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BY-NC-ND 3.0 license Open Access Published by De Gruyter Open Access November 2, 2016

Spatial Analysis of b-value Variability in Armutlu Peninsula (NW Turkey)

  • Tekin Yeken
From the journal Open Geosciences

Abstract

Spatial variations of b values were studied by means of 2376 earthquakes with a magnitude completeness of 2.7 in the Armutlu Peninsula (NW Turkey) during a 15-year period following the destructive earthquake on August 17, 1999 in Kocaeli. The b value of L6 for the entire Armutlu Peninsula represents a large value for a global value, but this analysis suggested that the distribution of b value around the Armutlu Peninsula varied extensively from 1.2 to 2.6. Several pockets of high bvalue reflected changes in the physical properties of the Armutlu Peninsula. The southern part of the peninsula represents a lower b value against the northern part of the peninsula. A high b value was observed around Termal and Armutlu towns where plenty of geothermal springs occur. Seismic tomography studies revealed a low velocity zone beneath the Termal area where the high b value was imaged in this study. A seismic swarm which is considered to be related with geothermal activity also occurred in 2014 at the same place. This observation suggests that it is possible to propose that the high b value in the northern part of the peninsula could be related to hydrothermal/geothermal activity which contributes to lowering the effective stress.

1 Introduction

Probabilistic forecasting of earthquake attempts to deliver the most accurate estimate of future seismicity at a given location and for a given magnitude range and period. The fundamental statistics characterizing the distribution of the number of earthquakes by their magnitudes is described by the [1] and [2] law:

log10N(M)=abM(1)

where N is the total number of the earthquakes with magnitude M (or 2 M for the cumulative number of the events); and a and b are constant coefficients. The a value depends on the area and time-window of investigation and describes the productivity, while the slope b describes the relative size distribution of earthquakes. This relationship is of critical importance in seismicity, seismotectonics and seismic hazard studies, including calculation of recurrence time intervals of earthquakes with different magnitudes, mapping subsurface magmatic chambers and investigation of induced seismicity. Several factors can cause perturbations of the b value and these include: increased material heterogeneity results in high b values [3]; an increase in applied shear stress [4]; [5], or an increase in effective stress [6] decreases the b value, In addition, an increase in the thermal gradient causes an increase in b [7]; [8], Also, the b value is an indicator of whether the bulk of the seismic energy is released in a large number of smaller events, or oppositely through a small number of larger events, A high b value means an abundance of smaller events with respect to larger ones, In tectonic areas, the b value is generally around 1.0 [9]. In contrast. volcanic areas and earthquake swarms are characterized by b values greater than 1.0 with values as high as 3.0 [10].

The question of the spatial heterogeneity in b values is closely related to hazard estimates. Even contemporary hazard mapping projects differ in their approach between assuming a constant b value. or a spatially varying one. It also relates to understanding the underlying physics of the system. The inverse relationship between the concentration of stress at an epicentral region prior to the occurrence of the earthquakes and b-value is evidently of particular interest in the prediction of major earthquakes. During the past century. the North Anatolian Fault Zone (NAFZ) in Turkey has produced a sequence of large earthquakes. The epicenters of these earthquakes show a westward migration towards the Marmara Sea. leaving a seismic gap of ∼150 km under the Marmara Sea close to Istanbul. one of the most populous and rapidly expanding cities in the world. Understanding crustal seismicity in the Marmara Sea is a main issue towards seismic hazard assessment for the region and for the city of Istanbul. The seismicity at the Armutlu Peninsula has an important role in understanding the formation of the next larger earthquake at the western extension of the 1999 rupture.

The Armutlu Peninsula (NW Turkey) is a tectonically complex and important region in North-western Turkey. Even though historic earthquake locations are controversial, in the period of AD 1500-1900, five M > 7 earthquakes (1509, 1719, 1754, 1766, 1894) occurred around the Armutlu Peninsula [11]; [12]. The last largest event occurred in 1419 with an estimated magnitude of M 6.8 [11, 13, 14]. However, the largest event in the last century in the Marmara Sea was the Cinarcik Earthquake (Ms 6.4) occurred on September 18, 1963 [15]. Recently, two moderate earthquakes occurred on October 24, 2006, in Gemlik (M=5.2) and March 12, 2008, in Cinarcik (M=4.3) on the Armutlu Peninsula [1618]. In addition to these events, thousands of smaller events have been located in the last decades [19]. The Armutlu Peninsula has also been known to be associated with thermal springs, accompanied with magma intrusion. Henceforth, the Armutlu Peninsula is an ideal place to analysis spatial changes in b value.

