Passive Seismic Studies Research Articles

Ambient seismic noise analysis of LARGE-N data for mineral exploration in the Central Erzgebirge, Germany

Abstract. Ambient seismic noise tomography is a novel, low-impact method for investigating the earth's structure. Although most passive seismic studies focus on structures on crustal scales, there are only a few examples of this technique being applied in a mineral exploration context. In this study, we performed an ambient seismic experiment to ascertain the relationship between the shallow shear wave velocity and mineralized zones in the Erzgebirge in Germany, one of the most important metal provinces in Europe. Late Variscan mineralized greisen and veins occurring in the Geyer-Ehrenfriedersdorf mining district of the Central Erzgebirge were mined from medieval times until the end of the 19th century. These occurrences represent a significant resource for commodities of high economic importance, such as tin, tungsten, zinc, indium, bismuth and lithium. Based on ambient noise data from a dense “LARGE-N” network comprising 400 low-power, short-period seismic stations, we applied an innovative tomographic inversion technique based on Bayesian statistics (transdimensional, hierarchical Monte Carlo search with Markov Chains using a Metropolis/Hastings sampler) to derive a three-dimensional shear wave velocity model. An auxiliary 3D airborne time-domain electromagnetic dataset is used to provide additional insight into the subsurface architecture of the area. The velocity model shows distinct anomalies down to approximately 500 m depth that correspond to known geological features of the study area, such as (a) gneiss intercalations in the mica schist-dominated host rock, imaged by a SW–NE striking low-velocity zone with a moderately steep northerly dip, and (b) a NW-trending strike-slip fault, imaged as a subvertical linear zone cross-cutting and offsetting this low-velocity domain. Similar to the velocity data, the electromagnetic data exhibit north-dipping (high-conductivity) structures in the mica schists, corresponding to the strike and dip of the predominant metamorphic fabric. An unsupervised classification performed on the bivariate 3D dataset yielded nine spatially coherent classes, one of which shows a high correspondence with drilled greisen occurrences in the roof zone of a granite pluton. The relatively high mean shear velocity and resistivity values of this class could be explained by changes in density and composition during greisen formation, as observed in other areas of the Erzgebirge. Our study demonstrates the great potential of the cost-efficient and low-impact ambient noise technology for mineral exploration, especially when combined with other independent geophysical datasets.

Insights Into the Source of Magmatic Hot-Lines: Forty Years of Geophysical Studies of the Cameroon Volcanic Line

The Cameroon Volcanic Line (CVL) of west Africa poses many challenges to common models of mantle processes and volcanic system genesis. Although the CVL superficially resembles the hot-spot trace of a mantle plume, there is no evidence for age progression of initiation or cessation of volcanic activity, making the CVL one of several hot-lines observed on Earth, which exhibit asynchronous volcanism along each line. This unusual form of volcanic system, together with the complexity of the surrounding geology, which includes a Mesozoic rift, an Archean Craton, a Proterozoic mobile belt, and a major continental-scale shear zone, have fueled numerous geophysical investigations of the crust and upper mantle structure within the region. This review highlights major findings from geophysical studies, including controlled source and passive source seismic studies and gravimetric modeling, conducted in the late 20th and early 21st centuries. Potential geodynamic models for the formation of the CVL are examined and evaluated based on the body of geophysical evidence. Finally, several key outstanding questions about the structure and formation of the CVL are highlighted, potentially guiding future studies of this region.

Passive seismic studies of the Capricorn Orogen, Western Australia

The study of incoming seismic waves from teleseismic earthquakes can be used to investigate the deep structure of the Earth’s crust and upper mantle. Resolving the deep crustal structure and locati...

