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ORIGINAL PAPER
Spatial variation of b-value, creep rate, and seismic moment release
along Chaman fault system
Junaid Ahmed
1
&Farhan Javed
1
&Waqar Ali Zafar
1
&Talat Iqbal
1
&Muhammad Ali Shah
1
Received: 26 August 2020 / Accepted: 15 July 2021
#Saudi Society for Geosciences 2021
Abstract
The current study analyses the earthquake catalog of Southern Pakistan from 1973 to 2016. The magnitude of completeness (M
c
)
is 4.9. The b-value and seismic moment release of the Chaman fault system have been calculated and compared with slip rates.
The results demonstrate that both seismic moment releases and b-value estimation are consistent with the geodetic slip rates
inferred from the InSAR (i.e., lower moment release and higher b-value were estimated on the creep section of the Chaman fault
system). The b-value is estimated approximately 1.2 in the northern (i.e., >200 km) and southern (i.e., between 25 and 100 km)
portions of Ghazaband fault, which likely depicts the partially creep sections and could not generate large earthquakes. Similarly,
the southernOrnach-Nal fault section between 25 and 100 km shows higher b-value (> 1.5), which reflects the creeping nature of
the fault. b-value are varied from 0.65 to 1.0 in the region where historic large earthquakes occurred, whereas the b-value is
estimated 1.68 for the partial creep section between 110 and 140 kmand 2.0 for the creeping section between 210 and 310 km of
the Chaman fault. Moreover, b-value of 0.7–1.0 has been found for the central section of both Ornach-Nal and Ghazaband faults.
Results reveal that Chaman fault system is likely to host large earthquakes on the section of the faults, which estimate b-value in
the range 0.65–1.1 and moderate size earthquakes where b-value is estimated between 1.1 and 1.5. Moreover, faults, which
estimate b-value >1.5, are most likely the safe regions from moderate to large earthquakes. In short, estimated b-value clearly
depicts the mechanical properties of fault, so that combination of mechanics of fault from geodetic studies and estimated b-value
from homogenized earthquake catalog improve the earthquake forecasting models.
Keywords Earthquake catalog .Magnitude of completeness (M
c
).b-value .Seismic moment release .Chaman fault system .
Z-map
Introduction
The potential of occurrence of future earthquakes on ac-
tive fault systems can be evaluated through the combina-
tion of analyzing the spatial variation of b-values and
moment release. These parameters have been computed
from the earthquake catalog, which contains the informa-
tion about spatial distribution of small to large events and
their magnitudes. The frequency of earthquake distribu-
tion, in most instances, is characterized by power law
(Gutenberg and Richter 1944;Ishimoto1939). The slope
of the power law is commonly known as b-value, which
defines the relative occurrence of large and small earth-
quakes. The b-value may vary from 0.6 to 1.6 as sug-
gested by Chan and Chandler (2001) for the global seis-
micity. A low b-value shows a small portion of large
earthquake (Schorlemmer et al. 2005;Schurretal.
2014), whereas higher values relate to either creeping
(Enescu and Ito 2003; Ghosh et al. 2008;Legrandetal.
2012;Vorobievaetal.2016) or presence of fluids like
waterormagma(Francis1968;Hill1977;McNutt1986;
Wiemer and McNutt 1997; Legrand et al. 2004,2011;
Kundu et al. 2012). b-values have been also suggested
to indicate as a stress sensor, with low b-values depicting
high stresses (Schorlemmer et al. 2005;Schurretal.
2014). A variation in b-values has been observed in the
source region before the occurrence of both Iquique earth-
quake (Schurr et al. 2014;GuliaandWiemer2019)and
Responsible editor: Longjun Dong
*Junaid Ahmed
junaid.ahmed@ncp.edu.pk
1
Centre for Earthquake Studies, National Centre for Physics (NCP),
Islamabad, Pakistan
Arabian Journal of Geosciences (2021) 14:1623
https://doi.org/10.1007/s12517-021-08032-z
2011 Tohoku earthquake (Nanjo et al. 2012). Similar tem-
poralandspatialvariationsofb-values have been reported
in other case studies (El-Isa and Eaton 2014;Tormann
et al. 2015;Rigoetal.2018).
