Active Tectonic Zones Research Articles

Active tectonic blocks and strong earthquakes in the continent of China

The primary pattern of the late Cenozoic to the present tectonic deformation of China is characterized by relative movements and interactions of tectonic blocks. Active tectonic blocks are geological units that have been separated from each other by active tectonic zones. Boundaries between blocks are the highest gradient of differential movement. Most of tectonic activity occurs on boundaries of the blocks. Earthquakes are results of abrupt releases of accumulated strain energy that reaches the threshold of strength of the earth’s crust. Boundaries of tectonic blocks are the locations of most discontinuous deformation and highest gradient of stress accumulation, thus are the most likely places for strain energy accumulation and releases, and in turn, devastating earthquakes. Almost all earthquakes of magnitude greater than 8 and 80%–90% of earthquakes of magnitude over 7 occur along boundaries of active tectonic blocks. This fact indicates that differential movements and interactions of active tectonic blocks are the primary mechanism for the occurrences of devastating earthquakes.

Teleseismic anistotropy and its features in the upper mantle beneath the Tibet Plateau and neighboring areas. Metallogenic implications

The extensive seismic studies carried out in Tibet and its neighboring areas in China provided teleseismic data revealing upper mantle structure and indicating structural features at varying levels down through the upper mantle. An example is included to contribute to the study of deep-rooted pathways guiding the ascent of magmatic and hydrothermal fluids potential to concentrate metals in the upper crust. Systematic seismic studies in Tibet and its neighboring areas started in June 1992 under the Tibetan Plateau Seismic Experiment - a joint Sino-French project by the Ministry of Geology and Mineral Resources of China (MGMR) and Institut National des Sciences d'Univers (CNRS), France (Jiang et al., 1995; Hirn et al., 1995). A seismic profile across Tibet, extending essentially NNE fromTingri in southern Tibet to Golmud in Qinghai Province on the north, included 108 digital seismic stations and revealed shear wave anisotropy beneath the Tibetan Plateau and the Qinghai Province (Shi et al., 1999). Our present study analyzed the characteristics of shear wave anisotropy achieved from three-component broadband teleseismic data collected in Tibetan Plateau and its neighboring areas. The time lag between fast and slow wave was usually found bigger than 1.0 s which can not be attributed to crust. The values bigger than 2.0 s may indicate anisotropic materials deep in the upper mantle. The strongest anisotropy appears along the margins of high velocity terrains, often associated with the partial molten material deep in the mantle. In these areas, the formation of anisotropy should have subtle relations with hydrothermal process, earthquake activity and uplifting of Tibetan Plateau. Near the strike-slip faults on the edge of these terrains, the anisotropy direction is consistent with the strikes of the fault systems. This consistency implies that the faults in surface geology may be effected by anisotropy features of upper mantle at the depths of at least 200 km. Figure 1a (after Jiang & Su, 1999) gives a seismic velocity image reaching the depths of 400 km, produced by means of tomography with a rich set of data acquired in western China, Tibet, Qinghai, and Xinjiang. Low-velocity bodies located in the active tectonic zones can be found all along the profile of thousands of kilometers, which are supposed to have close ties with mantle-derived materials, suggesting potential correlation with giant ore deposits. The interpreted mantle plume at the Kunlun Pass (Figure 1b) occupies a special structural position at the boundary between theTibet block and Qaidam Basin. Other preferred locations can be identified especially along the AltynTagh Fault at the boundary between theTibetan Plateau and Tarim Basin. The morphology of the broad region hosting the Tarim Basin (Fig. 1 in the paper by Wittlinger et al., 1998) indicates that some major NW-trending faults may intersect the Altyn Tagh Fault. Also the Magnetic Lineament Map of China (Jiang & Ma, 1991) shows major lineaments (especially the northwest ones) crossing the boundary between the Tibetan Plateau and Tarim Basin and can assist in defining structural intersections as possible targets for mineral exploration.

Magnetic fields over active tectonic zones in ocean

The aim of our work is to estimate the electromagnetic effects that can be detected in the submarine zones with hydrothermal activity. It is known that meso-scale flows appear in the regions over underwater volcanoes or hot rocks. Their origin is connected with heat flux and hot jets released from underwater volcanoes or faults in a sea bottom. Values of mean velocities and turbulent velocities in plumes were estimated. Quasiconstant magnetic fields induced by a hot jet and a vortex over a plume top are about 1–40 nT. Variable magnetic fields are about 0.1–1 nT. These magnetic disturbances in the sea medium create an additional natural electromagnetic background that must be considered when making detailed magnetic surveys.

