An anisotropic shear velocity model of the Earth's mantle using normal modes, body waves, surface waves and long-period waveforms

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.

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

Bulletin of the Seismological Society of America ◽

10.1785/bssa0810051900 ◽

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.

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Shear wave imaging from traffic noise using seismic interferometry by cross-coherence

Geophysics ◽

10.1190/geo2010-0188.1 ◽

2011 ◽

Vol 76 (6) ◽

pp. SA97-SA106 ◽

Cited By ~ 105

Author(s):

Norimitsu Nakata ◽

Roel Snieder ◽

Takeshi Tsuji ◽

Ken Larner ◽

Toshifumi Matsuoka

Keyword(s):

Surface Waves ◽

Traffic Noise ◽

Random Noise ◽

Shear Waves ◽

Shear Velocity ◽

Body Waves ◽

Seismic Interferometry ◽

Spectral Amplitude ◽

Interval Velocity ◽

Wide Range

We apply the cross-coherence method to the seismic interferometry of traffic noise, which originates from roads and railways, to retrieve both body waves and surface-waves. Our preferred algorithm in the presence of highly variable and strong additive random noise uses cross-coherence, which uses normalization by the spectral amplitude of each of the traces, rather than crosscorrelation or deconvolution. This normalization suppresses the influence of additive noise and overcomes problems resulting from amplitude variations among input traces. By using only the phase information and ignoring amplitude information, the method effectively removes the source signature from the extracted response and yields a stable structural reconstruction even in the presence of strong noise. This algorithm is particularly effective where the relative amplitude among the original traces is highly variable from trace to trace. We use the extracted, reflected shear waves from the traffic noise data to construct a stacked and migrated image, and we use the extracted surface-waves (Love waves) to estimate the shear velocity as a function of depth. This profile agrees well with the interval velocity obtained from the normal moveout of the reflected shear waves constructed by seismic interferometry. These results are useful in a wide range of situations applicable to both geophysics and civil engineering.

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Retrieving reflection arrivals from passive seismic data using Radon correlation

10.5194/egusphere-egu21-9179 ◽

2021 ◽

Author(s):

Diako Hariri Naghadeh ◽

Christopher J Bean ◽

Patrick Smith ◽

Sergei Lebedev ◽

Huda Mohamed

Keyword(s):

Surface Waves ◽

Correlation Method ◽

Velocity Model ◽

Body Waves ◽

Data Sets ◽

Dirac Delta ◽

Passive Seismic ◽

Strong Surface ◽

Time Offset ◽

Gas Guns

<p>Since explosive and impulsive seismic sources such as dynamite, air guns, gas guns, or even vibroseis can have a big impact on the environment, some companies have decided to record ambient seismic noise and use it to estimate the physical properties of the subsurface. Big challenges arise when the aim is extracting body-waves from recorded passive signals, especially in the presence of strong surface waves. In passive seismic signals, such body-waves are usually weak in comparison to surface waves which are much more prominent. To understand the characteristics of passive signals and the effect of natural source locations, three simple synthetic models were created. To extract body-waves from simulated passive signals we propose and test a Radon-correlation method. This is a time-spatial correlation of amplitudes with a train of time-shifted Dirac delta functions through different hyperbolic paths. It is tested on a two-layer horizontal model, three-layer model which includes a dipping layer (with and without lateral heterogeneity) and also on synthetic Marmousi model data sets. Synthetic tests show that the introduced method is able to reconstruct reflection events at the correct time-offset positions which are hidden in results obtained by the general cross-correlation method. Also, a depth migrated section shows a good match between imaged-horizons and the true model. It is possible to generate off-end virtual gathers by applying the method to a linear array of receivers and to construct a velocity model by semblance velocity analysis of individually extracted gathers.</p>

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

Earth Planets and Space ◽

10.1186/s40623-021-01408-1 ◽

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.

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Effect of a plume on long period surface waves computed with normal modes coupling

Physics of The Earth and Planetary Interiors ◽

10.1016/s0031-9201(99)00153-3 ◽

2000 ◽

Vol 119 (1-2) ◽

pp. 57-74 ◽

Cited By ~ 14

Author(s):

Yann Capdeville ◽

Eléonore Stutzmann ◽

Jean Paul Montagner

Keyword(s):

Surface Waves ◽

Normal Modes ◽

Long Period ◽

Modes Coupling

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

10.21203/rs.3.rs-120308/v1 ◽

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.

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Retrieving reflection arrivals from passive seismic data using Radon correlation

Journal of Geophysics and Engineering ◽

10.1093/jge/gxab004 ◽

2021 ◽

Vol 18 (2) ◽

pp. 1-15

Author(s):

Diako Hariri Naghadeh ◽

Christopher J Bean ◽

Florent Brenguier ◽

Patrick J Smith

Keyword(s):

Surface Waves ◽

Correlation Method ◽

Velocity Model ◽

Body Waves ◽

Data Sets ◽

Dirac Delta ◽

Passive Seismic ◽

Strong Surface ◽

Time Offset ◽

Gas Guns

Abstract Since explosive and impulsive seismic sources such as dynamite, air guns, gas guns or even vibroseis can have a big impact on the environment, some companies have decided to record ambient seismic noise and use it to estimate the physical properties of the subsurface. Big challenges arise when the aim is extracting body waves from recorded passive signals, especially in the presence of strong surface waves. In passive seismic signals, such body waves are usually weak in comparison to surface waves that are much more prominent. To understand the characteristics of passive signals and the effect of natural source locations, three simple synthetic models were created. To extract body waves from simulated passive signals we propose and test a Radon-correlation method. This is a time-spatial correlation of amplitudes with a train of time-shifted Dirac delta functions through different hyperbolic paths. It is tested on a two-layer horizontal model, a three-layer model that includes a dipping layer (with and without lateral heterogeneity) and also on synthetic Marmousi model data sets. Synthetic tests show that the introduced method is able to reconstruct reflection events at the correct time-offset positions that are hidden in results obtained by the general cross-correlation method. Also, a depth migrated section shows a good match between imaged horizons and the true model. It is possible to generate off-end virtual gathers by applying the method to a linear array of receivers and to construct a velocity model by semblance velocity analysis of individually extracted gathers.

