Evidence of strong long-period ground motions of engineering importance for Nankai Trough plate boundary earthquakes: comparison of ground motions of two moderate-magnitude earthquakes
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KeywordsNankai Trough Plate boundary earthquake Philippine Sea Plate Long-period ground motions Accretionary prism Tonankai earthquake Ground motion prediction equations
centroid moment tensor
finite difference method
full-range seismograph network
ground motion prediction equation
Japan Meteorological Agency
Japan integrated velocity structure model
Japan Seismic Hazard Information Station
Kiban Kyoshin network
National Research Institute for Earth Science and Disaster Resilience
peak ground acceleration
peak ground velocity
The Nankai Trough is a prominent geological structure resulting from the continuous subduction of the Philippine Sea Plate beneath the southwest Japan. Large earthquakes with magnitudes around 8 have occurred repeatedly in the region and have caused tremendous damages in southwest Japan (e.g., Ando 1975; Mochizuki and Obana 2003). The Headquarters for Earthquake Research Promotion (HERP), Japan, has estimated probabilities of 70–80% for the next magnitude 8–9 class earthquake to occur within the next 30 years in the Nankai Trough area, based on certain assumptions (HERP 2018). Moderate-magnitude earthquakes are not very frequent in the Nankai Trough compared with those in the Japan Trench area in northeast Japan. On April 1, 2016, an Mw 5.8 earthquake occurred in the coseismic slip area of the 1944 Tonankai earthquake, which was independent of the 2004 southeast off-Kii peninsula earthquake sequence. Ground motions for these earthquakes were recorded by hundreds of strong-motion stations operated by the National Research Institute for Earth Science and Disaster Resilience (NIED). In the present study, we processed and compared the observed ground motion recordings for two moderate-magnitude earthquakes in the Nankai Trough: one having an Mw of 6.5 (an aftershock of the 2004 southeast off-Kii peninsula earthquake) and the other having an Mw of 5.8 (the 2016 southeast off-Mie Prefecture earthquake). According to the revised list of region names for earthquake information by Japan Meteorological Agency (JMA), both the 2004 and 2016 events belong to the same region: Mie-ken Nanto-Oki, which means the southeast off-Mie Prefecture. However, we used the two distinct names for the earthquakes to separate the 2016 event, which presumably occurred as a plate boundary event, from the 2004 earthquake sequence that occurred inside the subducting Philippine Sea Plate as intraslab events. Moreover, the region name for the 2004 event referred to in this paper is consistent with the previous region name (see Availability of data and materials) and the published literature discussed below.
The 2004 Mw 6.5 earthquake occurred as an aftershock of the 2004 off-Kii peninsula earthquake sequence. The mainshock (Mw ~ 7.5) in the sequence occurred on September 5 23:57 JST (UTC + 9 h), while the aftershock analyzed in this study occurred on September 7 08:29 JST. These earthquakes did not overlap with but occurred near the source region of the anticipated megathrust earthquake in the Nankai Trough. As the ground motions from the earthquakes were recorded by the nationwide dense seismic observation network (Okada et al. 2004), the events drew huge attention from the seismological community. The ground motions for the mainshock and the largest foreshock (Mw ~ 7.2) were analyzed by a number of researchers (e.g., Hayakawa et al. 2005; Miyake and Koketsu 2005; Yamada and Iwata 2005; Furumura et al. 2008). Because the earthquakes occurred offshore, about 120 km from inland settlements, the damage done by the earthquakes was limited. On the other hand, significant sloshing of liquid and damage to the gauge pole in a large oil storage tank located on the Kanto Plain, about 400 km from the epicenter of the mainshock, was reported (Hatayama and Zama 2005). Miyake and Koketsu (2005) noted that the 2004 mainshock provided a timely warning of damaging long-period ground motions from future great magnitude Nankai Trough earthquakes. Furumura et al. (2008) showed by computer simulations of long-period ground motions that earthquakes in the Nankai Trough are expected to be the most disastrous as they can produce extraordinarily large and long-duration shakings in the Kanto Plain. However, ground motions from the relatively smaller magnitude aftershock events were not analyzed in detail in previous studies.
