Three-dimensional upper mantle structure beneath the intraplate Atlas and interplate Rif mountains of Morocco

Copyright 1996, American Geophysical Union. See also: http://www.agu.org/pubs/crossref/1996/95JB03112.shtml; http://atlas.geo.cornell.edu/morocco/publications/seber1996JGR.htmWe integrate observations based on teleseismic P wave travel times and available geologic data to infer that the lithosphere beneath the intraplate Atlas mountains is thin and/or it is characterized by lower P wave velocities, while beneath the interplate Rif mountains and the adjacent Alboran Sea a previously thickened lithosphere has been delaminated into the upper mantle. Using surface geology and geochronology data, previous studies have proposed that lithospheric delamination took place in this region. In thsi study we show through analysis of teleseismic P wave residuals the existence of a high velocity (>3%) upper mantle body, which is interpreted to be the delaminated, rigid lithosphere. This high-velocity layer is overlain by a very low velocity uppermost mantle material replacing the delaminated lithosphere. Teleseismic P waves recorded by a recently installed digital seismic network and an older analog network in Morocco provide the residuals database. A total of 734 P wave residuals from 92 selected teleseismic earthquakes are used to document the spacial pattern of upper mantle velocity structure beneath northern Morocco and the Alboran Sea. Subsequent use of these residuals in a tomographic inversion scheme produced a three-dimensional velocity image of the upper mantle. We infer from the P residuals that strong upper mantle velocity anomalies exist beneath both the Rif and Atlas regions. The Rif stations show negative residuals (- 1-1.5s) for ray paths from the east and northeast and show positive residuals (~ 1-1.5s) for raypaths from the northwest and southwest. Tomographic results indicate the existence of a high-velocity body (~ 3% higher velocities) in the upper mantle beneath the eastern Rif and Alboran Sea, extending approximately from subcrustal depths down to a depth of at least 350 km. In the western Rif, however, 1-2% lower velocity material is imaged in the upper mantle. The residuals of the Atlas stations also show azimuthal variations. In general, most of the P waves that travel beneath the High and Middle Atlas have about 0.5-1.0s delays. In contrast, the rays that travel beneath the northwestern margin of the Atlas mountains and the adjacent Moroccan Meseta area show negative residuals (~1s), suggesting that higher velocity material exists beneath the platform area adjacent to the Atlas mountains. Tomographic results indicate that beneath most of the Atlas system the uppermost mantle has about 1% lower velocities. Beneath the Alboran Sea region, however, reported low uppermost mantle Pn velocities contrast strongly with higher velocity upper mantle velocities obtained by our analysis. Low-velocity uppermost mantle beneath the Alboran Sea underlain by a high-velocity upper mantle material is used to support earlier interpretations of lithospheric delamination beneath the Rif and Alboran Sea Regions. The enigmatic occurrence of subcrustal earthquakes in these regions is also consistent with this active delamination mechanism

Deterministic Seismic Hazard Assessment for North Morocco

The purpose of this work is to evaluate the regional seismic hazard for Morocco, following the deterministic approach proposed by Costa et al [1], based on the computation of complete P-SV and SH synthetic seismograms. The input for the computations is represented by source and structural models. Seismic sources are parameterized using the knowledge about past seismicity and the tectonic regime. The regional structural model we adopted is the one proposed by Cherkaoui [2], modified in its shallower part to account for the effects of the uppermost sedimentary layers. Maps of peak acceleration, velocity, and displace- ments are used for the general representation of the hazard. Accelerations are in good agreement with the values determined by Jimenez et al [3] with the standard probabilistic approach

Sn to Sg conversion and focusing along the Atlantic margin, Morocco: Implications for earthquake hazard evaluation

