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We study the two North African earthquakes, May 26, 1994 (Mw=6.0) and February 24, 2004 (Mw=6.4) earthquakes that affected the Al Hoceima region of northern Morocco with the available In-SAR data collected from both the ascending and descending orbits. Being the strongest earthquakes ever to be recorded instrumentally in the region, the analysis of the earthquakes has an important role in the tectonics of the region. The Rif Cordillera belong to the E-W trending thrust-and-fold system of north Africa that results from the collision between Africa and Eurasia. The system includes the Tell Atlas mountain ranges of Algeria and Tunisia along the Mediterranean coast to the east, and forms a collision tectonics strip along the African-Eurasian plate boundary. The ongoing shortening rate between the two plates decreases towards the west from 6.3 to 2.3 mm/yr from Sicily to northern Morocco. The seismicity along the boundary is rather complex and varies significantly from west to east. The Al Hoceima sequence indicate that the Rif is being deformed under a strike-slip tectonic regime. However adjacent regions in northern Algeria to the east and the Gulf of Cadiz to the west are subject to thrust faulting. Neotectonic features of the Rif consist of the major Nekor and Jebha, NESW trending left-lateral strike slip faults accompanied by N-S trending normal faults that form a graben-like structure east of Al Hoceima and a conjugate network of relatively small NW-SE and NE-SW strike-slip faults. The transpressive tectonics and existence of a complex fault network with different types of faulting in the Rif probably reflect the rapidly changing local tectonic regime with block rotations during the Neogene and Quaternary. Several strong earthquakes are known to have occurred in the historical times. The exact location, kinematics and relationships between the 1994 and 2004 earthquakes are poorly known since neither of them produced surface ruptures. Using Synthetic Aperture Radar interferometry (InSAR) we mapped the surface displacement field of the two earthquakes to characterize their seismic source parameters. The availability of ascending and descending interferograms for both earthquakes allows us to constrain their rupture parameters with high confidence. We used European Space Agency's (ESA) ERS and Envisat SAR data, respectively. While the ERS Level-0 (raw) SAR data were processed using the JPL ROI̱PAC software, the Envisat Level-1 (single-look) ASAR data were processed using DORIS SAR processing software, and precise satellite orbits from Delft University. The effect of topography which depends on the perpendicular separation between orbital trajectories is removed from the interferograms using the SRTM 3-arc-second (∼90 m) posting digital elevation model. The interferograms were also filtered using a weighted power spectrum technique. We modelled the manually unwrapped fringes derived from the processed interferograms by using slip inversions on triangular fault patches instead of commonly used rectangular ones which enabled us to use nonplanar more realistic fault models for the earthquakes. … [ABSTRACT FROM AUTHOR]

The North Anatolian Fault (NAF) forms the northern boundary of the Anatolian plate and is characterized by a right-lateral strike slip motion. The fault extends between 26° and 40° E longitudes and forms a broad arc roughly parallel to the coast of the Black Sea following a former suture zone. The only visible perturbations to the smooth geometry of the NAF are, at around 34-37°E longitude, two main splay faults named Ezinepazar-Sungurlu Fault and Suluova Fault with several related minor fault segments that bifurcate from the main fault line, possibly due to the convexity of the NAF geometry. These secondary fault structures show remarkable morphological expressions accompanied with elongated basin formations and significant microseismicity. The NAF, together with these splay faults, form a broad wedge-shaped shear zone called Amasya Shear Zone (ASZ) where major growing cities of central Anatolia such as Amasya, Çorum and Tokat are located. These cities, which today have total a population close to 500000 with significant industrial infrastructure, were subject to destructive earthquakes during both historical (such as 1579, 1794 and 1668) and instrumental (such as 1939, 1942a, 1942b, 1943 and 1996) seismic periods. Recent paleoseismological studies revealed the rupture history of NAF in a time span of 2000 years (for a review see Hartleb et al. 2006). However, the historical earthquake database of the study area contains unlocated destructive earthquakes (such as 1579 and 1794 events) which are proposed to be nucleated on the splay faults of the ASZ. This statement reveals that there is an unevaluated seismic hazard potential of the region. In