Download Time-dependent analysis of aftershock events and structural impacts

Cent. Eur. J. Geosci. • 5(3) • 2013 • 423-434
DOI: 10.2478/s13533-012-0141-8
Central European Journal of Geosciences
Time-dependent analysis of aftershock events and
structural impacts on intraplate crustal seismicity of
the Van earthquake (Mw 7.1, 23 October 2011),
E-Anatolia
Research Article
Mustafa Toker1∗
1 Yuzuncuyıl University, Department of Geophysical Engineering, Van/Turkey
Received 2013 May 19; accepted 2013 July 31
Abstract: The Van earthquake (MW 7.1, 23 October 2011) in E-Anatolia is typical representative of intraplate earthquakes.
Its thrust focal character and aftershock seismicity pattern indicate the most prominent type of compound earthquakes due to its multifractal dynamic complexity and uneven compressional nature, ever seen all over Turkey.
Seismicity pattern of aftershocks appears to be invariably complex in its overall characteristics of aligned clustering events. The population and distribution of the aftershock events clearly exhibit spatial variability, clusteringdeclustering and intermittency, consistent with multifractal scaling. The sequential growth of events during time
scale shows multifractal behavior of seismicity in the focal zone. The results indicate that the extensive heterogeneity and time-dependent strength are considered to generate distinct aftershock events. These factors have
structural impacts on intraplate seismicity, suggesting multifractal and unstable nature of the Van event. Multifractal seismicity is controlled by complex evolution of crustal-scale faulting, mechanical heterogeneity and seismic
deformation anisotropy. Overall seismicity pattern of aftershocks provides the mechanism for strain softening
process to explain the principal thrusting event in the Van earthquake. Strain localization with fault weakening
controls the seismic characterization of Van earthquake and contributes to explain the anomalous occurrence of
aftershocks and intraplate nature of the Van earthquake.
Keywords: the van earthquake• intraplate • aftershock seismicity • multifractal behavior • strain softening
© Versita sp. z o.o.
1.
Introduction
The Van earthquake (Mw 7.1, 23 October 2011) occurred
in E-Turkey, typically "intraplate" (plate boundary related)
earthquake [1]](Fig. 1b). Its epicenter is located nearby
"volcanic intraplate environment", surrounded by active
∗
E-mail: [email protected]
volcanism [2, 3]. Thrust faulting process [4] and aftershock
seismicity pattern of the Van earthquake indicate the most
prominent type of compound-complex earthquakes [1] due
to its multifractal dynamic complexity and uneven compressional nature, ever seen all over Turkey.
The 23 October 2011 Van earthquake (Mw, 7.1) was followed by the 9 November 2011 Edremit (Mw, 5.6) earthquake (Fig. 1b), reported by Kandilli Observatory and
Earthquake Research Institute (KOERI). Hypocentral and
source parameters of these earthquakes estimated by dif-
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Time-dependent analysis of aftershock events and structural impacts on intraplate crustal seismicity of the Van earthquake
The 2011 Van earthquake is unique for its unusual focal
depth (below 5 km) with a peak slip of about 5.5 m, vertical
displacement of 60 cm, the total seismic moment (4.6x1019
Nm) [4] and volcanic intraplate location. The main rupture
zone striking N700E and its main cluster was confined to
a northeast -southwest area of 70 kmCE 20 km in map
view seen in Fig. 2, with about 80% primary slip concentrated at depths of 5-15 km. Several strong events
including magnitudes (Mw≥5.0) occurred along the mainshock area, between Lake Van and Lake Erçek (see LE in
Fig. 2). This area appears almost ellipsoidal, also including the causative fault. It is apparent that the 2011 Van
and Edremit events seen in Fig. 1b are caused by the
prevailing regional N-S compression [3]. Similarly, two
intermediate earthquakes with oblique thrust type having
moment magnitudes of 5.4 on 1988 June 25 and 5.3 on
2000 November 15 occurred in the south-eastern part of
Lake Van (Fig. 1a).
