An MW 7.1 intraslab earthquake within the Pacific/Yakutat slab underlying the North American Plate struck Anchorage, southern Alaska, on November 30, 2018. The ground-motion records very close to the source region of the Anchorage earthquake provide an important opportunity to better understand the source characteristics of intraslab earthquakes in this subduction zone. We estimated the kinematic rupture process during this earthquake using a series of strong-motion waveform (0.05–0.4 Hz) inversions. Our inversions clearly indicate that the Anchorage earthquake was a rare intraslab event with simultaneous rupture on two conjugate faults, which are worldwide recognized sometimes for shallow crustal earthquakes but rarely for deep intraslab earthquakes. Interestingly, one of the conjugate faults is located in a zone that had low aftershock activity. This fault extends to great depth and may reflect a deep oceanic Moho or a local low-velocity and high-VP/VS zone within the oceanic mantle. Even though the Anchorage earthquake was a rare event due to the conjugate faults, we found that its kinematic source parameters were not abnormal compared to the average parameters of global intraslab earthquakes. The normal source parameters suggest that the larger low-frequency (0.33-Hz) ground-motion amplitudes than predicted by the ground-motion prediction equation observed in downtown Anchorage were primarily due to site amplification effects associated with a sedimentary basin, not source effects. Because such normality of the source parameters was also found for another large intraslab earthquake in the subducting Pacific/Yakutat slab, this normality is likely an important source characteristic common to this subduction zone.
Full paper
Simultaneous Rupture on Conjugate Faults During the 2018 Anchorage, Alaska, Intraslab Earthquake (MW 7.1) Inverted from Strong-Motion Waveforms
https://doi.org/10.21203/rs.3.rs-42861/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 23 Nov, 2020
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2018 Anchorage earthquake
Intraslab earthquake
Subduction zone
Ground motion
Waveform inversion
Source rupture process
Conjugate fault
Source scaling relationship
At 17:29 Coordinated Universal Time (UTC) on November 30, 2018, an MW 7.1 earthquake occurred in southern Alaska ~20 km north of Anchorage at a depth of ~50 km (Figure 1). Under southern Alaska, the Yakutat terrane, which has characteristics of an oceanic plateau (e.g., Christeson et al. 2010), and the Pacific Plate are moving at the same slip rate but in slightly different directions with respect to the overlying North American Plate (Elliott et al. 2010). Even though the exact location of the interface between the Pacific and Yakutat slabs has not yet been completely defined (e.g., Kim et al. 2014), there is no doubt that the 2018 Anchorage earthquake was an intraslab event within the subducting Pacific or Yakutat slabs. The hypocenter of this earthquake was located in the upper part of the transition area from shallow-dipping to steep-dipping subduction, and its focal mechanism indicates normal faulting.
Southern Alaska and Anchorage recently experienced two large intraslab earthquakes, the 1999 Kodiak Island (MW 7.0) and 2016 Iniskin (MW 7.1) earthquakes (Figure 1). Compared to these two events, the 2018 Anchorage earthquake caused stronger ground motions resulting in structural damage to downtown Anchorage because of its closer proximity to Anchorage (e.g., West et al. 2020). As depicted in Figure 2, during the Anchorage earthquake, the seismograms recorded by the stations of the United States National Strong-Motion (NP) network and the Alaska Regional (AK) network had significantly larger low-frequency (0.33-Hz) peak amplitudes than those predicted by the ground-motion prediction equation (GMPE; Atkinson and Boore 2003). Most of the stations in Figure 2 are located above both the cold fore-arc region extended by the low-angle subduction and the Cook Inlet sedimentary basin (Figure 1). Even though propagation path effects resulting from variations in the seismic velocity and attenuation associated with subducting slabs have been reported worldwide for intraslab earthquakes (e.g., Furumura and Kennett 2005; Oth et al. 2008; Chen et al. 2013), these effects primarily affect high-frequency (≥1-Hz) ground motions (Cramer and Jambo 2020; Mann and Abers 2020). Therefore, the increased peak amplitudes at 0.33 Hz can be interpreted as being due to site amplification effects and/or source effects. Moschetti et al. (2020) demonstrated that the low-frequency (<1-Hz) amplification was caused by the Cook Inlet Basin. Even though source characteristics such as the slip amplitude and rupture propagation pattern can also significantly affect ground motions, there have been few detailed insights into the relationship between the source characteristics of the Anchorage earthquake and the observed low-frequency (long-period) ground motions. To determine whether site effects or source effects were dominant, it is necessary to examine the source characteristics of the earthquake. To achieve this goal, we estimate the kinematic source rupture process by performing waveform inversions of the strong-motion data (0.05–0.4 Hz).