2 Study Area

Armutlu Peninsula is situated in the eastern Marmara Region, and is located at the western end of the 1999 Kocaeli earthquake rupture; it is bounded by the northern and southern branches of the NAFZ zone (Figure 1). The region has very complex tectonics and active seismicity. The neotectonic period began in Anatolia by the collision of the Arabian and Eurasia plates in the Early-to-Middle Miocene [2023]. As a result of this collision and the crustal deformation, the East Anatolian crust thickened and the NAFZ and East Anatolian Fault Zone (EAFZ) systems were formed [24, 25]. With the impact of the NAFZ and EAFZ systems and the collision, Anatolia moved westward. Associated with this movement, an extensional regime is seen in the Aegean region [24, 26, 27], which caused a horst and graben structure in this region, whereby the Armutlu Peninsula represents a horst between two branches of the NAFZ system; this has resulted in a complex dextral zone. According to [28], the active faults are Riedel shears within a right lateral shear zone that is rotated clockwise. Armutlu Peninsula has several geothermal areas. The hottest thermal sources are located in the north (Yalova Termal) and on the western end of the peninsula (Armutlu), and they have surface temperatures of 60°C to 70°C. Other geothermal sources of Gemlik, Orhangazi, Keramet and Sogucak, are located in the south and east of the region, and these are also characterized by water temperatures of 20°C to 30°C. By associating the locations of the thermal sources with the regional faults, it can be seen that the thermal sources are related to the north and south branches of the NAFZ [17, 28].

Figure 1 (a) Marmara Sea and study area of the Armutlu Peninsula. Fault segments of the NAFZ. Dashed black lines indicate surface rupture of the Kocaeli 1999 earthquake. Black arrows indicate GPS velocity vectors [29]. Gray stars indicate larger earthquakes (Ms≥6.8) since 18th century. (b) tectonic setting of the Marmara Sea region [14].
Figure 1

(a) Marmara Sea and study area of the Armutlu Peninsula. Fault segments of the NAFZ. Dashed black lines indicate surface rupture of the Kocaeli 1999 earthquake. Black arrows indicate GPS velocity vectors [29]. Gray stars indicate larger earthquakes (Ms≥6.8) since 18th century. (b) tectonic setting of the Marmara Sea region [14].

3 Data Processing

In seismicity studies, it is frequently necessary to use the maximum number of events available for high-quality results. The data used in this study were provided by National Earthquake Monitoring Center at Bogazici University Kandilli Observatory and Earthquake Research Institute (KOERI). It covers 15 years data starting from 01.01.2000 to 31.12.2014 with magnitude completeness (Mc) level of 2.7. Fig. 2 shows an epicenter map of the Armutlu Peninsula. The largest magnitude was 5.2 which occurred on October 24, 2006 in Gemlik Bay.

Figure 2 Distribution of the earthquakes in the study area from 2000 to 2014, z is focal depth of earthquakes and different colors indicate different depths given in legend.
Figure 2

Distribution of the earthquakes in the study area from 2000 to 2014, z is focal depth of earthquakes and different colors indicate different depths given in legend.

The homogenization of earthquake catalog involves expressing the earthquake magnitudes in one common scale. Practical problems, such as seismic hazard assessment, necessitate use of homogenized catalog. As such a consistent magnitude should be used for investigating the frequency magnitude distribution (FMD). The earthquake catalog provided by KOERI includes different magnitudes such as duration magnitude and local magnitude. The duration magnitude is mostly problematic for microearthquakes due to dependency of noise level. Also, there is lack of the local magnitude definition in the catalog. All earthquakes have the duration magnitude whereas only a small amount of them have the local magnitude definition. On the other hand, a Moment Magnitude (Mw) is preferred because of its applicability for all ranges of earthquakes; large or small, far or near, shallow or deep focused. In order to construct a homogeneous earthquake catalog, a moment magnitude-duration magnitude relationship of [30] was used in this study.