Figure 1.
a) recorded events (Mw 3.0-7.3 for the last 111 years) in
Lake Van and its vicinity and focal mechanisms of some
earthquakes recently occurred (KAN, USGS, EMSC) [43],
b) Epicentral distribution and focal mechanism solutions
of the October 2011 Van and the November 2011 Edremit
earthquakes and the main aftershocks (Mw¿5.0) by different institutions (EMSC), (KAN: B.U. Kandilli Observatory
and Earthquake Research Institute (KOERI); EMSC: European Middle East Seismology Center; AZUR: Nice University, GeoAzur Laboratory, France; GFZ: Geoforshung
Zentrum, Potsdam, Germany; ERD: Disaster Management and Emergency Presidency, Ankara Turkey; HARV:
Harvard CMT; INGV: Insituto Nazionale di Geofisicae Vulcanologia, Italy; USGS: United States Geological Survey
[43].
ferent organizations and seismological institutes are summarized in map view given in Fig. 1b.
The Van event is the largest thrust earthquake known to
have occurred in Van area and Turkey, since at least the
1976 Çaldıran-Muradiye event of Ms, 7.3 (Fig. 1a) [4, 5].
The 1976 event caused the uplifting (about 16 cm) of the
northern shore of Lake Van and was followed by several
Ms, 5.0 aftershocks in November 1976 and January 1977.
The 9 November Mw 5.6, 2011 Edremit earthquake (5-7
km depth) occurred offshore, in the south of Van along the
north dipping, Edremit fault, a normal oblique-strike-slip
fault (Fig. 1b). The epicentre locations and the source
mechanism solutions of these earthquakes indicate that
they occurred on different faults.
Aftershock data (Fig. 2) reveals that intraplate nature of
the Van earthquake can’t be explained with linearly uniform, elastic fracture mechanics, because the time delays
between the observed individual clusters in aftershock
seismicity are too long to result from elastic processes. It
is clear that the Van earthquake dynamically loads the
surrounding volcanic region. Such a compound earthquake can result from viscoelastic relaxation in the immediate postseismic period. This results in nonlinear and
heterogeneous redistribution of loads, such as volcanicmagmatic coupling within the accretionary crustal blocks.
Seismicity pattern of Van earthquake, as well seen in most
compound earthquakes [1], often has delay times of hours,
days and months (Fig. 1). In the case of delay times
of months, seismic coupling may be important, as shown
by the earthquake waveforms recorded in Van and high
seismic b-values [6–8]. The point is that Van earthquake
as a composite system has input parameters more than
one and strongly characterized by composite sequences
of event instabilities. These are well recognized in timedependent distribution of aftershock seismicity (Fig. 2).
Aftershocks seen in Fig. 2 associated with the 2011 Van
mainshock still continue to occur till today and provide
6500 events during October 2011-August 2012. This gives
a reliable dataset of aftershocks that enabled this study
to carry out a detailed investigation of events in the focal region of the 2011 Van earthquake. Aftershock patterns used in this study contain all of event features for
a given time scale (Fig. 2). These patterns can be examined for evidence of precursors and characteristic features
in overall seismicity [1, 9–11]. Aftershocks are the most
ubiquitous, observed to follow almost all shallow tectonic
earthquakes of any significant size [1]. They have the most
well-defined characteristics of any of the earthquake se-
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Figure 2.
Topographic map of Lake Van area and its vicinity showing major provinces, structural features and epicenter distribution of all events
(5304 events) between 2011.10.23 and 2012.03.28 (157 days) extracted from KOERI catalogs. An inset with Turkey shows KOERI
seismic network area in E-Anatolia and defines two stations (VANB and VANT) located in Van (VANB) and Edremit (VANT) areas.
The colored and dashed line shows Muž Suture (MS) zone, separating Bitlis Pötürge Massive in S from Eastern Anatolia Acretionary
Complex in N, (NV: Nemrut Volcano; SV: Suphan Volcano; PVD: Parasitic Volcanic Domes; CC: Collapsed Cones; ÇSZ: Çarpanak Spur
Zone; LE: Lake Erçek; F: Fault). Variation of the magnitude, depth and frequency of occurrence (foo) of 5516 events, as a function of
time (177 days) during the period from 2011.10.23 to 2012.04.17 within the focal zone. Note that the figure locations used are shown
as lines including their time intervals in the foo.
quences. In particular, the decay of aftershock sequences
follows the Omori law [12].
