The geometry of Turkish strikeslip faults has been reviewed and described. From this data set it appears that fault geometry (the distribution of discontinuities such as bends and stepovers along the main fault trace) plays an important role in controlling the location of large earthquake rupture segments along the fault zones. Large earthquake ruptures generally do not propagate past individual stepovers that are wider than 5 km or bends that have angles greater than about 30 ø . More important than the size of each discontinuity, however, is the total "geometric pattern," i.e., the distribution of adjacent bends and stepovers based not only on distance from one another but also on relative discontinuity size. Certain geometric patterns (restraining single or double bends) are particularly common and can be viewed as responsible for strain accumulation along portions of the fault zone. Fault geometry not only plays a role in the extent of earthquake rupture but also in characteristics of earthquake behavior. For example, large earthquake epicenters often occur near restraining bends or double bends. Furthermore, aftershocks and swarm activity can often be associated with releasing areas
Electron auroral energy flux is characterized by electron hemispheric power (Hpe) estimated since 1978 from National Oceanic and Atmospheric Administration (NOAA) and Defense Meteorological Satellite Program (DMSP) satellites after the estimates were corrected for instrumental problems and adjusted to a common baseline. Similarly, intersatellite adjusted ion hemispheric power (Hpi) estimates come from one MetOp and four NOAA satellites beginning in 1998. The hemispheric power (Hp) estimates are very crude, coming from single satellite passes referenced to 10 global activity levels, where the Hpi estimates are the difference between the total and the electron Hp (Hpi = Hpt‐Hpe). However, hourly averaged NOAA/DMSP Hpe and Hpi estimates correlate well with hourly Polar Ultraviolet Imager (UVI) Hpt and Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) far ultraviolet (FUV) Hpe and Hpi estimates. Hpe winter values were larger than summer values ∼65% of the time (when geomagnetic activity was moderate or higher), and Hpe were larger in the summer ∼35% of the time (typically for low geomagnetic activity). Hpe was ∼40% larger at winter solstice than summer solstice for the largest Hp from mostly nightside increases, and Hpe was ∼35% larger in summer than winter for the smallest Hp owing to dayside auroral enhancements. Ion precipitation differed from electron precipitation because it was almost always larger in summer than winter. Hpe and Hpi increased with Kp, solar wind speed (Vsw), and negative Interplanetary Magnetic Field (IMF) Bz, similar to previous studies. Hpi also increased strongly with positive Bz. For quiet conditions, Hpe increased with increasing 10.7‐cm solar flux (Sa), while Hpi increased with Sa up to Sa ∼115 for all conditions.
In this study we investigate crustal and uppermost mantle physical properties that characterize some of the continental plateaus of the Middle East. This is done as part of a larger effort to map and compare high‐frequency wave propagation at regional distances across the earth's continental plateaus. Thousands of short‐period WWSSN seismograms recorded at stations located in the Middle East and produced by earthquakes with epicentral distances less than about 20° were examined visually in an effort to study lateral variations of high‐frequency (0.5–2 Hz) seismic wave propagation across this area, particularly to the north of the zone of continental collision between the Africa‐Arabian and Eurasian plates. Variations of frequencies and amplitudes of Sn and Lg relative to P are examined and mapped throughout the region, and this work is supplemented by a study of velocities of Pn, Sn, and Lg. Sn amplitude variations are very striking in this area. An important observation of this study is that Sn propagates efficiently beneath a major part of the Turkish and Iranian plateaus. Sn is strongly attenuated, however, in the northernmost portion of the plateaus south of the Black and Caspian seas and in an area between the two seas. These regions are characterized, in general, by active tectonism, including volcanism, faulting, and folding. However, this active tectonism is not restricted to the areas of high Sn attenuation but appears to extend beneath other parts of the Iranian and Turkish plateaus. Patterns of lateral variations in the propagation of Lg are not as consistent as those for Sn. Lg propagates efficiently across Turkey, Iran, and adjacent regions, but the Lg waves that cross the Turkish and Iranian plateaus are weak and have relatively long predominant periods of about 2–5 s. The Lg phase is not observed when the path of propagation crosses the southern Caspian and the Black seas, consistent with the evidence of oceanic‐type crustal structure beneath these seas. Lg is also not observed from subcrustal events located in the Hindu Kush region. The velocity of Pn beneath most of the plateau areas, and particularly in the regions of Sn attenuation, ranges between 8.0 and 8.2 km/s. Velocities for Sn and Lg throughout the Middle East are about 4.5 and 3.4 km/s, respectively. It is possible to interpret the efficient Sn propagation and the relatively normal uppermost mantle P and S velocities beneath a major part of the Turkish‐Iranian plateau as indicating a partial underthrusting of the Arabian continental plate beneath the Iranian and Turkish continental blocks. Alternatively, the mapped regions of efficient Sn propagation and high Sn attenuation may result from a differential cooling of a past thermal anomaly. In this case the isostatic compensation of the plateaus could be due to an anomalously hot uppermost mantle that developed behind the mid‐to‐upper Tertiary subduction zone(s) in this region and to a subsequent crustal shortening after the collision of the Arabian and Eurasian plates. Our results cannot ...
