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Numerical earthquake model of the 20 April 2015 southern Ryukyu subduction zone M6.4 event and its impact on seismic hazard assessment
© Lee. 2015
- Received: 31 July 2015
- Accepted: 1 October 2015
- Published: 8 October 2015
The M6.4 earthquake that took place on the 20 April 2015 off the shore of eastern Taiwan was the largest event in the vicinity of Taiwan during 2015. The mainshock was located in the southern Ryukyu subduction zone, which is the interface between the Philippine Sea Plate and the Eurasian Plate. People in Taipei experienced strong ground shaking for more than 40 s, even though the epicenter was located more than 150 km away. In order to understand the origin of ground motions from this earthquake and how it caused such strong shaking in Taipei, a numerical earthquake model is analyzed, including models of source rupture and wave propagation. First, a joint source inversion was performed using teleseismic body wave and local ground motion data. Source inversion results show that a large slip occurred near the hypocenter, which rapidly released seismic energy in the first 2 s. Then, the rupture propagated toward the shallow fault plane. A large amount of seismic energy was released during this rupture stage that slipped for more than 8 s before the end of the rupture. The estimated stress drop is 2.48 MPa, which is consistent with values for subduction zone earthquakes. Forward simulation using this inverted source rupture model and a 3D seismic velocity model based on the spectral-element method was then performed. Results indicate that the strong ground motion in Taipei resulted from two factors: (1) the Taipei basin amplification effect and (2) the specific source radiation pattern. The results of this numerical earthquake model imply that future subduction zone events that occur in offshore eastern Taiwan are likely to cause relatively strong ground shaking in northern Taiwan, especially in the Taipei metropolitan area.
- Ryukyu subduction zone
- Source rupture process
- Source radiation
- Ground motion simulation
- Basin effect
On the 20 April 2015, a moderately large earthquake with a magnitude of M L 6.38 (M w 6.4) struck offshore eastern Taiwan (hereafter referred to as the 0420 earthquake). The earthquake report of the Central Weather Bureau (CWB) showed an epicenter located at 122.441° E/24.022° N with a hypocentral depth of 30.57 km. The mainshock was close to Yilan county (about 80 km from the epicenter), where a CWB intensity scale of 4 (25–80 gal) was experienced. A CWB intensity of 4 was also recorded in Taipei City, even though it is located over 150 km from the epicenter. People in Taipei experienced strong long-period ground shaking for more than 40 s. After the earthquake, the Japan Meteorological Agency immediately issued an alert for the southern Okinawa island chain regarding a possible 1-m tsunami, but the alert was lifted after an hour, with no sign of high waves or damage. Unfortunately, the strong ground shaking of this event caused the indirect death of one person and injuries to several others in Taiwan.
List of M L > 6 earthquake events in offshore eastern Taiwan, recorded between 1994 and 2015
The centroid moment tensors (CMT) of the 0420 earthquake provided by the Real-time Moment Tensor (RMT) Monitoring System (Lee et al., 2013) showed thrust faulting mechanisms with two west–east striking nodal planes, one dipping toward the north with a shallower dipping angle, and the other dipping toward the south with a steeper fault plane (Fig. 1). Several noticeable aftershocks were recorded after the mainshock. The largest aftershock (M L 6.15) occurred around 10 h after the mainshock. The epicenters of this earthquake sequence follow a northwest-to-southeast alignment (Fig. 1). On the vertical profile (C–C’ in Fig. 1), the patterns of aftershock distributions appear to be consistent with the background seismicity, which presents a clear northward-dipping subduction Benioff zone. In addition, the depth of the 0420 event (30.57 km) also fits well with the fault geometry of the subducting plate in this area (Wu et al., 2009). From this information, the 0420 event is likely to be an inter-plate earthquake of the Ryukyu subduction zone, which lies at the boundary between the Philippine Sea Plate and the Eurasian Plate.
