Systematic difference between first-motion and waveform-inversion solutions for shallow offshore earthquakes due to a low-angle dipping slab
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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.
KeywordsFocal 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.
Observed first-motion polarization and propagation during the 2016 Off Mie earthquake
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).
Simulation of seismic wave propagation in the 3D heterogeneous model
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.
Discussion and conclusions
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.
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