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[Display omitted] • 3-D crustal structure in the 1969 earthquake doublet area is first investigated. • The 1969 earthquake doublet was induced by magmatic fluids from the Hainan plume. • Earthquake swarms in the Hainan area are related to the magmatism of the plume. Mantle plumes are one of the most influential first-order tectonic systems on Earth, playing a prominent role in controlling local magmatic activity and seismicity. Recent studies have shown that the Hainan mantle plume originates from the lower mantle and its volcanic activity is still intense during the Holocene. However, the effect of the Hainan plume on local seismicity is still unclear. In this study, we conducted the first wide-angle seismic survey in the source area of the 1969 earthquake doublet (M 5.1 and 5.2) offshore Hainan Island. A high-resolution 3-D P-wave velocity model of the crust is obtained using a large amount of Pg and PmP arrival times recorded by a portable array consisting of 17 ocean bottom seismographs (OBSs). Our results show that the 1969 earthquake doublet occurred in high-to-low velocity transitional zones which correspond to active faults. Prominent low-velocity anomalies are revealed in the crust of the study area, which may reflect magmatic intrusion channels. On the basis of multiple geophysical evidence including multi-channel seismic reflection profiles, geothermal data, and seismic tomography results, we infer that the low-velocity anomalies in the source area may reflect magmas and fluids from the Hainan plume, and high-angle normal faults in the crust serve as conduits for ascending of the magmas and fluids. The 1969 earthquake doublet offshore Hainan Island can be attributed to the magma-hydrothermal alteration processes in the fault zones, which led to the significant reduction of the fault strength and friction coefficient, facilitating the shear failure propagation. Furthermore, the seismicity in and around Hainan Island exhibits distinct swarm-like behaviors, which is also evidence for the Hainan plume affecting the crustal seismicity. [ABSTRACT FROM AUTHOR]

Between 2010 and 2013, the Pollino Mountains region (south Italy), already proposed as a seismic gap, was affected by a seismic crisis of more than 5000 small-to-moderate earthquakes (maximum magnitude ML 5.0). Preliminary analyses performed in a previous work highlighted that this activity can be ascribed to normal faulting on north-northwest-trending west-dipping dislocation surfaces consistent with the general seismotectonic frame of the southern Apennines. This work contributes additional data and a more sophisticated analyses that highlight new features of the seismic swarm and support a new interpretation for the study area. We obtained high-precision locations and focal mechanisms using the double-difference method and the cut-andpaste waveform inversion method, respectively. The 3D patterns of hypocenters and focal mechanisms consistently image an ~10-km-long north-northwest-striking and west-dipping fault zone between 5 and 10 km depth, with predominantly extensional kinematics. The high-resolution data show that this zone broadens from north to south as a result of secondary faulting. The depicted geometry, with preliminary geological observation, leads to the hypothesis of multiple seismogenic normal faults rooted into more regional shallow-dipping detachments inherited from the pre-existing Apennine thrust tectonics. [ABSTRACT FROM AUTHOR]

More than 60 small earthquakes (ML 0.7-3.0) were detected from December 2011 to March 2012 north of Cardston, Alberta, an area with little evidence for previous seismic activity. The timing of these events closely correlates (>99.7% confidence) with hydraulic fracturing completions of the Devonian-Mississippian-age Exshaw Formation at a nearby horizontal well. Unanimous waveform multiplicity within the swarm suggests that the events share a similar origin and source mechanism. This observation is corroborated by the point-like collocation of hypocenters within the crystalline basement during robust, double-difference relocations. Furthermore, the presence of a pre-existing fault is confirmed via formation-top offset mapping and inteipreted to be a Fate Cretaceous extensional fault. The confirmation of this fault at depth provides a plausible pathway for rapid hydraulic communication from the fracturing interval into the crystalline basement. Consistent with structural interpretations and available stress information, moment tensor inversion of the largest magnitude event (Mw 3.0) indicates reactivation of a basement fault with normal slip. We conclude that the genesis of this earthquake swarm was likely caused by increased pore pressure, within the basement fault, as a result of fracturing stimulation. [ABSTRACT FROM AUTHOR]