Source parameters for the earthquakes used in the paper
Origin time (JST)
September 7, 2004, 08:29
April 1, 2016, 11:39
The JMA magnitude and moment magnitude are determined differently. The JMA magnitude is an amplitude-based scale and is determined from the maximum displacement amplitude, high-pass-filtered at a corner frequency of 6 s (Katsumata 2004). When an earthquake is so small that the magnitude cannot be determined based on the displacement amplitude, the JMA magnitude is determined based on the velocity amplitude using the magnitude formula given by Funasaki and Division (2004). The moment magnitude, however, is determined from the seismic moment by modeling long-period seismic waves. For example, the F-net moment magnitudes are determined by modeling seismic waves in the period range of 20–100 s for moderate events, and 50–200 s for events larger than Mj 7.5 (Kubo et al. 2002). The average JMA magnitude is larger than the moment magnitude by about 0.1 or less in the Mw range of about 5–7 for earthquakes with focal depths smaller than 100 km (Utsu 2002). The reason for the difference between the JMA and moment magnitudes is generally well understood for inland crustal events (e.g., Furumura and Kennett 2001). However, similar phenomena for plate boundary earthquakes have been discussed less in the literature. In this regard, it is important to distinguish the ground motion characteristics of the 2004 and 2016 events for the sake of reliably predicting ground motions of future large plate boundary earthquakes in the Nankai Trough subduction zone.
In this paper, we first describe selection and processing of the data used in the present study. Next, we set up the essential elements for sections that follow by introducing and describing example recordings at two sites common to both events. Thirdly, we compare the peak ground accelerations (PGAs) and peak ground velocities (PGVs) obtained from strong-motion recordings of the two events with reference to the existing ground motion prediction equations (GMPEs). Then, we compare the absolute acceleration response spectra (ARS) of the two events at selected periods between 0.1 and 10 s with reference to GMPEs that employ moment magnitude in their prediction model. Absolute velocity response spectra (AVRS) of the two events at selected periods are then compared with reference to GMPEs that employ JMA magnitude in their prediction model. Following our presentation of the comparisons, we summarize them quantitatively based on an analysis of residuals. We also compare PGVs from broadband recordings at different passbands between 5 and 100 s. Finally, we discuss the implications of our findings for future ground motion predictions for Nankai Trough subduction zone earthquakes.
Data selection and processing
The hypocenter locations and magnitudes of the 2004 and 2016 events used in this paper are listed in Table 1. The hypocenters are taken from the unified JMA hypocenter catalog, and centroid depths and moment magnitudes are taken from F-net moment tensor catalog (Kubo et al. 2002). Figure 1 depicts the epicenter locations for the events. Based on the focal and centroid depths, both events are categorized as intraplate earthquakes. However, in this paper, we treat the 2016 event as a plate boundary event and the 2004 event as an intraslab event based on the published literature, of which some papers were introduced in the previous section. We used the strong-motion data recorded by K-NET and KiK-net and broadband ground motion data recorded by F-net (Okada et al. 2004). The locations of the strong-motion and broadband stations used in this study are depicted in Fig. 1. We selected the strong-motion recordings that contained the arrival of S-waves. The S-wave arrival times were estimated based on a 1D velocity model (JMA 2001, Ueno et al. 2002), and the waveforms were visually inspected to confirm the S-wave arrivals. We also removed records dominated by long-period noises in the target periods by visual inspection of acceleration Fourier spectra, which normally fall off smoothly at longer periods (Brady 1988).
The PGAs, PGVs, and response spectra were calculated from the bandpass-filtered strong-motion seismograms with a cutoff frequency of 0.07 Hz. We computed two types of response spectra: absolute acceleration response spectra and absolute velocity response spectra for a 5% damping ratio. The response spectra were computed following the method of Nigam and Jennings (1969). The values of the PGAs and PGVs were taken as the maximum values of the vector sum (i.e., the square root of the sum of squares) of two horizontal component acceleration and velocity time histories, respectively, over the available time steps, while the response spectra were obtained as the maximum values of the vector sum of two horizontal component response time histories computed for the corresponding periods. The F-net broadband recordings were processed to correct for the effects of instrumental response, and PGVs were computed at different passband frequencies from the two horizontal component recordings similar to the PGVs of strong-motion recordings.
