scholarly journals Observation of the Long-Period Monotonic Seismic Waves of the November 11, 2018, Mayotte event by the Iranian Broadband Seismic Stations

2020 ◽  
Author(s):  
Hossein Sadeghi ◽  
Sadaomi Suzuki
Keyword(s):  
Surface Waves ◽  
Rayleigh Waves ◽  
Seismic Waves ◽  
Maximum Amplitude ◽  
Body Waves ◽  
Unusual Event ◽  
Long Period ◽  
Phase Velocities ◽  
Clear Peak ◽  
Rayleigh And Love Waves

Abstract On November 11, 2018, an event generating long-lasting, monotonic long-period surface waves was observed by seismographs around the world. This event occurred at around 09:30 (UTC) east of the Mayotte Island, east Africa. This event is unusual due to the absence of body waves in the seismograms and people’s lack of sense. The purpose of this study is to investigate this unusual event using the waveforms recorded by the Iranian National Broadband Seismic Network. The network consisted of 26 stations in operation on November 11, 2018. The stations are located from 4542 km to 5772 km north-northeast of the event’s epicentre. The arrival of monochromatic long-period signals is visible around 10 UTC in the recordings of all the stations and lasts for more than 30 minutes. Frequency analysis of the seismograms shows a clear peak at 0.064 Hz (15.6 sec/cycle). The maximum amplitude of the transverse components is less than a half of the radial components. This is in agreement with the theoretical radiation pattern of Rayleigh and Love waves at a frequency of 0.06 Hz from a vertical Compensated Linear Vector Dipole (CLVD) source mechanism. The average apparent phase velocities are calculated as 3.31 km/s and 2.97 km/s, in the transverse and radial directions, corresponding respectively to the Love and Rayleigh waves in the range of 0.05 to 0.07 Hz. The surface wave magnitude of Ms 5.07 ± 0.22 was estimated. Just before the monochromatic signal, there is some dispersion in the surface waves. This observation may suggest a regular earthquake that triggered the strange Mayotte event.

scholarly journals Observation of the long-period monotonic seismic waves of the November 11, 2018, Mayotte event by Iranian broadband seismic stations

2021 ◽  
Vol 73 (1) ◽  
Author(s):  
Hossein Sadeghi ◽  
Sadaomi Suzuki
Keyword(s):  
Surface Waves ◽  
Seismic Waves ◽  
Maximum Amplitude ◽  
Body Waves ◽  
Dipole Source ◽  
Arrival Times ◽  
Unusual Event ◽  
Long Period ◽  
Clear Peak ◽  
The Difference

AbstractOn November 11, 2018, an event generating long-lasting, monotonic long-period surface waves was observed by seismographs around the world. This event occurred at around 09:28 UTC east of the Mayotte Island, in the Indian Ocean off the coast of East Africa. This event is unusual due to the absence of body waves in the seismograms and no feeling of earth shaking by people locally. The purpose of this study is to investigate this unusual event using the waveforms recorded by 26 stations of the Iranian National Broadband Seismic Network. The stations are located at epicentral distances ranging from 4542 to 5772 km north-northeast of the event’s epicenter. The arrival of monochromatic long-period signals is visible around 10 UTC in the recordings of all the stations and the signals lasted for more than 30 min. Frequency analysis of the seismograms shows a clear peak at 0.064 Hz (15.6 s/cycle). The maximum amplitude of the transverse components is less than a half of the radial components. This is in agreement with the theoretical radiation pattern of Rayleigh and Love waves at a frequency of 0.06 Hz for a vertical compensated linear vector dipole source mechanism. The average apparent phase velocities were calculated as 3.31 and 2.97 km/s, in the transverse and radial directions, corresponding, respectively, to Love and Rayleigh waves in the frequency range of 0.05–0.07 Hz. A surface wave magnitude of Ms 5.07 ± 0.22 was estimated. Just before the monochromatic signal arrives, there is some dispersion in the surface waves. This observation may suggest a regular earthquake of Ms 4.3 ± 0.11 that triggered the November 11, 2018, event. The difference between the arrival times of the recorded surface waves of the two events is estimated to be less than 31 s, and most likely of ~ 7 s only.

