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Determination of Site Response in Anchorage, Alaska, on the Basis of Spectral Ratio Methods

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This paper deals with the site response (SR) in the Anchorage basin in south-central Alaska. The investigation is based on the analysis of seismograms of 114 earthquakes recorded by 22 weak-motion stations and 46 earthquakes recorded by 19 strong-motion stations in the study area. We. have computed SR for 41 sites, using standard spectral ratio and horizontal-to-vertical spectral ratio methods in the ftequency range from 0.5 to 11 Hz. Based on these results, we have calculated band-average site response values in two frequency ranges: low frequency (from 0.5 to 2.5 Hz) and high frequency (from 3 to 7 Hz). There is a good correlation between SR values and surficial geology of the Anchorage area in the low-frequency range. SR values increase by a factor of three from the foothills of the Chugach Mountains in the east to the west towards the deeper part of the basin. The highest site response values (SR>2.5) in the same frequency range are observed in the west-central part of the city, which is underlain by cohesive facies of the Bootlegger Cove formation. The SR has a good correlation with the uppermost 30-m time-average shear-wave velocity with a correlation coefficient of 0.82. Moreover, the low-frequency SR values are close to the NEHRP site coefficients for I sec. However, high-frequency SR values lack correlation with 30-m average shear-wave velocity and short-period NEHRP site coefficients.
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Determination of Site Response in Anchorage,
Alaska, on the Basis of Spectral Ratio Methods
A. Martirosyan,
M.EERI,
U. Dutta, N. Biswas,
M.EERI,
A. Papageorgiou,
M.EERI,
and R. Combellick
This paper deals with the site response (SR) in the Anchorage basin in south-
central Alaska. The investigation is based on the analysis of seismograms of 114
earthquakes recorded by 22 weak-motion stations and 46 earthquakes recorded by
19 strong-motion stations in the study area. We have computed SR for 41 sites,
using standard spectral ratio and horizontal to vertical spectral ratio methods in
the frequency range from 0.5 to 11 Hz. Based on these results, we have calculated
band-average site response values in two frequency ranges: low frequency (from
0.5 to 2.5 Hz) and high frequency (from 3 to 7 Hz). There is a good correlation
between SR values and surficial geology of the Anchorage area in the low
frequency range. SR values increase by a factor of three from the foothills of
Chugach Mts. in the east to the west towards the deeper part of the basin. The
highest site response values (SR > 2.5) in the same frequency range are observed
in the westcentral part of the city, which is underlain by cohesive facies of the
Bootlegger Cove formation. The SR has a good correlation with the uppermost 30
m time-average shear wave velocity with a correlation coefficient of 0.82.
Moreover, the low frequency SR values are close to the NEHRP site coefficients
for 1 sec. However, high frequency SR values lack correlation with 30 m average
shear wave velocity and short-period NEHRP site coefficients.
INTRODUCTION
Anchorage lies within the highly active seismogenic zone of south-central Alaska. The
first-order feature of the seismotectonic setting of this section of Alaska consists of the
underthrusting of the Pacific plate beneath the North American plate. Geologically, most
parts of the study area lie in a sedimentary basin, with the sedimentary formations abutting
metamorphic bedrocks exposed in the Chugach Mts. to the east (Figure 1a). The formations
of the basin dip towards the west as shown by an east-west cross-section (Figure 1b) along
AA in Figure 1a. An important Quaternary unit of the Anchorage basin is called the
Bootlegger Cove formation (BCF); it consists of glacial deposits of interbedded clay, silt and
sand. This formation underlies lowland areas in westcentral Anchorage, including the
downtown area, in a north-south oriented zone several kilometers wide (Updike and Ulery
1986, Combellick 1999). The eastern extent of about 10 m thick BCF deposits is shown in
Figure 1a as a broken line (Combellick 1999). Updike and Ulery subdivided BCF into two
facies - cohesive and noncohesive. The cohesive facies, consisting of silty clay and/or clayey
silt, are susceptible to failure. They suffered extensive ground failure during the 1964 Prince
William Sound earthquake (M
W
= 9.2).
Following the catastrophic results in and around Anchorage from the 1964 earthquake
__________________________
(AM, UD, NB) Geophysical Institute, University of Alaska Fairbanks, AK 99775-7320
(AP) State University of New York at Buffalo, NY 14260-4300
(RC) Alaska Division of Geological and Geophysical Surveys, 794 University av., Fairbanks, AK 99709
A'
-150.05 -149.95 -149.85 -149.75 -149.65
61.10
61.15
61.20
61.25
A
Scale
1 2 3 km
(a)
(b)
A
A'
C
h
u
g
a
c
h
M
t
s
.
0m
100m
200m
300m
Figure 1. (a) Surficial geology of Anchorage. Thick broken line is approximate 10 m isopach of the
Bootlegger Cove formation, which dips from east to west; (b) Geological cross-section along AA',
shown in Figure 1a (reproduced from Selkregg et al. 1972).
and the vigorous socio-economic growth of the area during the past two decades, seismic
hazard has become a major concern to the inhabitants of the area. At the same time, there is
a severe lack of information on the characteristics of strong ground motion. In order to
address this problem, we implemented a seismic microzonation project for the metropolitan
area of Anchorage. The determination of S-wave site response (SR) constitutes an important
part of this project. Herein we report the SR results obtained for this area.
NETWORK AND DATABASE
Two seismological databases, namely, weak-motion (WM) and strong-motion (SM) have
been used to investigate the site response in the study area. Figure 2 shows the locations of
WM and SM stations, and their coordinates are given in Table A1 of Appendix A. A
summary of the data used in this study is given in Table 1.
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61.05
61.10
61.15
61.20
61.25
61.30
Scale
1 2 3km
Gulf of Alaska
Knik Arm
Turnagain Arm
C
h
u
g
a
c
h
M
t
s
.
Figure 2. The inset shows the location of the study area with respect to the rest of the Alaska-
Aleutian region. The locations of Anchorage weak-motion (circles) and strong-motion (triangles)
stations are shown on the background of main roads (solid lines).
The WM network was operated temporarily for about six months with 22 IRIS-
PASSCAL instruments; they were all installed in the free-field. Each station consisted of a 3-
component seismometer (Mark product L-28) connected to a Reftek recorder with a GPS
receiver; data were recorded at a 50 Hz sampling rate. The stations were located in areas of
various geological units. During six months of operation of the network, 114 earthquakes in
the local magnitude (ML) range from 1.5 to 5.5 were recorded with good signal-to-noise
ratio. The average number of stations that recorded an event simultaneously was about 19.
The epicentral locations of the 114 events selected for the study are shown in Figure 3 as
circles, which were noted from the catalog of the Alaska Earthquake Information Center.
Table 1. Summary of the Anchorage weak-motion and strong-motion networks and databases
used in this study.
Weak-motion network Strong-motion network
Operational period July – December 1996 Since November 1995
Instruments
IRIS-PASSCAL short period
sensors and Reftek recorders
Kinemetrics Altus K2
sensors
Sampling rate (Hz) 50 (continuous mode) 200 (trigger mode)
Number of stations 22 19
Number of events 114 46
Number of records 2209 512
Magnitude range (ML) 1.5 – 5.5 3.1 – 6.3
Depth range (km) 1 – 160 10 – 140
-154 -150 -146 -142
59
61
63
65
Scale
M4
M5
M6
G u l f o f A l a s k a
50 100 150km
Figure 3. Epicenter map of earthquakes used in this study. Weak-motion events are shown as circles
and strong-motion events as triangles. Symbol size is proportional to the event magnitude. Large
black circle indicates the network location.