The New Zealand GeoNet P-phase picker's performance can be significantly improved, by up to 70%, when Mexican hat and Haar wavelet scale thresh-olding (WST) is implemented in the preprocessing stage of manually picked events. In total, 811 events with the required spatial and quality criteria from the 2008 Matata swarm were studied. The WST was applied to manually processed wave-forms recorded by key stations, including 48 waveforms recorded by URZ; 47 by stations LIRZ, MARZ, and OPRZ; 64 by EDRZ; 33 by KARZ; and 47 by OPRZ. The signal-to-noise ratios of these key stations were dominated by additive noise rather than weaknesses introduced by the radiation pattern. We found that the Mexican hat WST produced the largest amount of retrieved and revised P onsets for station EDRZ (70.3%). The next best station was KARZ, which had 66.7% of picks showing improvement and only 3% of picks adversely affected. The main best scales (i.e., the wavelet's degree of compression) for stations EDRZ and KARZ were scales 1 and 2, respectively. Haar WST for the OPRZ recordings resulted in a total improvement greater than 50%, with the lowest adversely affected picked on-sets of 2.1%. Similarly, the Haar WST retrieved a considerable number of missed on-sets at the TGRZ and LIRZ stations (25.5% and 20%, respectively), with a minor effect on the performance. The results confirmed that WST served to significantly enhance the performance of the GeoNet P picker for the chosen Matata earthquakes due to the frequency localization of the WSR compared to the current Butterworth filtering scheme. [ABSTRACT FROM AUTHOR]

Over the last few years, seismic activity in the Pollino area (a sector of the Calabro-Lucanian Apennines in southern Italy known as a seismic gap) has been very weak. However, in 2011 the seismicity gradually intensified, culminating in an earth-quake of Mw 5.0 occurred on 25 October 2012. The depth of the 2011-2012 earthquake hypocenters ranges between 2 and 10 km; the seismicity results in two separate clusters and traces a north-northwest-south-southeast fracture more evident in the western sector. In this area, an MT station was installed on 26 September 2012 by the Institute of Methodologies for Environmental Analysis, National Research Council of Italy (IMAA-CNR), Italy, at about 50 km from another MT station operating since 2003 in the Agri Valley (Tramutola, southern Italy). Such a seismic swarm occurred in the Pollino area (more than 3600 events in last two years with local magnitude ML >0.1). It has provided a rare opportunity to study the earthquake-related temporal patterns of electromagnetic (EM) signals, and is potentially informative about ongoing seismogenic processes. In this study, we present several cases of EM field variations associated with the passage of seismic waves. The maximum amplitude of the electrical signals registered at the two MT sites and the earthquake magnitude are related by an attenuation factor that depends on the distance between the hypocenter and the MT station. Furthermore, at the two MT sites the maximum electrical anomalies seem to be more appreciable predominantly in different directions, indicating a certain directivity in the propagation of the electric field. A deep analysis of EM time series recorded during the mainshock Mw 5.0 was performed. In particular, by applying time-frequency misfit criteria based on the continuous wavelet transform, we compared the electric field with seismic recordings, and we found a good waveform similarity between signals. Moreover, we also found an EM signal that significantly anticipates the theoretical first P-wave arrival at the Tramutola MT station. [ABSTRACT FROM AUTHOR]

Highlights: [•] The French colonial Empire was in crisis during WWII. [•] Efforts to control Desert Locusts helped resolve this crisis. [•] Responses to locust invasions enabled new modes of government. [•] Resulting field allowed transnational federation of techno-scientific benevolence. [•] Fit or mismatch between institutions and ecological dynamics is unstable, contingent. [Copyright &y& Elsevier]

Southwest Japan lies above an area where the Philippines Sea plate subducts beneath the Eurasian plate. The region experiences relatively high levels of crustal seismicity in addition to the interplate and intraplate seismicity associated with subduction. Crustal earthquakes are common, for example, in the Wakayama district of southwest Japan where they occur as swarmlike seismic activity. Swarm activity is common in volcanic areas but there are no Quaternary volcanic features in the Wakayama district. This report interprets crustal structure in the Wakayama district and its relationship to the swarm activity observed there. We use a new imaging method based on amplitudes of Sp-converted waves to image the spatial distribution of velocity discontinuities at high resolution. We detected four distinct velocity discontinuities at depths of 2-5, 8-13, 12-22, and 20-27 km. We interpret the 2 - 5 km discontinuity as the bottom of the sedimentary layer, the 12-22 km feature as the Conrad discontinuity, and the 20-27 km feature as the Moho discontinuity. The 8-13 km velocity discontinuity is detected only beneath the swarm region. The thickness of the upper crust in the swarm region was estimated to be greater than that of surrounding areas. Contrasts in the distribution of velocity discontinuities between the swarm region and surrounding areas suggest that swarm activity is generated by increased pore pressure of fluids escaping from a magmatic body ascending from the mantle. [ABSTRACT FROM AUTHOR]

We investigated the effect of autonomous underwater vehicle (AUV) dynamics and navigation on underway submarine gravimetry. Our research was motivated by the need to obtain spatially dense marine gravity measurements close to the source of subkilometer-scale geologic features in the shallow oceanic crust. Such measurements have been previously obtained, for instance, with piloted submarines and towed sleds; however, the high cost and, in the case of on-bottom measurements, poor spatial sampling preclude routine acquisition of these measurements. Continuous underway gravity surveys with AUVs is a compelling cost-effective option, but this method requires separating the AUV accelerations from the measured gravity. We show that AUVs with a large distance between the center of buoyancy and the center of gravity have lower vertical accelerations than torpedo-shaped AUVs and consequentially are better suited for underway gravity surveys. Furthermore state estimators, which combine sensor measurements and models of the vehicle's motion, provide superior estimates of the vehicle's vertical accelerations than methods used in previous underway submarine gravity surveys. We simulated the use of these navigation methods in detecting dike swarms at the East Pacific Rise. Analysis showed that we can shorten filters used in reducing gravity data and consequentially provide improved measurements of the free-water anomaly with a minimal detectable spatial wavelength approximately 65% lower than previously reported results. [ABSTRACT FROM AUTHOR]

