- Open Access
Systematic difference between first-motion and waveform-inversion solutions for shallow offshore earthquakes due to a low-angle dipping slab
© The Author(s) 2016
- Received: 2 June 2016
- Accepted: 20 August 2016
- Published: 31 August 2016
Systematic difference between first-motion and waveform-inversion solutions for shallow offshore earthquakes was examined by using the seismograms of the 2016 Off Mie (Mw 5.8) earthquake occurred at a depth of 14 km southeast off of the Kii peninsula, central Japan. Observed seismograms illustrated first arrivals with an apparent velocity of 7.2 km/s, which is faster than crustal P waves. The apparent velocity and polarization pattern of the first arrivals were reproduced by a finite-difference method simulation incorporating the three-dimensional Philippine Sea slab. The first arrivals consist of P waves radiated downward from the source, passing the oceanic Moho as head waves. Thus, a first-motion analysis, assuming a one-dimensional structure, causes incorrect estimations of the focal mechanisms and hypocenter depths, which tend to be deeper than the actual ones. Our result possibly indicates that the seismicity above the oceanic Moho was underestimated in the previous catalogs.
- Focal mechanism
- First-motion polarization
- Head wave
- Nankai Trough
- Philippine Sea slab
The Philippine Sea slab (PHS) is subducting beneath southwestern Japan along the Nankai Trough at a rate of 2–6 cm per year (e.g., Seno et al. 1993; Heki and Miyazaki 2001). Due to the subduction of the PHS, large (M > 8) interplate earthquakes repeatedly occurred at recurrence intervals of approximately 100–150 years (e.g., Ando 1975). Indeed, in this area, a slip deficit was widely documented based on GPS Earth Observation Network (GEONET) and seafloor geodetic observations (e.g., Hashimoto et al. 2004; Yokota et al. 2016). This means that stress is being accumulated in preparation for future large earthquakes. On the basis of the current slip deficit rate and assuming other geophysical parameters such as the recurrence interval and the friction law, significant tsunamigenic earthquake scenarios have been proposed (e.g., Hori et al. 2004; Hok et al. 2011; Kim et al. 2016). In addition to seismic velocity structure and seismicity, several phenomena such as non-volcanic tremors, very low-frequency earthquakes, and slow-slip events have been extensively studied around the subducting PHS slab in order to understand the mechanisms of the large interplate earthquakes (e.g., Ozawa et al. 2002; Obara 2002; Shiomi et al. 2006, 2008; Shelly et al. 2007; Hirose et al. 2008; Citak et al. 2012; Matsuzawa et al. 2013; Kim et al. 2016; Kita and Matsubara 2016; Takagi et al. 2016).
Using Hi-net waveforms during the 2016 Off Mie earthquake, we propose that the misestimation of focal mechanisms and depths for first-motion polarization analysis is caused by subsurface structure related to the geometry of the low-angle dipping slab. Our hypothesis is validated via finite-difference method (FDM) simulations of seismic wave propagation using a three-dimensional (3D) heterogeneous velocity structure model. We also discuss the effects of the dipping slab on hypocenter location and seismicity by first-motion analysis in the other subduction zones.
During the 2016 Off Mie earthquake, almost all stations observed upward polarizations in the first motion (Fig. 1b), which could not be reproduced by the F-net MT solution in a homogeneous medium. Similar features were found in four other earthquakes (Additional file 1: Figure S2). Furthermore, although most hypocenter depths estimated by first-motion analysis were deeper than the depth of the oceanic Moho of the PHS (~19 km), the MT solutions indicated that the earthquakes occurred at the interface or within the oceanic crust of the PHS (see Additional file 1: Table S1 and colors of focal spheres in Additional file 1: Figure S1).
Since the high-velocity PHS exists at shallower depths (10–15 km) beneath the epicenters of the analyzed offshore earthquakes, the velocity structure is completely different from the 1D velocity structure model. In the case where a high-velocity oceanic mantle exists beneath the hypocenters, the rays of the first arrivals in land-area stations should pass through the oceanic Moho of the PHS as a head wave (hereafter called “P PHS”) with a faster apparent velocity. This might cause misestimations of the takeoff angles from a hypocenter calculated with the Hi-net 1D velocity structure. Furthermore, to fit such fast apparent velocity around land area, the hypocenter depths might be estimated to be deeper than the actual ones (Additional file 1: Figure S4).
