Prominent arrivals in the coda of seismograms from the wider Alpine area can be associated with l... more Prominent arrivals in the coda of seismograms from the wider Alpine area can be associated with lateral reflections of Love waves at the northern Apennines mountain chain (Italy), where structural heterogeneity causes an abrupt contrast in phase velocity. We discuss an approach to image lateral heterogeneity from reflected surface waves using intermediate-period, threecomponent coda waveforms as sources for an adjoint wavefield that propagates the reflections backward in time. We numerically compute three-dimensional sensitivity kernels for the dependence of coda waveforms on P velocity, S velocity and density, based upon correlations between the adjoint and the regular forward wavefields. We consider synthetic coda waveforms for a simplified model of the northern Apennines, as well as real coda observations from five moderate magnitude earthquakes (M W 4.6-5.6) in the southern Alps. Wave propagation is simulated using the spectral-element method, for which a 3-D regional earth model is used in the case of real data. Single and combined event sensitivity kernels provide clear images of the reflectivity associated with the northern Apennines in kernels for density and S-wave speed. The kernels show that surface wave reflections occur near the axial zone of the mountain chain. Apart from the Apennines, the approach is able to image other smaller reflectivity patches from the coda waveforms, like the Ivrea zone in the southern Alps. Our coda misfit kernels can be integrated in a gradient-based waveform tomography, where they could enhance the sharpness of the model at lateral discontinuities.
Late wave arrivals in the intermediate period coda of regional seismograms in the wider Alpine ar... more Late wave arrivals in the intermediate period coda of regional seismograms in the wider Alpine area can be associated with lateral reflection of surface waves at the northern Apennines mountain chain (Italy). Frequently, these coda waveforms are dominated by a ...
1] During the TopoIberia experiment, a total of 26 seismic broadband stations were recording in n... more 1] During the TopoIberia experiment, a total of 26 seismic broadband stations were recording in northern Morocco, providing for the first time extended regional coverage for investigating structure and seismotectonics of the southern branch of the Betic-Rif arc, its foreland, and the Atlas domain. Here, we analyze P-to-S converted waves in teleseismic receiver functions to infer gross crustal properties as thickness and V p /V s ratio. Strong lateral variations of the crustal thickness are observed throughout the region. Crustal thicknesses vary between 22 and 44 km and display a simple geographic pattern that divides the study area into three domains: entire northwestern Morocco underlain by a thickened crust with crustal thicknesses between 35 and 44 km; northeastern Morocco affected by significant crustal thinning, with crustal thicknesses ranging from 22 to 30 km, with the shallowest Moho along the Mediterranean coast; and an extended domain of 27-34 km thick crust, farther south which includes the Atlas domain and its foreland regions. V p /V s ratios show normal values of $1.75 for most stations except for the Atlas domain, where several stations give low V p /V s ratios of around 1.71. The very sharp transition from thick crust in northwestern Morocco to thin crust in northeastern Morocco is attributed to regional geodynamics, possibly the realm of present-day subcrustal dynamics in the final stage of western Mediterranean subduction. Crustal thicknesses just slightly above 30 km in the southern domain are intriguing, showing that high topography in this region is not isostatically compensated at crustal level.
The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, NWSE ... more The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, NWSE convergence between Africa and Eurasia at a rate of 5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in ...
