Abrupt Subsidence Research Articles

Experimental investigation on dynamic deformation characteristics of the overlying strata in backfilling strip mining at different time scales.

The dynamic subsidence disaster caused by underground mining of coal resources is a complex spatiotemporal process, which is a common disaster in mining areas. The backfilling strip mining technology is a green and sustainable coal mining method, which has been commonly used to reduce the subsidence disaster of the overlying strata and protect surface buildings. The transient deformation is the main reason of surface buildings damage; therefore, in this study, the similar material model was used to research dynamic deformation characteristics of the overlying strata in backfilling strip mining at different time scales, and the optical image method was employed to monitor and obtain the movement data of the overlying strata automatically. The data analysis shows that there is a time-scale effect in mining subsidence. The deformation of the overlying strata increases instantaneously at a certain time under the monitoring of small time scale, and this phenomenon gradually disappears as time scales increase. According to the subsidence velocity of small time scale, the subsidence state of the overlying strata can be further divided into the abrupt subsidence state and the gentle subsidence state. This is really significant for promoting the development of the backfilling strip mining technology and preventing the damage of surface buildings.

Relative Sea Level and Coastal Vertical Movements in Relation to Volcano-Tectonic Processes at Mayotte Island, Indian Ocean

During the last 10 kyr, significant subsidence and uplift occurred on Mayotte Island in the Comoros archipelago (Indian Ocean), but the role of volcanic processes in Holocene vertical movements has been neglected in the research so far. Here, we show that an abrupt subsidence of 6–10 m occurred between 9.4 and 10 kyr ago, followed by an uplift of the same amplitude at a rate of 9 mm/yr from 8.1 to 7 kyr ago. A comparison of the relative sea level of Mayotte and a reference sea level curve for the global ocean has been conducted using a modeling approach. This shows that an increasing and decreasing pressure at depth, equivalent to the process caused by a deep magma reservoir (50–70 km), was responsible for ~6–10 m subsidence and 6–10 m uplift, whereas loading by new volcanic edifices caused subsidence during the last few thousand years. Surface movements and deep pressure variations may be caused by pulses from the deep mantle, related to superplume activity, but uncertainties and unknowns about these phenomena are still present and further studies are needed. A better understanding of the volcano-tectonic cycle may improve assessments of volcanic hazards.

MAPPING AND PREDICTION OF LAND SUBSIDENCE

The exact location and timing of a land subsidence-related calamity cannot be foreseen with any degree of confidence. This is true for both progressive subsidence caused by fluid (groundwater, oil or gas) withdrawal and abrupt subsidence on surface caused by underground mine collapse. The best way to deal with these dangers is to mitigate them. In an ideal world, all regions prone to such risks would be well-known, and measures would be made to either prevent causing the problem if it is human-caused, or to avoid inhabiting such areas if they are prone to natural subsidence. Extensometers, levelling, hydrogeology modelling, and GPS are common methods for monitoring subsidence, although they require precise field data and are time-consuming. PSI (Persistent Scatterer Interferometry) is a strong radar-based remote sensing technique for measuring and monitoring surface displacements over time. The techniques of PSI using microwave sensors has made monitoring of earth deformation precisely more reliable and generates large number of Persistent Scatterer Candidates (PSC’s) or sampling points. Although, all the above techniques have their own merits and de-merits but mapping and prediction of susceptible subsidence prone zones is an issue. This paper deals with the application of the Exploratory Data Analysis (EDA), a powerful environment in Data Science provided a set of tools that made it easier to interpret PSI processed Big Data and also improved the capability of validation, management, visualization, and presentation of results with greater reliability.

Enhanced static simulation process of caisson sinking using the coupled Euler-Lagrange method

Caisson sinking is a vital process in marine engineering, and accurate simulations are crucial for predicting and preventing potential accidents or failures. Current numerical simulations necessitate measured engineering parameters, constraining their applicability. This study utilized the coupled Euler-Lagrange (CEL) method to compute specific parameters. These parameters served as substitutes for the engineering measurement values to enhance the static simulation process for the incremental height sinking of caissons, thereby addressing the constraints set by engineering measurements. The CEL method was initially utilized to simulate a dynamic permeation process, generating a series of valuable parameters such as the degree of caisson sinking and mud surface depth at the cavity bottom. Using these parameters instead of engineering measured parameters, a stable state model for incremental height sinking of caissons was established, followed by a static analysis. The improved method was subsequently applied to simulate the construction process involving caisson sinking, demonstrating its feasibility. This enhanced static simulation provides a novel approach for examining the equilibrium state of caisson subsidence, eliminating the need to wait for the actual construction phase to be completed. The proposed method exhibits the critical advantage of favorable predictability, allowing for the anticipation of potential mishaps, such as abrupt subsidence, and supplying timely cautionary information to facilitate proactive measures, ultimately reducing the incidence of engineering accidents or failures.