On tectonic problems of the Okinawa Trough

The Okinawa Trough is a very active tectonic zone at the margin of the Northwest Pacific and is typical of back-are-rifting at the young stage of tectonic evolution. Many scientists from Japan, China, Germany, France, the U.S.A. and Russia have done a lot of geologic and geophysical investigations there. It is well known that the Okinawa Trough is an active back-are rift with extremely high heat flow, very strong hydrothermal circulation, strong volcanic and magmatic activity, frequent earthquakes, rapid subsidence and rifting, well-developed fault and central graben. But up to now, there are still some important tectonic problems about the Okinawa Trough that require clarification on some aspects such as the type of its crust, its forming time, its tectonic evolution, the distribution of its central grabens, the relationship between its high heat flow and tectonic activity. Based on the data obtained from seismic survey, geomagnetic and gravity measurements, submarine sampling and heat flow measurements in the last 15 years, the author discusses the following tectonic problems about the Okinawa Trough: (1) If the Okinawa Trough develops oceanic crust or not. (2) Is the South Okinawa Trough tectonically more active than the North Okinawa Trough with shallower water and few investigation data on it. (3) The formation time of the Okinawa Trough and its tectonic evolution. The Okinawa Trough has a very thin continental crust. Up to now, there is no evidence of oceanic crust in the Okinawa Trough. The North, Middle and South Okinawa Trough are all very strongly active areas. From 6 Ma B. P., the Okinawa Trough began to form. Since 2 Ma, the Okinawa Trough has been very active.

An attempt to determine the tritium, 22Na, 36Cl and radon in territory of mud volcano in Taman

Little is known about the radio-isotopic geochemistry of mud volcanoes in the Taman area, Krasnodar region, in the tectonic active zone of the Caucasus range. In 1998, we have started the determination of radon in soil air using solid state nuclear track detectors on a monthly basis. Tritium was determined out of water sampled at the mud volcano vents by means of both mass spectrometry and liquid scintillation. 22Na, 36Cl were also determined by means of a low background gamma spectrometer placed at a depth of 50 m in our underground laboratory. It was found that mud volcano fluids are of partial deep origin.

The seismotectonics of the Marmara region (Turkey): results from a microseismic experiment

The Marmara region is an active tectonic zone characterised by the transition between the dextral strike-slip regime of the North Anatolian Fault (NAF) and the extension regime of the Aegean Sea. Strong historical earthquakes ( M>7) and the presence of known seismic gaps imply a high level of seismic hazard. A synthesis of recent studies of active tectonics in the region is presented, including inland and underwater observations. The branching of the NAF is explained in terms of increasing influence of the extension. Historical information and instrumental seismicity are reinterpreted in order to have a critical appraisal of the existence of large seismic gaps in the central and eastern Marmara Sea. Focal mechanisms of strong earthquakes are used to obtain orientation and shape factor of the deviator of the stress tensor. The resulting tensor is in shear regime ( σ 2 vertical) but close to extension ( R=0.93) with σ 1 oriented N145°0. A microseismic experiment with 48 stations distributed around the Marmara Sea was carried out in October–December 1995. A total of 137 microearthquakes were located and 23 of those were selected to obtain focal mechanism solutions. The epicentral distribution indicates activity along the system of pull-apart basins north of the Marmara Sea. The segment between Marmara Sea and the Saros Bay, activated in 1912, and the Gulf of Izmit, site of the 1754 earthquake, are now silent. Seismic activity is very linear along the northern branch of the NAF, but it is more diffused on the Bursa and Iznik branches, southeast of the Marmara Sea. The stress tensor obtained from the focal mechanisms of the micro-earthquakes is compared to the one inverted from teleseismic data. The microseismic stress is compatible with a shear (intermediate) regime like the one obtained from strong earthquakes, though not as well constrained, one of the acceptable solutions having the same orientation but different shape ( R=0.5).