The long-time anomalous occurrence of the aftershock
events in Van region is typically related to very heterogeneous crustal activity, dynamic thrust faulting and strain
incompatibility problems [13, 14]. Crustal heterogeneity is
characterized by major irregularity in the event rate and
unstability of event propagation in aftershock seismicity.
This brings an idea that is intraplate seismicity of accretions. This kind of seismicity may give a possibility of
the extensive investigation of post-collisional [3] rheology
of accretionary complex of E-Anatolia and also the Van
earthquake. However, in this study, I briefly aim to show
some structural impacts on intraplate crustal seismicity of
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Time-dependent analysis of aftershock events and structural impacts on intraplate crustal seismicity of the Van earthquake
the Van earthquake by analyzing of available aftershock
data and to expect clear ideas to comment on this principal
thrust event.
2.
Seismicity of Lake Van basin
Lake Van basin indicates seismotectonic paradigm and extreme intraplate seismic deformation for crust-forming process in Turkic-type orogens [3]. The lake is subjected to
various seismic activities as given by historical and instrumental records (Fig. 1) [4, 5, 15–19]. Recorded events
with magnitudes (3.0≤M≤7.3) for time interval (19002011) are also given in Fig. 1a. These events include
the earthquakes with magnitudes M ≥5.0 [4], reflecting
results of the extensive compilation from the studies given
above.
In Lake Van basin, from 1970 to the 23 October 2011 Van
mainshock, the events with magnitudes 4.0≥M<5.0 are
reported [20, 15, 4] (Fig. 1a). The epicentre distribution
of these events, their source parameters and focal mechanisms (M ≤5.0) are extensively detailed and documented
[4]. The past strong events (M≤5.0) in the instrumental
period are reported to be a sequence that started in June
1900 and lasted several months [4, 21–23]. These events
are reported as to be felt strongly in the Van [4], regarding
the reported macroseismic effects of the previous studies
[21] (Fig. 1a).
The 1903 Malazgirt (MS 7.0) is one of the largest earthquakes that occurred in the study area in the instrumental period [4, 24, 25]. This earthquake had been strongly
felt across Van and Malazgirt area [26] The 1913 Tatvan
event was instrumentally located 27 at about 14 km south
of Tatvan area (Fig. 1a) and the 1915 (MS 5.6) Nemrut
event occurred NE of the Nemrut volcano (Fig. 1a) [4, 20].
The 1941 Erciž (MS 5.9-6.0) earthquake caused damage
in the N-NW of Lake Van (Fig. 1a) [4, 24, 25]. The most
destructive event located along the E-shore of the lake is
the 1945 Van earthquake sequence as the largest shock
occurred on 20 November (MS 5.5-5.8) [21, 22, 24, 25,
4]. The 1972 Gevaž and 1976 Van earthquakes (MS 5.0)
appear to be felt strongly at Van, Edremit and Gevaž [25]
and their focal solutions are also reported [4, 28].
The 1976 Çaldıran (MS 7.3) earthquake is the largest one
that occurred in the area (Fig. 1a). This large event
ruptured two-segmented fault zone about 40 km N-NW
of Lake Van with a 55 km-long [4, 24, 29]. The 1976
Çaldıran event was followed by several MS 5.0 aftershocks
in November 1976 and January 1977 [4]. After the 1988
Van earthquake in the lake [30] (Fig. 1a), the last recordable event occurred in the region on the November 15,
the 2000 Van earthquake [4, 31] (Fig. 1a). A small part
(35 s) of tremor-like signals was recorded on 13 and 14
November 2000, a few days before the Van earthquake of
15 November 2000, Mw 5.3. This event occurred at the
southern shore of Lake Van (Fig. 1a) [6]. The spectral
analysis of earthquake data from Lake Van provided three
types of seismic events, hybrid event, long period event
and tremor, nearby the mainshock area of the October 23
Van earthquake [6].