If we assume that site amplification effects are similar for both events, the similarity between the low-frequency components of the ground motions may be related to earthquake type and path effects. Previous studies have shown relatively smaller PGVs (e.g., Si and Midorikawa 1999) and smaller response spectra at frequencies higher than about 0.1 Hz (e.g., Dhakal et al. 2010; Morikawa and Fujiwara 2013) for inter-plate events than for intraslab events at distances shorter than about 300 km. Since the observations for the 2016 event are contrary to previous results, i.e., the observed values are larger than expected for the moment magnitude of the event, the path effect seems to be a probable reason for the similarity of the spectral amplitudes at frequencies between about 0.1 and 1 Hz (1–10 s). More recently, Uetake (2017) suggested that the existence of thick sedimentary layers above the source region of the shallow earthquake could be the reason for the stronger excitation of long-period surface waves than in the case of similar events lacking thick sedimentary layers above the source. According to the JIVSM, there is a thick (~ 5 km) low-velocity sedimentary layer (Vs = 1.0 km/s) above the centroid location for the 2016 event, while the low-velocity layer is less than 1 km thick above the centroid location for the 2004 event (Fig. 2). Therefore, the effect of thick unconsolidated sediments near the source may be an important contributing factor to the observed large long-period ground motions for the 2016 event.
Comparison of PGAs and PGVs
Morikawa and Fujiwara (2013) updated the database used by Kanno et al. (2006) with additional data and obtained GMPEs for PGAs, PGVs, JMA intensities, and acceleration response spectra between 0.05 and 10 s applicable to different tectonic environments, as well as sites located on deep sediments in Japan. Because of their applicability to a wide range of site conditions, we selected the GMPEs in Morikawa and Fujiwara (2013) and adjusted the observed values for amplification due to deep and shallow sediments following the correction terms in the GMPEs. We adopted the hypocentral distance as a measure of the distance between the source fault and the site for use in the GMPEs, considering the moderate size of the earthquakes, and relatively longer distances from the epicenters to the recording stations (> 50 km for the 2016 event and > 100 km for the 2004 event). The data for the 2004 event were adjusted for the anomalous path effects for west Japan due to the effect of Philippine Sea Plate as described by Morikawa and Fujiwara (2013). These anomalous path effects necessitate general corrections for larger seismic intensity in the fore-arc regions observed during deep-focus events due to effective propagation of high-frequency seismic waves that results from a high Q in the oceanic plates (e.g., Utsu and Okada 1968; Furumura and Kennett 2005). The anomalous path effects considered here are different from those due to unconsolidated sediments in the accretionary prisms discussed later in this study. The adjusted PGAs and PGVs are plotted as functions of hypocentral distance in Fig. 9a, b, respectively. Prediction curves with the ranges of one standard deviation are also plotted. The observed PGAs for the 2016 event (red circles) generally follow the trend of the prediction curve (pink line), and most data lie well within the range of one standard deviation (dashed pink lines), suggesting that the PGAs are typical of those from previous plate boundary events. Unlike the 2016 event, the 2004 event is an intraslab event and the prediction curves plotted are also for intraslab events. The observed PGAs (black circles) for the 2004 event are noticeably underestimated by the GMPEs, with many data points more than one standard deviation above the curve (dashed gray lines). This suggests that the radiation of short-period ground motions was stronger for this event than the median values for previous intraslab events. The PGVs also generally follow a trend similar to that for the PGAs for the 2016 event, while the PGVs for the 2004 event are better explained by the prediction curves, unlike the PGAs. In summary, the PGAs and PGVs for the 2004 event are generally larger than those for the 2016 event, and while the PGAs for the 2016 event are consistent with the prediction curves, the PGA values for the 2004 event suggest event-specific radiation.