Seismograms are the most comprehensive and quantitative documents of ground motion produced by ea... more Seismograms are the most comprehensive and quantitative documents of ground motion produced by earthquakes. First preserved records account for more than 100 years of instrumental seismology already, outperforming the time-span covered by modern broad-band seismic networks. But their uniqueness, as a document, prior to the generalization of massive methods of copy and distribution, limits the usability and availability of the earliest seismograms for research purposes. Contemporaneous analysis of old seismograms predated fundamental developments in quantitative seismology, as well as the digital revolution, suggesting the reanalysis of these unique and valuable records with modern seismological tools for the direct calculation of earthquake source parameters, at least for the most relevant events. However, this is not straightforward: Early seismograms have been recorded at instruments with low dynamic range and narrow frequency band. Many times the complementary information required to process the records and to recover ground displacement, like instrument calibration and time accuracy, has been lost or is doubtful. In fact, procedures to make old seismograms useful for quantitative analysis are, in many aspects, similar to those needed to process and to use old macroseismic information. The present contribution reviews the main topics and methodologies leading to a proper use of old seismograms and related documents, including the location and distribution of the original seismograms and recording system information, as well as the sequence from the original paper seismogram to digital ground displacement, involving digitization, trace correction and deconvolution of the instrument response. We discuss the potential and the limitations of such treatments, and review some applications of recovered records in retrieving earthquake source parameters through full waveform analysis.
... García-Dueñas, V., Balanyá, JC and Martínez-Martínez, JM, 1992. Miocene extensional detachmen... more ... García-Dueñas, V., Balanyá, JC and Martínez-Martínez, JM, 1992. Miocene extensional detachments in the outcropping basement of the Northern Alboran Basin (Betics) and their tectonic implications. Geo-Mar. Lett., 12, 88–95. ...
The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, ∼ NW-S... more The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, ∼ NW-SE convergence between Africa and Eurasia at a rate of ∼ 5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in the Alboran-Betic region are not straightforward to understand in the context of regional shortening, and evidence complexity of deformation at the plate contact. We estimate 86 seismic moment tensors (M W 3.3 to 6.9) from time domain inversion of near-regional waveforms in an intermediate period band. Those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors. The solutions associated to the Trans-Alboran shear zone show predominantly strike-slip faulting, and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction (σ 1 at N350°E). At the N-Algerian and SW-Iberian margins, reverse faulting solutions dominate, corresponding to N350°E and N310°E compression, respectively. Over most of the Betic range and intraplate Iberia, we observe predominately normal faulting, and WSW-ENE extension (σ 3 at N240°E). From GPS observations we estimate that more than 3 mm/yr of African (Nubian)-Eurasian plate convergence are currently accommodated at the N-Algerian margin, ∼ 2 mm/yr in the Moroccan Atlas, and ∼ 2 mm/yr at the SW-Iberian margin. 2 mm/yr is a reasonable estimate for convergence within the Alboran region, while Alboran extension can be quantified as ∼ 2.5 mm/yr along the stretching direction (N240°E). Superposition of both motions explains the observed left-lateral transtensional regime in the Trans-Alboran shear zone. Two potential driving mechanisms of differential motion of the Alboran-Betic-Gibraltar domain may coexist in the region: a secondary stress source other than plate convergence, related to regional-scale dynamic processes in the upper mantle of the Alboran region, as well as drag from the continental-scale motion of the Nubian plate along the southern limit of the region. In the Atlantic Ocean, the ∼ 3.5 mm/ yr, westward motion of the Gibraltar Arc relative to intraplate Iberia can be accommodated at the transpressive SW-Iberian margin, while available GPS observations do not support an active subduction process in this area.
... Fig. 1. Distribution of permanent (filled triangles, operated by IGN, IAG, IGC, IM/FCUL, Geof... more ... Fig. 1. Distribution of permanent (filled triangles, operated by IGN, IAG, IGC, IM/FCUL, Geofon/ROA/UCM, IST, MedNet, IRIS or Observatori de l'Ebre) and temporary (open triangles, operated by the TopoIberia team or by IAG) seismic broadband stations in the IberiaMaghreb ...
The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, NWSE ... more The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, NWSE convergence between Africa and Eurasia at a rate of 5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in ...
... The macroseismic map was drawn by Rodriguez Navarro and is published in its original form in ... more ... The macroseismic map was drawn by Rodriguez Navarro and is published in its original form in Galbis (1940) and in an abridged form in Mezcua ... Nodal planes have orientations of 123°/39°/-86° (strike/dip/rake) and 298°/51°/-93°, with a near vertical P axis at 188°/83° (strike ...
