Earthquake Clusters in Corinth Rift European Geosciences Union General Assembly 2013 Vienna | Austria | 07 – 12 April 2013 Mesimeri M.D. 1 , Papadimitriou E. E. 1 , Karakostas V. G. 1 and Tsaklidis G. M. 2 1. Geophysics Department, Aristotle University of Thessaloniki, GR54124 Thessaloniki, Greece 2. Department of Statistics and Operational Research, Aristotle University of Thessaloniki, GR54124 Thessaloniki, Greece mmesimer@geo.auth.gr, ritsa@geo.auth.gr, vkarak@geo.auth.gr, tsaklidi@math.auth.gr 2.0≤M<3.0 3.0≤M<4.0 4.0≤M<5.0 M≥5.0 Figure 1 - Seismicity map of the complete catalogue (Mc≥2.0) for the region of Corinth Gulf (2008-2012). Earthquakes are denoted by circles increasing in size according to the magnitude. Figure 2 - Spatial distribution of earthquake clusters centres (denoted by star) for 2008-2012. The num- bers are referring to Tables 1 and 2 for the eastern and western part respectively. 0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 100 200 300 400 500 600 700 Corinthiakos Eastern Part case2 (2008-2012) Days Cummulative number All data M≥4.0 Clusters Declustered Figure 3 - a) Seismicity map of the complete catalogue (left), the declustered catalogue (right) and the clusters (bottom) for the Eastern part of Corinth Gulf (2008-2012). Earthquakes are denoted by circles increasing in size according to the magnitude. b) Seismicity rates for the three data sets for the eastern part of Corinth Rift. Solid, dotted and dashed lines cor- respond to complete, declustered and clusters catalogues, respectively. Stars denote events with M≥4.0. a) b) S/N Starting Time Duration (days) M max Mean Epicenter #of events d max (Km) S K t max B1 04/02/2008, 20:25 0.634 5 21.93 38.092 11 9.41 1.910 16.220 0.000 B2 24/4/2008, 06:22 4.502 3.3 21.706 38.216 21 4.95 -0.528 1.845 1.602 B3 17/07/2008, 11:25 8.5 2.9 21.959 38.456 33 5.83 0.022 2.927 1.117 B4 19/7/2008, 01:58 9.619 3.9 21.897 38.289 35 2.82 -1.624 4.914 1.691 B5 23/10/2008, 14:44 12.092 3.7 21.967 38.452 44 3.95 1.425 4.022 0.000 B6 06/03/2009, 03:19 10.727 4 21.866 38.346 43 6.13 2.079 5.957 0.737 B7 17/03/2009, 12:44 0.56 3 22.428 38.418 11 3.6 0.612 1.781 0.275 B8 18/06/2009, 10:27 21.923 3.8 22.024 38.307 71 7.2 -0.760 2.025 1.493 B9 16/11/2009, 20:45 11.114 2.8 21.991 38.413 22 3.9 0.871 2.183 0.143 B10 11/12/2009, 16:13 13.688 3.1 21.978 38.411 20 7.25 0.653 2.474 0.743 B11 18/01/2010, 09:24 35.149 5.5 21.967 38.415 481 11.38 4.554 51.757 0.040 Table 2 – Basic properties of the clusters of the western part of Corinth rift shown by asterisks in map of Figure 2. Figure 7 - Maximum distance from the first event in a sequence compared to maximum magnitude of it. Solid circles and squares correspond to sequences with event N≥10. Open circles and squares corre- spond to sequences with events 5≤N<10. 2 2.5 3 3.5 4 4.5 5 5.5 0 2 4 6 8 10 12 Magnitude Distance (Km) swarms N>10 swarms N<10 Ms-AS N>10 Ms-AS N<10 Table 1 – Basic properties of the clusters of the eastern part of Corinth rift shown by asterisks in map of Figure 2. S/N Starting Time Duration (days) M max Mean Epicenter #of events d max (Km) S K t max A1 21/10/2008, 15:20:14.40 3.46 3.6 22.581 38.155 10 3.8 -1.01 5.83 1.625 A2 16/5/2009, 12:56:19.6 6.17 4.5 22.672 38.123 17 5.7 5.92 74.8 0.316 A3 2/9/2009, 08:17:36.9 4.88 4.4 23.279 38.093 29 5.3 8.83 88.8 0 A4 27/1/2011, 11:58:57.7 0.5675 3.3 23.159 38.212 15 2.7 -1.4 3.54 1.522 A5 22/9/2012, 03:52:24.8 1.75 4.9 22.705 38.078 15 6.2 30.66 997.22 0 A6 1/10/2012 , 23:56:59.6 1.13 3.2 22.682 38.093 47 3.4 1.07 2.47 0.99 In order to identify earthquake clusters from a catalogue and distinguish MS-AS from earthquake swarms the following process was followed for each subregion. Methods Clusters Identification Statistical Parameters Firstly, data were collected from the monthly bulletins of the Geophysics De- partment of the Aristotle University of Thessaloniki for the time 01/01/2008 to 31/12/2012 and a threshold magnitude was identified. Then, Reasenberg’s (1985) algo- rithm, as it has been written in Matlab lan- guage in Zmap software (Wiemer, 2001), was applied in order to extract earthquake clusters from the complete catalogue. A swarm is defined if the following criteria proposed by Mogi (1963) are met: i) the maximum of the daily number of events in the sequence (N) is greater than twice the square root of the swarm duration in days (T): N>2√T and ii) the total number of