Earthquake nucleation and fault slip complexity in the ... · occurring over the course of about 10...
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Articleshttps://doi.org/10.1038/s41561-018-0144-2
Earthquake nucleation and fault slip complexity in the lower crust of central AlaskaCarl Tape 1*, Stephen Holtkamp 1, Vipul Silwal 1, Jessica Hawthorne 2, Yoshihiro Kaneko 3, Jean Paul Ampuero 4,5, Chen Ji 6, Natalia Ruppert 1, Kyle Smith 1 and Michael E. West 1
1Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA. 2Department of Earth Sciences, University of Oxford, Oxford, UK. 3GNS Science, Lower Hutt, New Zealand. 4Seismological Laboratory, California Institute of Technology, Pasadena, CA, USA. 5Université Côte d’Azur, IRD, CNRS, Observatoire de la Côted’Azur, Géoazur, France. 6Department of Earth Science, University of California, Santa Barbara, CA, USA. *e-mail: [email protected]
© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
SUPPLEMENTARY INFORMATION
In the format provided by the authors and unedited.
NATuRE GEoSCiENCE | www.nature.com/naturegeoscience
Supplementary Information for
Earthquake nucleation and fault slip complexity in the lower crust
of central Alaska
Carl Tape, Stephen Holtkamp, Vipul Silwal, Jessica Hawthorne, Yoshihiro Kaneko,
Jean Paul Ampuero, Chen Ji, Natalia Ruppert, Kyle Smith, Michael E. West
correspondence to: [email protected]
Contains: Supplementary Figures 1–23,
Supplementary Tables 1–5, and Supplementary Text
S1 The 2015 VLFE spectra as a sum of smaller events
It is possible to model the 2015 MW 3.8 VLFE as the sum of about 10 MW 3.2 earthquakes
occurring over the course of about 10 seconds, following the approach Gomberg et al. (2016)
used to model VLFEs as sums of much smaller LFEs.
To see this, imagine the VLFE is the sum of N subevents, all of which have moment M0s
and corner frequency fcs, and source spectra
Ss(f) = M0s
1
1 + (f/fcs)2. (S1)
At frequencies much lower than 0.05 Hz—far lower than 1 over the VLFE duration—all of the
subevents will occur during a small portion of the period of interest and combine constructively,
giving low-frequency VLFE spectral amplitude proportion to the VLFE moment
Sv(f ≪ 0.05 Hz) = NSs(f) = NM0s. (S2)
At frequencies much higher than 0.05 Hz—far higher than 1 over the VLFE duration—the
subevents will occur during random portions of the period of interest and may combine con-
structively or destructively. In this case, the VLFE amplitude will be given by
Ss(f ≫ 0.05 Hz) =√NSs(f) =
√NM0s
1
1 + (f/fcs)2. (S3)
These are the two end member regimes in the approach of Gomberg et al. (2016). At intermediate
frequencies, the VLFE amplitude Sv(f) will be between√NSs(f) and NSs(f).
These VLFE source spectra may be observed via displacement spectra at the station. The
1
displacement spectra at a given frequency are given by the source spectra Sv(f) multiplied by
a path effect G(f). So to analyze the 2015 VLFE, it is useful to compare its spectra with the
spectra of a nearby MW 3.5 earthquake. We model the spectra of the MW 3.5 earthquake with
moment M0e and corner frequency fce as
Se(f) = M0e
1
1 + (f/fce)2. (S4)
At frequencies larger than 1 Hz, the 2015 VLFE and MW 3.5 earthquake have similar displace-
ment spectra, as seen in Figure S6d.
Equating the high-frequency VLFE spectrum to the MW 3.5 spectrum and assuming they
have the same path effect gives
Ss(f ≫ 0.05 Hz)G(f) = Se(f)G(f) (S5)√NM0s
1
1 + (f/fcs)2
= M0e
1
1 + (f/fce)2. (S6)
Since the two spectra have similar shapes at high frequency, we may assume that they have
similar corner frequencies fcs and fce, and estimate that
√NM0s = M0e, (S7)
where M0e is the moment of a MW 3.5 earthquake. However, we also have inferred from the
analysis of the low-frequency VLFE signal (or from the ≪ 0.05 Hz VLFE amplitude, if you
prefer) that the VLFE moment M0v is equivalent to a MW 3.8 earthquake. Matching the VLFE
moment with N subevents with moment M0s gives
NM0s = M0v = moment of Mw 3.8 earthquake. (S8)
Combining the high-frequency and moment constraints (equations (S7) and (S8)) gives
√NM0v
N= M0e (S9)
√N =
M0v
M0e
(S10)
N =
(
moment of Mw 3.8 earthquake
moment of Mw 3.5 earthquake
)2
≈ 8. (S11)
So it is possible to match the data by constructing the MW 3.8 VLFE from 8 MW 3.2 earthquakes.