The observed seismograms suggest that the P PHS generated from the down-going P waves becomes first motions at land stations and is a major cause of the incorrect estimations of focal mechanisms and depths for first-motion solution. Our hypothesis was examined using 3D FDM simulations of seismic wave propagation for the 2016 Off Mie earthquake, in which we incorporated the 3D geometry of the subducting PHS. The 3D model of the FDM simulation covered a volume of 512 × 512 × 128 km3 (enclosed by red square in Fig. 1a), which was discretized with grid intervals of 0.2 and 0.1 km in the horizontal and vertical directions, respectively. Technical details of the simulation, such as FDM scheme, solid/air boundary conditions, and formulation of the anelastic attenuation are described in Takemura et al. (2015a).
The 3D velocity structure model was constructed based on the Japan Integrated Velocity Structure Model (JIVSM; Koketsu et al. 2008, 2012), which is widely used in many seismological analyses across the Japan Islands (e.g., inversion of source rupture process, evaluation of strong ground motion, and simulation of seismic wave propagation) (e.g., Koketsu et al. 2011; Iwaki et al. 2013; Maeda et al. 2013; Takemura et al. 2015b, c). Although the velocity model in the offshore region has relatively large uncertainties, upper surface of the PHS from JIVSM is consistent with other models (e.g., Hirose et al. 2008; Citak et al. 2012; Nakamura et al. 2015). Since we focused our attention on the first motions and their apparent velocities at land Hi-net stations, our model did not include low-velocity (V S < 2.9 km/s) sediments and seawater layers (V P = 1.5 km/s). The physical parameters of each layer are listed in Additional file 1: Table S2. Our 3D FDM was able to examine seismic wave propagation for frequencies less than 2 Hz under these settings.
The seismic source for the 2016 Off Mie earthquake was represented by a single-cycle Küpper wavelet function (Mavroeidis and Papageorgiou 2003) with a dominant frequency of 1 Hz. A double-couple point source for this event was assumed following the Hi-net first-motion and F-net MT solutions (see Event C in Additional file 1: Table S1), which were located within the oceanic crust of assumed velocity structure model. Here we note that since our simulation did not include small-scale velocity heterogeneity within the crust and realistic source time function, which might be required to achieve more accurate simulation for higher frequencies (≥1 Hz), we focus our attention on first-motion polarization, apparent velocity, and its transition.
We conclude that the systematic difference between first-motion and waveform-inversion solutions for shallow offshore earthquakes is mainly caused by the subducting PHS, which generates a P PHS phase with an apparent velocity of 7.2 km/s and causes the misestimations of the takeoff angles and hypocenter depths. To fit such a fast apparent velocity around land areas in the conventional one-dimensional studies, the hypocenter depth is overestimated compared to the actual one.
Figure 5 shows the vertical seismograms derived from the 2D simulations for various dip angles of subducting slab (θ = 5, 10, 20 and 30°). Since explosion source was employed, upward first motions clearly propagated along the profile. In the cases of dip angle θ = 5°, which is gentler dip angle of the PHS model used in 3D simulations, P PHS with an apparent velocity of approximately 7.2 km/s (Fig. 5, red solid line) was widely observed at epicentral distances of 60–180 km. As the dip angle θ increased, direct P waves propagating through the crust (Fig. 5, blue line) became dominant. In particular, for dip angles greater than θ = 30°, P PHS was only observed within a narrow distance range (140–180 km). This indicates that the effects of P PHS propagation on conventional determination of the hypocenter location and mechanism could be negligible in the following cases: (1) subduction zones with high (>20–30°) dip angles (e.g., Kuril, Izu-Bonin-Mariana and Tonga subduction zones) and (2) earthquakes occurring near/beneath land area.
In other subduction zones with low-angle (<20°) dipping slabs such as Cascadia, Mexican, and Peru–Chile subduction zones (e.g., Hayes et al. 2012), large (M > 8) thrust earthquakes have also repeatedly occurred at recurrence intervals of several 100 years. Seismicity and focal mechanisms near the slab interface are very important for considering such large earthquakes. Our findings suggest the possibility of underestimation of the seismicity above the oceanic Moho in such subduction zones with low-angle dipping slabs. Furthermore, low-angle dipping slabs also have a potential to affect seismic wave propagations and strong ground motions (e.g., Furumura and Singh 2002; Takemura et al. 2015b, c). In future studies, it would be important to precisely estimate the seismicity and characteristics of seismic wave prorogation by using the appropriate 3D subsurface structure model in order to overcome the misestimation of the source mechanisms and the hypocenter locations around low-angle dipping slabs.
