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The Easter (Rapanui) microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on Earth is occurring today. Recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets.During the January 1987 Rapanui expedition on the N/O “Jean Charcot” we conducted a Sea Beam/magnetics/ gravity survey of a portion of the microplate boundaries. The method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships. The northern terminus of the East Rift or eastern boundary of the microplate is expressed as a series of parallel NW-SE trending valleys including what appears to be, with 5890 m depth, the deepest active rift axis mapped in the Pacific today (Pito Rift).The northern end of the Pito Rift merges with an E-W to 083° narrow band of linear faults interpreted to be a transform fault between the Nazca and Easter (Rapanui) plates.The northern triple junction between the Easter (Rapanui), Nazca and Pacific plates is a RFF type with the two transform faults colinear along an approximately E-W direction.The southwestern boundary of the Easter (Rapanui) microplate is marked by a series of en-echelon offsets, outlined by depressions, which merge into an approximately E-W zone where shear must be predominant.The southern triple junction is a RRF junction with an overlapping ridge system.The structural data acquired during the survey provide strong constraints for kinematic models of the microplate. The structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate.  相似文献   
2.
Results of the R/V “Thomas Washington” Pascua 3 expedition provide evidence for the existence of the Juan Fernandez microplate just north of the junction between the East Pacific Rise (EPR) and the Chile fracture zone. Prior to Pascua 3, the microplate in the region had been hypothesized from the pattern of seismicity. The eastern and western boundaries of the Juan Fernandez microplate are well defined and represent north-south trending spreading centers characterized by very slow and very fast rates of accretion respectively. In agreement with the rates, the eastern boundary is represented by a rift valley and the western boundary by an EPR-type axial ridge. The northern boundary of the Juan Fernandez microplate is a 100°-trending wide fracture zone complex which may have resulted from northward transform fault migration. The fracture zone fails to meet the zone of accretion at the Pacific-Nazca-Juan Fernandez triple junction. In this area the zone of accretion displays a double ridge with a large overlap. The southern boundary of the Juan Fernandez microplate is still poorly constrained. The plate geometry derived from SEABEAM differs from that derived by Anderson-Fontana et al. (1986) [14] from a plate motion inversion scheme using primarily earthquake first-motion solutions together with limited bathymetric and magnetic data.  相似文献   
3.
During the spring of 1983, the R/V “Thomas Washington” surveyed an area located north of the Antarctic-Nazca-Pacific triple junction at 35°S.Magnetic and SEABEAM bathymetric data collected during the survey confirmed the existence of the Juan Fernandez microplate. This paper presents an analysis of the magnetic anomaly data.The western boundary of the microplate is a fast spreading center which has existed since 2 Ma and where the accretion rate has been about 14.5 cm/yr for the last 0.7 Ma.The eastern boundary of the microplate is characterized by a slow spreading center which separates the Juan Fernandez and Nazca plates. The accretion rate has been about 7.0 cm/yr between 0.7 and 0.4 Ma and about 1.6 cm/yr for the last 0.4 Ma.The two spreading centers are connected in the north and south by transform faults.Between the Juan Fernandez and Rapanui microplates, the East Pacific Rise is well defined between 30 and 32°S. In this region the axis displays a record accretion of about 17.2 cm/yr.South of the Juan Fernandez microplate, one magnetic profile (Oceano 7008) indicates that the opening rate is about 12.0 cm/yr on the Antarctic-Pacific ridge.The birth of the microplate is dated at about 2 Ma when the western boundary started to accrete. The evolution of the microplate corresponds to a transfer of accretion from the eastern boundary to the western axis. This is revealed by the net decrease of the opening rate from 7.0 cm/yr to 1.6 cm/yr observed at the eastern ridge where a small jump occurred at 0.4 Ma.  相似文献   
4.
In December 2007, the Mila region of northeastern Algeria experienced thousands of microearthquakes (0.8????Md????3.9) recorded by eight temporary stations, in addition to permanent stations. Most of the events were too small to be located precisely, but a set of 122 precisely located events shows an alignment of epicenters, extending mainly in a horizontal band at about 1?C2?km depth in a NNW?CSSE direction and concentrated in a small area, 3?km southeast of Jebel Akhal, a small rocky hill between the Beni Haroun dam/reservoir and the Oued Athmania reservoir. The reservoirs are connected by pipelines, and a pumping station ensures water can be transferred between them at transient pressures of up to 80?bars. During the pumping in 2007, only 45?% of the transferred water (~600,000?m3 per day) was recovered at the Oued Athmania reservoir, and a large amount of the slightly pressurized water leaked through defective joints in a tunnel that passes through the Jebel. This water penetrated deeply into the soil with the assistance of preexisting fractures, faults, and karsts. Nine days after the first pumping started, a local increase in pore fluid pressures at shallow depths triggered seismicity southeast of Jebel Akhal, where the faults were probably close to failure. The focal mechanisms show a near vertical N?CS strike-slip fault plane under regional NW?CSE tectonic compression. One of the fault plane solutions is consistent with the NNW?CSSE direction along which the seismic events are aligned. Furthermore, a long-term comparison of the seismic activity in the region versus water levels behind the dam and the pumping of water shows that the earthquake swarm was a one-off event related to the pumping operation.  相似文献   
5.