A tale of two orogens: Crustal processes in the Proterozoic Trans‐Hudson and Grenville Orogens, eastern Canada

The Precambrian core of North America was assembled in the Proterozoic by a series of collisions between Archean cratons. Among the orogenic belts, two stand out due to their significant spatial extent. The Paleoproterozoic Trans‐Hudson Orogen (THO) and Mesoproterozoic Grenville Orogen extend for thousands of kilometers along strike and hundreds of kilometers across strike. Both have been compared to the present‐day Himalayan‐Karakoram‐Tibetan Orogen (HKTO). Over the last 20–30 years, active and passive source seismic studies have contributed a wealth of information about the present‐day crustal structure and composition of the two orogens in Canada. The Proterozoic orogenic crust is generally thicker than that of neighboring Archean terranes, with a more variable Moho character, ranging from relatively sharp to highly diffuse. Both orogens have a prominent high‐velocity lower crustal layer, consistent with long‐term preservation of a partially eclogitized root at the base of the crust and similar to that inferred beneath the western HKTO. Crustal structure in the northern THO strongly resembles the lower crustal structure of the HKTO, suggesting that Moho depths may have reached 60–70 km when the orogen was active. A prominent midcrustal discontinuity beneath the central Grenville Province and changes in the patterns of seismic anisotropy in the THO crust beneath Hudson Bay provide geophysical evidence that lower crustal flow likely played a role in the evolution of both orogens, similar to that inferred beneath the present‐day HKTO. The seismic evidence from Canada supports the notion of tectonic uniformitarianism, at least as far back as the Paleoproterozoic.

Feasibility of using molecular-electronic seismometers in passive seismic prospecting: Deep structure of the Kaluga ring structure from microseismic sounding

Field experiments involving molecular-electronic seismometers, along with conventional pendulum geophones, were performed to study the deep structure of the upper crust (in the area of the Kaluga ring structure) by passive seismic methods. The microseismic sounding survey was carried out along a geophysical profile crossing the central part of the structure with simultaneous data acquisition by molecular-electronic and conventional seismometers at each measurement point. Experimental data on the propagation of Rayleigh waves along the curvilinear surface have been collected. The feasibility of using molecular-electronic seismometers for passive seismic studies has been confirmed by the results of comparative analysis of the vertical geophysical cross sections, which reveal upper crustal heterogeneities, and by the results of a series of laboratory tests.

Passive seismic studies show configuration of Paleoproterozoic subduction zones and their role in craton assembly in Western Australia

The Capricorn Orogen Passive source Array (COPA) is the passive source seismic component of a major Science and Industry Endowment Fund (SIEF) project, “The distal footprints of giant ore systems: Capricorn case study”. COPA focuses on the deep crustal and shallow lithospheric structure in the Capricorn Orogen with the aim to better understand the tectonic amalgamation of the Western Australian Craton. The objective of COPA is to produce 3D multiple scale seismic images across the orogeny, that together with other geological and geophysical datasets help constrain the timing and kinematic evolution of Capricorn Orogen’s fault zones, and their role in the metallogenic history.

Mapping the indentation between the Iberian and Eurasian plates beneath the Western Pyrenees/Eastern Cantabrian Mountains from receiver function analysis

In the last decades, active seismic profiling in the northern part of the Iberian Peninsula has evidenced that the Alpine collision between the Iberian and Eurasian plates resulted in a complex crustal structure, with the Iberian crust underthrusting the Eurasian crust and reaching depths of at least 45–50km beneath the Pyrenean chain and the Cantabrian Mountains. In the transition between these two zones the situation is particularly complex, as evidenced in previous wide-angle and passive seismic studies. This contribution focuses in getting new clues on the crustal structure of this transitional zone through receiver function (RF) analysis of teleseismic data recorded at permanent and temporary stations located in both the Spanish and French sides of the Western Pyrenees. Different techniques (H-κ stacking, pseudo-migration, synthetic 2D modeling) have been considered in the analysis. Passive seismic data from previous temporary deployments in the zone have been reworked and added to the discussion. A first order result is that passive seismic data are broadly consistent with the indentation of the Iberian and Eurasian crusts inferred from active seismic profiling, thus providing a completely independent confirmation of this feature. For the first time, an Iberian Moho underlying the Eurasian crust is documented from RF beneath the stations located at the Northern side of the Pyrenean range. Moreover, clear indications of dipping interfaces are observed at some stations. The new RF results suggest that in the crustal indentation beneath the Basque Massifs area, the Eurasian crust extends farther south with respect to the image inferred from active seismic data. This new geometry implies that the Pamplona transfer zone has played a major role in the regional geodynamic history.

