ACCELERATION SPECTRA FROM GISBORNE - 2ZG SITE
Figure 1: Three component spectra for Gisborne 2ZG record
The ground motion record from the above station has been reduced to motion spectra in the
E/W orientation (primary component PGA=0.25g) and the N/S orientation (secondary
component PGS 0.2g) with the vertical spectral response also presented for completeness).
(Note: the orientation selected are not necessarily the most severe from any orientation – work
is ongoing on this aspect).
The location of the recorder is two blocks south of the CDB and approximately 3 blocks east
of the river. Site soil classification is considered to be Class D, although it is close to being
Class E).
Notable Features
 the sharp spikes present at around 0.35 seconds in the E/W component where
Sa(0.35)1.15g.
 A less intense spike occurs in both records at approx T=0.5 sec with components
Sa(0.5)0.87g E/W and Sa(0.5)0.93g N/S.
 Between the period ranges T=0.25 and T=0.7 both records indicate acceleration
spectral levels generally in excess of 0.6g.
 Between T=1 and T=1.5 second, acceleration levels fall from 0.2g to 0.15g and
rapidly continue to decline into the higher period ranges.
Recorded vs Code 5% Damped Deep Soil Spectra
1.2000
1.0000
SA(T) (g)
0.8000
2ZG EW
2ZG NS
NZS1170 Deep Soil Z=0.36 R=1
NZS1170 Deep Soil Z=0.36 R=0.67
NZS1170 Deep Soil Z=0.36 R=0.5
0.6000
0.4000
0.2000
0.0000
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Period T(s)
Figure 2: Recorded vs Code
The two components of the recorded record are compared with the Deep Soil ground motion
spectra with R=1 (1/500), R=0.67 (1/150) and R=0.5 (1/100) (Note bracketed figures
representing annual probabilities or exceedence).
Notable features:
 Within the period range T=0.25 to T=0.8, the measured response components are
reasonably aligned with the R=0.67 ground motion projection, indicating that the
motion could be considered to be approximately coincident with motions associated
with a 1/150 annual probability or exceedence or motions approximately 2/3 those of
ULS design level.
 The Sa(0.35) peak value is exception in that it approaches the 1/500 design value
although is limited to a very narrow period band.
 Beyond T=1 second motions approximately align to R=0.25 (1/25) being levels of
motion at which the onset of damage could be expected (while recognising that few
elements and components will experience significant excitation over that period band.
Recorded vs Modelled 5% Damped Deep Soil Spectra
1.2
Magnitude 6.9 Intraslab
earthquake
Epicentral Distance 47 km
Depth 44 km
Hypocentral distance 64 km
1
SA(T) (g)
0.8
Deep Soil 50-percentile
Deep Soil 84-percentile
0.6
2ZG EW
2ZG NS
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Period T(s)
Figure 3: Recorded vs Modelled
The two component records are compared with those projected by running the event (location,
depth, magnitude and earthquake type) through the NZ seismic hazard model.
Notable features:
 The recorded accelerations match relatively closely the 84% deep soil projections in
the short period range T<0.4 seconds, exceed the projected motions over the mid
period range 0.4<T<1 second and are below projected motions for the long period
range T>1 second.
 The stronger motions within the mid-period range are abnormal and not included in
the model
Recorded vs Hazard 5% Damped Deep Soil Spectra
10
10yrs
25yrs
50yrs
100yrs
250yrs
500yrs
1000yrs
2500yrs
5000yrs
10000yrs
2ZG EW
2ZG NS
SA(T) (g)
1
0.1
0.01
0.01
1
0.1
10
Period T(s)
Figure 4: Recorded vs Hazard
The recorded motions have been superimposed over various hazard spectra for visual
comparison.
Notable features:
 The recorded values are deficient in both short (T<0.4 sec) and long (T>1 sec) ranges,
but relative strong within the mid-period range where motions align approximately
with motions expected from an event with an annual probability of exceedence of
1/250.