Shallow Source Depth Research Articles

Shallow water source depth discrimination based on a vertical linear array using deep learning

This study aims to explore source depth discrimination in shallow water waveguides using a vertical line array. Due to the similarity in physical characteristics of features and environmental parameters, the feature distributions of shallow and deep sources overlap in feature space. This overlap is further exacerbated by strong background noise, reducing detection reliability. Therefore, a deep learning-based source depth discrimination (DL-SDD) scheme is proposed. Specifically, this scheme is based on a residual structure, embedding channel attention mechanisms into the deep structure to eliminate noise-related information gradually. Furthermore, a specially designed loss function considers inter-class and intra-class distances to achieve compact and mutually distant distributions of source features. When this loss function is applied, the overlap of source feature distributions is suppressed in end-to-end feature learning, leading to a high detection probability. The numerical simulations demonstrate that the proposed DL-SDD outperforms traditional method, achieving a 7 % increase in detection probability and a 15 % decrease in false alarm rate, even near the discrimination depth. Furthermore, the discrimination depth is reduced by nearly half. Experimental results from the South China Sea validate the effectiveness of the proposed DL-SDD.

Gravity patterns and crustal architecture of the South-Central Indian Ridge at 22°-17°S: Evidence for the asymmetric ridge accretion

The present study examines the gravity-derived structural fabric and crustal structure beneath the southern portion of the Central Indian Ridge at 22°-17°S. We examined the structures using enhanced edge detection techniques like gradient amplitude of the vertical derivative (HGVD), improved horizontal tilt angle (ITDX), gradient amplitude of the NTilt (HGNTilt) and the fast sigmoid function (FSED) on synthetic examples and the gravity data of the study region. The gravity-derived lineaments describes the lineaments that are parallel to fracture zones trending in ENE-WSW, parallel to segments/major ridge axis, near circular lineaments probably depicting ascending magma sources. We estimated the depth to the gravity-derived lineaments using the tilt_depth technique, which reveals the presence of lineaments with shallow source depth range at 2.5–7 km. The south-Central Indian Ridge is characterized by numerous lineaments with random depths that shows variable spreading rates and continuous ridge jumps. The major lineament depths are rooted into the lower crust beneath the major axis and the upper mantle in the region away from it. In addition, gravity inversion of low-pass filtered Bouguer anomaly data is used to estimate the crustal thickness. The gravity inversion shows that the Moho depth varies from 5 to 14.5 km, which shows thicker crust along the principal ridge axis and thinner in the surrounding regions. The structural fabric in combination with the crustal thickness, depth of the lineaments, age of the ridge crust, earthquake data suggests discrete minor/local ridge jumps along the major ridge axis and asymmetric accretion along the ridge axis.

Interpretation of aeromagnetic and remote sensing data of Auchi and Idah sheets of the Benin-arm Anambra basin: Implication of mineral resources

Abstract The need for mineral resources for economic development is key in both developing and developed countries. However, miners usually resort to random excavation of mineral deposits without proper investigation to identify structures of interest in target areas. This usually leads to land depletion and abandonment. The aim of this study is to assess the mineral potential in part of the Benin-arm of the Anambra basin by investigating the geophysical characteristics of the area using remote sensing and aeromagnetic data. Surface and subsurface regional structures, including faults and zones of mineralisation, were mapped by integrating aeromagnetic and remote sensing data. The mineral bearing zones that show high prospects of mineral deposits in the region were identified. The rose diagram revealed that the surface lineaments are aligned in the NW–SE, N–S, NE–SW, E–W, NNE–SSW, ENE–WSW, and ESE–WNW directions. The orientations of the subsurface lineaments are aligned mostly in the NE–SW, N–S, and E–W directions. The magnetic intensity ranged between −431.38 and 399.82 nT, while reduction-to-pole magnetic intensity ranged from –416 to 664.45 nT. The first vertical derivative showed magnetic intensity which ranged from –0.5863 to 0.9060 nT/km2. The total horizontal derivative magnetic intensity ranged from –0.00031 to 0.762691 nT/km2, while the analytic signal showed magnetic intensity ranging from 14.0664 to 394,607.3438 nT/cm2. The windowed Euler deconvolution depth to magnetic source showed depth range of <20 to 2,000 m. Many of the features delineated in the study area were characterised by shallow magnetic source depths (<500 m); this is a common characteristic of basement complex terrains. Deeper magnetic source depths (>1,000 m) were observed in the sedimentary terrain. Mineral exploration should be focused in areas with high lineament concentration, as lineaments are potential conduits for economic minerals deposition.