Seismic Arrays on Drifting Ice Floes: Experiences from Four Deployments in the Arctic Ocean

In the last three decades our knowledge about the seismic structure and the seismicity of the world's oceans has increased tremendously through the common availability of ocean bottom seismometers for active and passive seismic studies ( e.g. , Orcutt et al. 1976; Toomey et al. 1985; Tolstoy et al. 2008). The safe recovery of the instruments is essential to access the acquired data. Comparable studies of the polar oceans do not exist, because the perennial sea-ice cover in polar regions prevents a safe recovery of ocean bottom instruments. Hence, the seismic structure and the earthquake activity of large parts of the Arctic Ocean, including the Gakkel Ridge, one of the most prominent members of ultraslow-spreading mid-ocean ridges (< 20 mm/y full rate; Dick et al. 2003), are largely unknown. Mounting conventional land seismometers on drifting sea ice is the only way to acquire seismic data in the Arctic Ocean. The difficult access to the survey area, which requires icebreaking vessels and helicopters, and the cold temperatures and a constantly moving seismometer platform, represent highly unsuitable survey conditions. However, Kristoffersen et al. (1982) made first attempts at recording regional earthquakes in the Arctic by attaching a sonobuoy array to an ice floe. Jokat et al. (2003) demonstrated that the acquisition of refraction seismic data is possible with seismic stations mounted on drifting ice floes. Okal and MacAyeal (2006) report on a deployment of seismometers on icebergs in Antarctica recording a variety of ice-induced (cryogenic) signals (MacAyeal et al. 2009). We deployed seismic arrays on drifting sea ice during four expeditions in 2001, 2007, 2008, and 2009, recording local and regional earthquakes along Gakkel Ridge. In addition, the records contained many sorts of cryogenic signals (“icequakes”) stemming possibly from collisions between ice floes, crack formation, or disintegration of ice floes …

Broadband Seismic Deployments for Imaging the Upper Mantle Structure in the Lützow-Holm Bay Region, East Antarctica

Broadband seismic deployments have been carried out in the Lützow-Holm Bay region (LHB), Dronning Maud Land, East Antarctica. The recorded teleseismic and local events are of sufficient quality to image the structure and dynamics of the crust and mantle of the terrain. Passive seismic studies by receiver functions and shear wave splitting suggest a heterogeneous upper mantle. Depth variations in topography for upper mantle discontinuities were derived from long period receiver function, indicating a shallow depth discontinuity at 660 km beneath the continental area of LHB. These results provide evidence of paleo upwelling of the mantle plume associated with Gondwana break-up. SKS splitting analysis anticipated a relationship between “fossil” anisotropy in lithospheric mantle and past tectonics. Moreover, active source surveys (DSSs) imaged lithospheric mantle reflections involving regional tectonic stress during Pan-African and succeeding extension regime at the break-up. By combining the active and passive source studies of the mantle structure, we propose an evolution model of LHB for constructing the present mantle structure.

Towards a better understanding of African topography: a review of passive-source seismic studies of the African crust and upper mantle