ST conducted waveform analysis for both observation and simulation and drafted this manuscript. KS and TK participated in the study design and interpretation of the results. TS participated in the considerations for wave propagation. All authors helped drafting the manuscript. All authors read and approved the final manuscript.
The Hi-net waveform data, Hi-net hypocenter catalog, and F-net MT solutions were provided by the National Research Institute for Earth Science and Disaster Resilience, Japan (NIED), via the Institute website. We also used the unified hypocenter catalogs provided by the Japan Meteorological Agency (last accessed April 28, 2016). Bathymetric depth data were obtained from ETOPO1 (Amante and Eakins 2009). The software for sensor response correction by Maeda et al. (2011) is available via Dr. T. Maeda’s website (http://www.eri.u-tokyo.ac.jp/people/maeda/w/doku.php/codes/hinet_decon). Large-scale FDM simulations were conducted on the supercomputer system at NIED and the Earth Simulator on Japan Agency for Marine-Earth Science and Technology. Generic Mapping Tools (Wessel and Smith 1998) were used to prepare the figures. The JIVSM is available via the website (http://www.jishin.go.jp/main/chousa/12_choshuki/dat/). Depth data of upper surface of the Philippine Sea plate are available via Dr. F. Hirose’s website (http://www.mri-jma.go.jp/Dep/st/member/fhirose/ja/PlateData.html).
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
- Amante C, Eakins BW (2009) ETOPO1 arc-minute global relief model: procedure, data sources and analysis: NOAA technical memorandum NESDIS NGDC-24, National Geophysical Data Center, NOAA. doi:10.7289/V5C8276M
- Ando M (1975) Source mechanisms and tectonic significance of historical earthquakes along the Nankai Trough. Jpn Tectonophys 27(2):119–140View ArticleGoogle Scholar
- Baba T, Tanioka Y, Cummins PR, Uhira K (2002) The slip distribution of the 1946 Nankai earthquake estimated from tsunami inversion using a new plate model. Phys Earth Planet Int 132(1):59–73. doi:10.1016/S0031-9201(02)00044-4 View ArticleGoogle Scholar
- Citak SO, Nakamura T, Nakanishi A, Yamamoto Y, Ohori M, Baba T, Kaneda Y (2012) An updated model of three-dimensional seismic structure in the source are of the Tokai–Tonankai–Nankai earthquake, in Abstract of AOGS-AGU (WPGM) Joint Assembly, Singapore, 12–17 August 2012, Abstract No. OS-6-A015Google Scholar
- Fukuyama E, Ishida M, Dreger DS, Kawai H (1998) Automated seismic moment tensor determination by using on-line broadband seismic waveforms. Zisin 51:149–156 (in Japanese with English abstract) Google Scholar
- Furumura T, Singh SK (2002) Regional wave propagation from Mexican subduction zone earthquakes: the attenuation functions for interplate and inslab events. Bull Seismol Soc Am 92(6):2110–2125. doi:10.1785/0120010278 View ArticleGoogle Scholar
- Hashimoto C, Fukui K, Matsu’ura M (2004) 3-D modeling of plate interfaces and numerical simulation of long-term crustal deformation in and around Japan. Pure Appl Geophys 161:2053–2068. doi:10.1007/s00024-004-2548-8 View ArticleGoogle Scholar
- Hayes GP, Wald J, Johnson RL (2012) Slab 1.0: a three-dimensional model of global subduction zone geometries. J Geophys Res 117:B01302. doi:10.1029/2011JB008524 View ArticleGoogle Scholar
- Heki K, Miyazaki S (2001) Plate convergence and long-term crustal deformation in central Japan. Geophys Res Lett 28:2313–2316. doi:10.1029/2000GL012537 View ArticleGoogle Scholar
- Hirose F, Nakajima J, Hasegawa A (2008) Three-dimensional seismic velocity structure and configuration of the Philippine Sea slab in southwestern Japan estimated by double-difference tomography. J Geophys Res 133:B09315. doi:10.1029/2007JB005274 Google Scholar
