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

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>

scholarly journals ML:M0 as a regional seismic discriminant

1993 ◽  
Vol 83 (4) ◽  
pp. 1167-1183 ◽  
Author(s):  
Bradley B. Woods ◽  
Sharon Kedar ◽  
Donald V. Helmberger
Keyword(s):  
Surface Waves ◽  
Seismic Moment ◽  
Body Wave ◽  
Detection Threshold ◽  
Data Set ◽  
Waveform Data ◽  
Long Period ◽  
Source Type ◽  
Short Period ◽  
Source Excitation

Abstract The mb:MS ratio determined by teleseismic observations has proven to be an effective discriminant, for explosive sources tend to be significantly richer in short-period energy than are earthquakes. Unfortunately, this method is limited by the detection threshold of teleseismic surface waves. However, recent advances in instrumentation allowing low amplitude surface wave measurements coupled with new analytical techniques make it feasible to use regional waveform data to determine the long-period source excitation level of low magnitude events. We propose using the ratio of ML (local magnitude) to M0 (scalar seismic moment) as an analogous regional discriminant. We applied this criterion to a data set of 299 earthquakes and 178 explosions and found that this ratio seems to be diagnostic of source type. For a given M0, the ML of an explosion is more than 0.5 magnitude units larger than that of an earthquake. This separation of populations with respect to source type can be attributed to the fact that ML is a short-period (1 Hz) energy measurement, whereas seismic moment is determined from long-period body wave phases (period > 4 s) and surface waves (10 to 40 sec). Using regional stations with sources 200 to 600 km away, the effective threshold for magnitude measurements for this discriminant is found to be ML = 3.1 for earthquakes and ML = 3.6 for explosions. This method does require the determination of regional crustal models and path calibrations from master events or by other means.

Frequency‐wavenumber analysis of geothermal microseisms at Norris Geyser basin, Yellowstone National Park, Wyoming

Geophysics ◽  
1980 ◽  
Vol 45 (5) ◽  
pp. 952-963 ◽  
Author(s):  
David H. Oppenheimer ◽  
H. M. Iyer
Keyword(s):  
Phase Velocity ◽  
Surface Waves ◽  
Yellowstone National Park ◽  
National Park ◽  
Body Wave ◽  
Body Waves ◽  
Spectral Techniques ◽  
Velocity Information ◽  
Deep Source ◽  
Array Aperture

Microseisms were recorded by two separate arrays within 5 km of Norris Geyser basin, Yellowstone National Park, Wyoming. The data were analyzed using frequency‐wavenumber (f-k) spectral techniques to investigate whether the microseisms are originating at the geyser basin and, if so, whether body waves emanating from a deep source could be distinguished from surface waves on the basis of phase velocity. Array aperture and seismometer spacing were systematically varied to examine a continuous wavenumber range of 0 to 100 cycles/km. Results from high‐resolution f-k analysis show that the microseisms indeed originate at the geyser basin in the frequency range 1.5 to 6.3 Hz with phase velocities of 1.1 to 2.5 and 2.0 to 4.0 km/sec on arrays southwest and east of the geyser basin, respectively. Although we could not distinguish between surface waves and body waves originating near the surface solely on the basis of phase‐velocity information, observed velocities clearly preclude the possibility that a deep hydrothermal system is responsible for body‐wave microseisms in this area.

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.

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.

LONG‐PERIOD SIGNAL PROCESSING RESULTS FOR THE LARGE APERTURE SEISMIC ARRAY

Geophysics ◽  
1969 ◽  
Vol 34 (3) ◽  
pp. 305-329 ◽  
Author(s):  
J. Capon ◽  
R. J. Greenfield ◽  
R. T. Lacoss
Keyword(s):  
Signal Processing ◽  
Surface Wave ◽  
Maximum Likelihood ◽  
Surface Waves ◽  
Body Wave ◽  
Seismic Array ◽  
Seismic Events ◽  
Long Period ◽  
Large Aperture ◽  
Period Data