Abstract Explaining the cause and support of Africa's varied topography remains a fundamental question for our understanding of the long-term evolution of the continent. As geodynamical modelling becomes more frequently used to investigate this problem, it is important to understand the seismological results that can be incorporated into these models. Crustal thickness estimates are crucial for calculating components of topography that are isostatically compensated. Variations in seismic velocity help constrain variations in subsurface temperature and density and thus buoyancy; measurements of anisotropy can also be used to determine the contribution of the mantle flow field to dynamic topography. In this light, we review the results of passive seismic studies across Africa. At the continental scale there are significant differences in crustal models, meaning large uncertainties in corrections for isostatic topography. In east Africa, multiple seismic experiments have provided firm constraints on crustal and mantle structure. Tomographic images illuminate a broad ( c. 500 km wide) low-velocity region in the upper mantle, with possible connection to the African Superplume in the lower mantle. These observations, alongside the variations in radial anisotropy, strongly suggest that the mantle flow field contributes significantly to the uplift of the region. Beneath southern Africa, low velocities are observed near the base of the continental lithosphere; the depth to transition zone discontinuities however suggests that they are not linked to the superplume beneath. It is thus less clear what role the sublithospheric mantle plays in supporting the region's high topography. Many of Africa's secondary topographic features (e.g. Atlas, Hoggar, Bie Dome) are underlain by slow velocities at depths of 100–150 km and are adjacent to rapid changes in lithospheric thickness. Whether these variations in lithospheric structure promote small-scale convection or simply guide the larger-scale mantle flow field remains ambiguous.

Joint Passive/Controlled Source Seismic Experiment Across Taiwan

Scientists who study Earth structures and interactions at tectonic plate boundaries illuminate the three‐dimensional (3‐D) subsurface primarily through passive or active source seismic studies. In passive source studies, natural earthquakes are observed through arrays of seismic stations, allowing seismologists to image structures at depth and infer physical properties of the Earth. In active (controlled) source studies, scientists provide an artificial seismic source to be recorded by seismic networks, allowing more detailed views of fine‐scale structures buried within plate boundaries.The use of “continuous” modes on large‐storage‐capacity seismic recording systems enables seismologists to blur traditional lines between active and passive source seismology in regions of high earthquake activity. In addition to generating seismograms from controlled sources, a deployed dense array can record local and teleseismic earthquakes with exceptionally coherent signals across its aperture. As a result of such joint recording, the scientific value of the collected data increases. Moreover, whether or not the seismic sources are natural or engineered becomes less of a factor for data storage, sorting, and analysis methods. Local earthquakes can supplement controlled sources, multichannel processing and imaging methods traditionally used in active seismic studies become amenable to dense sets of passive source data, and active source data with known source origin times and locations are easily incorporated into regional seismic 3‐D tomography and earthquake location studies.

The crust-mantle transition and the Moho beneath the Vogtland/West Bohemian region in the light of different seismic methods

The structure of the crust and the crust-mantle boundary in the Vogtland/West Bohemian region have been a target of several seismic measurements for the last 25 years, beginning with the steep-angle reflection seismic studies (DEKORP-4/KTB, MVE-90, 9HR), the refraction and wide-angle experiments (GRANU’95, CELEBRATION 2000, SUDETES 2003), and followed by passive seismic studies (receiver functions, teleseismic tomography). The steep-angle reflection studies imaged a highly reflective lower crust (4 to 6 km thick) with the Moho interpreted in a depth between 30 and 32 km and a thinner crust beneath the Eger Rift. The refraction and wide-angle reflection seismic studies (CELEBRATION 2000) revealed strong wide-angle reflections in a depth of 26–28 km interpreted as the top of the lower crust. Long coda of these reflections indicates strong reflectivity in the lower crustal layer, a phenomenon frequently observed in the Caledonian and Variscan areas. The receiver function studies detected one strong conversion from the base of the crust interpreted as the Moho discontinuity at a depth between 27 and 37 km (average at about 31 km). The discrepancies in the Moho depth determination could be partly attributed to different background of the methods and their resolution, but could not fully explain them. So that new receivers function modelling was provided. It revealed that, instead of a first-order Moho discontinuity, the observations can be explained with a lower crustal layer or a crust-mantle transition zone with a maximum thickness of 5 km. The consequent synthetic ray-tracing modelling resulted in the model with the top of the lower crust at 28 km, where highly reflective lower crustal layer can obscure the Moho reflection at a depth of 32–33 km.