Hsu et al. (2012) proposed that the southernmost Ryukyu subduction zone needs to consider the potential M w 7.5–8.7 tsunami earthquakes generated by shallow ruptures. Nakamura (2014) used seismic tomography and receiver function analysis to investigate the relation between the slab structure in the southern Ryukyu arc region and the occurrence of slow-slip events that repeat biannually. Result suggests that the thermal condition of the southern Ryukyu arc is similar to the case of a hot-slab area where slow-slip events occur. The repeated occurrence of slow-slip event results in stress accumulation in the up-dip part of the fault, which could potentially induce megathrust earthquakes. Lin et al. (2014) indicated that the occurrence of a potential Sumatra-like earthquake in the south Ryukyu arc is highly likely and the rupture will mainly propagate continentward to fulfill the region of low seismicity. Thus, investigating the origin of the 0420 event and how it caused strong ground shaking in the Taipei metropolitan area is an important topic for scientific research, since it could be an indication of future hazardous large subduction zone events.
This study analyzes a numerical earthquake model of the 0420 event. The numerical earthquake model includes models of source rupture and 3D wave propagation. The entire time history of earthquake is going to be rebuilt through fully 3D simulations based on advanced numerical methods in computational seismology. Both the inverse (source inversion) and forward (ground motion simulation) studies are performed with parallel computing techniques to improve computing performance and enhance the resolution of model in both time and space. First, source rupture process analysis is carried out using joint source inversion. Then, the inversion result is considered as the input source model for a forward ground motion simulation. Since the same regional 3D Green’s function is employed in the source inversion and ground motion simulation, the 3D path effects are identical and well included in both the inverse and forward processes. Finally, the origin of the prolonged strong ground shaking in Taipei during this event and its impact on seismic hazard assessment are discussed.
In matrix A, the teleseismic P wave Green’s functions were calculated using the approach developed by Kikuchi and Kanamori (1982), where the near source structure was given by a 1D average crust model of Taiwan (Chen and Shin, 1998) while the receiver side structure used a global 1D IASPEI91 model (Kennett and Engdahl, 1991). The teleseismic synthetic waveforms were filtered between 0.01 and 0.5 Hz. The 3D synthetic Green’s functions for near-field ground motion stations were generated using the spectral-element method (SEM) (Komatitsch and Tromp 1999; Komatitsch et al. 2004), with a tomography velocity model taken from Huang et al. (2014). A complete description of the SEM mesh model is provided in the next section. Due to the lack of detailed 3D velocity structure in offshore eastern Taiwan, a lower frequency band was considered. The seismic synthetic waveforms were filtered between 0.02 and 0.1 Hz in the same frequency range as for the observed data. Lee et al. (2014) proposed that even in a lower frequency band, the full-waveform regional seismic data can still provide constraints for the source rupture process in both the spatial and temporal distributions of the slip.
In the inversion, 16 time windows of 0.8 s in length are used, each window overlapping by 0.4 s, leading to each subfault having a possibility to slip within 6.8 s after the rupture begins. A maximum rupture velocity, V r = 3.0 km/s, was assumed; thus, the inverted rupture velocity will be between 0 and 3 km/s. A northwest–southeast strike and northern dipping fault plane characterized by thrust movement (strike 295°, dip 26°, and rake 128°) taken from the RMT (Lee et al., 2013) were considered. The fault planes were divided into 156 subfaults, each with a size of 3 × 3 km2. The slip on each subfault was divided into rake ± 45° in the inversion. By doing so, the variation of rake angle in each subfault can be determined by the combination of these two slip components. To determine the slip amplitude from the inverted seismic moment, the empirical relationships between μ, V p, V s, and density (Brocher, 2005) were used to generate shear modulus on each subfault.
