Ahmet M. AKOĞLU, Ziyadin ÇAKIR

1994-2004 El Hüseyma (Fas) depremleri: eşlenik faylanmanın SAR interferometrisi ile tayini

Fas 'ın El Hüseyma şehri 10 yıl arayla 26 Mayıs 1994 ve 24 Şubat 2004 tarihlerinde meydana gelen iki deprem (Mw=6.0; Mw^-6.5) ile sarsılmıştır. Gömülü doğrultu atımlı faylarda meydana gelen bu depremlerde yüzey kırığı oluşmamıştır. Aktif doğrultu atımlı faylar boyunca gözlenen morfotektonik yapıların topoğrafyada çok belirgin olmaması ve sismik gözlemlerin yetersizliği nedeniyle bu iki depremin hangi faylar üzerinde oluştuğu, kinematikleri ve birbirleri arasındaki ilişki ortaya çıkartılamamıştır. Bu çalışmada Avrupa Uzay Ajansı 'nın (ESA) Envisat ve ERS uydularına ait radar görüntüleri Sentetik Açıklık Radar İnterferometrisi (InSAR) yöntemi ile işlenip bu iki depremin yeryüzünde meydana getirdiği yüzey deformasyonu farklı bakış açılarından haritalanmış ve atım dağılımları modellenmiştir. Elde edilen kosismik interferogramlar ve nihai fay modelleri depremlerin kör eşlenik faylar üzerinde meydana gelmiş olduğunu; 1994 depreminin sol yanal olup K23°D doğrultusunda, 2004 depreminin ise sağ yanal ve K45°B doğrultusunda olduğunu göstermektedir. Bu sonuç önceki araştırmalarda sismolojik analizlerden elde edilen çıkarımlar ile çelişmektedir. Son zamanlarda oluşan doğrultu atımlı faylarla ilişkili deprem aktivitesi göstermektedir ki Kuzey Afrika-Avrasya levha sınırındaki Rif bölgesi doğusunda ve batısında bindirme faylarıyla ilişkili depremlerin etkisi altındaki kuzey Cezayir ve Kadiz bölgelerinden sismotektonik açıdan farklılık göstermektedir. El Hüseyma ve civarında topoğrafyadaki hakim morfotektonik yapıların halen sıkışma rejimi ürünleri olmaları bu bölgedeki D-B yönelimli açılma ile uyumlu doğrultu atımlı tektonik rejimin yakın zamanda başladığı fikrini desteklemektedir.

The 1994-2004 Al Hoceima (Morocco) earthquake sequence: conjugate fault ruptures deduced from InSAR

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 InSAR 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, NE-SW trending left-lateral strike slip faults accompanied by N-S trending normal faults that form a gra-ben-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 m complex fault network with different types offaulting 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 ROIJPAC 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 non-planar more realistic fault models for the earthquakes.Analysis of the interferograms and subsequent elastic modeling suggest that the two mainshocks occurred on blind conjugate strike-slip faults; the 1994 quake on a N23°E trending left-lateral and 2004 quake on a N45°W trending right-lateral fault. It is worthwhile to mention that, especially for the 2004 event, InSAR result contradicts previous inferences on the kinematics, location and rupture geometry of the earthquakes deduced from conventional analyses of seismic waveforms and aftershocks distribution which suggest a left-lateral fault plane instead of a right-lateral one.The InSAR analysis reveals the fragmentation of the Rif Mountain throughout a complex network of conjugate blind faults consistent with the transpression tectonics along the plate boundary in North Africa. Although the two earthquakes took place in the Rif thrust-and-fold belt, the late Quaternary deformation indicates E-W extension in agreement with the NW-SE and NE-SW trending conjugate strike-slip faulting.

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