3.
Aftershock data and method
Earthquake data is provided by KOERI catalogs, prepared by the National Earthquake Monitoring Center
of Turkey that has a gradual increase of the number of stations operating in E-Anatolia since 1970 (see
Fig. 2). Aftershock data used is recorded at VANB
(38.595◦ N, 43.388◦ E) broadband station of KOERI seismic
network (Fig. 2). The aftershock activity including recent
large event in Van area, E-Anatolia (38.10◦ N-39.20◦ N,
41.90◦ E-44.03◦ E) was monitored by 19 stations; 14 shortperiods, 5 broadband seismometers for the time period between October, 2011 and August, 2012 (Fig. 2). Due to
the single-active broadband station located at the Van city
(VANB), localization errors of aftershocks are considered
to be about ± 4-8 km, as reported by KOERI.
To show structural nature of the Van earthquake and its
aftershocks in E-Anatolia, I used earthquake data set from
KOERI [32] and seismic reflection profiles collected in
Lake Van (ICDP-2004, PaleoVan project) [33–35]. I illustrate the aftershock clustering patterns and event rates
in the focal zones of the Van (Mw 7.1) and Edremit (Mw
5.6) earthquakes [4]. I used the hypocenter depths and
seismic density patterns collected from the seismograms
of 6500 aftershocks. I present here a summary of timedependent aftershock observations that have been made
in the clustering earthquake areas of Lake Van. This focuses on characteristic sub-sequences for the origin and
generation of the long-time occurrence of the repeated
events. It is observed that aftershocks spatially extend
over the focal zone and its vicinity larger than the entire
rupture zone of the earthquake (Fig. 2).
3.1. The scaling process and seismicity parameters
Anomalous effects of aftershocks recorded that persist
for periods of a few months after the Van earthquake
are briefly reviewed with selected examples in this study
(Figs. 2-7). The scaling process of aftershock pattern as
a function of time includes a composite data set. This
data is composed of a wide range of punctual events with
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Figure 3.
Variation of the magnitude, depth and frequency of occurrence (foo) of 321 events, as a function of time (1,25 days or 31 hours) during the
period from 2011.10.23 to 2011.10.24 within the focal zone (see the foo in Fig. 1 for time interval of this figure). The Van earthquake (Mw
7.2) and the following aftershocks (Mw 5.7 and 4.8) are also shown by plot of magnitude variation, recorded in 23.10.2011. Topographic
map shows epicenter distribution of 321 events, trending northeast-southwest into the lake basin.
The main approach of this research deals with detection
of clusters (C) and their temporal variation by examining
large groups of aftershocks statistically for achieving highquality results with various key intervals (see Fig. 2 for
intervals) to show the event clusters that shift to local
clusters.
of events (foo) (Fig. 4). These intervals are well enough
to illustrate dynamic fluctuation and propagation of clustering events and strong local interactions for short time
period. The 21th, 38th and 75th days for long-range activity are particularly taken into account to show low resolution pattern of considerable and irregular drop of events
(foo), firstly below about 100 (Fig. 5), continuous sharp
drop below 50 (Fig. 6) and slight drop below 30 (Fig. 7).
The continual drop of events remains almost stable below
50 for the rest of time (Fig. 2). Hence, these key intervals appear to be enough to illustrate long-range dynamic
fluctuation, propagation of clustering events, and particularly the event migration and/or shifting to a series of
local clusters for long time period.
The first three days with 683 events for short-range activity are considered to show high resolution pattern of
the first day maximum (Fig. 3) and initial sudden drop
In sequential analysis of aftershock data, the behavior of
a sequence of aftershocks is well classified by detecting
the event variations of the interevent times (∆t), given
available information between them. Seismicity parameters used are compiled to show variation and correlative
relation in magnitude, focal depth, the frequency of occurrence (foo) and the clustering events in the focal area
(Fig. 2). These parameters well illustrate time-dependent
evolution and propagation of aftershock events. This can
exhibit an overview of seismic property on a wide range
of scales in and around the mainshock.
