4.1 Case 1. The 2003 Northern Miyagi earthquake
The 2003 Northern-Miyagi earthquake occurred at the eastern edge of the Miocene northern Honshu rift system (Sato et al., 2004a), and the damaged area is the boundary between the Neogene rocks and the Paleozoic and Mesozoic rocks of the Kitakami Mountains (Kato et al., 2004). Marine Miocene sedimentary rocks spread widely in the Asahiyama and Sue Hills. The Miocene basin fill is subdivided into two units; the Matsushima-wan Group composed of mainly syn-rift sediments such as volcanic rocks, fluviolucastrine sediments and marine-interbedded sandstone and siltstone in ascending order and the Shida Group composed of post-rift sediments such as a shallow marine sandstone (Ishii et al., 1982; Kato et al., 2004).
Hypocenters are located at downward extension of a west-dipping, inverted reverse fault (Sue fault) (Umino et al, 2003; Nishimura et al., 2003; Kato et al., 2004; Kimura and Okamura, 2009). Both the Sue fault was originally formed in the extensional stress field during the Miocene opening Sea of Japan and reactivated in the compression stress field due to the subduction of the Pacific plate during Quaternary (Sato et al., 2004b).
Background seismicity before the mainshock was relatively quiet so that D95 was undetermined due to a lack of sufficient data. However, there were at least three major historical earthquakes occurred in 1861, 1900, and 1962 in the north extension of the study region. We assumed that these historical earthquakes and the 2003 Northern Miyagi earthquake are related to each other, so that we borrowed D95 value from the north extension region. The mainshock depth and D95 turn out to be compatible in this case, however, D250-450deg in this seismically inactive area is going to be critical to understand the seismogenic depth. We re-estimated D250-450deg from latest method (Matsumoto et al., 2022), which included a careful analysis for climate change correction and crustal thermal structure based on the sedimentary layers, rather than a uniform subsurface structure model, the temperature dependence of thermal conductivity, and the difference in heat generation by lithology.
D300deg maps in both Figure 1 (b) and Figure 1 (c) show similar patterns with D95 (Figure 1 (a)) being shallower toward to the west. However, a new D300deg shows better fit with D95 than the original result, particularly more moderate change in depth towards the northwestern region. D250-450deg cross-sections in both Figure 1 (e) and (f) appear compatible with Vp/Vs and aftershocks patterns as relatively a flat and constant depth. However, because of new D250deg-450deg (Figure 1 (f)) and high Vp/Vs (around 1.8) being shallower in the south than the north, new result is better fit with the Vp/Vs pattern. Because the new D450deg (Figure 1 (f)), in the epicentral region, is shallower about 5 km than the original result (Figure 1 (e)), the location of the mainshock falls onto D450deg isotherm. The kinks in the D250-450deg isotherms at about 12 km to the north from the epicenter may be artifact since it is due to the one extreme value at the temperature measurement point.
4.2 Case 2 The 2004 Chuetsu earthquake
The 2004 Chuetsu earthquake occurred in the Niigata Basin that is filled by > 6km thick sedimentary and volcaniclastic rocks (Niigata Prefectural Government, 2000). Structural analyses and stratigraphy of Neogene strata in this region suggest that reverse faults and fault-related folds have grown in the last 2 to 3 million years due to EW compression (Sato, 1992).
We re-estimated D250-450deg from latest method (Matsumoto et al., 2022), which included a careful analysis for climate change correction and crustal thermal structure based on the sedimentary layers, rather than a uniform subsurface structure model, the temperature dependence of thermal conductivity, and the difference in heat generation by lithology. D95 in Figure 2 (a) and D300deg maps for both Figure 2 (b) and Figure 2 (c) show similar patterns as being deeper toward Sea of Japan. Particularly, D95 reaches deeper than 30 km near the Sea of Japan.
D250-450deg in both Figure 2 (e) and (f) appears compatible with Vp/Vs and aftershocks patterns as deeper in the southwest. However, new D250deg-450deg in Figure 2 (f) shows better fit with D95 (Figure 2 (a)), particularly aftershocks and Vp/Vs are having a concave upward pattern in the southwest from the mainshock.
4.3 Case 3 The 2008 Iwate-Miyagi earthquake
The 2008 Iwate-Miyagi earthquake struck with a magnitude of 7.2 near the volcanic front in the eastern flank of the Ou Backbone Ranges in Northern Japan. It recorded over 4G at the ground surface in the source region (Aoi et al., 2008). The epicenter was located to the northeast of Mt. Kurikoma, a Quaternary active volcano, and this event caused many landslides, debris flows and landslide dams. Similar to the 2003 Northern Miyagi earthquake which occurred in the south from the 2008 Iwate-Miyagi earthquake, this earthquake occurred near the eastern edge of the Miocene failed rift (Sato, 1994). In the source region of this earthquake, there are many west-dipping Miocene normal faults reactivated as reverse faults under the EW compressional stress field since Pliocene (Kato et al., 2006). The basement rocks of the source region are Cretaceous granitoids (Sasada, 1985).
We re-estimated D250-450deg from latest method (Matsumoto et al., 2022), which included a careful analysis for climate change correction and crustal thermal structure based on the sedimentary layers, rather than a uniform subsurface structure model, the temperature dependence of thermal conductivity, and the difference in heat generation by lithology. Isotherms of D250deg-450deg are much shallower than D95 (Figure 3). These extremely shallow isotherms may be affected by active volcanoes close to this region.
