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TABLE 11-D layered model in the near field of the M 6.2 February 4,2011 Canterbury earthquake. The fault is embedded in thesecond layer. H-S denotes the half-space underneath thelayering.Thickness (km) Vp (km/s) Vs (km/s) ρ (g/cm 3 )0.02 0.3 0.165 1.50.98 4.5 2.6 1.89.0 5.6 3.2 2.4H-S 6.24 3.6 2.9ing the effect of attenuation of shear waves on the accelerationseismograms. We define a 1-D layered medium representingthe Canterbury region close to the fault that ruptured in theMw 6.2 February 2011 aftershock based on a regional subsetof the New Zealand–wide 3-D velocity model (Eberhart-Phillips et al. 2010) and shallow velocity structure determinedby microtremor analysis (Stephenson et al. 2011). The fault isrepresented by a 9-km-long and 8-km-wide rectangular thrustfault dipping 65° and striking 70° (from north), rupturing with3.2 km/s rupture velocity and 120° rake. This fault is embeddedin the second layer of a four-layered medium of elastic parameterslisted in Table 1. The modeling is accomplished with a discretewavenumber numerical scheme (Bouchon 1979) in whichattenuation is introduced by the factor exp(2π*f / Q s ) for eachplane wave in the layer (Aki and Richards 1980), where f is frequencyand Q s is the shear wave quality or attenuating factor.We apply attenuation to the top 20-m-thick soil layer to testthe hypothesis that the observed differences in frequency contentbetween the horizontal and vertical components is due tostrong shear wave attenuation in the shallow subsurface. Ideally,modeling the anelastic attenuation would include Biot’s modelof fluid-solid interaction in our wave propagation algorithm.Geli et al. (1987) successfully incorporated Biot’s model in theAki-Larner (1970) numerical technique to study the responseof smooth 2-D basins with water-saturated sediments. Theirwork prescribes a characteristic frequency ( fc) of about 5 Hzfor unconsolidated coarse sands and gravels with a permeabilityon the order of 10 –8 m 2 . This frequency depends on fluidand solid parameters such as permeability, viscosity, density,and pore density. For frequencies lower than fc, the attenuationof shear waves is stronger than for primary waves, and for frequenciesabove fc, the attenuation of primary P waves increasessignificantly with frequency due to the rise of a secondary Pwave (termed P 2 ) that results from the solid-fluid coupling.Based on an anelastic representation of attenuation in whichvelocities are estimated from the fluid and solid parameters,Geli et al. (1987) prescribe the dependency of Q on frequency,showing only weak dependency in the range 1–10 Hz for permeabilityand porosity in the range of the shallow subsurface ofChristchurch (10 –8 and 35%, respectively). We therefore applyonly shear wave attenuation (between Q s = 1 and Q s = 10 for0 Hz–7 Hz) to our range of simulations.The top panel of Figure 3 shows the three components ofdisplacement and acceleration at a station located 2 km fromthe fault, when there is no attenuation (Q s = 1,000). Although(A)(B)▲ ▲ Figure 3. Synthetic wavefield calculated with a discrete wavenumber method at a distance of 2 km from the 22 February rupture.U is east-west, V is north-south, and W is vertical. A) Results from a simulation with zero attenuation. B) Results from inclusion of aquality factor to represent the attenuating effects of the local groundwater table.848 Seismological Research Letters Volume 82, Number 6 November/December 2011
▲ ▲ Figure 4. Vertical acceleration waveforms from Figure 1. Waveforms show larger positive accelerations than negative ones. Manyof the negative acceleration troughs are also broader than the narrow positive acceleration spikes.the high acceleration may be unrealistically large (about 5 g),we recall that this is the result of a kinematic model combiningthe effect of directivity and the presence of the thin (20 m), softtop layer in the near field. The bottom panel of Figure 3 showsthe results for the case of Q s = 5. Noticeably, high frequencieshave been attenuated in the horizontal components but notin the vertical component. This is compatible with numerousobservations of this characteristic recorded during the aftershocksequence (e.g., Figure 2).VERTICAL COMPONENT ASYMMETRYAnother notable characteristic of many of the recordings fromthe February event and some recordings from other strong localevents is the occurrence of asymmetric accelerations and spikesin the vertical direction. In this observational paper, we documentan example of a newly discovered phenomenon in thevertical components of the acceleration seismograms that haspreviously been recognized in a handful of records from strongshallow earthquakes and nuclear explosions. Many accelerogramsrecorded in the Mw 6.2 earthquake exhibit maximumPGA on the vertical component (Figure 1). The asymmetricalrecordings are confined to within ~6–10 km of the epicenter,suggesting that either very strong near-field motions are necessaryto generate them or that they result from high-frequencywaves generated during source processes that subsequentlyattenuate at greater distances. Most of the high-accelerationvertical records are asymmetric with maximum accelerationsin the upward direction (>1 g) exceeding accelerations in thedownward direction (
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- Page 54 and 55: (A)6.146.13(B)6.246.36Misfit6.156.1
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- Page 60 and 61: Luo, Y., Y. Tan, S. Wei, D. Helmber
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- Page 67 and 68: Depth (km)Coulomb Stress Change(bar
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- Page 77 and 78: DISCUSSIONThe 2010-2011 Canterbury
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- Page 91 and 92: REFERENCESAvery, H. R., J. B. Berri
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- Page 101 and 102: (A)Magnitude, M w876542009 NZdataba
- Page 103 and 104: Scale0.5 g5 seconds▲▲Figure 4.
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- Page 107 and 108: Vertical-to-horizontal PGA ratio543
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- Page 111 and 112: REFERENCESAagaard, B. T., J. F. Hal
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- Page 119 and 120: TABLE 1Mean (μ LLH ) and standard
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▲ ▲ Figure 4. Vertical acceleration waveforms from Figure 1. Waveforms show larger positive accelerations than negative ones. Manyof the negative acceleration troughs are also broader than the narrow positive acceleration spikes.the high acceleration may be unrealistically large (about 5 g),we recall that this is the result of a kinematic model combiningthe effect of directivity and the presence of the thin (20 m), softtop layer in the near field. The bottom panel of Figure 3 showsthe results for the case of Q s = 5. Noticeably, high frequencieshave been attenuated in the horizontal components but notin the vertical component. This is compatible with numerousobservations of this characteristic recorded during the aftershocksequence (e.g., Figure 2).VERTICAL COMPONENT ASYMMETRYAnother notable characteristic of many of the recordings fromthe February event and some recordings from other strong localevents is the occurrence of asymmetric accelerations and spikesin the vertical direction. In this observational paper, we documentan example of a newly discovered phenomenon in thevertical components of the acceleration seismograms that haspreviously been recognized in a handful of records from strongshallow earthquakes and nuclear explosions. Many accelerogramsrecorded in the Mw 6.2 earthquake exhibit maximumPGA on the vertical component (Figure 1). The asymmetricalrecordings are confined to within ~6–10 km of the epicenter,suggesting that either very strong near-field motions are necessaryto generate them or that they result from high-frequencywaves generated during source processes that subsequentlyattenuate at greater distances. Most of the high-accelerationvertical records are asymmetric with maximum accelerationsin the upward direction (>1 g) exceeding accelerations in thedownward direction (
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