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Seismic Monitoring of Permafrost in Svalbard, Arctic Norway

Seismological Research Letters ◽

10.1785/0220200470 ◽

2021 ◽

Author(s):

Julie Albaric ◽

Daniela Kühn ◽

Matthias Ohrnberger ◽

Nadège Langet ◽

Dave Harris ◽

...

Keyword(s):

Surface Waves ◽

Seismic Noise ◽

Green's Functions ◽

Seismic Velocity ◽

Green’S Functions ◽

Velocity Model ◽

Body Waves ◽

Ambient Seismic Noise ◽

Seasonal Temperature ◽

Velocity Variations

Abstract We analyze data from passive and active seismic experiments conducted in the Adventdalen valley of Svalbard in the Norwegian Arctic. Our objective is to characterize the ambient wavefield of the region and to investigate permafrost dynamics through estimates of seismic velocity variations. We are motivated by a need for early geophysical detection of potentially hazardous changes to permafrost stability. We draw upon several data sources to constrain various aspects of seismic wave propagation in Adventdalen. We use f-k analysis of five years of continuous data from the Spitsbergen seismic array (SPITS) to demonstrate that ambient seismic noise on Svalbard consists of continuously present body waves and intermittent surface waves appearing at regular intervals. A change in wavefield direction accompanies the sudden onset of surface waves when the average temperature rises above the freezing point, suggesting a cryogenic origin. This hypothesis is supported further by our analysis of records from a temporary broadband network, which indicates that the background wavefield is dominated by icequakes. Synthetic Green’s functions calculated from a 3D velocity model match well with empirical Green’s functions constructed from the recorded ambient seismic noise. We use a shallow shear-wave velocity model, obtained from active seismic measurements, to estimate the maximum depth of Rayleigh wave sensitivity to changes in shear velocity to be in the 50–100 m range. We extract seasonal variations in seismic velocities from ambient noise cross-correlation functions computed over three years of SPITS data. We attribute relative velocity variations to changes in the ice content of the shallow (2–4 m depth) permafrost, which is sensitive to seasonal temperature changes. A linear decreasing trend in seismic velocity is observed over the years, most likely due to permafrost warming.

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The 11 November 2018 Mayotte event was observed at the Iranian Broadband seismic stations

10.5194/egusphere-egu2020-9767 ◽

2020 ◽

Author(s):

Hossein Sadeghi ◽

Sadaomi Suzuki

Keyword(s):

Phase Velocity ◽

Surface Waves ◽

Earthquake Engineering ◽

Body Wave ◽

Body Waves ◽

Least Square ◽

International Institute ◽

Broadband Seismometer ◽

Waveform Data ◽

Long Period

<p>The 11 November 2018 Mayotte event was first introduced in the media by Maya Wei-Haas (2018) on National Geographic Magazine as a strange earthquake of which seismic waves were recorded by instruments around the world, but unusually nobody felt them. The Mayotte event in the absence of body waves caused long-duration, long-period surface waves traveling around the globe. Cesca et al. (2020) by analyzing regional and global seismic and deformation data suggested drainage of a deep magma reservoir. Tono Research Institute of Earthquake Science recorded the data with the broadband seismometer (STS-1) and gravimeter (gPhone) installed in Mizunami, Japan (Murakami et al., 2019). The records by Iranian broadband stations clearly showed the long-period seismic signals around 10 (UTC) on November 11, 2018. We studied records by 26 stations distributed throughout the country. The stations are operated by National Center of Broadband Seismic Network of Iran, International Institute of Earthquake Engineering and Seismology (IIEES). Since the frequency content of Fourier amplitude spectra appeared the signal of the surface waves as a peak around 0.06 Hz, we applied a bandpass filter of 0.05-0.07 Hz to the waveform data. To separate Rayleigh from love in surface waves, the filtered horizontal components were rotated to the radial and transverse components based on an assumed epicenter location at the latitude of 12.7S and longitude of 45.4E degrees. The stations considered as an array and the investigation was carried out in two ways. First, the position of each station was taken as the reference point of the array coordinate, and arrival delay times at the other stations relative to the reference were calculated. The phase velocity and the back-azimuth of each station were estimated through the least-square regression method. The estimated back azimuths were within 13 degrees from the back azimuths from the assumed epicenter. The average phase velocity for Rayleigh and Love phases are calculated as 2.97 and 3.31 km/sec, respectively. Second, we applied semblance analysis to six stations with the shortest spacing distances. However, the distance between the adjacent stations relative to the signal wavelength was not enough short to prevent spatial aliasing. Nevertheless, the interesting was that the semblance results were different for radial and transverse components. We calculated surface-wave magnitude (Ms) for the event and a number of recorded earthquakes occurring in the Mayotte area from May 13 to June 1, 2018. Linear regression was used to define relationships between the calculated Ms and the USGS body-wave magnitude (mb) and the local magnitude by BRGM catalog (Bertil et al. , 2019), and the moment magnitude (Mw) from the CMT solutions of HRVD and USGS.</p>

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