Crust and upper mantle structure of a continental backarc: central North Island, New Zealand

SUMMARY Near vertical and wide-angle seismic data provide evidence for a gradational crust‐mantle boundary in a depth range of 15‐20 km beneath the Central Volcanic Region (CVR), New Zealand. This volcanic area includes the Taupo Volcanic Zone and is a direct extension of Tonga-Kermadec oceanic backarc spreading into continental lithosphere. Long-range seismic refraction data show velocities of 6 km s −1 and less within the top 15 km of the crust of the CVR. At a depth of 15 km compressional seismic velocities increase to 6.8 km s −1 , and then to 7.4 ± 0.2 km s −1 at ∼20 km depth. These 7.4 km s −1 seismic wave speeds are interpreted as anomalous upper mantle as beneath this level passive seismic studies show similar Pn wave speeds that increase slowly to ∼7.8 km s −1 at about 80 km deep. We interpret rocks between 15 and 20 km to be a layer of new crust formed by underplating. The strongest reflection observed, and what might also be interpreted as a reflection Moho, is from the top of the proposed underplated layer at 15 km depth. At 20 km depth no such distinct reflection is observed. Rather, wide-angle reflection data show a continuum of low-level reflectivity between 15 and at least 35 km depth, indicating some heterogeneity and/or structure within the lower crust and upper mantle. Thus the transition from lower crust to upper mantle is broad, and a conventional reflection Moho does not exist beneath the CVR. Buoyancy force calculations based on rock uplift for the central North Island indicate that the subjacent mantle, to a depth of 80‐100 km is ∼70 kg m −3 or 2 per cent less dense than normal mantle. Best estimates attribute half of this density anomaly to the effects of increased temperature with additional contributions from partial melt (∼1.2 per cent) and melt residuum.

Gravity modelling of the Hellenic subduction zone — a regional study

The Hellenic subduction zone is clearly expressed in the arc-shaped distribution of earthquake epicenters and gravity anomalies, which connect the Peloponnesos with Crete and Anatolia. In this region, oceanic crust of the African plate collides northward with continental crust of the Aegean microplate, which itself is pushed apart to the south–west by the Anatolian plate and, at the same time, is characterised by crustal extension. The result is an overall collision rate of up to 4 cm/year and a retreating subduction process. Recent passive and active seismic studies on and around Crete gave first, but not in all details consistent, structural results useful for supporting gravity modelling. This was undertaken with the aim of presenting the first 3D density structure of the entire subduction zone. Gravity interpretation was based on a Bouguer map, newly compiled using data from land, marine and satellite sources. The anomalies range from + 170 mGal (Cretan Sea) to − 10 mGal (Mediterranean Ridge). 3D gravity modelling was done applying the modelling software IGMAS. The computed Bouguer map fits the low frequency part of the observed one, which is controlled by variations in Moho depth (less than 20 km below the Cretan Sea and extending 30 km below Crete) and the extremely thick sedimentary cover (partly up to 18 km) of the Mediterranean Ridge. The southernmost edge of the Eurasian plate, with its more triangular-shaped backstop area, was traced south off Crete. Only 50 to 100 km further to the south, the edge of the African continent was traced as well. In between these boundaries there is African oceanic crust, which has a clear arc-shaped detachment line situated at the Eurasian continental edge. The subduction arc is open towards the north, its slab separates hotter mantle material (lower density) below the updoming Moho of the Cretan Sea from colder one (higher density) in the south. Subjacent to the upper continental crust of Crete is a thickened layer of lower crust followed by the subducted oceanic crust with some mantle material as intermediate layer. The depth of the oceanic Moho below Crete is 50 km. The presence and structure of subducted or underplated sediments remains uncertain.