Very long-period data from the geoscope network: Preliminary results on great circle averages of fundamental and higher Rayleigh and Love modes

1984 ◽  
Vol 74 (6) ◽  
pp. 2221-2243
Author(s):  
G. Roult ◽  
B. Romanowicz
Keyword(s):  
Rayleigh Waves ◽  
Quality Factors ◽  
Preliminary Results ◽  
The Third ◽  
Long Period ◽  
Phase Velocities ◽  
Rayleigh And Love Waves ◽  
Period Data ◽  
Available Information ◽  
Large Earthquakes

Abstract We have analyzed the long-period Rayleigh and Love waves that were generated by four recent large earthquakes (New Ireland of 18 March 1983; Costa Rica of 4 April 1983; Honshu of 26 May 1983; and Chagos Islands of 30 November 1983). Our records are those of the three-component very long-period stations (SSB, PCR, PAF, and TAM) of the GEOSCOPE network, that were in operation in 1983. Available information from transverse and radial components is obtained; preliminary results concerning the fundamental modes of Love and Rayleigh waves, the first toroidal overtone, the second, and the third spheroidal higher mode are presented. For each of these particular modes, the phase velocities and sometimes the Q quality factors have been obtained, and the methods of data reduction are described in detail for future reference. The examples shown demonstrate, in particular, the benefits to be expected from a modern three-component very long-period network, as well as from original station locations.

scholarly journals Local Coupling and Conversion of Surface Waves due to Earth’s Rotation. Part 2: Numerical Examples

2020 ◽  
Author(s):  
Christoph Sens-Schönfelder ◽  
Ebru Bozdağ ◽  
Roel Snieder
Keyword(s):  
Surface Wave ◽  
Surface Waves ◽  
Normal Mode ◽  
Coriolis Force ◽  
Rayleigh Waves ◽  
Love Wave ◽  
Resonant Coupling ◽  
Phase Velocities ◽  
Rayleigh And Love Waves ◽  
Similar Phase

Summary Rotation of the Earth affects the propagation of seismic waves. The global coupling of spheroidal and toroidal modes by the Coriolis force over time is described by normal-mode theory. The local action of the Coriolis force on the propagation of surface waves can be described by coefficients for the coupling between propagating Rayleigh and Love waves as derived by (Landau & Lifshitz 1959). Using global wavefield simulations we show how the Coriolis force leads to coupling and conversion between both surface wave types depending on latitude, propagation direction, frequency, and local velocity structure. Surface wave coupling is most efficient for periods where the modes have similar phase velocities, a condition that is equivalent to the selection rules of the angular degree in the normal-mode framework, a phenomenon that we refer to as resonant coupling. In the time-domain, resonant coupling gradually converts energy from one wave type–Rayleigh waves or Love wave–into the other, which then propagates independently. Due to the lateral heterogeneity, the condition of equal phase velocity renders the rotational coupling location-dependent. East-west oriented ray path segments and segments at high latitudes (across the Poles) only weakly couple the fundamental mode Rayleigh and Love waves while coupling is strongest for propagation along the meridians across the equator. At 250 s period, where Love and Rayleigh waves have similar phase velocities, the net energy transfer from Rayleigh to Love wave reaches about 10% for one orbit.

The rupture process and aftershock distribution of the 6 April 1992 MS 6.8 earthquake, Offshore British Columbia

1995 ◽  
Vol 85 (3) ◽  
pp. 716-735 ◽  
Author(s):  
John F. Cassidy ◽  
Garry C. Rogers
Keyword(s):  
Surface Waves ◽  
Triple Junction ◽  
Rupture Process ◽  
Body Waves ◽  
Ocean Bottom Seismometer ◽  
Strain Accumulation ◽  
Transform Fault ◽  
Junction Region ◽  
Long Period ◽  
Initial Rupture

