Seismicity of Sinai Peninsula, Egypt

The Sinai Peninsula has a triangular shape between the African and Arabian Plates and is bounded from the western and eastern borders by the Gulf of Suez and Gulf of Aqaba–Dead Sea rift systems, respectively. It is affected by strong and destructive earthquakes (e.g., March 31, 1969 and November 22, 1995) and moderate earthquakes (m _b?>?5) throughout its history. After the installation of the Egyptian National Seismic Network (ENSN), a great number of earthquakes has been recorded within and around Sinai. Consequently, the seismogenic source zones and seismotectonic behavior can be clearly identified. Available data, including both historical and instrumental (1900–1997), have been collected from national and international data centers. While the data from 1998 till December 2007 are gathered from ENSN bulletins. The seismogenic source zones that might affect Sinai Peninsula are defined more precisely in this work depending on the distribution of earthquakes, seismicity rate (a value), b value, and fault plane solution of the major earthquakes. In addition, the type of faults prevailed and characterized these zones. It is concluded that the Gulf of Aqaba zone–Dead Sea transform zone, Gulf of Suez rift zone, Cairo–Suez District zone, and Eastern Mediterranean dislocation zone represent the major effective zones for Sinai. Furthermore, there are two local seismic zones passing through Sinai contributing to the earthquake activities of Sinai, these are the Negev shear zone and Central Sinai fault (Themed fault) zone. The source parameters, a and b values, and the maximum expected moment magnitude have been determined for each of these zones. These results will contribute to a great extent in the seismic hazard assessment and risk mitigation studies for Sinai Peninsula to protect the developmental projects.     

http://www.sciencedirect.com/science/article/pii/S1674987112001247

The Sinai Peninsula has been recognized as a subplate of the African Plate located at the triple junction of the Gulf of Suez rift, the Dead Sea Transform fault, and the Red Sea rift. The upper and lower crustal structures of this tectonically active, rapidly developing region are yet poorly understood because of many limitations. For this reason, a set of P- and S-wave travel times recorded at 14 seismic stations belonging to the Egyptian National Seismographic Network (ENSN) from 111 local and regional events are analyzed to investigate the crustal structures and the locations of the seismogenic zones beneath central and southern Sinai. Because the velocity model used for routine earthquake location by ENSN is one-dimensional, the travel-time residuals will show lateral heterogeneity of the velocity structures and unmodeled vertical structures. Seismic activity is strong along the eastern and southern borders of the study area but low to moderate along the northern boundary and the Gulf of Suez to the west. The crustal V_p/V_s ratio is 1.74 from shallow (depth ≤ 10 km) earthquakes and 1.76 from deeper (depth > 10 km) crustal events. The majority of the regional and local travel-time residuals are positive relative to the Preliminary Reference Earth Model (PREM), implying that the seismic stations are located above widely distributed, tectonically-induced low-velocity zones. These low-velocity zones are mostly related to the local crustal faults affecting the sedimentary section and the basement complex as well as the rifting processes prevailing in the northern Red Sea region and the ascending of hot mantle materials along crustal fractures. The delineation of these low-velocity zones and the locations of big crustal earthquakes enable the identification of areas prone to intense seismotectonic activities, which should be excluded from major future development projects and large constructions in central and southern Sinai.     

The present-day active deformation in the central and northern parts of the Gulf of Suez area,Egypt, from earthquake focal mechanism data

The average seismic strain rate is estimated for the seismotectonic zone of the northern/central parts of the Gulf of Suez. The principal strain rate tensor and velocity tensor were derived from a combination of earthquake focal mechanisms data and seismic moment of small-sized earthquakes covering a time span of 13 years (1992–2004). A total of 17 focal mechanism solutions have been used in the calculation of the moment tensor summation. The local magnitudes (MLs) of these events range from 2.8 to 4.7. The analysis indicates that the dominant mode of deformation in the central and northern parts of the Gulf of Suez is extension at a rate of 0.008 mm/year in N28°E direction and a small crustal thinning of 0.0034 mm/year. This low level of strain means that this zone experienced a little seismic deformation. There is also a right lateral shear motion along the ESE–WNW direction. This strain pattern is consistent with the predominant NW–SE normal faulting and ESE–WNW dextral transtensive faults in this zone. Comparing the results obtained from both stress and strain tensors, we find that the orientations of the principal axes of both tensors have the same direction with a small difference between them. Both tensors show a predominantly extensional domain. The nearly good correspondence between principal stress and strain orientations in the area suggests that the tectonic strength is relatively uniform for this crustal volume.     

