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I recently reported temporal proof that Mw5.6+ strong earthquakes occur due to (as the lithosphere rides on) vast waves of the tidally driven and gravitationally aided 1–72h long-periodic Earth body resonance (EBR). Here I report a methodologically independent spatial proof of EBR, conclusively showing that tremors are not the only earthquake type caused by mechanical resonance: observations of actual EBR waves in solid matter using continuous Global Positioning System (cGPS) and of their triggering Mw5.6+ earthquakes. Superharmonic resonance periods from the EBR’s 55’–15 days (0.303 mHz–0.7716 μHz) band are thus recoverable in spectra of International Terrestrial Reference Frame (ITRF2014) positional components solved kinematically from 30-s cGPS samplings. The signal is so pure, strong, and stable that even daylong components are constantly periodic at or above 99%-significance, with very high statistical fidelity, ϕ>>12, and ϕ<<12 characterizing overtones or undertones. cGPS stations have diurnal EBR fingerprints: unique sets of ~13–18 EBR frequencies, most clearly formed during ~Mw6+ quiescence, enabling depiction of EBR orientation for real-time EBR mapping. Furthermore, weeklong component time series reveal complete EBR and expected undertones as the signature of EBR’s companion sympathetic resonances, with very high ϕ>>12. Also, I demonstrate EBR mapping using the Mexico City–Los Angeles–San Francisco cGPS profile alongside a tectonic plate boundary, successfully depicting the preparation phase of the 2020 Puerto Rico Mw6.4–Mw6.6 earthquakes sequence. I finish by showing that the EBR triggered the 2019 Ridgecrest Mw6.4–Mw7.1 earthquakes sequence. EBR maps can now be produced for seismic prediction/forecasting and unobscuring (decoupling EBR frequencies) from geophysical observables like stress and strain. EBR engulfs the Earth’s crust, forming the resonance wind whose role and incessantness demote mantle convection from the working hypothesis of geophysics and whose applications include geophysical prospecting and detection at all scales and times. A previously unaccounted-for fundamental force of geophysics, the impulsive EBR spans the vastest energy bands, invalidating any previous claims of seismic detections of gravitational wave signals from deep space, such as by the LIGO experiment.
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Homogenizing earthquake catalogs is an effort critical to fundamentally improving seismic studies for next-generation seismology. The preparation of a homogenous earthquake catalog for a seismic region requires scaling relations to convert different magnitude types, like the mb and Ms, to a homogenous magnitude, such as the seismic moment scale, Mwg, and energy magnitude scale, Me. Several recent studies addressed the preparation of homogenized earthquake catalogs, usually involving the estimation of proxies of moment magnitude Mw from local, ML, and teleseismic (Ms and mb) magnitude estimates. Instead of the standard least squares (SLR), most of such studies used the general orthogonal regression (GOR), while some used the Chi-square regression method. Here we critically discuss GOR and Chi-square regression theory and find that both are the same for the linear case — as expected since both stem from the same mathematical concept. Thus to foster an improved understanding of seismicity and seismic hazard, we used GOR methodology and derived global scaling relations individually between body, surface, energy, and seismic moment magnitude scales. For that purpose, we have compiled 13,576 and 13,282 events for Ms from ISC and NEIC, respectively, mb magnitude data for 1,266 events from ISC, 614 events from NEIC, and Mwg magnitude values for 6,690 events from NEIC and GCMT. We have also derived MS,ISC-to-Me and MS,NEIC-to-Me conversion relations in magnitude ranges of 4.7≤MS,ISC≤8.0 and 4.5≤MS,NEIC≤8.0, respectively. Likewise, we obtained mb,ISC-to-Me and mb,NEIC-to-Me conversion relations for ranges of 5.2≤mb,ISC≤6.2 and 5.3≤mb,NEIC≤6.5. Since the number of data points was insufficient to derive the relations, we considered mb,NEIC up to M6.5. Finally, we derived an MWg-to-Me conversion relation for the 5.2≤Mw≤8.2 range of magnitudes with focal depths <70 km. Our scaling relations can be used for homogenizing earthquake catalogs and conducting seismicity and seismic hazard assessment studies with enhanced realism.
