Magnetic history of a dyke on Mount Etna (Sicily)

During the 1989 eruption of Mount Etna, two fracture systems, trending c. N45°E and N150°E, opened at the foot of its 3000 m high SE Crater and propagated quickly downslope to distances of ≈3 and 7 km, respectively. The northeastern fracture fed a flank eruption, whereas the southeastern fracture remained dry and offered contrasting volcanological and geophysical evidence of the presence of magma at a shallow depth. During the opening of this non-eruptive fracture system, a differential magnetic network was set up on a short profile across its distant extremity. Initially, the magnetic field did not display any change along the profile between frequent surveys. However, repeated measurements at intervals of about 3 months for two years revealed the slow build-up of a 130 nT anomaly. The anomaly vanishes laterally within 0.2 km of the surface expression of the fracture system. This exceptional set of observations constrains the location and time of cooling of a shallow dyke. The increase in magnetization of the dyke inferred by the rate of growth of the anomaly leads to the interpretation that the dyke was emplaced near the end of the eruption.     

Seismic image of a CO2 reservoir beneath a seismically active volcano

Mammoth Mountain is a seismically active volcano 200 000 to 50 000 years old, situated on the southwestern rim of Long Valley caldera, California. Since 1989 it has shown evidence of unrest in the form of earthquake swarms (Hill et al. 1990), volcanic 'long-period' earthquakes (Pitt & Hill 1994), increased output of magmatic ³He (Sorey et al. 1993) and the emission of about 500 tonnes day ⁻¹ of CO₂ (Farrar et al. 1995; Hill 1996; M. Sorey, personal communication, 1997), which has killed trees and poses a threat to human safety. Local-earthquake tomography shows that in mid-1989 areas of subsequent tree-kill were underlain by extensive regions where the ratio of the compressional and shear elastic-wave speeds V_P/V_S was about 9 per cent lower than in the surrounding rocks. Theory (Mavko & Mukerji 1995), experiment (Ito, DeVilbiss & Nur 1979), and experience at other geothermal/volcanic areas (Julian et al. 1996) and at petroleum reservoirs (Harris et al. 1996) indicate that V_P/V_S is sensitive to pore-fluid compressibility, through its effect on V_P . The observed V_P/V_S anomaly is probably caused directly by CO₂, and seismic V_P/V_S tomography is thus a promising tool for monitoring gas concentration and movement in volcanoes, which may in turn be related to volcanic activity.     

Geophysical and structural signatures of syntectonic batholith construction: the South Mountain Batholith, Meguma Terrane, Nova Scotia

The Late Devonian South Mountain Batholith is a very large (7000 km²) composite peraluminous granitoid complex situated within the Meguma Terrane of the northern Appalachians. It is made up of two suites of granodioritic to leucogranitic plutons emplaced at approximately 380–370 Ma during the Acadian Orogeny, i.e. during the collision of Gondwana with the eastern margin of North America. A significant geophysical and geological database makes the South Mountain Batholith a type example of a very large syntectonic batholith emplaced within a collisional orogen. Gravity models reveal the plutons have flat or gently dipping floors at approximately 7.0 km depth and aspect ratios >6:1. They are underlain by deeper (>10 km) elongate northeast–southwest-trending roots that may indicate magma feeder zones. Dyke transport of granitic magma and the progressive construction of plutons by sheet injections are supported by field observations and by mapping of the anisotropy of magnetic susceptibility at the pluton scale. The very narrow deformation aureole within the country rocks suggests lateral spreading of the plutons was not the main space creation mechanism during emplacement; space was mostly created by vertical displacements of country rocks. The data are consistent with a laccolithic model for syntectonic batholith assembly. The laccolithic plutons may have been emplaced at the base of the Meguma Supergroup metasedimentary rocks, suggesting a maximum thickness of approximately 7.0 km for the supracrustal rocks in the Meguma Terrane.     