Unraveling contributions of Greenland's seasonal and transient crustal deformation during the past two decades

Contemporary research on Greenland surface mass balance (SMB) is largely focused on the characteristics of decadal or longer trends, periodic oscillations, and acceleration. However, the specific components of the SMB such as snowfall (SF), rainfall (RF) and runoff (RU), and their corresponding temporal and spatial variability remain poorly understood. Here, we explore the respective contributions of SF, RF, and RU to the seasonal and transient crustal deformations of Greenland during the past two decades using GPS network and satellite gravimetry (GRACE) datasets, and regional climate model output. Our study unraveled that the largest annual vertical displacement caused by precipitations is in southeastern Greenland, reaching 7.27 mm. The largest surface displacement caused by RU is in western Greenland, reaching 19.82 mm. Ice mass gain/loss in Greenland shows a clear correlation between latitude and temperature, with greater variations in the south compared to the north. The transient deformation signals in Greenland mainly manifested in terms of abrupt subsidence in 2010, followed by uplift in 2014. The 2014 uplift can mainly be attributable to the combined effect of SF, RF, and RU. The largest transient signal occurs in the southeast subregions, with peak-to-peak amplitude exceeding 10 mm. Transient crustal deformation is mainly caused by precipitation in southeastern Greenland, while the contribution of RU dominates most of the time and in most subregions. We find that even though RF is increasing due to an increasingly warmer climate, its effect on SMB is still negligible, when compared with SF and RU. In some subregions and some periods, SF could become the primary contributor to transient SMB variations in Greenland.

Abrupt Subsidence of Seasonal Influenza after COVID-19 Outbreak, Hong Kong, China.

The onset of the 2019–20 winter influenza season in Hong Kong coincided with the emergence of the coronavirus disease epidemic in neighboring mainland China. After widespread adoption of large-scale social distancing interventions in response to the impending coronavirus disease outbreak, the influenza season ended abruptly with a decrease to a low trough.

The Impact of Pre‐Salt Rift Topography on Salt Tectonics: A Discrete‐Element Modeling Approach

AbstractGravity‐driven salt tectonics along passive margins is commonly depicted as comprising domains of updip extension and downdip contraction linked by an intermediate, broadly undeformed zone of translation. This study expands on recently published physical models using discrete‐element modeling to demonstrate how salt‐related translation over pre‐salt rift structures produce complex deformation and distribution of structural styles in translational salt provinces. Rift geometries defined by horsts and tilted fault‐blocks generate base‐salt relief affecting salt flow, diapirism and overburden deformation. Models show how flow across pairs of tilted fault‐blocks and variably‐dipping base‐salt ramps associated with pre‐salt faults and footwalls produce abrupt flux variations that result in alternation of contractional and extensional domains. Translation over tilted fault‐blocks defined by basinward‐dipping normal faults results in wide, low amplitude inflation zones above footwalls and abrupt subsidence over steep fault‐scarps, with reactive diapirs that are squeezed and extrude salt as they move over the fault. Translation over tilted‐blocks defined by landward‐dipping faults produces narrow inflation zones over steep fault‐scarps and overall greater contraction and less diapirism. As salt and cover move downdip, structures translate over different structural domains, being inverted and/or growing asymmetrically. Our models allow, for the first time, a detailed evolution of these systems in cross‐section and demonstrate the effects of variable pre‐salt relief, salt sub‐basin connectivity, width and slope of base‐salt ramps. Results are applicable to syn‐ and post‐rift salt basins; ultimately improving understanding of the effects of base‐salt relief on salt tectonics and working as a guide for interpretation of complex salt deformation.