Kinematic rupture models of the Anchorage earthquake have already been derived from the inversion of Liu et al. (2019) using geodetic, teleseismic, and strong-motion data and that of He et al. (2020) using geodetic and teleseismic data. Their models indicate that a westward steep-dipping fault rupture is preferred over an eastward shallow-dipping rupture. Even though these studies focused on the rupture during the Anchorage mainshock, the fault geometries in their rupture models insufficiently reproduce the spatially complex aftershock pattern (Figure 3), as detailed later. Based on this pattern, Ruppert et al. (2020) and West et al. (2020) suggested the possibility of a simultaneous rupture on two conjugate faults during the mainshock. The fault geometry in our inversion considers this possibility and decreases the level of disagreement between the mainshock fault geometry and the aftershock distribution.
We determined the fault planes in our inversion based on the mainshock and the aftershock distributions as relocated by Ruppert et al. (2020). As shown in Fig. 3, the Anchorage mainshock was followed by a vigorous aftershock sequence with ~ 1000 aftershocks (M ≥ 2) within the three days following the mainshock. Because these aftershocks exhibit no clear spatiotemporal migration (Figure S1) [see Additional file 1], the fault planes assumed for the inversion should cover the entire range of these aftershocks. Most of the aftershocks are concentrated north of the mainshock hypocenter at a depth range of 45–55 km. Meanwhile, the mainshock hypocenter (149.9715° W, 61.3342° N) has a depth of 55.7 km. Even though the relocated hypocenter depth of the mainshock is deeper than the depth of 46.7 km estimated by the United States Geological Survey (USGS 2019), the depth of ~ 55 km better matches the arrival times of the observed depth phases in the teleseismic waveforms (Liu et al. 2019). The relocated aftershocks form two evident clusters: a southern cluster as marked by a dashed ellipse and a northern cluster as marked by a dotted ellipse in Fig. 3. The southern cluster dips eastward, and the northern cluster dips westward (Fig. 3b). In general, the aftershocks exhibiting a spatially complex pattern reflect a mainshock rupture on two or more fault planes with different directions, which results both in a difference between the moment tensor and first motion mechanisms and in a low percentage of double-couple component in the moment tensor mechanism, as was the case in the 2008 northern Iwate, Japan, intraslab earthquake (e.g., Suzuki et al. 2009). However, for the Anchorage mainshock, the difference between the moment tensor and first motion mechanisms is very small and the percentage of double-couple component is significantly high, ~ 90% (USGS 2019). These facts suggest that the Anchorage mainshock was an event with two conjugate faults corresponding to the two aftershock clusters. The suggestion of Ruppert et al. (2020) that there are no systematic differences in the focal mechanisms between the southern and northern aftershock clusters also supports this fault configuration.