Quaternary normal faulting in southeastern Sicily (Italy):a seismic source for the 1693 large earthquake

We present geological and morphological data, combined with an analysis of seismic reflection lines across the Ionian offshore zone and information on historical earthquakes, in order to yield new constraints on active faulting in southeastern Sicily. This region, one of the most seismically active of the Mediterranean, is affected by WNW–ESE regional extension producing normal faulting of the southern edge of the Siculo–Calabrian rift zone. Our data describe two systems of Quaternary normal faults, characterized by different ages and related to distinct tectonic processes. The older NW–SE-trending normal fault segments developed up to ≈400  kyr ago and, striking perpendicular to the main front of the Maghrebian thrust belt, bound the small basins occurring along the eastern coast of the Hyblean Plateau. The younger fault system is represented by prominent NNW–SSE-trending normal fault segments and extends along the Ionian offshore zone following the NE–SW-trending Avola and Rosolini–Ispica normal faults. These faults are characterized by vertical slip rates of 0.7–3.3  mm  yr ⁻¹ and might be associated with the large seismic events of January 1693. We suggest that the main shock of the January 1693 earthquakes ( M ~ 7) could be related to a 45  km long normal fault with a right-lateral component of motion. A long-term net slip rate of about 3.7  mm  yr ⁻¹ is calculated, and a recurrence interval of about 550 ± 50  yr is proposed for large events similar to that of January 1693.     

Large quaternary landslides in the central appalachian valley and ridge province near Petersburg, West Virginia

Geological mapping and photointerpretation of side-looking airborne radar images and color-infrared aerial photographs reveal two large Quaternary landslides in the Valley and Ridge province of the central Appalachians near Petersburg, W. Va. The Elkhorn Mountain rock avalanche occurs on the thrust-faulted northwestern flank of the Elkhorn Mountain anticlinorium. A minimum of 7 × 10⁶ m³ of quartzite colluvium was transported more than 3 km from a 91 m high escarpment of Silurian Tuscarora Quartzite. The extensively vegetated deposit may owe, in part, its transport and weathering to periglacial conditions during the Pleistocene. In contrast, the Gap Mountain rock block slide is a single allochthonous block that is 1.2 km long, 0.6 km wide, and at least 60 m thick. The 43 × 10⁶ m³ block is composed of limestone of the Helderberg Group and the Oriskany Sanstone of Early Devonian age. Planar detachment probably occurred along a dissolution bedding plane near the Shriver Chert and the Oriskany Sandstone contact. Failure probably was initiated by downcutting of the South Branch Potomac River during the Pleistocene. Landslides of this magnitude suggest accelerated erosion during periglacial climates in the Pleistocene. The recognition of these large slope failures may provide evidence of paleoclimatic conditions and, thereby, increase our understanding of the geomorphologic development of the Valley and Ridge province.     

Relocation of M ≥ 2 events of the 1989 Dôbi seismic sequence in Afar: evidence for earthquake migration

The Ethiopian side of central Afar was struck in August 1989 by the largest seismic sequence (three 6.1 ≤ M _s ≤ 6.3 events, 15 with M _s or m _b ≥ 5.0) since that of Serdo in 1969. Using the Djibouti seismological network, we relocated 297 of the events of that sequence. As most of the large events took place outside the network, we assessed the accuracy and stability of earthquake relocations by using three different velocity models and two relocation codes to try to relate individual shocks to distinct faults and surface breaks. A majority of the events apparently occurred underneath the floor of the Dôbi graben, an area about 45  km long and 15  km wide, rupturing boundary and inner floor faults, in agreement with the surface cracks and scarps that we mapped in the area. The relocation shows that the principal events propagated about 50  km northwestwards along the graben in the first 40  hr. A day and a half after the beginning of the sequence, smaller events ( M ≤ 4) started to propagate more than 55  km eastwards, towards Asal Lake. Using two three-component stations installed near the Ethiopian border, we could determine reliable depths for 21 events. The depths are compatible with a seismogenic crust about 14  km thick in the Dôbi and Hanle graben area. Although the Dôbi sequence ruptured about 50  km of the fault array extending from Serdo to Asal, where the regional stress was released by earthquakes in 1969 and 1978, respectively, a seismic gap about 50  km long still subsists along the northern part of the Gaggade region (Der'êla half-graben).     

