Open Geosci. 2016; 8:548–555 Research Article Open Access Tekin Yeken* Spatial Analysis of b-value Variability in Armutlu Peninsula (NW Turkey) DOI 10.1515/geo-2016-0048 and a and b are constant coecients. The a value de- Received February 25, 2016; accepted June 12, 2016 pends on the area and time-window of investigation and describes the productivity, while the slope b describes Abstract: Spatial variations of b values were studied by the relative size distribution of earthquakes. This rela- means of 2376 earthquakes with a magnitude complete- tionship is of critical importance in seismicity, seismo- ness of 2.7 in the Armutlu Peninsula (NW Turkey) during tectonics and seismic hazard studies, including calcula- a 15-year period following the destructive earthquake on tion of recurrence time intervals of earthquakes with dif- August 17, 1999 in Kocaeli. The b value of L6 for the en- ferent magnitudes, mapping subsurface magmatic cham- tire Armutlu Peninsula represents a large value for a global bers and investigation of induced seismicity. Several fac- value, but this analysis suggested that the distribution of tors can cause perturbations of the b value and these in- b value around the Armutlu Peninsula varied extensively clude: increased material heterogeneity results in high b from 1.2 to 2.6. Several pockets of high bvalue reected values [3]; an increase in applied shear stress [4]; [5], or changes in the physical properties of the Armutlu Penin- an increase in eective stress [6] decreases the b value, In sula. The southern part of the peninsula represents a lower addition, an increase in the thermal gradient causes an b value against the northern part of the peninsula. A high increase in b [7]; [8], Also, the b value is an indicator of bvalue was observed around Termal and Armutlu towns whether the bulk of the seismic energy is released in a large where plenty of geothermal springs occur. Seismic tomog- number of smaller events, or oppositely through a small raphy studies revealed a low velocity zone beneath the number of larger events, A high b value means an abun- Termal area where the high b value was imaged in this dance of smaller events with respect to larger ones, In tec- study. A seismic swarm which is considered to be related tonic areas, the b value is generally around 1.0 [9]. In con- with geothermal activity also occurred in 2014 at the same trast. volcanic areas and earthquake swarms are charac- place. This observation suggests that it is possible to pro- terized by b values greater than 1.0 with values as high as pose that the high b value in the northern part of the penin- 3.0 [10]. sula could be related to hydrothermal/geothermal activity The question of the spatial heterogeneity in b values which contributes to lowering the eective stress. is closely related to hazard estimates. Even contemporary hazard mapping projects dier in their approach between assuming a constant b value. or a spatially varying one. 1 Introduction It also relates to understanding the underlying physics of the system. The inverse relationship between the concen- Probabilistic forecasting of earthquake attempts to deliver tration of stress at an epicentral region prior to the oc- the most accurate estimate of future seismicity at a given currence of the earthquakes and b-value is evidently of location and for a given magnitude range and period. The particular interest in the prediction of major earthquakes. fundamental statistics characterizing the distribution of During the past century. the North Anatolian Fault Zone the number of earthquakes by their magnitudes is de- (NAFZ) in Turkey has produced a sequence of large earth- scribed by the [1] and [2] law: quakes. The epicenters of these earthquakes show a west- ward migration towards the Marmara Sea. leaving a seis- log N(M) = a − bM, (1) 10 mic gap of ∼150 km under the Marmara Sea close to Istan- where N is the total number of the earthquakes with mag- bul. one of the most populous and rapidly expanding cities nitude M (or 2 M for the cumulative number of the events); in the world. Understanding crustal seismicity in the Mar- mara Sea is a main issue towards seismic hazard assess- ment for the region and for the city of Istanbul. The seis- *Corresponding Author: Tekin Yeken: Kocaeli University, Depart- micity at the Armutlu Peninsula has an important role in ment of Geophysics, 41040, Kocaeli, Turkey © 2016 Tekin Yeken, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. The article is published with open access at www.degruyter.com. Spatial Analysis of b-value Variability in Armutlu Peninsula (NW Turkey) Ë 549 understanding the formation of the next