Source parameters of earthquakes from the 1987 and 1993 swarms and crustal deformation study during 1987, Aswan Lake area, Egypt

Shallow earthquake swarms of focal depths <10 km and magnitude <4 occurred beneath Aswan Lake in 1987 and 1993. The 1987 events are located along the E-W Kalabsha fault and their focal mechanism solution is strike-slip. Events of the 1993 sequence are distributed along a fault segment trending in the N-S direction and show a strike slip fault plane solution, too. Source parameters of 29 earthquakes from these two sequences are determined using a software developed by the Institute of Geophysics, Polish Academy of Science. The seismic moment and corner frequency data indicate weakness of the focal region of these swarms. The weakness may be produced by the presence of a fractured zone. Horizontal movements, which were collected by Kalabsha local geodetic network, are analyzed for epochs 1984-February 1987, February 1987-September 1987 and September 1987-November 1988. The results show different spatial patterns and magnitudes of the horizontal displacement. The large magnitude of the horizontal displacements suggests that the crustal deformation measurements are mixture of seismic and aseismic components. Moreover, correlation between the geodetic data and the stress pattern deduced from seismicity exists

Identification of local seismicity observed south of Aswan City — Egypt

Suddenly, Aswan Seismic Network detected and observed a number of events beside Aswan city. The magnitude of these events ranged from 0.9 to 2.6. These events were considered very important events because they were located not far from the Aswan High Dam. The main purpose of this work is to investigate and identification of these events using different seismic methods. Small earthquakes and explosions have the same flat and displacement spectrum for short-period P waves and discrimination of small events rests on detection and location. The final result appears that the source of these events is not an artificial type

Seismicity, arrival time delays of the seismic phases and slowness characteristics study in Abu Dabbab area, Egypt

The temporal variations of seismicity from the Abu Dabbab area, 25 km west of the Red Sea coast, are collected from the Egyptian national seismic network (ENSN), which has magnified the detection capability in that area to ML < 1 earthquakes. These data show a sequence of the micro earthquake swarm during 2003–2011. This area has experienced larger shocks up to M = 6 during the 20th century and its seismicity is concentrated in a narrow spatial volume. We analyze the digital waveform data of about 1000 seismograms, recorded by portable network of 10 vertical component seismographs that are employed in a temporary survey experiment in the Abu Dabbab area in 2004, and the results indicate: firstly, there are similar waveform seismograms, which are classified into three groups. In each group a master event is identified. Then, the arrival time delays of the P and S phases (Δtp and Δts, respectively) are measured between the master event and its slave events. Δtp and Δts range between −0.01 and 0.02 s, respectively. These values are used to relocate the studied events. Secondly, the slowness vector (Δs) in 3-dimensional pattern, which is estimated using the genetic algorithms, is found Δsx = 0.0153, Δsy = 0.00093 and Δsz = 0.2086 s/km in the three spatial coordinates (X, Y and Z), respectively. These analyses demonstrate the inhomogeneities within the upper crust of the study area. Also, Δs shows little dependence of lateral distances and reasonably high slowness along the depth extent, which is consistent with the seismic velocity structure variations

Detection of the seismic quiescence along the seismic active faults in Kalabsha area, west of Lake Nasser, Aswan, Egypt

The spatial and temporal distribution of seismicity around the northern part of Lake Nasser in Aswan has been continuously monitored by a short-period seismograph network since 1982. Data of this network, which consists of 13 field stations distributed around the Kalabsha fault, has demonstrated occurrences of three swarm sequences during the past 26 years. The first swarm occurred in June 1987 on a fault segment about 8 km in length beneath the lake water. The largest magnitude of this swarm was 3.7. Its hypocenters were located within the shallow depth’s interval (i.e., 2 to 5 km from the ground surface). The estimated b-value for this swarm was 0.63. The second swarm sequence took place at the intersection of the Seiyal fault with Kurkur fault about 10 km to the north of the June 1987 sequence. This swarm occurred during the period from August to December 2004 with largest magnitude of 4.1. The focal depths of this swarm also ranged from 2 to 5 km. The b -value of this swarm was found to be 0.69. The third swarm occurred recently in April 2007 with largest magnitude of 4.2 at the same location of 1987 sequence but its focal depths ranged from 6 to 8 km and has the b -value of 0.58. The composite focal mechanism study of these three swarm sequences revealed right-lateral strike slip faults with horizontal stress axes pattern. The relationship between the lake water level and the swarm sequences showed that both 1987 swarm and 2007 swarm occurred during the decreasing of water level, whereas the 2004 swarm occurred during the increasing of the water level. The study of these three swarm sequences illustrates an interesting phenomena, that there is a seismic quiescence preceding each swarm sequence. The duration period of the seismic quiescence ranges from 3 to 5 months

Earthquakes focal mechanism and stress field pattern in the northeastern part of Egypt

Egypt is characterized by moderate size seismicity where earthquakes are distributed within several active regions. In the present study, we investigated the source mechanism of earthquakes using the digital waveform data recorded by the Egyptian National Seismic Network (ENSN) during the period from 2004 to 2008. The focal mechanisms are constructed with high reliability based on the polarity of the first motion of P-wave. These solutions are used to examine the mode of tectonic deformation and the present-day stress field pattern affecting on different tectonic provinces in the northern part of Egypt. The results demonstrate mainly a normal faulting mechanism with minor strike slip component generally trending parallel to the northern Red Sea, the Suez rift, Aqaba rift with their connection with the great rift system of the Red Sea and the Gulf of Suez and Cairo-Alexandria trend. The inversion technique scheme is used also in the present study for determining the regional stress field parameters for earthquake focal mechanism solutions based on the grid search method of Gephart and Forsyth (1984). The Results of the stress tensor using focal mechanisms of recent earthquakes show a prevailed tension stress field in N52°E, N41°E and N52°E for the northern Red Sea, Gulf of Suez and Gulf of Aqaba zone respectively