The results of a series of off‐line signal processing experiments are presented for long‐period data obtained from the Large Aperture Seismic Array (LASA) located in eastern Montana. The signal‐to‐noise ratio gains obtained with maximum‐likelihood processing, as well as other simpler forms of processing, are presented for body‐wave as well as surface‐wave phases. A discussion of the frequency‐wavenumber characteristics of the noise which led to these results is also given. On the basis of these experiments, several recommendations are made concerning optimum long‐period array configurations and on‐line or off‐line processing methods. The usefulness of maximum‐likelihood processing in suppressing an interfering teleseism is demonstrated. An experiment is given in which maximum‐likelihood processing achieved about 20 db suppression of an interfering teleseism, while simpler forms of processing such as beam‐forming obtained about 11 db. The matched filtering of surface waves using chirp waveforms is shown to be highly effective. A useful discriminant for distinguishing between natural seismic events and underground nuclear explosions, using both the long‐period and short‐period data, was found to be the relationship between the surface‐wave and body‐wave magnitudes. Measurements of this discriminant made on events from four tectonic regions of the earth are presented. It is shown that 60 and 100 percent detectability of surface waves for natural seismic events from the Central Asian‐Kurile Islands‐Kamchatka region occurs at about LASA body‐wave magnitudes 4.5 and 4.9, respectively.

scholarly journals Inversion of the body waves from the Borrego Mountain earthquake to the source mechanism

1976 ◽  
Vol 66 (5) ◽  
pp. 1485-1499 ◽  
Author(s):  
L. J. Burdick ◽  
George R. Mellman
Keyword(s):  
Body Wave ◽  
High Stress ◽  
Time Function ◽  
The Body ◽  
Body Waves ◽  
Polarization Angle ◽  
Long Period ◽  
Amplitude Data ◽  
Short Period ◽  
Wide Range

abstract The generalized linear inverse technique has been adapted to the problem of determining an earthquake source model from body-wave data. The technique has been successfully applied to the Borrego Mountain earthquake of April 9, 1968. Synthetic seismograms computed from the resulting model match in close detail the first 25 sec of long-period seismograms from a wide range of azimuths. The main shock source-time function has been determined by a new simultaneous short period-long period deconvolution technique as well as by the inversion technique. The duration and shape of this time function indicate that most of the body-wave energy was radiated from a surface with effective radius of only 8 km. This is much smaller than the total surface rupture length or the length of the aftershock zone. Along with the moment determination of Mo = 11.2 ×1025 dyne-cm, this radius implies a high stress drop of about 96 bars. Evidence in the amplitude data indicates that the polarization angle of shear waves is very sensitive to lateral structure.

Surface waves in structures with locally irregular boundaries

1980 ◽  
Vol 70 (3) ◽  
pp. 791-808
Author(s):  
Anne Suteau ◽  
Louis Martel
Keyword(s):  
Surface Waves ◽  
Body Wave ◽  
Complex Structure ◽  
Additional Term ◽  
The Body ◽  
Body Waves ◽  
First Order ◽  
Irregular Region ◽  
Regular Medium ◽  
General Method

abstract The transmitted field due to surface waves incident on a local irregularity of a plane-layered medium has been studied. A perturbation method to the first order and the Born approximation can be used if the variations in the thickness of the layers are sufficiently smooth and the wavelengths are long when compared to the size of the irregularities. The spectrum of the perturbed part of the displacement field at the surface is a sum over the surface-wave modes for the regular medium, with an additional term involving the scattered body waves. Numerical computations have been performed for structures composed of a layer overlying a half-space. The contribution of the various modes to the transmitted Love or Rayleigh fields has been studied for several structures. A general method has been obtained to analyze the effect of a complex structure as the superposition of the fields due to simpler ones. When the layer thickness is kept unchanged, the incident mode is not perturbed to the first order. Synthetic seismograms, computed at stations sufficiently close to the irregular region, show how the perturbation of the signal depends on distance. A comparison has been made for Love waves with a finite element method. Both methods give very similar results when the stations are not too close to the irregularities so that the body-wave contribution is negligible. The local phase velocity shows departures from the curves for a regular model.

Large Explosions at Sea

1971 ◽  
Vol 8 (2) ◽  
pp. 243-247
Author(s):  
Goetz G. R. Buchbinder
Keyword(s):  
Surface Wave ◽  
Continental Shelf ◽  
Surface Waves ◽  
Small Amplitude ◽  
Body Wave ◽  
Surface Wave Magnitude ◽  
Underwater Explosions ◽  
Long Period ◽  
Short Period ◽  
The Difference

Two large unannounced events occurred at sea in aseismic areas in the Atlantic. Comparison of these with the announced event Chase III shows them to be explosions.Large explosions at sea may be recognized by the relatively small amplitude of long period surface waves with periods up to 10 s. Energy of longer periods is absent for events mb ≤ 5.5. The surface wave magnitudes for the events are at least 1.5 smaller at 10 s than those of underground explosions of equal mb, at 20 s they are at least 0.9 smaller. At longer periods the difference between body wave and surface wave magnitude is larger than 0.9 but larger explosions are needed to determine the separation. Underwater explosions on or near the continental shelf are very efficient in the generation of higher mode short period waves.

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.

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