Frequency–magnitude statistics and spatial correlation dimensions of earthquakes at Long Valley caldera, California

Intense earthquake swarms at Long Valley caldera in late 1997 and early 1998 occurred on two contrasting structures. The first is defined by the intersection of a north-northwesterly array of faults with the southern margin of the resurgent dome, and is a zone of hydrothermal upwelling. Seismic activity there was characterized by high b -values and relatively low values of D , the spatial fractal dimension of hypocentres. The second structure is the pre-existing South Moat fault, which has generated large-magnitude seismic activity in the past. Seismicity on this structure was characterized by low b -values and relatively high D . These observations are consistent with low-magnitude, clustered earthquakes on the first structure, and higher-magnitude, diffuse earthquakes on the second structure. The first structure is probably an immature fault zone, fractured on a small scale and lacking a well-developed fault plane. The second zone represents a mature fault with an extensive, coherent fault plane.     

Coseismic deformation revealed by inversion of strong motion and GPS data: the 2003 Chengkung earthquake in eastern Taiwan

A moderate earthquake of   M _w= 6.8  occurred on 2003 December 10. It ruptured the Chihshang Fault in eastern Taiwan which is the most active segment of the Longitudinal fault as a plate suture fault between the Luzon arc of the Philippine Sea plate and the Eurasian plate. The largest coseismic displacements were 13 cm (horizontal) and 26 cm (vertical). We analyse 40 strong motion and 91 GPS data to model the fault geometry and coseismic dislocations. The most realistic shape of the Chihshang fault surface is listric in type. The dipping angle of the seismic zone is steep (about 60°–70°) at depths shallower than 10 km and then gradually decreases to 40°–50° at depths of 20–30 km. Thus the polygonal elements in Poly3D are well suited for modelling complex surfaces with curving boundaries. Using the strong motion data, the displacement reaches 1.2 m dip-slip on the Chihshang Fault and decreases to 0.1 m near surface. The slip averages 0.34 m, releasing a scalar moment of 1.6E26 dyne-cm. For GPS data, our model reveals that the maximal dislocation is 1.8 m dip-slip. The dislocations decrease to 0.1 m near the surface. The average slip is 0.48 m, giving a scalar moment of 2.2E26 dyne-cm. Regarding post-seismic deformation, a displacements of 0.5 m were observed near the Chihshang Fault, indicating the strain had not been totally released, as a probable result of near-surface locking of the fault zone.     

A microearthquake survey at the junction of the East Pacific Rise and the Wilkes (9°S) fracture zone

Summary. An ocean bottom seismograph survey of the junction of the East Pacific Rise and the Wilkes fracture zone detected only three microearthquakes beneath the rise crest during seven days of recording. In contrast, during the same period 41 events were detected on the fracture zone, all at distances greater than 10 km from the junction. These results suggest that near the rise crest the thin crust can support sufficient stress only to generate infrequent small earthquakes and that most faulting may take place by aseismic slip. At 10 km from the rise axis part of the crest has become competent enough to support stress, resulting in earthquakes probably at depths of up to 5 km below the sea-bed. Gear 5-waves on the seismometer records indicate that a magma chamber, if it exists near the junction, is less than 10 km across.     

Earthquake processes of the Himalayan collision zone in eastern Nepal and the southern Tibetan Plateau

Focal mechanisms determined from moment tensor inversion and first motion polarities of the Himalayan Nepal Tibet Seismic Experiment (HIMNT) coupled with previously published solutions show the Himalayan continental collision zone near eastern Nepal is deforming by a variety of styles of deformation. These styles include strike-slip, thrust and normal faulting in the upper and lower crust, but mostly strike-slip faulting near or below the crust–mantle boundary (Moho). One normal faulting earthquake from this experiment accommodates east–west extension beneath the Main Himalayan Thrust of the Lesser Himalaya while three upper crustal normal events on the southern Tibetan Plateau are consistent with east–west extension of the Tibetan crust. Strike-slip earthquakes near the Himalayan Moho at depths >60 km also absorb this continental collision. Shallow plunging P -axes and shallow plunging EW trending T -axes, proxies for the predominant strain orientations, show active shearing at focal depths ∼60–90 km beneath the High Himalaya and southern Tibetan Plateau. Beneath the southern Tibetan Plateau the plunge of the P -axes shift from vertical in the upper crust to mostly horizontal near the crust–mantle boundary, indicating that body forces may play larger role at shallower depths than at deeper depths where plate boundary forces may dominate.     