To get a rough idea on the strength of high-frequency radiation, we estimated the approximate values of the corner frequencies for the two events using the S-wave recordings at two selected F-net stations located at rock sites. An additional file is provided that deals with the estimation of the corner frequencies for the two events (see Additional file 1). We found that the corner frequencies for the 2004 event are higher than or similar to those for the 2016 event. Given the similar corner frequencies for the two events, the stress drop for the 2004 event is expected to be much larger because the seismic moment of the 2004 event is larger by a factor of about 12 than that for the 2016 event (Brune 1970, 1971). Stress drop may be considered as one of the parameters that control the strength of high-frequency radiation (Boore 1983). In fact, as mentioned above, the observed PGAs were clearly underestimated by the GMPEs for the 2004 event, while the prediction was reasonable for the 2016 event. Together, these observations may indicate that the 2004 event was a higher stress drop event.
Comparison of ARS
Comparison of AVRS
Unlike the moment magnitude, the JMA magnitude for both the 2004 and 2016 events is 6.5. Considering the faster estimation of the JMA magnitude, Dhakal et al. (2015) constructed GMPEs for absolute velocity response spectra at periods between 1 and 10 s from the viewpoint of early warning of earthquake long-period ground motions using the JMA magnitude, hypocentral distance, and site correction terms. They used events having focal depths shallower than 50 km and treated all events identically despite their different tectonic settings. Site parameters, namely AVS30 and depth of deep sediments, were employed to obtain the site correction coefficients. The site parameters were identical to those employed by Morikawa and Fujiwara (2013). The AVRS, relative velocity response spectra, and pseudo-velocity response spectra are identical at the peak response period and differ gradually at longer or shorter periods (Dhakal et al. 2014). In this section, we compare the observed AVRS for the two events with reference to the GMPEs of Dhakal et al. (2015).
Despite the difference in the number of recordings, the general distribution pattern is identical at a period of 10 s for the two events. Large response values can be seen at sites located in sedimentary basins such as the Osaka basin for both events. At equal distances, such as between radii of 200 and 300 km, the observed values and their distribution are similar for the two events. This suggests that seismic waves with a period of about 10 s were similarly excited and/or transmitted from areas close to the sources. We discuss the similarity of ground motions at longer periods further in later sections.
To augment the qualitative comparisons described above, we computed mean residuals and standard deviations with respect to the GMPEs to summarize the comparisons numerically. Because the difference between the two events is obvious at short periods, two statistical parameters (the mean and standard deviation of the residuals) are computed for selected periods between 1 and 10 s. The data were divided into three groups based on hypocentral distances: those smaller than 100 km, those between 100 and 200 km, and those larger than 200 km. The mean residuals and standard deviations were computed at periods of 1, 2, 5, 7, and 10 s. The mean residuals are computed for each group of data from the logarithmic differences between the observed values and those predicted using the GMPEs for the ARS and AVRS described above. Positive values indicate underprediction (i.e., the observed values are larger than the predictions), on average, over the range of analysis, while negative values indicate the opposite. Similarly, the standard deviations are computed from the logarithmic differences of the observed and predicted values as root-mean-square residuals in each group of data.
Comparison of PGVs from broadband recordings
In the previous section, we described the results of some statistical analyses of the residuals between the observed strong-motion data and median predictions determined using GMPEs. The strong-motion data were recorded by K-NET and KiK-net and were mostly for distances within 400 km of the epicenters (for example, see Figs. 10 and 12). The K-NET and KiK-net recordings have limitations at much longer periods such as 20–100 s due to either a low signal-to-noise ratio or a limited duration for moderate earthquakes. The F-net waveform data, on the other hand, are continuous and are recorded in a low-noise system and environment. As a result, ground motions at much longer periods, such as 50–100 s, and for much longer distances, at which strong-motion stations were not triggered, can be evaluated using the F-net recordings.