The SM network (22 stations) has been operational since 1995 in the metropolitan area of
Anchorage. The stations were installed on the concrete floors of small buildings, such as the
city fire station or similar structure, which are not expected to significantly influence our
results below 10 Hz. Each station consisted of a Kinemetrics Altus K2 accelerograph. Of
these, two stations (K2-17 and K2-18) are located outside Anchorage proper to obtain a
distance coverage of about 60 km in the NE direction. Moreover, the station K2-19 has
recorded very few events, and data from this station were not considered in this study. We
selected 46 earthquakes recorded by the SM network for this study; their epicentral locations
are shown in Figure 3 as triangles. The largest event in this group is ML 6.3. However, the
maximum peak ground acceleration (75 gal) has been recorded for an event of ML 5.1,
having a focal depth of 36 km and located 50 km from the center of the network.
The spatial coverage (about 16 x 16 km) obtained by the WM and SM networks is
approximately the same with average station separation of about 2.5 km. Two stations - An01
(WM) and K2-16 (SM) - have been installed along the Chugach Mts., where metamorphic
rocks are exposed, and they have been considered as reference sites for this study. There
were four sites with collocated WM and SM instruments, with station-to-station distances
less than 150m; the others could not be collocated due to logistic difficulties. Despite the fact
that the operational periods of the two networks entirely overlapped, there were only 5 WM
events, which triggered some of the SM stations. The azimuthal coverage and focal depth
distribution in the WM and SM datasets are comparable, while the average epicentral
distance of the WM events is relatively greater, that is, 115 km compared to 80 km for the
SM events.
DATA PROCESSING
The ground motion at a given site is a convolution of earthquake source, path and site
factors. In this study, we are concerned with the site response, and for its determination we
used two spectral ratio methods, namely, standard spectral ratio (SSR) and horizontal to
vertical spectral ratio (HVR). The SSR method was introduced by Borcherdt (1970) and
productively used later in site response studies. It requires simultaneous recordings of
earthquakes at rock and soil sites. Assuming that the rock site has negligible site response,
the amplitude spectral ratio of the record at the soil site to that at the rock site provides an
estimate of site response at the soil site under the assumption that the source and path effects
are common to the rock and soil sites. However, this assumption may not be true over the
entire frequency range of interest. The other technique is the HVR method, first used by
Nakamura (1989) for the interpretation of data of microtremor measurements. Lermo and
Chavez-Garcia (1993) showed correlation of HVR results with those obtained by the SSR
method. However, Lachet et al. (1996) and Bonilla et al. (1997) among others showed that
the HVR method provides an estimate of the dominant frequency, but not the actual level of
the site response for a given site.
In processing the data of the present study, we first compared in the time domain the
amplitudes between the S-wave envelope (S) and pre-event trace (B). If the condition S 3B
was satisfied, the seismogram was selected, otherwise it was rejected for further analysis. The
selected seismograms were corrected for instrument response and bandpass-filtered with
bandwidth from 0.1 to 16 Hz. The record lengths considered in the WM and SM
seismograms were 20 and 7 sec, respectively, with the Hanning taper. The pre-event data
were processed in the same manner and used to correct the signal spectrum for each record.
The corrected amplitude spectra were smoothed using a triangular window of 0.5 Hz half-
width. The smoothed spectral values were interpolated using a spline interpolator at 0.05 Hz
interval from 0.5 to 15 Hz frequency range. Next, the root mean square of the amplitude
spectra were computed from:
() ()
(
()
)()
2/1
22
2fHfHfH
EWNS
+= (1)
where f is the frequency, H
NS
(f) and H
EW
(f) represent smoothed amplitude spectra of the
windowed and filtered data of the N-S and E-W components. The H(f) values were used to
obtain spectral ratios at different sites.
RESULTS AND DISCUSSION
Since the SR values can be approximated by a lognormal distribution (Field and Jacob
1995), we computed the logarithmic event-average of individual spectral ratios obtained from
the WM and SM network data; the results are shown in Figures 4 and 5. The results show
considerable variations of SR in the basin. At some sites (An02, An15, K2-13, K2-21, K2-
08), SR tends to approach a value of 6, while others (An06, An17, An18, K2-09, K2-22) are
associated with relatively low values (1-3) of SR. Moreover, some sites (An02, K2-01) show
sharp peaks of SR while others (An07, An12, K2-06) show broadly distributed highs as a
function of frequency. Spatially, SR values in general increase from the foothills of Chugach
Mts. in the east towards the west in the deeper part of the basin.
Figure 4. Site response as a function of frequency (from 0.5 to 11 Hz) for the weak-motion sites. The
solid and dashed lines represent the results based on the SSR and HVR methods, respectively.
The mean of the logarithmic standard deviations of the SR values obtained by the SSR
and HVR methods are 0.32 and 0.23, respectively. In Mexico, Lermo and Chavez-Garcia
(1993) also observed less scatter in HVR results compared to those for SSR. Moreover, we
obtained a decrease in standard deviation with the increase in focal depths of earthquakes.
This relationship is shown in Figure 6 for the SSR case. A similar result was obtained for the
HVR case. The decrease is by a factor of about two for earthquakes with hypocenter depths
greater than 50 km compared to those shallower than 50 km.
Figure 5. Site response as a function of frequency (from 0.5 to 11 Hz) for the strong-motion sites.
The solid and dashed lines represent the results based on the SSR and HVR methods, respectively.
The comparison of site responses yielded from the WM and SM datasets is of particular
interest. Earlier, we mentioned that besides the reference site, there were three more sites
with collocated WM and SM stations: K2-01 and An15 (distance – 100 m), K2-12 and An05
(distance – 130 m), and K2-06 and An12 (distance – 145 m). With the exception of one case
(K2-06 and An12), the SR variations in Figure 4 and 5 show that site responses of collocated
stations are more or less consistent, especially between HVR results. An12 shows
considerably higher amplifications above 6 Hz compare to the collocated site K2-06 in both
SSR and HVR results. This result is more than likely caused by variations of local geology at
short distances. Similarly, the SSR results for An15 in northern Anchorage exceeds one for
the collocated SM site K2-01 by a factor of 1.3-1.4 in a wide frequency range from 0.5 to 9
Hz. At the reference site, the two HVR-based estimations are in accord, gradually increasing
with the frequency up to a factor of 3 at 10 Hz. This trend indicates that the reference site has
site responses at higher frequencies. We particularly analyzed five events, simultaneously
recorded by both WM and SM stations at collocated sites, in order to compare the spectral
ratios yielded from the velocity and acceleration records of the same event. However, the
results show large scatter, preventing any specific conclusion.
0 20 40 60 80 100 120 140 160
Focal depth
(
km
)
0.1
0.2
0.3
0.4
0.5
0.6
Logarithmic standard deviation of SR
Figure 6. Relationship between logarithmic standard deviation of SR values and focal depths of
corresponding events.