Physics of the Earth and Planetary Interiors, 2008
On 29 January 2005, a MW 4.8, Imax VI–VII, earthquake occurred near the village of La Paca, SE Sp... more On 29 January 2005, a MW 4.8, Imax VI–VII, earthquake occurred near the village of La Paca, SE Spain. The aftershock sequence shows distinct heterogeneity, manifested in a non-uniform temporal decay of activity and different source mechanisms for the main shock and the largest aftershocks. We analysed 262 earthquakes of the seismic series in order to characterize the active seismic
1] During the TopoIberia experiment, a total of 26 seismic broadband stations were recording in n... more 1] During the TopoIberia experiment, a total of 26 seismic broadband stations were recording in northern Morocco, providing for the first time extended regional coverage for investigating structure and seismotectonics of the southern branch of the Betic-Rif arc, its foreland, and the Atlas domain. Here, we analyze P-to-S converted waves in teleseismic receiver functions to infer gross crustal properties as thickness and V p /V s ratio. Strong lateral variations of the crustal thickness are observed throughout the region. Crustal thicknesses vary between 22 and 44 km and display a simple geographic pattern that divides the study area into three domains: entire northwestern Morocco underlain by a thickened crust with crustal thicknesses between 35 and 44 km; northeastern Morocco affected by significant crustal thinning, with crustal thicknesses ranging from 22 to 30 km, with the shallowest Moho along the Mediterranean coast; and an extended domain of 27-34 km thick crust, farther south which includes the Atlas domain and its foreland regions. V p /V s ratios show normal values of $1.75 for most stations except for the Atlas domain, where several stations give low V p /V s ratios of around 1.71. The very sharp transition from thick crust in northwestern Morocco to thin crust in northeastern Morocco is attributed to regional geodynamics, possibly the realm of present-day subcrustal dynamics in the final stage of western Mediterranean subduction. Crustal thicknesses just slightly above 30 km in the southern domain are intriguing, showing that high topography in this region is not isostatically compensated at crustal level.
1] We use small and moderate aftershocks of the 24 February 2004 Al Hoceima earthquake as empiric... more 1] We use small and moderate aftershocks of the 24 February 2004 Al Hoceima earthquake as empirical Greens' functions (EGFs) to retrieve the rupture history of the main event. The magnitude, depth, and geometry of faulting were estimated for the main shock and 20 small and moderate aftershocks (M w between 3.9 and 5.2) computing moment tensor solutions. For the main shock, we obtain a moment magnitude of M w = 6.3 and a nearly pure double-couple source (4% compensated linear vector dipole) with predominately strike-slip faulting. The preferred fault plane has a strike of N11°E, a dip of 72°, and a rake of À17°. Fourteen aftershocks, having depths (±4 km) and faulting geometries (±11°for the strike of the preferred plane) similar to the ones found for the main event, were used as EGFs. Apparent source time functions (ASTFs) of the Al Hoceima event were retrieved at twelve regional stations using an iterative time domain deconvolution on the P waveforms. At each station, time functions are enhanced by stacking of the deconvolved waveforms for different EGFs. ASTFs show total durations of 5 to 6 s, two consecutive subevents separated by about 3 s, and only a weak effect of directivity. We further investigate the source history by inverting the set of ASTFs for the distribution of fault slip and rupture propagation. Two different source models were selected for presentation, one involving one single fault plane and the other involving two parallel fault planes. The two-fault model fits more accurately the observations and leads to a more plausible slip distribution. We estimate rupture dimensions of about 10 Â 10 km 2 and 8 Â 8 km 2 and moment magnitudes of M w = 6.2 and 6.0 for the first and second subevent, respectively.
[18] Small to moderate tectonic earthquakes are usually thought as simple shear failure events co... more [18] Small to moderate tectonic earthquakes are usually thought as simple shear failure events corresponding to double-couple force systems. Most of the computed moment tensors have small non double couple components (< 20% for 42 and< 30% for 53 out of ...
