KOERI has a long history of earthquake seismology,
beginning its observations right after the devastating earthquake on 10 July 1894 in Istanbul, by deploying the first seismograph in the region.
Naturally, its seismic network and earthquake catalog evolved since that
time, in harmony with the progress in the science of seismology. Currently,
the seismic network consists of 242 stations that record approximately 1500
earthquakes per month during periods of regular seismicity. Magnitude is one
of the most critical parameters in determining the size of an earthquake,
especially in seismic hazard assessment studies. The objective of this study
is to homogenize the magnitudes of the KOERI catalog between 2008 and 2018.
For this aim, we computed the Magnitude of Completeness (Mc) for two
different time periods between 2008–2011 and 2012–2018 by taking into
account the duration magnitude (Md) and local magnitude (Ml), where these
parameters might not be available jointly for the both time periods
considered. As a result, we present a relationship of Md and Ml magnitudes
derived from and applicable to KOERI's earthquake catalogs.
Introduction
Turkey and surrounding regions form one of the most seismically active
regions in the world. Moderate (5.0–5.9) to strong (6.0–6.9) earthquakes
occur frequently in the region, where a strong earthquake of M>6.0 is experienced annually or biennially, and a major earthquake of
M>7.0 every 7–8 years. Seismicity in Turkey and its surrounding
regions is monitored by a number of different establishments. RETMC with the
FDSN Network Code: KO (Boğaziçi University Kandilli Observatory And
Earthquake Research Institute, 2001), is the oldest seismological
observation center in Turkey monitoring the earthquake activity 24/7
(Louderback, 1948; Fettahoğlu, 2012; Kalafat, 2017; Cambaz et
al., 2019). KOERI is one of the core participants and corporate founder of
Observatories & Research Facilities for European Seismology (ORFEUS) and
also one of the primary nodes of the European Integrated Data Archive
(EIDA), which is an initiative within ORFEUS
(http://www.orfeus-eu.org/data/eida/, last access: October 2019). RETMC is also an accredited Tsunami
Service Provider of the Intergovernmental Coordination Group for the Tsunami
Early Warning and Mitigation System in the North-eastern Atlantic, the
Mediterranean and connected seas (ICG/NEAMTWS), providing services to
Eastern Mediterranean, Aegean, Marmara and Black Seas since 2012).
Seismic networks evolve in time. When dealing with an earthquake catalog, it
is important to know the details of this evolution to avoid any
misinterpretation of the data. Lack of information documenting the
homogeneity and completeness of the data set makes it difficult to reliably
interpret data. The main motivation of this study is to find a way to
homogenize KOERI's earthquake catalog of different time periods and
magnitude types.
Station distribution used in earthquake location from RETMC
network (triangles) and other stations from neighbourhood countries (reverse
triangles). The red rectangle in the inset figure shows the monitoring area
of KOERI-RETMC (30.000–48.000∘ N, 22.000–44.000∘ E).
Seismic network
The installation of first seismological sensor of KOERI dates back to 10 July 1894 earthquake in Marmara Region. According to the historical studies (Louderback, 1948; Fettahoğlu, 2012), Giovanni Agamennone came to
İstanbul by the invitation of the Government of the Ottoman Empire in
1895 to study the 1894 earthquake in the Marmara region, and remained two
years to install two seismographs in a specially constructed housing and to
conduct relevant studies. The first mechanical station installed after this
date was a Mainka seismograph, deployed in Istanbul Kandilli, where KOERI
resides today, in 1938 by the government of the Republic of Turkey. KOERI's
stations started to became a network starting in the early 1970's as a
result of steady increase of station installations all over Turkey, growing
considerably mainly after the 1999 İzmit M7.4 earthquake, accompanied
also with the transition from short period stations to broadband stations,
witnessing also changes in operational procedures, such as the type of
magnitudes to be considered. In comparison to 35 stations in 1999, and 123
in 2010, the KOERI network consist of 242 stations today, composed of 135
broadband, 93 strong motion and 14 short period sensors (Fig. 1).
