THE EFFECT OF SOIL CHARACTERISTICS
ON NFGM AND FFGM GROUND MOTION
RESPONSE SPECTRA
Ghada Haitham Razzaq*
Civil Engineering Department, Al-Nahrain University, Baghdad1001, Iraq
Gedo0haitham@gmail.com
Hussam K. Risan
Civil Engineering Department, Al-Nahrain University, Baghdad1001, Iraq
Reception: 30/10/2022 Acceptance: 26/12/2022 Publication: 22/01/2023
Suggested citation:
H. R., Ghada and K. R., Hussam. (2023). The Effect of Soil Characteristics On
NFGM and FFGM Ground Motion Response Spectra. 3C TIC. Cuadernos de
desarrollo aplicados a las TIC, 12(1), 29 - 4 5 . https://doi.org/
10.17993/3ctic.2023.121.29-44
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ABSTRACT
This study aims to achieve the following goals: 1- To distinguish between the inelastic
responses of buildings under earthquakes with NFGM and FFGM. 2- To inspect the
effect of soil shear velocity on the response spectra. Several earthquake events, with
different characteristics, are brought from the PEER website for “Pacific Earthquake
Engineering Research” and analyzed using PRISM software to achieve these goals.
The research found that the acceleration, velocity, and displacement response of the
selected near-fault ground motions on the structure has a higher effect than the far-
field ground motion response in both types of soils and this difference is displayed
more noticeably in long periods (periods after 0.2 sec). And when comparing
responses of the two types of soils (the soft soil type and the rock type) it shows that
the geological and geotechnical aspects of the soil deposits majorly affect the
response spectra of the free field surface motions
KEYWORDS
Earthquake, near-fault ground motion, far-field ground motion, shear velocity,
response spectra, PEER, PRISM.
PAPER INDEX
ABSTRACT
KEYWORDS
INTRODUCTION
METHOD OF RESPONSE SPECTRUM
CHOSEN GROUND MOTION RECORDS OF SPECIFIC EARTHQUAKES
ELASTIC RESPONSE OF NFGM AND FFGM
THE EFFECT OF SOIL TYPE ON SDOF ELASTIC SEISMIC RESPONSE
CONCLUSIONS
REFERENCES
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INTRODUCTION
The concentration in this paper is on the seismic ground motion of a general single-
degree-of-freedom structural system by comparing the change in the produced
responses when varying several parameters like distance to the rupture plane,
average soil shear velocity, earthquake components, etc.
From the “Pacific Earthquake Engineering Research Center” (PEER) Various
numbers of earthquake events were taken to study, evaluate, process and compare
with each other. These ground motions were divided into four groups. Each group is
different from the other group in a certain aspect. the dynamic response spectrum is
then calculated using PRISM software for all the ground motions in these groups and
then a graph is drawn to better illustrates the results. Finally, the results will be
discussed to get a broad understanding of the behavior of the structure under seismic
ground motion.
An earthquake is an inevitable natural phenomenon that poses a great danger with
the uncertainty of what time it is going to strike. Though, with preparation measures in
place, the effects of disaster-destruction can be retained to a minimum and the effects
of damage can be restricted, whereby emergency response and rescue labors can be
made more effective during the outcome with a mixture of aid facilities, public spaces
and shelters [1].
Tectonics-induced earthquakes are common and critical earthquakes. They will
happen anyplace within the earth where adequate stored elastic strain energy is
present to enterprise breakage propagation lengthwise a fault plane, and typically
initiate by an initial rupture at a point on the fault plane [2].
Near-fault ground motions can be well-defined as the ground motions that occur
near the fault of the earthquake. Near fault motions that get noted close to the
epicenter are called near-epicenter records, whereas the ones that get recorded along
the fault of the earthquake in the rupture direction can be arbitrated to being forward
directivity [3].
Far-field ground motion is known as the ground motion that occurs far away from
the fault of the earthquake. The distance by which the earthquake can be defined as
NFGM or FFGM can vary from research to research but mostly we can take 20 m as
an acceptable number to divide between the two. Even though in this paper the
search is tightened to get a better result and to show the difference between the two
types of GM better.
