Mapping b-values beneath Abu Dabbab from June to August 2004 earthquake swarm

摘要

Abu Dabbab area is considered as one of the most active earthquake sources in Egypt. It is defined by its swarm type activity, and complicated stress pattern. This study was conducted to evaluate the two and three dimensional spatial distribution of b -value at Abu Dabbab area (Margin of the northern Red Sea Rift, Egypt). The gridding technique of Wiemer and Wyss (1997) was used to compute b -value using ZMAP software. The b -value is calculated from a catalog consisting of 850 well-located earthquakes, which were recorded from 1st June to August 2004, using the maximum likelihood method. These earthquakes were recorded by temporary digital seismic network, with magnitudes ranging from ?1 to 3.4 ML. It is important to mention that the variations of b -value with time cannot be easily detected for a short period. Hence, this study has been carried out to examine the variations of b - value in space. The computed b -value in the Abu Dabbab area does not follow a uniform distribution. A small volume of anomalously high b -value ( b 1.8) exists in the central part of the area at a depth between 6 and 9 km. This seems to agree with the reported low velocity value derived from previous P-wave travel time tomography studies (Hosny et al., 2009) and the low Q value (Abdel-Fattah et al., 2008). The existence of an anomalously high b -value region may be attributed to the presence of a magma reservoir or dyke zone beneath the northern Red Sea Rift that causes an intensively heterogeneous fractured crust or unusually high pore pressure.