Furthermore, we computed network-average site responses from the WM and SM
datasets, assigning equal weight to each individual spectral ratio from both datasets. The
results are shown in Figures 7a and 7b. It may be noted that the differences lie within the ±σ
range, where σ refers to the standard deviation of SR values. Similar comparison of SR
values obtained by the SSR and HVR methods from the data of both networks is displayed in
Figures 8a and 8b. The results show that below 6 to 7 Hz, HVR values are higher by factors
of about 1.3 and 1.7 than SSR values for WM and SM cases, respectively. For higher
frequencies, these differences increase slightly. The source of the difference between the WM
and SM cases is not clear at this stage.
Since the HVR method provides a good estimation of predominant frequency (f
o
) of the
sedimentary column at the site of measurement (Theodulidis and Bard 1995, Field and Jacob
1995, Riepl et al. 1998), we determined f
o
from the HVR results for most of the WM and SM
sites, where the peaks with maximum amplitudes are well defined (dashed curves in Figures
4 and 5). These values are shown in Figure 9, where in most cases f
o
decreases from about 3
to 4 Hz along the foothills of Chugach Mts. to a minimum of 0.7 Hz on the western side of
the basin. This result is in accord with the variation of the thickness of the Quaternary
sediments (Figure 1b), which increases from the east to the western side of the basin where
the Quaternary sediments are several hundreds meters in thickness. However, predominant
frequencies shown in Figure 9 correspond to the ground motion level less than 0.1 g. At
higher levels of ground motions these values may change considerably due to nonlinear
effects.
The network average SR values obtained from both WM and SM data by the SSR method
show deamplification above 7 to 8 Hz (Figure 7a). A computation of the normalized response
spectra with the SM data showed a cross-over of the rock site spectrum by those of the soil
sites around 7 Hz. Aki (1988) has pointed out that SR of soil sites is about 2 to 3 times higher
than that of rock sites below 5 Hz. This relationship reverses at higher frequencies (>5 Hz).
This may be due to higher attenuation at the soil sites with respect to the rock sites. The
overall effect of this reversal will be exhibited by the deamplification of SR values yielded by
the SSR method at high frequencies.
012345678910111
Frequency (Hz)
2
2
3
4
5
6
7
8
9
2
3
4
5
6
1.0
Site Response
SSR
WM
(a)
SM
012345678910111
Frequency (Hz)
2
2
3
4
5
6
7
8
9
2
3
4
5
6
1.0
Site Response
HVR
WM
(b)
SM
Figure 7. Network-average site responses; (a) - by SSR method and (b) - by HVR method. Thick gray
lines correspond to WM results, thin black lines to SM results. Shaded areas represent one standard
deviation limits of SR values.
012345678910111
Frequency (Hz)
2
2
3
4
5
6
7
8
9
2
3
4
5
6
1.0
Site Response
WM
HVR
SSR
(a)
012345678910111
Frequency (Hz)
2
2
3
4
5
6
7
8
9
2
3
4
5
6
1.0
Site Response
SM
HVR
SSR
(b)
Figure 8. Network-average site responses; (a) - from WM records and (b) - from SM records. Thick
gray lines correspond to HVR method, thin black lines to SSR method. Shaded areas represent one
standard deviation limits of SR values.
Recently, the drilling of a special purpose borehole of 9 m depth located close to the
reference sites (An01 and K2-16; Figures 4 and 5) showed the presence of about 4 m thick
weathered and fractured metamorphic rock. We anticipate that the weathered layer is the
result of repeated freezing and thawing of the insitu formations. It appears that this weathered
layer, whose spatial extent is not known, may play a part in the amplification of the ground
motion at reference sites at higher frequencies (Steidl et al. 1996).
In addition to the factors referred so far, source directivity and topographic effects may be
thought of as potential sources of the above observed phenomenon (Kawase and Aki 1990;
Pedersen et al. 1994; Zhang et al. 1998). However, as shown in Figure 3, the earthquakes
considered for this study are distributed in different azimuths and epicentral distances with
respect to the networks. Therefore, the effects of these two factors on the event-average site
response are assumed to be minimal. Nevertheless, for further discussion of SR values in the
Anchorage area, we restricted them to 7 Hz for the reasons mentioned above.
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61.05
61.10
61.15
61.20
61.25
61.30
4.2
4.0
5.3
0.8
0.9
2.2
6.0
3.3
2.1
1.1
1.5
3.3
2.4
1.4
0.8
7.1
1.0
1.8
4.5
0.7
1.2
5.2
0.7
1.1
0.9
2.6
5.7
3.2
2.1
3.4
0.8
0.9
Figure 9. Predominant frequencies (in Hz) for some of the WM and SM sites, estimated from HVR
results.
SPATIAL DISTRIBUTION OF BAND-AVERAGE SR VALUES
In order to facilitate usage of the SR by practicing engineers, we computed frequency
band-averages of the SR values. For this purpose, a low frequency (LF) range from 0.5 to 2.5
Hz and a high frequency (HF) range from 3.0 to 7.0 Hz with approximate geometric center
frequencies of about 1 and 5 Hz, respectively, were selected. Moreover, Dutta et al. (2000b)
obtained SR values by the generalized inversion method (GI) for the Anchorage basin, and
these values were found similar to those obtained by the SSR method of the present study.
We, thus, considered the three sets (SSR, HVR, and GI) of SR values and assigned weights
of 0.45, 0.10 and 0.45, respectively, to obtain weighted average values. The relatively low
weight given to the HVR results is due to their likely overestimation of SR values over the
frequency range considered, as mentioned earlier.
Figures 10a and 10b show the spatial distribution of SR values at 0.5-unit interval
contours over a geological background (Combellick 1999) of the study area. The contours are
based on grid values of dimensions 0.6 x 0.6 km, which were obtained by the Kriging method
(Stein 1999). At 1 Hz, the SR values increase from 1.5 from the eastern side of Anchorage to
3 and slightly above in the westcentral and northwestern parts of the city. At 5 Hz, the trend
of the spatial distribution of SR changes, where the areas of SR 3 is concentrated in the
south and southeastern parts of the city adjoining the Turnagain Arm. Moreover, an area with
SR values greater than 2 is located near the center of the city.
In the LF range, the areas with SR 1.5 are characterized by deposits grouped as older
glacial drift (GD), consisting of heterogeneous undifferentiated till, stony glaciomarine,
glaciofluvial deposits, etc. GD is the dominant geological unit in the eastern side of
Anchorage. The areas with SR > 2.0 are underlain by the Bootlegger Cove formation (BCF),
which predominates in the western side of the city.
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61.10
61.15
61.20
61.25
Knik Arm
Turnagain Arm
A - Pt. Woronzof
B - Pt. Campbell
C - International Airport
C
C'
(a)
1.5
2.0
2.5
2.5
3.0
3.0
A
B
C
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61.10
61.15
61.20
61.25
Knik Arm
Turnagain Arm
A
1.5
1.5
1.5
2.0
2.0
2.0
2.0
1.0
2.5
2.5
3.0
3.0
(b)
A - Pt. Woronzof
B - Pt. Campbell
C - International Airport
B
C
Figure 10. Contour maps of band-average SR values at 1 Hz (a) and 5 Hz (b) are shown on the
background of Anchorage surficial geology.
