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

Analysis of the Eastern Mediterranean Earthquake of 20 May 1202 183do not record body waves well and the best results are obtained by using the timeof the reported maximum phase, presumed to be a surface wave travelling with avelocity of about 2.9 to 3.1 km/sec. Simple graphical location methods have beenused. For some later events, readings of P- and S-phases from other instruments arealso used to support those of phase maxima. Such relocations are not accurate bymodern standards, but an earthquake’s epicenter may usually be estimated withina few degrees and felt information may be found to help resolve ambiguities. Whenthe existence of such an event is established in a certain region and teleseismicamplitudes and periods are available, a magnitude can be estimated even if theposition is unreliable to within several degrees, since calibration functions do notchange rapidly with distance in the teleseismic range (Ambraseys and Adams, 1986).2.2. MagnitudesFor earthquakes of the period 1898 to 1914, surface-wave magnitudes may beestimated using trace maxima recorded by undamped Milne seismograms. Thismethod has been applied to a number of geographically restricted regions in theMiddle East and North Africa with excellent results, along much the same lines asdiscussed by Abe (1987). For the majority of early earthquakes in Europe and theMiddle East, data from the more advanced instruments are available from 1903,so that the need to resort to the undamped Milne trace amplitudes for magnitudedetermination is minimal.Magnitudes are often given without indications as to whether they are local (ML),surface (Ms) or body wave (m) magnitudes and the indiscriminate amalgamationof such values from existing catalogues leads to a body of non-homogeneous data.Similarly, the use of empirical correlations to convert rn or ML into Ms and viceversa is quite unacceptable in areas such as the Middle East and Southeast Europe,where the seismotectonic characteristics of the region vary so rapidly and where thevalues of m, ML and Ms reported by local stations are based on procedures thathave changed many times during the last few decades.For events of the pre-instrumental period, surface wave magnitudes may be estimatedquite consistently using a set of macroseismic observations such as the radiiof the isoseismals of a given intensity grade (see for example Equation (1) below).The use of uniformly processed macroseismic data may also lead to an estimate offocal depth (Ambraseys, 1985b). In practice, however, there are often considerableproblems in assessing intensities and felt areas from historical information, particularlyin areas of low seismicity or low population density and also the further backin time one goes, since the available documentary sources become both fewer andmore laconic.2.3. IntensitiesWhatever the limitations imposed on the type of data available by the particularcultural characteristics of a given age or area, intensities should be assesseduniformly by one or more observers, either on a scale such as the MSK scale orusing a simpler one designed for early events in a specific region. Conversion fromone intensity scale to another, using empirical formulae, has been found to lead tounacceptably large errors and should be avoided.