Zhang and Iwan (2002) discussed that the near-fault ground motions produce twice
as high a dynamic response as the far field ground motion, and they also found that
the damage resulting from NFGM earthquakes is done due to a small number of large
inelastic deformation cycles, but the damage resulting from by FFGM earthquakes is
due to many high-frequency cycles [4].
Anil K. Chopra and Chatpan Chintanapakdee studied the difference between the
NFGM and FFGM. They studied two groups of ground motions, one with the
characteristics of NFGM and the other with FFGM. They choose 15 NFGM and 15
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FFGM and studied them concerning the linear response spectrum of pseudo-
acceleration, pseudo-velocity, and displacement. And figured out that sensitivity
regions in the cases of NFGM are much wider [5].
The research done by Hall et al. (1995) discussed that structures with a significant
height are very weak against severe NFGM. The increase of the effect of high mode
shapes and the wave propagation with its outcome on the deformation of the structure
is reported, due to the exposure of the tall building to the 1994 Northridge earthquake
[6].
Ali A. Muhsin and Hussam K. Risan also did a paper on the elastic response of a
35 stories reinforced concrete building under NF and FF excitation. And the results
show that the NF excitations will cause the structure to surpass its life safety
performance level. Also, the near-fault (NF) earthquake intensity measurements were
explored in this paper for three different frame buildings with 6, 13, and 20 stories. And
found that the building’s vibration period and the equation that was used to calculate
the IM highly affect the accuracy of the IM [7].
In this paper, a great number of earthquake events were taken randomly as a
sample to evaluate this paper’s goals. Four earthquake groups were made, each with
specific characteristics. And analyzing these GM using PRISM to result in the
responses that will be reviewed and discussed in this research.
METHOD OF RESPONSE SPECTRUM
The RSM expression is used to refer to the “Response spectrum method” and is
known as the first logical act of scientific development of the design for earthquake-
resistant. It can also be defined as an outline of absolute values of the structure’s
maximum response (that consist of displacement, accelerations, velocities, etc.) which
is known as a function of the natural time period of the structure.
This method is the first actual technique for developing earthquake resistance
design. For a structure to withstand an earthquake, it is required to be built around the
notion that it is adept to endure a force equal to . where m is the mass of the
structure and Samax is the maximum expected acceleration that a structural body will
be subjected to under a specific ground acceleration, and it is also the function of the
individual time period of the structural body [8].
If a building system is considered an elastic system with MDOF (multiple degrees of
freedom), then we can separate it into singular components of an SDOF (single
degree of freedom). for each of these singular components, it is probable to find the
peak response if we can measure the ground acceleration resulting from a shock-like
earthquake, then The overall response can then be calculated by the superimposition
of these singular responses. Using this method [8].
Fig.1 shows the acceleration time history of an NFGM for the " El Mayor-Cucapah"
earthquake recorded in Mexico in 2010. This record is taken from PEER, having
magnitude ( = 7.2 ), epicentral distance (Rrup = 11.44 Km ), and PGA of 0.255g
acceleration time history (near-fault). This acceleration time history is converted to an
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elastic spectrum response using PRISM software to get the spectral (acceleration,
velocity, and displacement) as in Fig.2. Three Damping ratios (
) values of 3%, 5%,
and 10% to be used in the spectrum analysis and different values are taken because
damping depends on the material used to build the structure as well as the number of
joints and restraints. As for a typical value, 5% is correct for most concrete structures.