Earthquakes occur in clusters, which classically are described as foreshock-mainshock-aftershock sequences or swarms. In this paper, every earth-quake in a seismicity catalog is assigned to a cluster if it is separated from at least one other event in the cluster by less than At in time and less than Ar in space. The minimum cluster size is one earthquake. For catalogs that are complete to small magnitudes, this approach is successful in capturing the full spatial extent of an ex-tensive cluster even for Ar much smaller than the actual cluster dimension. The de-clustered catalogs are much closer to Poissonian distribution than the originals. This was applied to seismicity catalogs for Japan, Southern California, and Nevada. Cluster sizes measured by the number of earthquakes in the cluster exhibit an approximate power-law frequency distribution. An upper bound to cluster durations is proportional to K0.5, where K is the number of earthquakes in the cluster. This paper demonstrates an analytical approach suitable for selecting values of At and Ar that are appropriate for the earthquake catalog. [ABSTRACT FROM AUTHOR]

The San Diego trough fault zone (SDTFZ) is part of a 90-km-wide zone of faults within the inner California Borderland that accommodates motion between the Pacific and North American plates. Along witii most faults offshore southern California, the slip rate and paleoseismic history of the SDTFZ are unknown. We present new seismic reflection data that show that the fault zone steps across a 5-km-wide stepover to continue for an additional 60 km north of its previously mapped extent. The 1986 Oceanside earthquake swarm is located within the 20-km-long restraining stepover. Farther north, at the latitude of Santa Catalina Island, the SDTFZ bends 20° to the west and may be linked via a complex zone of folds with the San Pedro basin fault zone (SPBFZ). In a cooperative program between the U.S. Geological Survey (USGS) and the Monterey Bay Aquarium Research Institute (MBARI), we measure and date the coseismic offset of a submarine channel that intersects the fault zone near the SDTFZ-SPBFZ junction. We estimate a horizontal slip rate of about 1.5 ± 0.3 mm/yr over the past 12,270 yr. [ABSTRACT FROM AUTHOR]

A detailed record of earthquake frequency and distribution is essential to understanding regional tectonic strain and seismic hazard, particularly in regions of low, but significant, seismicity levels. Comprehensive analyses of seismicity within Arizona have not been previously possible due to a lack of seismic stations in many regions, contributing to the perception that earthquakes within Arizona are rare and generally limited to the north-central and northwestern portions of the state. The EarthScope USArray Transportable Array (TA) was deployed within Arizona from April 2006 to March 2009 and provided the opportunity to examine seismicity on a statewide scale. In this study, we developed a streamlined workflow for producing a comprehensive earthquake catalog using TA data. We combined our new catalog with historical earthquake catalogs from several sources to produce the first comprehensive historical earthquake catalog for the state of Arizona. The TA-derived catalog is complete to local magnitude (ML) ∼ 1.2, contains crustal events as small as ML 0.0, and includes events located within several previously unidentified areas of seismic activity in Arizona. We also identified 16 earthquake clusters, many of which have swarmlike behavior. These earthquake clusters account for 42% of the events identified during the study period, and they occur in all physiographic, geophysical, and tectonic settings. We suggest that swarms and clusters, such as those documented in this study, may represent an important mechanism for small-scale tectonic strain release within intraplate regions with otherwise apparently low seismicity levels. [ABSTRACT FROM AUTHOR]

Magnetic anomalies provide insights into the tectonic implications of a swarm of ∼1500 shallow (∼1 km deep) earthquakes that occurred in 2009 on the Hanford site, Washington. Epicenters were concentrated in a 2 km2 area near Wooded Island in the Columbia River. The largest earthquake (M 3.0) had first motions consistent with slip on a northwest-striking reverse fault. The swarm was accompanied by 35 mm of vertical surface deformation, seen in satellite interferometry (InSAR), interpreted to be caused by ∼50 mm of slip on a northwest-striking reverse fault and associated bedding-plane fault in the underlying Columbia River Basalt Group (CRBG). A magnetic anomaly over exposed CRBG at Yakima Ridge 40 km northwest of Wooded Island extends southeastward beyond the ridge to the Columbia River, suggesting that the Yakima Ridge anticline and its associated thrust fault extend southeastward in the subsurface. In map view, the concealed anticline passes through the earthquake swarm and lies parallel to reverse faults determined from first motions and InSAR data. A forward model of the magnetic anomaly near Wooded Island is consistent with uplift of concealed CRBG, with the top surface <200 m below the surface. The earthquake swarm and the thrust and bedding-plane faults modeled from interferometry all fall within the northeastern limb of the faulted anticline. Although fluids may be responsible for triggering the Wooded Island earthquake swarm, the seismic and aseismic deformation are consistent with regional-scale tectonic compression across the concealed Yakima Ridge anticline. [ABSTRACT FROM AUTHOR]