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Time-dependent analysis of aftershock events and structural impacts on intraplate crustal seismicity of the Van earthquake
Figure 4.
Variation of the magnitude, depth and frequency of occurrence (foo) of 683 events, as a function of time (3 days) during the period
from 2011.10.23 to 2011.10.26 within the focal zone (see the foo in Fig. 1 for time interval of this figure). Clusters are shown by C.
Topographic map shows epicenter distribution of 683 events, trending northeast-southwest and clustering in the lake basin. Note that
uneven peaks (Clusters, C) at the foo of events form very distinct series.
Figure 5.
Variation of the magnitude, depth and frequency of occurrence (foo) of 2553 events, as a function of time (21 days) during the period
from 2011.10.23 to 2011.11.13 within the focal zone (see the foo in Fig. 1 for time interval of this figure). Clusters are shown by C.
Topographic map shows epicenter distribution of 2553 events, trending northeast and clustering in northeastern end of the lake. Note
that the last 49 events occurred within 6 hours resulted in a prominent cluster (C) at the foo. The epicenters of these 49 events are
numbered from 1 to 49, shown in the map by curved line (see arrow).
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by average values of the interevent times (Cv). If Cv is
larger than 1, the event distribution is referred to as clustered in time, known as the temporal behavior of cluster
(C) [36]. In this study, it is considered that analysis of
the event time intervals is more appropriate for detecting
and quantifying temporal clustering. Detailed characterizations of the temporal statistics of clusters are given by
specifying the distribution of the time intervals between
all events with magnitude bigger than magnitude cutoff
(Mc). The variations in minimum magnitude for the entire KOERI catalog roughly range between 1.4 and 2.3,
simply giving Mc ≈ 2.5 for this study [37]. This value
represents the threshold of events over which a temporal
cluster is defined in analysis of aftershocks. Thus, clusters
of repeating events with peaked statistics are interpreted
to understand fault-related heterogeneity. Previous studies also found that small asperities generated clusters of
highly repeating small earthquakes with peaked statistics
centered on events (M ≈ 1.0) [36, 38].
In the interpretation of aftershock data, I used an approach of the range of size scales (ROSS). Since, it is the
key physical parameter in the effects of heterogeneities
(fault instabilities and asperities) on earthquake dynamics [36, 39]. The ROSS also plays the significant role of
a tuning parameter [40, 41]. It is reported that extrapolations of statistics based on low-magnitude seismicity
to behavior of large events are valid only for disordered
systems with a wide ROSS [36, 40, 41]. Model realizations with heterogeneities are also characterized by a
wide ROSS, representing strong geometrical disorder and
disordered immature fault zones [36, 39]. Such model realizations can produce frequency-size statistics following
the Gutenberg-Richter relation over the entire temporal
statistical range of events and clustered (or random) temporal distribution of large aftershock events (see Fig. 2).
Moreover, small events can follow in all cases power law
frequency-size distribution and are also clustered in time
[36, 39] (Figs. 3 and 4). In this study, the effective analysis and interpretation of aftershocks with a wide ROSS
approach well provides a bridge between strongly disordered individual fault zones and broad regions with a
diverse population of faults and/or asperities [36, 39].
4. Time-dependent seismicity analysis of aftershock events
Time period characteristics (magnitude-depth-frequency
of occurrence) and seismic properties of aftershock sequences of the Van earthquake are given in Figs. 2-7.
Figs. 3 and 4 show magnitude and depth configuration
of the repeated 321 events occurred in the first 31 hours
(Fig. 3) and 683 events occurred in the first 3 days (Fig.
4). Frequency of occurrence (foo) is clearly irregular and
continually shifting to local clusters (C) during time scale
(Figs. 3 and 4). Configurational pattern of aftershock
events seen in Figs. 3 and 4 is systematically continued
and well observed in different time windows.
