Influence of Crustal Lithology and the Thermal State on Microseismicity In
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Maeda et al. Earth, Planets and Space (2021) 73:173 https://doi.org/10.1186/s40623-021-01503-3 EXPRESS LETTER Open Access Infuence of crustal lithology and the thermal state on microseismicity in the Wakayama region, southern Honshu, Japan Sumire Maeda1* , Shinji Toda2, Toru Matsuzawa3, Makoto Otsubo4 and Takumi Matsumoto1 Abstract Here we investigate the infuence of the lithology and thermal state of the upper crust on earthquake distributions beneath the Wakayama region, southern Honshu, Japan, to better understand the infuence of crustal conditions on regional seismogenesis. The earthquakes are concentrated in the deeper sections of mafc belts and shallower sec- tions of pelitic belts, based on a comparison of relocated hypocenters and estimated subsurface geological structures. We compare the frictional properties of pelitic rocks and basalt, as obtained from petrological experiments, with the hypocenter depth distributions in pelitic and mafc belts to assess the control of crustal lithology on the depth extent of regional seismicity. The earthquake distributions are consistent with the temperature ranges over which the respec- tive rock types are expected to exhibit a velocity-weakening behavior, based on the petrological experiments. The results suggest that the occurrence of shallow intraplate earthquakes is controlled by the temperature- and lithology- dependent friction of the upper crust. Keywords: Lithological heterogeneity, Seismicity, Seismogenic depth, Lithospheric thermal structure, Geological structure, Kii Peninsula Introduction lithological properties of the crust controls the depth Te upper crust is generally considered a zone of seis- extent and magnitude of earthquakes. mic heterogeneity. Tis heterogeneity has been linked to Te strength profle and rock frictional properties of physical conditions within the upper crust, including the the crust are closely related to the observed seismicity heterogeneous distributions of active faults and subsur- (e.g., Scholz 1988). Te relationship between the lower face lithologies, and spatial heterogeneities in the stress limit of the seismogenic zone and the depth to the brit- and temperature felds. Previous studies have reported tle–ductile transition in the crust has been extensively a correlation between the distributions of inland earth- investigated in seismically active lithotectonic belts (e.g., quakes and the physical conditions (e.g., temperature, Albaric et al. 2009; Hauksson and Meier 2019). However, structure, and lithology) of lithotectonic belts (e.g., Mag- these previous studies did not discuss whether these istrale and Zhou 1996; Albaric et al. 2009; Hauksson lithotectonic belts extend downward through the seismo- and Meier 2019). Tese studies have suggested that the genic layer. Sibson (1977) originally proposed a crustal model based on a crustal strength profile derived from *Correspondence: [email protected] geological observations and experimental stud- 1 National Research Institute for Earth Science and Disaster Resilience, 3-1 Tennodai Tsukuba, Ibaraki 305-0006, Japan ies. Improved models have since been developed Full list of author information is available at the end of the article by comparing this model with natural earthquake © The Author(s) 2021. 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Earth, Planets and Space (2021) 73:173 Page 2 of 10 observations and additional experimental studies (e.g., Geological background Sibson 1982; Scholz 1988; Verberne et al. 2020). The Te area to the south of the Median Tectonic Line (MTL) crustal strength profile varies with the predominant in southwestern Japan is known as the “Outer Zone of minerals in the crust (e.g., Scholz 1988; Wintsch and Southwest Japan.” Tis outer zone consists mainly of Yeh 2013), which indicates that the crustal strength an accretionary prism (sediments, volcanic rocks, and profile is dependent on crustal lithology. equivalent metamorphic rocks) above a descending slab. Another fundamental property of the crust is the Te Sambagawa, Chichibu, and Shimanto belts strike frictional nature of crustal rocks (e.g., Dieterich 1979a, E–W in the Wakayama region, and are separated by b; Ruina 1983; Scholz 1988, 1998). Scholz (1988) pro- the MTL, Aridagawa Tectonic Line (ATL), and Butsuzo posed a