Seismic risk assessment at the proposed site of Gemsa wind power station,southwestern coast of Gulf of Suez,Egypt

Gemsa has been chosen as the site for one of a new generation of power stations along the south-western margin of the Gulf of Suez. This site has been affected by a number of destructive earthquakes (M_w> 5), in addition to large number of earthquakes with magnitudes of less than 5. In this study seismic activities in the region were collected and re-evaluated, and the main earthquake prone zones were identified. It is indicated that this site is affected by the southern Gulf of Suez, northern Red Sea and Gulf of Aqaba source zones. The southern Gulf of Suez source zone is the nearest to the proposed site. The stochastic simulation method has been applied to estimate the Peak GroundAcceleration at the site of the proposed Gemsa power plant. It was noticed that the pseudo-spectral acceleration (PSA) reaches 175 cm/sec2 resulting from the southern Gulf of Suez seismic source. In addition, the response spectrum was conducted with a damping value of 5% of the critical damping, and the predominant period reached 0.1sec at the site. These results should be taken into consideration by civil engineers and decision-makers for designing earthquake resistant structures.     

Source parameters and ground motion of the Suez-Cairo shear zone earthquakes,Eastern Desert,Egypt

Three felt earthquakes with local magnitudes 4.0 (June 29th, 2000), 4.2 (July 07th, 2005) and 3.7 (October 30th, 2007) occurred to the southeast of Cairo along the Suez-Cairo shear zone. Being the most well recorded events by the Egyptian National Seismic Network (ENSN) in this area, they provide us an excellent opportunity to study the tectonics, the stress field, the source parameters, and the expected ground motion at Cairo. The focal mechanisms of the three events based on the first motion P-wave polarities indicate mainly normal faulting with a slight strike-slip component along nodal planes trending nearly EW and NW–SE. The three focal solutions imply a rejuvenation of the pre-existing EW and NW–SE faults due to a partly transfer of rifting deformation from the Red Sea-Gulf of Suez along these trends. Dynamic source parameters of these events are estimated from the P-wave spectra of the closest ENSN stations. The average values of the seismic moment, stress drop, rupture radius, and fault dislocation are estimated from six stations and interpreted in the context of the tectonic setting. These parameters in addition to the effects of site and path are used to synthesize the ground motion seismograms using stochastic modeling technique at the recorded stations and at some strategic sites.     

Moment Tensor Solutions of some Selected Local Events: Implications towards the Present-day Tectonics of the Kachchh Rift zone

The Kachchh rift zone of the northwestern India lies near to the India-Arabia and the India-Eurasia plate boundaries, which has experienced many devastating earthquakes in the past, namely the 1819 Allah Bund earthquake, the 1956 Anjar earthquake and the 2001 Bhuj earthquake. These earthquakes claimed the lives of about 17,000 people. To understand the current seismo-tectonic scenario, moment tensor inversion on the broadband data of fourteen Kachchh events of M_w 3.5–4.6 (during 2009–2015) from 5–12 three-component seismograph stations of the National Geophysical Research Institute (NGRI), Hyderabad, India was applied. Here deviatoric moment tensor inversion of multiple point sources (10–20s) for regional (or local) earthquakes, developed by Zahradnik et al. (2005) was used. The study reveals that modeled focal mechanisms range between reverse and normal oblique strikeslip while no pure normal dip-slip mechanism is found. However, only four out of fourteen events show oblique normal faulting with a minor strike-slip component. Thus, the modeling proposed in this study suggests that the oblique-reverse strike-slip, reverse and strike-slip type focal mechanisms are found to be dominant in the Kachchh rift zone. This observation indicates that the region is presently under compression.     