Long-range ground deformation monitoring by InSAR analysis

Abstract. InSAR (Interferometric Synthetic Aperture Radar) analysis is an effective technique to map 3-dimensional surface deformation with high spatial resolution. The aim of this study was to evaluate the capability of InSAR analysis when applied to ground monitoring of an environmental disaster. We performed a time series InSAR analysis using ENVISAT/ASAR and ALOS/PALSAR data and commercial software to investigate subsidence around the Kanto District of Japan. We also investigated techniques for efficient early detection of landslides in Kyushu using time series analysis that incorporated synthetic aperture radar (SAR) images. ENVISAT/ASAR data acquired from 2003–2010 and ALOS/PALSAR data acquired from 2006–2011 were used to detect poorly expressed geomorphological deformation by conducting time series analyses of periodically acquired SAR data. In addition, to remove noise caused by geographical feature stripes or phase retardation, we applied median filtering, histogram extraction processing, and clarification of the displacement with a Laplacian filter. The main functions of the InSAR time series analysis are the calculation of phase differences between two images and the inversion with smoothness constraint for the estimation of deformation along the line of sight. The results enabled us to establish criteria for the selection of suitable InSAR data pairs, and provided the final error estimation of the derived surface deformation. The results of the analysis in the Kanto District suggested that localized areas of uplift and subsidence have occurred at irregular intervals in this area. Furthermore, the method offers the possibility of early warning of environmental disasters such as landslide and abrupt subsidence. Our results confirm the effectiveness of InSAR analysis for the monitoring of ground deformation over wide areas via the detection of localized subsidence and landslides.

Tectonic history and structural development of the Zallah-Dur al Abd Sub-basin, western Sirt Basin, Libya

The Zallah-Dur al Abd Sub-basin area lies in the western part of the Sirt Basin of Libya. 2D seismic data covering an area of about 32,000 km² were studied along with the formation tops from 240 wells. We mapped a complex network of normal and probable strike-slip faults, generally striking NNW-SSE that control the asymmetry of the basin. Subordinate NE–SW structures acted as transverse faults controlling local depocentres that segment the Zallah-Dur al Abd Sub-basin.A number of active faults in the intra-basin area have been identified in seismic sections with generally moderate to high dip angles, and displaying evidence for positive and negative flower structures. The bordering extensional fault (the Gedari fault) passes at depth into a moderately SW-dipping structure crossing most of the Upper Mesozoic to Cenozoic stratigraphic section. Thickness variations adjacent to other major faults suggest also an original extensional system where inherited high-angle faults were reactivated throughout this time.A detailed analysis of the available seismic reflection and drill hole data shows that an obliquely rifted, multi-cyclic, NNW-SSE trending basin developed during the complex Upper Mesozoic Cenozoic rearrangement of Mediterranean tectonics. Multiple phases of rifting can be observed in the study area affecting a number of different horizons from Upper Cretaceous to Eocene.In the study area, the basin was initiated as a result of a Tethyan oblique extensional rift system that began in the Early Cretaceous and peaked in the Late Cretaceous. The basin reached its rift maturation phase during the Upper Cretaceous as a result of the continuing extensional tectonics on the marginal bounding NNW-SSE trending normal growth faults. During the Alpine-related tectonic pulses of Middle–Late Eocene the Sirt Basin underwent compression resulted in northward tilting of the basin, causing abrupt subsidence in the north and uplift on the basin southern shoulders, possibly driving a late stage of minor regional subsidence.

Deformation of Sandy Beach and Inundation on Iwama-Sanuka Coast in Fukushima Prefecture due to the 2011 Tohoku Earthquake Tsunami

The massive earthquake with a magnitude of 9.0 occurred at 14:46 on March 11, 2011, with an epicenter 130 km offshore from the Oshika Peninsula in Japan’s northeastern Miyagi Prefecture. After the earthquake, large tsunamis were generated owing to abrupt crustal subsidence and uplift, which inundated Japan’s eastern Pacific Ocean coastline. We carried out field observations to investigate the deformation of a previous river-mouth bar by comparing oblique photographs, and investigated the damage to seawalls and the tsunami inundation depth on the Iwama-Sanuka coast, located north of the Same River in southern Fukushima Prefecture. Here, the results of the field observations on the deformation of the sandy beach and the inundation of the Iwama-Sanuka coast are reported.