Abstract: This study proposes three models to explain the mechanism of the three major types of mafic dyke swarms. Parallel dyke swarms form in response to a regional stress field, e.g. the mafic dyke swarms in the North China Craton, whereas small radiating dyke swarm forms due to stress constructions around a plutonic or volcanic edifice, such as the dyke swarm at Spanish Peak, USA. The third type of radiating dyke swarm is giant fan-shaped dyke swarm such as the Mackenzie dyke swarm. Fractures that formed prior to magmatism may play a vital role in dictating the dyke swarm geometry. In most of the cases, the pre-existing fractures are induced by tectonic stresses and not by magma injection though magma injection can increase the fracture size by propagation at the dyke tip. [Copyright &y& Elsevier]

We examined characteristics of routinely cataloged seismicity from 1970 to the present in and around the Yakima fold-and-thrust belt (YFTB) in eastern Washington to determine if the characteristics of contemporary seismicity provide clues about regional-scale active tectonics or about more localized, near-surface processes. We employed new structural and hydrologic models of the Columbia River basalts (CRB) and found that one-third to one-half of the cataloged earthquakes occur within the CRB and that these CRB earthquakes exhibit significantly more clustered, and swarmlike, behavior than those outside. These results and inferences from published studies led us to hypothesize that clustered seismicity is likely associated with hydrologic changes in the CRB, which hosts the regional aquifer system. While some general features of the regional groundwater system support this hypothesis, seismicity patterns and mapped long-term changes in groundwater levels and present-day irrigation neither support nor refute it. Regional tectonic processes and crustal-scale structures likely influence the distribution of earthquakes both outside and within the CRB as well. We based this inference on qualitatively assessed alignments between the dominant northwest trends in the geologic structure and the seismicity generally and between specific faults and characteristics of the 2009 Wooded Island swarm and aseismic slip, which is the only cluster studied in detail and the most vigorous since regional monitoring began. [ABSTRACT FROM AUTHOR]

April 1998, a swarm of ~1800 microearthquakes near the village of Iznajar (southern Spain) was recorded at the Granada basin short-period seismic network. Focal mechanisms from local P-wave polarities are poorly constrained and cannot characterize the seismotectonics of the series. Here we combine polarity information and multiplet relocation to address this issue. We use waveform cross correlation on P and S arrivals to identify events with highly similar seismograms, group our detections into multiplet clusters, and invert the cross-correlation time delays to obtain precise relative locations. Relative locations have errors of several tens to a few hundreds of meters horizontally and vertically, and define strike and dip of active fault patches with an accuracy of ~20°-30°. We introduce the multiplet fault plane orientations into focal mechanism inversion, now yielding mostly well-constrained solutions, in addition to resolving the nodal plane symmetry. We observe mainly north-south left-lateral strikeslip faulting and a few north-northwest-south-southeast normal faulting solutions, illustrating the kinematic complexity of the swarm, and pointing to a local deformation style different from the nearby Granada basin. [ABSTRACT FROM AUTHOR]

We observed six components of ground rotational and translational motions in a near-field region during an earthquake swarm in April 1998 offshore Ito, Izu Peninsula, Japan. To check the reliability of our observation using a MotionPack sensor, we installed an 8301F sensor, which is an inertial angular displacement sensor with a higher sensitivity than MotionPack. The noise level of the MotionPack is much higher than that of the 8301F, but the waveforms of these signal components are quite similar to each other, suggesting that the MotionPack records rotational rate correctly but with higher noise. Our observation made known a linear correlation between the maximum rotational displacements around the vertical axis and the maximum translational velocities. The waveforms of rotational motion around the vertical axis are analogous to those of translational horizontal velocities, suggesting the linear correlation between maximum values. These rotational motions are much larger than those calculated by array data at the San Andreas fault. This large discrepancy might be explained by one or more of the following effects: the difference in the spatial scale of rotational motion by single gyro measurement and by array observation, the effect of the precipitous topography at the station of the offshore Ito swarm, and the difference of the degree of maturation between the San Andreas fault and the swarm regions of offshore Ito. [ABSTRACT FROM AUTHOR]