MW=1.27MDave1.12(2)

A seismic source zone is defined as a seismically homogeneous area. A complete understanding of the historical and instrumental seismicity, tectonics, geology, paleoseismology, and other neotectonic properties of the considered region are necessary for an ideal delineation of seismic source zones. Microseismicity and seismotectonics of the eastern part of the Marmara Sea, have been discussed by several researchers following the 1999 Izmitearthquake (e.g., [14, 16, 17, 3145]). Using all available research results, earthquake epicenters from 2000 to 2014, with a magnitude greater than Mw ≥ 0.4, active fault maps prepared by various scientists and combining the available information from previous studies, three seismic zones have been defined in the Armutlu Peninsula. All zones (northern and southern) are related with the North Anatolian Fault Zone.

4 Estimation of b-value

To investigate the spatial variability of b values in the Armutlu Peninsula, the gridding technique [46] using the ZMAP software package [47] was applied. The maximum likelihood bvalue was calculated using the equation [4850]

b=log10(e)/(mmeanmmin)(3)

where mmean is the average magnitude and mmin is the minimum magnitude of the given sample. The magnitudes in the catalog were rebinned with ΔM = 0.01 into new magnitude bins with ΔM = 0.1. This step is necessary because computing magnitude of completeness is based on the non-cumulative frequency-magnitude distribution [46]. Me was corrected by Δm/2 to compensate the bias of rounded magnitudes to the nearest Δm bin, thus mmin = McΔm/2 [48, 51, 52] suggested a bootstrap method [53] to estimate the associated standard deviation, δb, of b value. The approach involves computation of b value repeatedly for a number of times, each time employing different replacement events drawn from the associated catalog wherein any event can be selected more than once. The error is then estimated as the standard deviation associated with the computed values. The maximum like-lihood method often gives slightly lower values compared with the least squares approach, but it is found to be more stable [54].

To identify periods of different recording quality and catalog completeness, changes in the slope of the plot of cumulative number of events per time were investigated [55]. Plotting the cumulative number of all events in the catalog showed only a slight change in the slope at 2006, 2011, 2013 and 2014 (Figure 3). From 2011 to 2014, recording completeness dropped significantly for small events. This indicated an overall completeness level of Me which is higher than the catalog in the period from 2011 to 2014.

Figure 3 Cumulative number of earthquakes from 2000-2014.
Figure 3

Cumulative number of earthquakes from 2000-2014.

In this study, the distance between the grid points were chosen as 0.02°. The effective resolution of the technique depends on the density of earthquakes surrounding each grid node, which varies with position. The b value at each node was estimated from the ensemble of the 250 nearest earthquakes, from which a minimum of 15 must be above the completeness threshold.

5 Results and Discussion

The occurred number of earthquakes versus time in the study region is shown in Figure 4a. The number of earthquakes as a function of time between 2000 and 2002 is larger than the 2003-2008 period. During this period (2003-2008) the seismic activity decreased. It was only the October 24, 2006 Gemlik (M=5.2) event and their aftershock that increased the seismic activity. The seismic activity between 2000 and 2002 might probably contains aftershocks of the August 17, 1999 Kocaeli (M=7.4) earthquakes. Then observed seismic activity was increased again, after 2008. The increasing of the number of earthquakes after 2008, also depends on the increasing number of installed stations in Armutlu peninsula. Looking at Figure 4b, it can be seen that recorded magnitudes are descending to around 1 after 2012. However, the maximum increase in the number of earthquakes was observed in the year 2014. An earthquake swarm occurred in 2014 and more than 2000 earthquakes were located within two months [19].

Figure 4 (a) The occurred number of earthquakes versus time and (b) recorded magnitudes versus time in the study region.
Figure 4

(a) The occurred number of earthquakes versus time and (b) recorded magnitudes versus time in the study region.

Figure 5 shows the histogram of magnitude distribution for the data set of the region. The magnitudes of most earthquakes were from 2 to 3.3, and a maximum was observed at ML=5.2 (ML is local magnitude). Completeness magnitude (Me) generally showed stable values from 2000 to 2012, then it decreased to 1.9 in the beginning of 2012 (Figure 6).

Figure 5 Magnitude - occurrence number of histogram of events used in the present study.
Figure 5

Magnitude - occurrence number of histogram of events used in the present study.

Figure 6 Magnitude completeness (Me) versus time.Asharp decreasingstarted around 2011 parallelto increasingofnumberof stations and number of recorded small earthquakes.
Figure 6

Magnitude completeness (Me) versus time.Asharp decreasingstarted around 2011 parallelto increasingofnumberof stations and number of recorded small earthquakes.