Seismic tomography of the crust and lithospheric mantle in the Betic Cordillera and Alboran Sea

The Alboran Domain lies between the South of Spain and the North of Morocco and contains a Miocene extensional basin which formed during continuous compression between the African and Eurasian plates. We have performed a 1-D and 3-D passive velocity tomography study of the structure of the crust and lithospheric mantle of the Betic Cordillera and Alboran Sea region. An important drawback encountered in most passive seismic tomography studies is that the quality of the data set used is inhomogeneous and it is often impossible to measure the error. This occurs as the data set needs to be very large which implies using several years’ worth of data from different seismic networks with different station noise levels and phase travel time picking accuracy. We present a simple method to quantitatively determine the errors in the travel time data typically used in passive seismic tomography studies. We use this method to determine the standard error of our data set and present the tomography results with quantitatively determined error bars. An additional advantage of performing the error analysis is that the data can be weighted in terms of its standard error, and hence the contribution of noisy data can be reduced in the inversion, resulting in substantially enhanced tomography images. The main structural results obtained in the 1-D inversion of the data are: (1) the average velocity of the crust in the Betic Cordillera is V p=6.0±0.05 km/s and V s=3.5±0.05 km/s, indicating an average V p/ V s ratio of 1.71 for the crust. (2) The average Moho depth in the Betic Cordillera is less than 36 km. (3) The average crustal velocity in the Alboran Sea is V p=6.4±0.1 km/s and V s=3.4±0.2 km/s, indicating an average V p/ V s ratio of 1.88 for the crust. (4) The average depth of the Moho in the Alboran Sea is greater than 20 km. The 3-D inversion provides new structural information of the Alboran basin over the depth range 24–60 km. For the depth range 24–31 km we image the centre and north western margin of the basin and observe velocities of the order of 7.2–7.5±0.45 km/s, which could correspond to lower crust material or else anomalous low velocity mantle material. For the depth range 31–40 km we image the centre and the southern margin of the basin and observe P phase velocities of 8.0–8.2±0.55 km/s which we interpret as normal mantle material. Below this layer, in the depth range 40–60 km we image the centre and northern margin of the basin and observe low P velocities ( V p=7.8±0.55 km/s). At present the preferred model used to explain the formation of the Alboran Sea Basin makes use of the process of Lithospheric Delamination. The low velocity layer observed in the mantle starting at approximately 40 km depth could be interpreted to be hot mantle material or the asthenosphere, however, the normal mantle velocities observed in the 31–40 km layer above it do not support replacement of the lithosphere by hot asthenospheric material in the lower crust or immediately beneath the crust. If this is so, then delamination must have occurred within the lithospheric mantle at an approximate depth of 40 km, or else not occurred at all.

Introduction to special issues: Lithospheric structure and evolution of the Rocky Mountains (Parts I and II)

Two issues (Parts I and II) of Rocky Mountain Geology (RMG) are an attempt to summarize the structure and evolution of the continental lithosphere in a Rocky Mountain transect from Wyoming to New Mexico (Fig. 1). After decades of geologic work, our understanding of the complex history of this region is still incomplete. As in many other regions, first-order questions remain about the deep structure, processes of formation, and evolution of the continental lithosphere. Fundamental new insights are most likely to come through integration of a broad range of data, involving close collaboration among geologists, geochemists, and geophysicists. Figure 1. Proterozoic rocks and boundaries of the North American Southwest, adapted from Karlstrom and Bowring (1993, fig. 7). The Wyoming province contains Archean crust. The Mojave province contains Archean crustal material incorporated into Proterozoic crust. The Yavapai province contains 1.75-Ga juvenile volcanic-arc crust. The Mazatzal province contains 1.65-Ga crust. The Grenville province contains 1.4- to 1.1-Ga crust. Stippled zones are transitions between provinces. Proposed seismic reflection lines are: (1) the Cheyenne belt; (2) a proposed Yavapai–Mazatzal boundary in southern Colorado; and (3) the Jemez lineament, which corresponds to a proposed Mazatzal–Yavapai province boundary in New Mexico. The Continental Dynamics program of the National Science Foundation has provided funding recently for a collaborative effort in the Rocky Mountains that involves many of the contributors to these theme issues of RMG . Plans for coordinated activities include acquisition of about 600 km of seismic reflection and refraction data (in 1999), passive seismic studies, xenolith studies, and a range of geologic and geochronologic studies. We hope that the papers in the two issues of RMG will promote discussion of the best design for our upcoming seismic …