These 8 subevents are consistent with the 10 or so peaks observed in the deconvolution of
the 2015 VLFE (Figure S4a).
To match the spectral shape, those MW 3.2 earthquakes would have to have corner frequen-
cies similar to the MW 3.5 earthquake analyzed for comparison. Commonly observed corner
frequency scaling suggests that MW 3.2 earthquakes should have corner frequencies a factor of
2
1.4 higher than MW 3.5 earthquakes, so the lower subevent corner frequencies might suggest
slightly lower stress drop or rupture velocities for the VLFE subevents, but that level of variation
in the corner frequency, coupled with variability in the spectra, is within the range observed in
earthquakes, so it is plausible that the VLFE subevents could be normal earthquakes.
We note, however, that it is very rare to observe 8 MW 3.2 earthquakes within a 10-second in-
terval without a strong external forcing. So any physical model of VLFEs as a sum of subevents
would likely require a driving mechanism distinct from that commonly seen in normal earth-
quakes, be it fluid diffusion, off-fault microcracking, delayed nucleation, or aseismic slip.
Indeed, we should note that our preferred slow slip interpretation and the VLFE as a sum
of subevents are end members, and one could model part of the VLFE moment via subevents
and part of the moment as aseismic slip. In that case, the total VLFE moment M0v modeled as
subevents in equation (S11) would be smaller, and a larger number of smaller subevents would
be expected. However, if such smaller subevents exist, they would be expected to have higher
corner frequencies, and the VLFE spectra would also be expected to have more power at higher
frequencies—to have a slower high-frequency decay than the MW 3.5 earthquake used for com-
parison. Such a model would appear to contradict the observations. However, the contradiction
could arise because the model is overly simplistic. It may not be appropriate to assume that there
are 100 or more earthquakes of a similar size. If there are many events of different sizes, the
high-frequency spectra could have a different shape, determined by the subevent size distribu-
tion rather than their average corner frequency.
S2 The low-frequency foreshock signal of the 2016 event
S2.1 Low-frequency onset of the 2016 event
Figures S10–S12 show low-frequency causal filtered seismograms and high-frequency envelopes
for all 17 stations within 60 km of the 2016 earthquake. The origin time of the 2016 earthquake
(dashed line corresponding to the origin of the x-axis) is shown along with P and S phase picks
on the envelope plot. On the low-frequency plots, we pick the onset of the low-frequency waves
associated with the event. Low-frequency picks preceding the P-wave arrival of the mainshock—
interpreted as the precursory VLFE—are present on 8 of the 17 closest stations (Table S3, Fig-
ures S10–S12).
S2.2 Relative timing of foreshock signals for the 2016 event
The onset times of HFF and LFF for the 2016 event are listed in Table S4. The column LF-HF
shows the difference between the onsets, while the last column (LF-HF-10) shows the difference
between the onsets, but with a 10-second correction applied to LF on the basis of Figure S9.
Here we consider the three possibilities for the relative timing of the HFF and LFF signals for
the 2016 event.
3
• Option 1 (preferred): HFF and LFF are simultaneous. The last column in Table S4,
which has the 10-second correction (Figure S9), shows values that are near 0 s, indicating
that the HFF and LFF are nearly simultaneous.
• Option 2: LFF before HFF. It is possible that the LFF signals occurred before HFF. Three
factors could promote an earlier initiation of LFF:
1. The delay in the measured onset time is greater than the 8–12 s implied by Figure S9.
This would push the actual LFF onset time earlier.
2. A portion of LFF is below our detection capabilities. This would imply that the
actual LFF started earlier than it was detected. The LFF has a simple long-period
pulse (Figure 2b), but it is barely detectable on even the closest stations, whereas
HFF is easily detected on all stations (Table S3).
3. The delay could be a consequence of differences in seismic velocities. Although
we have rotated the seismograms to analyze the transverse component, the earliest
arriving HFF signal is likely P waves, while the LFF signal is comprised of shear
or surface waves. Given the depth of the event and monitoring network, this could
introduce a delay of the LFF arrivals of a few to several seconds.
• Option 3: HFF before LFF. If the delay in the measured onset time of LFF is less than
the 8–12 s implied by Figure S9, then HFF would appear to start before LFF is observed.
For the 2015 event, option 3 is clearly the case: we see that LF-HF-10 > 20 for most stations
within 50 km of the event (Table S5), indicating a clear delay of LF with respect to HF. We also
see that the low-frequency signal started near or just after the peak in the high-frequency waves,
evidenced from the positive values of LF in Table S5.
