Seismotectonic Movements Research Articles

Seismotectonic Movements in the Minute Range of Periods Before the Catastrophic Earthquake in Japan March 11, 2011

The vertical movements measured by a broadband seismic stations located around Tohoku earthquake 11.03. 2011 with magnitude 9 are calculated. It is shown that during the 15 years before the earthquake the closest to epicenter MAJO station located 386 km from the epicenter the quiet daily variations was demonstrated. Seismic pulses with the durations of a few minutes and amplitudes bigger 10% of the diurnal variations of tidal velocities were revealed in 2009 year. They appeared under quiet meteorological and geomagnetic conditions. These pulses are not found on the records of the stations remoted more 700 km from epicenter. It is hypothesized that sharp changes in the low frequency seismic noise reflect the tectonic deformations in the lithosphere of Japan and adjoin part of Pacific Ocean.

Seismic and Tectonic Correspondence of Major Earthquake Regions in Southern Ghana with Mid-Atlantic Transform-Fracture Zones

For four centuries now, southern Ghana has been known to be seismically active, and there is no clear geological explanation for the cause of the seismicity. By evaluating new field data and information with re-interpreted historical earthquake data of southern Ghana, the nature of the seismicity of southern Ghana has been elucidated. The mutual connection between the earthquake epicentres and the remote causes by Mid-Atlantic transform faults and fracture zones has been established. The seismic regions of southern Ghana have been linked separately to tectonic faults and activities of the St. Paul’s and Romanche transform-fracture zone systems offshore in the Gulf of Guinea to onshore. It is concluded that the seismicity of southern Ghana is due to tectonic activities of the St. Paul’s and Romanche transform-fracture systems. The Accra region earthquakes originate from reactivation of faults in the Romanche transform-fracture zone, and propagate onshore through Accra and environs. The Axim region earthquakes come from reactivated faults linked to the St Paul’s fracture zone, which go through southern Cote D’Ivoire to Ghana. Seismotectonic movements along the St Paul’s transform and fracture zones have quieted since 1879. But movement along the Romanche Transform fault and Fracture zone is active, causing ongoing seismicity of southern Ghana.

Deep structure and seismotectonics of the Southern Sea of Okhotsk region along a profile from southern Sakhalin to the southern Kuril Islands

The results of deep seismic profiling through Southern Sakhalin, the southern Sea of Okhotsk, and the Southern Kuril Islands allowed the identification of deep fault zones, the hypocenter locations, the features of the stress state, and the types of seismic dislocations at the earthquake sources. The east side of the fault was upthrown relative to the west side beneath the southern part of the Tatar Strait and Sakhalin Island and led, as a result of multiple thrusting events along the fault over the geologic history, to the rise and 5-8 km displacement of the seismic boundaries. The true uplift of the Greater Kuril arc block was determined using the focal mechanism solutions. The seismoctectonics and present-day dynamics of the crustal blocks were estimated using a detailed joint analysis of the position of the structural boundaries at the seismic section and the seismotectonic movements according to the earthquake focal mechanisms.

Relative sea-level rise across the Eastern Border fault (Branford, Connecticut): evidence against seismotectonic movements

Basal peat age–depth data from four salt-marsh sites located 500–1500 m south of the Eastern Border fault (EBF) near Branford, Connecticut, document an error envelope for the lower limit of relative mean high water (MHW) rise during the past 3300 yr. The long-term rate of relative lower-limit-MHW rise during the past 3000 yr (1.1 mm yr −1) corresponds with late Holocene rates of relative MHW rise found elsewhere in coastal Connecticut (0.9–1.2 mm yr −1). This result invalidates repeated claims that multiple seismotectonic lowering of a crustal block towards the south has occurred along a segment of the EBF near Branford shortly after 815 cal yr BC and during the past 1200 yr and calls for a non-tectonic interpretation of biostratigraphic evidence of local short-term sea-level variations.

Seismic imaging of upper crustal basement faults in the Skagerrak Sea

The continental upper crystalline crust appears almost non-reflective on many deep seismic reflection profiling sections. Typically, only the major faults and lithological boundaries are imaged, whereas smaller-scale faults are usually not identified. A deep seismic survey (0–16 s TWT) covering the Skagerrak Sea was undertaken in 1987. Although the northeastern part of the Skagerrak has only thin or no sediment cover, the data in the depth range 0–6 s TWT were reprocessed to commercial exploration standards. Close inspection of these high quality sections provides evidence of abundant, moderately steep dipping (20–50°) faults that can be traced to depths of 10–13 km. Fault geometry, which can be inferred at profile intersections, is in accordance with the general tectonic fabric observed on land and beneath the Skagerrak Sea. These areas were deformed during the Proterozoic Sveconorwegian orogeny and during the formation of the Permian Oslo Rift. Due to post-Permian erosion of the Skagerrak Sea proper, the relative age and movements of the basement faults could not be determined. This study shows that under favorable conditions, faults in the upper crystalline crust are mappable by seismic means. Such measurements have the potential for resolving variations in block geometry with depth and also recent seismotectonic movements.

A simple 2-D model for the evolution of an island-arc system

Slow seismotectonic movements along inclined deep fault planes under compressive horizontal stresses are supposed to be the principal mechanism controlling the structure and processes in island-arc systems. In order to treat the stress variations caused by this mechanism, a simple geomechanical model is investigated. We consider a shearing surface crack embedded in a homogeneous elastic half-space. The key element of the model is viscous interaction between the sides of the crack, the viscosity varying with depth. The model differs from the classical steady-state mode of subduction by nonstationary creep processes on deep faults and possibly by cyclical evolution of island-arc systems. The results of our numerical analysis are in good agreement with geological, geophysical and seismological data. (i) Vertical displacements of the free surface in the model fit well with the typical topography of a trench—arc-basement rise—back-arc basin system. (ii) The Benioff seismic zone is supposed to be formed due to the concentration of shear stresses near the fault plane. The characteristic patterns of seismicity, the fine geometry of Benioff zones, and their double-planed structure can be explained in terms of our model. (iii) A zone of considerable heat generation caused by viscous dissipation along the fault plane is found within a narrow area in the depth range 100–200 km. Moreover, the island-arc basement rise is characterized in the model by a relative tension of a few tens or even hundreds of bars, while at depths of 100–150 km below the surface, additional compression of the same order of magnitude acts. The magmatic plumbing system may be visualised as a “toothpaste tube” or a sponge filled with magma which is squeezed from the depths to the surface due to the redistribution of the tectonic stresses only. This can explain the physical origin of island-arc magmatism and the typical position of volcanic belts.