Based on the cross-sectional view perpendicular to the strike angle (5°) of the moment tensor solution (Fig. 3b), we found that the southern and northern clusters dip with angles of ~ 30° and ~ 60°, respectively. These dip angles are consistent with those of the two nodal planes in the focal mechanisms for the mainshock and aftershocks (Ruppert et al. 2020). The mainshock hypocenter is likely on the plane of the southern cluster rather than on that of the northern cluster. We also checked another cross-sectional view (Figure S2) [see Additional file 1], which was perpendicular to the apparent strike angle (340°) of the southern cluster in the map view (Fig. 3a), and confirmed that the dip angles in this direction had large gaps relative to those of the two nodal planes in the focal mechanisms for the aftershocks. Based on the directional superiority of Fig. 3b over Figure S2, our inversion for the Anchorage mainshock assumed two fault planes, one with a strike of 5° and a dip of 30° and the other with a strike of 185° and a dip of 60°. These planes are perpendicular to each other, that is, they are conjugate (Fig. 4). As explained later, these planes were divided into three segments (S1, S2, and S3) for our inversion strategy. Segments S1 and S2 represent the deep and shallow portions of the eastward shallow-dipping fault, respectively, and segment S3 represents the westward steep-dipping fault. These segments have identical fault lengths of 34 km. The fault width of segment S1 is 18 km, and segments S2 and S3 have fault widths of 16 km. The hypocenter is located on segment S1. We ruled out the possibility of the hypocenter being located at the intersection of the three segments because the shortest distance from the hypocenter to the intersection (3 km) is greater than the relocation error (< 0.5 km) in Ruppert et al. (2020). The rigidity of the fault planes was assumed to be 56 GPa (Eberhart-Phillips et al. 2006, 2019) without a depth dependence.
Our inversion selected nine strong-motion stations (Fig. 5): seven AK network stations and two NP network stations. The station distribution provides good azimuthal coverage except southwest of the source region. Even though there is a southwestern station (Fig. 5) located above the thickest sediments of the Cook Inlet Basin (Shellenbaum et al. 2010), we excluded this station because strong site effects (e.g., Mann and Abers 2020) complicate the seismic waves even at extremely low frequency (~ 0.1 Hz) and therefore can adversely affect our inversion results. The observed three-component acceleration waveforms were integrated to velocity waveforms, band-pass filtered between 0.05 Hz and 0.4 Hz, and resampled with a sampling frequency of 5 Hz. The waveforms lasting 20–30 s from the P-wave onsets were used as the inversion data.
We used a multi-time-window linear waveform-inversion method (Hartzell and Heaton 1983), where the rupture process was spatially discretized by dividing the source faults into subfaults and temporally discretized on each subfault by representing the slip histories in two orthogonal slip directions with multiple time windows. All three fault segments, S1, S2, and S3, were divided into 2 km × 2 km subfaults. Three time windows, each of which was a smoothed ramp function with a duration of 2 s separated from each other by 1 s were used on each subfault. These time windows permit a maximum slip duration of 4 s. The model parameters of the waveform inversion were the slip amplitudes in the two slip directions of each time window on each subfault. These model parameters were obtained by limiting the rake angle (slip direction) to be within ± 45° centered at 270° (pure normal faulting) and solving a non-negative least-squares problem (Lawson and Hanson 1974). For the inversion stability, a spatiotemporal smoothing constraint (e.g., Sekiguchi et al. 2000) was also imposed on the model parameters. The appropriate weight of the smoothing constraint was selected by referring to the Akaike Bayesian information criterion (Akaike 1980). The constant rupture velocity triggering the first time window on each subfault was determined based on the fit between the observed and synthetic waveforms.