Figure 1. Acceleration time history of El Mayor-Cucapah (2010) earthquake
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Figure 2. Acceleration, velocity, and displacement response spectrum of El Mayor-Cucapah
(2010) earthquake
CHOSEN GROUND MOTION RECORDS OF SPECIFIC
EARTHQUAKES
The data selected in this research are all shallow earthquakes in active tectonic
areas (Shallow earthquakes are between 0 and 70 km deep) with a moment
magnitude (Mw) in the range of (4-9) and all types of fault mechanisms were
considered. Two soil types were chosen for this chapter with Vs30 (soil shear velocity)
values of (100-300) m/s for the first type (soft soil) and (600-800) m/s for the second
type (soft rocks). Those ranges were taken because most of the shear velocity values
for the Iraqi soils stand in those ranges. And for every type of soil NFGM and FFGM
records are considered. For the NFGM distance of < 15 km and the events had to
have a noticeable peak-like record to be considered whereas for the FFGM distance
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of >50 km with no apparent peak was taken and this is to have a better comparison
between the two ground motions.
By applying the conditions mentioned previously on the PEER search page four
different groups of earthquake records are found, and the criteria for those groups as
well as the number of events taken are listed in Table 1. To avoid the control of
earthquakes with a large number of station records and satisfy the independent
conditions, only one station was chosen for each earthquake event.
These earthquake event characteristics for groups 1 to 4 are listed in Tables 2 to 5
respectively. Groups 1 and 2 are built for Vs30 ranging from 100-300 m/s for both
NFGM and FFGM. While groups 3 and 4 are built for Vs30 ranging from 600-800 m/s
for both NFGM and FFGM. The fault type in all four groups was irrelevant to this
research and that’s why it was not specified.
Table 1. Ground motion groups’ criteria
Group Magnitude Vs30 GM
No. of
records
1
4-9
100-300
NFGM
15
2
FFGM
11
3
600-800
NFGM
20
4
FFGM
10
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Table 2. the first group’s earthquakes events
NO. Earthquake event Year Station Magnitude Rrup Vs30
1 "Central Calif-02" 1960 "Hollister City Hall" 5.00 09.02 198.77
2
"Managua_
Nicaragua-02"
1972 "Managua_ ESSO" 5.20 04.98 288.77
3 "Hollister-03" 1974 "Hollister City Hall" 5.14 09.39 198.77
4 "Mammoth Lakes-09" 1980 "Hot Creek (HCF)" 4.85 12.01 295.93
5 "Westmorland" 1981
5.90 06.50 193.67
6 "Coalinga-02" 1983 "SUB (temp)" 5.09 12.31 270.41
7 "Imperial Valley-06" 1979
6.53 07.31 192.05
8 "Loma Prieta" 1989 "Gilroy Array #2" 6.93 11.07 270.84
9"Chi-Chi_ Taiwan" 1999 "CHY101" 7.62 09.94 258.89
10 "Duzce_ Turkey" 1999 "Bolu" 7.14 12.04 293.57
11 "Tottori_ Japan" 2000 "TTR008" 6.61 06.88 139.21
12 "Parkfield-02_ CA" 2004
6.00 03.79 260.63
13
"Christchurch_ New
Zealand"
2011
6.20 05.13 141.00
14
"El Mayor-Cucapah_
Mexico"
2010
7.20 11.26 196.88
15
"Darfield_ New
Zealand"
2010 "TPLC" 7.00 06.11 249.28
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Table 3. the second group’s earthquakes events
NO. Earthquake Year Station Magnitude Rrup Vs30
1 "Yorba Linda" 2002 "Calstate Bakersfield" 4.265 199.89 275.00
2 "Parkfield-02_ CA" 2004
"Milpitas Fire Station
4"
6.000 195.59 263.76
3"El Alamo" 1956 "El Centro Array #9" 6.800 121.70 213.44
4 "Borrego Mtn" 1968 "LB - Terminal Island" 6.630 199.84 217.92
5 "Tabas_ Iran" 1978 "Kashmar" 7.350 194.55 280.26
6 "San Fernando" 1971
"Bakersfield - Harvey
Aud"
