Intermediate and deep earthquakes in Spain

Recent improvements in the seismological networks on the Ibero-Maghrebian region have permitted estimation of hypocentral location and focal mechanisms for earthquakes which occurred at South Spain, Alboran Sea and northern Morocco of deep and intermediate depth, with magnitudes between 3.5 and 4.5. Intermediate depth shocks, range from 60 to 100 km, with greater concentration located between Granada and Málaga. Fault-plane solutions of 5 intermediate shocks have been determined; they present a vertical plane in NE-SW or E-W direction. Seismic moments of about 10¹⁵ Nm and dimensions of about 1 km have been determined from digital records of Spanish stations.P-wave forms are complex. This may be explained by the crustal structure near the station, discontinuities in the upper mantle and inhomogeneities near the source. Deep activity at about 650 km has only 3 shocks since 1954 (1954, 1973, 1990). Shocks are located at a very small region. Fault-plane solutions show a consistent direction of the pressure axis dipping 45° in E direction. For the 1990 shock seismic moment is 10¹⁶ Nm and dimensions 2.6 km. TheP-waves are of simpler form with a single pulse. The intermediate and deep activities are not connected and no activity has been detected between 100 and 650 km. The intermediate shocks may be explained in terms of a recent subduction from Africa under Iberia in SE direction. The very deep activity must be related to a sunk detached block of lithospheric material still sufficiently cold and rigid to generate earthquakes.     

Velocity field and tectonic strain in Southern Spain and surrounding areas derived from GPS episodic measurements

The goal of this paper is to study the velocity field and deformation parameters in Southern Spain and surrounding areas (Ibero-Maghrebian region) using GPS episodic measurements. Results are compared to those previously published as well as deformation parameters derived from seismic data. For this purpose, a geodetic GPS network of 12 stations was observed during eight field campaigns from 1998 to 2005 by the San Fernando Naval Observatory (ROA), Spain. Relative GPS velocities in the Gulf of Cadiz with respect to the stable part of Eurasia are ～4.1 mm/yr in a NW–SE to NNW–SSE direction. In the Betics, Alboran Sea and North of Morocco, velocities are ～4.4 mm/yr in a NW–SE direction, and they are ～2.3 mm/yr in a N–S direction in the eastern part of the Iberian Peninsula. These results are in agreement with the anticlockwise rotation of the African plate. GPS strain tensors are determined from the velocity model, to obtain a more realistic crustal deformation model. The Gulf of Cadiz is subjected to uniform horizontal compression in a NNW–SSE direction, with a rotation to N–S in the Alboran Sea and Northern Morocco. An extensional regime in a NW–SE direction, which rotates to W–E, is present in the Internal Betics area. In the Betic, Alboran Sea and North of Morocco regions we compare seismic deformation rates from shallow earthquakes with the determined GPS deformation rates. The comparison indicates a seismic coupling of 27%, while the remaining 73% might be generated in aseismic processes. Deformations measured in the Ibero-Maghrebian region with GPS could be interpreted in terms of either elastic loading or ductile deformation.     

Source mechanism studies of earthquakes in the Ibero-Maghrebian region and their tectonic implications

Seismicity of the Ibero-Maghrebian region includes the occurrence of shallow, intermediate depth, and very deep earthquakes. This is a very rare occurrence for a region not associated to an active subduction zone. Detailed studies of the source mechanism of these three types of earthquakes have been made possible through the collaboration with Prof. Madariaga. They give important information about the complex tectonic of the region. Shallow earthquakes at the west and east ends of the region have predominant reverse faulting with NW-SE trending horizontal pressure axes. The center part is the most tectonically complex. At the Strait of Gibraltar, there is a change on focal mechanisms from reverse faulting to strike-slip motion in northern Morocco, conserving the horizontal compression on NW-SE direction. In the Alboran Sea, mechanisms are of normal faulting with E-W trending horizontal tension axes, and in south Spain, mechanisms are of mixed solutions. The intermediate depth earthquakes (40–130 km) are located at both sides of the Strait of Gibraltar, at the western part distributed in E-W direction. The most important concentration, however, is located at the east of Gibraltar in a N-S trending thin vertical body and has different mechanisms. The very deep earthquakes (650 km) are concentrated at a small volume, and their mechanism corresponds to N-S vertical planes or horizontal ones. A tectonic model for the region is presented to explain the shallow, intermediate, and deep earthquakes.     