We assumed 1D layered velocity structure models and calculated the Green’s functions for each subfault using the discrete wavenumber method (Bouchon 1981) and the reflection/transmission coefficient matrix method (Kennett and Kerry 1979). For each station, the 1D model was extracted from the 3D velocity structure model of Eberhart-Phillips et al. (2006, 2019). Because their 3D model primarily imaged deep structures such as the crust and mantle and therefore did not sufficiently include near-surface low-velocity structures such as sedimentary basins, which amplify and complicate the observed low-frequency (< 1-Hz) waveforms (e.g., Boore 2004; Moschetti et al. 2020), we replaced the shallowest portion of the model of Eberhart-Phillips et al. (2006, 2019) with four layers with S-wave velocities of 0.40–2.25 km/s referring to Dutta et al. (2007). We determined the thicknesses of these four layers via trial and error such that the phases and amplitudes in the synthetic waveforms (0.1–0.4 Hz) for two moderate-sized aftershocks (MW 5.0 at 7:57 UTC and MW 4.5 at 12:44 UTC on December 1, 2018; Fig. 3 and Table S1) satisfactorily reproduced the observations (Figure S3) [see Additional file 1]. Double-couple point sources with smoothed ramp functions and focal information based on Ekström et al. (2012) and Ruppert et al. (2020) were assumed to calculate the synthetic waveforms. Q values based on the formula of Brocher (2008) were used for the calculations. The calibrated 1D velocity structure models (Fig. 5 and Table S2 [see Additional file 1]) show thick low-velocity layers relative to the initial model (Eberhart-Phillips et al. 2006, 2019) for the stations AHOU, AWCH, and SKN, located inside the sedimentary basins and away from the basin edges.
To verify if a simultaneous rupture on two conjugate faults occurred during the Anchorage mainshock, we performed waveform inversions for three cases (Fig. 4). Case A assumes a rupture on a single, eastward shallow-dipping fault (segments S1 and S2), which is similar to the eastward dipping fault model in the supporting information of Liu et al. (2019). Meanwhile, cases B and C assume a conjugate rupture both on the eastward shallow-dipping and westward steep-dipping faults (segment S3). The difference between the two cases is the absence (case B) or presence (case C) of the shallow portion of the eastward shallow-dipping fault (segment S2). In cases B and C, we assume that the rupture on segment S3 radiated without a time delay from the intersection point (the black star in Fig. 4), with the rupture propagating on segment S1 arriving first. We also assumed that the slip between the westward steep-dipping (segment S3) and eastward shallow-dipping (segments S1 and S2) faults could be discontinuous. These assumptions follow the inversions of earthquakes with multiple fault planes (e.g., Suzuki et al. 2009).
In all three cases, the down-dip extension of segment S1 (Fig. 4), which resulted in a significantly larger depth relative to the aftershock clusters, was determined via a preliminary inversion trial. Figure 6 shows that, if segment S1 has a small fault width (6 km) that does not extend into the eastern zone with low aftershock activity, an extremely large slip is estimated at its eastern edge. This means that segment S1 should be further expanded. We tested several fault widths of segment S1 and determined its width to be 18 km, as mentioned earlier.
Table 1 summarizes our inversion results, Figs. 7 (cases A, B, and C), 8a (case A), 9a (case B), and 10a (case C) show the final slip distributions, and Figs. 8b (case A), 9b (case B), and 10b (case C) show the distributions of the maximum slip velocities and moment rate functions. The optimal rupture velocities triggering the first time windows for all three cases were determined to be 2.9 km/s, which corresponded to 69% of the average S-wave velocity around the source faults. The total seismic moments are 1.2–1.4 times as large as that of the Global Centroid Moment Tensor solution (Ekström et al. 2012).
Table 1 Parameters of the Three Inversion Cases.
|
Case A |
Case B |
Case C |
||
|
Segments |
S1, S2 |
S1, S3 |
S1, S2, S3 |
|
|
Seismic moment |
6.6 × 1019 Nm (MW 7.1) |
5.9 × 1019 Nm (MW 7.1) |
6.8 × 1019 Nm (MW 7.2) |
|
|
Average slip |
All segments |
1.1 m |
0.96 m |
0.75 m |
|
Segments S1 and/or S2 |
1.1 m |
1.1 m |
0.80 m |
|
|
Segment S3 |
- |
0.84 m |
0.65 m |
|
|
Rupture velocity |
2.9 km/s |
2.9 km/s |
2.9 km/s |
|
|
Variance reduction |
0.65 |
0.65 |
0.73 |
|
The spatial distributions of the estimated large slips and aftershocks are complementary to each other (Fig. 7). On segments S1 and S2 (cases A and C), large slips occur east (~ 60 km in depth) and north (~ 50 km in depth) of the hypocenter, even though the northern slip in case C is slightly uncertain. These large slips surround most of the southern aftershock cluster. On segment S3 (cases B and C), the large slip is located at a depth of ~ 50 km just south of the densest clustering of northern aftershocks. We can directly compare the slip distribution of case A with that of the eastward shallow-dipping model in Liu et al. (2019) because the two have similar fault geometries. The slip distribution of case A is consistent with that of Liu et al. (2019) in that the large slips are located a small distance from the hypocenter.