Distribution of the energy release, b-values and seismic hazard in Egypt

A review of the seismicity and seismic history of Egypt indicates areas of high activity concentrated along Oligocene-Miocene faults. This supports the idea of recent activation of the Oligocene-Miocene stress cycle. There are similarities in the spatial distribution of recent and historical epicenters. Destructive earthquakes in Egypt are mostly concentrated in the highly populated areas of the Nile Valley and Nile Delta. Some big earthquakes located near the plate boundary as far away as Turkey and Crete were strongly felt in Egypt. The distribution of the energy release shows a possible tectonic connection between active zones in Egypt and the complicated tectonic zones in Turkey and Crete through geologically verified fault systems. The distribution of intensity shows a strong directivity along the Nile Valley. This is due to the presence of a thick layer of loose sediments on top of the hard rock in the Nile Valley graben. The distribution of b-values indicates two different zones, comparable with stable and unstable shelf areas. Stress loads in the northern Red Sea and northern Egypt are similar. Geologically, northern Egypt is a part of the Unstable Shelf area. The probability to have an earthquake with intensity V or larger within 94 years is more than 80% in the Nile Valley and Nile Delta areas, Egypt-Mediterranean coastal area, Aswan High Dam area, Gulf of Aqaba-Levant Fault zone and in the oil fields of the Gulf of Suez. The maximum expected intensity in these areas and within the same period is V–VI for a 80% probability and VII–VIII+ for a 10% probability. Intensity VIII–IX has been reported for several earthquakes in both historical and recent time.     

Stress and faulting in southeast Australia as derived from the strongest earthquakes in the region

In an attempt to clarify the state of stress in Southeast Australia, the spatial and dynamic behaviour of the strongest events in the region (with magnitude greater than 5.0) has been studied in the light of the most recent numerical modelling of the crust. Although the local seismicity has little obvious spatial correlation with the mapped structures, the strongest events line up in a very narrow strip with a NW–SE direction. Composite fault-plane solutions, based on data from all of these earthquakes, allows the general characteristics of the local stress field to be determined, and shows that the orientation of the main compressive stress lies in an E–W direction. Taking into account the number of inconsistent data (40%) for this composite solution and the fault-plane solutions for some individual events, an improved result can be obtained by domain analysis, which shows that northern and southern zones have different orientation of the compressional axes. Although the composite solution for the events in the northern domain shows the same E–W orientation of the main compression as for the whole region, the composite solution for the southern zone reveals a rotation of the compression axis to an azimuth of 120–130°. From the relation between the energy and seismic moment the maximum value of the stress-drop for the whole of SE Australia is estimated as ∼2.0 MPa.     

Effect of azimuthal coverage of an earthquake on moment tensor solutions estimated by waveform inversion

The purposes of this seismological investigation are to understand and describe the effect of decrease in the azimuthal coverage of an earthquake on moment tensor solution estimated by waveform inversion. It will be very useful and worthwhile as mostly seismological networks are sparse and in case when only one or two station data are available. In this work, we have used two moderate earthquakes, 21 September 2009 (near Uttarakashi) and 3 May 2010 (near Ghansali). The waveform inversion has been carried using ISOLA software. The moment tensor is first estimated by using all station data and then by removing the stations so that the azimuthal coverage changes. The results show that strike of both nodal planes is varying with the change in azimuthal coverage. However, the slip and dip of both nodal planes remain quite stable against the variation in azimuthal coverage for these two earthquakes analyzed. The effect of decrease in the azimuthal coverage showed increase in double-couple percentage (DC %) and decrease in compensated linear vector decomposition (CLVD %). The other focal parameters such as T-axis azimuth, P-axis azimuth, T-axis plunge, and P-axis plunge have been found stable against the variation in azimuth coverage. The study also demonstrates that the moment tensor solutions obtained from waveform inversion using single station are almost similar to those estimated using maximum azimuthal coverage data and by polarity inversion.     