Abstract On 6 April 1992, a magnitude 6.8 (MS) earthquake occurred in the triple-junction region at the northern end of the Cascadia subduction zone. This was the largest earthquake in at least 75 yr to occur along the 110-km-long Revere-Dellwood-Wilson (RDW) transform fault and the first large earthquake in this region recorded by modern broadband digital seismic networks. It thus provides an opportunity to examine the rupture process along a young (<2 Ma) oceanic transform fault and to gain better insight into the tectonics of this triple-junction region. We have investigated the source parameters and the rupture process of this earthquake by modeling broadband body waves and long-period surface waves and by accurately locating the mainshock and the first 10 days of aftershocks using a well-located “calibration” event recorded during an ocean-bottom seismometer survey. Analysis of P and SH waveforms reveals that this was a complex rupture sequence consisting of three strike-slip subevents in 12 sec. The initial rupture occurred 5 to 6 km to the SW of the seafloor trace of the RDW fault at 50.55° N, 130.46° W. The dominant subevent occurred 2 to 3 sec later and 4.3 km beneath the seafloor trace of the RDW fault, and a third subevent occurred 5 sec later, 18 km to the NNW, suggesting a northwestward propagating rupture. The aftershock sequence extended along a 60- to 70-km-long segment of the RDW fault, with the bulk of the activity concentrated ∼30 to 40 km to the NNW of the epicenter, consistent with this interpretation. The well-constrained mechanism of the initial rupture (strike/dip/slip 339°/90°/−168°) and of the largest aftershock (165°/80°/170°) are rotated 15° to 20° clockwise relative to the seafloor trace of the RDW fault but are parallel to the Pacific/North America relative plate motion vector. In contrast, the mechanisms of the dominant subevent (326°/87°/−172°), and the long-period solution derived from surface waves aligns with the RDW fault. This suggests that small earthquakes (M < 6) in this area occur along faults that are optimally aligned with respect to the regional stress field, whereas large earthquakes, involving tens of kilometers of rupture, activate the RDW fault. For the mainshock, we estimate a seismic moment (from surface waves) of 1.0 × 1026 dyne-cm, a stress drop of 60 bars, and an average slip of 1.2 m. This represents only 21 yr of strain accumulation, implying that there is either a significant amount of aseismic slip along the RDW fault or that much of the strain accumulation manifests itself as deformation within the Dellwood and Winona blocks or along the continental margin.

Short-period seismic noise

1967 ◽  
Vol 57 (1) ◽  
pp. 55-81
Author(s):  
E. J. Douze
Keyword(s):  
Surface Waves ◽  
Rayleigh Waves ◽  
Seismic Noise ◽  
Body Waves ◽  
Deep Hole ◽  
Period Range ◽  
Short Period ◽  
The Subject ◽  
Hole Arrays

abstract This report consists of a summary of the studies conducted on the subject of short-period (6.0-0.3 sec period) noise over a period of approximately three years. Information from deep-hole and surface arrays was used in an attempt to determine the types of waves of which the noise is composed. The theoretical behavior of higher-mode Rayleigh waves and of body waves as measured by surface and deep-hole arrays is described. Both surface and body waves are shown to exist in the noise. Surface waves generally predominate at the longer periods (of the period range discussed) while body waves appear at the shorter periods at quiet sites. Not all the data could be interpreted to define the wave types present.

scholarly journals Surface waves in multilayered elastic media I. Rayleigh and Love waves from buried sources in a multilayered elastic half-space

1964 ◽  
Vol 54 (2) ◽  
pp. 627-679
Author(s):  
David G. Harkrider
Keyword(s):  
Surface Waves ◽  
Frequency Domain ◽  
Half Space ◽  
Rayleigh Waves ◽  
Elastic Half Space ◽  
Love Waves ◽  
Displacement Fields ◽  
Matrix Formulation ◽  
Total Displacement ◽  
Rayleigh And Love Waves

ABSTRACT A matrix formulation is used to derive integral expressions for the time transformed displacement fields produced by simple sources at any depth in a multilayered elastic isotropic solid half-space. The integrals are evaluated for their residue contribution to obtain surface wave displacements in the frequency domain. The solutions are then generalized to include the effect of a surface liquid layer. The theory includes the effect of layering and source depth for the following: (1) Rayleigh waves from an explosive source, (2) Rayleigh waves from a vertical point force, (3) Rayleigh and Love waves from a vertical strike slip fault model. The latter source also includes the effect of fault dimensions and rupture velocity. From these results we are able to show certain reciprocity relations for surface waves which had been previously proved for the total displacement field. The theory presented here lays the ground work for later papers in which theoretical seismograms are compared with observations in both the time and frequency domain.

Observations of Loma Prieta aftershocks from a dense array in Sunnyvale, California

1991 ◽  
Vol 81 (5) ◽  
pp. 1900-1922
Author(s):  
Arthur Frankel ◽  
Susan Hough ◽  
Paul Friberg ◽  
Robert Busby
Keyword(s):  
Surface Waves ◽  
Body Waves ◽  
Love Waves ◽  
Apparent Velocity ◽  
Dense Array ◽  
S Wave ◽  
Small Aperture ◽  
Long Period ◽  
Santa Clara Valley ◽  
Loma Prieta