The Chaman fault system is one of the longest strike
slip fault systems in the world, extending from the
shore of the Makran subduction zone through the
Southern Pakistan to the Afghanistan. It is the result
of the relative motion between the Indian and Eurasian
plates. The Indian plate moves at 29–36 mm/year rela-
tive to the Eurasian plate near 30°N (Argus et al. 2011).
Geological studies suggested that the Chaman fault sys-
tem accommodates most of the relative plate motion
(Lawrence and Yeats 1979;Jadoonetal.1994;Ul-
Hadi et al. 2013). Moreover, present-day deformation
is not restricted along any single fault, but it is distrib-
uted over Chaman, Ghazaband, Hoshab, and Ornach-Nal
fault (Thatcher 2009; Molnar and Dayem 2010;Szeliga
et al. 2009; Szeliga et al. 2012; Fattahi and Amelung
2016,2015; Barnhart 2017). Large historical earth-
quakes with magnitude M7+ have been reported on
Ghazaband (Gupta and Singh 1980;Ambraseysand
Bilham 2003;Szeligaetal.2009)andHoshabfaults
(Avouac et al. 2014; Jolivet et al. 2014), whereas few
events of magnitude M6.5+ have been observed on
locked portion of the Chaman fault (Bilham et al.
2017; Barnhart 2017). It is also noted that no large
earthquakes have been documented on the Ornach-Nal
fault to date (Bilham et al. 2017). Recent geodetic stud-
ies including campaign GPS surveys (Szeliga et al.
2012) and InSAR (Fattahi and Amelung 2016;
Barnhart 2017) marked a heterogeneous distribution of
shallow fault creep and interseismic locking along the
entire length of Chaman fault. Barnhart (2017)marked
through InSAR studies that a ~95-km-long segment of
theChamanfaultfrom~30.7to~31.5°Nislocked.The
creeping nature of the fault is resumed at north of
31.5°N, as well as in the south of 30.7°N (Barnhart
2017). This south creeping section is also known as
Nushkai creeping segment (Barnhart 2017). The 95-
km-long locked segment, estimated fault slip rates of
8.5 mm/year and a locking depth of 3.4 km (Szeliga
et al. 2012; Barnhart 2017), has already caused an
earthquake with a magnitude of M 6.5 in 1892
(Griesbach 1893). For the Ghazaband fault, below with
a locking depth of 10.6-km strain accumulation rate is
16 mm/year (Fattahi and Amelung 2016).
The aim of this study is the computation of b-value variation
and seismic moment release along major active faults of the
Chaman fault system. For this purpose, we analyze homoge-
nized earthquake catalog of Southern Pakistan and compare the
results with mechanical properties of faults in order to evaluate
the potential of future earthquakes on Chaman fault system.
Data and methodology
Earthquake catalog
Earthquake catalog provides the first-order information about
the dominant focal mechanism as well as the quiescence and
seismic activation of the region. Earthquake catalog contain
all types of events such as induced seismicity due to hydro-
carbon extraction, volcanic activity, and small magnitude
man-made events due to mining activity in the region of active
fault system (Ma et al. 2018 and 2019). For Southern Pakistan,
earthquake catalog spanning from 1973 to 2016 is explored in
this study. A reliable and good database in a region is required
to analyze the behavior of seismic activity along the active
tectonic zone. For this purpose, a homogenized catalog has
been used, which was prepared mainly by combining the cat-
alogs from the US Geological Survey (USGS), International
Seismological Centre (ISC), Pakistan Metrological
Department (PMD), and local network (Khan et al. 2018).
