Similarity and Cluster Analysis of Intermediate Deep Events in the Southeastern Aegean

Abstract:

In order to gain a better understanding of geodynamic processes in the Hellenic subduction zone (HSZ), in particular in the eastern part of the HSZ, we analyze a cluster of intermediate deep events in the region of Nisyros volcano. The events were recorded by the temporary seismic network EGELADOS deployed from September 2005 to March 2007. The network covered the entire Hellenic subduction zone and it consisted of 23 offshore and 56 onshore broadband stations completed by 19 permanent stations from NOA, GEOFON and MedNet. The cluster of intermediate deep seismicity consists of 159 events with local magnitudes ranging from magnitude 0.2 to magnitude 4.1 at depths from 80 to 200 km. The events occur close to the top of the slab at an about 30 km thick zone. The spatio-temporal clustering is studied using three component similarity analysis.

Single event locations obtained using the nonlinear location tool NonLinLoc are compared to relative relocations calculated using the double-difference earthquake relocation software HypoDD. The relocation is performed with both manual readings of onset times as well as with differential traveltimes obtained by separate cross-correlation of P- and S-waveforms. The three-component waveform cross-correlation was performed for all the events using data from 45 stations. The results of the similarity analysis are shown as a function of frequency for individual stations and averaged over the network. Average similarities between waveforms of all event pairs reveal a low number of highly similar events but a large number of moderate similarities. Interestingly, the single station similarities between the event pairs show (1) in general decreasing similarity with increasing epicentral distance, (2) reduced similarities for paths crossing boundaries of slab segments, and (3) the influence of strong local heterogeneity leading to a considerable reduction of waveform similarities e.g. in the center of the Santorini volcano.