Total of 2376 earthquakes have been used with Me 2.7, and b-value was estimated for 250 events/windows. A spatial grid of points with a distance of 0.02° was assumed. With the Me value assumed to be 2.7, the b-value was then calculated as 1.6 ± 0.05. The bvalue that obtained for the study area was larger than typical b value for the whole Earth (Figure 7). Earthquakes are characterized by the b-value mostly in the range of 1.02 ± 0.03 for the whole Earth [56]. However, on a local scale, the b-value has been reported to show a relatively wide range of variations (0.3 to 2.5 or more; e.g. [4, 5760]).

Figure 7 The maximum likelihood solution of log10N(M) = a + bMrelationship for the study area. The vertical axis indicates cumulative earthquake occurrence number (logarithmic) while horizontal axis indicates magnitude of earthquakes.
Figure 7

The maximum likelihood solution of log10N(M) = a + bMrelationship for the study area. The vertical axis indicates cumulative earthquake occurrence number (logarithmic) while horizontal axis indicates magnitude of earthquakes.

The spatial distribution of b-value for the Armutlu Peninsula is presented in Fig. 8. The spatial b-value map for the Armutlu Peninsula considered individually indicated that the b-values of seismicity for the regions were not homogeneous. The b value for the southern peninsula was lower than the northern part. The b value obtained for the southern part of the peninsula was around 1.2-1.4 while 2.2-2.5 for the northern side. These temporal variations in b-value appeared to be also associated with changes in local and regional stresses. It has long been known that normal faults attain the highest b-values, whereas reverse faults are associated with the lowest (e.g. [52, 62, 63]). The focal mechanism studies in Armutlu Peninsula showed that most earthquakes are characterized by normal faulting [14, 34, 36, 44]. Also, variations are also due to different crustal structures, thickness, rock types, physical properties and ultimately its degree of heterogeneity. Examples of studies that document this class of b-variations include [6468]. [69] have suggested that the increase in b-value is caused by the presence of fluids at a particular depth in the Koyna - Warna region. According to [70], large thermal gradients that exist especially at shallower depths (< 100 km) could create a stress field with associated seismicity characterized by high b-value. Lower shear stress that can give rise to high pore pressure exhibits high b-value [6]. The northern inland part mention above and the western inland part of the peninsula have plenty of geothermal springs. High b value in inland part of the northern peninsula is probably due to earthquake swarm which occurred in 2014 which is related with geothermal activity [19]. Thus, it can be concluded that these reasons could be a clue for the high b value for the northern part of the study region.

Figure 8 Spatial distribution b value in and around the Armutlu peninsula (upper: northern part of the peninsula, middle: middle section of the peninsula, lower: southern part of the peninsula).
Figure 8

Spatial distribution b value in and around the Armutlu peninsula (upper: northern part of the peninsula, middle: middle section of the peninsula, lower: southern part of the peninsula).

6 Conclusions

The b value of 1.6 for the entire Armutlu Peninsula represents a large value for a global value, but this analysis suggests that the distribution of b value around the Armutlu Peninsula varies extensively. Several pockets of high b value reflect changes in the physical properties of the Armutlu Peninsula. The northern part of the peninsula represents a higher b value against the southern part of the peninsula. Around Termal and Armutlu towns, a high b value was observed. These areas have plenty of geothermal springs. Seismic tomography studies revealed a low velocity zone beneath Termal area where the high bvalue was imaged in this study. A seismic swarm which is considered related with geothermal activity also occurred in 2014 at the same place. This observation suggests that the h ydrothermal or geothermal activity contributes to lowering the effective stress in these areas (Termal and Armutlu) in the Armutlu Peninsula. Thus, it is possible to propose that the high b value in the northern part of the peninsula could be related to high crustal heterogeneity as discussed above and hydrothermal/geothermal activity which contributes to lowering the effective stress.

Acknowledgement

The author thanks the journal reviewers for their critical reading of the manuscript and providing valuable comments on the initial manuscript. The author also grateful to Mutala Mohammed for checking the language.

Data and Resources

All earthquakes data used in this article were accessed from the National Earthquake Monitoring Center of the Kandilli Observatory and Earthquake Engineering Institute catalog (http://www.koeri.boun.edu.tr/sismo/2/deprem-verileri/deprem-katalogu/; last accessed April, 2015).

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Received: 2016-2-25
Accepted: 2016-6-12
Published Online: 2016-11-2
Published in Print: 2016-10-1

© 2016 Tekin Yeken, published by De Gruyter Open

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

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