Lithospheric Structure of the Arabian Plate and Surrounding Regions

Copyright 2003, Khaled Al-Damegh See also: http://atlas.geo.cornell.edu/SaudiArabia/publications/Al-Damegh%20Dissertation%202004.htmContinuous waveform recording from a newly established broadband seismic network in Saudi Arabia, in addition to data produced by other stations in the region, were used to map regional seismic wave propagation (Lg and Sn) and Pn attenuation. Moreover, crustal thickness in the Arabian plate was also estimated based on receiver function analysis. Zone blockage and inefficient Sn propagation is observed along and to the east of the Dead Sea fault system and in the northern portion of the Arabian plate (south of the Bitlis suture)., We observed Sn blockage across some segments of the Red Sea. These regions of high Sn attenuation have anomalously hot and possibly thin lithospheric mantle (i.e., mantle lid). Consistent with our Sn attenuation findings, we also observed low Qpn along the western portion of the Arabian plate and along the Dead Sea fault system. Our results imply the presence of a major anomalously hot and thinned lithosphere in these regions that may be caused by the extensive upper mantle anomaly that appears to span most of east Africa and western Arabia. These mapped zones of high attenuation closely coincide with an extensive Neogene and Quaternary volcanic activity. We found that the average crustal thickness of the Arabian shield is 39 km. The crust thins to about 23 km along the Red Sea coast and to about 25 km along the Gulf of Aqaba. We observed a dramatic change in crustal thickness between the topographic escarpment of the Arabian shield and the shorelines of the Red Sea. We compared our results in the Arabian shield to nine other Proterozoic and Archean shields that include reasonably well-determined Moho depths. We do not observe a significant difference between Proterozoic and Archean crustal thickness. Our observations show that the transition from oceanic to continental crust along the Red Sea margin occurs over a relatively short distance compared to a typical west Atlantic continental margin. We argue that the anomalous nature of the Red Sea margin may be one of the consequences of the presence of a mega plume that extends from the core-mantle boundary into the upper mantle beneath east Africa, the Red Sea, and the western portion of the Arabian plate. In addition, the site where the sea-floor spreading of the Red Sea occurred was a Proterozoic suture and a zone of weakness. These observations combined may explain the relatively abrupt breakup of the Arabian plate and the anomalous nature of the Red Sea margin

Crustal structure of the Arabian Plate: New constraints from the analysis of teleseismic receiver functions

An edited version of this paper was published by Elsevier Science. Copyright 2005, Elsevier Science. See also: http://dx.doi.org/10.1016/j.epsl.2004.12.020; http://atlas.geo.cornell.edu/SaudiArabia/publications/Al-Damegh%202005.htmReceiver functions for numerous teleseismic earthquakes recorded at 23 broadband and mid-band stations in Saudi Arabia and Jordan were analyzed to map crustal thickness within and around the Arabian plate. We used spectral division as well as time domain deconvolution to compute the individual receiver functions and receiver function stacks. The receiver functions were then stacked using the slant stacking approach to estimate Moho depths and Vp/Vs for each station. The errors in the slant stacking were estimated using a bootstrap re-sampling technique. We also employed a grid search waveform modeling technique to estimate the crustal velocity structure for seven stations. A jackknife re-sampling approach was used to estimate errors in the grid search results for three stations. In addition to our results, we have also included published receiver function results from two temporary networks in the Arabian shield and Oman as well as three permanent GSN stations in the region. The average crustal thickness of the late Proterozoic Arabian shield is 39 km. The crust thins to about 23 km along the Red Sea coast and to about 25 km along the margin of the Gulf of Aqaba. In the northern part of the Arabian platform, the crust varies from 33 to 37 km thick. However, the crust is thicker (41?53 km) in the southeastern part of the platform. There is a dramatic change in crustal thickness between the topographic escarpment of the Arabian shield and the shorelines of the Red Sea. We compared our results in the Arabian shield to nine other Proterozoic and Archean shields that include reasonably well determined Moho depths, mostly based on receiver functions. The average crustal thickness for all shields is 39 km, while the average for Proterozoic shields is 40 km, and the average for Archean shields is 38 km. We found the crustal thickness of Proterozoic shields to vary between 33 and 44 km, while Archean shields vary between 32 and 47 km. Overall, we do not observe a significant difference between Proterozoic and Archean crustal thickness. We observed a dramatic change in crustal thickness along the Red Sea margin that occurs over a very short distance. We projected our results over a cross-section extending from the Red Sea ridge to the shield escarpment and contrasted it with a typical Atlantic margin. The transition from oceanic to continental crust of the Red Sea margin occurs over a distance of about 250 km, while the transition along a typical portion of the western Atlantic margin occurs at a distance of about 450 km. This important new observation highlights the abruptness of the breakup of Arabia. We argue that a preexisting zone of weakness coupled with anomalously hot upper mantle could have initiated and expedited the breakup

Studies on the source mechanism and SES for 28 earthquakes in Greece

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references: p. 82-85.Issued also on microfiche from Lange Micrographics.The VAN telemetric network used to forecast earthquakes in Greece has been in operation since January 1983. Eighteen VAN stations were established in various locations within Greece to detect Seismic Electric Signals (SES) which are thought to precede the onset of earthquakes. During the past thirteen years, success in predicting the three main parameters (the epicenter, time, and magnitude) of impending large magnitude (mb> 5.0) earthquakes has been remarkable. Two VAN stations, Ioannina (IOA) and Pyrgos (PIR), which have been in operation since 1983 and cover overlapping seismic areas, detected SES before the onset of 25 out of 28 different but spatially grouped earthquakes. Among the 25 predicted earthquakes, 15 were preceded by SES at IOA station and IO were preceded by SES at PIR station. Because this study focuses on SES at the IOA station only, SES detected at the PIR station were not analyzed. Source mechanisms for the 28 earthquakes predicted from SES at the IOA and PIR stations are analyzed for their relationship to the SES as well as the location of these two SES stations. Other stations have not been included because they cover different areas and the data currently available from them are not abundant enough to draw any significant conclusions. A comparison of the 28 earthquakes reveals that all earthquakes with reverse mechanisms were preceded by SES at the IOA station. The parameters (i.e., polarity, magnitude, time lag, and duration) for these SES are, however, dissimilar. All normal or strike slip earthquakes west of Kefallinia Island were predicted by SES at the PIR station. South of the PIR station, all earthquakes with focal mechanism parameters similar to the earthquakes west of Kefallinia Island were also preceded by SES at the PIR station. Hence, different earthquakes with similar mechanisms were preceded by SES with different polarities and vice versa, the polarity of the SES does not seem to be affected by the earthquake's mechanism. The polarity distribution suggests a possible correlation with tectonic zones in Greece. There is also clearly a correlation between type of focal mechanism, location of the earthquake and the station at which SES preceded the event