larger earthquake ized by water temperatures of 20°C to 30°C. By associat- at the western extension of the 1999 rupture. ing the locations of the thermal sources with the regional The Armutlu Peninsula (NW Turkey) is a tectonically faults, it can be seen that the thermal sources are related complex and important region in North-western Turkey. to the north and south branches of the NAFZ [17, 28]. Even though historic earthquake locations are controver- sial, in the period of AD 1500-1900, ve 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]. How- ever, 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 earth- quakes occurred on October 24, 2006, in Gemlik (M=5.2) and March 12, 2008, in Cinarcik (M=4.3) on the Armutlu Figure 1: (a) Marmara Sea and study area of the Armutlu Peninsula. Fault segments of the NAFZ. Dashed black lines indicate surface Peninsula [16–18]. In addition to these events, thousands rupture of the Kocaeli 1999 earthquake. Black arrows indicate of smaller events have been located in the last decades [19]. GPS velocity vectors [29]. Gray stars indicate larger earthquakes The Armutlu Peninsula has also been known to be asso- (Ms≥6.8) since 18th century. (b) tectonic setting of the Marmara Sea ciated with thermal springs, accompanied with magma region [14]. intrusion. Henceforth, the Armutlu Peninsula is an ideal place to analysis spatial changes in b value. 2 Study Area 3 Data Processing Armutlu Peninsula is situated in the eastern Marmara In seismicity studies, it is frequently necessary to use Region, and is located at the western end of the 1999 the maximum number of events available for high-quality Kocaeli earthquake rupture; it is bounded by the north- results. The data used in this study were provided by ern and southern branches of the NAFZ zone (Figure 1). National Earthquake Monitoring Center at Bogazici Uni- The region has very complex tectonics and active seis- versity Kandilli Observatory and Earthquake Research micity. The neotectonic period began in Anatolia by the Institute (KOERI). It covers 15 years data starting from collision of the Arabian and Eurasia plates in the Early- 01.01.2000 to 31.12.2014 with magnitude completeness M to-Middle Miocene [20–23]. As a result of this collision ( c) level of 2.7. Fig. 2 shows an epicenter map of the Ar- and the crustal deformation, the East Anatolian crust mutlu Peninsula. The largest magnitude was 5.2 which oc- thickened and the NAFZ and East Anatolian Fault Zone curred on October 24, 2006 in Gemlik Bay. (EAFZ) systems were formed [24, 25]. With the impact of The homogenization of earthquake catalog involves the NAFZ and EAFZ systems and the collision, Anatolia expressing the earthquake magnitudes in one common moved westward. Associated with this movement, an ex- scale. Practical problems, such as seismic hazard assess- tensional regime is seen in the Aegean region [24, 26, 27], ment, necessitate use of homogenized catalog. As such which caused a horst and graben structure in this region, a consistent magnitude should be used for investigating whereby the Armutlu Peninsula represents a horst be- the frequency magnitude distribution (FMD). The earth- tween two branches of the NAFZ system; this has resulted quake catalog provided by KOERI includes dierent mag- in a complex dextral zone. According to [28], the active nitudes such as duration magnitude and local magnitude. faults are Riedel shears within a right lateral shear zone The duration magnitude is mostly problematic for micro- that is rotated clockwise. Armutlu Peninsula has several earthquakes due to dependency of noise level. Also, there geothermal areas. The hottest thermal sources are located is lack of the local magnitude denition in the catalog. All in the north (Yalova Termal) and on the western end of earthquakes have the duration magnitude whereas only a the peninsula (Armutlu), and they have surface tempera- small amount of them have the local magnitude denition. M tures of 60°C to 70°C. Other geothermal sources of Gem- On the other hand, a Moment Magnitude ( w) is preferred lik, Orhangazi, Keramet and Sogucak, are located in the because of its applicability for all ranges of earthquakes; south and east ofthe region, and these are also character- large or small, far or near, shallow or deep focused. In or- 550 Ë Tekin Yeken where mmean is the average magnitude and mmin is the minimum magnitude of the given sample. The magni- tudes in the catalog were rebinned with ∆M = 0.01 into new magnitude bins with ∆M = 0.1. This step is nec- essary because computing magnitude of completeness is based on the non-cumulative frequency-magnitude distri- bution [46].