Abstract
To determine whether recent seismicity around the High Dam Lake (Lake Nasser) in the Aswan region is induced or triggered by the lake reservoir or is of natural origin, we analyzed moment tensor solutions and source spectra of recent recorded earthquakes in the area. The earthquakes' focal mechanisms, including source mechanism parameters and source spectra, can give important information to assist in discriminating between triggered and natural seismic events. In the current work, we computed moment tensors and stress drop values for six recently triggered earthquakes recorded by the Egyptian National Seismic Network (ENSN) around Lake Nasser, Aswan area, with local magnitudes between 3.0 and 4.6, as well as 40 nearby earthquakes in and around Egypt with local magnitudes ranging from 4.0 to 5.5, that are known as natural events. We performed full waveform inversion for the studied seismic events, including the dominant double-couple (DC) signature, and also completed moment tensor solutions. Our results show that most triggered events demonstrated significant non-double-couple components. The focal depths of most Aswan seismic events calculated in the current study are significantly shallower than the estimated focal depths for inter-plate and intraplate earthquakes in and around the area under investigation. The focal depths of 80% of the triggered events are shallower than 10 km, while 80% of the tectonic earthquakes are deeper than 15 km. On the other hand, stress and source spectra of the studied events are used as another key to deeply study the source mechanics and physics of natural and triggered events in this area.
Highlights
Aswan earthquakes are regarded as triggered seismicity (Gahalaut and Hassoup 2012), controlled by the pressure of pore fluid due to the water impoundment in the second largest reservoir in the world, Lake Nasser
We investigated full waveform moment tensor solutions for 46 earthquakes in and around Egypt, including six triggered seismic events that took place in the Aswan seismic zone and 40 earthquakes of natural origin such as in the Gulf of Aqaba, Triple Junction, Red Sea, and inland seismicity zones
The static stress drop for 1200 earthquakes including triggered and natural events were calculated based on the generalized inversion technique solved by the least square approach using P- and S-wave displacement source spectra constrained by the reference site
Summary
Aswan earthquakes are regarded as triggered seismicity (Gahalaut and Hassoup 2012), controlled by the pressure of pore fluid due to the water impoundment in the second largest reservoir in the world, Lake Nasser. Lizurek (2017) observed that most of the seismic events in the area of Czorsztyn Lake have at least 20% of non-DC components, and they concluded that full moment tensor inversion is a practical technique for distinguishing reservoir-triggered earthquakes from natural and purely induced seismicity. We limited our analyses to discriminate between Aswan seismicity and those of tectonic origin in Egypt based on full moment tensor solutions and decomposition into a DC part and compensated linear vector dipole part. We calculated the static stress drop of a mixed data set including triggered and natural seismic events using the spectral inversion technique. All the mentioned data will be involved in the evaluation of the stress drop, and only the earthquakes with magnitude [ 3 will be involved in the moment tensor inversion and decomposition analyses
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