Abstract: For natural analogue study of CO2 leakage, the Matsushiro field in central Japan was selected to investigate hydrogeochemical behavior due to CO2 leakage from a fault zone during the period of the Matsushiro Earthquake Swarm between 1965 and 1967. One of the main objectives of the modeling study is to understand effects of hydrogeological characteristics of shallow aquifers and seal mechanism due to mineral precipitation in a fault zone. Reactive geochemical transport simulations were carried out using TOUGHREACT. Based on the historical data including discharge rate, chemical composition of water and seismic activity, we developed a numerical model to reproduce the geochemical behavior during and after the swarm earthquakes. A two-dimensional model (2000 m×1800 m) for the fault zone using the Multiple Interacting Continua (MINC) method was set up to evaluate geochemical effects during degassing and mixing of brine upwelling along the fault zone. The simulation results can capture the observed trend at the Matsushiro field. Supercritical and gaseous CO2 occur in the deep and the shallow fracture zones, respectively. During the earthquake swarm, CO2 saturated brine moves up along the fracture zone and CO2 degassing occurs in the shallow zone. Subsequently, water pH increases and a small amount of calcite precipitates. Lateral groundwater flow in shallow aquifers, which transports the CO2 away from the fault zone, makes CO2 surface discharge small. At the same time, calcite dissolves after the swarm due to lateral fresh water recharge. The result is consistent with the fact that no calcite observed in the core from the 80 m borehole drilled during the course of this study, indicating that CO2 discharge is likely affected by lateral shallow groundwater flow. Measurements of surface CO2 flux and soil gas indicated that little seepage of CO2 occurs in the area of the fan deposits at present, 40 years after the earthquake swarm. Using the same model, long term leakage simulations (if supercritical CO2 leaked along fault zone from the storage reservoir) were carried out to test whether CO2 surface leakage occurs or not. Two simulations were performed, with CO2 upflow rates of 50,000 and 100,000 kg/year in an area of 25 m×25 m. CO2 surface leakage does not occur in the first case, while CO2 leakage occurs in the second case. CO2 surface leakage is reduced because CO2 dissolves in groundwater flowing laterally in shallow aquifer. These simulations indicate that it is important to understand hydrogeological characteristics of aquifers underlain by reservoir as well as cap rocks. [Copyright &y& Elsevier]

This paper describes the procedure of building a probabilistic decision tree on the basis of the integration of data from multiple sources, conditional probabilities, and the application to map fracture corridors (FCs) in a mature oil field with abundant production data. An FC is a tabular, subvertical, fault-related fracture swarm that intersects the entire reservoir and extends laterally for several tens or hundreds of meters. Direct indicators of FCs, such as image logs, flow profiles, well tests, and seismic fault maps, are sometimes insufficient to map all FCs in a field. It is also necessary to use indirect FC indicators from well data, such as productivity index (PI), gross rate, water cut, and openhole logs. FCs from indirect indicators can be inferred by a probabilistic decision tree, which makes predictions by integrating data from multiple sources, giving preference to the indicators with the highest relevance. Decision trees are constructed by use of a training set that includes measurements of both direct and indirect FC indicators. In this study, wells with borehole images, production logs (flow profiles), and injector/producer short cuts are selected as the training set. The resulting decision trees reveal that total losses, gross production rates, and water cuts are the three most effective indirect indicators of fracture corridors in the test field. [ABSTRACT FROM AUTHOR]

The economy of northern Ontario, Canada, is heavily dependent on mining, so accurate knowledge of seismicity is important for the safe design and operation of mines and other critical facilities, including a proposed underground repository for nuclear waste. In this study, we analyzed 537 cataloged earthquakes that occurred from 1980 to 2006. Seismicity is mainly concentrated in topographically elevated Archean terranes northwest of Lake Superior and in the James Bay and Kapuskasing regions. We analyzed waveforms to determine the focal depth for 331 recorded events, using the regional depth-phase modeling (RDPM) method coupled with surface-wave relative-amplitude analysis. The majority of events are shallow (<6 km) and concentrated in areas of relatively high elevation (>350 m), although in the eastern part this pattern breaks down and some deeper earthquakes (>12 km) are observed. Based on a moving-window event-counting technique, we show that distinct spatial clusters of seismicity can be delineated that are statistically significant relative to background seismicity levels. A particularly active cluster is located within James Bay, where focal depths range from a few kilometers to more than 20 km. Another cluster near Kapuskasing contains deep-focus events and may occur along a hot spot track that runs through western Quebec. Near Dryden, a shallow (∼1 km) earthquake swarm concentrated in a 1 x 1 km region commenced in May 2002, faded, and then started up again in February 2003. Shallow mining-induced events are also common around Sudbury, a major world center for nickel mining. The overall pattern of seismicity appears to correlate with upper-mantle P-wave velocity anomalies, suggesting that lateral variations in mantle rheology may play a significant role in controlling intraplate seismicity of shield areas. It is also likely that crustal stresses caused by glacial isostatic adjustment are an important factor, although the correlation of seismicity with uplift rate is not as clear. Online Material: Focal depth solutions for earthquakes in northern Ontario, Canada. [ABSTRACT FROM AUTHOR]