Heterogeneous structure around the Jemez volcanic field, New Mexico, USA, as inferred from the envelope inversion of active-experiment seismic data

We analyse active-experiment seismic data obtained by the 1993 Jemez Tomography Experiment (JTEX) programme to elucidate the heterogeneous structure of the Jemez volcanic field, which is located at the boundary between the Colorado Plateau and the Rio Grande Rift. Using a single isotropic scattering assumption, we first calculate the envelope Green's functions for the upper and lower crust and the uppermost mantle. By fitting the theoretical envelopes with the observed three-component data, we estimate depth-dependent features of the scattering coefficients around Valles Caldera. We estimate the ratios of scattering coefficients, rather than scattering coefficients themselves, because of the uncertainty of the seismic efficiency of the explosive sources and knowledge of absolute site-amplification factors. The strongest scattering coefficients are observed at a shallow depth beneath the Valles Caldera. This is considered to be related to the complex structure caused by two episodes of caldera formation and the ensuing resurgent uplift in the caldera, etc. The depth-dependent characters of the scattering coefficients for the Colorado Plateau and the Rio Grande Rift are similar to each other: a transparent upper crust and a heterogeneous lower crust (small and large scattering coefficients, respectively). However, the scattering coefficients beneath the Rio Grande Rift are several times larger than those beneath the Colorado Plateau. Depths of the lower crust and the Moho boundary beneath the Rio Grande Rift are shallower than those of the Colorado Plateau. From their geological settings and other geophysical results around the region, we infer that the larger scattering coefficients of the rift are associated with rift formation and volcanic activity, such as magma ascent from the upper mantle to the crust.     

Upper-mantle structure of the Baltic Shield below the Swedish National Seismological Network (SNSN) resolved by teleseismic tomography

Upper-mantle structure under the Baltic Shield is studied using non-linear high resolution teleseismic P -phase tomography. Observed relative arrival-time residuals from 52 teleseismic earthquakes recorded by the Swedish National Seismological Network (SNSN) are inverted to delineate the structure of the upper mantle. The network consists of 47 (currently working) three-component broad-band stations located in an area about 450 km wide and 1450 km long. In order to reduce complications due to possible significant three-dimensionality of Earth structure, events chosen for this study lay close to in-line with the long-axis of the array  (±30°)  . Results indicate P -wave velocity perturbations of ±3 per cent down to at least 470 km below the network. The size of the array allows inversion for structures even at greater depths, and lateral variations of velocity at depths of up to 680 km appear to be resolved. Below the central part of the array (60°–64° N), where ray coverage is best, the data reveals a large region of relatively low velocity at depths of over about 300 km. At depths less than about 250–300 km, the models include a number of features, including an apparent slab-like structure dipping gently towards the north.     

Anisotropy of magnetic susceptibility investigations of the St Malo dyke swarm (Brittany, France): emplacement mechanism of doleritic intrusions