A part of the deep seismic near normal-incidence Münchberg–Vogtland–Erzgebirge (MVE) observations... more A part of the deep seismic near normal-incidence Münchberg–Vogtland–Erzgebirge (MVE) observations located in the Saxo-Thuringian Belt at the northern rim of the Bohemian Massif has been reprocessed using the method of amplitude preserving 3D prestack depth migration. The advantage of this method is its ability to deliver a quantitative and geometrically correct image of the crustal reflectivity without preference of
Time reversal of seismic wavefields recorded at active volcanoes may provide images of the source... more Time reversal of seismic wavefields recorded at active volcanoes may provide images of the source process for long-period (LP) or volcanic tremor events directly, independent of assumptions about extent, geometry and mechanics of the source. Time reversal methods involve propagating the original time series from many receiver locations back into the medium. The reversed signals then focus at their origin
1] An eleven-month deployment of 25 ocean bottom seismometers provides an unprecedented opportuni... more 1] An eleven-month deployment of 25 ocean bottom seismometers provides an unprecedented opportunity to study low-magnitude local earthquakes in the complex transpressive plate boundary setting of the Gulf of Cadiz, known for the 1755 Lisbon earthquake and tsunami. 36 relocated earthquakes (ML 2.2 to 4.8) concentrate at 40-60 km depth, near the base of the seismogenic layer in ∼140 Ma old oceanic mantle lithosphere, and roughly align along two perpendicular, NNE-SSW and WNW-ESE striking structures. First motion focal mechanisms indicate compressive stress for the cluster close to the northern Horseshoe fault termination which trends perpendicular to plate convergence. Focal mechanisms for the second cluster near the southern termination of the Horseshoe fault indicate a strike-slip regime, providing evidence for present-day activity of a dextral shear zone proposed to represent the Eurasia-Africa plate contact. We hypothesize that regional tectonics is characterized by slip partitioning. Citation: Geissler, W. H., et al. (2010), Focal mechanisms for sub-crustal earthquakes in the Gulf of Cadiz from a dense OBS deployment, Geophys.
Prominent arrivals in the coda of seismograms from the wider Alpine area can be associated with l... more Prominent arrivals in the coda of seismograms from the wider Alpine area can be associated with lateral reflections of Love waves at the northern Apennines mountain chain (Italy), where structural heterogeneity causes an abrupt contrast in phase velocity. We discuss an approach to image lateral heterogeneity from reflected surface waves using intermediate-period, threecomponent coda waveforms as sources for an adjoint wavefield that propagates the reflections backward in time. We numerically compute three-dimensional sensitivity kernels for the dependence of coda waveforms on P velocity, S velocity and density, based upon correlations between the adjoint and the regular forward wavefields. We consider synthetic coda waveforms for a simplified model of the northern Apennines, as well as real coda observations from five moderate magnitude earthquakes (M W 4.6-5.6) in the southern Alps. Wave propagation is simulated using the spectral-element method, for which a 3-D regional earth model is used in the case of real data. Single and combined event sensitivity kernels provide clear images of the reflectivity associated with the northern Apennines in kernels for density and S-wave speed. The kernels show that surface wave reflections occur near the axial zone of the mountain chain. Apart from the Apennines, the approach is able to image other smaller reflectivity patches from the coda waveforms, like the Ivrea zone in the southern Alps. Our coda misfit kernels can be integrated in a gradient-based waveform tomography, where they could enhance the sharpness of the model at lateral discontinuities.
Late wave arrivals in the intermediate period coda of regional seismograms in the wider Alpine ar... more Late wave arrivals in the intermediate period coda of regional seismograms in the wider Alpine area can be associated with lateral reflection of surface waves at the northern Apennines mountain chain (Italy). Frequently, these coda waveforms are dominated by a ...