The 10 m isopach of BCF (Figure 1a), approximately follows the SR 2. As mentioned
earlier, Updike and Ulery (1986) have subdivided BCF into cohesive (CBCF) and
noncohesive (NBCF) facies. The western edge of the city (Pt. Woronzof and Pt. Campbell),
including the western part of the International Airport, is underlain by NBCF with SR 2.5.
Areas in westcentral and northwestern parts of the city are underlain by CBCF with SR
values of 2.5 to 3.5. This correlation of CBCF and relatively high SR values is further
illustrated in Figure 11 along a N-S cross-section (CC, Figure 10a). Note that values of SR >
2 are associated with the thick deposits of CBCF in the areas at the two ends of the cross-
section.
In the HF range, there is a lack of correlation between the spatial distribution of SR
values and geological trend. According to the quarter wavelength estimation (Kramer 1996),
SR values around 5 Hz are mostly controlled by the physical properties of the shallower 10 to
20 m thick soil column (assuming 200 to 400 m/s shear wave velocity) which are highly
variable across the Anchorage basin. This variability is interpreted as the primary source of
the lack of correlation at 5 Hz.
Distance
(
km
)
1.0
1.5
2.0
2.5
3.0
3.5
Site Response
(a)
C
C'
0
105
15
Glacial Drift
CBCF
CBCF
Tertiary
Sedimentary Rocks
C
C'
0m
100m
200m
(b)
Figure 11. (a) Site response along CC as shown in Figure 10a; (b) Geological cross-section along
CC.
RELATIONSHIP BETWEEN SR AND UPPERMOST 30 M AVERAGE SHEAR WAVE
VELOCITY
In the Anchorage area, the distribution of the uppermost 30 m time-average shear wave
velocity (β
30
) was obtained at 36 sites (including 15 SM sites) by surface measurements in
cooperation with the Vibration Instrument Company of Tokyo, Japan and Ensole Corporation
of Raleigh, North Carolina. The details of the method have been given by Rodriguez-
Ordonez (1994), Nath et al. (1997), and Dutta et al. (2000a) and are not repeated here.
The results of these measurements are shown in Figure 12, where the β
30
values range
from 220 to 600 m/s. On the basis of the NEHRP site classification (Building Seismic Safety
Council 1997), it appears that the Anchorage basin consists of site class C (360 < β
30
760
m/s) and D (180 < β
30
360 m/s). Moreover, though there is no velocity data coverage along
the Chugach Mts., we anticipate site class B (760 < β
30
1500 m/s) in this area from rock
exposures and samples obtained from the special purpose 9 m borehole mentioned earlier.
Following the known subsurface lithologic variation from the foothills of the Chugach
Mts. toward the west side of Anchorage, it is necessary to introduce a transition zone (C/D)
between the areas of site classes C and D. The β
30
in C/D ranges from 320 to 410 m/s and this
zone extends in the N-S direction with an eastward bent around downtown Anchorage
(Figure 12). SR values in this zone approximately lie between 1.5 and 2.0 in the LF range
(Figure 10a) and this zone includes the 10 m isopach of BCF. The band-average SR values,
β
30
,
and site classes obtained for the WM and SM station sites are listed in Table A2 of
Appendix A.
The relationships between band-average SR values and β
30
at 1 and 5 Hz, respectively,
are shown in Figures 13a and 13b. At 1 Hz, the two parameters correlate with correlation
coefficient of 0.82. A power law fit to the data (Figure 13a) yielded the relation:
2.0)7.7)ln(2.1()ln(
30
±+=
βSR
(2)
Dutta et al. (2000b) also obtained a similar relation as in (2) for the Anchorage basin,
using weak-motion data only. However, at 5 Hz there is a lack of correlation between band-
average SR values and β
30
. The extensive lateral variation of the geologic formations in the
uppermost 10 to 20 m, as mentioned earlier, may be the source of this lack of correlation at
high frequency.
C
C/D
D
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61.05
61.10
61.15
61.20
61.25
238
366
474
309
284
491
270
274
582
269
394
514
354
524
412
448
398
491
239
303
227
256
277
514
421
448
504
451
313
520
265
408
445
377
404
410 m/s
320 m/s
Figure 12. The shear wave velocity measurement sites and
β
30
values in m/s. Two contour lines,
which represent velocities 320 and 410 m/s, separate site classes C, C/D and D. Broken line is
approximate 10 m isopach of deposits of BCF (Combellick 1999).
200 250 300 350 400 450 500 550 600
30m average shear wave velocity (m/s)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Band-average SR
(a)
Site Class C/D
Site Class D
Site Class C
1 Hz
200 250 300 350 400 450 500 550 600
30m average shear wave velocity (m/s)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Band-average SR
(b)
Site Class C/D
Site Class D
Site Class C
5 Hz
Figure 13. Relationship between band-average site response values and uppermost 30 m average
shear wave velocities for the LF (a) and HF (b) ranges. The velocity values for sites with no direct
velocity measurements were interpolated.
COMPARISON WITH THE NEHRP SITE COEFFICIENTS
The two frequency intervals for the band-average SR values were selected such that the
average values can be directly related to the NEHRP site coefficients (Building Seismic
Safety Council 1997). It may be noted that the NEHRP site coefficients were derived from
both Fourier and response spectra (Borcherdt 1994, Dobry et al. 2000). Although our band-
average SR values were obtained from Fourier spectra, we have also computed SR from the
response spectra of SM records for 5% damping, using the SSR method. The results closely
agree with those obtained from Fourier spectra, as shown in Figure 14 for two SM sites,
although the response spectra-based estimations in general are smoother, especially at higher
frequencies. Furthermore, a good correspondence between band-average SR values is
illustrated in Figure 15 for both LF and HF intervals.
In Table 2, the characteristics of site classes obtained from this study are compared with
those given by the NEHRP provisions of 1997 for a ground motion level less than 0.1 g. In
the LF range, we have obtained class-average SR values of 1.31, 1.84 and 2.67 for site
classes C, C/D, and D, respectively. With respect to these values, the NEHRP recommended
site coefficients at 1 sec are higher for site class C (F
v
=1.7) and lower for site class D
(F
v
=2.4). These differences are attributed to the additional subdivision between soil classes C
and D used here; the site class C/D occupies the lower SR values, compared to the NEHRP-
specified site class D (i.e. sites with β
30
between 320 and 360 m/s) and the higher SR values
from site class C (360
β
30
< 410 m/s, see Figure 13a). At 5 Hz, the lack of spatial
correlation between β
30
and SR seems to yield values of SR close to each other for the three
site classes -1.68, 1.66, and 1.70 for C, C/D, and D, respectively.
012345678910111
Frequency (Hz)
2
0
1
2
3
4
Site Response
(a)
012345678910111
Frequency (Hz)
2
0
1
2
3
4
Site Response
(b)
Figure 14. Site responses for two SM sites: K2-01 (a) and K2-02 (b). Black lines are based on
Fourier spectra, gray lines are based on response spectra.
13024
Band-average site amplification
(from response spectra)
1
3
0
2
4
Band-average site amplification
(from Fourier spectra)
Figure 15. Comparison of band-average SR values, obtained from Fourier and response spectra.
Filled and open triangles correspond to LF and HF ranges, respectively.
Table 2. Site class characteristics; N is the number of sites in a given site class used in this study.