That catalog is homogenized in terms of moment magnitude
(M
w
) and reports 2783 events which are bounded by the geo-
graphical limits 25–35°N and 64–70°E. The final catalog con-
tains only events which are related to tectonic activity along
the Chaman fault system. The seismotectonic map of
Southern Pakistan as shown in Fig.1gives a detailed descrip-
tion of the seismicity in the area. As evident from the figure,
the seismicity is concentrated along the major faults like
northern portion of the Chaman fault, Ghazaband fault,
Ornach-Nal fault, and Pab fault (Kazmi and Jan 1997). The
two most prominent earthquakes that are shown in the seis-
micity map of Southern Pakistan are the Quetta earthquake
1935 of M
w
7.7 and Awaran earthquake 2013 of M
w
7.7 that
occurred on Chaman fault system in the Baluchistan region of
Pakistan (Avouac et al. 2014; Jolivet et al. 2014; Barnhart
et al. 2015). There are a total of four events with magnitudes
M
w
7.0 reported along the Chaman fault system.
Magnitude completeness (M
c
)andb-value
In order to analyze the seismicity changes along the Chaman
fault system, magnitude completeness (M
c
)andb-valuesare
estimated. Both of them are important parameters for evalua-
tion of the seismicity parameters in a particular region. The M
c
is the minimum magnitude in a particular earthquake catalog
above which all of the recorded events can be considered as
completelyrecorded within a specificregion. To guarantee the
fairness of the results, the events are studied only with mag-
nitude (M
w
) equal to or greater than the magnitude of com-
pleteness. M
c
calculated in this study is 4.9, whereas the
b-value is found to be 1.12 with an uncertainty of
±0.04. The M
c
value of the whole catalog along with
b-value is demonstrated in Fig. 2a.
1623 Page 2 of 9 Arab J Geosci (2021) 14:1623
The calculation of an accurate b-value is very critical
for hazard analysis and physical understanding of active
fault system. In a given area, the b-value is a measure
of the relative number of small to large earthquakes that
occur in a given period of time (Farrell et al. 2009).
The slope of the frequency magnitude distribution
(FMD) is the b-value of the region (Gutenberg and
Richter 1956; Lee and Stewart 1981)foragiven
population of earthquakes. The time series and the
FMD of the earthquake catalog is shown in Fig. 2b.
Various studies have revealed that the b-value also
changes with applied stresses (Scholz 1968;Wyssand
Lee 1973; Urbancic et al. 1992; Wiemer and Wyss
2000; Wiemer and Wyss 2002; Schorlemmer et al.
2004; Schorlemmer et al. 2005), thermal gradient
(Warren and Latham 1970), and material heterogeneity
Fig. 1 Seismotectonics map of
the Southern Pakistan, where
black lines indicate the main
tectonic features in the region.
Rectangle and ellipses show the
fault area of the historical and
instrumental large earthquakes,
respectively. Earthquake catalog
from 1973 to 2016 has been
shown by colored circles
according to magnitude (Khan
et al. 2018). The faults shown on
map are after Kazmi and
Jan 2011. Large earthquakes with
magnitude M7+ have been shown
by a yellow star.
Fig. 2 aMagnitude completeness was calculated using maximum likelihood method and found that M
c
equals to 4.9 with b-value of 1.12+/−0.04 for
earthquake catalog. bThe cumulative number of events from the catalog selected for instrumental period only, i.e., from 1973 to 2016.
Arab J Geosci (2021) 14:1623 Page 3 of 9 1623
(Mogi 1962). The b-value in tectonic areas is approxi-
mately 1.0 (Frohlich and Davis 1993). In comparison,
the areas having volcanic activities are identified by b-
values in the range of 1.0–3.0 (McNutt 2005).