Abstract: The 1.3– 1.2 Ga fan- shaped Mackenzie dyke swarm and other similar- aged dyke swarms in the Canadian Shield constitute the subswarms of a Late Mesoproterozoic giant radiating swarm. The Late Mesoproterozoic mafic dyke swarms in Australia and East Antarctica might constitute additional subswarms of the giant radiating dyke swarm. A possible Late Mesoproterozoic mantle plume is placed at the focal area of the giant radiating dyke swarm between North America and the landmass (West Australia– East Antarctica). This mantle plume triggered the continuous extension at ca. 1.3–1.2 Ga, which extended into much of Columbia and led to the final fragmentation of the Columbia supercontinent. [Copyright &y& Elsevier]

Abstract: Remnants of 2.1–1.8 Ga orogens can be recognized in nearly every craton assembled within reconstructions of the Rodinia supercontinent, although no particular pattern of laterally extensive orogenic belts emerges. These cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early Earth history. Paleomagnetic constraints have played a limited role in many previous reconstructions of the early Precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles. The geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the Pre-Rodinian supercontinent. Some early unmetamorphosed and undeformed mafic dyke swarms in North China, Southern Peninsular India and North America share coeval age of ~1.85 Ga. If these continents are assembled, the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm, with a piercing point between the Cuddapah rift in South India and the Xiong''er aulacogen in North China. This suggests that the North China Craton, Indian Craton and Canadian Shield may be fragmented from the same landmass. The reconstruction of ~1.85 Ga giant radiating dyke swarm attempted in this paper suggests that the North China Craton, India Craton and Canadian Shield were united together to form a landmass within the Columbia supercontinent before its extension and break-up. The 1.90–1.85 Ga Andean-style North Orogenic Belt suggests that a subduction zone existed on the northern margin of the North China Craton. Wopmay Orogenic Belt developed in a 1.88–1.84 Ga continental volcano-plutonic arc, which is interpreted as the product of eastward subduction of oceanic lithosphere. The Transantarctica Orogenic Belt in East Antarctica may be another candidate to be linked with the subduction zone. The North Orogenic, Wopmay Orogenic and Transantarctica Orogenic Belts could be connected to form a continuous subduction zone. The orogenic comparison strengthens the configuration of the Columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm. The Laurentia, West Australia and East Antarctica were relatively stable from 1.85 Ga to 1.20 Ga as inferred from the data on dyke swarms, magmatism and the paleomagnetism. These continents constituted the core of the Columbia supercontinent in the Late Paleoproterozoic time. The North China Craton and Indian Craton were however separated from the Columbia supercontinent during the Mesoproterozoic time. [Copyright &y& Elsevier]

The article presents a study on the effect of habitat fragmentation and degradation on flocks of African ant-following birds in the middle altitude of Congo-Guinean rain forest in Western Kenya. The bird community was analyzed and differentiated quantitatively into five species of specialized ant-followers and 51 species of opportunistic ant-followers. Results of the study showed that the richness and size of bird flocks declined with the decreasing size of the forest fragments and appeared higher in the undergraded habitat. This was due to the decline of the species' richness and number of specialized ant-followers at swarms. The findings also showed that the opportunistic ant-follower group was affected by habitat fragmentation and degradation.

In 2003, a magma intrusion event occurred at 25-30 km in the lower crust under the northwestern corner of Lake Tahoe, as evidenced by both an earthquake swarm and a geodetic displacement. This study examines the seismicity associated with that event and subsequent seismicity in the upper crust. HYPODD relocations showed that the deep swarm of approximately 1600 microearthquakes at the intrusion site was concentrated on a planar area with a strike of N42°W, dipping at 39° to the northeast. The largest microearthquake in this swarm was M 2.2, and an anomalously high b-value of 2.0 is seen in the recurrence-versus-magnitude plot. The swarm progressed over this plane in a somewhat irregular pattern for a period of roughly 5 months. Focal mechanisms of the deep-swarm events are highly variable and do not reflect the known regional stress field. Two months after the deep-swarm activity started, a shallow swarm of approximately 1100 microearthquakes began at 10-12-km depths in the shallow crust almost immediately above the deep swarm and continued through 2005. This swarm had a maximum M of 2.4 and a relatively high b-value of 1.5. Based on HYPODD relocations, hypocenters in this swarm are concentrated in a narrow pipelike volume, and event depths progressed steadily upward over the more than 2 yr of observation. Focal mechanisms in this shallow swarm are more consistent with the regional stress field than those of the deep swarm.Within one focal depth horizontally of the deep swarm, postintrusion seismic activity increased significantly compared to prior years. Stress triggering from the deep magma intrusion, although based on sub-bar stress changes in the shallow crust, is a feasible explanation of the observed increase. [ABSTRACT FROM AUTHOR]