earthquakes in a sequence is at least 10. Seismic swarms are distinguished from typical MS-AS by their unique seismicity patterns: the larg- est swarm events tend to occur later in the sequence, swarms contain several events as opposed to one clear mainshock, and swarm seismicity is more pro- longed in time. One simple way to quantitatively identify earthquake clusters with swarm-like proper- ties is through characterizing the timing of the largest event relative to the rest of the seismicity. To accom- plish this, we calculate the skewness of moment re- lease history (Mo) and the kurtosis for each of the se- quences that we analyze. As described by Roland and McGuire (2009), a larger positive value is observed for pure aftershock sequences (approximately 30) while a lower or even negative value is observed for swarms (between -2 to 2). In addition, Mesimeri et al. (2013), proposed that a large value of kurtosis is ex- pected for MS-AS (kurtosis≥3) and a lower one for swarm-like sequences (kurtosis≤3) by the definition of kurtosis. Chen and Shearer (2011) classify clusters with tmax≤0.4 as early Mmax (more similar to MS-AS) and clusters with tmax≥0.4 as late Mmax (more swarm like), where tmax is the normalized timing of the largest event. Introduction Clusters commonly occur as main shock–aftershock (MS–AS) sequences but also as earthquake swarms, which are empirically defined as an increase in seismicity rate above the background rate with- out a dominant main shock .The Corinth Rift (Fig.1), which was selected as our target area, constitutes an asymmetric half-graben with the southern footwall being uplifted (Armijo et al., 1996) and appears to be the most recent extensional structure, with an extension rate of about 15mm/yr in the west and about 10mm/yr in the east (Briole et al.,2000). Due to this property and significant difference in seis- micity, we divide the region of Corinth rift into two subregions, the westerrn Part (21.50-22.50/38.00- 38.50) and eastern Part (22.50-23.50/38.00-38.50) and examine seismicity manifestation separately. References Armijo, R., B. Meyer, G. C. P. King, A. Rigo, and D. Papanastassiou. 1996. Quaternary evolution of the Corinth rift and its implications for the late Cenozoic evolution of the Aegean. Geo- physical Journal International 126 (1): 11-53 (accessed 1 April 2013). Briole, P., A. Rigo, H. Lyon-Caen, J. C. Ruegg, K. Papazissi, C. Mitsakaki, A. Balodimou, G. Veis, D. Hatzfeld, and A. Deschamps. 2000. Active deformation of the Corinth rift, Greece: Re- sults from repeated global positioning system surveys between 1990 and 1995.Journal of Geophysical Research B: Solid Earth 105 (B11): 25605-25 (accessed 2 April 2013). Chen, X., and P. M. Shearer. 2011. Comprehensive analysis of earthquake source spectra and swarms in the Salton Trough, California. Journal of Geophysical Research B: Solid Earth 116 (9) Cristelli, M., A. Zaccaria, and L. Pietronero. 2012. Universal relation between skewness and kurtosis in complex dynamics. Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 85 (6). Hatzfeld, D., V. Karakostas, M. Ziazia, I. Kassaras, E. Papadimitriou, K. Makropoulos, N. Voulgaris, and C. Papaioannou. 2000. Microseismicity and faulting geometry in the gulf of Corinth (Greece). Geophysical Journal International 141 (2): 438-56. Mesimeri, M., E. Papadimitriou, V. Karakostas, and G. Tsaklidis. 2013. Earthquake clusters in NW Peloponnese. Bulletin of the Geological Society of Greece, vol. XLVII .Proceedings of the 13th International Congress, Chania. Mogi, K. 1963. Some discussions on aftershocks,foreshocks and earthquake swarms - the fracture of a semi-infinite body caused by an inner stress origin and its relation to the earthquake phenomena. Bulletin of the Earthquake Research Institute, University of Tokyo 40 : 831-53. Reasenberg, P. 1985. Second-order moment of central California seismicity, 1969–1982. Journal of Geophysical Research: Solid Earth 90 (B7): 5479-95. Roland, E., and J. J. McGuire. 2009. Earthquake swarms on transform faults. Geophysical Journal International 178 (3): 1677-90. Sattin, F., M. Agostini, R. Cavazzana, G. Serianni, P. Scarin, and N. Vianello. 