The inversion results of the data fitting for the 0420 event are shown in Fig. 2. The inverted teleseismic synthetic waveform compared with observed data showed that the peak amplitudes and later phases fit sufficiently well (Fig. 2a). The teleseismic waveform misfit determined from Eq. (1) is 0.20. The regional synthetic waveforms versus observed velocity waveforms are shown in Fig. 2b. Most of major phases can be explained not only for stations near the source area but also for the stations far from it. However, some shifts in time can be found, especially for the stations located in southern Taiwan. The regional ground motion waveform misfit is 0.38.
The source model derived from joint inversion was adopted to perform a forward 3D ground motion simulation based on the SEM. In the past studies, source inversion and forward wave propagation are typically carried out individually and use different velocity structure models (Sørensen et al., 2007; Bjerrum et al., 2010; Maeda et al., 2013). In this study, to ensure path effects are identical in the inversion and forward processes, the same SEM mesh and velocity model are used in both inverse and forward ground motion simulations.
It is noted that when seismic waves propagate through the offshore seabed and mountainous areas on land, scattered seismic waves produced by steep bathymetry and topography can be observed, especially at the Ryukyu trench. The sudden change in depth of the seabed across the Ryukyu trench produced strong scattered seismic waves parallel to the trench. This phenomenon can be observed clearly between approximately 25 and 70 s. These scattered waves were then propagated toward the north and south, and parts of them propagated through the island of Taiwan, which prolonged the ground shaking further, especially in northern Taiwan.
Island-wide waveform comparison
In order to obtain higher frequency (>1 Hz) ground motion simulation and synthetic waveforms, a more precise 3D velocity model will be required. Indeed, there are several important structures that will need to be confirmed by the next generation of tomographic studies, for example, shallow sedimentary structures, the exact plate boundary and the Moho depth distribution beneath Taiwan. Thus, building accurate numerical wave-speeds and structure models for Taiwan will continue to be a topic of future interest.
Long-period strong ground motion in northern Taiwan
The seismic intensity in Taipei City (150 km from the epicenter) during the 0420 event was 4 (25–80 gal, defined by CWB intensity scale), which is comparable to that observed in Yilan (intensity 4) and Hualien (intensity 4), even though the epicentral distance to the city was approximately twice that of these areas. From wave propagation snapshots (see Fig. 8 and Additional file 1), it can be seen that the seismic waves were amplified when they propagated through the Taipei basin (green circles as shown in Fig. 8 at 46 s). However, relatively large S wave and surface waves were also found in northern Taiwan before they propagated into Taipei (as indicated by the light green ellipse in Fig. 8 at 34 s). The amplified ground motion could be due to several factors, including path effects (e.g., basin amplification, structure focusing, and so on) and source effects (e.g., source radiation and rupture directivity). In order to clarify which factor dominates waveform amplification in northern Taiwan as well as in Taipei, a forward simulation test was performed that considered four different cases: (A) a full 3D velocity model with an inverted finite-fault source model, (B) a homogeneous half-space model (flat surface) with an inverted finite-fault source model, (C) a homogeneous half-space model (flat surface) with a double-couple point source, and (D) a homogeneous half-space model (flat surface) with an explosive source.
A comparison between case A (3D velocity model) and case B (half-space model) indicates that the localized strong ground shakings in the Taipei basin and Yilan plain could be due to the low velocity sedimentary deposits; the strong motion that occurred in the Hualien area might result from complex path effects, such as sedimentary deposits in Longitudinal Valley, topographic scattering (Lee et al., 2009), or subducting structures (Furumura and Kennett, 2005; Chen et al., 2013). A stronger ShakeMap pattern can be observed between Yilan and Hualien in case A compared to that in case B. This might be due to the topographic effect which could cause amplified ground motion at tops and ridges of mountain (Lee et al., 2009). The finite-fault ruptured from the deep to the shallower subduction zone (from north to south) also caused a remarkable directivity effect, which produced a southern-extension ShakeMap pattern from the epicenter in cases A and B, but it was not present in case C. It is noted that a northwesterly strong-motion extension can be found in both the finite-fault model (cases A and B) and the double-couple point source (case C), compared to that with a simple geometric decay (explosive source in case D) where there is no such extension. This implies that the source effect, especially the radiation pattern, is one of the dominant factors that caused strong ground shaking in northern Taiwan.