Combining AIRSAR data of Huon peninsula and Hawaii MR1 data of Huon Gulf, Papua New Guinea, for studies of tectonic and sedimentary processes

Two swaths of airborne synthetic aperture radar (AIRSAR) imagery, acquired over the southern Huon Peninsula, Papua New Guinea, have been digitally merged with aerial photogrammetric topography and with Hawaii MR] acoustic side‐scan imagery and bathymetry of the adjacent seabed of Huon Gulf. This example of land‐sea integration provides a means of examining relationships between tectonic lineaments and sediment transport pathways on land and on the subjacent seabed. Imaging methods and geometries dictate both the regions covered and the information content of the data. The principal limitations in merging the two data sets arise because the coastal and inner shelf zone, in water depths less than 500 m, is inaccessible to the imaging techniques that were used. Indirect methods allow some inferences to be made about the geomorphology of this zone, using surface sound channel data and sea surface imagery. The area examined is part of an active tectonic collision zone in which the northern margin of the Australian plate is being subducted beneath the South Bismarck plate. The edge of the South Bismarck plate has been uplifted 4,000 m since the late Pliocene, forming the Finisterre Range on Huon Peninsula. The record of relative emergence and submergence is seen in subaerially exposed Quaternary marine sediments and in drowned carbonate reefs. The merged radar and sonar imagery clearly indicate rapid erosion and deformation, both on land and under water. The imagery shows details of sediment transport and depositional systems, which are generating large terrestrial fluvial/alluvial plains, coastal fan deltas, and submarine channel deposystems. Fault structures and outcrops of bedrock units are clearly delineated both on land and on the seabed. These observations help explain patterns of onshore and submarine outcrops in terms of interactions between modern offshore sedimentation patterns and progressive deformation.

Regional gas geochemistry in an active tectonic zone, Erzurum Basin, eastern Turkey

Abstract The Karasu Plain is an intermontane sedimentary basin with a Miocene—Quaternary volcanicbasement, andesitic-basalticlava flows and fissure eruptions of basaltic lava. It was filled in the early Quaternary by lacustrine fan-delta deposits, and later by fluvial-alluvial fan deposits. The basin is characterized by NNE-SSW trending sinistral wrench faults on its eastern margin and ENE-WSW trending reverse faults on its southern margin. Both systems of active faults intersect very near to Erzurum, which is considered to be the most likely site for the epicentre of a probable future large earthquake. Regional gas geochemistry has therefore been undertaken adjacent to the city of Erzurum, in order to characterize radon levels over the varied lithologies, examine the potential for geological fault mapping and assess the status of fault activity. Initially,41 sites were sampled and analysed for Rn, CO 2 , O 2 and CH 4 in soils along three traverses to the east (8.26km and 4.5km length) and to the west (6.8km length) of Erzurum. During the first investigation, 495 Rn determinations were made from 165 samples at 41 sites over a ten-day period, during which a small earthquake (M=2.95) occurred. Another 211 sites have been subsequently used for Rn investigations including one traverse through the centre of the city. Additionally,water samples were degassed for Rn and soils tested at five key sites around the Erzurum Plain. Radon was readily detected in soils at all sites. Both 222 Rn and 220 Rn were present at most. Soils over major lithologies gave characteristic signals and, in all traverses, anomalously high values coincided with active faults. Little correlation was found between spring gases (thermal and cold) and adjacent soils. De-gassed spring water Rn values proved highest along the southern margin of the basin and CO 2 values were highest along the northern edge.

First palaeoseismological results from Greece

This is a brief account of our approach to trends in palaeoseismological research that are currently emerging in Greece, with the application of modern investigative methods practised world-wide in active tectonic zones. These techniques include fault stratigraphy in trenches, and dating recent fault reactivations in order to understand and distinguish palaeoseismic events along specific segments of active faults, named Gerakarou (Mygdonia Basin, Thessaloniki area), Palaeochori Fault (Kozani-Grevena, Western Macedonia), Souli (Petousi, Epirus), and Volos-Larissa (Thessaly plain). Four palaeoearthquakes were investigated on the Gerakarou fault segment, with a uniform slip rate and return period around 7000 years. In Western Macedonia "aseismic areas" the Servia-Palaeochori recently activated fault shows a -3500 year quiescence. Three coseismic events on the Souli Fault pre- and post-date to 26000 year deposits.

