A microearthquake survey at the junction of the East Pacific Rise and the Wilkes (9°S) fracture zone

Summary. An ocean bottom seismograph survey of the junction of the East Pacific Rise and the Wilkes fracture zone detected only three microearthquakes beneath the rise crest during seven days of recording. In contrast, during the same period 41 events were detected on the fracture zone, all at distances greater than 10 km from the junction. These results suggest that near the rise crest the thin crust can support sufficient stress only to generate infrequent small earthquakes and that most faulting may take place by aseismic slip. At 10 km from the rise axis part of the crest has become competent enough to support stress, resulting in earthquakes probably at depths of up to 5 km below the sea-bed. Gear 5-waves on the seismometer records indicate that a magma chamber, if it exists near the junction, is less than 10 km across.     

Geophysical surveys on the East Pacific Rise — Galapagos Rise system

Summary. The East Pacific Rise at 12–15° S is topographically smooth with a crestal horst or linear volcanic peak marking the present axis of spreading. The Galapagos Rise at 14–17° S is topographically rough with a possible central graben marking the extinct spreading axis. The seafloor spreading magnetic anomalies on the East Pacific Rise are of low amplitude, but fracture-zone anomalies at 13–14° S have amplitudes of up to 1250 nT. Anomalies of this amplitude at the magnetic equator must be formed within the fracture zone by some combination of block reversal boundaries, anomalously-high magnetic intensities, and/or anomalously-large thicknesses of the magnetic layers within the fracture zone. Magnetization and major-element chemical analyses of basalts dredged from four locales along the fracture zone indicate that the large magnetic-anomaly amplitudes are caused by the high iron and titanium content of these ferrobasalts. The magnetic-anomaly profiles from the Galapagos Rise and its fracture-zone system are of normal amplitude and are extremely difficult to correlate internally or with the geomagnetic timescale.
Eighty-one heat-flow measurements indicate that the values measured are controlled by sediment thickness. Where the thickness of the sediment blanket is greater than 100 m, high heat flow is measured and possibly is representative of the total heat transfer at the seafloor. Where the sediment thickness is less than 100 m, seawater circulation in the oceanic crust is thought to remove most of the heat convectively; thus causing low conductive heat-flow values to be measured by the usual heat-flow apparatus. The heat loss by convective processes is probably a function also of topographic roughness and sediment permeability.     

The crustal structure of the Reykjanes Ridge at 59° 30'N

Summary. In order to examine the development of the oceanic crust in the neighbourhood of a slowly spreading ridge, a seismic refraction experiment was carried out at 59° 30'N on the Reykjanes Ridge. Three 120 km long overlapped split profiles were shot parallel to the trend of the ridge, on the eastern flank, and recorded on up to five recording sonobuoys. The profiles were at distances of 0, 30 and 90km from the ridge axis, corresponding to approximate crustal ages of 0, 3 and 9 Myr. Data from the main profiles were supplemented by using a large chamber air gun during recovery of the buoys.
The analysis of the data combined standard travel-time interpretation, the 'tau' method of systematic travel-time inversion and detailed amplitude modelling using the Reflectivity Method to calculate synthetic seismograms. Detailed velocity-depth models were constructed for each of the profiles.
There is no indication of a significant magma chamber at the ridge crest, although a slight velocity inversion in layer 3 suggests a zone of elevated temperature. Away from the crest there was a slight positive velocity gradient in layer 3. Layer 2 was most effectively modelled by a region of varying velocity gradients, which thinned with age and the transition to layer 3 is marked by a sharp change in velocity gradient. The transition to mantle velocities is also best modelled by a high velocity gradient rather than an interface.
Although some lateral variation in properties is apparent along the profiles, the lateral velocity gradients were sufficiently weak to allow an effective analysis in terms of laterally uniform models.     