Site class C (N = 18) C/D (N = 6) D (N = 19)
β
30
interval (m/s) (present study)
410 - 760 320 - 410 180 - 320
NEHRP
β
30
interval (m/s)
360 - 760 - 180 - 360
Band-average SR value at 1 Hz (present study)
1.31 ± 0.30 1.84 ± 0.33 2.67 ± 0.53
NEHRP site coefficient at 1 sec (F
v
) 1.7 - 2.4
Band-average SR value at 5 Hz (present study)
1.68 ± 0.50 1.66 ± 0.53 1.70 ± 0.55
NEHRP short-period site coefficient (F
a
) 1.2 - 1.6
CONCLUSIONS
The sedimentary section of the Anchorage metropolitan area exhibits considerable spatial
variations of seismic site response (SR). Band-average SR values in the low frequency (LF)
range from 0.5 to 2.5 Hz increase by a factor of three from the foothills of Chugach Mts. in
the east to the west towards the deeper part of the basin. The areas of relatively higher site
response values (SR > 2.5) in the LF range are observed in westcentral and northwestern
parts of the city underlain by the cohesive facies of the Bootlegger Cove formation.
The SR values yielded by the weak-motion and strong-motion datasets are comparable
and lie within the one standard deviation limits. The results obtained by the standard spectral
ratio (SSR) method are more consistent than those yielded by the horizontal to vertical
spectral ratio (HVR) method, which tends to overestimates SR values. However, there is less
scatter in HVR results with respect to that of SSR. Also, the logarithmic standard deviation of
the SR values decrease by a factor of about two for earthquakes with focal depths greater
than 50 km compared to those shallower than 50 km. The HVR method provides a good
estimation of predominant frequency of the sedimentary column, which in general decreases
from about 3 Hz to 4 Hz along the foothills of Chugach Mts. to a minimum of 0.7 Hz on the
western side of the basin. This is in accord with the variation of the thickness of Quaternary
sediments.
The site responses computed by the SSR method from the Fourier spectra closely agree
with those obtained from the response spectra for 5 percent damping.
Following the known subsurface lithologic variation and the uppermost 30 m time-
average shear wave velocity (β
30
) in the area, it is necessary to introduce a transition zone of
C/D between areas of site classes C and D in the Anchorage basin. The width of the zone
C/D is about 2-3 km and β
30
in this zone varies from 320 to 410 m/s.
The frequency band-average SR values at 1 Hz have good correlation with β
30
having
correlation coefficient of 0.82. SR values at this frequency (1 Hz) are consistent with the
NEHRP specified site coefficients. However, frequency band-average SR values at 5 Hz lack
correlation with the trend of β
30
.
ACKNOWLEDGMENTS
The authors are grateful to Toshifumi Kono for operation and initial data reduction of the
weak-motion data and maintenance of the Anchorage strong-motion network, and David
Cole of Dowl Engineers of Anchorage for useful discussion. We also thank Mitch Robinson
and Kent Lindquist of the Geophysical Institute for assisting in computer related problems
and locations of earthquakes used in this study. We are grateful to three anonymous
reviewers and the editor for their comments and suggestions. This work was supported in part
by the Alaska Science and Technology Foundation Project #91-2-125 and #97-3-131 and in
part by the Geophysical Institute, University of Alaska Fairbanks.
APPENDIX A
Table A1. Anchorage strong-motion and weak-motion station coordinates. Three SM stations, K2-04,
K2-11, and K2-14 have been relocated to nearby locations after a few years of operation. The old
locations are listed at the end of the table, marked with star.
SM station
code
Latitude
(N)
Longitude
(W)
WM station
code
Latitude
(N)
Longitude
(W)
K2-01 61.235 149.869 An01 61.098 149.687
K2-02 61.224 149.822 An02 61.075 149.807
K2-03 61.219 149.718 An03 61.114 149.820
K2-04 61.177 150.010 An04 61.101 149.863
K2-05 61.200 149.911 An05 61.156 149.794
K2-06 61.191 149.822 An06 61.155 150.052
K2-07 61.160 150.001 An07 61.127 149.933
K2-08 61.177 149.919 An08 61.157 149.987
K2-09 61.185 149.744 An09 61.153 149.929
K2-10 61.130 149.928 An10 61.189 150.015
K2-11 61.149 149.855 An11 61.186 149.875
K2-12 61.156 149.792 An12 61.191 149.824
K2-13 61.113 149.856 An13 61.181 149.720
K2-14 61.124 149.766 An14 61.210 149.909
K2-15 61.087 149.750 An15 61.235 149.870
K2-16 61.099 149.685 An16 61.249 149.818
K2-20 61.155 150.053 An17 61.223 149.726
K2-21 61.153 149.949 An18 61.206 149.787
K2-22 61.088 149.834 An19 61.174 149.846
K2-04* 61.178 150.015 An20 61.217 149.849
K2-11* 61.157 149.869 An21 61.187 149.938
K2-14* 61.140 149.781 An22 61.137 149.889
Table A2. Characteristics of the SM and WM sites. Site classes for the sites with no direct velocity
measurements are defined according to their positions relative to the velocity contour lines 320 and
410 m/s.
SM
station
code
β
30
(m/s)
Site
class
SR in
LF
range
SR in
HF
range
WM
station
code
Site
class
SR in
LF
range
SR in
HF
range
K2-01 238 D 2.58 1.28 An01 B Ref. Ref.
K2-02 366 C/D 1.94 0.86 An02 C 0.80 3.56
K2-03 474 C 1.34 1.46 An03 C 1.95 2.93
K2-04 - D 1.96 1.38 An04 C/D 2.51 1.84
K2-05 284 D 2.84 1.75 An05 C 1.14 1.21
K2-06 491 C 1.26 2.53 An06 D 2.31 1.43
K2-07 270 D 2.07 1.41 An07 D 3.58 3.57
K2-08 274 D 2.58 1.45 An08 D 3.19 2.90
K2-09 582 C 1.19 1.01 An09 D 3.92 2.21
K2-10 269 D 2.24 1.61 An10 D 2.86 1.80
K2-11 - C 1.50 1.82 An11 C 1.48 1.29
K2-12 514 C 0.90 1.33 An12 C 1.44 3.15
K2-13 354 C/D 1.65 1.85 An13 C 1.02 2.22
K2-14 - C 1.38 1.34 An14 D 3.01 1.50
K2-15 412 C 1.28 2.16 An15 D 3.49 1.85
K2-16 - B Ref Ref An16 D 2.67 1.04
K2-20 - D 2.06 1.87 An17 C 1.55 1.02
K2-21 - D 2.71 2.20 An18 C 1.68 0.90
K2-22 - C 1.62 1.64 An19 C 1.60 1.56
K2-04* 309 D 2.17 1.39 An20 D 2.36 1.01
K2-11* 394 C/D 1.75 2.07 An21 D 3.20 2.45
K2-14* 524 C 1.14 1.75 An22 C/D 1.87 1.89
REFERENCES CITED
Aki, K., 1988, Local site effects on strong ground motion, Proceedings of Earthquake Engineering &
Soil Dynamics II GT Div/ASCE, Park City, Utah, June 27-30, 1988.
Bonilla, F. L., Steidl, J. H., Lindley, G. T., Tumarkin, A. G., and Archuleta, R. J., 1997, Site
amplification in the San Fernando Valley, CA: variability of site effect estimation using the S-
wave, coda and H/V methods, Bull. Seism. Soc. Am., 87, 710-730.