this work, a 6 step procedure is followed for a complete earthquake hazard assessment for the study area: (1) First, the faults of ASZ are mapped in detail with field studies to understand the behavior and segmentation of the major fault systems (SuFS and EzSFS). (2) A detailed catalogue of instrumental seismic activity is prepared to understand the seismicity of the region and to define the local clusters concentrated on the fault segments. (3) Information of historical earthquakes are investigated through the literature and field study in major cities to identify the possible related fault segment, this is accomplished by comparing intensity distributions based on historical data and modeled scenarios. (4) The results of recently published analysis of a local GPS campaign (Yavasoglu et. al., 2009) is used to determine the annual slip rate resolved on the overall shear zone to estimate the earthquake recurrence interval. (5) For each active fault segment, the maximum magnitude of a possible earthquake is calculated with the relations between the rupture length and magnitude (Wells and Coppersmith, 1994). (6) GIS based earthquake scenarios based on attenuation relations (Tüysüz, 2003) are prepared to model the geographic intensity distribution for each calculated earthquake. … [ABSTRACT FROM AUTHOR]

Large earthquakes (M>7) occur with intervals more than hundreds of years. Quantitative data of large earthquakes are available through recordings of modern (digital) seismographs since the last 40 years. However historical (analog) seismograMs encompass nearly 100 years of seismicity and document significant large earthquakes of the world. Contemporary analyses of old seismograMs predate fundamental developments in quantitative seismology, while present techniques and methods in modern seismology allow comprehensive analysis of the earthquake phenomena (Kanamori and Brodsky, 2004). A reanalysis of old seismograMs, integrated with modern methods will expand considerably the instrumental period of earthquake seismology and may provide key information for regional tectonics, kinematics of faults, earthquake parameters, recurrence intervals and seismic hazard (Batllo et al., 2008).In this work, it is here provided a short guideline how to collect and process historical seismograMs by presenting a case study of the 1912 earthquake sequence along the North Anatolian Fault. The 9 August 1912 Mürefte earthquake (Ms=7.4) occurred at an early stage of seismological research. At that time, at least one primitive seismograph was active at 143 stations. 73 seismic records have been collected for the 9 August, 10 August and 13 September 1912 shocks, which occurred along the Ganos Fault. The majority of the records are from European stations located northwest and west of the epicentre. However, other recordings from Japan, Australia and Russia were also obtained. The SISMOS online database was the major source of the collected seismograMs.The TESEO software has been used to vectorize and correct the waveforMs and to convert them to SAC files. Signals were processed using the Empirical Green Function approach proposed by Vallee (2004), which avoids complexities due to unknown instrument parameters. The method uses the signal of a smaller event to model the Green function of the main earthquake. Comparable signal pairs 9 August and 13 September shocks from the Bergen, Ebro, and Taranto stations were digitized and corrected. 15 digital seismic signals have been modelled with the related method. Due to limitations by the applied method and majorly because of inadequate signal corrections our modelling revealed reliable results only for the Taranto station. The modelling of the 9 August and 13 September shocks provide a relative source time function and indicate that the moment ratio between the two shocks is about 30 which corresponds to 1 degree difference in magnitude and infer 40 second source duration for the 9 August. In addition to the modelling, we use the P-wave polarities at 5 stations to construct a focal mechanism. The vertical component seismograMs from Göttingen, Toledo, Ebro, Pulkovo and Hongo and field based N68°E fault strike allow us to construct the focal mechanism solution for the 9 August 1912. The pure strike slip solution we obtained is in agreement with the known fault kinematics and slip measurements. The 40 second source duration implies ∼ 120 km coseismic rupture length if a unilateral rupture propagation at 3 km/sec is attributed to the 9 August earthquake. On the other hand, the size of the 9 August shock corresponds to 1.5 x 1027 dyn*cm seismic moment which also suggests a 120 ± 20 km rupture length, assuming an average slip of 2.5 m and a fault width of 15 km. The size of the 13 September shock (Ms=6.9) implies 30 ± 10 km of surface rupture with similar fault parameters. … [ABSTRACT FROM AUTHOR]