Asymmetric Delamination and Convective Removal Numerical Modeling: Comparison with Evolutionary Models for the Alboran Sea Region

Convective removal and mantle delamination are geodynamical mechanisms proposed to explain the presence of extension in the Alboran Sea within a regional context of compression. Using a new thermo-mechanical algorithm, we present here a quantitative evaluation and comparison of conceptual models based on these geodynamical mechanisms. In contrast to the in situ convective removal process, the laterally propagating delamination mechanism is shown here to be consistent with first-order features of the Alboran Sea such as the thinning/thickening distribution, intermediate-depth seismicity and upper mantle structure imaged by seismic tomography. The lower crust is predicted to reach depths of 100–150 km in some areas, due to mechanically-driven viscous drag of the downwelling mantle.     

Stress fields in the Iberian-Maghrebi region

This study concerns the present stressfield between the Eurasian and Africanplates in the Iberian-Maghrebi region(Portugal, Spain, Morocco, Algeria andTunisia). In addition to an up-to-datecatalogue of earthquakes in this area, acatalogue of the focal mechanisms composedof 486 solutions of fault planes,standardized in terms of notation andinformation type, was used. These data wereused applying the right-dihedron method ofAngelier and Mechler (1977), to obtaindifferent zones with homogeneous stress.The results obtained for shallowearthquakes (h < 30 km) coincide, in themajority of cases, with the general stressfields proposed by numerous authors forthis region, according to which there isNW-SE compression. However, the stressorientation appears to vary in certainareas, perhaps perturbed by the opening ofthe Atlantic Ocean, the approach of Iberiaand Africa, or the extension of the AlboranSea.For the intermediate earthquakes (30 < h< 150 km) no general pattern was found,and the P and T axes seem to be randomlyoriented for the depth intervalsconsidered. For the very deep earthquakes(h > 600 km), however, the P axis lies ina NNW-SSE direction, dipping towards theSSE, while the T axis is subhorizontal in aNE-SW direction.The determinations from the focalmechanisms highlight the existence of aregional stress field with a subhorizontalcompression axis trending NW-SE. Superimposed are others that specificallyaffect particular sectors; these arerelated to the opening of the AtlanticOcean, the extension of the BeticCordillera and the Alboran Sea, and eventhe present compression between the Iberianand European plates.     

Seismic anisotropy and velocity structure beneath the southern half of the Iberian Peninsula

Travel times of 11,612 Pn arrivals collected from 7675 earthquakes are inverted to image the uppermost mantle velocity and anisotropy structure beneath the southern half of the Iberian Peninsula and surrounding regions. Pn phases are routinely identified and picked for epicentral distances from 200 to 1200 km. The method used in this study allows simultaneous imaging of variations of Pn velocity and anisotropy. The results show an average uppermost mantle velocity beneath the study area of 8.0 km/s. The peninsular area covered by the Iberian massif is characterized by high Pn velocity, as expected in tectonically stable regions, indicating areas of the Hercynian belt that have not recently been reactivated. The margins of the Iberian Peninsula have undergone a great number of recent tectonic events and are characterized by a pronouncedly low Pn velocity, as is common in areas greatly affected by recent tectonic and magmatic activity. Our model indicates that the Betic crustal root might be underlined by a negative anomaly beneath the southeastern Iberian Peninsula. In the Atlantic Ocean, we find a sharp variation in the uppermost mantle velocities that coincides with the structural complexity of the European and African plate boundary in the Gulf of Cadiz. Our results show a very pronounced low-velocity anomaly offshore from Cape San Vicente whereas high velocities are distributed along the coast in the Gulf of Cadiz. In the Alboran Sea and northern Morocco, the direction of the fastest Pn velocity found is almost parallel to the Africa–Eurasia plate convergence vector (northwest–southeast) whereas to the north, this direction is almost parallel to the main trend of the Betic Cordillera, i.e. east–west in its central part and north–south in the curvature of the Arc of Gibraltar. This suggests that a significant portion of the uppermost mantle has been involved in the orogenic deformation that produced the arcuate structure of the Betic Cordillera. However, we assume that the Neogene extension had no major influence on a lithospheric scale in the Alboran Sea. Our results also show a quite complex pattern of anisotropy in the southwest Iberian lithospheric mantle since the relationship between the direction of fastest Pn velocity and major Hercynian tectonic trends cannot be directly established.     

