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Jul 20, 2017 - as mentioned in previous chapter. There is no clear anomaly between UTC 00:00 and UTC 06:00. The global. TEC disturbance shows widely ...
9th Congress of the Balkan Geophysical Society Antalya, Turkey, 05-09 November 2017

Erman Şentürk1*, Murat Selim Çepni1, Evrim Yavuz2 1 Kocaeli

University, Department of Surveying Engineering, Kocaeli, Turkey erman.senturk/[email protected] 2 Kocaeli University, Department of Geophysical Engineering, Kocaeli, Turkey [email protected]

Introduction

Data observation

Investigation of the earthquake related ionospheric anomalies as the precursor of an earthquake is engaging topic since the last six decades. The first studies were reported at Good Friday Alaska earthquake in 1964 (Davies and Baker, 1965; Leonard and Barnes, 1965). Other subsequent studies showing the formation of ionospheric anomalies before strong earthquakes (Yuen et al., 1969; Weaver et al., 1970) was published and, particularly, seismic-ionospheric studies have become more popular with the increasing diversity and quality of developing technology and measurement methods. It was introduced satellitebased data the early 1980s (Gokhberg, 1983) and the GPS technology was firstly applied on January 17, 1994, at Mw 6.7 Northridge earthquake (Calais and Minster, 1995). Some remarkable studies have been carried out on the seismic-ionospheric anomalies of earthquakes in Turkey (Doğan et al., 2011; Arıkan et al., 2012; Ulukavak and Yalçınkaya 2016).

The CODE-GIM data was used to find the temporal variations of TEC values at the epicenter and to analyze the spatial distribution of the anomalies. The IONEX files are downloaded from the NASA website (ftp://cddis.gsfc.nasa.gov/gps/products/ionex/). The CODE-GIM data covers ±87.50 latitude and ±1800 longitude ranges with 2.50 x 50 spatial resolutions (5184 cells) and 2-h temporal resolutions. The TEC values of the epicenter are calculated by the bivariate interpolation from four nearest grid point of epicenter coordinates (Schaer et al, 1998).

In this study, the pre-earthquake ionospheric anomalies of Bodrum earthquake were investigated by CODE-GIM data. The TEC values of the epicenter were obtained from IONEX files using a simple 4-point bivariate interpolation. The ionospheric anomalies were detected by using a quartile-based process during 15 days before the earthquake. Kp, Dst geomagnetic storm indices and F10.7 solar index were utilized to cross-check the effects of geomagnetic and solar activities on TEC anomalies.

Kp, Dst and F10.7 indices were used to cross-check the effects of geomagnetic and solar activities on the anomalies and to define the seismo-ionospheric disturbances (SID) more accurately. The Kp and Dst geomagnetic indices were obtained from http://wdc.kugi.kyoto-u.ac.jp/wdc/expdata.html#index and F10.7 solar index were obtained from ftp://ftp.swpc.noaa.gov/pub/indices/quar_DSD.txt.

Analysis method A quartile-based process (Liu et al., 2009) was applied to determine the TEC anomalies before 15 days of the earthquake. The median Md of every consecutive 15 days of the TEC was calculated to find the divergence between the observed TEC on the 16th days and the calculated median Md. To determine the level of the divergence, the lower quartile (LQ) and the upper quartile (UQ) were computed. In order to describe an anomaly at the TEC variation, the lower and upper bounds were defined as LB = Md - 1.5 (Md - LQ) and UB = Md + 1.5 (UQ - Md), respectively. When an observed TEC on the 16th day is exceeding its previous 15-day based UB or LB, the positive or negative abnormal TEC signal is approved, respectively. The observed TEC value between the UB and LB defines no abnormal condition in the ionosphere.

Results

Figure 1 The TEC variation of the epicenter, Kp, Dst and F10.7 index in the 15 pre-earthquake days (UTC) in which the vertical black line shows the time of Bodrum earthquake. a) TEC variation of epicenter b) Kp index c) Dst index and d) F10.7 index between 6 July 2017 and 20 July 2017.

Mw 6.6 Bodrum earthquake on July 20, 2017 A normal faulting and shallow depth Mw 6.6 earthquake occurred in southeast of Bodrum, Turkey on July 20, 2017 22:31:11 UTC. The location of the earthquake is about 250 km north of the closest main plate boundary, where the Africa plate moves to the north approximately 10 mm/yr with respect to Eurasia. Therefore, the earthquake is considered an intraplate event. In the ~20 hours since the earthquake, 9 M ≥ 4.0 aftershocks occurred in fault zone and M 4.7 is the largest one (USGS, 2017). The epicenter was estimated by the Kandilli Observatory and Earthquake Research Institute Regional Earthquake-Tsunami Monitoring Center (KOERI-RETMC) located at 36.96200N, 27.40530E, within a 5-km depth (KOERI, 2017).

Figure 1 shows the temporal variations of the TEC at the epicenter and the Kp, Dst and F10.7 indices from 15 days before the Bodrum earthquake. In Figure 1(a), the anomalies were obtained by the quartile-based process and only anomalies of longer than 6 hours were shown. In the detailed analysis of Figure 1, it was observed that daily ∑Kp values are over 25 and Dst values exceeded 30 nT on 9, 16 and 17 July. The F10.7 solar index values continue to be quiet (