1] During the TopoIberia experiment, a total of 26 seismic broadband stations were recording in n... more 1] During the TopoIberia experiment, a total of 26 seismic broadband stations were recording in northern Morocco, providing for the first time extended regional coverage for investigating structure and seismotectonics of the southern branch of the Betic-Rif arc, its foreland, and the Atlas domain. Here, we analyze P-to-S converted waves in teleseismic receiver functions to infer gross crustal properties as thickness and V p /V s ratio. Strong lateral variations of the crustal thickness are observed throughout the region. Crustal thicknesses vary between 22 and 44 km and display a simple geographic pattern that divides the study area into three domains: entire northwestern Morocco underlain by a thickened crust with crustal thicknesses between 35 and 44 km; northeastern Morocco affected by significant crustal thinning, with crustal thicknesses ranging from 22 to 30 km, with the shallowest Moho along the Mediterranean coast; and an extended domain of 27-34 km thick crust, farther south which includes the Atlas domain and its foreland regions. V p /V s ratios show normal values of $1.75 for most stations except for the Atlas domain, where several stations give low V p /V s ratios of around 1.71. The very sharp transition from thick crust in northwestern Morocco to thin crust in northeastern Morocco is attributed to regional geodynamics, possibly the realm of present-day subcrustal dynamics in the final stage of western Mediterranean subduction. Crustal thicknesses just slightly above 30 km in the southern domain are intriguing, showing that high topography in this region is not isostatically compensated at crustal level.
The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, NWSE ... more The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, NWSE convergence between Africa and Eurasia at a rate of 5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in ...
Seismograms are the most comprehensive and quantitative documents of ground motion produced by ea... more Seismograms are the most comprehensive and quantitative documents of ground motion produced by earthquakes. First preserved records account for more than 100 years of instrumental seismology already, outperforming the time-span covered by modern broad-band seismic networks. But their uniqueness, as a document, prior to the generalization of massive methods of copy and distribution, limits the usability and availability of the earliest seismograms for research purposes. Contemporaneous analysis of old seismograms predated fundamental developments in quantitative seismology, as well as the digital revolution, suggesting the reanalysis of these unique and valuable records with modern seismological tools for the direct calculation of earthquake source parameters, at least for the most relevant events. However, this is not straightforward: Early seismograms have been recorded at instruments with low dynamic range and narrow frequency band. Many times the complementary information required to process the records and to recover ground displacement, like instrument calibration and time accuracy, has been lost or is doubtful. In fact, procedures to make old seismograms useful for quantitative analysis are, in many aspects, similar to those needed to process and to use old macroseismic information. The present contribution reviews the main topics and methodologies leading to a proper use of old seismograms and related documents, including the location and distribution of the original seismograms and recording system information, as well as the sequence from the original paper seismogram to digital ground displacement, involving digitization, trace correction and deconvolution of the instrument response. We discuss the potential and the limitations of such treatments, and review some applications of recovered records in retrieving earthquake source parameters through full waveform analysis.
... García-Dueñas, V., Balanyá, JC and Martínez-Martínez, JM, 1992. Miocene extensional detachmen... more ... García-Dueñas, V., Balanyá, JC and Martínez-Martínez, JM, 1992. Miocene extensional detachments in the outcropping basement of the Northern Alboran Basin (Betics) and their tectonic implications. Geo-Mar. Lett., 12, 88–95. ...