On August 21st and 22nd 1856, two strong earthquakes occurred off the seaport of Djidjelli, a small city of 1000 inhabitants, located 300 km east of Algiers (capital of Algeria). In relation to these two earthquakes, an important tsunami (at least one) affected the western Mediterranean region and the eastern Algerian coastline between Algiers and La Calle (Algero-Tunisian border). Based on historical information as well as on data recently collected during the Maradja 2 survey conducted in 2005 over the Algerian margin, we show that the tsunami could have been generated by the simultaneous rupture of a set of three en echelon faults evidenced off Djidjelli. From synthetic models, we point out that the area affected along the Algerian coast extended from Bejaia to Annaba. The maximum height of waves reached 1.5 m near the harbor of Djidjelli.  相似文献   
6.
On March 20, 2006, a moderate-magnitude earthquake of Mw 5.2 hit the region of the Babors mountains, a region located two hundred km east of Algiers (capital of Algeria). More precisely, the epicentre occurred 40 km south-east of Béjaïa, the second most important city in the Kabylian region. The earthquake impacted an area with a radius of about 70 km and caused the death of four persons and injured 68. Damage was mainly observed in Lâalam village (district of Kherrata) where some buildings and old houses were affected. Damage was mainly due to a moderate landslide triggered by the earthquake. Rock falls were also observed near the main cliffs. The maximum observed intensity I0 was estimated to VII (EMS-98 scale). According to the main shock focal mechanism solution and the spatial distribution of aftershocks, the main shock was reliably found to have been generated by an NS sinistral strike-slip fault. The compressive stress axis σ1, oriented N325, is in agreement with the direction of convergence between Eurasia and Africa.  相似文献   
7.
The Cheliff region has experienced some significant earthquakes in the last century (1937, 1954, and 1980). The most destructive one is that of El Asnam on October 10, 1980, Ms = 7.3 (Io = IX), which destroyed the Chlef city (formerly El Asnam) and its surrounding villages. On December 16, 2006 a moderate earthquake (Mw = 5.0) hit the Cheliff region. The maximum observed intensity (Io = V: MSK-scale) was observed at Abou El Hassen, Benaria, Bouzghaïa and Tadjena. No damages or human losses were recorded. Nevertheless, minor cracks on walls of the old school at Tadjena were observed. The point source focal mechanism of the event was determined by inverting the waveforms of three regional broadband stations of the ADSN (Algerian Digital Seismic Network). It corresponds to thrust-reverse faulting with a strike-slip component. The stress tensor obtained by the inversion of the 15 focal mechanisms available in the Cheliff region exhibits a well constrained compression axis σ1 horizontal and trending N145°. The NW dipping nodal plane indicating a NE–SW thrust fault with a right-lateral component (strike, dip, rake = 249, 38, 137) is more compatible with the regional stress tensor than the steep dipping NNE-SSW nodal plane showing reverse faulting with a left-lateral component (strike, dip, rake = 15, 65, 60). Accordingly, the Tadjena moderate size earthquake can be related to the Boukadir active fault bordering the lower Cheliff basin to the north, a situation similar to that of the El Asnam fault bordering the middle Cheliff basin to the north.  相似文献   
8.
Starting on 14 May 2010 and lasting several months, the village of Beni-Ilmane (Msila District, North-Central Algeria) and its surroundings were struck by an important seismic crisis marked by three successive moderate shocks (5.0 ≤ M d ≤ 5.2). This sequence of events caused severe damage in the Beni-Ilmane village and in the epicentral area. The poor quality of masonry construction and the cumulative effects of the large number of aftershock events played a key role in the destruction. To follow this earthquake sequence, 11 temporary seismic stations, in addition to the permanent stations of the Algerian seismic network, were deployed in the region. A representative set of well located aftershocks in the period of maximum activity (lasting 18 days) were selected. The horizontal distribution of the aftershocks shows two main earthquake clusters located near Beni-Ilmane village, one cluster oriented E–W and the other oriented NNE–SSW, crossing the first cluster at its eastern tip. The aftershocks distribution suggests that the three main shocks ruptured two distinct and adjacent fault segments of about 8 km length. The focal mechanisms of the first and third events, located in the NNE–SSW cluster, show near-vertical left-lateral strike-slip fault planes. In the second cluster, oriented E–W, focal mechanisms show a high-angle reverse fault. A field survey, initiated immediately after the first main shock, identified surface fissures generated by the three largest events in the sequence. The fissures, concentrated in a narrow area at the western termination of the NE–SW Jebel Choukchot anticline (location of Beni-Ilmane village), showed several orientations which were mainly related to gravity instabilities. The 2010 Beni-Ilmane earthquake sequence, located in the Bibans–Hodna Mountains transition zone, demonstrates that the Tellian Atlas–High Plateaus border region is an active seismic zone marked by moderate and possibly strong earthquakes; thus, a reevaluation of the seismic hazard in the region is needed.  相似文献   
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