Cases A and B have comparable total variance reductions (Table 1):
where the total variance reduction measures the degree of the fit between the observed and synthetic data. Here, 
and 
represent the observed and synthetic waveforms, respectively. However, we favor case B, that is, the westward steep-dipping fault in the upper half of the source (segment S3), based on its better reproduction of the observed main pulses at station AHOU, which lies adjacent to the northern edge of the source fault and recorded considerable amplitudes. Two large pulses can be seen at a lapse time of 10–15 s in the east–west component (Figure 11b). Segment S3 in case B reproduces the amplitudes of both pulses reasonably well, even though segment S2 in case A reproduces only the second pulse. This is primarily due to the upward rupture propagation on the steep-dipping segment S3. Supporting our preference, analyses of interferometric synthetic aperture radar images (e.g., He et al. 2020; West et al. 2020) indicate that the westward steep-dipping fault is required to explain the significant uplift on the eastern side of the source region.
Subsequently, we compared case B to case C. In case B, the peak values of the large slips on the conjugate segments S1 and S3 are comparable (Figures 7 and 9a). In case C, the added segment S2 results in a slip separation from S3 to S2 and therefore in a reduction of the large slip on S3. The spatial slip patterns on segments S2 and S3 in case C are similar to those in cases A and B, respectively. In case C, even though the large slip on segment S3 is reduced, its peak value is still ~1.5 times greater than the maximum slip on segment S2 (Figures 7 and 10a). Table 1 shows that the variance reduction of case C is significantly higher than that of case B, which means that the total waveform fit (Figure 11) is improved by the addition of segment S2. However, this improvement is not surprising because the added segment S2 increases the number of model parameters and therefore allows a more detailed waveform fit. In general, a model with more degrees of freedom and a higher variance reduction is not always better in that it may not correspond to other datasets. To judge whether case B or C is better, we calculated the values of the Akaike information criterion (AIC; Akaike 1974), which considers both the data fit and the model complexity/simplicity and is often used for model selection. We found that case B (2.0 × 102) has a smaller AIC value than case C (9.7 × 102). Therefore, we suggest that case B is the better model and that most of the actual rupture during the Anchorage mainshock was restricted to segments S1 and S3 and did not extend to segment S2. Even though the complex aftershock distribution and the large focal depth made it difficult to precisely determine the extent of the source faults during the Anchorage mainshock, the above comparisons lead us to conclude that case B is the most plausible of the three cases.