Seismicity Catalog
KOERI-RETMCs earthquake monitoring areas is confined within 30.0–48.0∘ N, 22.0–44.0∘ E, also for the purposes of identifying any tsunamigenic earthquake in
the Eastern Mediterranean and its connected Seas (Fig. 1). The recent
seismicity rate is almost constant around 50 earthquakes per day during
routine seismicity.
Computation of Md from analog records had been initiated in 1992 just after
the 13 March 1992 (M6.7) Erzincan earthquake. The magnitude type of the catalog
was changed from duration magnitude (Md) to local magnitude (Ml) in 2012.
However, Mb, Ml and Mw magnitudes are still given as reference for the
important and widely felt earthquakes (Kalafat et al., 2011). High amount of
quarry blasts are also recorded in the region. These events are removed
regularly as a result of detailed discrimination studies based on satellite
imagery, temporal and spatial properties, maximum peak amplitude ratio
(S / P), power ratio, and spectral amplitude ratio of the vertical component
of the seismograms (Kekovalı, 2009; Kekovalı et al., 2011, 2012; Kekovalı and Kalafat, 2014).
Seismicity map of the region with red dots representing the
174 285 earthquakes recorded between 1 January 2008–31 December 2018 and the black
lines representing the active faults (Emre et al., 2013).
In this study, the area of consideration is bounded by 34.0–43.0∘ N, 23.0–46.0∘ E, as shown in Fig. 2. Almost 200 000 events were recorded in this area
during the time period 1 January 2008–31 December 2018. Quarry blasts, mine explosions
and other suspicious events were eliminated as mentioned above in detail
(Kekovalı, 2009) and we used 174 285 earthquakes for statistical analysis
using ZMAP (Wiemer, 2001). Figure 3 shows the earthquake-time histogram for
this between 2008 and 2018. The seismicity rate is around 1000 events per
month in a period of normal seismic activity, specifically after the sharp increase in
the number of stations in 2012. But an up to tenfold increase in the
seismicity can also be seen after the moderate earthquakes e.g., after four
moderate earthquakes (Mw=5.2 to Mw=5.4) which occurred within a week in Çanakkale in February 2017, after the Mw=6.3 Lesvos earthquake of 12 June 2017 and the Mw=6.6 Gökova earthquake of 21 July 2017. These
earthquakes were followed by an intense aftershock activity in the following
months, reaching up to thousands of additional earthquakes. Figure 4 shows
the depth histogram of the earthquakes between 2008 and 2018, which shows that the earthquake activity is mainly observed within 0 to 30 km depth with more
than half of the earthquakes having hypocentral depths in the first 20 km of
earth's crust. Deeper earthquakes generally exist at the southernwest part
of Turkey along the Hellenic arc, which represents the boundary between the
African and Anatolian plates in the Eastern Mediterranean Region.
Earthquake – time histogram for the time interval
1 January 2008–31 December 2018.
Earthquake – depth histogram for the time interval
1 January 2008–31 December 2018.
Comparison of Duration and Local Magnitude Catalogs
Duration magnitude, which relies on the length of the recorded earthquake
seismic coda, was the main magnitude scale used in the estimation of
magnitude in KOERI prior 2012. Despite its fast and easy to use, the
earthquake magnitude of consideration was changed to Ml at the beginning of 2012 as a more scientifically sound parameter considering the needs of a national earthquake network. Currently, Ml is used as the primary magnitude scale for the routine earthquake magnitude estimation but Mw and MwS is also
used in order to avoid the saturation problem of Ml for earthquakes greater
than 6.0.