Zircon U-Pb ages and Hf isotope characteristics of granitic rocks from the Shapinggou molybdenum deposit, Dabie Shan, China

The giant Shapinggou porphyry molybdenum deposit in Jinzhai county, Anhui province is located in eastern Dabie Shan, but its genesis, genetic type and tectonic setting are still debated. New zircon U − Pb age data presented in this contribution show that the granitic rocks at Shapinggou area have been episodically formed at ∼ 135 Ma, ∼127 Ma, ∼117 Ma and ∼ 113 Ma in Early Cretaceous, and can be classified into the early-stage (135–127 Ma) granodiorite and monzogranite plutons, and the late-stage (117–113 Ma) quartz-orthophyre stock and cryptoexplosive breccia pipes. Mo mineralization is closely associated with the late-stage shallow-seated granitic stock or pipes. Granitic rocks of both stages have high contents of SiO2, K2O + Na2O and Al2O3, low contents of TiO2, CaO and MgO, showing a metaluminous to peraluminous, high-K calc-alkaline to shoshonite affinity. They are generally featured by enrichments of LILE (Rb, U, Th) and LREE, and depletions of HFSE (Nb, Ta, Ti) and HREE, with high (La/Yb)N ratios. The early-stage granitic plutons show trace element geochemical signatures of partial melting of over-thickened continental crust (>50 km), whereas element geochemistry of the late-stage small-sized granitic intrusions suggests a shallow source depth (ca. 35 km) under post-collisional extension setting. The Hf isotope data indicate that the Shapinggou granitic rocks are originated from a crustal source predominated by the Kuanping Group mixed with other components such as the mantle. The southward subduction and partial melting of the southern North China Block (including Kuanping Group) beneath the Dabie Shan lead to the generation of the Shapinggou porphyry Mo system in the post-collisional extension setting. Hence the Shapinggou Mo system distinguishes for its crustal source and post-collisional tectonic setting from the porphyry Mo deposits of Climax- or Endako-types, and should belong to collision-type or Dabie-type porphyry Mo deposit.

Phase fluctuation of cross-spectrum by dividing a vertical linear array for shallow water source depth discrimination

This study proposes a depth-dependent characteristic for discriminating source depth by dividing a vertical linear array (VLA) in shallow water with a typical downward-refracting sound-speed profile. Theoretical analysis of the phase fluctuation of cross-spectrum between the wide-band acoustic field recorded by the upper-half VLA and that recorded by the lower-half VLA was conducted. Results showed that the number of zero crossing points of the phase fluctuation of cross-spectrum had significant separability for the surface and submerged sources. The effects of velocity gradient, SNR, thermocline depth fluctuation, and array aperture on depth separability were investigated using simulation analysis, which is important for the application of depth discrimination method in real marine environments. Furthermore, the effect of array tilt on depth separability was illustrated and improved using a modified sound field summation method. On this basis, the VLA acoustic data collected in the South China Sea were processed to verify the detection ability of the current method on submerged sources.