crustal model of rock frictional properties Tectonic Line (BTL) from north to south, respectively based on experimental studies, with the seismogenic (Fig. 1). Te Sambagawa belt consists mainly of pelitic layer corresponding to the region, where the friction and mafc schists (e.g., Takagi 1986), and is tradition- rate parameter, a–b (Dieterich 1979a, b; Ruina 1983), ally divided into three zones (spotted, non-spotted, and is negative. Numerous experimental results have pro- Mikabu zones) based on lithology, geological structure, vided information on rock frictional properties for a and metamorphic grade (e.g., Kurimoto 1993, 2013; variety of crustal lithologies (e.g., Blanpied et al. 1991; Nakayama 1983). Te spotted zone is composed mainly He et al. 2007; Hartog et al. 2012; Zhang et al. 2017; of relatively high-grade mafc schist (metabasalt) that Verberne et al. 2020), and it is becoming possible to is are especially common around Mt. Ryumon (Fig. 1). discuss the relationship between the seismogenic layer Te non-spotted zone is composed mainly of relatively and rock frictional properties in a variety of crustal low-grade pelitic schist. Te Mikabu zone is composed lithology. mainly of relatively low-grade mafc rocks (basalt) and It is generally thought that the lower limit of the pelitic rocks. Te mafc rocks typically occur north of the background seismicity is controlled by the frictional ATL in the Mikabu zone, whereas the pelitic rocks are properties of the rock and the limit is located shallower mainly distributed south of the non-spotted zone. Te than the brittle–ductile transition zone (Scholz 1988). weakly metamorphosed Chichibu belt consists mainly of Terefore, the background seismicity should be sen- alternating marine sandstone and mudstone (e.g., Wal- sitive to the change in the frictional properties better lis and Okudaira 2016). Te non-metamorphosed Shim- than the brittle–ductile transition although the rup- anto belt consists mainly of sedimentary and mixed rocks ture of large earthquakes can reach down to the transi- (e.g., Taira et al. 1988). tion zone (Scholz 1988). Moreover, the upper limit of the background seismicity cannot be explained by the vertical strength profle expected from a simple brittle Methods model, while a rate- and state-dependent friction model Hypocenter relocation suggests the existence of the upper limit as indicated by We used the double-diference hypocenter location tech- Scholz (1988). nique (hypoDD; Waldhauser and Ellsworth 2000) to relo- Here we attempt to quantitatively interpret the vari- cate the earthquakes, because this approach provides ations in the depth extent of shallow inland seismicity better constraints on the detailed structures of active by focusing on diferences in lithotectonic belts and faults in a high-seismicity area than absolute location temperature-dependent rock friction to better under- methods (e.g., Bouchaala et al. 2013). We relocated 8761 stand the relationship between the depth extent of seis- events (M ≥ 0.0) that occurred during the period from 1 mogenesis and the lithological properties of the crust January 2015 to 31 December 2016 using the JMA2001 at depth. We frst describe the geological background one-dimensional seismic velocity model (Ueno et al. of the study area, the Wakayama region, southern Hon- 2002) and the compressional- and shear-wave arriv- shu, Japan (Fig. 1). We then compare the relocated als from the JMA unifed catalog (left panel of Fig. 2). hypocenters, which are constrained via the double- Because the seismicity in the Wakayama region is almost diference relocation technique (Waldhauser and Ells- stable in the long period as shown in Additional fle 1: worth 2000), with the estimated subsurface geological Fig. S1, we regarded this 2-year seismicity as the repre- structure, as derived from Bouguer gravity anoma- sentative seismicity in this region. Te inset in Additional lies, to clarify the detailed relationship between the fle 1: Figure S2a shows the locations of the seismograph hypocenters and lithotectonic belts. We conclude by stations (https:// www. data. jma. go. jp/ svd/ eqev/ data/ discussing the possibility of this relationship being con- bulle tin/ deck. html) used to relocate the hypocenters in trolled by the temperature-dependent rock friction of this study. For further details on the hypoDD relocation lithotectonic belts. method, see Waldhauser and Ellsworth (2000).