake Data File of the U.S. National Geophysical Data Center(1972), we have:OCV 1933 11/04 04:15 Felt Cdcuta-Pamplona, and Venezuelan AndesCGS 1933 11/04 08:41:15.0 9.000 N 72.000 W 000 0.0 M 101GUT 1933 11/04 08:41:17.0 8.500 N 72.000 W 000 6.0 Ms 101.Adding 4h 30m to the arrival time of an unknown phase recorded at OCV, we mustsubtract 3m 45s to obtain the origin time given by CGS, or 3m 43s for the GUTepicenter, which is located 55 km south of the CGS epicenter.The total duration of the seismogram recorded by OCV was 1 minute and theepicentral distances are 580 km (CGS), 600 km (GUT) and 700 to 750 km (regionCdcuta-Pamplona, where the real epicenter was first assumed). The duration of1 minute excludes the possibility that P-phases were recorded, because just thetime intervals of the arrival (Sg - Pg) or (Sn - Pn) for epicentral distances of580 km or more are longer than the mentioned duration of 1 minute. Probablythe small P-phases were not recognized by the station operator of that time andthe given onset therefore corresponds to the Sg-wave with a travel time of 2m 55scorresponding to the velocity of propagation of 3.3 km/sec. The error in origin timecalculated from that Sg-wave is 50 sec and this corresponds to only 9.19 mm on therecord, which is not bad if there were no minute marks. The main failing was in theinterpretation of the seismogram, and because of this, it appears that practicallyall seismograms from the former Cagigal Observatory (OCV) must be reanalyzed,if these important records are to be meaningfully used with other network data toimprove seismic maps and risk calculations.Between November 1933 and December 1953, the two-component Wiechert seismographat OCV recorded about 149 earthquakes, of which about 68 events (or 46percent) can be related to earthquakes published in the data files of GUT, CGS,and others.Table 1 gives the arrival time at Cagigal (OCV in this paper) in UTC, and theepicentral region of earthquakes for which the Pasadena (PAS) magnitude is 7 orlarger. Origin times are taken from the Earthquake Data File of the U.S. NationalGeophysical Data Center (1972). The earthquakes listed in Table 1 occurred in theregion 5's to 21'N and 55OW to 1OO"W. Because the OCV arrival times are not inall cases P-wave arrivals, reevaluation of the records is essential.