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Spatiotemporal Variations in the Stress Field In Maeda et al. Earth, Planets and Space (2020) 72:117 https://doi.org/10.1186/s40623-020-01245-8 EXPRESS LETTER Open Access Spatiotemporal variations in the stress feld in the northeasternmost part of the NE Japan arc: constraints from microearthquakes Sumire Maeda1* , Toru Matsuzawa2 , Tomomi Okada2, Hiroshi Katao3, Takeyoshi Yoshida2 , Masahiro Kosuga4 and Makoto Otsubo1 Abstract We determined focal mechanism solutions of microearthquakes and examined the stress feld in the low-seismicity region from southern Hokkaido to eastern Aomori, NE Japan. The stress felds determined in this study comprise (1) a reverse faulting stress regime in southern Hokkaido with the axis of maximum compressional stress (σ1) being sub- horizontal and trending WNW–ESE, and (2) a stress regime in eastern Aomori to Tsugaru Strait that shows temporal variations and diferential stress of less than tens of MPa. The spatiotemporal variation in stress from eastern Aomori to Tsugaru Strait might refect the efects of the upper-plate bending and the 2011 Mw 9.0 Tohoku-Oki earthquake. It also indicates that the compressional stress caused by the descending Pacifc plate is relatively weak, which is similar to other areas in eastern parts of the NE Japan arc. Keywords: Focal mechanism, Stress feld, Low-seismicity area, Microseismicity, Forearc, Northeastern Japan, Pacifc plate subduction, Kurile arc Introduction in low-seismicity areas. However, to properly understand Te stress feld in inland areas of NE Japan is generally earthquake generation mechanisms, it is necessary to recognized as being associated with a WNW–ESE com- investigate the stress feld in low-seismicity areas as well pressional reverse-faulting stress regime (e.g., Townend as in areas with high seismicity. and Zoback 2006; Terakawa and Matsu’ura 2010; Yuku- In Japan, the High-Sensitivity Seismograph Network take et al. 2015) related to subduction of the Pacifc (Hi-net) has been deployed nationwide by the National plate. However, recent studies have revealed that there Research Institute for Earth Science and Disaster Resil- is a localized stress feld in these areas which is diferent ience (NIED) since the 1995 Mj 7.2 Hyogo-ken-Nanbu from that expected from the subduction of the Pacifc (Kobe) earthquake, which has been greatly improved Plate and that the stress feld is spatially inhomogene- the ability for detecting microearthquakes (Obara et al. ous (e.g., Terakawa and Matsu’ura 2010; Imanishi et al. 2005). Applying the inversion method developed by 2012, 2013; Yoshida et al. 2012, 2015, 2019). Given that Michael (1984, 1987) to micro- and small-earthquake the stress distribution is commonly estimated using focal focal mechanism data, Yoshida et al. (2015) defned the mechanism data, it is difcult to estimate the stress feld stress regime in the forearc region of the NE Japan, where the stress feld had not previously been defned in detail owing to the low seismicity in the region. *Correspondence: [email protected] Here, we focus on the low-seismicity area from 1 Geological Survey of Japan, AIST Tsukuba Central, National Institute southern Hokkaido (region N) to eastern Aomori of Advanced Industrial Science and Technology, 7, Higashi-1-1-1, Tsukuba, Ibaraki 305-8567, Japan (region S), Japan (Fig. 1), where the stress feld was not Full list of author information is available at the end of the article previously estimated by Yoshida et al. (2015, 2019). Tis © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Maeda et al. Earth, Planets and Space (2020) 72:117 Page 2 of 7 46.0˚ 0 Mj 4 Mj 3 5 00 200 3 42.5˚ Mj 2 250 Mj 1 10 44.0˚ 0 15 20 Hokkaido 150 30 Depth (km) 90 20 80 70 60 50 40 42.0˚ ) 0 30 25 42.0˚ Region N (southern Hokkaido) 250 sugaru Strait 41.5˚ Aomori Pref. 0 0 30 60 50 40 70 150 80 90 20 20 ) 0 1 40.0˚ Region C (T 5 Iwate Pref. Aomori 41.0˚ 10 38.0˚ 15 0 Honshu 25 Depth (km) Region S (eastern 40.5˚ 40 km 20 1 0 12 100 80 70 km 60 50 0 30 140 40 0 100 36.0˚ 120 25 138.0˚140.0˚142.0˚144.0˚146.0˚ 141.0˚ 141.5˚ 142.0˚ Fig. 1 Tectonic setting and hypocenter distribution in and around the study area. (Left) Hypocenter distribution in NE Japan. The red rectangle indicates the location of the study area shown in the right-hand panel. Thin broken black and orange lines show the