ARK: https://n2t.net/ark:/88439/x063005
Permalink: https://geophysicsjournal.com/article/304
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The electromagnetic responses of a shallow channel (Hainan Strait), an island (Hainan Island) in a shallow coastal sea, and a large flat-topped seamount (Zhongsha Islands) in a deep ocean were studied using a scaled laboratory analogue model. To examine the responses, in-phase and quadrature Hz and HY magnetic fields are presented for traverses over the channel, the island and the seamount for simulated geomagnetic variations with periods in the range 5–500 min. The in-phase Hz and HY channel and island responses were found to decrease rapidly with increasing period, reaching negligible values at about 60-min periods, while the in-phase Hz and HY seamount responses had significant values over the entire 5–500-min period range. The quadrature Hz and HY channel and island responses had maxima at a period of approximately 20 min when the 0.25 km depth ocean in the channel and surrounding the island was 0.025 skin depth (δ). The shapes of the quadrature Hz and HY seamount response curves showed a transition from a "channel response" to an "island response" at a period of approximately 20 min, the same period at which maximum in-phase responses occur. At this period the surrounding ocean depth is 0.2–0.4 δ and the ocean depth over the seamount is 0.05 δ. The quadrature Hz and HY seamount responses were each at a maximum at approximately 100 min, when the surrounding ocean depth was 0.1–0.2 δ and the ocean over the seamount is 0.025 δ. The addition of a conducting plate to simulate conducting mantle structure at a depth of 100 km led to much less attenuation for the case of the seamount than for the channel or island due to the deep ocean surrounding the seamount, effectively shielding the conducting mantle from the overhead primary source field.
ARK: https://n2t.net/ark:/88439/y035553
Permalink: https://geophysicsjournal.com/article/302
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Climate-related extreme geophysical events are among critical global challenges, and Sri Lanka is the second most-affected nation. To minimize disaster impacts and enhance the livability of human settlements, the concept of building community resilience has become crucial in disaster management and preparedness. This paper presents key results and recommendations from an integrated approach to post-disaster recovery interventions and improvements in preparedness activities, to reduce the impact of future disasters and associated risks. We tackled this goal by undertaking a reflective assessment using a case of post-disaster recovery interventions after the floods and landslides of May 2017 in three districts of Sri Lanka. This study emphasizes the need for capitalizing the immediate post-disaster response period to integrate risk reduction and resilience-building activities from the early stages of the recovery timeline. Preparedness and resilience enhancement activities need to align with the Sri Lanka Community Resilience Framework as it can help optimally utilize time and resources to enhance resilience in resources-limited contexts.
ARK: https://n2t.net/ark:/88439/x059916
Permalink: https://geophysicsjournal.com/article/301
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The first systematic collection of magnetotelluric, magnetovariational and frequency sounding results for the Baltic Shield is presented. The great variations in data processing and interpretation methodology make it difficult to present any unified summary of the existing results. Conductive regions in the crust occur in all parts of the shield; sometimes in connection with schist zones, variations in bedrock structure and composition, and sometimes as apparent conductive layering in the depth range 15-25 km. The extent of this layer, as well as the conductive layer at asthenospheric depths in the northern parts of the shield, cannot yet be established. The usefulness of combining electromagnetic and other geophysical data is indicated by the example from central Finland, where deep seismic sounding results seem to be able to explain long-period anisotropies of magnetotelluric sounding curves.
ARK: https://n2t.net/ark:/88439/y024024
Permalink: https://geophysicsjournal.com/article/295
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We investigate the effect of low-velocity waveguides on ground motion. The computation of eigenvalues and eigenfunctions of Rayleigh waves up to the frequency 10.0 Hz allows an analysis of the seismic response which is source independent. The main conclusions of this study are: (1) Extending the model to depths greater than that of the waveguides allows the exact computation of leaking modes. (2) A source in a layered structure generates P waves of higher frequency than S waves even if the structure is purely elastic. (3) At high frequencies (10.0 Hz) the modal components of P waves separate from those of SV waves. (4) Strong surface waves are generated by shallow sources in sedimentary basins, also at a source distance of a few tens of kilometres. These waves do not appear in structures without sediments. (5) The polarization of strong ground motion at the surface of low-velocity layers is mainly horizontal. (6) For oceanic models, the contribution of the sedimentary layers is separable from that of the water layer only at high frequencies.