Sedimentary records of rift to pull-apart tectonics in the Miocene Eoil Basin, SE Korea

The Miocene Eoil Basin, SE Korea, is a pull-apart basin formed by dextral crustal deformation during the backarc opening of the East Sea. The Gampo Conglomerate, the lowermost formation of the basin, provides significant information on how a pull-apart basin initiates and is filled by sedimentary strata. The formation consists of nonmarine conglomerates, sandstones, mudstones, and a variety of volcaniclastic deposits that were deposited in alluvial fans, delta slopes, braided streams, mouth bars, and lakes. These deposits are organized into several stratal packages (SP) with contrasting geometries. SP I at the base consists of mouth-bar to delta-front and volcaniclastic deposits, and shows rapid facies variations along the southeastern normal fault margins. SP I suggests that the early basin was bounded by multiple, incipiently developed normal faults with variable displacement and accommodation creation. SP II consists of thick alluvial-fan conglomerates and overlies SP I abruptly, suggesting sudden input of coarse-grained sediment from the uplifted footwalls. It is thickest near the central part of the southeastern normal fault margins, suggesting development of a large alluvial fan near a transfer zone. SP III consists mostly of mouth-bar sandstones, indicating abrupt subsidence of the basin and the transition from subaerial to subaqueous environments. SP IV consists of braided-stream conglomerates, suggesting gradual progradation of coarse-grained fluvial systems toward the basin when the extension and subsidence of the basin was slowed. SP V shows upward-deepening facies changes from braided-stream to mouth-bar deposits, suggesting resumption of basin subsidence. Notably, the cumulative thickness of SP III to SP V is thickest near the northeastern and southwestern strike-slip margins, suggesting migration of the loci of maximum subsidence from the normal fault margins towards the strike-slip margins after deposition of SP II. The change in stratal architecture suggests that the activity of the normal faults was dominant in the early stage of the basin evolution whereas the activity of the strike-slip faults became more pronounced afterwards. Overall, the stratal architecture of the Gampo Conglomerate appears to record the transition tectonics from rift to pull-apart during the incipient stage of a pull-apart basin development. The Eoil Basin is thus interpreted to have been hybrid in nature, influenced by the dynamic changes of the basin-margin faults, which were either normal or strike-slip.

Dating of late Holocene beach shoreline positions by regional correlation of coseismic retreat events in the Columbia River littoral cell, USA

Beaches and barrier spits of the Columbia River littoral cell (CRLC), 160 km in length, experienced substantial progradation, 0.5–2.5 km in width, during the late Holocene. The accreted beach plains and barriers preserved evidence of episodic catastrophic retreat that was forced by coseismic subsidence. The abrupt subsidence events (0.5–1.5 m submergence) are associated with slip on the underlying Cascadia megathrust, with a mean recurrence interval of ∼ 500 yr. Ground penetrating radar (GPR) profiles taken across the barriers and beach plains show high-angle beach retreat scarps (3–15 m vertical relief) and foredune-ridge slump scarps (5–15 m vertical relief). The upper-shoreface retreat features are correlated throughout the littoral cell by radiocarbon age, position relative to abandoned foredune ridges, and distinct sequences of large and small scarps. Particularly large retreat scarps (10–15 m in height) are associated with the first oldest and fourth oldest prehistoric scarps, ‘A’ and ‘D’, respectively. A maximum of 10 retreat scarps in the Long Beach subcell date from 216 ± 211 calRCYBP to 4756 ± 114 calRCYBP. The youngest seven scarps (A–G) are correlated between all four subcells, showing a regional response to the coseismic strain cycles. Backshore disturbance features (1–3 m vertical relief) are observed in the GPR profiles, but they are not identified here due to their discontinuities between adjacent profiles at ∼ 5 km longshore spacing. The seven regional scarps (A–G) are directly correlated to seven coseismic subsidence events, reported to date between 0.3 and ∼ 3.2 ka. The correlation of scarps to earthquake events provides high-resolution proxy ages for shoreline positions in late Holocene time. The proxy shoreline positions are dated at 0.3, 1.1, 1.3, ∼ 1.7, ∼ 2.5, ∼ 2.8, and ∼ 3.2 ka, based on buried wetland sequences from adjacent tidal basins. The paleoshoreline dating demonstrates that the onset of net shoreface progradation (between 4.7 and 0.3 ka) was delayed with increasing distance (0–100 km) from the Columbia River sand source. The history of beach progradation reflects the sequential filling of bay and shoreface accommodation spaces located down-drift of the Columbia River mouth. These late Holocene shoreline changes portend future redistributions of existing sand within the littoral system as the limited sand reserves continue to migrate away from the Columbia River mouth source.