In this study, we divided the catalog into two different time intervals
corresponding to the change from Md to Ml, namely from 1 January 2008–31 December 2011
for the Md and from 1 January 2012–31 December 2018 for the Ml. Magnitude histograms are presented both for Md and Ml for a better understanding of
the variation these parameters. Figure 5 shows the magnitude histogram for
Md in between 2008 and 2011. Magnitudes of earthquakes range from 1.5 to 4.5 with most of the earthquake magnitudes occurring between 2.0 to 4.0. Figure 6 shows the equivalent histogram for Ml between 2012 and 2018. It is noticeable that Ml varies across a wider range as compared to Md. Local
magnitudes of earthquakes range from 0 to 6.0 with most of the earthquake
magnitudes occurring between 1.0 to 4.0.
Earthquake – magnitude histogram of earthquakes with duration
magnitude, Md, in between the time interval 1 January 2008–31 December 2011.
Earthquake – magnitude histogram of earthquakes with local
magnitude, Ml, in between the time interval 1 January 2012–31 December 2018.
Magnitude of Completeness
Magnitude of completeness (Mc) is a critical parameter for seismicity studies, determination of the b value and seismic hazard analysis, and it
can be simply defined as the lowest magnitude above which all events can be
considered to be fully detected (Wiemer and Wyss, 2000). It varies as a
function of space and time but also varies with artificial changes such as
network configuration and magnitude estimation methods. Mc computation by using a maximum likelihood solution which is based on the maximum curvature method of (Wiemer and Wyss, 2000; Woessner and Wiemer, 2005) is available in ZMAP (Utsu, 1999; Wiemer and Katsumata, 1999). Maximum curvature is a fast and reliable estimate of Mc, in order to define the point of the maximum curvature as a magnitude of completeness, by computing the first derivative
of the frequency magnitude curve. This method matches the magnitude bin with
the highest frequency of events in the non-cumulative frequency-magnitude
distribution (Woessner and Wiemer, 2005).
Frequency-magnitude distribution of the catalog in between the
time interval 1 January 2008–31 December 2011 with magnitude of completeness Mc= 2.7.
Kalafat (2016) computed the Mc values of Turkey and surrounding regions by
using the KOERI catalogue in the time period 1975–2015. Due to lesser number
of stations during the time period considered, especially in the earlier
parts, Kalafat (2016) observed significantly higher Mc values in the same study area. Mc values were mainly changing between 2.6–2.9 for the selected regions. However, Mc has significantly decreased to Mc=2.0 with the
installation of recent stations in the study area. Cambaz et al. (2019)
presented the dynamic variations of Mc with respect to time and earthquakes sequences in the region for the time period 2013–2017. They computed the magnitude of completeness as Mc=2.0 for almost all parts of Turkey with a b value of 1.01±0.05. They also presented the variations of Mc with time and region.
Mc computation was performed for both of the Md and Ml catalog. The first
period comprises almost 45 000 earthquakes and the second period comprises
approximately 130 000 earthquakes. Figures 7 and 8 shows the
frequency-magnitude distributions of the catalogs for the two different time
intervals considered. We obtained a magnitude completeness Mc=2.7 for earthquakes in Md catalog and Mc=2.0 for earthquakes in the Ml catalog. The Mc=2.0 value computed for the time interval 2012–2018 is compatible with the Mc value (Mc=2.0) obtained by Cambaz et al at a similar time
interval (2013–2017). However, a significant variation between the two Mc values for the catalogs pre and after 2012 is observed, which may be related not only with the change of magnitude scale from Md to Ml, but also to network geometry. Indeed, the number of sensors operated by KOERI was 123 in 2010, but increased almost twice in number up to 2019. The number of
earthquakes are quite different in the catalogs for these two different time
periods. The tremendous increase in station number and hence in the number
of detected earthquakes cause a drastic difference in Mc values obtained for these two catalogs. Strong variations were also observed on the computation
of b values, namely a b value of 1.65 for the Md catalog and 0.83 for the Ml
catalog. Variations in b value can be affected by the occurrence of large
events and amount of small events, generally. Also a cut off or threshold
magnitude in the analysis of b value usually result in higher b values.
Frequency-magnitude distribution of the catalog in between the
time interval 1 January 2012–31 December 2018 with magnitude of completeness Mc= 2.0.