Fig. 5 illustrates magnitude, depth and the foo variation
of 2553 events for 21 days, showing strong and chaotic
pattern of aligned clusters (C). The last 49 events occurred
within 6 hours resulted in the cluster seen in Fig. 5.
Irregular variation in magnitude, depth and foo is also
distinct for 38 days (Fig. 6). The event rate in the clusters
considerably lowers below 20 events in the foo. As seen
in Fig. 7, total 4147 events for 75 days are recorded
during time scale, showing irregular peaks at events and
local clusters (C). This shows that the foo continually shifts
to a series of local clusters. It is clear that aftershocks
with magnitudes (Mw ≥ 4.0) frequently occurred for the
first 3 days (Figs. 3 and 4) and that they continued to
occur for 21 days (Fig. 5). This produced the second
large earthquake (Mw 5.6, the 9 November 2011, Edremit
earthquake, southern part of the mainshock area) (see the
interval between 16th days and 18th days in Fig. 5).
The events with magnitude, larger than Mw 4.0, are also
distinctly recorded for 38 days (Fig. 6) and 75 days (Fig.
7).
It is observed from data that the foo shifts to local clusters (Fig. 3). This shifting produces a distinct seismicity
pattern, characterized by propagation and fluctuation of
clustering events for a given time scale (Fig. 4). This
seismicity pattern appears to be densely covered by a series of clusters by sudden increase in the event rate (foo)
and magnitude (Fig. 5). The event rate (foo) suddenly
decreases on 25th days and remains unchanged (below
20) (Fig. 6). In Fig. 7, irregular pattern of clusters with
peak at about 20 events, from 25th days to 75th days, is
also observed. This pattern is characterized by the events
with magnitudes (Mw ≤ 5.0).
5.
Interpretation and discussion
5.1. Anomalous occurrence and distribution
of aftershock seismicity
Aftershock pattern seen in Fig. 2 indicates the longer
sequence of events. This exhibits two extremes of seismic temporal pattern, typically the mainshock-aftershock
pattern [42] with many smaller events and sub-sequences
of similar magnitude, within a small focal area (Fig. 2).
This pattern reveals sequential duration, evolution and
propagation of the aligned events and includes subse-
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Figure 6.
Variation of the magnitude, depth and frequency of occurrence (foo) of events, as a function of time (38 days) during the period from
2011.10.23 to 2011.11.30 within the focal zone (see the foo in Fig. 1 for time interval of this figure). Topographic map shows epicenter
distribution of events, clustering in eastern and northeastern ends of the lake. Note that the last event (5.0) occurred, together with
smaller-sized events (2.0≤Mw≤3.0), nearby Van city, shown in the map by curved line.
Figure 7.
Variation of the magnitude, depth and frequency of occurrence (foo) of 4147 events, as a function of time (75 days) during the period
from 2011.10.23 to 2012.01.06 within the focal zone (see the foo in Fig. 1 for time interval of this figure). Clusters are shown by C.
Topographic map shows epicenter distribution of 4147 events, clustering in the further east of the lake. Note that the last event (4.3)
occurred, together with the last 10 events (2.0≤Mw≤3.0), nearby Lake Erçek (see LE in Fig. 1 for the location), shown in the map by
curved line. A series of these last events on 75th days produced a prominent cluster (C) at the foo.
quences such as swarm-like response (120th-125th days)
and typical earthquake series (128th-183th days). Fig. 2
shows that the aftershocks densely take place at shallow
depth between 5 and 10 km. The minimum magnitudes
are mostly 1.9-2.0. Aftershock seismicity in Fig. 2 also
displays an aftershock decay pattern and dense clusters,
with most of the activity within the first few days (see and
examine Figs. 5-7 for time scale). However, detailed pattern of aftershock seismicity seen in Figs. 3-7 has varying
short-range characteristics, with a rapid development and
decay, tendency for enhanced, repeated seismicity after
the large-medium events (Mw≥4.0), and a very high bvalue (>2.0).