Aeromagnetic anomalies and magnetic domains of the Jalisco Block, western Mexico

The Jalisco Block (JB) in western Mexico has been considered as a margin terrane being detached from the North American plate along the Tepic-Zacoalco rift and Colima graben. In this study, an aeromagnetic analysis of the JB and surrounding areas is performed applying a series of geophysical filters and VOXI inverse model. The JB is characterized by two aeromagnetic domains (AMD). AMD-Ia located throughout the south-central section that borders the continental margin is characterized by positive magnetic anomalies extending from the Armería river to Banderas Bay. The AMD-Ia corresponds to the Puerto Vallarta batholith emplaced in the late Cretaceous. The AMD-IIa in the northern section is marked by negative magnetic anomalies over volcanic and volcano-sedimentary rocks, extending to the southern Sierra Madre Occidental. We propose that JB tectonic boundaries correspond to the aeromagnetic AMD-Ia eastern limit and AMD-IIa northern limit. Estimated depths from the radially average power spectrum for deep magnetic sources in the AMD-Ia are in the range of 5.4 km, and for shallow sources from 0.8 to 1.2 km. For AMD-IIa depths are 10.45 km for deep sources and 0.13 km for shallow sources. Depths for deep sources from the Euler deconvolution for the AMD-Ia range from 3.3 to 4.5 km and from 3.5 to 4.9 km for the AMD-IIa. Shallow source depths in the AMD-Ia are 0.2–0.3 km and near surface for the AMD-IIa. The integrated analysis shows five aeromagnetic domains, two associated with the JB and three with the surrounding areas. The VOXI inverse model allows to estimate the dimensions and location of magnetic sources in the reduced to the pole and upward continuations.

Inflation and Asymmetric Collapse at Kīlauea Summit During the 2018 Eruption From Seismic and Infrasound Analyses

AbstractCharacterizing the large M4.7+ seismic events during the 2018 Kīlauea eruption is important to understand the complex subsurface deformation at the Kīlauea summit. The first 12 events (May 17–May 26) are associated with long‐duration seismic signals and the remaining 50 events (May 29–August 2) are accompanied by large‐scale caldera collapses. Resolving the source location and mechanism is challenging because of the shallow source depth, significant nondouble‐couple components, and complex velocity structure. We demonstrate that combining multiple geophysical data from broadband seismometers, accelerometers, and infrasound is essential to resolve different aspects of the seismic source. Seismic moment tensor solutions using near‐field summit stations show the early events are inflationary. Infrasound data and particle motion analysis identify the source of inflation as the Halema'uma'u reservoir. For the later collapse events, two‐independent moment tensor inversions using local and global stations consistently show that asymmetric slips occur on inward‐dipping normal faults along the northwest corner of the caldera. While the source mechanism from May 29 onwards is not fully resolvable seismically using far‐field stations, infrasound records, and simulations suggest there may be inflation during the collapse. The summit events are characterized by both inflation and asymmetric slip, which are consistent with geodetic data. Based on the location of the slip and microseismicity, the caldera may have failed in a “see‐saw” manner: small continuous slips in the form of microseismicity on the southeast corner of the caldera, compensated by large slips on the northwest during the large collapse events.

Characterizing Near‐Field Surface Deformation in the 1990 Rudbar Earthquake (Iran) Using Optical Image Correlation

AbstractThe 1990 Rudbar earthquake (Mw 7.3) broke along three right‐stepping segments of the left‐lateral Rudbar fault, in the western Alborz mountains (Iran), producing ∼80 km of surface rupture. Previously reported horizontal (∼60 cm) field offsets were surprisingly low, and the vertical (∼95 cm) unexpected, given the relatively large moment release, shallow source depth, vertical fault geometry, and left‐lateral rake. We characterize the surface displacement from optical image correlation of satellite and aerial images. A maximum displacement of ∼6.5 m is observed in the eastern part of the rupture consistent with both a sharp pulse in the moment release rate ∼10 s after the rupture initiation, and the macroseismic isoseismal curves. We find ∼2.2 m mean surface slip, larger than field measurements, but consistent with empirical scaling laws based upon the seismological moment release. Estimates of fault zone width based on correlation of 1 m resolution aerial photos (and corrected for smoothing bias) are <200 m; these values increase artificially with lower resolution imagery, which is more affected by smoothing bias. The large mismatch between field and correlation displacements suggests the Rudbar earthquake probably featured significant off‐fault deformation, consistent with its low structural maturity. Finally, we document a surface rupture along a structural continuation of the Kabateh segment, east of the Sefid Rud river. Slip along this segment (1–2 m), is less than the neighboring Zard Geli segment (1.5–6 m). The two faults converge at Sefid Rud, which appears to represent a major structural feature controlling fault segmentation in this region.