- Hok S, Fukuyama E, Hashimoto C (2011) Dynamic rupture scenarios of anticipated Nankai–Tonankai earthquakes, southwest Japan. J Geophys Res 116:B12319. doi:10.1029/2011JB008492 View ArticleGoogle Scholar
- Hori S (2002) Comparison of earthquake mechanism solutions obtained from first motion analysis with those with waveform analysis. Zisin 55:275–284 (in Japanese with English abstract) Google Scholar
- Hori T, Kato N, Hirahara K, Baba T, Kaneda Y (2004) A numerical simulation of earthquake cycles along the Nankai Trough in southwest Japan: lateral variation in frictional property due to the slab geometry controls the nucleation position. Earth Planet Sci Lett 228:215–226. doi:10.1016/j.epsl.2004.09.033 View ArticleGoogle Scholar
- Iwaki A, Morikawa N, Maeda T, Aoi S, Fujiwara H (2013) Finite-difference simulation of long-period ground motion for the Sagami Trough megathrust earthquakes. J Disaster Res 8(5):926–940View ArticleGoogle Scholar
- Kato A, Sakai S, Obara K (2011) A normal-faulting seismic sequence triggered by the 2011 off the Pacific coast of Tohoku earthquake: wholescale stress regime changes in the upper plate. Earth Planets Space 63(7):745–748. doi:10.5047/eps.2011.06.014 View ArticleGoogle Scholar
- Kim SB, Saito T, Fukuyama E, Kang TS (2016) The Nankai trough earthquake tsunamis in Korea: numerical studies of the 1707 Hoei earthquake and physical-based scenarios. Earth Planets Space 68:64. doi:10.1186/s40623-016-0438-9 View ArticleGoogle Scholar
- Kita S, Matsubara M (2016) Seismic attenuation structure associated with episodic tremor and slip zone beneath Shikoku and the Kii peninsula, southwestern Japan, in the Nankai subduction zone. J Geophys Res 121:1962–1982. doi:10.1002/2015JB012493 View ArticleGoogle Scholar
- Koketsu K, Miyake H, Fujiwara H, Hashimoto T (2008) Progress towards a Japan integrated velocity structure model and long-period ground motion hazard map. In: Proceedings of the 14th world conference on earthquake engineering, Beijing, China, October 12–17Google Scholar
- Koketsu K, Yokota Y, Nishimura N, Yagi Y, Miyazaki S, Satake K, Fujii Y, Miyake H, Sakai S, Yamanaka Y, Okada T (2011) A unified source model for the 2011 Tohoku earthquake. Earth Planet Sci Lett 310(3):480–487. doi:10.1016/j.epsl.2011.09.009 View ArticleGoogle Scholar
- Koketsu K, Miyake H, Suzuki H (2012) Japan integrated velocity structure model version 1. In: Proceedings of the 15th world conference on earthquake engineering, Lisbon, Portugal, September 24–28Google Scholar
- Maeda T, Obara K, Furumura T, Saito T (2011) Interference of long-period seismic wavefield observed by the dense Hi-net array in Japan. J Geophys Res 116:B10303. doi:10.1029/JB008464 View ArticleGoogle Scholar
- Maeda T, Furumura T, Noguchi S, Takemura S, Sakai S, Shinohara M, Iwai K, Lee SJ (2013) Seismic- and tsunami-wave propagation of the 2011 off the Pacific coast of Tohoku earthquake as inferred from the tsunami-coupled finite-difference simulation. Bull Seismol Soc Am 103(2B):1456–1472. doi:10.1785/0120120118 View ArticleGoogle Scholar
- Matsuzawa T, Shibazaki B, Obara K, Hirose H (2013) Comprehensive model of short- and long-term slow slip events in the Shikoku region of Japan, incorporating a realistic plate configuration. Geophys Res Lett 40:5125–5130. doi:10.1002/grl.51006 View ArticleGoogle Scholar
- Mavroeidis GP, Papageorgiou AS (2003) A mathematical representation of near-field ground motions. Bull Seismol Soc Am 93(3):1099–1131. doi:10.1785/0120020100 View ArticleGoogle Scholar
- Nakamura T, Takenaka H, Okamoto T, Ohori M, Tsuboi S (2015) Long-period ocean-bottom motions in the source areas of large subduction earthquakes. Sci Rep 5:16648. doi:10.1038/srep16648 View ArticleGoogle Scholar