Borcherdt, R. D., 1970, Effects of local geology on ground motion near San Francisco Bay, Bull.
Seism. Soc. Am., 60, 29-61.
Borcherdt, R. D., 1994, Estimates of site-dependent response spectra for design (methodology and
justification), Earthquake Spectra, 10, 617-653.
Building Seismic Safety Council (BSSC), 1998, 1997 Edition NEHRP Recommended Provisions for
Seismic Regulations for New Buildings and Other Structures, FEMA 302/303, Part I (Provisions)
and Part II (Commentary), Washington, DC.
Combellick, R. A., 1999, Simplified geologic map and cross sections of central and east Anchorage,
Alaska: Alaska Div. Geol. Geophys. Surveys Preliminary interpretive report 1999-1, Fairbanks,
AK.
Dobry, R., Borcherdt, R. D., Crouse, C. B., Idriss, I. M., Joyner, W. B., Martin, G. R., Power, M. S.,
Rinne, E. E., and Seed, R. B., 2000, New site coefficients and site classification system used in
recent building seismic code provisions, Earthquake Spectra, 16, 41-67.
Dutta, U., Biswas, N. N., Martirosyan, A. H., Nath, S., Dravinski, M., Papageorgiou, A., and
Combellick, R., 2000a, Delineation of spatial variation of shear wave velocity with high-
frequency Rayleigh waves in Anchorage, Alaska, GJI, 143, 1-16.
Dutta, U., Martirosyan, A. H., Biswas, N. N., Papageorgiou, A., Dravinski, M., and Combellick, R.,
2000b, Estimation of S-wave Site Response in Anchorage, Alaska from Weak Motion Data Using
Generalized Inversion Method, Bull. Seism. Soc. Am.,91, 335-346.
Field, E. H. and Jacob, K. H., 1995, A comparison and test of various site-response estimation
techniques, including three that are not reference-site dependent, Bull. Seism. Soc. Am., 85, 1127-
1143.
Kawase, H. and Aki, K., 1990, Topography effect at the critical SV-wave incidence: Possible
explanation of damage pattern by the Whittier Narrows, California, earthquake of 1 October
1987, Bull. Seism. Soc. Am., 80, 1-22.
Kramer, S. L., 1996, Geotechnical Earthquake Engineering, Prentice Hall, Upper Saddle River, NJ.
Lachet, C., Hatzfeld, D., Bard, P-Y., Theodulidis, N., Papaioannou, C., and Savvaidis, A., 1996, Site
effect and microzonation in the city of Thessaloniki (Greece). Comparison of different
approaches, Bull. Seism. Soc. Am., 86, 1692-1703.
Lermo, J. and Chavez-Garcia, F. J., 1993, Site effect evaluation using spectral ratios with only one
station, Bull. Seism. Soc. Am., 83, 1574-1594.
Nakamura, Y., 1989, A method of dynamic characteristics estimation of subsurface using
microtremor on the ground surface, QR RTRI, 30, 25-33.
Nath, S., Chatterjee, D., Biswas, N., Dravinski, M., Cole, D., Papageorgiou, A., Rodriguez, J., and
Poran, C., 1997, Correlation study of shear wave velocity in near surface geological formation in
Anchorage, Alaska, Earthquake Spectra, 13, 55-76.
Pedersen, H. A., Sanchez-Sesma, F. J., and Campillo, M., 1994, Three-Dimensional Scattering by
Two-Dimensional Topographies, Bull. Seism. Soc. Am., 84, 1169-1183.
Riepl, J., Bard, P-Y., Hatzfeld, D., Papaioannou, C., and Nechtschein, S., 1998, Detailed Evaluation
of Site Response Estimation Methods across and along the Sedimentary Valley of Volvi (Euro-
Seistest), Bull. Seism. Soc. Am., 88, 488-502.
Rodriguez-Ordonez, J. A., 1994, A new method for interpretation of surface wave measurements in
soils, PhD dissertation, North Carolina State University, Raleigh, NC.
Schmoll, H. R. and Dobrovolny, E., 1972, Generalized geologic map of Anchorage and vicinity,
Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-787-A, 1 sheet,
scale 1:24,000.
Selkregg, L. L., Buck, E. H., Buffler, R. T., Cote, O. E., Evans, C. D., and Fisk, S. G., 1972,
Environmental atlas of the greater Anchorage area borough, Alaska, Arctic Environmental
Information and Data Center, University of Alaska, Anchorage.
Stein, M. L., 1999, Interpolation of Spatial Data: Some Theory for Kriging, Springer Series in
Statistics, Springer Verlag, NY.
Steidl, J. H., Tumarkin, A. G., and Archuleta, R. J., 1996, What is a Reference Site?, Bull. Seism. Soc.
Am., 86, 1733-1748.
Theodulidis, N. and Bard P-Y., 1995, Horizontal to vertical ratio and geological conditions: an
analysis of strong motion data from Greece and Taiwan (SMART-1), Soil Dyn. Earthquake Eng,
14, 177-197.
Updike, R. G. and Ulery, C. A., 1986, Engineering Geologic Map of Southwest Anchorage, Alaska:
Alaska Div. Geol. Geophys. Surveys Professional Report 89, 1 sheet, scale 1:15,840.
Zhang, B., Papageorgiou, A. S., and Tassoulas, J. L., 1998, A Hybrid Numerical Technique,
Combining the Finite Element and Boundary Element Methods, for Modeling the 3D Response of
2D Scatterers, Bull. Seism. Soc. Am., 88, 1036-1050.
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Anchorage, Alaska, is located in one of the most active tectonic settings in the world. The city and region were significantly impacted by the MW 9.2 Great Alaska Earthquake in 1964, and they were recently shaken by a MW 7.1 event in 2018. The city was developed in an area underlain by complex soil deposits of varied geological origins and stiffnesses, with the deposits’ thicknesses increasing east to west. Situated at the edge of the North American Plate, with the actively subducting Pacific Plate below, Anchorage is susceptible to both intraslab and interface earthquakes, along with crustal earthquakes. Strong-motion stations were installed across the city in an attempt to capture the variability in site response. Several previous studies have been performed to evaluate that variability but have not included larger magnitude events and have not benefited from the current density of instrumentation. The work presented here provides background information on the geology and tectonic setting of Anchorage and presents details related to the dataset and methods used to perform the site-response analysis. This study has collected strong-motion recordings from 35 surface stations across Anchorage for 95 events spanning from 2004 to 2019, including the MW 7.1 Anchorage Earthquake in 2018. The more than 1700 three-component recordings from those 95 events with moment magnitudes ranging from 4.5 to 7.1 were used to evaluate site response variability across the city. Using the Generalized Inversion Technique and a reference rock site, spectral amplifications were calculated and analyzed for frequencies between 0.25 and 10 Hz for each strong-motion station. The study results were used to develop contour maps at 1 Hz and 5 Hz, using logarithmic-band averages, to describe the variability of spectral amplifications at these two frequencies of interest. The results were also compared to geologic conditions across Anchorage, and the overlaying of different soil deposits can be seen to have an impact on the spectral amplification at the sites. The results of this study provide improvements on past microzonation studies and, using sensitivity analyses, offer support for the use of small and moderate earthquakes to evaluate spectral amplifications.