FORAMINIFERAL EVIDENCE OF HOLOCENE SUBSIDENCE AND FAULT DISPLACEMENTS, COASTAL SOUTH OTAGO, NEW ZEALAND

Foraminiferal assemblages preserved in estuarine sedimentary sequences along the coast of South Otago, New Zealand, provide evidence for three Holocene fault-displacements and associated earthquakes within the 250 km-wide continental collision zone along the Australian-Pacific plate boundary. The modern analogue technique was used to estimate paleotidal elevations of the fossil foraminiferal faunas, based on an analogue set of 334 modern New Zealand faunas (including data from three new intertidal transects taken from the studied estuaries). These elevation estimates provide a record of abrupt changes in tidal elevation, which are inferred to have been produced by vertical displacements on nearby faults. The foraminiferal record in cores (0.75–2.8 m long) from Akatore Estuary identify the previously recognized last rupture (,1100 cal yrs BP) on the Akatore Fault, with ,0.4 m of subsidence on the western side produced by actual downthrow or shaking-related compaction of underlying sediment. The foraminiferal record in cores (1.4– 7.0 m long) and in an uplifted terrace section from tidal Catlins Lake provides the first evidence of Holocene fault rupture and major earthquake risk south of the Clutha River mouth. Three displacement events on the Settlement Fault, which forms the eastern margin of Catlins Lake, are recognized. The youngest (,1000 cal yrs BP) resulted in 0.4 m of subsidence (downthrow or compaction) on its west side. The penultimate (,3600 cal yrs BP) resulted in ,1.2 m of uplift of an extensive terrace on the east side of the fault with consequent narrowing of the estuarine entrance and reduced salinity in the tidal lake (recorded by foraminiferal faunas). The earliest event (,5000–4500 cal yrs BP) is recognized by ,1 m of abrupt subsidence on the west (Catlins Lake) side of the fault. Foraminiferal faunas in the upper parts of the majority of cores (from high tidal salt meadows) also document ,0.3 m of sea-level rise within the last 150 years.

Palaeoenvironments and palaeoecology of the Early Pennsylvanian Lancaster Formation (‘Fern Ledges’) of Saint John, New Brunswick, Canada

The Pennsylvanian Lancaster Formation (‘Fern Ledges’) of Saint John, New Brunswick, Canada contains a world-famous fossil biota. Largely unstudied since the works of Hartt, Dawson, Matthew, and Stopes in the mid-nineteenth to early twentieth century, we present new data concerning biostratigraphy, taxonomy, palaeoenvironments, and palaeoecology. Megafloral assemblages suggest a mid- to late Langsettian age for the succession, making it approximately coeval with the classic Joggins Formation of nearby Nova Scotia. Facies analysis suggests deposition on a tectonically influenced coastal plain whose braided channels drained into a shallow brackish gulf. Most of the historical fossil collections are derived from flooding surfaces formed by abrupt subsidence events along the coastline. Three communities are recognized. Phoronids, crustaceans, and xiphosurans lived in brackish coastal waters. A lowland community of gastropods, insects, arachnids, and myriapods inhabited coastal forests. Coastal vegetation was dominated by shrubby cordaitaleans and pteridosperms whereas ferns, sphenopsids, and lycopsids were rare. An upland or dryland community, discernible from allochthonous assemblages, comprised forests of giant cordaitaleans, archaic pteridosperms, and plants of uncertain affinity.

Depositional controls on Palaeoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA

AbstractShallowing‐upward, decametre‐scale, Palaeoproterozoic iron formation cycles in northern Wisconsin record the combined effects of tectonism and changing oceanographic conditions on a storm‐dominated shelf. Cycles consist of a lower unit of laminated, Fe‐ and Si‐rich chemical mudstone that is transitional into an upper unit dominated by trough cross‐stratified chert grainstone. Grainstone lenses become progressively thicker upwards in cycles with the largest at cycle tops, where they are sharply overlain by a unit of slumped chemical mudstone. The cycles developed through progradation when offshore‐directed storm currents transported chert sand intraclasts that were formed in nearshore settings into middle and distal shelf environments. Abrupt subsidence events, probably resulting from normal faulting associated with extensional tectonism, repeatedly terminated chert grainstone accumulation and may also have generated the slumped units at cycle boundaries. The episodic storm currents are also interpreted to have transported biologically oxygenated waters from the shallow‐water, inner shelf into otherwise anoxic bottom waters of the strongly stratified distal shelf. The consequence of such transport and mixing was rapid deposition of chemical mud, mainly as precipitated Fe‐oxide. In many cases, the resultant decrease in Fe2+ in the water column, together with pelagic inorganic precipitation of SiO2 and rainout of terrigenous clays, resulted in submillimetre‐ to millimetre‐thick, chemically graded laminae. The concomitant decreasing Fe2+/Mn2+ ratio also led to increasing Mn‐compound precipitation and enrichment in the upper portions of some chemically graded layers.