Source parameters of March 10 and September 13, 2007, United Arab Emirates earthquakes

On March 10 and September 13, 2007 two earthquakes with moment magnitudes 3.66 and 3.94, respectively, occurred in the eastern part of the United Arab Emirates (UAE). The two events were widely felt in the northern Emirates and Oman and were accompanied by a few aftershocks. Ground motions from these events were well recorded by the broadband stations of Dubai (UAE) and Oman seismological networks and provide an excellent opportunity to study the tectonic process and present day stress field acting in this area. In this study, we report the focal mechanisms of the two main shocks by two methods: first motion polarities and regional waveform moment tensor inversion. Our results indicate nearly pure normal faulting mechanisms with a slight strike slip component. We associated the fault plane trending NNE–SSW with a suggested fault along the extension of the faults bounded Bani Hamid area. The seismicity distribution between two earthquake sequences reveals a noticeable gap that may be a site of a future event. The source parameters (seismic moment, moment magnitude, fault radius, stress drop and displacement across the fault) were also estimated from displacement spectra. The moment magnitudes were very consistent with waveform inversion. The recent deployment of seismic networks in Dubai and Oman reveals tectonic activity in the northern Oman Mountains that was previously unknown. Continued observation and analysis will allow for characterization of seismicity and assessment of seismic hazard in the region.     

Source parameters of some recent earthquakes in the Gulf of Aqaba, Egypt

Gulf of Aqaba is recognized as an active seismic zone where many destructive earthquakes have occurred. The estimation of source parameters and coda Q attenuation are the main target of this work. Fifty digital seismic events in eight short-period seismic stations with magnitude 2.5–5.2 are used. Most of these events occurred at hypocentral depths in the range of 7–20 km, indicating that the activity was restricted in the upper crust. Seismic moment, M _o, source radius, r, and stress drop, Δσ, are estimated from P- and S-wave spectra using the Brune’s seismic source model. The average seismic moment generated by the whole sequence of events was estimated to be 4.6E?+?22 dyne/cm. The earthquakes with higher stress drop occur at 10-km depth. The scaling relation between the seismic moment and the stress drop indicates a tendency of increasing seismic moment with stress drop. The seismic moment increases with increasing the source radius. Coda waves are sensitive to changes in the subsurface due to the wide scattering effects generating these waves. Single scattering model of local earthquakes is used to the coda Q calculation. The coda with lapse times 10, 20, and 30 s at six central frequencies 1.5, 3, 6, 12, 18, 24 Hz are calculated. The Q _c values are frequency dependent in the range 1–25 Hz, and are approximated by a least squares fit to the power law [ $ {Q_c}(f) = {Q_o}{(f/{f_o})^\eta } $ ]. The average of Q _c values increases from 53?±?10 at 1.5 Hz to 700?±?120 at 24 Hz. The average of Q _o values ranges from 13?±?1 at 1.5 Hz to 39?±?4 at 24 Hz. The frequency exponent parameter η ranges between 1.3?±?0.008 and 0.9?±?0.001.     

Stress field in the central and northern parts of the Gulf of Suez area,Egypt from earthquake fault plane solutions

Earthquake focal mechanism solutions from 18 events in the central and northern parts of the Gulf of Suez with local magnitudes ranging from 2.8 to 5.2 and occurring between 1983 and 2004 are used to determine the type of motion and stress pattern of the region. Fault plane solutions show mostly normal component; pure normal faulting mechanisms and normal faulting with a strike-slip component. Only some mechanisms show pure strike-slip faulting. The fault planes strike in NW, WNW, NNE and ENE directions, in conformity with the geologically observed striking faults in the northern and central parts of the gulf. The principal stress orientation is also estimated by inverting the selected focal mechanism solutions. The results show that the northern part of the Gulf is subjected to NE–SW to NNE–SSW extension, with a horizontal σ₃ (plunge 3°) and subvertical σ₁ (plunge 80°). This means that the horizontal extensional stresses are still present in the central/northern Gulf of Suez.     

