Application of cluster analysis based on waveform cross-correlation coefficients to data recorded by ocean-bottom seismometers: results from off the Kii Peninsula

Abstract

Waveform cross-correlation coefficients (CCs) have often been used to investigate clustering of earthquakes. Such techniques, however, have rarely been applied to waveform data recorded by ocean-bottom seismometers (OBSs). The data recorded by some OBSs are strongly influenced by site effects such as the existence of unconsolidated sediment layers. The resulting waveforms tend to have a monotone frequency so that the calculated CCs stay artificially high, and false delay times are obtained at certain multiples of the period. This effect also varies from place to place. To overcome such problems, CC measurements were first performed using multiple time windows with different lengths in order to reject unstable results. Station-specific CC thresholds were then objectively determined based on the CC distributions. This method was applied to seismic measurements obtained by OBSs deployed off the Kii Peninsula from 2003 to 2007 in order to identify characteristic hypocenter patterns. Two types of earthquake clusters were found. The first occurred within the oceanic mantle, and the component events were linearly aligned along the NNE direction. Such earthquakes are considered to occur on preexisting faults due to dehydration of the serpentinized mantle. The second type of cluster consisted of interplate earthquakes that occurred at the southern tip of the Kii Peninsula. These are thought to be the result of stress caused by local structural heterogeneity of the dense rock body.

The 2016 Mw 5.9 earthquake off the southeastern coast of Mie Prefecture as an indicator of preparatory processes of the next Nankai Trough megathrust earthquake

“…Gray triangles represent locations of DONET stations. The ellipsoid labeled with "A" designates the region of interplate seismic activity reported by Akuhara and Mochizuki (2014) our routine hypocenter determinations using a 1D velocity structure (Fig. 3).…”

“…Present seismic activities in the eastern Nankai Trough are mostly within the subducting or overriding plates, and there is very low activity in the rupture area of the megathrust earthquakes (Obana et al 2004;Obana and Kodaira 2009;Nakano et al 2015). However, Akuhara and Mochizuki (2014) observed continuous activity of an earthquake cluster with magnitudes smaller than 4 along the plate boundary west of the Kii Peninsula (labeled "A" in Fig. 1a).…”

“…1 and Additional file 1: Figure S1) has facilitated continuous monitoring of offshore seismic activities Kawaguchi et al 2015). These observations revealed that, in recent years, earthquake activities along the Nankai Trough have mostly occurred in the oceanic crust or upper mantle of the subducting PSP (Obana et al 2004Obana and Kodaira 2009;Mochizuki et al 2010;Akuhara et al 2013;Nakano et al 2013Nakano et al , 2014Nakano et al , 2015Akuhara and Mochizuki 2014). The most recent notable events during this time have been the 2004 M JMA 7.1, 7.4, and 6.5 earthquakes off the Kii Peninsula, which occurred in the subducting plate (e.g., Sakai et al 2005;Seno 2005).…”

Preface for the special issue of “New Perspective of Subduction Zone Earthquakes”

“…(1)Framework and setting of the seismogenic zone from the view point of seismicity in the Nankai trough (Idehara et al 2014, Akuhara andMochizuki 2014), Japan Trench (Aochi and Ide 2014), Hikurangi margin , or from the geological and geophysical points of view for the Nankai Trough (Tsuji et al 2014, Sugihara et al 2014) and Japan Trench (Boston et al 2014, Koge et al 2014) (2)Material and physical properties or conditions of the fault for the Nankai Trough (Takahashi et al 2014, the Japan Trench (Sawai et al 2014), and the Costa Rica margin (Namiki et al 2014) (3)Analysis of water-rock interaction with faulting in an ancient accretion complex and fault in the Shimanto Belt, Japan (Schumann et al 2014, Hamahashi et al 2015, Chelunpu Fault, Taiwan , Alaska , Median Tectonic Line, Japan , crustal rocks on land (Kogure et al 2014), and theoretical treatment of fracturing, friction, and earthquakes (Nishiyama et al 2014, Kame et al 2014 Traditional research on earthquakes is physics-based, but physico-chemical processes and their geological record in fault rocks, both in active and inactive fossilized rocks, are of recent concern. Many studies of natural records of seismic faulting have been conducted in terms of their physico-chemical processes.…”

“…(1)The Nankai trough (14 papers; Sugihara et al 2014, Tsuji et al 2014, Idehara et al 2014, Akuhara and Mochizuki 2014, Hyodo et al 2014, Takahashi et al 2014, Yamada and Shibanuma 2015, Takeshita et al 2014, Hino et al 2015, and Toki et al 2014) (2)The Japan trench (6 papers; Aochi and Ide 2014, Koge et al 2014, Sawai et al 2014, and Boston et al 2014) (3)Other trenches and fault zones (4 papers; Maekawa et al 2014, Namiki et al 2014) and ancient accretionary complexes and faults on land (7 papers; Schumann et al 2014, Hamahashi et al 2015, Kogure et al 2014, and Hashimoto and Yamano 2014) (4)Theoretical treatments of fracture and earthquake (2 papers; Kame et al 2014 andNishiyama et al 2014) The papers are also categorized into three scientific issues, focusing on different regions within each topic:…”