6.610 113.02 241.41
7 "Landers" 1992
"Anaheim - W Ball
Rd"
7.280 144.90 269.29
8"Duzce_ Turkey" 1999 "Bursa Tofas" 7.140 166.07 289.69
9 "Hector Mine" 1999 "Newhall - Fire Sta" 7.130 198.13 269.14
10
"Chi-Chi_
Taiwan-03"
1999 "KAU066" 6.200 123.57 214.97
11
"Chi-Chi_
Taiwan-04"
1999 "KAU015" 6.200 109.50 233.21
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Table 4. the third group’s earthquake events
No. Earthquake Year Station Magnitude Rrup Vs30
1 "Morgan Hill" 1984 "Gilroy Array #6" 6.19 09.87 663.31
2 "L'Aquila_ Italy" 2009 "L'Aquila - Parking" 6.30 05.38 717.00
3 "10319993" 2008 "Hector" 4.14 06.41 726.00
4 "21522424" 2006 "Anderson Dam" 4.30 13.62 600.00
5
"Umbria Marche
Italy"
1997
"Nocera Umbra-
Salmata"
5.50 12.45 694.00
6 "Fruili_ Italy-03" 1976 "Tarcento" 5.50 06.30 629.08
7
"Chi-Chi_
Taiwan-02"
1999 "TCU089" 5.90 12.02 671.52
8 "Gilroy" 2002 "Gilroy Array #6" 4.90 14.39 663.31
9 "San Juan Bautista" 1998
"Hollister - SAGO
Vault"
5.17 07.04 621.20
10 "14239764" 2006
"Joshua Ridge:
China Lake"
4.02 13.75 623.00
11 "30226086" 2003
"Geyserville; Warm
Springs Dam;
Downstream"
4.00 08.38 760.00
12 "14282008" 2007
"Joshua Ridge:
China Lake"
4.11 06.72 623.00
13 "21455182" 2005 "Atlas Peak" 4.14 12.85 652.29
14 "Sierra Madre" 1991
"Mt Wilson - CIT
Seis Sta"
5.61 10.36 680.37
15 "Oroville-01" 1975
"Oroville
Seismograph
Station"
5.89 07.99 680.37
16 "Coyote Lake" 1979 "Gilroy Array #6" 5.74 03.11 663.31
17
"Anza (Horse
Canyon)-01"
1980
"Anza - Terwilliger
Valley"
5.19 12.28 617.78
18
"Mammoth
Lakes-09"
1980
"USC McGee
Creek"
4.85 09.18 653.56
19 "Coalinga-07" 1983
"Sulphur Baths
(temp)"
5.21 12.11 617.43
20 "Hollister-04" 1986
"SAGO South -
Surface"
5.45 12.32 608.67
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Table 5. the fourth group’s earthquake events.
ELASTIC RESPONSE OF NFGM AND FFGM
Elastic Seismic responses for the NFGM and FFGM are computed by scaling the
chosen records in the four groups mentioned in the tables previously to a similar PGA
(peak ground acceleration) which is chosen to be 0.3g then after processing all the
data, the response spectra are then determined using Prism software as mentioned
before. An average value of the SDOF response results of the spectral acceleration,
spectral velocity, and spectral displacement ground motions for the earthquake events
is done. Finally, the responses spectra are graphed using excel to better illustrate the
results and to show the difference between NFGM and FFGM in acceleration, velocity,
and displacement in elastic conditions.
Fig. 3 shows the differences in the Far-field ground motion and the near-fault
ground motion for the soft soils (groups one and two). While Fig. 4 shows the
difference in the Far-field ground motion and the near-fault ground motion for the soft
rocks (groups three and four).
As illustrated above in Fig. 3 we can see that the general response of the near-
fault ground motion in all three figures is much higher than the responses of far-field
ground motion. Both the start of NFGM and FFGM are approximately equal in the
short period where T is less than 0.3 sec. then, they start to depart from each other as
the NFGM response starts to escalate until the end of the 4 sec period. It’s obvious
that the NFGM has the higher response on the structure with max NFGM
acceleration, velocity, and displacement values being 18%, 182%, and 265% higher
than the responses of FFGM for soft soils. As for the values of acceleration, velocity,
and displacement of NFGM are 7%, 200%, and 501% higher than for FFGM for rocks
as in fig. 4.