Earthquake source parameters at the sumatran fault zone: Identification of the activated fault plane

Fifteen earthquakes (M_w 4.1–6.4) occurring at ten major segments of the Sumatran Fault Zone (SFZ) were analyzed to identify their respective fault planes. The events were relocated in order to assess hypocenter uncertainty. Earthquake source parameters were determined from three-component local waveforms recorded by IRIS-DMC and GEOFON broadband lA networks. Epicentral distances of all stations were less than 10°. Moment tensor solutions of the events were calculated, along with simultaneous determination of centroid position. Joint analysis of hypocenter position, centroid position, and nodal planes produced clear outlines of the Sumatran fault planes. The preferable seismotectonic interpretation is that the events activated the SFZ at a depth of approximately 14–210 km, corresponding to the interplate Sumatran fault boundary. The identification of this seismic fault zone is significant to the investigation of seismic hazards in the region.     

Implementation of integrated multi-channel analysis of surface waves and waveform inversion techniques for seismic hazard estimation

In this study, an integrated multi-channel analysis of Surface Waves (MASW) technique is applied to explore the geotechnical parameters of subsurface layers at the Zafarana wind farm. Moreover, a seismic hazard procedure based on the extended deterministic technique is used to estimate the seismic hazard load for the investigated area. The study area includes many active fault systems along the Gulf of Suez that cause many moderate and large earthquakes. Overall, the seismic activity of the area has recently become better understood following the use of new waveform inversion method and software to develop accurate focal mechanism solutions for recent recorded earthquakes around the studied area. These earthquakes resulted in major stress-drops in the Eastern desert and the Gulf of Suez area. These findings have helped to reshape the understanding of the seismotectonic environment of the Gulf of Suez area, which is a perplexing tectonic domain. Based on the collected new information and data, this study uses an extended deterministic approach to re-examine the seismic hazard for the Gulf of Suez region, particularly the wind turbine towers at Zafarana Wind Farm and its vicinity. Alternate seismic source and magnitude-frequency relationships were combined with various indigenous attenuation relationships, adapted within a logic tree formulation, to quantify and project the regional exposure on a set of hazard maps. We select two desired exceedance probabilities (10 and 20 %) that any of the applied scenarios may exceed the largest median ground acceleration. The ground motion was calculated at 50th, 84th percentile levels for both selected probabilities of exceeding the median.     

The earthquakes and related tsunamis of October 6, 1944 and March 7, 1867; NE Aegean Sea

During two distinct earthquakes occurred on March 7, 1867 and October 6, 1944, tsunami waves were also observed at some localities around the Gulf of Edremit, NE Aegean Sea. The first event (M _w = 6.8) mostly affected the city of Mitilini of Lesvos Island while the Gulf of Edremit-Ayvacık earthquake (M _S = 6.8) largely affected the northern and eastern coastal areas of the Gulf of Edremit. In 1944 earthquake, numerous surface cracks and water gushes were reported. The coastal neighborhoods of the town of Ayvalık in the east were flooded by tsunami waves. At the WSW extend of the main fault observed on land, which is parallel to the present-day slip vectors, some normal-oblique faults were observed close and subparallel to the northern coast. On the basis of historical documents, reports, interviews, geological setting, field observations and marine seismic reflection data, the 1944 earthquake was not triggered by one of the main fault segments but by a secondary fault or fault group which was described in this study. Depending on the distribution of tensional and compressional forces in the region, which rotates clockwise under the control of the middle strand of the North Anatolian fault, secondary fault groups become important. The moment tensor parameters of such small-size events have been determined and have obtained consistent results with the faults proposed in this study.     