The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, ∼ NW-S... more The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, ∼ NW-SE convergence between Africa and Eurasia at a rate of ∼ 5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in the Alboran-Betic region are not straightforward to understand in the context of regional shortening, and evidence complexity of deformation at the plate contact. We estimate 86 seismic moment tensors (M W 3.3 to 6.9) from time domain inversion of near-regional waveforms in an intermediate period band. Those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors. The solutions associated to the Trans-Alboran shear zone show predominantly strike-slip faulting, and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction (σ 1 at N350°E). At the N-Algerian and SW-Iberian margins, reverse faulting solutions dominate, corresponding to N350°E and N310°E compression, respectively. Over most of the Betic range and intraplate Iberia, we observe predominately normal faulting, and WSW-ENE extension (σ 3 at N240°E). From GPS observations we estimate that more than 3 mm/yr of African (Nubian)-Eurasian plate convergence are currently accommodated at the N-Algerian margin, ∼ 2 mm/yr in the Moroccan Atlas, and ∼ 2 mm/yr at the SW-Iberian margin. 2 mm/yr is a reasonable estimate for convergence within the Alboran region, while Alboran extension can be quantified as ∼ 2.5 mm/yr along the stretching direction (N240°E). Superposition of both motions explains the observed left-lateral transtensional regime in the Trans-Alboran shear zone. Two potential driving mechanisms of differential motion of the Alboran-Betic-Gibraltar domain may coexist in the region: a secondary stress source other than plate convergence, related to regional-scale dynamic processes in the upper mantle of the Alboran region, as well as drag from the continental-scale motion of the Nubian plate along the southern limit of the region. In the Atlantic Ocean, the ∼ 3.5 mm/ yr, westward motion of the Gibraltar Arc relative to intraplate Iberia can be accommodated at the transpressive SW-Iberian margin, while available GPS observations do not support an active subduction process in this area.
... Fig. 1. Distribution of permanent (filled triangles, operated by IGN, IAG, IGC, IM/FCUL, Geof... more ... Fig. 1. Distribution of permanent (filled triangles, operated by IGN, IAG, IGC, IM/FCUL, Geofon/ROA/UCM, IST, MedNet, IRIS or Observatori de l'Ebre) and temporary (open triangles, operated by the TopoIberia team or by IAG) seismic broadband stations in the IberiaMaghreb ...
The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, NWSE ... more The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, NWSE convergence between Africa and Eurasia at a rate of 5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in ...
... The macroseismic map was drawn by Rodriguez Navarro and is published in its original form in ... more ... The macroseismic map was drawn by Rodriguez Navarro and is published in its original form in Galbis (1940) and in an abridged form in Mezcua ... Nodal planes have orientations of 123°/39°/-86° (strike/dip/rake) and 298°/51°/-93°, with a near vertical P axis at 188°/83° (strike ...
Physics of the Earth and Planetary Interiors, 2008
On 29 January 2005, a MW 4.8, Imax VI–VII, earthquake occurred near the village of La Paca, SE Sp... more On 29 January 2005, a MW 4.8, Imax VI–VII, earthquake occurred near the village of La Paca, SE Spain. The aftershock sequence shows distinct heterogeneity, manifested in a non-uniform temporal decay of activity and different source mechanisms for the main shock and the largest aftershocks. We analysed 262 earthquakes of the seismic series in order to characterize the active seismic
1] During the TopoIberia experiment, a total of 26 seismic broadband stations were recording in n... more 1] During the TopoIberia experiment, a total of 26 seismic broadband stations were recording in northern Morocco, providing for the first time extended regional coverage for investigating structure and seismotectonics of the southern branch of the Betic-Rif arc, its foreland, and the Atlas domain. Here, we analyze P-to-S converted waves in teleseismic receiver functions to infer gross crustal properties as thickness and V p /V s ratio. Strong lateral variations of the crustal thickness are observed throughout the region. Crustal thicknesses vary between 22 and 44 km and display a simple geographic pattern that divides the study area into three domains: entire northwestern Morocco underlain by a thickened crust with crustal thicknesses between 35 and 44 km; northeastern Morocco affected by significant crustal thinning, with crustal thicknesses ranging from 22 to 30 km, with the shallowest Moho along the Mediterranean coast; and an extended domain of 27-34 km thick crust, farther south which includes the Atlas domain and its foreland regions. V p /V s ratios show normal values of $1.75 for most stations except for the Atlas domain, where several stations give low V p /V s ratios of around 1.71. The very sharp transition from thick crust in northwestern Morocco to thin crust in northeastern Morocco is attributed to regional geodynamics, possibly the realm of present-day subcrustal dynamics in the final stage of western Mediterranean subduction. Crustal thicknesses just slightly above 30 km in the southern domain are intriguing, showing that high topography in this region is not isostatically compensated at crustal level.