Anomalous occurrence and sequential pattern of aftershock events are also prominently characterized by individual and peculiar clusters of events. The 49 events
seen between 20th and 21th days (Fig. 5) occurred in 6
hours only and caused a distinct cluster with a prominent
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group of events (M ≈ 4.0). The occurrence order of these
49 events is curve-linearly aligned and concentrated in
the epicentral area. This implies strong structural heterogeneity of the ruptured zone. It is to note that this
cluster is distinctly observed within time interval of irregular and discontinuous drop of events below about 100
(Fig. 5), indicating chaotic picture of events. Similarly,
distinct clusters with two significant events (5.0 and 4.3)
are also seen in Figs. 6 and 7. These clusters are interpreted as indicating fault-related heterogeneities and/or
small asperities that generate clusters of highly repeating
small events with peaked statistics centered on events (M
≈ 3.0).
The aftershock data used for the all days of the events
make it easy to get a reliable indication of changes or
variations in aftershock seismicity. Hence it seems from
the plot of magnitude with time (Fig. 2) that it has a
decreasing average magnitude with time. It appears that
there is also a tendency for the foo of per event to decrease
with time. The resulting distribution has a shape similar to
that found for the clustering of natural seismicity [1]. Thus,
a progressively organized pattern of events develops even
when the initial heterogeneity is set to be random [12].
Irregular variation of aftershock events in magnitude and
the foo in Fig. 2 suggests time-dependent long-range interactions of events in the focal zone. The shifting clusters
result in the prominent and variable sequential pattern
of events, suggesting dynamic fluctuation, propagation of
clustering events and strong local interactions. These interactions are probably resulted from dynamic event instabilities and faulting complexity. Aftershock events are
interacted by triggering of seismicity with local nearest
neighbor interactions, implying that event instability dynamically takes place in aftershock pattern. The event
instability indicates that aftershocks appear to have a continuing trigger mechanism, either by a strain transfer, or
by a slow irregular slip event, in the mainshock area. As
well seen by Figs. 3 and 4, the events drop and/or increase is continually redistributed to its nearest neighbors
during a time scale, suggesting complex and multifractal
seismic nature of the Van earthquake (see Fig. 2).
Briefly, this study of aftershock seismicity shows timedependent distribution of events, event instabilities and
their local interactions. The population and distribution
of the aftershock events clearly exhibit spatial variability,
clustering-declustering and intermittency, consistent with
multifractal scaling [1, 12]. It is found that the distribution
of clusters of events observed is power-law in both space
and time. The sequential growth of aftershock events during time scale is considered as multifractal behavior of
seismicity in and around the focal zone. It is suggested
that multifractal seismicity can be dominated by strength
heterogeneity of the dip-slip faults (thrusting) and multifractal pattern of the accretionary crust, or slip instability of the failed elements. These are probably driven
by complex evolution of intraplate crustal faults, mechanical heterogeneity and seismic deformation anisotropy, as
discussed below.
6. Structural impacts on intraplate
crustal seismicity
All the events are clustered and centered in the epicentral
area of the mainshock where the causative thrust faulting
prominently resulted in an uplifting structure (ÇSZ-F-LE
line, together with NE-delta in Fig. 2). This was accompanied by rapid subsidence in central Tatvan basin.
Seismicity pattern of aftershocks appears to be invariably
complex in its overall characteristics of aligned events and
clusters (Figs.2-7). This pattern is interpreted as the geometrical irregularity of many faults and extreme heterogeneity in the various physical parameters in the rupture
process [1, 12]. This may be resulted from both in irregularity in the propagation of the rupture and in the distribution of stress release within. It is obvious from the data
that the heterogeneity may be considered to generate distinct aftershock events and various subevents recognized
in this study. It is reported by previous studies that environmental effects on time-dependent strength are important for the generation of aftershocks [1]. These effects
can result in variable friction as a function of time and
also allow subcritical crack growth to occur, causing the
event instability. Thus, it is considered that these effects
significantly control an aftershock picture of post-seismic
behavior and seismic triggering in the focal area.