Abstract: The study area in the northwest Sinai represents one of the most significant regions in the Egyptian basement intensely invaded by post-orogenic calc-alkaline dyke swarms. Two post-orogenic dyke swarms have been recognized in NW Sinai namely: (1) mafic dykes of basalt, basaltic andesite and andesite composition and (2) felsic dykes of dacite, rhyodacite and rhyolite composition. These basaltic to rhyolitic dykes intruded contemporaneously and shortly after the intrusion of the post-orogenic leucogranite. The mafic and felsic dykes are enriched in incompatible elements, especially in the large ion lithophile elements (e.g. K, Rb, Ba) and depleted in high field strength elements with negative P, Ti and Nb anomalies. Major and trace element geochemistry indicates that investigated mafic and felsic magma types are not related via fractional crystallization. The protoliths of the mafic and felsic dykes appear to have evolved from different parental magmas. The incompatible trace element patterns favour a derivation of the mafic dykes from melting of a garnet peridotite source followed by fractional crystallization of olivine, clinopyroxene amphibole and zircon. The felsic dykes, on the other hand, could be generated by melting of garnet-free source modified subsequently by fractional crystallization of plagioclase, apatite and titanomagnetite. This implies variable source characteristics at the end of the Pan-African in the NW Sinai. The mafic and felsic dykes can be related to an intracontinental setting and that this was accompanied by a chemical evolution of the subcontinental lithosphere. Magma generation and ascent in the area was favoured by extensional movements, which is already known from other areas in NE Africa. [Copyright &y& Elsevier]

Abstract: A half-century of investigations are summarized here on the youngest Hawaiian volcano, Lṑihi Seamount. It was discovered in 1952 following an earthquake swarm. Surveying in 1954 determined it has an elongate shape, which is the meaning of its Hawaiian name. Lṑihi was mostly forgotten until two earthquake swarms in the 1970s led to a dredging expedition in 1978, which recovered young lavas. The recovery of young lavas motivated numerous expeditions to investigate the geology, geophysics, and geochemistry of this active volcano. Geophysical monitoring, including a real-time submarine observatory that continuously monitored Lṑihi''s seismic activity for 3 months, captured some of the volcano''s earthquake swarms. The 1996 swarm, the largest recorded in Hawaìi, was preceded earlier in the year by at least one eruption and accompanied by the formation of a ∼300-m deep pit crater, Pele''s Pit. Seismic and petrologic data indicate that magma was stored in a ∼8–9km deep reservoir prior to the 1996 eruption. Studies on Lṑihi have altered conceptual models for the growth of Hawaiian and other oceanic island volcanoes, and refined our understanding of mantle plumes. Petrologic and geochemical studies of Lṑihi lavas showed that the volcano taps a relatively primitive part of the Hawaiian plume, producing a wide range of magma compositions. These compositions have become progressively more silica-saturated with time, reflecting higher degrees of partial melting as the volcano drifts toward the center of the hotspot. Helium and neon isotopes in Lṑihi glasses are among the least radiogenic found at ocean islands, and may indicate a relatively deep and undegassed mantle source for the volcano. The north–south orientation of Lṑihi rift zones indicates that they may have formed beyond the gravitational influence of the adjacent older volcanoes. A new growth model indicates that Lṑihi is older, taller and more voluminous than previously thought. Seismic and bathymetric data have clarified the importance of landsliding in the early formation of ocean island volcanoes. However, a fuller understanding of Lṑihi''s internal structure and eruptive behavior awaits installation of monitoring equipment on the volcano. The presence of hydrothermal activity at Lṑihi was initially proposed based on nontronite deposits on dredged samples that indicated elevated temperatures (31°C), water temperature, methane and 3He anomalies, and clumps of benthic micro-organisms in the water column above the volcano in 1982. Submersible observations in 1987 confirmed a low temperature geothermal system (15–30°C) prior to the 1996 formation of Pele''s Pit. The sulfide mineral assemblage (wurtzite, pyrrhotite, and chalcopyrite) deposited after the pit crater collapsed are consistent with hydrothermal fluids with temperatures >250°C, although the highest measured temperature was ∼200°C. Vent temperatures decreased to ∼60°C during the 2004 dive season indicating a waning of the current phase of hydrothermal activity. [Copyright &y& Elsevier]