2009. About the parabolic relation existing between the skewness and the kurtosis in time series of experimental data. Physica Scripta 79 (4). Wiemer, S. 2001. A software package to analyze seismicity: ZMAP. Seismological Research Letters 72 (3): 373-82. The mean epicenters of the 26 clusters are presented in Fig.2 for the entire region of Corinth Rift and additional information for each cluster, such as staring time, duration and mean epicenter, is provided in Tables 1 and 2 for the eastern and western part, respectively. Spatial Distribution Eastern Part The complete catalogue which contains the events occurred in the eastern part of Corinth rift, was declustered by dividing the initial catalogue into three datasets, (i) the entire complete catalogue, (ii) the one that contains only the clusters (236 events) and (iii) the de- clustered catalogue (482 events), as shown in Fig.3a. Also, in Fig. 3b the seismicity rate for the three datasets is shown, along with the events with M≥4.0, denoted by stars on the demonstrating the entire catalogue. For the period 2008-2012, 6 clusters were identi- fied in the region of eastern Corinth Rift. Western Part The same procedure was followed for the western part (Fig.4). As shown in Fig.4a the 3 datasets are again (i) the complete catalogue, (ii) the one that contains only the clusters (2250 events) and (iii) the de- clustered (1709 events. The difference from the eastern part is that here the cata- logue with the clusters contains more events than the declustered one. This indi- cates that the seismicity in the western part of Corinth rift is strongly clustered. Skewness-Kurtosis Combining the skewness of seismic moment release history of each sequence with the value of tmax we distinguish MS-AS from earthquake swarms. The earthquake swarms observed here have tmax<0.4 and skewness between [-2, 2] .On the other hand MS-AS have tmax≥0.4 and skewness greater than 2. With those criteria applied we consider as MS-AS the following clusters: A2, A3, A5, B1, B11, B14, B17 and the rest as earthquake swarms. Fur- thermore, the value of kurtosis of seismic moment release history for earthquake swarms is less than 12 and for MS-AS is greater than 12. Finally, in order to quantify the relationship between kurtosis and skewness, the kurtosis of seismic moment release is plotted against the skewness (Cristelli et al., 2012; Sattin et al., 2009). The parabolic relation is presented in Fig.5 and the linear relation in Fig.6. In both figures we used additional data from our study area, which are clusters with events 5≤N<10 and duration of less than a day. The parabolic relation that exists for the financial markets is also ob- served here. The MS-AS are placed in the right branch of the parabola and the earthquake swarms are placed in the center and left branch of it. Discussion For the period 2008-2012, 26 clusters were identified in the region of Corinth Rift. Most of them (20) occurred in western part, where the seismicity is strongly clustered both in space and time. The separation between earthquake swarms and MS-AS was based on the skewness of seismic moment release and the time of occurrence of the greater event in a sequence. Kurtosis was also calculated and so its relation with skewness. Furthermore, clusters with 5≤N<10 events are used for that comparison, indicating that independently of their short dura- tion they present MS-AS or swarm characteristics. Finally, in Fig.7 the maximum distance from the first event in a sequence is plotted against the maximum magnitude of the sequence. The clusters characterized as MS-AS seem to have a linear re- lation between the magnitude and the maximum distance. In contrast, earthquake swarms are scattered. MINISTRY OF EDUCATION & RELIGIOUS AFFAIRS, CULTURE & Acknowledgments This research has been co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework NSRF) - Research Funding Program: THALES. Investing in knowledge society through the European Social Fund. 0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 500 1000 1500 2000 2500 3000 3500 4000 Corinthiakos Western Part case 2 (2008-2012) Days Cummulative number All data M≥4.5 Clusters Declustered Figure 4 - Same as figure 3 for the western part of Corinth Rift. a) b) 0 100 200 300 400 500 600 700 800 900 1000 0 200 400 600 800 1000 1200 (Skewness) 2 Kurtosis N>10 N<10 Figure 6 - Linear elation between skewness and kurtosis of seismic moment release. y=1.074x+4.56 -5 0 5 10 15 20 25 30 35 0 200 400 600 800 1000 1200 1400 Skewness Kurtosis Figure 5 - Parabolic relation between skewness and kurtosis of seismic moment release. N>10 N<10 y=1.032x 2 +1.17x+3.06