1] We use small and moderate aftershocks of the 24 February 2004 Al Hoceima earthquake as empiric... more 1] We use small and moderate aftershocks of the 24 February 2004 Al Hoceima earthquake as empirical Greens' functions (EGFs) to retrieve the rupture history of the main event. The magnitude, depth, and geometry of faulting were estimated for the main shock and 20 small and moderate aftershocks (M w between 3.9 and 5.2) computing moment tensor solutions. For the main shock, we obtain a moment magnitude of M w = 6.3 and a nearly pure double-couple source (4% compensated linear vector dipole) with predominately strike-slip faulting. The preferred fault plane has a strike of N11°E, a dip of 72°, and a rake of À17°. Fourteen aftershocks, having depths (±4 km) and faulting geometries (±11°for the strike of the preferred plane) similar to the ones found for the main event, were used as EGFs. Apparent source time functions (ASTFs) of the Al Hoceima event were retrieved at twelve regional stations using an iterative time domain deconvolution on the P waveforms. At each station, time functions are enhanced by stacking of the deconvolved waveforms for different EGFs. ASTFs show total durations of 5 to 6 s, two consecutive subevents separated by about 3 s, and only a weak effect of directivity. We further investigate the source history by inverting the set of ASTFs for the distribution of fault slip and rupture propagation. Two different source models were selected for presentation, one involving one single fault plane and the other involving two parallel fault planes. The two-fault model fits more accurately the observations and leads to a more plausible slip distribution. We estimate rupture dimensions of about 10 Â 10 km 2 and 8 Â 8 km 2 and moment magnitudes of M w = 6.2 and 6.0 for the first and second subevent, respectively.
[18] Small to moderate tectonic earthquakes are usually thought as simple shear failure events co... more [18] Small to moderate tectonic earthquakes are usually thought as simple shear failure events corresponding to double-couple force systems. Most of the computed moment tensors have small non double couple components (< 20% for 42 and< 30% for 53 out of ...
A part of the deep seismic near normal-incidence Münchberg–Vogtland–Erzgebirge (MVE) observations... more A part of the deep seismic near normal-incidence Münchberg–Vogtland–Erzgebirge (MVE) observations located in the Saxo-Thuringian Belt at the northern rim of the Bohemian Massif has been reprocessed using the method of amplitude preserving 3D prestack depth migration. The advantage of this method is its ability to deliver a quantitative and geometrically correct image of the crustal reflectivity without preference of
Time reversal of seismic wavefields recorded at active volcanoes may provide images of the source... more Time reversal of seismic wavefields recorded at active volcanoes may provide images of the source process for long-period (LP) or volcanic tremor events directly, independent of assumptions about extent, geometry and mechanics of the source. Time reversal methods involve propagating the original time series from many receiver locations back into the medium. The reversed signals then focus at their origin
1] An eleven-month deployment of 25 ocean bottom seismometers provides an unprecedented opportuni... more 1] An eleven-month deployment of 25 ocean bottom seismometers provides an unprecedented opportunity to study low-magnitude local earthquakes in the complex transpressive plate boundary setting of the Gulf of Cadiz, known for the 1755 Lisbon earthquake and tsunami. 36 relocated earthquakes (ML 2.2 to 4.8) concentrate at 40-60 km depth, near the base of the seismogenic layer in ∼140 Ma old oceanic mantle lithosphere, and roughly align along two perpendicular, NNE-SSW and WNW-ESE striking structures. First motion focal mechanisms indicate compressive stress for the cluster close to the northern Horseshoe fault termination which trends perpendicular to plate convergence. Focal mechanisms for the second cluster near the southern termination of the Horseshoe fault indicate a strike-slip regime, providing evidence for present-day activity of a dextral shear zone proposed to represent the Eurasia-Africa plate contact. We hypothesize that regional tectonics is characterized by slip partitioning. Citation: Geissler, W. H., et al. (2010), Focal mechanisms for sub-crustal earthquakes in the Gulf of Cadiz from a dense OBS deployment, Geophys.
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Papers by Daniel Stich