Source properties of the 1997–98 Central Italy earthquake sequence from inversion of long-period and broad-band seismograms

We study source properties of the main earthquakes of the 1997–98 Umbria-Marche (central Italy) sequence by analysis of regional-distanceand teleseismic long period and broadband seismograms recorded by MedNet and IRIS/GSN stations. We use a modified Harvardcentroid-moment tensor (CMT) algorithm to allow inversion of long period waveforms, primarily Rayleigh and Love waves, for small earthquakes (4.2 M_W 5.5) at local to regional distances (<15^°). For the seven largest earthquakes (M_W>5.2) moment tensors derived from local and regional data agree well with those determined using teleseismic waveforms and standard methods of analysis. We also determine moment tensors for a foreshock and 12 other aftershocks, that were too small for global analysis. Focal depth and rupture propagation are analyzed for three largest shocks by inversion of teleseismic broadband body waves. The earthquakes are generally located at shallow depth (5 km or shallower) and are characterized by normal faulting mechanisms, with a NE-SW tension axis. The presumed principal fault plane dips at a shallow angle towards the SW. Only one of the events analyzed has an entirely different faulting geometry, indicating instead right-lateral strike-slip motion on a plane approximately E-W, or left-lateral faulting on a N-S plane. The other significant exception to the regular pattern of mechanisms is represented by the March 26, 1998, event, located at 51 km depth. Its connection with the shallow earthquake sequence is unclear and intriguing. The time evolution of the seismic sequence is unusual,with the mainshock accounting for only approximately 50% of the total moment release. The broadband teleseismic waveforms of the main, September 26, 09:40, earthquake are very complicated for the size of the event and suggest a complex rupture. In our favored source model, rupture initiated at 5 km depth, propagated updip and was followed, 3 seconds later, by a shallower subevent with a slightly rotated mechanism.     

大华北浅源地震与日本海西部及我国东北深震的关系

分析了大华北浅源地震与日本海西部及我国东北深震的关系,认为本世纪来日本海西部—我国东北深震经历了5个相对活跃期,大华北各地震区相应经历这5个活跃期的影响期。根据大华北M≧6级浅源地震与深震活动的相关性,建立了太平洋板块楔形俯冲带端部重大深震事件导致大华北浅源M≧6级地震发生的板块俯冲模型,应变波传播速度约94km/年,地表视速度约100km/年。重大深震事件突出、模型稳定性强,预测实验表明模型公式可做大华北地震监测参考。用本模型可以解释浅源地震迁移、各地震区地震与深震活动相关等现象。     

Source mechanism of earthquakes in Peru

The source mechanism of 19 earthquakes that occurred in Peru (1990–1996) is studied using broad band data. Focal mechanisms are obtained using polarities of P wave and body wave form inversion. Shallow earthquakes show complex source time functions, intermediate and deep depth shocks have simpler ones. Stress distributions have been obtained from focal mechanisms estimated in this study and previous studies. Shallow earthquakes show reverse faulting with an ENE-WSW to E-W oriented pressure axes. Intermediate depth shocks indicate horizontal extension on E-W direction, normal to the Peru-Chilean trench. Earthquakes with foci at very deep depth show horizontal extension in the E-W direction in Peru-Brazil and N-S in Peru-Bolivia borders. This difference in stress orientation may indicate a different origin for deep activity at each region.     

