GNGTS 2013 - Atti del 32° Convegno Nazionale

Source inversion of the M6.3 1927 Jericho earthquake, possible repetition of the biblical earthquake of 1473 B.C. L. Sirovich, F. Pettenati Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, OGS, Trieste, Italy Introduction. According to the Bible and the Torah (Joshua [Giosuè] 6:1-21), God made the walls of Jericho fall down, perhaps with an earthquake, to help Joshua to conquer the city. The battle would have taken place in 1473 B.C.. This hypothesis found some archaeological confirmations (e.g., Garstang and Garstang, 1940; Keller, 1956), but it is still controversial. However, the M6.25 earthquake of 11 July 1927 (Ben-Menahem et al. , 1976) heavily hit also the area of Jericho and could perhaps be the repetition of the hypothetic biblical event. Vivid descriptions of earthquakes in the region are found in the Bible. In particular, as regards the area of study, Ambraseys (2009) pointed out that the descriptions by prophets Amos and Zacharias allow the interpretation of an earthquake about in 766 B.C.; the Zacharias’ words even comply with a sinistral strike-slip movement. The epicentre and causative faults of the 1927 destructive earthquake is still very controversial (see Tab. 1). Ben-Menahem et al. (1976) located it north of Jericho. According to Avni et al. (2002), however, the location north of the city was also based upon some secondary macroseismic evidence by Garstang (1931) and became one of the most accepted facts. In particular, this author reported the collapse of the banks of Jordan River about 20 km north of Jericho, damming thereby the Jordan for twenty-one hours. This damming became a crucial evidence for locating the epicentre by Ben-Menahem et al. (1976). However, upon close examination of the daily reports of the British Police of the time, Avni et al. (2002) concluded that: i) these detailed reports registered all happenings but did not even mention an important happening such as a one-day damming of the river (and the following flooding); ii) Garstang was not a witness of the phenomenon because, at the time of the 1927 earthquake, he went already back to Great Britain and iii) he had a personal religious interest «to relate natural disasters to miraculous biblical events» (Avni et al. , 2002; p. 471). Then, Shapira et al. (1993) noticed that many authors reported the (32.0°; 35.5°) epicentre of Tab. 1 without re-evaluating it and thinks that that approximate location was not obtained using the many recordings collected by the International Seismological Summary, ISS, but was estimated by the Ksara, Lebanon, station using only the Ksara data. Instead, Shapira et al. (1993) used the ISS data and applied standard location procedures. But there is a question that puts some doubts on the Shapira et al. (1993) epicentre itself: the difficulties associated with synchronization of the mechanical clocks in 1927 and the relatively low sensitivity of the seismic stations at the time, which produces high residuals (they omitted those greater than ±10 s). More recently, Zohar and Marco (2012) used the intensities, I , provided by Avni (1999) to relocate the 1927 epicentre. To find their best solution, they corrected the data for the site effects and, then, correlated the intensities spatially with a logarithmic variant of the epicentral distance. They explain that the optimum site corrections had the effect of moving their first epicentre some 25 km west, on the Dead Sea transform DST. Instead, exagerated corrections moved the epicentre 60 km east of DST. Their epicentre is close to that by Shapira et al. (1993). We comment that, in our opinion, an objective criterion for evaluation of all the sites is required. We avoid situations where only selected data are examined or where part of the data are modified, because this could drive the inversion results. It should also be kept in mind that deamplifications, if any, usually do not attract the same attention as cases of amplification, even though they have the same importance as far as inversion is concerned (Pettenati and Sirovich, 2007; pp. 1591). For these reasons, we do not correct single I values following local site studies, if any. In California and in Italy, we attempted to search for site effects by splitting the logarithms of the epicentral distances of each observed I , according to the prevailing 124 GNGTS 2013 S essione 1.1