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

58 N. V. Kondorskaya and Yu. F. Kopnichev3. Interpretation of SeismogramsOne of the simplest and highly important elements in the interpretation of oldrecords is determination of dynamic parameters in groups of body (P and S) andsurface (L) seismic waves and applying corresponding calibrating functions. Inpreparing a New Catalog of the USSR territory from ancient times to 1977, Kondorskayaand Shebalin (1982) did not have access to world-wide seismograms, thusthey analyzed mainly seismological bulletins. In Russia, the Bulletin of PermanentCentral Seismological Commission (BPCSC), issued in 1902-1908 under the editorshipof G.V. Levitsky, is a very useful publication. Acknowledging the imperfectionof seismic instrumentation used at that time, those who compiled the BPCSC triedtheir utmost to preserve for the future the principal characteristics of these records- maximum amplitude and oscillation period. Without delving into the history ofthe development of the seismological observations in the USSR (see Kirnos et al.,1957), we will draw attention only to the basic factor which impeded work on thenew catalog, i.e., while substituting the old instruments of Bosch, Milne, Elert, andTselner with more up-to-date seismographs of B.B. Galitzin, the formal approachto observation data took the upper hand. Inability to accurately evaluate the actualground displacement was assumed enough to remove from the bulletins of 1911 alldynamic characteristics of seismic waves. This tendency is also apparent in wellknown global publications, such as the ISS and the BCIS bulletins. In 1923, thepublication of amplitude data in the ISS was terminated and for a long time afterwardno evaluations of earthquake size appeared except for indirect indications -the number of stations recording earthquakes. For measurements of (AIT),, in agroup of body and surface waves or of magnitudes, we advise the use of the recommendationsof the Commission on Practice (Recommendations of the Commissionon Practice, 1967), and the calibrating functions of Vanek et al. (1962) and Vaneket al. (1980).Evaluations of the duration T of earthquake waves on historical seismograms canprobably be used to estimate magnitudes. The following equations were obtainedby Kondorskaya and Shebalin (1982):M, = 2.4 logl, 7 + 1.6 (for shallow earthquakes)M, = 2.4 loglo F + 2.2 (for deep earthquakes),where 7 is the derived duration in minutes.The above duration 7 is calculated with regard to the effective magnification ofinstruments; correlations between 7 and T for various types of devices is specifiedexperimentally. M, is based on surface waves recorded on horizontal instruments forshallow earthquakes, while for deep earthquakes it is a conventional characteristicthe nature of which is still to be verified.Records of early years were in most cases obtained from short-period instruments.We believe that these records of historical seismograms can be used to estimate thespatial-temporal parameters of the sources from the characteristics of short-periodradiation of strong earthquakes.Kopnichev (1985) shows that relatively short-period (T < 3 sec) seismic wavescarry important information on incoherent radiation of strong earthquakes and