scholarly journals Observation and synthesis of spatially-incoherentweak-motion wavefields at Alfredton basin, New Zealand

1997 ◽  
Vol 30 (1) ◽  
pp. 14-31 ◽  
Author(s):  
A. J. Haines ◽  
Jaishun Yu
Keyword(s):  
New Zealand ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Soft Soil ◽  
Seismic Energy ◽  
Wave Impedance ◽  
3 Dimensional ◽  
Scale Experiment ◽  
Random Nature

To observe and model the detailed pattern of ground motion amplification in a small soft-soil basin an experiment was conducted at Alfredton, New Zealand. 19 seismometers were deployed for 5 weeks at closely spaced sites in and around a 400-500 m diameter, sediment-filled depression in soft, sandstone basement. During this period 112 earthquakes, with "weak" ground motions, were detected by at least some of the instruments, and 15 well-recorded events were selected for detailed analysis. Geotechnical data obtained to provide the parameters for the 3-dimensional modelling included measurements of the shear-wave velocity. Across the basin this is 60 m/s at the surface, increasing steadily to 300+ m/s at the bottom of the basin, and the shear-wave velocity in the basement is 850 m/s. Thus, there are no boundaries where the contrast in shear-wave impedance is especially large. In contrast to situations where there are large contrasts in shear-wave impedance to trap seismic energy in soft-soil layers, the amplifications observed in the basin at Alfredton were small. The small amplifications are confirmed by the 3-dimensional modelling. Another feature of the observed wavefields is that in all cases the incident motions, recorded at the basement sites around the basin, were spatially incoherent. In other words, the wavefields arriving at the basin were of a complex, seemingly random nature. This is the first occasion that the spatial coherency of wavefields has been measured in a fine-scale experiment in New Zealand. Apart from the small amplications and the observed lack of coherency between the basement sites, the most striking result, which was obtained from both the observations and the modelling of similarly incoherent wavefields, is that for short-duration events in which the main motions last for no more than a second, the amplifications in the basin are larger than for events in which the motions are of longer duration; that is, the extent to which differently propagating incoherent wave packets interfere destructively inside the basin increases with the duration of the wavefields.

Development of Deep Shear Wave Velocity Profiles in the Canterbury Plains, New Zealand

2018 ◽  
Vol 34 (3) ◽  
pp. 1065-1089 ◽  
Author(s):  
Michael R. Deschenes ◽  
Clinton M. Wood ◽  
Liam M. Wotherspoon ◽  
Brendon A. Bradley ◽  
Ethan Thomson
Keyword(s):  
New Zealand ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Joint Inversion ◽  
Three Dimensional ◽  
Strong Motion ◽  
Spectral Ratio ◽  
Deep Basin ◽  
Velocity Models

Deep (typically > 1,000 m) shear wave velocity ( V S) profiles were developed across the Canterbury region of New Zealand at nine strong-motion stations using a combination of active and passive surface wave methods. A multimode, multimethod joint inversion process, which included Rayleigh and Love wave dispersion and horizontal-to-vertical spectral ratio data, was used to develop the V S profiles at each site. A priori geologic information was used in defining preliminary constraints on the complex geologic layering of the deep basin underlying the region, including velocity reversals in locations where interbedded terrestrial gravels and marine sediments are present. Shear wave profiles developed as part of this study had characteristics comparable to the profiles from 14 Christchurch sites detailed in a separate study. The profiles developed in the two studies were combined to form region-specific V S profiles for typical deposits, which can be used to improve the accuracy of current three-dimensional (3-D) crustal velocity models of the region.

Shear Wave Velocity Investigation for Ten Representative Sites of National Capital Territory, New Delhi, India

2011 ◽  
Vol 2 (1) ◽  
pp. 29-43 ◽  
Author(s):  
A.K. Mahajan ◽  
A.K. Shukla ◽  
Ajit Pandey ◽  
Mukesh Chauhan ◽  
Neetu Chauhan ◽  
...  
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Soft Soil ◽  
River Sediments ◽  
Soil Classification ◽  
New Delhi ◽  
Site Class ◽  
Class B ◽  
Masw Method

In this paper, shear wave velocity (Vs) investigations are carried out using Multichannel analysis of surface waves (MASW) method at ten representative sites in the NCT region, New Delhi. The analysis shows that the Vs obtained from the sites located on Alwar quartzites of Delhi Super Group ranges from 770 m/s to 2800 m/s, whereas on other sites located on lake/river sediments (Nazafgarh, Balsava and Akshar Dham) have Vs less than 180 m/s. The sites located on thick sediments shows Vs of the order of 180 m/s to 250 m/s. According to the soil classification, the sites covered can be classified under three categories: Class ‘B’ (Vs30 as >760m/s; JNU site and Asola site), class ‘D’ (Vs30>180 m/sec-360; Bhavana, Suhalpur, Ghazipur and Kirbi cantt. sites), whereas the sites located near lake/river sediments are classified as class ‘E’ (with very soft soil) and will be prone to liquefaction potential during strong earthquake shaking.

Development of a Vs30 (NEHRP) map for the city of Ottawa, Ontario, Canada

2011 ◽  
Vol 48 (3) ◽  
pp. 458-472 ◽  
Author(s):  
D. Motazedian ◽  
J. A. Hunter ◽  
A. Pugin ◽  
H. Crow
Keyword(s):  
Shear Wave ◽  
Fundamental Frequency ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Seismic Refraction ◽  
Weighted Average ◽  
Spectral Ratio ◽  
Wave Impedance ◽  
Seismic Methods ◽  
The City

Four different seismic methods were used extensively to evaluate the shear wave velocity of soils and rock in the city of Ottawa, Canada, from which the travel-time weighted average shear wave velocity (Vs) from surface to 30 m in depth (Vs30) and the fundamental frequency (F0) were computed. Three main geological or geotechnical units were identified with distinct shear wave velocities: these consist of very loose thick post-glacial fine-grained sands, silts, and clays (Vs <150 m/s, thickness up to 110 m), firm glacial sediments (Vs ∼580 m/s, thickness ∼3 m), and very firm bedrock (Vs ∼1750–3550 m/s). The seismic methods applied were downhole interval Vs measurements at 15 borehole sites, seismic refraction–reflection profile measurements for 686 sites, high-resolution shear wave reflection “landstreamer” profiling for 25 km in total, and horizontal-to-vertical spectral ratio (HVSR) of ambient seismic noise to evaluate the fundamental frequency for ∼400 sites. Most of these methods are able to distinguish the very high shear wave impedance of and depth to bedrock. Sparse earthquake recordings show that the soil amplification is large for weak motion when the soil behaves linearly.

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