Dynamic Action of Rock Mass on the Powered Support Legs

The results are presented for the in-situ measurements of dynamic loads on powered support units. These measurements were made in four longwalls prone to abrupt roof subsidence. The cases of dynamic action of rock mass on canopies and bases of support units are identified. All recorded parameters have a precise statistical linear dependence between the maximum load on the support leg and its initial setting load.

On the convergence of myth and reality: examples from the Pacific Islands

Various (groups of) myths from the Pacific Islands are discussed. Generic groups considered are diluvian myths, myths involving abrupt subsidence, abrupt uplift, and simultaneous abrupt subsidence and uplift. Specific myths, recently validated, illustrate the superiority of the mythic explanation for recent volcanism over available geological information, and the possibility of myth recalling a migration which took place nearly 3000 years ago. The implications of the correct interpretation of the geographical basis of myths for an understanding of environmental change in the Pacific are explored. Particularly in a region where written history is relatively recent, myths have the potential for extending available chronologies of particular phenomena and allow consideration of issues such as the role of infrequent catastrophic events in landscape evolution and the role of (rapid) environmental change in cultural transformation to be considered more fully.

Geologic evidence of earthquakes at the Snohomish delta, Washington, in the past 1200 yr

Exposed channel banks along distributaries of the lower Snohomish delta in the Puget Lowland of Washington reveal evidence of at least three episodes of liquefaction, at least one event of abrupt subsidence, and at least one tsunami since ca. A.D. 800. The 45 measured stratigraphic sections consist mostly of 2‐4 m of olivegray, intertidal mud containing abundant marsh plant rhizomes. The most distinctive stratigraphic unit is a couplet comprising a 0.523-cm-thick, laminated, fining-upward, tsunami-laid sand bed overlain by 2210 cm of gray clay. We correlated the couplet, which is generally ;2 m below the modern marsh surface, across an ;20 km2 area. Sand dikes and sand-filled cracks to 1 m wide, which terminate upward at the couplet, and sand volcanoes preserved at the level of the sand bed record liquefaction at the same time as couplet deposition. Differences in the type and abundance of marsh plant rhizomes across the couplet horizon, as well as the gray clay layer, suggest that compaction during this liquefaction led to abrupt, local lowering of the marsh surface by as much as 50‐75 cm. Radiocarbon ages show that the tsunami and liquefaction date from ca. A.D. 800 to 980, similar to the age of a large earthquake on the Seattle fault, 50 km to the south. We have found evidence for at least two, and possibly as many as five, other earthquakes in the measured sections. At two or more stratigraphic levels above the couplet, sand dikes locally feed sand volcanoes. Radiocarbon ages and stratigraphic position gov. suggest that one set of these dikes formed ca. A.D. 910‐990; radiocarbon ages on a younger set indicate a limiting maximum age of A.D. 1400‐1640. We also interpret a sharp lithologic change, from olive-gray, rhizome-rich mud to grayer, rhizome-poor mud, ;1 m above the couplet, to indicate a second abrupt lowering of the marsh surface during an earthquake ca. A.D. 1040‐ 1400, but no conclusive liquefaction structures have been identified at this horizon. Two distinctive coarse-sand laminae, 30‐80 cm below the couplet, may record tsunamis older than A.D. 800. Thus, study shows that in the past ;1200 yr, this part of Washington’s Puget Lowland has been subjected to stronger ground shaking than in historic times, since ca. 1870.