The PGVs at different passbands are regressed separately with respect to the hypocentral distances for the corresponding events using the linear least square method, and the fitted lines are drawn in Fig. 15. The fitted lines clearly show different slopes for the different passbands, becoming gentler as the period increases. These results support the longer-period ground motions decaying slowly with distance. The difference between the fitted lines for the two events increases as the period increases at equal distances, while the difference remains generally constant for a given passband regardless of distance. As evidenced from the plot for the longest periods (50–100 s) analyzed in this study, we found that on average the ratio (~ 13) of the fitted values is similar to the ratio of the seismic moments (6.0E + 18 Nm to 4.9E + 17 NM ~ 12), estimated by F-net (NIED) for the two events.
Site amplification effects may be neglected at periods over 5 s at the F-net stations because the stations are set up inside tunnels constructed on stiff or hard rock. Both events were reverse fault events with similar dip amounts, and the station coverage is almost identical for the two events. There were more than 60 recordings covering all of Japan for each event. Therefore, the similarities at periods of 5–20 s for these two events that had a significantly large difference in moment magnitude may be attributed to factors closely related to path effects.
We compared the observed peak ground motions for the 2004 Mw 6.5 off-Kii peninsula and 2016 Mw 5.8 southeast off-Mie Prefecture earthquakes. The comparisons were complimented with GMPEs based on past earthquake recordings. Based on these comparisons, the two earthquakes are nearly identical in terms of their peak ground motions at periods of about 2–20 s, but their short-period (smaller than about 2 s) and much longer-period (larger than 20 s) peak ground motions are clearly different. Obviously, their tectonic locations and moment magnitudes are different based on results published by many researchers. The Mw for the 2016 event, estimated by the Global CMT project (e.g., Ekström et al. 2012), is 5.9, which is identical to that estimated for the 2016 event by Asano (2018) using the empirical Green’s function method (Irikura 1986; Irikura et al. 1997) based mostly on ocean bottom recordings, while the corresponding value from the F-net moment tensor catalog is 5.8. Similarly, the Mw for the 2004 event is 6.6 in the Global CMT catalog, while it is 6.5 in the F-net moment tensor catalog. Such differences in the value of the moment magnitude arise from differences between the data sets, crustal and subsurface velocity models, mathematical approximations, and other factors used in the simulations (e.g., Kubo et al. 2002). For example, moment magnitudes differing by up to 0.5 units have been reported for the same moderate-magnitude events under thick sediments during the 2012 Ferrara seismic sequence in northeast Italy. These discrepancies were caused mainly by differences in the velocity models employed in the inversions (e.g., Malagnini and Munafò 2017). More recently, Takemura et al. (2018) estimated the moment magnitude of the 2016 event in the Nankai Trough using the 3D Green’s function computed by 3D FDM employing a realistic 3D subsurface velocity model that includes both the unconsolidated soft sediments and subducting oceanic plates. Their estimated moment magnitude is 5.6 for the 2016 event, compared to its 5.8 F-net and 5.9 GCMT catalog magnitudes. If we took the 5.6 value into consideration in the GMPEs, the observed data for the 2016 event would be further underestimated by the GMPEs presented in Figs. 9, 10. This means that shallow plate boundary events in the Nankai Trough produce significantly large ground motions at periods of about 1–10 s in relation to the GMPEs of Morikawa and Fujiwara (2013). One of the reasons for the differences between the GMPEs (Morikawa and Fujiwara 2013) and the data, particularly for the 2016 event, is that the GMPEs for plate boundary-type events were constructed using mostly plate boundary events in northeast Japan due to the lack of corresponding records in the Nankai Trough subduction zone.