Contribution of serpentinized ultramafics to marine magnetic anomalies at slow and intermediate spreading centres: insights from the shape of the anomalies

Detailed characteristics of marine magnetic anomalies 33r and 20r suggest that the magnetization of the deeper magnetic layers, including the lower crust and possibly the uppermost mantle, is horizontally displaced with respect to that of the upper crust. We examine the possibility that serpentinization of ultramafics in the lower crust and possibly the uppermost mantle delays the acquisition of magnetization and introduces a shift between the upper- and lower-crustal magnetization patterns. Thermal evolution models and the resulting magnetization patterns of the oceanic lithosphere are calculated for a wide range of physical parameters such as the Nusselt number and the depth of hydrothermal circulation in the crust, and the temperature range of serpentinization. The models with moderate hydrothermal cooling of the whole crust and serpentinization temperatures ranging between 200 and 300 C successfully explain the anomalous skewness and the 'hook shape' of observed sea-level magnetic anomalies created at slow and intermediate spreading rates.     

Secular variation of the poloidal magnetic field at the core–mantle boundary

A region of enhanced conductivity at the base of the mantle is modelled by an infinitesimally thin sheet of uniform effective conductance adjacent to the core–mantle boundary. Currents induced in this sheet by the temporally varying magnetic field produced by the geodynamo give rise to a discontinuity in the horizontal components of the poloidal magnetic field on crossing the sheet, while the radial component is continuous across the sheet. Treating the rest of the mantle as an insulator, the horizontal components of the poloidal magnetic field and their secular variation at the top of the core are determined from geomagnetic field, secular variation and secular acceleration models. It is seen that for an assumed effective conductance of the sheet of 10⁸  S, which may be not unrealistic, the changes produced in the horizontal components of the poloidal field at the top of the core are usually ≤10 per cent, but corrections to the secular variation in these components at the top of the core are typically 40 per cent, which is greater than the differences that exist between different secular variation models for the same epoch. Given the assumption that all the conductivity of the mantle is concentrated into a thin shell, the present method is not restricted to a weakly conducting mantle. Results obtained are compared with perturbation solutions.     

Anomalous variations of the geomagnetic field in East Fergana — magnetic precursor of the May earthquake with M= 7.0 (1978 November 2)

Summary. A secular variation anomaly has been discovered at the north-east part of the Fergana vdey by repeated measurements every year or less. The change of total field Δ F at the 'magnetic epicentre' was 9 nT in 1977 and 16 nT in 1978 relative to the level of 1973. In 1977 an anomalous region was recognized, where according to the data from 25 observation points Δ F increased in the northern part up to 5.2 nT, and decreased by 4.7nTin the southern part according to a further 22 points. Permanent observations were begun at the epicentre in 1978 October. We normally observed variations of Δ F differences with magnitude ± 2–3 nT, which were not correlated with worldwide magnetic activity. Anomalous variations appeared on October 26 and rose to a maximum value of + 23 nT on October 30. The decrease of this anomalous field began on October 31. This made it possible to predict a potential earthquake. The Alay earthquake with M = 7.0 occurred on November 2 six hours after the prediction was issued; Δ F then returned to the initial level. Thus, using the geomagnetic field variations in the Fergana region, geophysicists were able to predict the moment of a strong earthquake.     

Structure and recent evolution of the Hazar Basin: a strike‐slip basin on the East Anatolian Fault,Eastern Turkey

The Hazar Basin is a 25 km‐long, 7 km‐wide and 216 m‐deep depression located on the central section of the East Anatolian Fault zone (eastern Turkey) and predominantly overlain by Lake Hazar. This basin has been described previously as a pull‐apart basin because of its rhombic shape and an apparent fault step‐over between the main fault traces situated at the southwestern and northeastern ends of the lake. However, detailed structural investigation beneath Lake Hazar has not been undertaken previously to verify this interpretation. Geophysical and sedimentological data from Lake Hazar were collected during field campaigns in 2006 and 2007. The analysis of this data reveals that the main strand of the East Anatolian Fault (the Master Fault) is continuous across the Hazar Basin, connecting the two segments previously assumed to be the sidewall faults of a pull‐apart structure. In the northeastern part of the lake, an asymmetrical subsiding sub‐basin, bounded by two major faults, is cross‐cut by the Master Fault, which forms a releasing bend within the lake. Comparison of the structure revealed by this study with analogue models produced for transtensional step‐overs suggests that the Hazar Basin structure represents a highly evolved pull‐apart basin, to the extent that the previous asperity has been bypassed by a linking fault. The absence of a step‐over structure at the Hazar Basin means that no significant segmentation boundary is recognised on the East Anatolian Fault between Palu and Sincik. Therefore, this fault segment is capable of causing larger earthquakes than recognised previously.