No. Earthquake Year Station Magnitude Rrup Vs30
1
"Chi-Chi_
Taiwan-05"
1999 "TAP081" 6.20 155.66 671.52
2 "San Simeon_ CA" 2003
"Frazier Park - Post
Office"
6.52 186.24 643.91
3
"El Mayor-
Cucapah_ Mexico"
2010
"Silent Valley -
Poppet Flat"
7.20 167.65 659.09
4 "Niigata_ Japan" 2004 "SIT012" 6.63 156.93 710.53
5
"Darfield_ New
Zealand"
2010 "ODZ" 7.00 180.55 638.39
6
"Christchurch_ New
Zealand"
2011 "RDCS" 6.20 172.19 628.04
7 "Parkfield-02_ CA" 2004
"Saint Joseph's
Hill"
6.00 181.51 690.97
8
"Chi-Chi_
Taiwan-02"
1999 "TTN025" 5.90 106.03 704.96
9 "Molise-01_ Italy" 2002 "Norcia" 5.70 183.86 678
10 "L'Aquila_ Italy" 2009 "Cassino" 5.40 116.68 630
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Figure 3. Comparison between NFGM and FFGM elastic response spectrum for the soft soils
a) Acceleration response, b) Velocity response, and c) Displacement response
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Figure 4. Comparison between NFGM and FFGM elastic response spectrum for the rock soil
a) Acceleration response, b) Velocity response, and c) Displacement response
THE EFFECT OF SOIL TYPE ON SDOF ELASTIC SEISMIC
RESPONSE
For the last two decades, the studies of numerous travelling wave solutions to the
nonlinear development equations have appealed the attentions of many scientist from
all over the world. Nonlinear evolution equations (NLEEs) are used in explaining
several complex phenomena that ascend on daily basis in the various fields of
nonlinear sciences, such as, quantum mechanics, plasmas physics, earthquake
waves and so on [9].
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Seismic waves can cause shaking which can result in damage or failure of the
structures. There are many characteristics of free field motion that can be
considerably modified by the Local soil deposits such as the amplitude, duration, and
frequency content. The nonlinear response of the site is displayed during the
transmittal of high-intensity ground motion waves through the horizontal soil layers
[10].
The records of acceleration in the near-field region are attained during earthquakes
at somewhat short distances from the desired site and records in the far-field regions
occurring far from the site demonstrated the huge influence of geotechnical site
conditions such as properties of soil layers and soil stratification on strong motion
characteristics at the ground surface.
In the near-field zones, the soil characteristics are very dominant and affect the
directional properties of the earthquake GM. So the forward-directivity ground motions
will enforce high deformation and high energy demands on structures.
As we can see in Fig.5, we can notice that the soft soil responses are much higher
than the responses of the rock because the ground motion gets amplified much more
in the regions where the soils are soft thus resulting in a higher structural response.
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Figure 5. comparison between soft soils and rocks elastic response spectrum for NFGM in a)
Acceleration response, b) Velocity response, and c) Displacement response.
CONCLUSIONS
from all the previous sections of the paper the following points are concluded:
1)
The overall response of the near-fault ground motion in all three parameters
(acceleration, velocity, displacement) is much higher than the responses of far-field
ground motion in all conditions.
2) The start of NFGM and FFGM response graphs are roughly equal in the short
period where T is less than 0.3 sec. after that, they begin to depart from each other
as the NFGM response jumps to escalate until the end of the 4 sec period.
3) For both soft soils and rocks the NFGM responses are higher than the FFGM
responses and the percentages of these differences are 18%, 182%, and 265%
(acceleration, velocity, and displacement) higher for soft soils. As for the values of
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acceleration, velocity, and displacement of NFGM are 7%, 200%, and 501% higher
than for FFGM for rocks.
4) In the near-field regions, the soil characteristics are extremely governing and affect
the directional properties of the earthquake GM.
5)
The soft soil responses are more advanced than the responses of the rock
because the ground motion becomes amplified further in the regions where the
soils are soft thus causing a higher structural response.
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