2:30 PM - 2:45 PM
[SSS10-10] Microtremor array explorations of shallow S-wave profiles in Nile Delta, Egypt
Keywords:Nile Delta, Site Effect, Microtremors, SPAC method, HVSR method
Nile Delta is one of the highest populated regions in Egypt. It was affected by several destructive earthquakes. The most recent earthquake that triggered damage was the 12 October 1992 earthquake with a magnitude of 5.9 (mb = 5.9). Although this event had a moderate magnitude, it caused great life and economic losses. The damage was primarily related to the amplification of the seismic waves in the local soil deposits beneath the damaged areas and the earthquake source rupture. Therefore, microtremor array explorations were carried out at eighteen sites in Nile Delta, to obtain 1-D S-wave velocity (Vs) profiles for the site effects evaluation. Two or three different-sized circular arrays were deployed at each site to cover a wide frequency range of phase velocity. The recording lasted between 25 to 60 minutes, and the sampling interval was 100 samples per second. The maximum array radius was 50 m, while the minimum radius was 5 m.
The Spatial Autocorrelation (SPAC) method was applied to the microtremor data to estimate the phase velocities of Rayleigh wave, which can be consequently inverted into 1-D Vs profiles. Previous studies provided about 87 borings sediment information in Nile Delta. The sites were measured on four various surface geological units influenced by the Nile Valley history and the remnants of the old formations due to erosion of subsequent streams and rainfalls. The observed dispersion curves were categorized into the four groups based on these units. Group A represents the sites distributed on the salt marshes of the lower delta plain along the north coastline. The resulted phase velocities ranged from 130 to 490 m/sec at frequencies of 0.9-13.5 Hz. Port Said site has low phase velocities (~260m/s) at low frequencies due to the deep unconsolidated sedimentary layers. The dispersion curve at Alexandria has a steep variation in increasing frequencies compared to the other sites with the highest velocities (~490m/s) at low frequencies. The sites in this group generally have similar phase velocities at high frequencies. Sites in group B, located on Nile alluvium deposits, have the phase velocities between 133 to 505 m/s at a frequency band of 1.3 to 14 Hz, which gradually increase with lower frequencies. Banha in the south of the delta has the highest velocity of 505m/s at lower frequencies. In contrast, Abo-Elmatamer, located on the border of the clays soils with the coastal rock remnants, has higher velocities at higher frequencies. In group C, the sites on the salt-affected soils of the lower delta plain, Kafrelshikh has the highest velocity of 447 m/s at the low frequency of 1.3 Hz, while Abo-Homos had the low velocity of 126 m/s at the higher frequency of 12.4 Hz. In group D, Borg El-Arab, located on the coastal limestone ridges, shows a very high velocity of 460-650 m/s in a frequency range of 1.4 to 23 Hz.
Microtremor horizontal-to-vertical spectral ratios (HVSR) were also calculated for all the sensor records at each site. The derived HVSRs in the arrays show similar curves for all the sensors at each site, indicating nearly a flat layered structure beneath each site. The results also indicate that the fundamental resonant frequencies for most of the sites ranged between 1 to 3.5 Hz. These frequencies should be considered during the construction of low- or medium-rise buildings at these sites in the Nile Delta region. This research aims to make a further step into better understanding the shallow structures of the Nile Delta and their impact on the behavior of seismic waves.
The Spatial Autocorrelation (SPAC) method was applied to the microtremor data to estimate the phase velocities of Rayleigh wave, which can be consequently inverted into 1-D Vs profiles. Previous studies provided about 87 borings sediment information in Nile Delta. The sites were measured on four various surface geological units influenced by the Nile Valley history and the remnants of the old formations due to erosion of subsequent streams and rainfalls. The observed dispersion curves were categorized into the four groups based on these units. Group A represents the sites distributed on the salt marshes of the lower delta plain along the north coastline. The resulted phase velocities ranged from 130 to 490 m/sec at frequencies of 0.9-13.5 Hz. Port Said site has low phase velocities (~260m/s) at low frequencies due to the deep unconsolidated sedimentary layers. The dispersion curve at Alexandria has a steep variation in increasing frequencies compared to the other sites with the highest velocities (~490m/s) at low frequencies. The sites in this group generally have similar phase velocities at high frequencies. Sites in group B, located on Nile alluvium deposits, have the phase velocities between 133 to 505 m/s at a frequency band of 1.3 to 14 Hz, which gradually increase with lower frequencies. Banha in the south of the delta has the highest velocity of 505m/s at lower frequencies. In contrast, Abo-Elmatamer, located on the border of the clays soils with the coastal rock remnants, has higher velocities at higher frequencies. In group C, the sites on the salt-affected soils of the lower delta plain, Kafrelshikh has the highest velocity of 447 m/s at the low frequency of 1.3 Hz, while Abo-Homos had the low velocity of 126 m/s at the higher frequency of 12.4 Hz. In group D, Borg El-Arab, located on the coastal limestone ridges, shows a very high velocity of 460-650 m/s in a frequency range of 1.4 to 23 Hz.
Microtremor horizontal-to-vertical spectral ratios (HVSR) were also calculated for all the sensor records at each site. The derived HVSRs in the arrays show similar curves for all the sensors at each site, indicating nearly a flat layered structure beneath each site. The results also indicate that the fundamental resonant frequencies for most of the sites ranged between 1 to 3.5 Hz. These frequencies should be considered during the construction of low- or medium-rise buildings at these sites in the Nile Delta region. This research aims to make a further step into better understanding the shallow structures of the Nile Delta and their impact on the behavior of seismic waves.