Publication Date:
2018-03-23
Description:
We investigate seismic signatures of fracturing in a newly ruptured strike-slip fault by determining
the wavefield polarization in the New Zealand Canterbury Plains area and across the Greendale Fault, which
was responsible for the 3 September 2010 Darfield Mw 7.1 earthquake. Previous studies suggested that
fractured rocks in fault damage zones cause directional amplification and ground motion polarization in the
fracture-perpendicular direction as an effect of stiffness anisotropy, and cause velocity anisotropy with shear
wave velocity larger in the fracture-parallel component. An array of 14 stations was installed following the
Darfield earthquake. We assess polarization both in the frequency and time domains through the
individual-station horizontal-to-vertical spectral ratio and covariance matrix analysis, respectively, and
compare the results to previously reported anisotropy measurements from shear wave splitting. Stations
installed in the Canterbury Plains have an amplification peak between 0.1 and 0.3 Hz for both earthquakes
and ambient noise.We relate the amplification to the resonance of a considerable thickness (c. 1 km) of soft
sediments lying over the metamorphic bedrock. Analysis of seismic events revealed the existence of
another peak in amplification between 2 and 5 Hz at two on-fault stations, which was not visible in the
noise analysis. In contrast to the lower frequency peak, the ones between 2 and 5 Hz are more strongly
anisotropic, attaining amplitudes up to a factor of 4 in the N52° direction. To interpret this effect we model
the fracture pattern in the fault damage zone produced by the fault kinematics. We conclude that the
horizontal polarization is orthogonal to extensional fractures, which predominate in the shallow layers
(〈2 km) with an expected strike of N139°. Fracture orientation is consistent with coseismic surface rupture
observations, confirming the reliability of the model. S wave splitting is produced by velocity anisotropy in
the entire rock volume crossed along the seismic path; thus, it is affected by deeper material than the
amplification study. We explain the rotation of S wave fast component observed by Holt et al. (2013) near
the fault in terms of the dominant synthetic cleavages at greater depths (〉2 km), expected in N101°
direction on the basis of the model. Thus, different fracture distribution at different depths may explain
different results for amplification compared to anisotropy. We propose polarization amplification analysis
as a complementary method to S wave splitting analysis. Polarization analysis is rapidly computed and
robust, and it can be applied to either earthquakes or ambient noise recordings, giving useful information
about the predominant fracture patterns at various depths.
Description:
Published
Description:
7048–7067
Description:
4T. Sismologia, geofisica e geologia per l'ingegneria sismica
Description:
JCR Journal
Keywords:
directional amplification,S-wave splitting, Greendale fault
;
04.06. Seismology
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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