Moment Tensors of Ring‐Faulting at Active Volcanoes: Insights Into Vertical‐CLVD Earthquakes at the Sierra Negra Caldera, Galápagos Islands

AbstractModerate earthquakes (Mw > 5) with moment tensors (MTs) dominated by a vertical compensated‐linear‐vector‐dipole (vertical‐CLVD) component are often generated by dip slip along a curved ring‐fault system at active volcanoes. However, relating their MTs to ring‐fault parameters has been proved difficult. The objective of this study is to find a robust way of estimating some ring‐fault parameters based on their MT solutions obtained from long‐period seismic records. We first model the MTs of idealized ring‐faulting and show that MT components representing the vertical‐CLVD and vertical strike‐slip mechanisms are resolvable by the deviatoric MT inversion using long‐period seismic waves, whereas a component representing the vertical dip‐slip mechanism is indeterminate owing to a shallow source depth. We then propose a new method for estimating the arc angle and orientation of ring‐faulting using the two resolvable MT components. For validation, we study a vertical‐CLVD earthquake that occurred during the 2005 volcanic activity at the Sierra Negra caldera, Galápagos Islands. The resolvable MT components are stably determined with long‐period seismic waves, and our estimation of the ring‐fault parameters is consistent with the ring‐fault geometry identified by previous geodetic studies and field surveys. We also estimate ring‐fault parameters of two earthquakes that took place during the 2018 activity at the caldera, revealing significant differences between the two earthquakes in terms of slip direction and location. These results show the usefulness of our method for estimating ring‐fault parameters, enabling us to examine the kinematics and structures below active volcanoes with ring faults that are distributed globally.

Observation of Higher-Mode Surface Waves from an Active Source in the Hutubi Basin, Xinjiang, China

ABSTRACTBasins with thick sediments can amplify and prolong the incoming seismic waves, which may cause serious damage to surface facilities. The amplification of seismic energy depends on the shear-wave velocity of the uppermost layers, which is generally estimated through surface wave analysis. Surface waves may propagate in different modes, and the mechanism of the mode development is not well understood. Exploiting a recently deployed permanent airgun source in the Hutubi basin, Xinjiang, northwest China, we conducted a field experiment to investigate the development of multimode surface waves. We observed surface waves at the frequency of 0.3–5.0 Hz with apparent group velocities of 200–900 m/s, and identified five modes of surface waves (three Rayleigh-wave modes and two Love-wave modes) through time–frequency and particle-motion analyses. We then measured 125 group velocity dispersion curves of the fundamental- and higher-mode surface waves, and further inverted the 1D S-wave velocity structure of the Hutubi basin. The S-wave velocity increases abruptly from 238 m/s at the surface to 643 m/s at 300 m depth. Synthetic seismograms with the inverted velocity structure capture the main features of the surface waves of the different modes. Synthetic tests suggest that the low velocity, high velocity gradient, and shallow source depth are likely the dominant contributing factors in the development of higher-mode surface waves.

Comparative depth estimates and modeling of magnetic anomalies over the Nkalagu area, Southeastern Nigeria

High-resolution aeromagnetic data of Nkalagu area, in the southern Benue Trough, Nigeria, were employed for the determination of sediment thickness and characterization of magnetic anomalies in the area for possible mineralization and hydrocarbon prospect. Four methods of depth estimates, namely Euler deconvolution, source parameter imaging (SPI), forward and inverse modeling and radially averaged power spectrum (RAPS), were utilized, and the results were compared. Standard Euler deconvolution for the structural index (SI) of 0.5, 1.0, 2.0 and 3.0 gave depths to magnetic sources that range from 0 to 1500 m, 300 to 3300 m, 0 to 4000 m and 200 to 4200 m, respectively. Depth estimates from source parameter imaging (SPI) range from 135 to 2876 m and spectral depth analysis from RAPS revealed deep and shallow magnetic sources with the shallow source depths ranging from 420 to 850 m, while the depth of deep magnetic sources ranges from 3170 to 5840 km. Four profiles were taken on the total magnetic intensity contoured map and were modeled using forward and inverse modeling techniques. The models suggest the existence of intermediate to basic intrusives at depths between 545 and 1977 m. Generally, it is deduced that pockets of thick sedimentary piles in the area are zones of possible hydrocarbon accumulation. Furthermore, the existence of clusters of straight and curvilinear magnetic lineaments with a predominant NE–SW trend deduced from the tilt derivative grid, reveals four zones of high lineament density which suggest favourable zones for follow-up exploration for base metal mineralization.