Modeling of source parameters and moment tensors of local earthquakes occurring in the eastern Indian shield

Earthquake source parameters and crustal Q are being estimated simultaneously through the inversion of S-wave displacement spectra from three-component recordings of ten local cratonic intraplate earthquakes from 3-6 broadband stations in the eastern Indian shield, wherein, an iterative Levenberg-Marquardt inversion technique is used. The estimated seismic moment (M_o) and source radii (r) vary from 7.4 x 10¹² to 7.1 x 10¹⁴ N-m and 144.2 to 211.3 m, respectively, while estimated stress drops (Δσ) and multiplicative factor (E_mo) values range from 0.11 to 4.13 MPa and 1.33 to 2.16, respectively. The corner frequencies range from 6.23 to 8.62 Hz while moment magnitudes vary from 2.44 to 3.57. The radiated seismic energy and apparent stresses range from 8.3 x 10⁶ to 2.0 x 10¹⁰ Joules and 0.06 to 0.94 MPa, respectively, wherein the estimated corner frequencies and seismic moment satisfy the relation Mo ∞ f _c ^–(3+ε) for ε = 12.7. Thus, the source scaling of these events clearly deviates from the self-similarity i.e. f^–3. Estimated Zuniga parameters reveal that all selected events satisfy the partial stress drop model, which is in good agreement with the global observations. Our estimated crustal S-wave quality factors vary from 1091 to 4926 with an average of 3006, suggesting a less heterogeneous crustal structure underlying the study region.We also perform moment tensor inversion of five selected local events using ISOLA software, which reveals that the dominant deformation mode for the eastern Indian shield is left-lateral strike slip motion with minor normal dip-slip component on an almost vertical plane. This observation suggests that neotectonic vertical movements might have played a key role in generating these earthquakes. Our modeling also depicts that the seismically mildly active Singhbhum shear zone and Eastern Ghats mobile belt are characterized by the left-lateral strike motion while two events in the Chotanagpur half graben belt suggest a normal dip-slip motion along a south dipping plane. A north-south orientation of P-axis is found to be dominant in the area, which is consistent with the prevailing north–south compression over the Indian plate.     

Source parameters of earthquakes in the reservoir-triggered seismic (RTS) zone of Koyna–Warna, Western India

New empirical relations are derived for source parameters of the Koyna–Warna reservoir-triggered seismic zone in Western India using spectral analysis of 38 local earthquakes in the magnitude range M _L 3.5–5.2. The data come from a seismic network operated by the CSIR-National Geophysical Research Institute, India, during March 2005 to April 2012 in this region. The source parameters viz. seismic moment, source radius, corner frequency and stress drop for the various events lie in the range of 10¹³–10¹⁶ Nm, 0.1–0.4 km, 2.9–9.4 Hz and 3–26 MPa, respectively. Linear relationships are obtained among the seismic moment (M ₀), local magnitude (M _L), moment magnitude (M _w), corner frequency (fc) and stress drop (?σ). The stress drops in the Koyna–Warna region are found to increase with magnitude as well as focal depths of earthquakes. Interestingly, accurate depths derived from moment tensor inversion of earthquake waveforms show a strong correlation with the stress drops, seemingly characteristic of the Koyna–Warna region.     

Local seismic array observations at north Evoikos, central Greece, delineate crustal deformation between the North Aegean Trough and Corinthiakos Rift

In order to better constrain and define the microseismic activity at the north Evoikos Gulf and its surrounding area we deployed an onshore/offshore seismic array consisting of 31 three-component seismic digital stations. The array was active from 30 June to 24 October 2003, and covered an area of 2500 km². We located more than 2000 seismic events ranging from 0.7 to 4.5 M_L by using six stations as a minimum in order to define the foci parameters. Recorded seismicity delineated three major zones of deformation: from south to north, the Eretria–Parnis–eastern Corinthiakos zone, the Psachna–Viotia zone, and the Northern Sporades–North Evia–Bralos zone. Alignments of the recorded seismicity follow the tectonic trends and their orientation in the above zones. The whole area accommodates the stress field between the North Aegean Trough and the Corinthiakos Gulf. Rate of deformation intensifies from north to south, as revealed also by historical and instrumental seismicity. The successive change of orientation between the two stress fields fragments the crust in relatively small units and the fault systems developed do not permit the generation of major earthquakes in the north Evoikos area and its immediate vicinity. This is also supported by the instrumental seismicity of the last century. Larger events reported in historical times are probably overestimated.Most seismic activity is crustal. Subcrustal events were recorded mainly below the Lichades area and are interpreted as the consequence of the subduction of the Ionian oceanic lithosphere below the Hellenides. The Lichades volcano is the most northern end of the Hellenic volcanic arc.At present the highest seismic activity is associated with the Psachna region of north Evia that has been continuously active since 2001. Considering, however, the development of the seismic activity during the last decade, there has been a sequence of large events, i.e., Parnis in 1999, Skyros in 2001 and Psachna in 2001–2003. This demonstrates the fact that the tectonic deformation in all this area is intense and important for the accommodation of the stress field of the North Aegean Trough to that of the Corinthiakos Rift.     