Furumura et al. (2008) performed 3D simulations of long-period ground motions for the 1944 Tonankai earthquake using the source model proposed by Yamanaka (2004) and a detailed 3D velocity model reconstructed by Tanaka et al. (2006) comprising the subducting Philippine Sea Plate and the overlying sedimentary layer in the accretionary wedge. The simulations of the Tonankai earthquake produced about ten times larger peak ground displacements and velocity response spectra at periods larger than 2 s in the Kanto Plain, located about 400 km from the epicentral area, than those for the observed Mw ~ 7.5 mainshock event of the 2004 off-Kii peninsula earthquakes. The large difference was largely due to the greater moment magnitude of the Tonankai earthquake (Mw 8.1). In contrast, in the present study, the 2016 event is smaller than the 2004 event by 0.7 moment magnitude units, but the observed ground motions at periods of about 2–20 s are as strong as those for the 2004 event. These observations may suggest that the subsurface velocity structure in the Nankai Trough subduction zone is favorable for producing much stronger long-period ground motions for plate boundary events than for intraplate events inside the Philippine Sea Plate due to its proximity to overlying unconsolidated sediments. More recently, Uetake (2017) compared ground motions for two shallow crustal events having identical JMA magnitudes (Mj 6.7), similar moment magnitudes (Mw 6.2 and 6.3), similar focal depths, similar focal mechanisms, and similar source-to-target-site distances in central Japan. The subsurface velocity structures were clearly different near the source regions of the two events. One event, which originated beneath a relatively thick, low-velocity sedimentary layer, excited surface waves of larger amplitudes with a dominant period of about 5 s. These waves propagated into the Kanto Plain, which further amplified and trapped the waves to produce large, long-duration shaking. By complementing the observations with simulations, Uetake (2017) confirmed the effects of the subsurface velocities in the source regions on long-period ground motions. The vertical cross sections of the JIVSM plotted in Fig. 2 show that the thickness of low-velocity sediments is much larger in the source region of the 2016 Mw 5.8 event than in the source region of the 2004 Mw 6.5 event. The 2004 event also generated significant long-period shaking, but the comparable peak amplitudes of ground motion at periods of 2–20 s for the smaller-moment-magnitude 2016 event are most likely due to differences in the subsurface velocity structures near the source regions of the two earthquakes.
An analysis of seismic moments from many shallow-focus, inland, and plate boundary earthquakes by Takemura (1990) suggested that the excitation of somewhat longer-period surface waves near the sources for shallow-focus and inland earthquakes could be the reason for the larger JMA magnitudes for these earthquakes even when their seismic moments are similar to ordinary plate boundary earthquakes in northeast Japan. The larger Mj values for crustal events away from subduction zones in western Japan are posited to be related to the efficient propagation of Lg-type waves in the region (Furumura and Kennett 2001). Seismic wave propagations for the 2016 event may also have been compounded by wave-guide effects due to the subducting Philippine Sea Plate in the offshore region, and Lg-type wave conversion and transmission through the inland crustal structure in southwest Japan (Furumura et al. 2014). Together, all these effects contributed to the JMA magnitude being larger by 0.7 magnitude units than its F-net catalog Mw value of 5.8.
Dhakal et al. (2015) employed Mj and Mw separately in identical ground motion prediction models for events having Mw > 6.5 and found that inter-event errors were significantly smaller using Mj than those using Mw at periods between 1 and 10 s. The results were obtained for events including a large proportion of data from plate boundary events with focal depths smaller than 50 km in northeast Japan as well as inland earthquakes. On average, the JMA magnitude was larger than the Mw for events having Mw values smaller than about 7 in their data set, and for some events, the difference was 0.4 or larger. It has now become apparent that the GMPEs by Dhakal et al. (2015) described the observed response spectra of the 2016 Mw 5.8 event well at periods between about 2 and 10 s without correction for propagation path effects caused by either thick sediments or surface waves generated near the source, while GMPEs employing Mw appear to be insufficient without correction for propagation path effects in the Nankai Trough, especially for events whose JMA and moment magnitudes are significantly different. The GMPEs of Dhakal et al. (2015) were constructed from an earthquake early warning viewpoint. Therefore, their GMPEs should be used with caution for other purposes such as the seismic hazard analysis. Nonetheless, the findings of this study clearly indicate the necessity of considering propagation path effects in the GMPEs for seismic hazard evaluation of long-period ground motions in the Nankai Trough. Moreover, evaluating the long-period ground motions of future Nankai Trough plate boundary earthquakes will be greatly assisted by further validation of the available velocity models against observed data for moderate-magnitude earthquakes.