Chemistry of hydrothermal quartz in the post-Variscan “Bavarian Pfahl” system, F.R. Germany

Chemical analyses of hydrothermal quartz by means of instrumental neutron activation, coulometric, dewatering, and decrepitation analysis are combined with previously published microthermometric and isotopic data in order to unravel the processes involved in the formation of the post-Variscan hydrothermal system “Bavarian Pfahl”. The Bavarian Pfahl is interpreted as a fossil zone of tectonic activity and large-scale channelized fluid movement in the upper continental crust. The predominance of K relative to Ca and Na in quartz as well as in fluid inclusions strongly supports the hypothesis that incongruous dissolution of feldspar during wall-rock alteration and K-metasomatism during fluid ascent along the shear zone accompanied by intense silicification are characteristic features of important fluid pathways in the crust. The element contents detected in quartz along a 70 km long profile can be correlated with variable wall-rock compositions. High concentrations of Ba, As, Rb, Sr, and Cs are found in the vicinity of sedimentary basins at the western margin of the Bohemian Massif. Elevated element contents in this part of the profile probably result from intense water-rock interaction within the adjoining sedimentary basin. High Ba-contents in the vicinity of the fluorite-barite mining district of Nabburg/Wölsendorf indicate a genetic relationship between both mineralizations. Low Cl/Br-ratios can be generated either by interaction with carbonaceous sediments in these basins or by interaction with graphite in basement rocks. The calculated salinities of the fluid inclusions vary widely, but correlate well with microthermometric data, and indicate mixing of basement or sedimentary brines with high salinity and meteoric water with low salinity. The effects of polyphase cataclasis, a result of seismic pumping, on the fluid inclusion population and the mechanical properties of quartz have been studied by combined dewatering and decrepitation analyses. Despite minor regional variations in the chemical composition of quartz along the profile, the overall uniform element pattern supports homogeneity of the hydrothermal system over distances of at least 70 km and probably over the total length of about 150 km. The fluid volume required for the formation of the Bavarian Pfahl is estimated to correspond to the volume of the Baltic Sea (approx. 23,000 km 3). In analogy with hydrothermal springs in the vicinity of recently active shear zones and faults the lifetime of the hydrothermal system “Bavarian Pfahl” is estimated to be about 160,000 years.

A cellular-automata, slider-block model for earthquakes II. Demonstration of self-organized criticality for a 2-D system

SUMMARY It has been suggested that distributed seismicity is an example of self-organized criticality. If this is the case, the Earth’s crust in an active tectonic zone is in a near-critical state and faults can interact over large distances. Observed seismicity and earthquake statistics are consequences of the dynamical interaction of seismic faulting over a wide range of scales. We address this problem by considering a two-dimensional array of slider blocks with static/dynamic friction. The twodimensional system is treated as a cellular automaton such that only one slider block is allowed to slip at a given time and interacts only with its nearest neighbours through connecting springs. Because of this treatment, the amount of slip for each failed block can be obtained analytically, and the system is deterministic with no stochastic inputs or spatial heterogeneities. In many cases, the slip of one block induces the slip of adjacent blocks. The size of an event is specified by the number of blocks that participate in the event. The number of small events with a specified size has a power-law dependence on the size with a power close to -1.36. The distributions of normalized recurrence times for most events are close to a Poisson process, and gradually deviate towards periodicity for large events. The recurrence time statistics are generally insensitive to parameter variations. Large events may occur at stress levels considerably lower than the failure strength of an individual block, and the stress drops associated with large events are generally small. This may provide an explanation for observed low stress levels in tectonically active

Environmental study of valley fill sediments

The study of Pleistocene and Holocene fluviatile deposits in river valleys is important for environmental protection. Well records allow the mapping of clay, sand, and gravel beds under recent valley floors. Such surveys can also locate active tectonic zones. This technique helps in assessing potential seismic hazards for large industrial sites. The study of fluviatile sediments under recent valley floors is relevant to environmental studies for another reason as well. The permeability characteristics of these formations have a decisive bearing on the spreading of various pollutants entering the rivers or the groundwater along the rivers. Thus, the knowledge of subsurface formations is essential for the siting of factories with dangerous products and deposits of hazardous waste. Fluviatile sediments along the Hungarian stretch of the Danube Valley have been explored by thousands of shallow boreholes. These allowed the geotectonical and hydrogeological study of sites for completed and planned dams and nuclear power stations and assessing potential environmental hazards.

