Sequence of deep-focus earthquakes beneath the Bonin Islands identified by the NIED nationwide dense seismic networks Hi-net and F-net
© The Author(s) 2017
Received: 28 November 2016
Accepted: 2 March 2017
Published: 8 March 2017
Observed seismic energy propagation during deep-focus earthquakes
Figure 2a shows vertical velocity seismograms for frequencies of 0.02–0.05 Hz (purple line) and 1–32 Hz (black line) recorded at Hi-net stations located near the Pacific coasts of Honshu and Hokkaido. Solid red and blue lines indicate the theoretical traveltimes of P- and S-waves, respectively. In the low-frequency seismograms, which are typically used in moment tensor analysis, a set of P- and S-wave propagations during the Mw 6.5 earthquake was observed, and we could not confirm occurrence of other earthquakes. However, observed high-frequency P- and S-wave seismograms showed spindle-shaped seismogram envelopes with delay of peaks and long-duration coda waves caused by the waveguide effect within the heterogeneous Pacific slab (e.g., Furumura and Kennett 2005; Takemura et al. 2016). Slight delays of P- and S-wave arrivals, which were caused by the low-velocity anomaly (Kita et al. 2010), were recorded in the Hokkaido region. In the high-frequency seismograms, coherent signals aside from those of the Mw 6.5 earthquake appear several times. Some of these signals (shown as dashed lines, parallel shifted from the solid lines) show apparent velocities similar to those of the P- and S-waves of the Mw 6.5 earthquake.
To investigate the sets of high-frequency P- and S-wave propagations, we calculated mean square (MS) seismogram envelopes of the sum of the three-component filtered seismograms, for frequencies of 1–32 Hz. An example MS envelope recorded at N.JUOH is shown in Fig. 2b. Although the typical coda amplitude during a single earthquake decays smoothly and monotonically with elapsed time (summarized in Sato et al. 2012, Ch. 2–3), the coda amplitude at N.JUOH does not show monotonic decay, but instead shows several sudden increases in amplitude within the coda envelope. Eight seismic phases can easily be identified in the logarithmic MS envelope (bottom of Fig. 2b) that are not apparent in a linear plot (upper part of Fig. 2b). Logarithmic plots of the high-frequency MS envelope enable detection of small earthquakes hidden in the coda waves of larger earthquakes. The time intervals and amplitude ratios for the phase pairs B–E, D–F, and G–H are very similar, which indicates the occurrence of a sequence of large-to-moderate earthquakes at similar hypocenter locations. We note that phases B and E, respectively, represent the P- and S-waves during the Bonin Mw 6.5 earthquake.
In the snapshots, two coherent seismic phases propagated through northeastern Japan from the Sea of Japan (marked A and C), which means that one deep-focus earthquake (Event 0) occurred beneath the Sea of Japan before the Mw 6.5 Bonin earthquake (Event 1). After Event 0, three additional P-wave propagations across Japan (B, D, and G) and three large S-wave propagations around northeastern Honshu and Hokkaido (E, F, and H) are found. Because the propagation patterns of the latter two P (D and G)- and S (F and H)-waves were very similar to the patterns of the former waves (B and E), we infer that two earthquakes (Events 2–3) occurred sequentially after Event 1, and their hypocenter locations are expected to be close to that of Event 1. Strong S-wave propagations to the NNE only (phases of E, F and H) may be related to heterogeneities within the Pacific slab (e.g., Takemura et al. 2016).
Earthquakes analyzed in this study
Origin time (JST)
M 6.8 (JMA)
M 6.5 ± 0.02
M 5.8 ± 0.02
Magnitude estimation from maximum-amplitude distributions
We examined the spatial distribution of the maximum S-wave amplitudes during three earthquakes that occurred around the Bonin Islands (Events 1–3). Maximum S-wave amplitudes were measured from the MS envelopes at F-net stations, using a 30-s time window starting from the S-wave arrival. Variations in site amplifications for high frequencies exist even for borehole Hi-net stations (Figure 7 of Takemoto et al. 2012). To minimize the effects of site amplifications, we analyzed the maximum S-wave amplitudes of the seismograms at the F-net stations, which are installed at rock outcrop sites. In addition, because the latter two earthquakes have hypocenter locations very close to that of Event 1, the maximum S-wave amplitude distributions are affected predominantly by differences in the source mechanisms and sizes of the events.
Discussion and conclusions
The observed Hi-net waveform records detected the occurrence of sequential deep-focus earthquakes around the Bonin Islands. Using snapshots of high-frequency seismic energy propagation from the dense Hi-net, we successfully identified Event 2 within the overlapping coda waves of the Mw 6.5 Bonin earthquake (Event 1); this event corresponds to an event magnitude of ~6, which was missed in both the JMA and F-net MT catalogs. The timings of Events 2 and 3 after Event 1 were about 60 and 230 s, respectively. Within 4 min, these M-6-class deep-focus earthquakes occurred sequentially at depths of around 480 km along the Izu–Bonin arc. The maximum-amplitude distributions of each event recorded at F-net stations enabled us to estimate the magnitudes of Events 2 and 3 precisely. The estimated magnitudes of Events 2 and 3 were 6.5 ± 0.02 and 5.8 ± 0.02, respectively. Recently, quantitative evaluations of the b value, stress accumulation, and internal deformation within the slab were conducted using source mechanism and magnitude data from a worldwide catalog of deep-focus earthquakes (e.g., Obayashi et al. 2017; Zhan 2017). Because the combined catalogs of the International Seismological Centre-Global Earthquake Model (ISC-GEM) (Storchak et al. 2013) and the USGS-NEIC include only 1287 moderate-to-large (M > 5.5) deep-focus earthquakes in the world, precise magnitude estimation in a certain subduction zone is important for such quantitative evaluations. Furthermore, only eight sequential occurrences of deep-focus earthquakes with M > 6 are listed in this catalog, and no sequence of three M-6-class deep-focus earthquakes has been listed.