Here, we detail the source characteristics of case B, our best model. Because large slips have a major influence on ground motions, an understanding of the characteristics of the large slips during the Anchorage earthquake will contribute to ground-motion predictions of future intraslab earthquakes. To identify the large slip area in case B, we used the criteria proposed by Somerville et al. (1999) as follows. First, we decided that the rupture area for case B was the same as the combined area of the modeled segments S1 and S3 because the average slips on the fault-edge rows/columns for case B (Figure 9a) were greater than 0.3 times the average slips over the modeled segments. Then, we defined the rectangular area of the subfaults with ≥1.5 times the average slip of the rupture area as a large slip area (called “asperity area” in Somerville et al. (1999)). As a result, we obtained two large slip areas in case B. One (A1) is on the down-dip side of the hypocenter on segment S1, and the other (A2) is in the central portion of segment S3 (Figure 9). A1 and A2 were generated 2.5–5 s and 5–10 s after the rupture initiation, respectively (Figure 12a). As illustrated in the contributions of A1 and A2 to the velocity waveforms at three near-source stations (Figure 12b), much of the amplitude can be explained by these large slip areas and the contribution of A2 occurs subsequent to that of A1. The synthetic peak amplitudes are produced by A2, and its value at the northern station AHOU is the largest due to the forward directivity resulting from the upward and northward rupture propagation of A2. At stations AHOU and K209, A1 yields smaller amplitudes than A2 because of both the downward rupture propagation and the larger depth of A1. Meanwhile, the amplitude from A1 is comparable to that from A2 at the eastern station K217, primarily because this station is located just at an antinode in the S-wave radiation pattern. The two large slip areas have a total dimension that is 17% of the rupture area. This percentage is equivalent to that calculated by Iwata and Asano (2011), who proposed scaling relationships of the source parameters for M 7–8-class intraslab earthquakes. Even though several global source scaling relationships for intraslab earthquakes have been published (e.g., Iwata and Asano 2011, Allen and Hayes 2017), these relationships were developed using no source parameter data from southern Alaskan (i.e., Alaska–Aleutian) earthquakes because of the low intraslab activity in this area compared to other subduction zones. In comparison with Iwata and Asano (2011), we found that the large slip areas of the Anchorage earthquake had a normal total dimension (Figure 13a).
The total dimension and average slip of the rupture area are also key parameters for constructing source models of scenario earthquakes. We compared these parameters of the Anchorage earthquake to source scaling relationships (Iwata and Asano 2011, Allen and Hayes 2017). Such scaling relationships can also account for the dimension (Figure 13b) and average slip (Figure 13c) of the rupture area of the Anchorage earthquake, as well as the dimension of the large slip area. Figure 13 also compares the Anchorage earthquake to the 2016 Iniskin earthquake (USGS 2018) in southern Alaska, the source parameters (Figure S4) [see Additional file 1] of which were also obtained using the criteria of Somerville et al. (1999). The previously described feature is shared by the Iniskin earthquake. Even though source parameters include regional perturbations associated with differences in the subduction environment (e.g., Stirling et al. 2013), we suggest that the source parameters of intraslab earthquakes in southern Alaska nearly equal the global average source parameters.
The static stress drop distribution was calculated by applying a computer program developed by Okada (1992) to the interpolated slip distribution for case B (Figure S5) [see Additional file 1]. The derived average static stress drop of the rupture area was 7.0 MPa, which is 1.5 times the global average (4.6 MPa) compiled by Iwata and Asano (2011). This slightly large value for the average static stress drop may be due to a difference in the calculation method between this paper and Iwata and Asano (2011). Our average static stress drop was calculated from an inhomogeneous static stress drop distribution (Figure S5), while Iwata and Asano (2011) calculated the average static stress drop based on an assumed asperity model (Boatwright, 1988). This difference in the calculation method makes it difficult to conclude whether the average static stress drop of the Anchorage earthquake in this paper was normal. Therefore, a characterization of the average static stress drop for global intraslab earthquakes based on the inhomogeneous static stress drop distribution is an important future task, similar to the case of inland crustal earthquakes in Japan (Asano and Iwata, 2011).
During the Anchorage earthquake, the large slip area A2 propagated in the direction away from downtown Anchorage (Figure 12), where most of the stations are concentrated (Figure 2). Despite the normal source parameters (except for the average static stress drop) and the backward direction, ground motions were observed with low-frequency amplitudes exceeding predictions (0.33 Hz in Figure 2). This fact suggests that the remarkably large low-frequency amplitudes were primarily due to site amplification effects associated with the Cook Inlet Basin, rather than source effects. Consequently, the low-frequency amplitudes in downtown Anchorage exhibited significant variation, where stations near the basin edge (ALUK and K215) had amplitude levels comparable to the GMPE while stations farther from the basin edge (8036 and K220) had larger amplitude levels (Figure 2).