Volcanic activity and crust-mantle exposure in the ultrafast Garrett transform fault near 13°28′S in the Pacific

The 130 km long Garrett transform fault, which offsets two ultrafast (14.5 cm/yr) EPR spreading segments, is the site of volcanic and tectonic activity. Evidence for recent volcanic events was observed during several dives on oblique ridges (3100–3500 m high) trending 040° (Alpha, Beta and Gamma), and on small constructional mounds located in the central transform basin and eastern valley at a depth of 3500–5000 m. The constructional features show a fissured terrain, suggesting a component of lithospheric opening. The strike-slip motion of the transform is inferred at several locations between the two EPR-transform intersection zones, from the presence of abundant sediment-free talus with crushed rock debris, fresh scarps, and well-developed striations on outcrops of serpentinized peridotites and metamorphosed volcanic breccia. The width of the active tectonic zone varies between 200 and 3000 m in the explored area near the eastern and western EPR-transform intersection zones, along the E-W trending Median Ridge walls in the eastern and western transform troughs of the central basin and at several sites on the south-facing wall of the transform domain first escarpment. The presence of recent volcanic activity implies that the Garrett transform behaves as a leaky transform fault. The most common volcanics, consisting of plagioclase and olivine phyric lava erupted in the transform, are compositionally more depleted [ K/Ti< 0.08 ] and less evolved (Mg#60–70) than the basalts encountered on the southern EPR segments [Mg#40–66, and K/Ti= 0.05–0.25 ]. This suggests that different magmatic conditions are operating in the transform domain. The transform strike-slip motion is responsible for the emplacement of a sliver of serpentinized peridotite located in an area about 2 km wide and 10 km long along the Median Ridge at the contact with outcrops of metamorphosed basaltic breccia. The ultramafics consist of residual harzburgites, often impregnated with irregular lenses and dykes of basaltic liquids (plagioclase-wehrlite and gabbro), and are found associated with troctolites, dunite and gabbros. The emplacement of the ultramafics and their gabbroic inclusions has resulted in the presence of metamorphosed tectonic breccia forming part of the Median Ridge in the central transform basin.

日本海中部地震発生による三陸沖地震活動の低下と東北日本のブロック構造

Seismicity in the east-off Sanriku region decreased conspicuously after 26 May 1983, when the central Japan Sea earthquake with a magnitude of 7.7 occurred on the west coast of Tohoku, northeastern Japan. The decrease was restricted to shallow seismicity in the depth range of 0-40 km. For deeper earthquakes in the depth range of 41-70 km rate of earthquake occurrence rather increased. Seismicity in the land area of northern Tohoku also decreased at the same time. However, seismic activity in the Oga-Oshika tectonic zone increased remarkably after the central Japan Sea earthquake. Increase of seismicity was also observed in the sea region corresponding to the extension of the Oga-Oshika tectonic zone. We consider that the coincidence of the notable decrease of shallow seismicity in the coupling region between the Pacific plate and the land plate with the central Japan Sea earthquake occurring on the west coast of Tohoku indicates the following tectonic regime. The northern Tohoku district constitutes one rigid-like block and stress fields in the both sides of the block, e. g., stress at the east-off Sanriku subduction zone and that at the active tectonic zone along the coast of the Japan Sea, influence each other. On the other hand the increase of seismic activity in the Oga-Oshika tectonic zone and in its extension after the central Japan Sea earthquake is considered to show that the zone corresponds to the southern boundary of the northern Tohoku block and change of stress is likely to be transmitted along the zone. Seismicity in the Oga-Oshika tectonic zone has a tendency to increase always when large earthquake occurs in the zone or in its extension. It should be noted that decrease of stress in the coupling region between the Pacific and the land plates due to the occurrence of the central Japan Sea earthquake was confined to the north of the Oga-Oshika tectonic zone. Seismicity in the subduction region to the south of the Oga-Oshika tectonic zone did not show any change when the central Japan Sea earthquake occurred. It is interesting to note as well that seismicity in the Pacific coast and sea regions did not change at all by the 1964 Niigata earthquake with a magnitude of 7.5 whose focal region was located on the east coast of the Japan Sea, but to the south of the Oga-Oshika tectonic zone. These facts suggest that the eastern Japan, eastern side to the Itoigawa-Shizuoka tectonic line, does not constitute a single microplate, but is separated mechanically at the Oga-Oshika tectonic zone.

GEOPHYSICAL CRITERIA FOR“RESUSCITATING”OF THE PANXI ANCIENT RIFT TECTONIC ZONE

Deep structure and properties and features of the geophysical fields of the Panxi region, in the middle of the Kang-Dian (Kangding-Yunnan) tectonic belt, are studied as a whole. The study is based on the detection of magnetic field (including paleomagnetic dating, aeromagnetics), geothermal field, natural earthquake, plate motion, explosion seismology and magneto-electrotelluric sounding.The results show that the Panxi tectonic zone is neither a completely withered ancient rift nor a modern rift system in the heyday having come into being since the Mesozoic or the Cenozoic era, but it is an active tectonic zone (i. e. a "resuscitating" ancient rift zone) formed and reformed by magma activity and tectonic movements in adjacent regions.