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The use of horizontal to vertical spectral ratios (HVSR) of earthquake ground motions has become a standard technique to characterize sites, especially those lacking subsurface measurements. Several studies have developed relationships between HVSR results and time-averaged shear-wave velocity in the upper 30 m (VS30). Other studies have utilized standard spectral ratios calculated from horizontal ground motion Fourier amplitude spectra to estimate VS30. Anchorage, Alaska (USA), has a network of strong-motion recording stations, many of which have no site-specific subsurface characterization. This study compares measured VS30 and HVSR results from 18 strong-motion stations to four regional models developed by others. A relationship between the 1 Hz band-averaged (0.5–2.5 Hz) spectral amplification results and VS30 is presented. VS30 estimates for the strong-motion stations are made, and a regional model is developed between HVSR and VS30, both in terms of fpeak (the frequency of the peak HVSR amplitude) and Apeak (the amplitude of the peak). In addition to the regional model, additional VS30 data from other sites in Anchorage, including 19 downhole VS30 measurements and 22 microtremor VS30 estimates from others, are used with the strong-motion station VS30 estimates to develop a VS30 contour map of Anchorage. The contouring represents the spatial distribution of the site classes of the local building code, which are based on VS30. This map may be incorporated into planning documents for future developments in the city.
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Anchorage, Alaska, is a natural laboratory for recording strong ground motions from a variety of earthquake sources. The city is situated in a tectonic region that includes the interface and intraslab earthquakes related to the subducting Pacific plate and crustal earthquakes from the upper North American plate. The generalized inversion technique was used with a local rock reference station to develop site response at >20 strong-motion stations in Anchorage. A database of 94 events recorded at these sites from 2005 to 2019 was also compiled and processed to compare their site response with those in the 2018 Mw 7.1 event (main event). The database is divided into three datasets, including 75 events prior to the main event, the main event, and 19 aftershocks. The stations were subdivided into the site classes defined in the National Earthquake Hazards Reduction Program based on estimated average shear-wave velocity in of the upper 30 m (VS30), and site-response results from the datasets were compared. Nonlinear site response was observed at class D and DE sites (VS30 of 215–300 and 150–215 m/s, respectively) but not at class CD and C sites (VS30 of 300–440 and 440–640 m/s, respectively). The relationship of peak ground acceleration versus peak ground velocity divided by VS30 (shear-strain proxy) was shown to further support the observation that sites with lower VS30 experienced nonlinear site response.
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The thermal state of the crust and mantle in subduction zones is controlled by the depth of the subducting plate. With low‐angle subduction, like at the eastern end of the Alaska subduction zone, the less attenuating fore‐arc is extended farther from the trench and can effect ground motions in addition to source and site effects. Recent crustal and subduction earthquakes in south‐central Alaska, including the 2018 M 7.1 Anchorage event, demonstrate these effects. Inslab earthquake waves in the subducting plate can propagate up the slab to the fore‐arc region with less attenuation, causing an increase in observed ground motions. Long‐period ground motions from the 2018 M 7.1 Anchorage earthquake are significantly higher than predicted ground motions from current subduction ground‐motion models within 50–100 km of the epicenter. At short periods, ground motions show reduced amplitudes due to nonlinear sediment effects in the Anchorage area, reducing the damage potential of the earthquake. At long periods, ground motions are little affected by sediment nonlinearity and remain higher than expected. The duration of shaking was too short for widespread liquefaction effects, unlike during the 1964 M 9.2 earthquake. Other historical earthquakes have produced similar increases in ground motions in the Cook Inlet and Kenai Peninsula region. At both short and long periods, ground motions from the 2016 Iniskin M 7.1 inslab earthquake are higher than expected in the Cook Inlet region. The 2015 Redoubt M 6.3 inslab earthquake also shows increased ground motions in the Cook Inlet region at all periods. Crustal Q estimates from Lg waves show less attenuation in south‐central Alaska at longer periods. In the larger south‐central Alaska region crustal Q(f)=336f0.34 compared to Q(f)=217f0.84 for all of Alaska with most of the decrease in attenuation at frequencies below 2 Hz.
Thesis
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The main objective of the present work is to establish a link between the scientific fields of engineering seismology and structural engineering. Substantially it deals with the application and enhancements of methods coming from engineering seismology as well as their junctions to the fields of structural engineering respectively earthquake engineering. Based on real occurred earthquake damage inflicted to multistoried reinforced-concrete frame buildings, the influence of local site effects on the grade of structural damage is worked out. This relying on comprehensive investigations conducted during numerous field missions of German TaskForce after damaging earthquakes in Venezuela and Türkiye. Instrumental investigations on both the structure and its local subsoil in order to identify the damage potential of seismic ground motion take center stage of the thesis. Thereby it is examined whether or not an estimated seismic demand representative in amplitude level and frequency characteristics is able to cause structural damage considering the vulnerability of the structure itself as well as the local site and subsoil conditions. Investigations are concentrated on selected RC frame structures with or without masonry infill walls.
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The paper compares various site-response estimation techniques using aftershock data of the 1989 Loma Prieta earthquake collected in Oakland, California. It examines and tests three site-response estimation techniques that do not rely on a reference site to estimate source and path effects. The first involves a parameterized source- and path-effects inversion. The second technique involves taking horizontal- to vertical-component spectral ratios of shear-wave aftershock data. The third estimate is formed by taking horizontal-to vertical-component ratios of ambient seismic noise, and these are shown to reveal the fundamental resonant frequency of the sediment sites. The highly frequency-dependent character of site response is well constrained, and the fact that non-reference-site-dependent methods are capable of revealing this is promising for site-specific hazard assessments in regions that lack adequate reference sites. -from Authors
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Many methods for estimating site response compare ground motions at sites of interest to a nearby rock site that is considered a "reference" motion. The critical assumption in these methods is that the surface-rock-site record (reference) is equivalent to the input motion at the base of the soil layers. Data collected in this study show that surface-rock sites can have a site response of their own, which could lead to an underestimation of the seismic hazard when these sites are used as reference sites. Data were collected from local and regional earthquakes on digital recorders, both at the surface and in boreholes, at two rock sites and one basin site in the San Jacinto mountains, southern California. The two rock sites, Keenwild and Piñon Flat, are located on granitic bedrock of the southern California peninsular ranges batholith. The basin site, Garner Valley, is an ancestral lake bed with water-saturated sediments, on top of a section of decomposed granite, which overlies the competent bedrock. Ground motion is recorded simultaneously at the surface and in the bedrock at all three sites. When the surface-rock sites are used as the reference site, i.e., the surface-rock motion is used as the input to the basin, the computed amplification underestimates the actual amplification at the basin site for frequencies above 2 to 5 Hz. This underestimation, by a factor of 2 to 4 depending on frequency and site, results from the rock sites having a site response of their own above the 2-to 5-Hz frequencies. The near-surface weathering and cracking of the bedrock affects the recorded ground motions at frequencies of engineering interest, even at sites that appear to be located on competent crystalline rock. The bedrock borehole ground motion can be used as the reference motion, but the effect of the downgoing wave field and the resulting destructive interference must be considered. This destructive interference may produce pseudo-resonances in the spectral amplification estimates. If one is careful, the bedrock borehole ground motion can be considered a good reference site for seismic hazard analysis even at distances as large as 20 km from the soil site.