Seismotectonic characteristics of the transition area between the Zagros continental collision zone and the Makran ocean‐continent convergence zone (the ‘Oman Line’) are carefully examined. A northeast trending zone of earthquakes terminates the Zagros belt of seismicity just west of the Oman Line. This seismic zone extends northeastward beyond the Main Zagros Thrust and coincides with a surface escarpment clearly visible on LANDSAT imagery. Events with accurately determined depths along this seismic zone occurred in the upper 20 km of the crust and possibly show a shallow (<10°) northeastward dip. East of the Oman Line, the Zendan‐Minab fault system is relatively aseismic. The Oman Line region may be characterized by underthrusting of a wedge of Arabian shelf edge beneath Iranian crust or by an indentation of Arabia into the Iranian crustal block as a promontory. Available seismological and geological data cannot uniquely distinguish between these two possible tectonic settings. The area located to the northwest of the Oman Line region, now a zone of continental collision, appears to have been characterized by an episode of Neogene subduction. A well‐located, intermediate‐depth earthquake occurred in this area to the northeast of the Main Zagros Thrust on November 9, 1970. It was accurately located at 107‐km depth beneath a line of Quaternary volcanoes. Its focal mechanism may indicate downdip tension, with nodal planes that strike closely parallel to the trend of the Zagros arc. Comparison of this event and other neotectonic features in this part of Iran with those in other active convergent zones suggests that a descending oceanic lithosphere beneath the Zagros volcanic arc may still be attached to the colliding Arabian plate.
Leveling data are combined with seismic observations from the November 23, 1977, Ms = 7.4 Caucete earthquake in western Argentina, in order to constrain models of crustal deformation in the Sierras Pampeanas tectonic province. The spatial pattern and amount of uplift ( ∼1 m) that accompanied this earthquake suggest 4 m of slip on a north‐south trending, west dipping fault with a dip of 35°, a downdip width of 24 km, and a length of 80 km. The top of the fault is 17 km below the surface. These fault parameters are generally consistent with the seismic focal mechanism and moment of the main shock, which occurred near the top of the fault. The fault length is derived from the distribution of local and relocated teleseismic aftershocks and from analysis of the main shock (a double event in which the two shocks occurred 21s apart and were separated by 65 km along the proposed fault plane strike). The fault width is determined from the locations of the larger hypocenters along the preferred fault plane. This fault is buried beneath the north‐south striking Sierra Pie de Palo, a Laramide style crystalline basement uplift in the western part of the Sierras Pampeanas province. The buried fault model is consistent with the lack of observed coseismic surface faulting and the broad, east‐west symmetric character of the overlying Sierra Pie de Palo. By combining the 1977 earthquake observations with topographic relief and with estimates of the age of deformation for the Sierras Pampeanas basement uplifts, we calculate a shortening rate of the order of 10 mm/yr for the past 5 m. y. for this Andean back arc province and estimate an average recurrence time of the order of 1000 years for earthquakes of this magnitude beneath the Sierra Pie de Palo.
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