The relationship between Md and Ml.
Relationship Between Md and Ml Catalogs
A homogeneous database of magnitude observations is a major requirement for
seismic hazard studies. In order to homogenize the catalogs, a relationship
between local and duration magnitude was computed. The dataset used in this
study consists of 21 543 earthquakes occurring from 2008 to 2011 and selected from the Md catalog. Ml magnitudes were computed by using the zSacWin (Yılmazer, 2012), which is a windows-based software providing easy usage of the
routine earthquake location package HYPO71 (Lee and Lahr, 1972) together
with the use of Kalafat et al. (1987) crustal model for the location of
earthquakes. Earthquakes computed with more than 5 stations were selected in
order to obtain a reliable and trustworthy relationship. The relationship
obtained as; Ml=1.0313Md-0.7677 between Ml and Md as in Fig. 9. As a comparison, Tuve et
al. (2015) obtained a relationship between Ml and Md as; Ml=1.164×Md-0.337
for the Mt Etna region. Brumbaugh (1989) stated a relation between Ml and Md
as; Ml=0.936Md-0.16±0.22 in North Arizona according to their linear regression analysis.
Results
Earthquake catalogs are one of the most important products of seismological
agencies. Quality, consistency and the homogeneity of the seismic catalogs
must be well defined for a healthy interpretation of seismic studies. In
this study we analysed the earthquake catalog in two different parts before
2012 and since 2012 by considering the types of earthquake magnitude
applicable, namely Md prior to 2012 and Ml after 2012. We presented the statistical analysis of the catalog by plotting the Md magnitude histogram and Ml magnitude histogram for the selected time intervals and computed the
Magnitude Completeness Mc in these two different time intervals. Magnitude of completeness was computed as Mc=2.7 for the Md catalog by using the earthquakes between 1 January 2008-31 December 2011 and Mc=2.0 for the Ml catalog by using the earthquakes in between 1 January 2012 and 31 December 2018. b values were
presented as 1.65 for Md catalog and 0.83 for Ml catalog. Differences in network status and earthquake catalogs result with strong variations in terms of b value and Mc. For reliable estimates of b and Mc values it is
necessary to make use of as much data as possible and homogenization of
earthquake catalogs of different types of earthquake magnitudes plays an
important role for this purpose. We derive an empirical relationship between
Md and Ml to accomplish this, which allows to back-extend the local magnitude dataset. The outcome is expected to better serve the needs of seismic hazard studies based on KOERI's earthquake catalogs.
Code and data availability
Seismicity
analysis of this catalog was made by using the ZMAP software (Wiemer, 2001)
which uses a number of scripts written in Matlab (http://www.mathworks.com, last access: October 2019). Some of the figures
were plotted using Generic Mapping Tools (Wessel et al., 2013).
The earthquake catalog and waveform data is available via Kandilli Observatory and Earthquake Research Institute (KOERI) web page
(http://www.koeri.boun.edu.tr/sismo/2/tr/, last access: October 2019, Boğaziçi University, 2019). KOERI waveform data can also be obtained through EIDA.
Author contributions
MDC, FT carried out the data analysis with the software support of MY. MDC, FT, ÖN created the figures. MY,ÖN, KK, DK evaluated the analysis. MDC prepared the manuscript, all authors read and approved the manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Improving seismic networks performances: from site selection to data integration (EGU2019 SM5.2 session)”. It is a result of the EGU General Assembly 2019, Vienna, Austria, 7–12 April 2019.
Acknowledgements
This article follows from the presentation given at EGU 2019, session
“Improving seismic networks performances: from site selection to data
integration” and the authorship therein. In addition, we thank all staff of
RETMC.
We are thankful to anonymous referees and Helle Pedersen for their most
valuable comments and criticism. We would like to thank the Topical Editor
Damiano Pesaresi for the support provided during the review process of this
manuscript.
Review statement
This paper was edited by Damiano Pesaresi and reviewed by two anonymous referees.
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