Introduction to mode coupling methods for surface waves

Maupin, Valérie

doi:10.1016/s0065-2687(06)48002-x

Cited by 15 publications

(10 citation statements)

References 51 publications

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“…Neither do our modeling results from Fig. 9 conform to local mode expectations (Maupin 2007) as the subarray velocity measurements do not represent the average structure below.…”

Section: Theoretical Considerationscontrasting

confidence: 57%

Anomalous azimuthal variations with 360° periodicity of Rayleigh phase velocities observed in Scandinavia

Mauerberger

Maupin

Guðmundsson

et al. 2020

Geophysical Journal International

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Summary We use the recently deployed ScanArray network of broadband stations covering most of Norway and Sweden as well as parts of Finland to analyse the propagation of Rayleigh waves in Scandinavia. Applying an array beamforming technique to teleseismic records from ScanArray and permanent stations in the study region, in total 159 stations with a typical station distance of about 70 km, we obtain phase velocities for three sub-regions, which collectively cover most of Scandinavia (excluding southern Norway). The average phase dispersion curves are similar for all three sub-regions. They resemble the dispersion previously observed for the South Baltic craton and are about 1% slower than the North Baltic shield phase velocities for periods between 40 and 80 s. However, a remarkable sin(1θ) phase velocity variation with azimuth is observed for periods >35 s with a 5% deviation between the maximum and minimum velocities, more than the overall lateral variation in average velocity. Such a variation, which is incompatible with seismic anisotropy, occurs in northern Scandinavia and southern Norway/Sweden but not in the central study area. The maximum and minimum velocities were measured for backazimuths of 120○ and 300○, respectively. These directions are perpendicular to a step in the lithosphere-asthenosphere boundary (LAB) inferred by previous studies in southern Norway/Sweden, suggesting a relation to large lithospheric heterogeneity. In order to test this hypothesis, we carried out 2D full-waveform modeling of Rayleigh wave propagation in synthetic models which incorporate a steep gradient in the LAB in combination with a pronounced reduction in the shear velocity below the LAB. This setup reproduces the observations qualitatively, and results in higher phase velocities for propagation in the direction of shallowing LAB, and lower ones for propagation in the direction of deepening LAB, probably due to the interference of forward scattered and reflected surface wave energy with the fundamental mode. Therefore, the reduction in lithospheric thickness towards southern Norway in the south, and towards the Atlantic ocean in the North provide a plausible explanation for the observed azimuthal variations.

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“…Neither do our modeling results from Fig. 9 conform to local mode expectations (Maupin 2007) as the subarray velocity measurements do not represent the average structure below.…”

Section: Theoretical Considerationscontrasting

confidence: 57%

Anomalous azimuthal variations with 360° periodicity of Rayleigh phase velocities observed in Scandinavia

Mauerberger

Maupin

Guðmundsson

et al. 2020

Geophysical Journal International

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“…Thus, the glacier thickness beneath the southern part of the network may be overrepresented in the dispersion measurement. Perhaps most importantly, the 3-D velocity structure beneath the seismic network can give rise to mode coupling of surface waves (Maupin 2007). If this were the case, our phase velocity measurements would be influenced by several Rayleigh wave modes, which differs from our assumption that the dispersion relationship includes only the fundamental mode.…”

Section: Methodsmentioning

confidence: 85%

Using glacier seismicity for phase velocity measurements and Green's function retrieval

Walter

Roux

Roeoesli

et al. 2015

Geophysical Journal International

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High-melt areas of glaciers and ice sheets foster a rich spectrum of ambient seismicity. These signals not only shed light on source mechanisms (e.g. englacial fracturing, water flow, iceberg detachment, basal motion) but also carry information about seismic wave propagation within glacier ice. Here, we present two approaches to measure and potentially monitor phase velocities of high-frequency seismic waves (≥1 Hz) using naturally occurring glacier seismicity. These two approaches were developed for data recorded by on-ice seasonal seismic networks on the Greenland Ice Sheet and a Swiss Alpine glacier. The Greenland data set consists of continuous seismograms, dominated by long-term tremor-like signals of englacial water flow, whereas the Alpine data were collected in triggered mode producing 1-2 s long records that include fracture events within the ice ('icequakes'). We use a matched-field processing technique to retrieve frequency-dependent phase velocity measurements for the Greenland data. In principle, this phase dispersion relationship can be inverted for ice sheet thickness and bed properties. For these Greenland data, inversion of the dispersion curve yields a bedrock depth of 541 m, which may be too small by as much as 35 per cent. We suggest that the discrepancy is due to lateral changes in ice sheet depth and bed properties beneath the network, which may cause unaccounted mixing of surface wave modes in the dispersion curve. The Swiss Alpine icequake records, on the other hand, allow for reconstruction of the impulse response between two seismometers. The direct and scattered wave fields from the vast numbers of icequake records (tens of thousands per month) can be used to measure small changes in englacial velocities and thus monitor structural changes within the ice.

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“…One of the possible reasons is that we observe a coupling of Love wave modes, probably due to reflection and diffraction at heterogeneities, where the fundamental mode is converted into higher modes. A detailed theory may be found in the paper by Maupin (2007). Waves of the same period can then propagate with different velocities, interfere with one another, and may cause the undulations we observe.…”

Section: Resultsmentioning

confidence: 89%

Shear wave crustal velocity model of the Western Bohemian Massif from Love wave phase velocity dispersion

2010

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We propose a new quantitative determination of shear wave velocities for distinct geological units in the Bohemian Massif, Czech Republic (Central Europe). The phase velocities of fundamental Love wave modes are measured along two long profiles (~ 200 km) crossing three major geological units and one rift-like structure of the studied region. We have developed a modified version of the classical multiple filtering technique for the frequency-time analysis and we apply it to two-station phase velocity estimation. Tests of both the analysis and inversion are provided. Seismograms of three Aegean Sea earthquakes are analyzed. One of the two profiles is further divided into four shorter sub-profiles. The long profiles yield smooth dispersion curves; while the curves of the sub-profiles have complicated shapes. Dispersion curve undulations are interpreted as perioddependent apparent velocity anomalies caused both by different backazimuths of surface wave propagation and by surface wave mode coupling. An appropriate backazimuth of propagation is found for each period, and the dispersion curves are corrected for this true propagation direction. Both the curves for the long and short profiles are inverted for a 1D shear wave velocity model of the crust. Subsurface shear wave velocities are found to be around 2.9 km/s for all four studied sub-profiles. Two of the profiles crossing the older Moldanubian and Teplá-Barrandian units are characterized by higher velocities of 3.8 km/s in the upper crust while for the Saxothuringian unit we find the velocity slightly lower, around 3.6 km/s at the same depths. We obtain an indication of a shear wave low velocity zone above Moho in the Moldanubian and Teplá-Barrandian units. The area of the Eger Rift (Teplá-Barrandian-Saxothuringian unit contact) is significantly different from all other three units. Low upper crust velocities suggest sedimentary and volcanic filling of the rift as well as fluid activity causing the earthquake swarms. Higher velocities in the lower crust together with weak or even missing Moho implies the upper mantle updoming.

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Introduction to mode coupling methods for surface waves

Cited by 15 publications

References 51 publications

Anomalous azimuthal variations with 360° periodicity of Rayleigh phase velocities observed in Scandinavia

Anomalous azimuthal variations with 360° periodicity of Rayleigh phase velocities observed in Scandinavia

Using glacier seismicity for phase velocity measurements and Green's function retrieval

Shear wave crustal velocity model of the Western Bohemian Massif from Love wave phase velocity dispersion

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