Focal mechanisms of recent seismicity at Campi Flegrei, Italy

The present study analysed volcano tectonic earthquakes that occurred in the caldera of Campi Flegrei, Italy from 2000 to 2019 to compute source focal mechanisms. We used data from a large number of seismic stations to estimate reliable fault plane solutions for 71 events of a maximum duration magnitude (MD) ranging between 0.5 ≤ MD ≤ 2.5. We found that most solutions were of a normal type and were rake-angle negative focal mechanisms in 86% of the cases. Only a few earthquakes occurred on normal dip-slip faults; the majority occurred on varying degrees of oblique slip. Only one event had a pure reverse-mechanism, and it was located far from the caldera centre. The spatial distribution of the computed mechanisms exhibited a remarkable mix of fault orientations without any relationship to the location area or source depth. The predominance of normal kinematics on high-angle and nearly vertical faults and the very shallow source depth exhibited good agreement with the fact that seismicity at Campi Flegrei occurred during periods of ground uplift. In fact, ground uplift elongated the shallowest crust, thus reducing the normal stress on existing locked faults; this facilitated earthquakes, particularly those characterised by normal, dip-slip, and strike-slip mechanisms. In contrast, ground subsidence produced a horizontal shortening of the upper crust, which increased the normal stress on high-angle faults and reduced earthquake occurrence. Our results indicate that the driving force of the recent volcano-tectonic seismicity in the study region is the ongoing ground uplift.

English

Structural features of Ikogosi Warm Spring and its environs have been delineated using qualitative and quantitative interpretation methods. Euler-Deconvolution and local wave number methods were adopted for quantitative and qualitative interpretation of aeromagnetic data sheet 243 by Geological Survey Agency of Nigeria (GSN) in 2008, respectively. Euler solution revealed depth range of -2.98 to -290 m to the magnetic sources, while local wave number method revealed depths to magnetic sources/basements ranging from -2.1 to -1311.4 m of the geologic lineaments in Ikogosi Warm Spring and its environs. Both methods revealed outcrop features characterized by their positive depth values above the mean ground level. Both methods proved efficient way of resolving shallow magnetic source depth giving a very plausible depth of -2.1 m for local wave number and -2.98 m for Euler solution in the area of study. Key words:  Warm-Spring, Ikogosi, lineament. &nbsp

Numerical modeling of underwater sound propagation in the presence of triangle obstacles at low frequency

Low frequency sound propagation from industrial activities in underwater is considered immensely important by many scientists. For reducing the destructive effects of low frequency sound at long range, different obstacles can be placed in the direction of sound propagation. However, experimental analysis of this phenomenon is very complex. This is mainly due to the fact that oceans have non-homogeneous environments, while the presence of various sediments, animals, and geological phenomena add to the complexity. Accordingly, numerical models are considered good alternatives for underwater acoustic simulation. In the current study, parabolic equation method is applied for numerically analyzing sound propagation with low frequencies in the presence of triangle obstacles in different situations and arrangements. To accomplish this task, the implicit finite difference approach is adopted to solve the governing parabolic equation and a computer program is developed in Python language. Furthermore, Greene source and Claerbout estimations are applied to simulate wide angle sound propagation. The effects of various physical parameters such as sound speed profiles, source depth, and frequencies are investigated. Results of the analyses performed on the effects of object geometry indicate that triangle obstacle is more effective than the square obstacle for increasing the transmission loss (TL). Investigation of the effects of two sound speeds of Munk (Etter, Underwater acoustic modeling and simulation, Taylor & Francis, New York 2003) and linear profiles on transmission loss shows that Munk (Etter, Underwater acoustic modeling and simulation, Taylor & Francis, New York 2003) profile causes a spatial shift in TL compared to that of linear profile. On the other hand, the conducted study on three arrangements of single, triple and quintuple triangles at different distances, and frequencies indicate that values of TL in the case of triple triangle arrangement are lower than those by the other two triangle arrangements at different frequencies. Therefore, this arrangement is found to be inappropriate for mitigation of sound travel range. The results of parametric studies on the effects of source depth on TL demonstrate that peak of the TL graph for the receiver depth in vicinity of the free surface, in view of the deeper source, is much larger than those of the shallow source depth.