ARK: https://n2t.net/ark:/88439/y014925
Permalink: https://geophysicsjournal.com/article/294
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Ground-based observations of locally confined, very intense, drifting current systems by the EISCAT magnetometer cross in correlation with GEOS-2 measurements will be explained in terms of kinetic Alfven waves. Particle and magnetic flux measurements on GEOS-2 indicate an excitation of the waves at the inner edge of the Earthward-drifting plasma sheet by resonance mode conversion from hydromagnetic surface waves. The collapsing tail-like field configuration itself is identified as the surface wave. The comparison of theoretically deduced quantities with observational results reveals a satisfactory agreement between observations and theory.
ARK: https://n2t.net/ark:/88439/y003986
Permalink: https://geophysicsjournal.com/article/293
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The symmetrized invariant formulae for the calculation of Fresnel zones or volumes are derived. It is assumed that an inhomogeneous medium with curvilinear interfaces is located between the source and/or the receiver and along the central ray within the Fresnel zone or volume. In the vicinity of the zone centre, the medium is considered locally homogeneous. The formula for the leading term of the field of a wave scattered by a bent body immersed in the above-mentioned medium is obtained by the Kirchhoff approximation. With the help of this formula and the expressions for the Fresnel radii for a particular case, the formulae for the Fresnel zones in the general case considered are obtained on the basis of the reciprocity relation. The formulae for the Fresnel zones are used to obtain the expressions for the Fresnel volumes. The physical consequences of the derived formulae with respect to the validity of the ray formulae and the resolution of seismic methods etc. are discussed.
ARK: https://n2t.net/ark:/88439/y093067
Permalink: https://geophysicsjournal.com/article/292
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The amplitude of the Pg phase, as recorded in explosion seismology studies, is analyzed with the aid of synthetic seismograms. Parameters such as source frequency, low-velocity cover above the crust (sediments or weathered layer), low-velocity layers within the upper crust, velocity gradients, thickness of the gradient zone, attenuation and Poisson's ratio strongly influence the amplitude-distance pattern of the Pg phase. A systematic study clearly shows that different models of the continental upper crust display distinct amplitude-distance characteristics. These models could not be distinguished by travel-time interpretation alone. In the presence of gradient zones the amplitude-distance curve shows different patterns depending on the source frequency. The higher the frequency, the more pronounced are the relative maxima in the amplitudes. The presence of a low-velocity cover at the surface accentuates the character of the amplitude-distance curves even if the cover is thin (a few hundred meters). Moreover, a low-velocity cover produces P to S conversions and multiples following the Pg which obscure possible secondary crustal phases. The thickness of the velocity gradient zone influences the amplitude decay and the width of the relative maxima. Low-velocity layers within the upper crust cause a faster drop-off of the amplitudes than would be expected from ray theory. Detailed Pg amplitude studies are thus useful in improving the knowledge of the physical properties of the upper continental crust. The application of the derived criteria to two sets of real data allow us to determine fine details of the velocity-depth function which are of great importance for the understanding of the Earth's crust.
ARK: https://n2t.net/ark:/88439/y083748
Permalink: https://geophysicsjournal.com/article/291
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About 190 mine tremors were recorded during a 120 day period in the eastern part of the Ruhr district (West Germany). Source parameters of 37 tremors (1.3 ≤ ML ≤ 2.3) are calculated. Hypocentral distance was about 2 km. A combined plot of first P-wave motions in an equal area projection of a focal sphere possibly suggests a dip slip mechanism. Displacement P-wave spectra of 13 events show a clear low frequency level and a high frequency asymptotic behaviour as f –3, where f is the frequency. Seismic moment varies from 2.3 x 1018 dyn cm to 2.3 x 1019 dyn cm. Corner frequencies are in the range of 12–16 Hz. Interpretation of spectra with Brune's (1970) model extended by Hanks and Wyss (1972) leads to source dimensions between 87 m and 120 m. Stress drops vary from 0.9 bars to 8.4 bars. Average dislocations range from 0.6 mm to 19 mm. Stress drop, dislocation, and seismic moment have a clear dependence on radiated seismic energy. Seismic energy of the mine tremors is between 1.4 x 1013 ergs and 2.2 x 1015 ergs. The energy magnitude relation log E = 11.8 + 1.5 M given by Gutenberg and Richter is in good agreement with the data. Source dimensions are in the same range as those found for mine tremors in Utah, South Africa, and Poland.
ARK: https://n2t.net/ark:/88439/y073859
Permalink: https://geophysicsjournal.com/article/290