During the earthquake cycle of some major earthquakes,
the decrease in the b-value has been reported for interseismic
periods (Scholz 1968;Gibowicz1973; Wyss and Lee 1973;
Fiedler 1974;Smith1981;ShiandBolt1982; Imoto and
Ishiguro 1986;Smith1986;Mainetal.1989;Hirataand
Imoto 1991;Urbancicetal.1992). The tectonic uniqueness
of a region also effects the variation in the b-value. The normal
faulting exhibits highest b-values, whereas reverse faulting
tends to lower side of calculating results (Amelung and King
1997; Nanjo et al. 2010; El-Isa and Eaton 2014). Along the
major faults, it is expected that b-value will be varied due to
the different degree of seismic coupling along the fault
(Powers and Jordan 2010).
In the present study, the maximum-likelihood method is
used for the calculation of b-value (Aki 1965; Utsu 1966;
Hirata 1989). By this method, the b-value is calculated by
the formula:
b¼log10e1
M−Mmin
ð1Þ
where Mand M
min
is the average and cutoff magnitude
respectively.
The seismic moment (M
0
) release was calculated by the
formula:
M0¼10 1:5Magnitudeþ16:1ðÞ ð2Þ
In order to compare the variation of b-value, seismic mo-
ment release, and creep rate along the major faults of the
Chaman fault system, we follow the following steps:
1) We subdivide the Chaman fault into several sub-
sections based on the known mechanism (i.e.,
locked, creep, or partial creep), which inferred from
the InSAR results (Barnhart et al. 2016; Fattahi
et al.2016). For sensitivity analysis on the estimated
b-value, we subdivide the Ghazaband and Ornach-
Nal faults into sub-faults with the length of 25 km
and 50 km.
2) Every event has been relocated to the nearest sub-fault by
computing the distance between the fault and the event.
3) b-value has been computed from original instrumental
catalog spanning from 1973 to 2016, whereas the histor-
ical earthquakes that occurred between 1765 and 1973
hasbeenusedinthecalculationofseismicmoment
release.
4) We then set up the criteria, by which the b-value
will be estimated on the segment of the fault, which
keep M
c
≥20 events.
Results and discussion
Analysis of magnitude’s completeness M
C
,b-value, and seis-
mic moment release from the past earthquakes in Southern
Pakistan depict the seismic potential on the major faults like
Ghazaband fault, Chaman fault, and Ornach-Nal fault. It also
has the implication in the seismic hazard assessment. Variation
in the b-value is one of the key components of any seismic
hazard map. Being a prerequisite of seismic hazard analysis, a
reliable earthquake catalog is an important ingredient. The com-
puted M
c
value is 4.9 in the study region, which is greater than
the values estimated for Northern Pakistan (Javed et al. 2016).
M
c
value for Northern Pakistan is usually varying from 3.7 to
4.0 (Parsons and Segou 2014; Javed et al. 2016). The computed
b-value is higher at several portions of the Chaman fault,
Southern Ghazaband, and couples of sections of Ornach-Nal
fault. b-value is estimated for both Ghazaband and Ornach-Nal
fault assuming 25-km (red circles) and 50-km (blue circles)
sub-fault segment. As shown in Fig. 3, along with the
Chaman fault, the b-value varies from 0.9 to 2.0. b-value varies
from 0.9 to 1.0 (red color) for the locked portion of the Chaman
fault, while it estimates 1.68 (green color) and 2.0 (blue color)
for both partial creep and fully creep section of the Chaman
fault, respectively. The computed b-value is highly variable in
Southern Pakistan. A lot of research work has previously
shown the spatiotemporal b-value variability; such variability
is related to the tectonic regimes of the region (Casado et al.
1995; Bayrak et al. 2002; Legrand et al. 2012; Nuannin et al.,
2005; Schorlemmer et al. 2005).Ithasalsobeendiscussedby
several authors (Amelung and King 1997;Moriand
Abercrombie 1997; Wiemer and Wyss 2000) that along fault
zones, the low b-values correspond with asperities, while high
b-values correspond to creeping sections of fault.