Geochemical trends in central and western North Greenland

Geochemical maps and geochemical cross-sections, based on chemical analyses of the &lt; 0.1 mm fraction of stream sediment samples collected at a density of approximately 1 sample per 30 km2 in central and western North Greenland, show that the distribution patterns for the major elements and some trace elements reflect the main lithological units of the North Greenland Palaeozoic platform and trough. By contrast the distribution patterns for S and Sr are different. High S values are correlated with zones of tectonic activity and are thought to indicate migration of H2S along faults. High Sr values are correlated with evaporitic rocks in the platform sequence and with deep sea carbonates. High BaO values occurring along the Silurian platform margin and in the Ordovician platform-slope sequence are the result of Ba enrichment in the sedimentary environment, combined with epigenetic vein-type baryte mineralisation.

Geoscientists will meet to organize transects project

The Global Geosciences Transects (GGT) project is a new activity of the International Lithosphere Program (ILP). The objective of GGT is to compile a series of 100 km‐wide crustal transects across major structures in all continents.The transects will be similar to those being produced by the North American Continent‐ Ocean Transects Program under the coordination of Robert C. Speed (Northwestern University, Evanston, 111.), except that they will include continental interiors and paleostructures (e.g., Precambrian sutures, rifts) as well as continental margins and active tectonic zones

Nankai Trough and Zenisu Ridge: a deep-sea submersible survey

Eight submersible dives between 3000 and 4200 m water depth were made off southern Japan in the eastern Nankai subduction zone. Benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone, at the toe of the accretionary prism. A benthic community was also discovered along a zone of active compression, at the foot of Zenisu Ridge, 30 km south of Nankai Trough. Temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there. Studies of water samples indicate advection of methane and light hydrocarbons. Specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption. Thus the benthic communities are related to overpressure-driven fluid advection along tectonic zones with active surface deformation. A 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280° direction which is close to the 305° average direction of subduction in this area. The dives establish further that compressive deformation is presently occurring at the foot of Zenisu Ridge. The previous interpretation of the Zenisu Ridge as a zone of recent north-south intraplate shortening, 40 km south of the Nankai Trench, is confirmed. We conclude that tectonic evolution might well lead to future detachment of the Zenisu Ridge and overthrusting of this large piece of oceanic crust over the continental margin. Such a process might be an efficient one to emplace ophiolites over continents.

Seismicity and tectonics of the far Western Aleutian Islands

Study of focal mechanisms of earthquakes in the Near and Komandorsky Islands indicate that there are several distinct zones of tectonic activity. South of the Near Islands, normal faulting occurs in the trench east of 172°E and low-angle thrusting dominates the Aleutian ridge. Mechanisms indicate underthrusting as far west as Mednyy Island with strike-slip faulting restricted to the south and west of Beringa Island. A zone of northeast striking left-lateral faulting near 1645.°E is proposed to separate the Aleutian Ridge from Kamchatka Peninsula. This motion, as well as faulting north of the Komandorsky Islands, may be related to the existance of a buffer plate comprising the Aleutian Ridge in the Komandorsky Islands. Active subduction terminates near 173°E and the faulting north of the Komandorsky Islands may, in part, be due to the bouyancy of a remnant slab. Depth phase modelling indicates bulletin-reported depths are overestimated due to a misidentification of depth phases.

On crustal seismicity of the Amazonian foothill of the central Peruvian Andes

A microearthquake survey of crustal seismicity conducted in 1985 on the Eastern Cordillera and the Amazonian foothills in Central Peru gives a description of the present tectonic activity related to the uplift of the Andes. Hypocenters on the Huaytapallana fault are shallower than 10 km, and their focal mechanism is in agreement with the fault trace corresponding to the earthquakes of 1969 and suggests a reverse strike slip movement on a plane striking N130°. An active tectonic zone is evidenced along a NW‐SE direction passing through the Amauta Subandean region. The vertical distribution of hypocenters and the comparison of shallow and deep focal mechanisms suggest a reverse fault dipping to the west. Depths vary between 0 and 20 km. and never exceed 32 km. The Huaytapallana Cordillera is uplifted by thrust faults on both sides.