Large-scale gravity anomaly in northern Norway: tectonic implications of shallow or deep source depth and a possible conjugate in northeast Greenland

Large-scale gravity anomaly in northern Norway: tectonic implications of shallow or deep source depth and a possible conjugate in northeast Greenland

Spectral Modulation Effect in Teleseismic P-waves from North Korean Nuclear Tests Recorded in Broad Azimuthal Range and Possible Source Depth Estimation

Three underground nuclear explosions, conducted by North Korea in 2006, 2009 and 2013, are analyzed. The last two tests were recorded by the Israel Seismic Network. Pronounced coherent minima (spectral nulls) at 1.2–1.3 Hz were revealed in the spectra of teleseismic P-waves. For a ground-truth explosion with a shallow source depth, this phenomenon can be interpreted in terms of the interference between the down-going P-wave and the pP phase reflected from the Earth’s surface. This effect was also observed at ISN stations for a Pakistan nuclear explosion at a different frequency 1.7 Hz and the PNE Rubin-2 in West Siberia at 1 Hz, indicating a source-effect and not a site-effect. Similar spectral minima having essentially the same frequency, as at ISN, were observed in teleseismic P-waves for all the three North Korean explosions recorded at networks and arrays in Kazakhstan (KURK), Norway (NNSN), Australia (ASAR, WRA) and Canada (YKA), covering a broad azimuthal range. Data of 2009 and 2013 tests at WRA and KURK arrays showed harmonic spectral modulation with three multiple minima frequencies, evidencing the clear interference effect. These observations support the above-mentioned interpretation. Based on the null frequency dependency on the near-surface acoustic velocity and the source depth, the depth of the North Korean tests was estimated about 2.0–2.1 km. It was shown that the observed null frequencies and the obtained source depth estimates correspond to P-pP interference phenomena in both cases of a vertical shaft or a horizontal drift in a mountain. This unusual depth estimation needs additional validation based on more stations and verification by other methods.

Comparison of Magnetic Basement Depth Values from Spectral Technique, SPITH and Slope Techniques using HRAM

The quantitative results obtained from the use of spectral analysis technique, Source parameter Imaging technique and empirical depth rule method (maximum and half slope techniques) in interpreting High Resolution AeroMagnetic of middle Benue trough data have been presented and compared. Before employing each technique, the regional residual seperation was done with least square method using polifit program to get the residual data for onward processing. The slope technique usually require that the residual values be contoured into map and then profiled along the most prominent anomaly to obtain the depth curves from where depth to magnetic sources could be calculated. The Spectral requires that the residual values be sub-divided into spectral blocks and radial energy spectrum be generated from where depth would be calculated from the slopes of the straight line plots. The SPI requires the griding of the residual values and the conversion of this to image map from where depth could be read off easily. The deep magnetic anomally source gotten ranges from 2.00 km to 6.29 km with an average depth of 3.25 km for SPI technique; 2.33 km to less than 5.66 km with an average depth of 3.65 km for Spectral analysis technique; and average of 3.74 km and 3.66 km for Half-slope and maximum slope technique respectively. These average depth could be taken as the magnetic basement depth and has the significant that if other conditions are met, this study area of marine sedimentary layer of the Albian Age is favourable for hydrocation accumulation. The shallow source depth gotten ranges from 0.02 km to 2.00 km with an average depth of 1.08 km for SPI; 0.05 km to less than 0.42 km with an average depth of 0.21 km for spectral analysis; and an average depth 0.80 km for Half slope technique. This can be viewed as magmatic intrussion into the sediment and could be responsible for the lead-Zinc mineralization found in the area. These methods thus compares favourably well and moreso the slope technique though empirical has compared favourably with more modern automatic source depth methods.