Our observations can be reconciled with our interpretations in Figure 4 and Table S2, which
propose that aseismic slow slip was responsible for the growing high-frequency signals of the
2015 and 2016 events. For the 2016 event, the duration of the aseismic slip could have been
absent, since the LFF—representing a VLFE—and HFF are nearly coincident. For the 2015
event, the duration of aseismic slip is inferred to be ∼20 s (Table S5) before transitioning into a
VLFE (e.g., Figure S1).
4
Supplementary Table S1: Events in Minto Flats fault zone in this study and in Tape et al.
(2015). The subregion corresponds to the designation in Tape et al. (2015). The depth is from
the moment tensor inversion, with the catalog depth in parentheses. The duration listed is the one
used within the moment tensor inversion, if performed. The bandpass for each moment tensor
inversion is listed as the periods T1 and T2. VLFE = very-low-frequency earthquake, HFS =
increasing high-frequency signal, EQ = earthquake. Y-I denotes that we interpret the event as a
VLFE based on the HFS.
origin time subregion latitude longitude Mw Ml depth duration T1 T2 VLFE HFS EQ
(km) (s) (s) (s)
events in Tape et al. (2015):
2000-11-29 10:35:47.2 S 63.90 -150.35 5.7 5.8 19 (16.4) 7.1 20 40 – – Y
2000-12-06 18:40:26.0 S 63.89 -150.31 4.9 5.0 10 (11.7) 2.8 20 33.3 – – Y
2001-03-25 11:34:50.9 E 64.63 -149.25 4.4 4.6 20 (22.2) 1.6 16.7 40 – – Y
2001-06-30 09:41:42.3 S 64.04 -150.15 4.6 4.4 16 (14.6) 2.0 14.3 25 – – Y
2008-07-16 10:12:00.6 W 64.59 -149.53 3.9 4.1 23 (30.5) 1.0 16.7 25 – – Y
2009-07-28 12:13:15.7 W 64.61 -149.49 3.8 3.7 23 (22.7) 1.0 14.9 25 – – Y
2012-04-11 09:21:57.4 W 64.92 -148.95 3.8 3.9 16 (19.3) 1.0 1.7 3.3 Y-I Y Y
2013-03-05 21:55:58.4 E 64.84 -148.73 3.4 3.4 20 (17.3) 1.0 16.7 28.6 – – Y
2013-06-05 18:58:23.3 SW 64.64 -149.68 3.9 4.0 10 (13.9) 1.0 16.7 33.3 – – Y
2013-07-12 07:59:17.0 N 65.09 -148.77 3.6 3.4 16 (17.2) 1.0 10 22.2 – – Y
2014-12-13 15:47:31.4 E 64.43 -149.38 3.4 3.3 17 (13.0) 1.0 11.1 28.6 – – Y
events in this study:
2002-12-29 20:38:30.2 E 64.95 -148.61 – 3.4 – (17.6) – – – – – Y
2004-11-17 11:29:00.3 W 64.89 -149.10 – 3.6 – (18.8) – – – – – Y
2011-11-18 10:46:23.5 W 64.94 -148.94 – 3.4 – (11.1) – – – – – Y
2013-03-12 07:39:50.2 E 64.72 -148.95 3.5 2.1 23 (1.0) 12 25 40 Y Y –
2015-09-12 03:24:12.2 N 65.13 -148.67 – 1.4 – (20.4) – – – Y-I Y –
2015-09-12 03:25:12.7 N 65.12 -148.66 3.8 2.6 21 (15.6) 10 20 50 Y Y –
2015-10-22 13:16:15.8 E 64.73 -149.04 2.6 2.7 18 (18.8) 1.0 5 20 – – Y
2015-10-31 02:56:35.6 S 64.43 -149.70 3.4 3.5 25 (23.9) 1.0 10 25 – ? Y
2016-01-14 19:04:10.7 W 64.68 -149.25 3.7 3.8 17 (22.7) 1.0 10 30 Y Y Y
5
Supplementary Table S2: Summary of seismic observations for six events in this study from
high-frequency waveforms (≥ 1 Hz; ) and low-frequency waveforms (≤ 0.1 Hz; ).