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During the months that followed the 17 January 1994 M 6.7 Northridge, California, earthquake, portable digital seismic stations were deployed in the San Fernando basin to record aftershock data and estimate site-amplification factors. This study analyzes data, recorded on 31 three-component stations, from 38 aftershocks ranging from M 3.0 to M 5.1, and depths from 0.2 to 19 km. Site responses from the 31 stations are estimated from coda waves, S waves, and ratios of horizontal to vertical (HIV) recordings. For the coda and the S waves, site response is estimated using both direct spectral ratios and a generalized inversion scheme. Results from the inversions indicate that the effect of Qs can be significant, especially at high frequencies. Site amplifications estimated from the coda of the vertical and horizontal components can be significantly different from each other, depending on the choice of the reference site. The difference is reduced when an average of six rock sites is used as a reference site. In addition, when using this multi-reference site, the coda amplification from rock sites is usually within a factor of 2 of the amplification determined from the direct spectral ratios and the inversion of the S waves. However, for nonrock sites, the coda amplification can be larger by a factor of 2 or more when compared with the amplification estimated from the direct spectral ratios and the inversion of the S waves. The H/V method for estimating site response is found to extract the same predominant peaks as the direct spectral ratio and the inversion methods. The amplifications determined from the H/V method are, however, different from the amplifications determined from the other methods. Finally, the stations were grouped into classes based on two different classifications, general geology and a more detailed classification using a quaternary geology map for the Los Angeles and San Fernando areas. Average site-response estimates using the site characterization based on the detailed geology show better correlation between amplification and surface geology than the general geology classification.
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Measurements of ground motion generated by nuclear explosions in Nevada were made for 37 locations near San Francisco Bay, Calif. The results were compared with the San Francisco 1906 earthquake intensities and the strong-motion recordings of the San Francisco earthquake of Mar 22 1957. The recordings show marked amplitude variations which are related consistently to the geologic setting of the recording site. Consistent correlations of the results from the nuclear recordings with the 1906 earthquake intensities and the spectral amplification curves for the 1957 earthquake suggest that areas of high amplification determined from small ground motions may also be areas of high intensity in future earthquakes.
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The generalized data inversion method has been used to estimate the spatial variation of site response (SR) in the Anchorage basin in south-central Alaska. The data analyzed represents weak motion of the horizontal components of S waves for local earthquakes. They were recorded by a temporary portable 22-station network (IRIS-PASSCAL equipment) that was operated for about six months. Setting the path effect from independent sources, the inversion of the data was carried out to determine SR. The SR values thus obtained were averaged (logarithmically) for two frequency bands, namely, a low frequency band (LFB) and a high frequency band (HFB) from 0.5 to 2.5 Hz and 3.0 to 7.0 Hz, respectively, with center frequencies of about 1.0 Hz and 5.0 Hz. In LFB, SR increases from 1.0 along the foothills of Chugach Mountains in the east to about 3.5 in the west in and around Campbell Lake and Government Hill. The areas with SR > 3.0 are associated with extensive ground failure during the Prince William Sound earthquake (M-w = 9.2) of 1964. In HFB, there are two small areas adjoining Tumagain Arm in the southern side of Anchorage with SR greater than 3.0. A comparison of the SR values obtained from the inversion with these reported by others for standard spectral ratio (SSR) and horizontal to vertical spectral ratio (HVR) showed (1:1) correspondence with the values of SSR but larger HVR values by a factor of about 2. Areas of soil class D in the study area are characterized by SR > 2.0, while those in C are characterized by SR less than or equal to 2.0 in LFB, but soil class and SR lack correlation in HFB.
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Recent code provisions for buildings and other structures (1994 and 1997 NEHRP Provisions, 1997 UBC) have adopted new site amplification factors and a new procedure for site classification. Two amplitude-dependent site amplification factors are specified: Fa for short periods and Fv for longer periods. Previous codes included only a long period factor S and did not provide for a short period amplification factor. The new site classification system is based on definitions of five site classes in terms of a representative average shear wave velocity to a depth of 30 m (V̄ s). This definition permits sites to be classified unambiguously. When the shear wave velocity is not available, other soil properties such as standard penetration resistance or undrained shear strength can be used. The new site classes denoted by letters A - E, replace site classes in previous codes denoted by S1 - S4. Site classes A and B correspond to hard rock and rock, Site Class C corresponds to soft rock and very stiff / very dense soil, and Site Classes D and E correspond to stiff soil and soft soil. A sixth site class, F, is defined for soils requiring site-specific evaluations. Both Fa and Fv are functions of the site class, and also of the level of seismic hazard on rock, defined by parameters such as Aa and Av (1994 NEHRP Provisions), Ss and S1 (1997 NEHRP Provisions) or Z (1997 UBC). The values of Fa and Fv decrease as the seismic hazard on rock increases due to soil nonlinearity. The greatest impact of the new factors Fa and Fv as compared with the old S factors occurs in areas of low-to-medium seismic hazard. This paper summarizes the new site provisions, explains the basis for them, and discusses ongoing studies of site amplification in recent earthquakes that may influence future code developments.
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Three-dimensional seismic responses of two-dimensional topographies are studied by means of the indirect boundary element method (IBEM). The IBEM yields, in the presented form, very accurate results and has the advantage of low computation cost. In IBEM, diffracted waves are constructed in terms of single-layer boundary sources. The appropriate Green's functions used are those of a harmonic point force moving along the axis of the topography in a full space. Obtained results are compared against those published by other authors. -from Authors
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The spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity. The purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station. The technique is presented briefly and it is discussed why it may be applicable to study the intense S-wave part in earthquake records. Results are presented for three different cities in Mexico: Oaxaca, Oax., Acapulco, Gro., and Mexico City. -from Authors
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Buildings damaged by the Whittier Narrows, California, earthquake of 1 October 1987 were concentrated in several areas, most of which were located 8 to 10 km from the epicenter (corresponding to the critical incidence for SV waves) and were near topographic irregularities, such as the northern part of Whittier, just south of Puente Hills. The purpose of this study is to show the possibility that this anomalous damage pattern is due to the amplification by the topographic irregularity when SV waves are near-critical incidence. The results show that the amplification due to the hill relative to the flat surface is more than 1.5 for all the source models. The combined effect of the topographic irregularity and critically incidence SV waves might be resonsible for the concentration of damage observed during the Whittier Narrows earthquake. -from Authors
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In the summer of 1995, surface measurements of shear wave velocity (β) was conducted at thirty six sites, approximately, in the 0-50 m depth range. Of these, at seven sites values of β, soil log and blow count (N) from borehole measurement were available from previous investigations by others. Using these seven sites for calibration, we compared the velocity profiles yielded by the surface and borehole measurements for these sites. The results show broad similarities. Using the soil logs and shear wave velocity variations at the seven sites, four site classes (SC-Ic, SC-II, SC-III and SC-IV) could be identified. The surface method corresponding to the mean value of β tends to underestimate β between about 1 and 18 percent for site classes SC-Ic, SC-II and SC-III compared to the downhole method. For SC-IV, β is overestimated by 11 percent using surface method. Moreover, the blow count (N) data for each site class shows a linear relationship with β obtained by the surface measurement.