position of the Japan trench and depth contours (in km) for the upper surface of the Pacifc slab, respectively (Nakajima and Hasegawa, 2006; Kita et al. 2010). The arrow shows the direction of plate motion of the Pacifc plate relative to the Okhotsk plate (Wei and Seno 1998). Red triangles denote active volcanoes. Circles denote hypocenters determined by JMA (3 June 2002 to 31 December 2017). (Right) Hypocenter distribution in the study area, with hypocenter depth shown by the color scale. The northern and southern parts of the study area are referred to as “region N” (southern Hokkaido) and “region S” (eastern Aomori), respectively. The region combining the southern half of region N and the northern half of region S is referred to as “region C” (Tsugaru Strait) region corresponds to the northeasternmost part of Hi-net, it is possible to estimate the stress feld in this the NE Japan arc (Fig. 1a), which is colliding with the area now. Kurile arc. Te stress feld in Iwate Prefecture, which is In this paper, we frst determine the focal mechanisms located to the south of region S, has been determined of micro and small earthquakes from southern Hokkaido by Yoshida et al. (2015, 2019). Although the detailed to eastern Aomori. We then examine the spatiotemporal stress distribution has not been determined because of variation in the stress feld in the study area. Finally, we the low seismicity in Hokkaido, including region N, the discuss the factors afecting the stress feld in the north- orientation of maximum horizontal compressive stress easternmost part of the NE Japan arc. (σHmax) has been estimated to be roughly parallel to the relative motion of the Pacifc plate (e.g., Ghimire et al. Data and methods 2005; Terakawa and Matsu’ura 2010; Yukutake et al. Hypocenter and focal mechanism data 2015). Previous estimates of the stress feld are also We used waveform data obtained by Hi-net for events consistent with the regional stress feld estimated from with Japan Meteorological Agency (JMA) magnitudes the strikes of ore veins (Watanabe 1986). However, greater than 2.0 for the period from June 2002 to Decem- because the seismicity in the study area is very low and ber 2017, for depths shallower than 25 km (Fig. 1b). no large events have occurred since the deployment of We determined hypocenters and focal mechanisms via Hi-net, the stress feld in this area has not been deter- these data and the following method. First, we picked mined in detail. Since the seismic waveform data have P-wave frst-motion polarities and onsets from the digi- accumulated over time since the deployment of the tal waveform data of each earthquake using the WIN system (Urabe and Tsukada 1992). Next, we determined Maeda et al. Earth, Planets and Space (2020) 72:117 Page 3 of 7 hypocenters using the Hypomh program (Hirata and For the period before the earthquake, the result of the Matsu’ura 1987) with the one-dimensional velocity stress tensor inversion in region N indicates that this model of Hasegawa et al. (1978). We then used the soft- region is characterized by a reverse faulting stress regime ware pick2mec (Katao and Iio 2004) to determine focal with σ1 axis being sub-horizontal, trending between E–W mechanisms using the phase data and the hypocenters. and WNW–ESE. Te stress ratio is < 0.5, which indicates Te pick2mec software uses a procedure proposed by that the magnitude of σ2 is closer to σ1 than to σ3 (region Maeda (1992) to determine the focal mechanisms. Tis N in Fig. 3a). In contrast, the results of the stress tensor software assigns each focal mechanism a score based on inversion in regions C and S indicate that these regions the ratio of consistent polarities to all polarities. Finally, are characterized by a reverse faulting stress regime with we retained only reliable solutions using the same quan- σ1 axis being sub-horizontal and trending between N–S titative criterion as employed by Maeda et al. (2018). and NNE–SSW. Te stress ratios show a wide distribu- Moreover, we evaluated the uncertainty of focal mecha- tion (regions C and S in Fig. 3a). nisms by measuring the extent of these multiple fault For the period after the Tohoku earthquake, region N plane solutions estimated using pick2mec with the Kagan is characterized by a reverse faulting stress regime with angle (Kagan 1991).
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