Fig. 3shows the comparison between the creep rate in-
ferred from the InSAR, b-values, and moment release. It is
shown that from above 230 km, b-value changes from 0.9 to
2.0 following the trend of the creep rate. But on the locked
portion of the Chaman fault, there is no report of earthquakes
that occurred on that portion since 1892. Similarly, along the
350-km-long profile, most of the seismic moment released at
75 km on which b-value becomes
approximately 0.96. At that portion, creep rate is also de-
creased to 2–3 mm/year. By combining all these evidence, b-
value clearly marks the physical characteristics (i.e., locking
or creeping) of the Chaman fault. The Chaman fault has ac-
commodated strain at a rate of 8 mm/year (Fattahi and
Amelung 2015; Fattahi and Amelung 2016). The seismic mo-
ment release along the Chaman fault is higher in the portion
where the creeping rate is low.The InSAR investigations have
shown that the central portion of the Chaman fault is locked.
The inference is supported by the seismic moment release as
well. It is also shown in Fig. 3that seismic moment release
during 1973–2016 did not filled the seismic gap in this portion
1623 Page 4 of 9 Arab J Geosci (2021) 14:1623
of the fault. Assuming the constant loading rate, the geodetic
moment accumulation on this segment of Chaman fault is
1.2e+26 dyne-cm, whereas the calculated seismic moment
release is 1.8e+25 dyne-cm; thereby, an accommodated mo-
ment is equivalent to the seismic moment released by M
w
6.6
earthquake.
Similarly, the b-values along the Ghazaband fault shows
relatively less value as compared to Chaman fault that endorses
the occurrence of large historic earthquakes (e.g., M
w
7.7, 1935;
M
w
7.4, 1935). Along the Ghazaband fault, the strain is accom-
modated at a rate of 16 mm/year (Fattahi and Amelung 2015;
Fattahi and Amelung 2016). On the central portion of the fault,
the maximum amount of the moment energy is released as
showninFig.4a–c. Along the Ghazaband fault, b-value varies
from 0.65 to 1.2. The variation of b-value manifests that lower
b-value coincides with the release of maximum moment ener-
gy, while the relatively higher b-value represents on the south-
ern portion of the fault that did not rupture with the past. The
higher b-values show that M6+ earthquakes on nearby active
faults decrease the stresses on these portions of the fault as
pointed out by Gulia et al. [2005]. Therefore, it illustrates that
northern and southern sections are either partially creep or not
close to failure in near future.
Along the southern portion of the Ghazaband fault, varia-
tion in b-value demonstrates that interseismic locking is vari-
ation with latitude, similar to Chaman fault. The potential of
the future earthquake on the southern part of the Ghazaband
fault is questionable and needs to be addressed. The geodetic
moment accumulation deduced from slip rate taken through
InSAR investigations along the Ghazaband fault is ~2.5e+26
dyne-cm, and calculatedseismic moment release is~1.06e+26
dyne-cm which could generate an earthquake of M
W
6.8 after
every 50 years.
The Ornach-Nal fault is the southern segment of the
Chaman fault system. Running nearly 250 km south of
the Ghazaband fault, the Ornach-Nal fault offsets primar-
ily Cenozoic mudstones and shale (Snead 1964;Lawrence
and Yeats 1979; Zaigham 1991). The Ornach-Nal fault is
interseismically deformed at a rate of 15.1 mm/year
(Szeliga et al. 2012). A locking depth of 2.9 km has been
estimated for this fault (Szeliga et al. 2012). The b-value
is estimated as 2.07, slightly higher for 50 km sub fault
sectionascomparedto1.72for25kmsubfaultsection
(Fig. 4b). The variation in b-value implies that Ornach-
Nal fault exhibits different degree of seismic coupling.
The higher b-value demonstrates that Ornach-Nal fault
Fig. 3 Comparison of seismic moment release and b-value with the creep rate along the Chaman fault and other tectonically active regions of Southern
Pakistan. Fig (3b) is taken from Fattahi and Amelung (2016).