Crustal heterogeneity and seismotectonics of the region around Beijing, China

A detailed three-dimensional (3-D) P-wave velocity model of the crust and uppermost mantle under the Chinese capital (Beijing) region is determined with a spatial resolution of 25 km in the horizontal direction and 4–17 km in depth. We used 48,750 precise P-wave arrival times from 2973 events of local crustal earthquakes, controlled seismic explosions and quarry blasts. These events were recorded by a new digital seismic network consisting of 101 seismic stations equipped with high-sensitivity seismometers. The data are analyzed by using a 3-D seismic tomography method. Our tomographic model provides new insights into the geological structure and tectonics of the region, such as the lithological variations and large fault zones across the major geological terranes like the North China Basin, the Taihangshan and the Yanshan mountainous areas. The velocity images of the upper crust reflect well the surface geological and topographic features. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocities, respectively. The Taihangshan and Yanshan mountainous regions are generally imaged as broad high-velocity zones, while the Quaternary intermountain basins show up as small low-velocity anomalies. Velocity changes are visible across some of the large fault zones. Large crustal earthquakes, such as the 1976 Tangshan earthquake (M=7.8) and the 1679 Sanhe earthquake (M=8.0), generally occurred in high-velocity areas in the upper to middle crust. In the lower crust to the uppermost mantle under the source zones of the large earthquakes, however, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids. The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes.     

Lateral variations of coda Q and attenuation of seismic waves in the Gulf of Suez, Egypt

Gulf of Suez consists mainly of three tectonic provinces that are separated by two accommodation zones. The southern edge of the gulf is bordered by N–S faults which mark the transition between the shallow water, Suez Basin and the deep northern Red Sea Basin. The sensitivity of coda Q measurements with respect to geological differences in the crust is demonstrated in three regions with a large variety of tectonic and geologic properties. The estimation of coda Q (Qc) is performed for 370 local earthquakes recorded at 12 digital seismic stations during the period from 2000 to 2007. The magnitudes of the earthquakes between 1.5 and ~4.5 have been used at central frequencies 1.5, 3, 6, 9, 12, 15, 18, and 24 Hz through three lapse time windows 10, 20, 30 s starting at once and twice the time of the primary S wave from the origin time. The time domain coda decay method of the single isotropic scattering model is employed to calculate frequency-dependent values of coda Q. The Qc values are frequency dependent in the range 1–25 Hz, and are approximated by a least squares fit to the power law [Qc(f) = Qo(f/fo)η]. The observed coda Q indicates that the area is seismically and tectonically active with high heterogeneities. The variation of the quality factor Qc has been estimated at different regions to observe the effect of different tectonic province. The average frequency-dependent estimated relations of Qc vary from 65f1.1 to 96f0.9 at 10 to 30 s window length, respectively. The decreasing value of the frequency parameter with increasing lapse time shows that the crust acquires homogeneity with depth. The variation of Qc with the variations in the geologic and tectonic properties of the crust was investigated. The frequency exponent η might be larger in active tectonic areas and smaller in more stable regions. In the northern region of the Gulf of Suez, the obtained value of η?=?0.8?±?0.011, which might indicate a low level of tectonic activity compared with η?=?1.1?±?0.005 and 1.3?±?0.009 for the central and southern regions of the gulf.