Three-dimensional image of the Moho undulations beneath the Gulf of Bothnia using wide-angle seismic data

The BABEL marine seismic experiment has been carried out to investigate the lithospheric structure and antecedent tectonic signatures of the Baltic Shield, including the Archaean-Proterozoic collisional structure in the northern part of the Gulf of Bothnia.
Lithospheric seismic-reflection streamer data and simultaneously recorded wide-angle reflection and refraction data collected in the Gulf of Bothnia as part of the BABEL project have been used for 3-D modelling. The distribution of land stations around the Gulf provides a good 3-D ray coverage of the PMP reflection data recorded at the eight stations in the area and allows an estimation of strikes and dips of the Moho boundary in the area. The traveltimes of reflected phases are calculated using a method that utilizes the finite-difference solution of the eikonal equation. The Moho wide-angle-reflection (PMP) traveltimes are modelled using an inversion method. A 2-D model from the Gulf of Bothnia extended into the third dimension is used as an initial model. During the inversion the velocity is kept constant and only the Moho boundary is allowed to vary. To estimate the strike of the Moho boundary and the stability of the inversion, two initial models with different strikes are examined.
The results indicate that the Moho depth in the Gulf of Bothnia undulates and has a maximum depth of 55 km in the south, rising to 42 km in the north. The Moho depth variations seem to be step-like. This change in the Moho depth coincides with the location of the presumed fossil subduction zone in the area. The crustal-thickness variations seem to be well approximated by a nearly 2-D structure striking parallel to a postulated subduction zone immediately to the south of the Skellefte area. The presence of the step at the crust/mantle boundary can be interpreted as a result of a plate-collision event at about 2 Ga.     

Critical evaluation and geological correlation of teleseismic phase data from Indian seismological stations

We evaluated the quality of seismic phase data from Indian seismological stations through the analysis of teleseismic travel times reported during 1976–83 and infer that only WWSSN stations (NDI, SHL, POO, KOD) apart from GBA and HYB can be rated satisfactory while the majority of stations (more than 40) produce very poor quality data sets. Detailed analysis of teleseismic P-wave travel time residuals shows that while the average structure of the upper mantle beneath India has high velocity (negative residuals) there are marked lateral variations. In particular, three zones of anomalous positive residuals (low velocity) are observed: one beneath the north western part of the Deccan trap, the second covering the southernmost peninsula (granulite terrain) and a third rather localized one, to the north of Delhi coinciding with Delhi-Haridwar ridge. New Delhi exhibits strong negative residuals in the E-SE quadrant along with negative station anomaly, implying that it is underlain by an anomalous high velocity crust/upper mantle. The negative residuals observed over India, continue beneath the Himalaya till the south of Lhasa but change sign further northward, suggesting the northern limit of the Indian upper mantle structure.     

Correction for water depth variations of marine crustal reflection seismic data using refraction statics

Water depth variations in marine reflection seismic profiling cause velocity push-down and, in regions of rapid fluctuations of the water-bottom slope, stack degradation. Static corrections are a very satisfactory and practical solution to these problems under typical survey conditions with water depths not exceeding a few hundred metres and relatively hard water bottoms. Static time shifts are best derived from a refraction analysis of first arrivals rather than from fathometer data, especially over underwater valleys where unconsolidated sediment of velocity close to that of water has been deposited unconformably onto the underlying lithified rocks. These points are illustrated by a field example from the GLIMPCE survey in Lake Superior. The availability of computer-effective algorithms such as the generalized linear inversion method allows the implementation of refraction statics during the initial processing of regional marine crustal surveys.LITHOPROBE Publication No. 263.     

Bending-related faulting of the incoming oceanic plate and its effect on lithospheric hydration and seismicity: A passive and active seismological study offshore Maule,Chile

We have studied the dependency between incoming plate structure, bending-related faulting, lithospheric hydration, and outer rise seismic activity offshore Maule, Chile. We derived a 2D Poisson's ratio distribution from P- and S-wave seismic wide angle data collected in the trench-outer rise. High values of Poisson's ratio in the uppermost mantle suggest that the oceanic lithosphere is highly hydrated due to the water infiltration through bending-related normal faults outcropping at the seafloor. This process is presumably facilitated by the presence of a seamount in the area. We conclude that water infiltrates deep into the lithosphere, when it approaches the Chile trench, producing a reduction of crustal and upper mantle velocities, supporting serpentinization of the upper mantle. Further, we observed a mantle Vp anisotropy of 8%, with the fast velocity axis running normal to the abyssal hill fabric and hence in spreading direction, indicating that outer rise processes have yet not affected anisotropy.The first weeks following the megatrust Mw = 8.8 Maule earthquake in 2010 were characterized by a sudden increase of the outer rise seismic activity, located between 34° S and 35°30′ S. We concluded that this phenomenon is a result of an intensification of the water infiltration process in the outer rise, presumably triggered by the main shock, whose epicenter was located some 100 km to the south east of the cluster.     

Capability of the penetrator seismometer system for lunar seismic event observation

We developed a seismometer system for a hard landing “penetrator” probe in the course of the former Japanese LUNAR-A project to deploy new seismic stations on the Moon. The penetrator seismometer system (PSS) consists of two short-period sensor components, a two-axis gimbal mechanism for orientation, and measurement electronics. To carry out seismic observations on the Moon using the penetrator, the seismometer system has to function properly in a lunar environment after a hard landing (impact acceleration of about 8000 G), and requires a signal-to-noise ratio to detect lunar seismic events. We evaluated whether the PSS could satisfactorily observe seismic events on the Moon by investigating the frequency response, noise level, and response to ground motion of our instrument in a simulated lunar environment after a simulated impact test. Our results indicate that the newly developed seismometer system can function properly after impact and is sensitive enough to detect seismic events on the Moon. Using this PSS, new seismic data from the Moon can be obtained during future lunar missions.     

中美青藏高原数字宽频带地震观测

1991年7月至1992年7月,中美合作在青藏高原进行了天然地震观测。布设了由11个台站组成的台阵,其中7个台站由北至南分布在青藏公路格尔木至拉萨段沿线,2个分别在其两侧,另外2个在青藏高原东部。此台阵使用REF TEK 72A-02记录器和宽频带STS-2地震计,进行事件触发及连续记录。此次观测为在青藏高原内采集地震数据提供了极好的机会。     

Seismicity in subduction zones from local and regional network observations

The data provided by local and regional seismograph networks are essential for the solution of many problems of subduction-zone seismology. The capabilities of such networks are limited by the instrumentation currently in common use and the unfavorable source-station geometry often imposed by the regional geography. Nevertheless, important contributions have come from the data gathered in many of the earth's subduction zones. The accuracy of hypocenter locations based on regional data is affected by the complex velocity structures characteristic of subduction zones, but the problems are now well-understood. Examples of numerous studies of the spatial configurations of the seismicity in subduction zones and consequent interpretations of seismogenesis and subduction processes are reviewed. Studies of the distributions of earthquakes in time and with magnitude, for events down to the microearthquake level, have the potential for clarifying the earthquake-generating processes and, possibly, a basis for earthquake prediction. Other uses of local and regional network data have been for investigations of coda-Q and the identification of asperities in subduction zones.