A high-quality aeromagnetic survey of northern Brittany (line spacing 250  m; flight elevation 150  m) has been used to delineate the Lower Carboniferous St Malo dyke swarm in detail. The dyke swarm fans at its northern and southern ends, and is affected by N60° sinistral transcurrent faults. After restoration of these offsets, the full structure trends in a N–S direction. Small dykes are not imaged, and only one-third of the swarm is evidenced by magnetism. Gravity and magnetic modelling shows that the swarm overlies a single N–S elongated magma chamber.
  The distribution of K _max inclinations of anisotropy of magnetic susceptibility (AMS) suggests that the dykes display a fanning magma flow in section. Computed K _max inclinations are usually shallower than the measured geological dips, probably because the flow becomes more disturbed as the dyke becomes shallower. We observe that the mean susceptibility values increase when the magma flow is steeper than about 70°.
  A detailed cross-section of St Briac dyke, which is part of the St Malo dyke swarm, reveals that the main carrier of magnetization is magnetite in the centre of the dyke and magnetite + maghaemite on its rims. The emplacement of the St Malo dyke swarm could have been contemporaneous with the N60° shearing which displaced the dyke swarm by about 20  km. The dyke swarm is cut at its southern end by an E–W-orientated fault which probably acted during Late Carboniferous times.     

Mantle transition zone thickness beneath Afar: implications for the origin of the Afar hotspot

The velocity spectrum stacking method is applied to receiver functions from stations ATD and AAE to image P -to- S converted phases originating at the 410 and 660 km discontinuities beneath Afar. A transition zone thickness of 244 ± 19 km is obtained, similar to the global average transition zone thickness. This result suggests that any broad thermal anomaly beneath Afar probably does not extend as far down as the transition zone. However, because of the 19 km uncertainty in the thickness estimate, a small thermal anomaly of ~100–150 K at mantle transition zone depths cannot be ruled out.     

Model of the geoelectrical structure of the mid- and lower mantle in the Europe–Asia region

The conductivity structure of the Earth's mantle was estimated using the induction method down to 2100  km depth for the Europe–Asia region. For this purpose, the responses obtained at seven geomagnetic observatories (IRT, KIV, MOS, NVS, HLP, WIT and NGK) were analysed, together with reliable published results for 11  yr variations. 1-D spherical modelling has shown that, beneath the mid-mantle conductive layer (600–800  km), the conductivity increases slowly from about 1  S  m⁻¹ at 1000  km depth to 10  S  m⁻¹ at 1900  km, while further down (1900–2100  km) this increase is faster. Published models of the lower mantle conductivity obtained using the secular, 30–60  yr variations were also considered, in order to estimate the conductivity at depths down to the core. The new regional model of the lower mantle conductivity does not contradict most modern geoelectrical sounding results. This model supports the idea that the mantle base, situated below 2100  km depth, has a very high conductivity.     

Fractal structure and characteristic scale in the distributions of earthquake epicentres, active faults and rivers in Japan

Using a box-counting method, we examined the fractal structures of the spatial distributions of three geological systems in Japan, namely those of earthquake epicentres, active faults, and rivers. Our results show that, in the scale range from 1 to 100  km, all geological systems have a common characteristic scale of ~13  km, which divides the spatial distribution into two bands: a smaller scale r < 13  km and larger scale r > 13  km (where r is the box size). In both bands, the three systems obey a power law distribution, and therefore it is proposed that all geological systems have a band-limited fractal structure. Since the characteristic scale of ~13  km is in agreement with the depth of the brittle–ductile transition zone of the crust, we suggest that it is a common feature of the heterogeneity of the crust.     

Ocean-bottom seismograph observations on the Mid-Atlantic Ridge near 45°N — further results

Summary. Analysis of data from events well recorded by ocean-bottom seismographs on the Mid-Atlantic Ridge near 45° N, indicates that most activity is centrally placed beneath the median valley floor. The results suggest a lithospheric thickness of 8 km under the median valley floor. The spatial and temporal behaviour of the largest swarm observed is indicative of active magma intrusion.     