Morphotectonics, Volcanism and Hydrothermal Activity on the East Pacific Rise between 21°12′ S and 22°40′ S

The morphotectonic setting of the East Pacific Rise (EPR) between21°12′ and 22°40′ S and its recent and past hydrothermalactivity were the focus of the Russian R/V Geolog Fersman’s expeditionin 1987–1988. The EPR axial zone in the study area is comprised of three segmentsseparated by overlapping spreading centers (OSCs) near 21°44′ and22°08′ S. The northern segment is the shallowest of three and hasa distinct massive axial ridge, trapeziodal in cross-section, toppedby a very wide flat summit surface and cut by a well-developedcentral graben. These features testify to intense magmatism and to avoluminous crustal magmatic chamber underlying the whole segment.Fine-scale segmentation is most clearly revealed in the structure ofthe central graben within which several 4th-order segments can bedistinguished. This scale of segmentation is also reflected on flanks of theaxis by variations in the character and intensity of faulting. According to structural and petrologic data, the magmatism is mostintense in the central part of the segment which is probably locateddirectly over a magmatic diapir supplying the melt to the whole segment.Magma migration at the subcrustal level from the center towards the ends ofthe segment with discrete injection into the crustal magmatic chamber ispresumed. The central segment is broken into two morphologically distinct partsseparated by a deval. In the subsided northern part, the wide summit of theaxial ridge is cut by a well-developed, intensely fractured axialgraben. In the southern part, the axial ridge is relatively elevated, butnarrow with an ephemeral graben along its crest. The character and intensityof faulting on the axial flanks are also considerably different in thenorthern and southern parts of the segment. Thus, the magmatic supply tothese two parts is thought to originate from two different sources. If so,then at present the magma chamber underlying the southern part of thesegment is probably at the stage of replenishment, while in the north it isat the stage of deep cooling. The southern segment is structurally similar to the central one. Howeverthere is considerably less intensive magmatic activity in this region,especially south of 22°30′ S where the axial ridge is narrow, andtriangular in cross-section. Both OSCs studied are marked by abrupt narrowing and sharp subsidence ofthe tips of axial ridges within the northern limbs. The southern OSC limbsare morphologically similar to normal sections of axial ridges. In bothcases the flanks are structurally and morphologically disrupted adjacent tothe OSCs and oblique structures can be traced far southward of the OSCflanks. Due to the spatial position of oblique structures on the the flanksit is presumed that the OSC near 22°07′ S is migrating northward.The 21°44′ S OSC zone has apparently undergone small spatialoscillations. In spite of the small amplitude of lateral displacement, thiszone is marked by prominent bathymetric anomalies. Numerous massive sulfide deposits were discovered atop the axial ridgealong the entire length of the uplifted and hydrothermally active northernsegment. Ore metal concentrations in near-bottom waters are maximumover the southern part of the northern segment, while maximum concentrationsof the same metals in surficial sediments are confined to the central partof the same segment. We surmise that there has been a recentalong-axis shift of the zone of maximum hydrothermal activity fromthe middle of the segment to its present position in the southern part ofthe segment. Considering sedimentation rates, the age of this shift can beapproximately estimated to be 5 to 10 thousand years before the present. The relatively Mg-enriched basalts of the middle part of thenorthern segment represent a tike of a more primitive pattern, while therelatively Fe-rich rocks of its southern part probably reflect alarge degree of fractionation at shallow crustal levels. Considering thistrend, in addition to morphotectonic data we presume that subaxial magmaflow from the middle to the southern part of the segment is responsible forthe along-axis shift of hydrothermal activity. In the central segment of the study area, massive sulfides have only beendiscovered south of the 21°55′ S deval, where the axial ridgeshoals and where the existence of a subjacent magma chamber is presumed. The very weak manifestations of recent volcanism within the southernsegment explain the absence of hydrothermal activity and sulfide depositswithin this segment.

Tree Ring Correlation Between Prehistoric Landslides and Abrupt Tectonic Events in Seattle, Washington

Radiocarbon ages of submerged trees on landslide deposits in Lake Washington, Seattle, indicate that the most recent slides in three separate areas may have occurred simultaneously about 1000 years ago. Tree ring crossdating shows that seven bark-bearing trees from one of these recent slides and a tree 23 kilometers to the northwest in a probable tsunami deposit on the shore of Puget Sound died in the same season of the same year. The close coincidence among the most recent lake landslides, a probable tsunami, abrupt subsidence, and other possible seismic events gives evidence for a strong prehistoric earthquake in the Seattle region.