Arab J Geosci (2021) 14:1623 Page 5 of 9 1623
between 50 and 110 km shows the partial creeping or
creeping properties of the fault. The northern portion of
the Ornach-Nal fault illustrates between 110 and 250 km
poses the possible seismic hazard in the region.
Assuming strain is accumulating at a rate of 15.1 mm/year
on the northern section of the fault and is locked to a depth of
2.9 km, the total geodetic moment accumulation is approxi-
mately equal to 1.0e+26 dyne-cm which could generate an
earthquake of M
W
~6.6. Historical earthquakes occurred in
the region where b-value is lower. It is noted that the
Chaman fault also reveals the same observations. The south-
ern portion between 0 and 50 km of this fault is
likely locked, and the maximum seismic moment was also
released on the same section as shown in Fig. 4d.Wefindthat
lower b-values on several sections of the Chaman fault system
are the indicator of the future impending moderate to large
earthquakes in the region. These results are quite consistent
with the finding of recent work by Gulia and Wiemer (2019)
that also demonstrated the decrease of b-value before the oc-
currence of Amatrice and Norcia mainshocks. We find that
Fig. 4 Comparison of moment
release and b-value along the a–c
Ghazaband fault and b–dOrnach-
Nal fault having an inverse corre-
lation at most of the sections
while moving from southwest to
northeast. b-value is estimated at
25 km (red circles) and 50 km
(blue circles) sub-fault segment.
1623 Page 6 of 9 Arab J Geosci (2021) 14:1623
there is a clear relationship between the mechanical properties
of the Chaman fault and variation in b-value, so that additional
information of mechanical nature of fault will reduce the un-
certainty of earthquake forecasting model. Moreover, we can
summarize the result using the following quantitative relation-
ships (Fig. 5).
Conclusions
In this study, the earthquake catalog spanning from 1973 to
2016 has been analyzed. The magnitude completeness (M
c
)
and b-value for earthquake catalog have been calculated. The
estimated magnitude of completeness is found to be 4.9. In
this investigation, the largest historical earthquakes have been
combined with earthquake catalog, and then the seismic mo-
ment release is estimated. It is observed that most of the seis-
mic moment released is on the portion of the fault where it is
interseismically locked. The result has also shown that the
lower b-value (i.e., 0.65–1.1) corresponds to the regions
where large earthquakes have been reported previously. The
inferred b-value estimated 1.68 and 2.0 for the creeping sec-
tion of the Chaman fault. The seismic moment release along
Chaman fault is 1.06e+26 dyne-cm which could generate an
earthquake of M
w
6.6 cyclically every 50 years. Along the
Ghazaband fault, the estimated b-value is varied from 0.65
to 1.2. The lower b-value is estimated at the central portion
of the fault. The slip rate of that fault is also quite high; so, this
is most likely the region of concern for the future large earth-
quakes. The northern and southern portions of the fault repre-
sent relatively higher b-value than central portion, which show
that these sections are mechanically different from central
portion of the fault. It demonstrates that these portions can
generate moderate size earthquakes or nearby faults decrease
the stresses on them, which result in slightly higher b-value.
The accumulated seismic moment is 2.5e+26 dyne-cm, which
after a cycle of 50 years would generate an earthquake of M
w
6.8. Similar results have been estimated for Ornach-Nal fault.
The lower b-value is estimated along that fault except the
portion between 25 and 100 km(i.e., b-value>1.5, which most
likely represents the creeping). In summary, it is demonstrated
that variation in b-value along Chaman fault system is well
consistent with the seismic moment release and InSAR de-
rived present-day slip rates. Our results capture the mechani-
cal properties (i.e., creeping, partially creeping, and locked) of
the fault. Our study highlights that variation of b-value, in
addition to mechanical properties of the fault, is a good indi-
cator for assessing the potential of future earthquakes. The
analysis presented in this work is only limited to tectonic
activity with M3.5+, and comprehensive analysis of small
events induced by other sources such as mining activity is
beyond the scope of this work.
Declarations
Conflict of interest The authors declare that they have no competing
interests.
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