Historical intermediate-depth earthquakes in the southern Aegean Sea Benioff zone: modeling their anomalous macroseismic patterns with stochastic ground-motion simulations

We model the macroseismic damage distribution of four important intermediate-depth earthquakes of the southern Aegean Sea subduction zone, namely the destructive 1926 M?=?7.7 Rhodes and 1935 M?=?6.9 Crete earthquakes, the unique 1956 M?=?6.9 Amorgos aftershock (recently proposed to be triggered by a shallow event), and the more recent 2002 M?=?5.9 Milos earthquake, which all exhibit spatially anomalous macroseismic patterns. Macroseismic data for these events are collected from published macroseismic databases and compared with the spatial distribution of seismic motions obtained from stochastic simulation, converted to macroseismic intensity (Modified Mercalli scale, I_MM). For this conversion, we present an updated correlation between macroseismic intensities and peak measures of seismic motions (PGA and PGV) for the intermediate-depth earthquakes of the southern Aegean Sea. Input model parameters for the simulations, such as fault dimensions, stress parameters, and attenuation parameters (e.g. back-arc/along anelastic attenuation) are adopted from previous work performed in the area. Site-effects on the observed seismic motions are approximated using generic transfer functions proposed for the broader Aegean Sea area on the basis of V_S30 values from topographic slope proxies. The results are in very good agreement with the observed anomalous damage patterns, for which the largest intensities are often observed at distances >?100 km from the earthquake epicenters. We also consider two additional “prediction” but realistic intermediate-depth earthquake scenarios, and model their macroseismic distributions, to assess their expected damage impact in the broader southern Aegean area. The results suggest that intermediate-depth events, especially north of central Crete, have a prominent effect on a wide area of the outer Hellenic arc, with a very important impact on modern urban centers along northern Crete coasts (e.g. city of Heraklion), in excellent agreement with the available historical information.     

Crustal seismic profiles in the Alboran Sea — Preliminary results

In September 1974, deep seismic sounding experiments were performed in the Alboran Sea. The crustal seismic profiles were carried out with shotpoints at sea along approximately the 36°N parallel and along 3°W and 5°W meridians with stations on land in Morocco and Spain following these three directions. The first interpretation of the data indicates a thinned continental crust with a Moho depth of 16 km on top of a slightly anomalous upper mantle (7.5<V _p<7.9 km/s) beneath the center of the Alboran Sea. Towards Spain the transition to the continental margin is characterized by a very rapid thickening of the crust. Towards Morocco a rather abrupt thickening is observed only for the Rif region, while in the eastern part (north-south profile along 3°W) the dip of the Moho is very slight.     

Source mechanism of intermediate and deep earthquakes in southern Spain

Focal mechanisms of 10 intermediate-depth earthquakes (30相似文献   

Source processes of large (M≥6.5) earthquakes of the Southeastern Caribbean (1926–1960)

We have conducted body waveform modeling studies of 13 historic earthquakes to provide a better understanding of the long-term spatial and temporal pattern of seismicity and deformation within a region extending from Barbuda, Lesser Antilles, to Cumana, Venezuela. Our results suggest that shallow earthquakes (<50 km deep) along the South American-Caribbean plate margin reflect right-lateral and extensional deformation. Intermediate depth events (100 km) show left-lateral strike-slip motion beneath the Paria peninsula of Venezuela. In the Lesser Antilles the 1960 Barbuda and 1946 Martinique earthquakes appear to be interplate thrust events, however the greatest moment release in the region has occurred at intermediate depths as a mixture of normal and strike-slip faulting, generally along trends oblique to the arc. The deformation rate estimated from the seismic moment release between 1926 and 1960 is only 1 to 10% of the estimated plate convergence rate for the region.     