Large forearc sedimentary basins (Hoping basin, Nanao basin, and East-Nanao basin) developed along the southwestern Ryukyu Trench could also amplify the ground motions. These forearc structures are not included in the 3D velocity model. In previous studies about the Taipei basin (Lee et al., 2008, 2009), the seismic wave amplification due to the basin effect is obvious for sites located in the basin. Outside the basin, the amplification is not apparent. Only second-order effects, such as reflections and scattering waves resulted from basin boundary can be found. These effects are minor that do not contribute much to the peak ground motion. This might also be true for the forearc basins which could amplify shaking in offshore sedimentary basins, but this effect will not contribute to the large ground motions observed in northern Taiwan.
From these results, it can be deduced that the strong ground shaking in Taipei during the 0420 earthquake was mainly caused by two factors: (1) Taipei basin amplification and (2) source radiation.
The 0420 event caused a long-period strong ground shaking in Taipei City, which is not only due to the basin effect but also the source radiation. On 31 March 2002, an M w 7.1 earthquake, called the 331 Hualien offshore earthquake, also occurred in this area (see Fig. 1). Again, although the hypocentral distance to Taipei City was more than 100 km, people in Taipei felt a very strong long-period ground shaking for more than 30 s. The focal mechanism of this event is similar to that of the 0420 earthquake, which might be related to the subduction zone structure. These events imply that the southernmost Ryukyu subduction zone earthquakes that occurred in offshore eastern Taiwan could be a critical issue for seismic hazard assessment in northern Taiwan.
The seismic hazards resulting from long-period ground motions have been observed in several large subduction zone events, such as the 1985 Michoacan M8.1, Mexico earthquake (Anderson et al., 1986; Mendoza and Hartzell, 1989) and the 2011 Tohoku M9.0 earthquake in Japan (Takewaki et al., 2011; Maeda et al. 2013). Hsu et al. (2012) inverted the slip-deficit rates and the geometric configuration of the southernmost Ryukyu subduction zone. Their preferred fault model dips 10° northward and extends around 70 km from the Ryukyu Trench to a depth of 13 km. The slip deficit rate exhibits a left-lateral motion of 78 mm/year and a normal motion of 36 mm/year on a 290° striking fault. This fault geometry is similar to the focal mechanism of 0420 event, having a strike of 295° and dipping 26° toward north.
As known from the source inversion results in this study, the events that occurred at the southernmost Ryukyu subduction zone could have mainly ruptured in the northwest-to-southeast direction, with right-lateral thrust and updip movements on the north-dipping subducting plate boundary. This source mechanism can cause a striking source radiation effect, which amplifies the shaking on the northwestern side from the source area. It leads to large ground motions propagating toward northern Taiwan and will be further amplified by the basin effect, as was observed in Taipei City during the 0420 earthquake. Thus, the type of subduction zone event that occurred in offshore eastern Taiwan should be seriously considered in future earthquake scenario analyses for the purposes of seismic hazard assessment.
The 24-bit real-time strong motion data for this paper are available from the Taiwan Rapid Earthquake Information Release System implemented by the Central Weather Bureau. The teleseismic data used were obtained from Incorporated Research Institutions for Seismology (IRIS). This research was supported by Academia Sinica, funded through the Taiwan Numerical Earthquake Model (TNEM) project, Grant Number 102-Investigator Award-02. This research was also supported by the Taiwan Earthquake Research Center, funded through the Ministry of Science and Technology by Grant Number MOST 103-2628-M-001-004-MY3. The TEC contribution number for this article is 00116.
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