We analyzed strong-motion and broadband ground motion data for the 2004 Mw 6.5 southeast off-Kii peninsula earthquake, an offshore aftershock event that occurred inside the Philippine Sea Plate near the Nankai Trough axis. We also analyzed similar data for the 2016 Mw 5.8 southeast off-Mie Prefecture earthquake, an independent offshore event in the source area of the 1944 Tonankai earthquake. Despite a large difference in the moment magnitudes for the two events, their magnitudes in the JMA magnitude scale were identical, both being 6.5. We found that the PGAs and ARS at periods smaller than about 0.5 s were noticeably underestimated by the GMPEs for the 2004 event, while the ARS at longer periods were generally well explained. In contrast, the PGAs and ARS at periods smaller than about 1 s were generally explained well by the GMPEs for the 2016 event, while the ARS at periods of about 2–10 s were significantly underestimated, especially at distances larger than about 100 km. We found that the ground motions for the 2016 event (Mw 5.8) were comparable to those for the larger-moment-magnitude 2004 event (Mw 6.5) at equal distances for periods of engineering importance of about 2–20 s over wide areas. We checked the existing subsurface velocity model, which suggested that the difference in the relative location of the two events, particularly with respect to the presence of a thick accretionary prism of low seismic velocity in the Nankai Trough, might have caused the difference between the two events in their excitation of seismic waves with periods of 2–20 s. Interestingly, it was found that the observed ARS are generally consistent with the GMPEs at distances shorter than about 100 km for the 2016 event. These observations suggested that the large response spectra at periods of about 2–10 s for the 2016 event at larger distances are not due to source effects. Moreover, GMPEs employing Mj generally described the observed AVRS well at corresponding periods for both events. These results suggest that the intensity of long-period ground motions at periods of about 2–10 s may be better represented by Mj than by Mw in GMPEs for shallow and moderate earthquakes in the Nankai Trough subduction zone, unless some adjustments are made in Mw-based GMPEs to account for propagation path effects due to the accretionary wedge in the Nankai Trough.
YPD analyzed the data, organized discussions, and drafted the manuscript. WS, TK, NM, TK, and SA provided insightful comments. All authors took part in revising the manuscript. All authors read and approved the final manuscript.
We would like to thank the Japan Meteorological Agency for providing us with hypocenter information for the earthquakes used in this study. We also used moment magnitudes from Global CMT project. We would like to thank Wessel and Smith (1998) for providing us with Generic Mapping Tools, which were used to make some figures in this paper. We would also like to thank the Headquarter for Earthquake Research Promotion, Japan, for providing us with the subsurface velocity model (JIVSM). We are thankful to editor, Dr. Kimiyuki Asano, reviewer, Dr. Dino Bindi, and an anonymous reviewer for their constructive comments which helped us significantly improve the manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The strong-motion (K-NET and KiK-net) and broadband (F-net) recordings used in this study were downloaded from the Web sites: http://www.kyoshin.bosai.go.jp/ and http://www.fnet.bosai.go.jp/freesia/top.php?LANG=en, respectively. The J-SHIS subsurface velocity models and JIVSM used in this study were downloaded from the Web sites: http://www.j-shis.bosai.go.jp/en/ and https://www.jishin.go.jp/evaluation/seismic_hazard_map/lpshm/12_choshuki_dat/, respectively. The hypocenter location and JMA magnitudes were taken from the Web site: https://www.data.jma.go.jp/svd/eqev/data/bulletin/hypo_e.html. The moment magnitudes and centroid depths were taken from the Web site: http://www.fnet.bosai.go.jp/event/joho.php?LANG=en. The region names of earthquakes may be found at https://www.jma.go.jp/jma/press/0609/20b/20060920chiiki.html, in Japanese. For other specific information such as the list of recordings used in the study, contact authors for data requests.
This study was supported by “Advanced Earthquake and Tsunami Forecasting Technologies Project” of NIED.
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