Neogene Through Quaternary Tectonic Reactivation of SW Iberian Passive Margin

Southwest Portugal, the Gulf of Cadiz and Morocco are under the potential threat of natural hazards linked to seismicity and tsunami generation. We report the results of two multi-channel seismic (MCS) surveys carried out in 1992 and 1998 along the continental margin and oceanic crust of SW Iberia. This MCS data set shows the evidence of the compressional deformation which involves both the continental and the oceanic crust of the study area. The area of deformation extends from the southern border of the Tagus Abyssal Plain to the Seine Abyssal Plain, encompassing the continental margin of SW Portugal. Most of the structures observed are probably related to a Mid-Miocene phase of Africa-Europe plate convergence. In this paper we discuss the recent advances on the identification of the tectonic structures that are still active and that may generate great earthquakes and tsunamis. The tectonic structures identified are located respectively at the Guadalquivir Bank, along the eastern border of the Horseshoe Abyssal Plain and along the southern continental slope of SW Portugal.BIGSETS Team: L. Mendes Victor, C. Corela, A. Ribeiro, D. Cordoba, J. J. Danobeitia, E. Grácia, R. Bartolomé, R. Nicolich, G. Pellis, B. DellaVedova, R. Sartori, L. Torelli, A. Correggiari, L. Vigliotti.     

东北地区地震危险趋势分析

利用丰富的震史资料，研究了日本海沟、日本海深震与中国东北浅震的时空特征。根据深震、浅震的相关活动特点，我们对东北浅震危险趋势进行了分析。     

Seismic evidence for a metastable olivine wedge in the subducting Pacific slab under Japan Sea

We apply a forward-modeling approach to high-quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting Pacific slab beneath the Japan Sea. Our results show that a finger-like anomaly exists within the subducting Pacific slab below 400 km depth, which has a P-wave velocity 5% lower than the surrounding slab velocity (or 3% lower than that of the normal mantle), suggesting the existence of a metastable olivine wedge (MOW) in the slab. The MOW top and bottom depths are 400 and 560 km, respectively. The MOW is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary. At 560 km depth the MOW is located at about 25 km below the slab upper boundary. Most of the deep earthquakes are located in the MOW. Our results favor transformational faulting as the mechanism for deep earthquakes.     

New evidence of delamination in the Western Alboran Sea: Geodynamic evolution of the Alboran domain and its margins

The presence of continuous upper crustal blocks between the Iberian Betics and Moroccan Rif in the western and middle Alboran Sea, detected with tomography, can add new information about the lithosphere structure and geodynamic evolution in this region. A large volume of seismic data (P and S wave arrival times) has been collected for the period between 1 December 1988 and 31 December 2008 by 57 stations located in northern Morocco (National Institute of Geophysics, CNRST, Rabat), southern Portugal (Instituto de Meteorologia, Lisbon) and Spain (Instituto Geografico National, Madrid) and used to investigate the lithosphere in the western Alboran Sea region. We use a linearized inversion procedure comprising two steps: (1) finding the minimal 1-D model and simultaneous relocation of hypocenters and (2) determination of local velocity structure using linearized inversion. The model parameterization in this method assumes a continuous velocity field. The resolution tests indicate that the calculated images give near true structure imaged at 5 km depth for the Tanger peninsula, the Alhoceima region and southern Spain. At 15, 30 and 45 km depth we observe a near true structure imaged in northern Morocco, and southern Spain. At 60 and 100 km, southern Spain and the SW region of the Alboran Sea give a near true structure. The resulting tomographic image shows the presence of two upper crustal bodies (velocity 6.5 km/s) at 5–10 km depth between the Betics, Rif, western and central Alboran Sea. Low velocities at the base of these two bodies favor the presence of melt. This new evidence proves that the Tethysian ocean upper crust was not totally collapsed or broken down during the late Oligocene–early Miocene. These two blocks of upper crust were initially one block. The geodynamic process in the eastern of the Mediterranean is driven by slab rollback. The delamination process of the lithospheric mantle terminates with the proposed slab rollback in the western part of the Mediterranean. This can be explained by the removal of the major part of the lithosphere beneath the area, except in the SW part of the Alboran Sea where a small part of the lithospheric mantle is still attached and is extends and dips to SE beneath the Rif, slowly peeled back to the west. A second detached lithospheric mantle is located and extends to eastern part of the Rif and dips to the SE. The removal of lithosphere mantle from the base of the crust was replaced and heated by extrusion of asthenospheric material coming from depth to replace the part of crust detached. A combination of isostatic surface/topographic uplift and erosion induced a rapid exhumation and cooling of deep crustal rocks.     