We present the results of an analysis of seismic data collected for seven years in the La Paz-Los Cabos region. The data set includes earthquakes with magnitudes of up to 3.6 and focal depths mostly between 2 and 14 km. The results show that some located epicenters correlated with known faults of the region, but others did not, suggesting faults that have not yet been recognized. In addition, even though earthquakes occurred all over the study area, zones of stress concentration were identified. In such zones, the seismicity often developed as earthquake swarms. Composite fault-plane solutions were prepared by using data of those areas. The resulting mechanisms for events of the northern part of the study area indicated normal faulting (east-side down). In such case, the P axis had a mean vertical angle of 55° in the N12° W direction, whereas the T axis was nearly horizontal and with a N68° E average trend. This part of the study area is found to be under a predominant tensional stress regime. To the southwest of the area, the focal mechanisms showed predominant components of strike-slip faulting. There, the T axes were still subhorizontal, but the P axes were more horizontal, resulting in a mean phage of about 20° in the N38° W direction. On the other hand, highly consistent directions of P and T axes from 13 representative events of the Gulf of California fault system led to average axes that are close approximations to the principal stresses that drive such a fault system. A comparison of these axes with average P and T axes of our region shows that the latter are about 16 to 23 degrees more westerly than the former. We propose then either that the earthquakes in the study area occur more in response to local tectonic forces, or that our inferred stress axes are only approximations to the principal stresses. The complex tectonic situation of the region makes the first possibility feasible, but the fact that the earthquakes occurred on pre-existing faults favors the second one. Future seismic data of the region will help to elucidate this uncertainty. Finally, the microearthquakes studied were not generated by the Gulf of California fault system that forms the boundary between the Pacific and North America plates. Our results, then, are evidence of a wide zone of deformation across which subsidiary faulting accommodates part of the relative plates motion. [ABSTRACT FROM AUTHOR]

A dataset comprising some 389 strong- and weak-motion records for 69 events from the Burakin 2001-2002 earthquake swarm, and additional recent events, is compiled to develop a regional ground-motion model for the Yilgarn Craton, southwestern Western Australia. Events range in size from moment magnitude 2.2 ≤ M ≤ 4.6. The decay of horizontal-component spectral amplitudes can be approximated by a geometrical attenuation coefficient of R-1.0 within 80 km of the source. The associated regional seismic quality factor can be expressed as Q(f) = 457 f0.37 for frequencies 1.07 ≤ f ≤ 25.0 Hz. Average corner frequencies for events with magnitude M >3.0 do not vary significantly with seismic moment M0, indicating a steep distribution of M0 versus corner frequency. This causes anomalously low estimates of stress drop for smaller magnitude events (M <4.0). Fourier spectral amplitudes, corrected for geometric and anelastic attenuation, were regressed with M to obtain quadratic attenuation coefficients. Modeled horizontal-component displacement spectra fit the observed data well. Amplitude residuals (predicted-observed amplitudes) are, on average, relatively small and do not vary significantly with hypocentral distance. Source spectra (i.e., at R = 1 km) predicted from the regression parameters give self-consistent amplitudes at low frequency (f less than approximately 2 Hz), equivalent to predictive models from eastern North America (ENA) for the same moment magnitude. However, our model predicts lower spectral amplitudes with increasing frequency as a consequence of the low-stress-drop events. This is particularly apparent for the smaller magnitudes. Western Australian source spectra begin to converge with ENA models at increasing magnitudes. If hypocentral distance is increased (i.e., R ≫ 1 km), the models begin to diverge at low frequencies owing to differences in geometrical attenuation coefficients. The bulk of these data were recorded from an earthquake swarm with very shallow focal depths (h < 2 km). Consequently, we suspect that the spectral shapes we observe may not be characteristic of isolated crustal earthquakes, in particular, for small magnitudes. [ABSTRACT FROM AUTHOR]

In the fall of 1996 the Alaska Volcano Observatory recorded an unprecedented level of seismic and volcanic activity. The following were observed: (1) a swarm at Iliamna Volcano (August 1996 to mid-1997); (2) an eruption at Pavlof Volcano (September 1996 to December 1996); (3) a swarm at Martin/Mageik volcanoes (October 1996); (4) a swarm at Strandline Lake, which continued for several years (1996-1999); and (5) deformation of Mt. Peulik (inflation begins after October 1996 and ends in fall 1998), based on interpretation of interferometric synthetic aperture radar data. The number of monitored volcanic areas in 1996 was thirteen. We conducted two formal statistical tests to determine the likelihood of four of these events occurring randomly in the same time interval. The tests are based on different conceptual probabilistic models (classical and Bayesian) that embrace a wide range of realistic tectonic models. The first test considered only the areas in which swarms or eruptions occurred (7 of 13 if Strandline Lake is included; 6 of 12 otherwise), by comparing the real catalog with 10,000 synthetic catalogs under the assumption that the sites are independent. The second method is a hierarchical Bayesian model in which the probability of a swarm at each of the 13 (or 12) areas is different but the parent population is the same. We found that the likelihood of the swarms and eruption occurring nearly simultaneously by chance alone is small for a wide range of probabilistic schemes and, consequently, for different tectonic scenarios. Therefore, we conclude that the events may be related to a single process. We speculate that a widespread deformation pulse or strain transient occurred, mainly in the east half of the arc, and may be the most likely candidate for causing such nearly simultaneous events. [ABSTRACT FROM AUTHOR]