In recent
years, there has been a great deal of interest to assess the local site
response, which is one of the key components of any seismic risk analysis. The
empirical method is one of the several techniques to compute the spectral
ratios of horizontal and vertical components (H/V) using microtremor records, a
method proposed by Nakamura (1989).
Recent studies
of large destructive earthquakes have shown that damage during the earthquakes
are often caused by the amplification of seismic waves in near-surface
sedimentary layers. Studies of historical earthquakes for the past few thousand
years demonstrate that the damaging earthquakes in Palestine were located along
the Dead Sea Transform (DST) fault. In the past century, a destructive
earthquake has occurred in the Jordan rift region at the boundary between the Arabian
and the Sinai–Palestine plates: Nablus earthquake of 11 July 1927 north Jericho
(M 6.2). The most recent earthquake of 11 February 2004 (Mb5.1) also in the
Dead Sea Region (about 16 km south of Jericho city with a focal depth of
21 km) caused
some partially damage cases; in Nablus city few historical buildings have been
affected with damages between grade 2 to grade 4 according to EMS-98.
A study on the
H/V ratio of microtremor motion at the rock substratum is presented using
records from 3-component digital seismograph installed in two Palestinian
cities: Nablus and Ramallah. Spectral ratios were computed for different
sedimentary sites, where the youngest sediments of Quaternary alluvium and
alternating layers of unconsolidated sedimentary materials give the highest
amplification factors.
Our results
show a good correlation between the site amplification and the damage areas of
Nablus earthquake of 11 July, 1927 and the disastrous effects of the recent
earthquake of 11 February 2004.