- Obara K (2002) Nonvolcanic deep tremor associated with subduction in southwest Japan. Science 296:1679–1681. doi:10.1126/science.1070378 View ArticleGoogle Scholar
- Okada Y, Kasahara K, Hori S, Obara K, Sekiguchi S, Fujiwara H, Yamamoto A (2004) Recent progress of seismic observation networks in Japan-Hi-net, F-net, K-NET and KiK-net. Earth Planets Space 56(8):15–28. doi:10.1186/BF03353076 View ArticleGoogle Scholar
- Ozawa S, Murakami M, Kaidzu M, Tada T, Sagiya T, Hatanaka Y, Yarai H, Nishimura T (2002) Detection and monitoring of ongoing aseismic slip in Tokai region, central Japan. Science 298(5595):1009–1012. doi:10.1126/science.107680 View ArticleGoogle Scholar
- Seno T, Stein S, Gripp AE (1993) A model for the motion of the Philippine Sea plate consistent with NUVEL-1 and geophysical data. J Geophys Res 98:17941–17948. doi:10.1029/93JB00782 View ArticleGoogle Scholar
- Shelly DR, Beroza GC, Ide S (2007) Non-volcanic tremor and low-frequency earthquake swarms. Nature 446(7133):305–307. doi:10.1038/nature05666 View ArticleGoogle Scholar
- Shiomi K, Obara K, Sato H (2006) Moho depth variation beneath southwestern Japan revealed from the velocity structure based on receiver function inversion. Tectonophysics 420:205–221. doi:10.1016/j.tecto.2006.01.017 View ArticleGoogle Scholar
- Shiomi K, Matsubara M, Ito Y, Obara K (2008) Simple relationship between seismic activity along Philippine Sea slab and geometry of oceanic Moho beneath southwest Japan. Geophys J Int 173:1018–1029. doi:10.1111/j.1365-246X.2008.03786.x View ArticleGoogle Scholar
- Storchak DA, Giacomo DD, Bondár I, Engdahl ER, Harris J, Lee WHK, Villaseñor A, Bormann P (2013) Public release of the ISC–GEM Global instrumental earthquake catalogue. Seismol Res Lett 84:810–815. doi:10.1785/02200130034 View ArticleGoogle Scholar
- Takagi R, Obara K, Maeda T (2016) Slow slip event within a gap between tremor and locked zoned in the Nankai subduction zone. Geophys Res Lett. doi:10.1002/2015GL066987 Google Scholar
- Takemura S, Furumura T, Maeda T (2015a) Scattering of high-frequency seismic waves caused by irregular surface topography and small-scale velocity inhomogeneity. Geophys J Int 201(1):459–474. doi:10.1093/gji/ggv038 View ArticleGoogle Scholar
- Takemura S, Yoshimoto K, Tonegawa T (2015b) Velocity increase in the uppermost oceanic crust of the Philippine Sea plate beneath the Kanto region due to dehydration inferred from high-frequency trapped P waves. Earth Planets Space 67:41. doi:10.1186/s40623-015-0210-6 View ArticleGoogle Scholar
- Takemura S, Yoshimoto K, Tonegawa T (2015c) Scattering of trapped P and S waves in the hydrated subducting crust of the Philippine Sea plate at shallow depths beneath the Kanto region, Japan. Geophys J Int 203(3):2261–2276. doi:10.1093/gji/ggv423 View ArticleGoogle Scholar
- Thurber C, Zhang H, Waldhauser F, Hardbeck J, Michael A, Eberhart-Phillips D (2006) Three-dimensional compressional wavespeed model, earthquake relocations, and focal mechanisms for the Parkfield, California, region. Bull Seismol Soc Am 96(4B):538–549. doi:10.1785/0120050825 View ArticleGoogle Scholar
- Ukawa M, Ishida M, Matsumura S, Kasahara K (1984) Hypocenter determination method of the Kanto-Tokai observational network for microearthquakes. Res Notes Natl Res Cent Disaster Prev 53:1–88 (in Japanese with English abstract) Google Scholar
- Wessel P, Smith WHF (1998) New, improved version of generic mapping tools released. EOS Trans Am Geophys Union 79(47):579. doi:10.1029/98EO00426 View ArticleGoogle Scholar
- Yokota Y, Ishikawa T, Watanabe S, Tashiro T, Asada A (2016) Seafloor geodetic constraints on interplate coupling of the Nankai Trough megathrust zone. Nature. doi:10.1038/nature17632 Google Scholar