According to the USGS catalog, the hypocenters of the target earthquakes show westward shifts of approximately 40 km and slight deepening over the 4 min in which this sequence occurred. Assuming a rupture velocity of 4 km/s, and a duration referred from the scaling relation (Figure 6.3 of Frohlich 2010), the spatial distribution of the rupture area associated with this sequence reaches 50 km in width, which corresponds well with the horizontal width of the metastable olivine wedge (MOW) at this depth within the Pacific slab (e.g., Jian et al. 2008; Furumura et al. 2016). Therefore, the target earthquakes may be interpreted as caused by transformational faulting within the MOW.
The detected sequence of M-6-class deep-focus earthquakes occurred after a large (Mw 7.9) deep-focus earthquake that occurred on May 30, 2015, at a depth of 680 km beneath the same region (e.g., Takemura et al. 2016; Ye et al. 2016). Although the hypocenter depth of the Mw 7.9 earthquake was approximately 150 km deeper than those of typical deep-focus earthquakes (400–520 km), the seismicity of typical earthquakes increased after this earthquake (bottom subfigure of Fig. 1c). The relation between the detected earthquake sequence and the large (Mw 7.9) deep-focus earthquake, which may provide key information about the cause of deep-focus earthquakes and the complex subduction system along the Izu–Bonin arc, remains an open question.
The mechanisms of deep-focus earthquakes at depths of 400–500 km beneath the Izu–Bonin arc, including the detected earthquake sequence, are characterized by down-dip compression (e.g., Alpert et al. 2010). Recent seismic surveys have revealed the complex configuration and subduction system of the Pacific slab beneath the Izu–Bonin arc (e.g., Miller et al. 2004; Fukao and Obayahashi 2013; Wei et al. 2015; Porritt and Yoshioka 2016). Such lateral tension events are consistent with the proposed slab geometries (e.g., Fukao and Obayahashi 2013; Wei et al. 2015; Obayashi et al. 2017). The hypocenters of the earthquake sequence are located in the transition region from slab stagnation to penetration. Obayashi et al. (2017) evaluated the stress accumulation within the slab due to deep-focus earthquakes beneath the Bonin Islands and revealed that vertical compressional stress is accumulating in the bottom part of the subducting Pacific slab. They suggested that vertical compressional stress accumulation may have caused the Mw 7.9 Bonin earthquake and promoted slab penetration into the lower mantle (green line in Fig. 4). Referring to the receiver function of the F-net N.OSWF (see map in Fig. 4), Porritt and Yoshioka (2016) also proposed a complex folded slab model. The detected sequence of earthquakes occurred just above the positive pulse of their receiver functions, which indicates a sudden change in the structural properties of the slab near this depth. Although different interpretations have been proposed for the configuration and the subduction system of the Pacific slab around the Izu-Bonin arc, the subducting Pacific slab is expected to be complex at the depths in which the detected M-6-class sequence occurred. To provide better insight into the subduction process in this region and the causes of these deep-focus earthquakes, evaluating the detailed configuration of the subducting Pacific slab and the precise characteristics of the seismicity in this region should be high priorities for future study.
ST conducted the waveform analysis and drafted the manuscript. TS and KS participated in designing the study and interpreting the results, and helped draft the manuscript. All authors read and approved the final manuscript.
The Hi-net and F-net waveform data, and the MT solutions of the F-net data, are available via the website of the National Research Institute for Earth Science and Disaster Resilience, Japan (last accessed May 30, 2016). The unified hypocenter catalog of the Japan Meteorological Agency (JMA) provides seismicity data from 1923 to July 2015. We also used the Preliminary Determined Earthquake catalog provided by the JMA (last accessed May 30, 2016) and the earthquake catalog provided by the United States Geological Survey, National Earthquake Information Center (http://earthquake.usgs.gov/earthquakes/search/ (last accessed February 18, 2016). Bathymetric data are from ETOPO1 (Amante and Eakins 2009). The software by Maeda et al. (2011) for sensor response correction is available via Dr. Maeda’s website (https://github.com/takuto-maeda/hinet_decon/releases). Generic Mapping Tools (Wessel and Smith 1998) and Seismic Analysis Code (SAC) were used for drawing figures and signal processing, respectively. TauP (Crotwell et al. 1999) is available via the Incorporated Research Institute for Seismology website (http://ds.iris.edu/ds/nodes/dmc/software/downloads/taup/). We also thank Dr. S. Padhy, an anonymous reviewer, and the editor Dr. A. Nishizawa for their careful reading and constructive comments, which have helped improve the manuscript.
The authors declare that they have no competing interests.
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