Abstract A small aperture (≈300 m), four-station array was deployed in Sunnyvale, California for 5 days to record aftershocks of the Loma Prieta earthquake of October 1989. The purpose of the array was to study the seismic response of the alluvium-filled Santa Clara Valley and the role of surface waves in the seismic shaking of sedimentary basins. Strong-motion records of the Loma Prieta mainshock indicate that surface waves produced the peak velocities and displacements at some sites in the Santa Clara Valley. We use the recordings from the dense array to determine the apparent velocity and azimuth of propagation for various arrivals in the seismograms of four aftershocks with magnitudes between 3.6 and 4.4. Apparent velocities are generally observed to decrease with increasing time after the S wave in the seismograms. Phases arriving less than about 8 sec after the S wave have apparent velocities comparable to the S wave and appear to be body waves multiply reflected under the receiver site or reflected by crustal interfaces. For times 10 to 30 sec after the direct S wave, we observe long-period (1 to 6 sec) arrivals with apparent velocities decreasing from 2.5 to 0.8 km / sec. We interpret these arrivals to be surface waves and conclude that these surface waves produce the long duration of shaking observed on the aftershock records. Much of the energy in the 40 sec after the S-wave is coming approximately from the direction of the source, although some arrivals have backazimuths as much as 60° different from the backazimuths to the epicenters. Two of the aftershocks show arrivals coming from 30 to 40° more easterly than the epicenters. This energy may have been scattered from outcrops along the southeastern edge of the basin. In contrast, the deepest aftershock studied (d = 17 km) displays later arrivals with backazimuths 30 to 40° more westerly than the epicenter. A distinct arrival for one of the aftershocks propagates from the southwest, possibly scattered from the western edge of the basin. Synthetic seismograms derived from a plane-layered crustal model do not produce the long-period Love waves observed in the waveforms of the ML 4.4 aftershock. These Love waves may be generated by the conversion of incident S waves or Rayleigh waves near the edge of the basin.

Statistical properties of Rayleigh waves due to scattering by topography

1967 ◽  
Vol 57 (1) ◽  
pp. 83-90
Author(s):  
J. A. Hudson ◽  
L. Knopoff
Keyword(s):  
Surface Waves ◽  
Rayleigh Waves ◽  
Seismic Noise ◽  
Forward Scattering ◽  
Statistical Properties ◽  
Body Waves ◽  
Simplified Model ◽  
Two Dimensional ◽  
Especial Reference ◽  
The Earth

abstract The two-dimensional problems of the scattering of harmonic body waves and Rayleigh waves by topographic irregularities in the surface of a simplified model of the earth are considered with especial reference to the processes of P-R, SV-R and R-R scattering. The topography is assumed to have certain statistical properties; the scattered surface waves also have describable statistical properties. The results obtained show that the maximum scattered seismic noise is in the range of wavelengths of the order of the lateral dimensions of the topography. The process SV-R is maximized over a broader band of wavelengths than the process P-R and thus the former may be more difficult to remove by selective filtering. An investigation of the process R-R shows that backscattering is much more important than forward scattering and hence topography beyond the array must be taken into account.

scholarly journals Crustal structure of central Myanmar (Burma) By surface wave dispersion

MAUSAM ◽  
2022 ◽  
Vol 44 (4) ◽  
pp. 347-352
Author(s):  
S. N. BHATTACHARYA
Keyword(s):  
Surface Waves ◽  
Fundamental Mode ◽  
Rayleigh Waves ◽  
Seismic Waves ◽  
Digital Data ◽  
Low Velocity ◽  
Surface Wave Dispersion ◽  
Gangetic Plain ◽  
Indo Gangetic Plain ◽  
Group Velocities

Digital records of seismic waves observed at Seismic Research Observatory, Cheng Mai. Thailand have been analysed for two earthquakes in western Nepal. Digital data are processed by the floating filter and phase equalization methods to obtain surface waves free from noise. Group velocities of Love and Rayleigh waves are obtained by frequency time analysis of these noise free surface waves. The period of group velocities ranges from 17 to 62 sec for fundamental mode Rayleigh waves and from 17 to 66 sec for fundamental mode Love waves. The wave paths cross both central Myanmar (Burma) and the Indo-Gangetic plain. The group velocity data of surface waves across central Myanmar (Burma) have been obtained after correction of the data for the path across the Indo-Gangetic plain. Inversion of data gives the average crustal and subcrustal structure of central Myanmar (Burma). The modelled structure shows two separate sedimentary layers each of  8 km thick, The lower sedimentary layer forms the low velocity zone of the crust. The total thickness of central Myanmar (Burma) crust is found to be 55 km

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