Evidence for periodic seismicity in the inner Aegean seismic zone

The most complete and reliable data of strong (M _s6.5), shallow (h<70 km) earthquakes which occurred in the inner Aegean seismic zone have been utilized to describe its seismicity time variation during 1800–1986 by two independent statistical models. The first is a sequentially stationary model of seismicity rates which shows that intervals of low seismicity rate, lasting for some 37 years, alternate with high rate intervals of 8–12 years duration. The second model is a statistical model according which seismic energy released within 5-year time windows approximates a harmonic curve within a period of about 50 years. This model is in agreement with the notion that the time series of strong earthquake occurrences in the inner Aegean seismic zone consists of a random (shocks withM _s=6.5–6.8) and a nonrandom component (M _s6.9). Maxima and minima of the harmonic curve coincide with the high and low rate intervals, respectively. A model of regional stationary accumulation of thermal stresses along certain seismic belts and their cyclic relaxation may explain this periodicity.     

M m: A variable-period mantle magnitude for intermediate and deep earthquakes

We extend to the case of intermediate and deep earthquakes the mantle magnitude developed for shallow shocks byokal andTalandier (1989). Specifically, from the measurement of the spectral amplitude of Rayleigh waves at a single station, we obtain a mantle magnitude,M _m, theoretically related to the seismic moment of the event through $$M_m = \log _{10} M_0 - 20.$$ The computation ofM _minvolves two corrections. The distance correction is the same as for shallow shocks. For the purpose of computing the frequency-dependent source correction, we define three depth windows: Intermediate (A) (75 to 200 km); Intermediate (B) (200–400 km) and Deep (over 400 km). In each window, the source correctionC _S is modeled by a cubic spline of log₁₀ T. Analysis of a dataset of 200 measurements (mostly from GEOSCOPE stations) shows that the seismic moment of the earthquakes is recovered with a standard deviation of 0.23 units of magnitude, and a mean bias of only 0.14 unit. These figures are basically similar to those for shallow events. Our method successfully recognizes truly large deep events, such as the 1970 Colombia shock, and errors due to the potential misclassification of events into the wrong depth window are minimal.     

Seismicity of Morocco for the Period 1987–1994

— A seismic data file of 3,740 earthquakes from January 1987 to December 1994 has been elaborated for Morocco and the border regions, with 10 main events registering magnitudes from 5 to 5.6. Such seismicity is particularly important for Morocco as the released seismic energy constitutes a considerable part of the total energy radiated during the 20th century. Relative seismicity maps confirm the persistence of the major features of the seismicity of Morocco. An important seismic activity is observed in the Alboran region continental crust, which absorbs the maximum deformation resulting from the convergence of the African and Iberian plates. However, in the longitude window 3.5°– 6° W at depths of 25 to 50 km, a seismic gap zone seems to take place. An explanation of this phenomenon may be provided by the slab breakoff model. Even if the seismicity of Morocco remains moderate, heavy damage is observed when the magnitude of earthquakes exceeds 4.5, especially in the case of traditional buildings.