Overall seismicity pattern of aftershocks and their anomalous distribution may provide a mechanism for strain softening process [1] to explain the principal thrusting event
in the Van earthquake. This suggests that the causative
thrust faulting may have become much more of a zone of
upper crustal weakness progressively during its deformational history. Strain is further localized by thrusting in
the focal zone for a given time scale. As a result, extreme
strain localization and fault weakening control the seismic
characterization of earthquake occurred and contribute to
explain the anomalous occurrence of aftershocks and intraplate nature of the Van earthquake where the rupture
process has high strength, moving slowly. This process
shows strong deformation anisotropy in the accretionary
complex sites, forming a typical fractal set of seismicity
[12].
From the combined interpretation of structural cases mentioned above and aftershock seismicity given here, it is
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to say that the slip event in the causative thrusting of
Van earthquake seems to have been much more irregular [1]. This reveals the much greater irregularity of
thrust fault topography in the slip-normal than the slipparallel direction [1] in the focal zone. It is also to note
that the causative thrusting event may have been segmentary with many smaller, secondary faults. Such a
structural case may result in a spectrum in the dynamic
rupture segmentation characteristics in the upper crustal
block [1]. This assumes that the spatial heterogeneity and
dynamic complexity of the causative thrusting and many
secondary faults can easily introduce many degrees of
freedom. Therefore, considerable irregularity of aligned
aftershock events, propagation of event instability and a
large number of events for a longer time scale should be
expected in multifractal aftershock seismicity of intraplate
Van earthquake. This is best characterized by a powerlaw size distribution of clusters observed and also by the
causative thrust faulting and its possible length distribution that obeys a power law and constitutes a fractal set.
7.
Conclusions
This study shows that there is a great amount of variation
in the aftershock seismicity pattern of the Van earthquake.
There are a large number of small events in a small focal
area, followed by the renewed clusters of aligned events.
It is possible to say that the anomalous pattern of aftershock seismicity may be a precursor of volcanic or tectonic
events (or both) as a result of intraplate crustal seismicity.
Seismicity parameters compiled of the data sampling procedure exerts a strong influence on the inferred fractal
seismicity of Van earthquake. Hence, aftershock events
used in this study are better interpreted as extensive heterogeneity in seismic deformation. This has structural impacts on intraplate seismicity, rather than the linear and
uniform fracture mechanics. This suggests multifractal and
unstable nature of the Van earthquake. This is supported
by inhomogeneous strain patterns and highly anisotropic
nature of accretionary crust where tectonic instability of
crustal blocks and thermal inequilibrium are frequently
observed. The reverse dip-slip character of Van earthquake, local strain softening mechanism and related strain
localization and fault weakening process well explain the
intraplate nature of the Van earthquake. The crustal heterogeneity and deformation anisotropy, associated with
the surrounding volcanoes contribute significant perturbation to intraplate strain regime. These findings have
played a key role in the formation of the continued aftershock seismicity since the occurrence of the 2011 Van
earthquake (Mw 7.1). This study of aftershock seismic-
ity contributes to a better understanding of the possible
physical processes and records of many small events. The
October 23, 2011 Van earthquake combined with an extensive examination of aftershocks provided new structural
impacts on intraplate crustal seismicity and different insights into seismic coupling between the principal thrusting event and its aftershocks. Finally, I suggest that aftershocks resulted from an intraplate earthquake appear to
be unique data sets. These may serve to retrieve detailed
information on the upper crustal structure of the focal zone
and related strain patterns.
8.
Acknowledgements
I would like to thank the anonymous reviewers for
their careful reviews and comments that improved the
manuscript. I sincerely thank Kandilli Observatory and
Earthquake Research Institute (KOERI) of Boğaziçi University (Turkey) for providing the earthquake catalogue
and G. Berkan Ecevitoğlu for useful discussions and comments about the aftershock data set. I am grateful to Alper
Çabuk of Anadolu University (Turkey), Research Institute
of Satellite and Space Sciences for his help in providing the seismological laboratory and related instruments
to work. This research was logistically supported by Research Institute of Satellite and Space Sciences, Anadolu
University (Eskižehir, Turkey).
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