Shaking intensity from injection-induced versus tectonic earthquakes in the central-eastern United States

Although instrumental recordings of earthquakes in the central and eastern United States (CEUS) remain sparse, the U. S. Geological Survey's “Did you feel it?” (DYFI) system now provides excellent characterization of shaking intensities caused by induced and tectonic earthquakes. Seventeen CEUS events are considered between 2013 and 2015. It is shown that for 15 events, observed intensities at epicentral distances greater than ≈ 10 km are lower than expected given a published intensity-prediction equation for the region. Using simple published relations among intensity, magnitude, and stress drop, the results suggest that 15 of the 17 events have low stress drop. For those 15 events, intensities within ≈ 10-km epicentral distance are closer to predicted values, which can be explained as a consequence of relatively shallow source depths. The results suggest that those 15 events, most of which occurred in areas where induced earthquakes have occurred previously, were likely induced. Although moderate injection-induced earthquakes in the central and eastern United States will be felt widely because of low regional attenuation, the damage from shallow earthquakes induced by injection will be more localized to event epicenters than shaking tectonic earthquakes, which tend to be somewhat deeper. Within approximately 10 km of the epicenter, intensities are generally commensurate with predicted levels expected for the event magnitude.

Experimental verification of enhanced sound transmission from water to air at low frequencies

Laboratory measurements of enhanced sound transmission from water to air at low frequencies are presented. The pressure at a monitoring hydrophone is found to decrease for shallow source depths in agreement with the classical theory of a monopole source in proximity to a pressure release interface. On the other hand, for source depths below 1/10 of an acoustic wavelength in water, the radiation pattern in the air measured by two microphones becomes progressively omnidirectional in contrast to the classical geometrical acoustics picture in which sound is contained within a cone of 13.4° half angle. The measured directivities agree with wavenumber integration results for a point source over a range of frequencies and source depths. The wider radiation pattern owes itself to the conversion of evanescent waves in the water into propagating waves in the air that fill the angular space outside the cone. A ratio of pressure measurements made using an on-axis microphone and a near-axis hydrophone are also reported and compared with theory. Collectively, these pressure measurements are consistent with the theory of anomalous transparency of the water-air interface in which a large fraction of acoustic power emitted by a shallow source is radiated into the air.

Curie isotherm depth from aeromagnetic data constraining shallow heat source depths in the central Aeolian Ridge (Southern Tyrrhenian Sea, Italy)

The Salina, Lipari, and Vulcano volcanic ridge and the surrounding sea sectors (Aeolian Archipelago, Southern Tyrrhenian Sea, Italy) are characterized by vents responsible for a recent (<40 ka—1889/1890 AD) effusive and explosive subareal activity and repeated, 56 to 7 ka in age, submarine explosive eruptions from source areas located between Lipari and Vulcano. A spectral depth estimation of the magnetic bottom using a fractal method on aeromagnetic data from Vulcano, Lipari, and Salina volcanic ridge allows us to constrain the Curie isotherm depth. The elevated portion of the isotherm is between 2 and 3 km below Salina and Vulcano and about 1 km below Lipari. The Curie depth results in the context of other geological and geophysical evidence suggest that the rise of the Curie isotherm is mainly due to the occurrence of shallow heat sources such as magma ponds and associated hydrothermal systems. The short-wavelength magnetic anomaly field reflects magnetic contrasts from highly magnetized volcanic bodies, low-magnetization sediments, and hydrothermally altered rocks. Borehole temperature data verify the Curie temperature derived from the magnetic methods on the island of Vulcano. We conclude that the whole Vulcano, Lipari, and Salina volcanic ridge is active and should be monitored.