Seismic noise cancellation in a geothermal field

High background seismic noise due to process machinery in production oil or geothermal fields can present a major problem for active seismic studies such as reflection and refraction surveys and passive seismic studies such as microearthquake monitoring. The general noise suppression problem is a difficult one since process noise may be due to a large number of sources distributed over a large region. In some situations, one or a few sources may dominate the noise field locally, presenting an opportunity for noise suppression by cancellation. In this note we describe an application of adaptive noise cancellation (Widrow, et al., 1975) in which we attempt to suppress noise recorded at a primary monitoring site using reference noise recorded at a major nearby noise source.

Microearthquakes Under and Alongside Ice Stream B, Antarctica. Detected By A New Passive Seismic Array

A new seismographic array with a band width of 500 Hz per channel and a dynamic range of 96 dB was developed for detecting natural events on glaciers. It was first deployed on ice stream B during the 1985–86 austral summer. The network consists of nine solar-powered seismographs, each monitoring three components of ground motion. Each of the seismographs is connected by up to 4 km of fiber-optic cable to a central node where seismic events are both detected and recorded. During 85 h of passive seismic monitoring on ice stream B, 25 microearthquakes were observed. Sixteen of these events were associated with shallow crevassing, mostly near the margins, although not within the zones of extreme shearing that bound the ice streams. Nine microearthquakes were associated with low-angle thrusting near the base of the ice stream. The principal initial result of these passive seismic studies is the demonstration that virtually none of the energy dissipated beneath ice stream B takes place through brittle fracture near the base. Nevertheless, fracture associated with microearthquakes may play a significant role in sub-glacial erosion.

Microearthquakes Under and Alongside Ice Stream B, Antarctica. Detected By A New Passive Seismic Array

A new seismographic array with a band width of 500 Hz per channel and a dynamic range of 96 dB was developed for detecting natural events on glaciers. It was first deployed on ice stream B during the 1985–86 austral summer. The network consists of nine solar-powered seismographs, each monitoring three components of ground motion. Each of the seismographs is connected by up to 4 km of fiber-optic cable to a central node where seismic events are both detected and recorded. During 85 h of passive seismic monitoring on ice stream B, 25 microearthquakes were observed. Sixteen of these events were associated with shallow crevassing, mostly near the margins, although not within the zones of extreme shearing that bound the ice streams. Nine microearthquakes were associated with low-angle thrusting near the base of the ice stream. The principal initial result of these passive seismic studies is the demonstration that virtually none of the energy dissipated beneath ice stream B takes place through brittle fracture near the base. Nevertheless, fracture associated with microearthquakes may play a significant role in sub-glacial erosion.

Microearthquakes Under and Alongside Ice Stream B, Antarctica. Detected By A New Passive Seismic Array

A new seismographic array with a band width of 500 Hz per channel and a dynamic range of 96 dB was developed for detecting natural events on glaciers. It was first deployed on ice stream B during the 1985–86 austral summer. The network consists of nine solar-powered seismographs, each monitoring three components of ground motion. Each of the seismographs is connected by up to 4 km of fiber-optic cable to a central node where seismic events are both detected and recorded. During 85 h of passive seismic monitoring on ice stream B, 25 microearthquakes were observed. Sixteen of these events were associated with shallow crevassing, mostly near the margins, although not within the zones of extreme shearing that bound the ice streams. Nine microearthquakes were associated with low-angle thrusting near the base of the ice stream. The principal initial result of these passive seismic studies is the demonstration that virtually none of the energy dissipated beneath ice stream B takes place through brittle fracture near the base. Nevertheless, fracture associated with microearthquakes may play a significant role in sub-glacial erosion.