Around the source region of the Anchorage earthquake, a laterally extended zone with low-velocity and high-VP/VS (>1.8), which is generally interpreted as being the oceanic crust and easy to trigger intraslab earthquakes, was detected at a depth of ~50 km (e.g., Eberhart-Phillips et al. 2006, 2019; Kim et al. 2014; Table S2 [see Additional file 1]). This means that the oceanic Moho of the subducting Pacific/Yakutat slab extends to at least ~50 km. In our case B, the large slip area A2 was identified at this depth. As shown in Figure 4, the lower limit of our conjugate fault model penetrated to a greater depth of 63 km. Note that, due to the eastward shallow-dipping fault (segment S1), this depth was shallower than the ~70 km found by Liu et al. (2019) and He et al. (2020), whose best models were on a single, westward steep-dipping fault. Our conjugate fault model suggests two possibilities (Figure 14). The first is that the oceanic Moho has a depth of 60–65 km at the edge of the shallow-dipping Pacific/Yakutat slab. If this is true, the total thickness of the oceanic crust from the slab surface (30–35 km) is ~30 km. The second possibility is that, even though the oceanic Moho is located at ~50 km, there is a local low-velocity and high-VP/VS zone within the oceanic mantle (e.g., Nakajima et al. 2011) around the source region of the Anchorage earthquake. To distinguish between these possibilities and to provide accurate information on source locations for ground-motion hazard assessments of intraslab earthquakes, it is important to conduct further investigations of seismic structure imaging with finer resolution.
We performed strong-motion waveform (0.05–0.4 Hz) inversions and proposed a kinematic rupture model of the 2018 Anchorage intraslab earthquake (MW 7.1) based on the aftershock distribution as relocated by Ruppert et al. (2020). Our inversions revealed a simultaneous rupture on two conjugate faults, an eastward shallow-dipping fault and a westward steep-dipping fault. The preferred rupture model suggests that the shallower side of the eastward shallow-dipping fault, where aftershocks formed a distinct cluster, experienced little rupture during the mainshock. Our conjugate faults also showed the possibility of a deep oceanic Moho or a local low-velocity and high-VP/VS zone within the oceanic mantle of the subducting Pacific/Yakutat slab. On each of the conjugate faults, we identified one large slip area that significantly contributed to the observed ground motions. Unlike shallow crustal earthquakes, there have been few identified large intraslab earthquakes with conjugate faults. We found that, even though the Anchorage earthquake was a rare intraslab event in terms of having conjugate faults, its source parameters, such as its slip amplitude and fault dimension, are consistent with the averages of global intraslab earthquakes. This suggests that the large low-frequency (0.33-Hz) ground motions observed in downtown Anchorage were primarily due to basin amplification effects, not source effects.
Additional file 1: This is a pdf file and consists of Tables S1–S2 and Figures S1–S5.
AIC
Akaike's Information Criterion
AK
Alaska Regional
GMPE
ground-motion prediction equation
NP
United States National Strong-Motion
PSA
pseudo-spectral acceleration
USGS
United States Geological Survey
UTC
Coordinated Universal Time
Availability of data and materials
The strong motion data recorded at the AK and NP Network stations were obtained from the Incorporated Research Institutions for Seismology Data Management Center at http://ds.iris.edu/wilber3/find_event.
Competing interests
The authors declare that they have no competing interests
Funding
This work is funded by the 2019 research project “the methodology of ground motion prediction for megathrust earthquakes” by the Nuclear Regulation Authority, Japan.
Authors' contributions
YG performed waveform inversions, interpretation and manuscript preparation. KM and MT provided supervision of work and manuscript evaluation. All authors read and approved the final manuscript.
Acknowledgements
Figures are drawn using the Generic Mapping Tools version 4.5.2 (http://www.soest.hawaii.edu/gmt/; Wessel and Smith 1998).
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
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