Copyright 1993, American Geophysical Union. See also: http://www.agu.org/journals/gl/; http://atlas.geo.cornell.edu/morocco/publications/seber1993.htmDigital data from a telemetered, short-period seismic network in Morocco provide a new perspective for understanding the cause of severe shaking and macroseismic reports in Morocco produced by large, offshore earthquakes located along the Azores-Gibralter seismic zone. Even though the earthquake epicenters are 500-1000 km away from the Moroccan coast, historical records show that such events are capable of producing considerable damage in inland areas. We analyze 15 earthquakes that occurred in this region. The records show multiple S phases with varying frequencies and amplitudes. The S phase with the largest amplitude, usually misinterpreted as Sn, has a phase velocity of 4.2-4.4 km/s. We show that these S arrivals can best be explained as Sn to Sg converted phases. Calculated locations of the conversion points for these phases exhibit two distinct zones almost parallel to the Atlantic coastline: one is located along the passive continental margin and the other is located about 100 km inland from the coastline. We interpret these two zones to be regions where a sudden change in crustal thickness occurs. Such zones act to focus and magnify the amplitudes of seismic phases. This interpretation explains the unusually strong felt reports within Morocco from such distant offshore events, and it has a significant effect on earthquake hazard evaluation and mitigation studies

The Al Hoceima earthquake of May 26, 1994 and its aftershocks: a seismotectonic study

The present paper focuses on the moderate earthquake that occurred on May, 26, 1994, at 8 h 27 min, which caused great damage and two deaths. The epicentre of the main shock was located at 35.280N, 3.990W. The focal depth was 13 km, and the magnitude (Md) attained 5.6. A field survey of the earthquake effects showed that the maximal intensity (VIII-IX EMS) follows an elongated corridor trending NNE-SSW, where 80% of the constructions were destroyed. During the 14-day survey carried out with the help of the temporary network established in the area, 512 shocks were located. The best constrained epicentres (68) are shallow and are largely distributed over a NNE-SSW trending cluster along an almost-vertical plane. Focal mechanisms determined for the main shock and for the 7 largest aftershocks, correspond to strike-slip faulting with a reverse (main shock and one aftershock) or normal component (6 events). The P axes have a NNW-SSE trend, with variable plunge, whereas the T axes are ENE-WSW with a slight plunge. The state of stress determined with the help of these mechanisms corresponds to a strike-slip regime with s1 oriented NNW-SSE and s3 ENE-WSW, which is in conformity with previous studies. The present study also shows that the Nekor fault remained inactive during the seismic crisis of 1994, as during the previous surveys, and this casts some doubt on the present-day role of this major fault. Instead, as proposed by some authors, a seismic zone trending NNE-SSW may be related to the faults of the same trend that appear to cross the Al Hoceima area towards the Alboran Sea

The Al Hoceima earthquake of May 26, 1994 and its aftershocks: a seismotectonic study

The present paper focuses on the moderate earthquake that occurred on May, 26, 1994, at 8 h 27 min, which caused great damage and two deaths. The epicentre of the main shock was located at 35.280N, 3.990W. The focal depth was 13 km, and the magnitude (Md) attained 5.6. A field survey of the earthquake effects showed that the maximal intensity (VIII-IX EMS) follows an elongated corridor trending NNE-SSW, where 80% of the constructions were destroyed. During the 14-day survey carried out with the help of the temporary network established in the area, 512 shocks were located. The best constrained epicentres (68) are shallow and are largely distributed over a NNE-SSW trending cluster along an almost-vertical plane. Focal mechanisms determined for the main shock and for the 7 largest aftershocks, correspond to strike-slip faulting with a reverse (main shock and one aftershock) or normal component (6 events). The P axes have a NNW-SSE trend, with variable plunge, whereas the T axes are ENE-WSW with a slight plunge. The state of stress determined with the help of these mechanisms corresponds to a strike-slip regime with s1 oriented NNW-SSE and s3 ENE-WSW, which is in conformity with previous studies. The present study also shows that the Nekor fault remained inactive during the seismic crisis of 1994, as during the previous surveys, and this casts some doubt on the present-day role of this major fault. Instead, as proposed by some authors, a seismic zone trending NNE-SSW may be related to the faults of the same trend that appear to cross the Al Hoceima area towards the Alboran Sea