Tectonic geomorphology of the Ash Hill fault, Panamint Valley, California

ABSTRACT
Panamint Valley, in eastern California, is an extensional basin currently bounded by active, dextral-normal oblique-slip faults. There is considerable debate over the tectonic and topographic evolution of the valley. The least-studied structure, the Ash Hill fault, runs for some 50 km along the valley's western edge, and active strands of the fault continue south into the neighbouring Slate Range. Vertical displacement on the fault is valley-side up, creating topography that conflicts with the gross morphology of the valley itself. We use this topography, along with kinematic and geological markers, to constrain the Quaternary slip rate and orientation of the Ash Hill fault. The fault offsets all but the active channel deposits in the valley, and slickenlines indicate a strike-slip to dip-slip ratio of 3.5:1. An offset volcanic unit dated at 4 Ma provides a minimum slip rate of 0.3±0.1 mm yr⁻¹, and a long-term strike-slip to dip-slip ratio of 5.2:1. Slip on the fault has warped a palaeolake shoreline within the valley. Simple elastic dislocation modelling of the vertical deformation of the shoreline suggests total fault slip of ≈60 m, valley-side up. The shoreline probably dates to 120–150 ka, implying a late Quaternary slip rate of 0.4–0.5 mm yr⁻¹. We suggest two possible mechanisms for the apparently anomalous slip behaviour of the Ash Hill fault. The fault may be a listric structure related to the proposed low-angle fault underlying Panamint Valley. Alternatively, the Ash Hill fault is a high-angle fault, implying that the valley is currently bounded by high-angle dextral-slip faults. Lack of detailed subsurface information precludes any knowledge of the true relationships between the presently active faults.     

Aftershock tectonics of the 1978 Tabas-e-Golshan (Iran) earthquake sequence: a documented active 'thin-and thick-skinned tectonic' case

Summary. In this study a locally recorded aftershock sequence of the 1978 Tabas-e-Golshan earthquake ( M_s = 7.4) was accurately located. Out of 1560 located events, 329 best-located aftershocks passed a strict quality criterion. These well-located aftershocks, which have uncertainties in epicentre and in focal depth of about 1 and 2 km respectively, together with the well-constrained focal mechanisms, provided a detailed picture of active continental deformation during an aftershock sequence.
Almost all aftershocks follow very closely the pattern of the earthquake faults at the surface and lie in the hanging-wall block of the active fault. The hypocentres occurred mainly at depths less than 23 km with a high concentration of seismic activity between 8–14 km depth. The aftershocks clearly demonstrate an active imbricate listric thrust system with fault planes flattening into a basement decollement zone, and the reactivation of different basement reverse faults in response to a considerable amount of shortening of the top sedimentary cover. The sense of motion was almost universally thrusting and the aftershocks shared the same tectonic causes as the main shock.
The study indicates that the development of the young fold-thrust mountain belts necessarily involves basement shortening (thin-and thick-skinned tectonics) and that the 'frontal reverse faults' in young active fold-thrust mountain belts are the most seismically active faults. Geological and seismic data propose that the active frontal reverse fault systems are possibly reactivated old normal faults and may add support to the contention of reversal of fault motion during re thickening of continental crust. The active 'thin-and thick-skinned tectonics' documented in this study may prevail in other young and active fold-thrust mountain belts which are characterized by a thick sequence of telescoped top sedimentary cover over a decollement detachment zone.     

Crustal structure and local seismicity in western Anatolia

Western Anatolia is one of the most seismically active continental regions in the world and much of it has been undergoing NS-directed extensional deformation since Early Miocene time. In a cooperative study, seismologists from Saint Louis University, USA and Dokuz Eylül University, Turkey, deployed five broad-band and 45 short-period seismic stations in western Anatolia between 2002 November and 2003 October. The present paper uses data collected by this network and the data from five permanent stations operated by the Kandilli Observatory and Earthquake Research Institute to map the hypocentral distribution of local earthquakes and to determine crustal structure of western Anatolia. We obtained a 1-D P -wave crustal velocity model using a generalized scheme for simultaneously obtaining earthquake locations and a crustal velocity model. Our velocity model is characterized by crustal velocities that are significantly lower than average continental values. The low velocities may be associated with high crustal temperatures, a high degree of fracture, or the presence of fluids at high pore pressure in the crust. We located 725 local earthquakes and classified them in three categories. We found that the level of seismic activity in western Anatolia is higher than previously reported. Station delays resulting from the inversion process correlate with near-surface geology and the thickness of sediments throughout the region. The hypocentral distribution of the events indicates that peak seismicity for the region occurs at depths of about 10 km.