D300deg maps in both Figure 3 (b) and Figure 3 (c) show similar patterns with D95 (Figure 3 (a)) being shallower at the epicentric region and the southwest from the epicenter. However, a new D300deg in Figure 3 (c) shows better fit with D95 than the original result (Figure 3 (b)), particularly concentrated shallow depth in the volcanic region. D250-450deg cross-sections in both Figure 3 (e) and Figure 3 (f) appear compatible with the pattern of high Vp/Vs (around 1.8) as relatively a flat depth while they are not alike to the aftershock’s patterns. Mainshock hypocenter is below the D450deg. Although new D250deg-450deg (Figure 3 (f) in the northeast give deeper depth which are slightly closer to the aftershocks in the area, new method for D250deg-450deg is not dramatically improve the original result. This result confirmed that an active volcanic region is challenging and need more consideration to estimate the seismogenic depth by heat flow data.
4.4 Case 4 the 2016 Kumamoto earthquake
The 2016 Kumamoto earthquake occurred at the Futagawa and Hinagu faults located at the topographic boundary between the Kumamoto and Yatsushiro plain and the foothills composed of Cretaceous accretionary complex and Jurassic metamorphic rocks. Aftershocks occurred along the Beppu-Shimabara graben and the southern side of the graben (National Institute of Advanced Industrial Science and Technology, 2016a).
While the most of the aftershocks occurred within the seismogenic layer, we found an exception in the aftershock distribution pattern during the 2016 Kumamoto earthquake below the southwestern strand of the Futagawa fault near the junction with the Hinagu fault (Figure 4); aftershocks apparently extend 5 km deeper than D95, which are significant considering that uncertainty of the hypocenter locations from the JUICE catalog is about 1 km in the depth direction. The estimated temperature of aftershock area is between 300°C and 450°C. The isotherm in this region is significantly deeper than that in the other part of the Futagawa Fault. This deep isotherm is constrained by the temperature data from station #1129 at Azumimachi, Uto (Sakagawa et al., 2005). The temperature profile taken from 100 m to 900 m for every 100 m in depth shows that subsurface temperature increases nearly linearly, indicating that the effect of local heat source on the data is apparently minimal. However, because the simple method we used in this analysis has limitation to estimate the crustal temperature in the complex system of extremely active groundwater flow region, extremely deep D250-450deg around a depth of 30 km may not be significant. The Vp and Vp/Vs values in the region where deep aftershocks occurred are approximately 6.5 km/s and 1.7, respectively (Matsubara et al. 2019). These ranges are not extra ordinary to be in the seismogenic zone since they are within the range in the other cases. Immediate deepening from pre-earthquake level was also recognized by the 1992 M 7.3 Landers earthquake. The seismic-aseismic transition changes to shallower as much as 3 km over the course of 4 years (Rolandone et al., 2004). They claimed that the brittle-ductile transition became deeper after Landers due to high postseismic stress and strain rates at the base of the seismogenic zone. The time-dependent changes of depth relation of the postseismic seismicity of Kumamoto earthquake may be important to monitor to understanding the mechanical behavior of rocks in the crust.
4.5 Case 5 the 2016 central Tottori earthquake
The mainshock and aftershocks of the 2016 central Tottori earthquake occurred where pre-Neogene basement rocks mainly composed by late Cretaceous and Paleogene granitic rocks are extensively exposed. While there are some minor active faults which are striking NE-SW and NW-SE, orthogonal to NE-SW, recognized around the mainshock (RGAFJ, 1991) and no major fault is known at the mainshock and aftershock region (National Institute of Advanced Industrial Science and Technology, 2016b).
4.6 Relationship between D95 and aftershock area, seismogenic layer, nucleation, and rupture process
Both background seismicity before mainshocks and seismicity after the mainshock tend to be occurred at Vp/Vs in the green color region (1.7-1.8) in Figure 1-5. Therefore, the analysis of Vp/Vs pattern is also shed light on understanding the seismogenic depth and forecasting the future aftershock regions.
Our observation confirmed that large earthquakes nucleated at the base of the deepest portion of the seismogenic layer (e.g., Sibson 1984) The nucleation depths for the main shocks and the deepest portion of the main co-seismic rupture are near or shallower than D95 for all 5 cases. Therefore, our results support that D95 is indeed a significant index for the base depth of the seismogenic layer.
For the solid crust, climate change correction and the temperature dependence of thermal conductivity and the difference in heat generation by lithology should be precisely applied in the equation at each study site. These parameters in equation 1 can be arguable since a continental crust, especially complicated island arcs as Japanese Islands, may not be suitable to estimate the crustal temperature. We partially applied the latest temperature models (Matsumoto et al, 2022) to the 2003 northern Miyagi, the 2004 Chuetsu, and the 2008 Iwate Miyagi earthquakes (Figure 1 (c and f), Figure 2 (c and f), Figure 3 (c and f)). These models are obtained by a crustal structure model that considers the temperature dependence of thermal conductivity, the difference in heat generation due to lithology, and local climate differences. New version of isotherms tends to fit better with Vp/Vs patterns and aftershock distributions.