Event locations are shown in Figure 1.
stage 2012 2013 2015 2015- 2015- 2016
observation in VLFE EQ VLFE VLFE 10-22 10-31 VLFE EQ
Fig. 4 EQ EQ
growing high-frequency 1
signal (Fig. 1b)
simultaneous large-amplitude 1
low-frequency signal
earthquake 2
(P, S, surface waves)
AEC† catalog magnitude (Ml), 3.9 2.1 2.6 2.7 3.5 3.8
from high frequencies
moment tensor magnitude (Mw), 3.8 3.5 3.8 2.6 3.4 3.7
from low frequencies†Alaska Earthquake Center
6
Supplementary Table S3: Polarities of waves (U = up, D = down) for the 2016-01-14 earth-
quake for stations within 200 km of the epicenter. LFFP = low-frequency foreshock polarity;
HFP = high-frequency P polarity from earthquake; LFP = polarity predicted from low-frequency
waveforms; parentheses denote stations that are nodal for the source mechanism (Fig. S3b). The
LFFP picks are based on waveforms shown in Figures S10–S12, which show the vertical com-
ponent of velocity, bandpass filtered 20–100 s. The amplitude listed is the amplitude at the peak
of the LFFP.
stations distance azimuth LFFP (nm/s) HFP LFP
(km) (◦)
F1TN XV 5.3 127 – – (D)
F2TN XV 6.2 62 – U U
FPAP XV 10.5 138 D (-13.7) D D
F3TN XV 11.0 26 – U (U)
FNN1 XV 12.5 173 D (-38.2) D D
NEA2 AK 13.2 140 D (-20.0) D D
FNN2 XV 15.2 218 U? (11.8) D (U)
FAPT XV 16.8 152 D (-25.2) D D
F4TN XV 17.4 15 D (-21.4) – (D)
FTGH XV 20.1 87 U (24.7) U U
F5MN XV 22.9 8 D (-26.0) D D
F6TP XV 25.6 325 D (-32.1) D D
F7TV XV 33.5 305 – D (D)
F8KN XV 33.8 286 – U U
I23K TA 52.1 354 – D D
BWN AK 56.9 182 – D D
MDM AK 57.3 57 – U U
WRH AK 60.2 112 – – U
CCB AK 69.0 93 – U U
COLA IU 69.3 72 – – U
POKR TA 98.6 60 – – U
BPAW AK 106.1 233 – – U
MCK AK 107.1 172 – – D
HDA AK 114.5 104 – – U
PS08 PS 117.1 97 – – U
I21K TA 140.7 294 – – U
RND AK 143.6 172 – – D
H24K TA 143.7 26 – – D
TRF AK 146.5 201 – – (D)
KTH AK 150.1 214 – – U
CHUM AK 173.3 240 – – U
CAST AK 197.6 226 – – U
PPD AK 198.2 60 – – D
H21K TA 199.0 305 – – U
7
Supplementary Table S4: Time picks for the 2016 event. HF = HFF start time (emerges from
background noise level). LF = LFF start time, only listed for the unequivocal polarity measure-
ments in Table S3. HF and LF are relative to the P time of the Mw 3.7 earthquake. The differential
time LF–HF is the delay in the LF onset relative to the HF onset. The time LF–HF–10 includes
a 10-second correction in LF that is based on Figure S9.
stations distance azimuth HF LF LF-HF LF-HF-10
(km) (◦) (s) (s) (s) (s)
F1TN XV 5.3 127 -19.3 NaN NaN NaN
F2TN XV 6.2 62 -18.8 NaN NaN NaN
FPAP XV 10.5 138 -19.3 -9.5 9.8 -0.2
F3TN XV 11.0 26 -18.3 NaN NaN NaN
FNN1 XV 12.5 173 -19.5 -11.0 8.5 -1.5
NEA2 AK 13.2 140 -20.6 -10.3 10.4 0.4
FNN2 XV 15.2 218 -21.7 NaN NaN NaN
FAPT XV 16.8 152 -16.5 -11.1 5.3 -4.7
F4TN XV 17.4 15 -18.9 -1.3 17.7 7.7
FTGH XV 20.1 87 -20.9 -11.6 9.4 -0.6
F5MN XV 22.9 8 -21.2 -12.1 9.1 -0.9
F6TP XV 25.6 325 -20.3 -10.8 9.5 -0.5
F7TV XV 33.5 305 -18.4 NaN NaN NaN
F8KN XV 33.8 286 -20.9 NaN NaN NaN
I23K TA 52.1 354 -19.6 NaN NaN NaN
BWN AK 56.9 182 -21.0 NaN NaN NaN
MDM AK 57.3 57 -19.6 NaN NaN NaN
WRH AK 60.2 112 -21.4 NaN NaN NaN
CCB AK 69.0 93 -19.6 NaN NaN NaN
COLA IU 69.3 72 -19.7 NaN NaN NaN
POKR TA 98.6 60 -18.4 NaN NaN NaN
BPAW AK 106.1 233 -19.7 NaN NaN NaN
MCK AK 107.1 172 -19.0 NaN NaN NaN
HDA AK 114.5 104 -18.8 NaN NaN NaN
PS08 PS 117.1 97 -18.5 NaN NaN NaN
I21K TA 140.7 294 -19.2 NaN NaN NaN
RND AK 143.6 172 -17.7 NaN NaN NaN
H24K TA 143.7 26 -18.1 NaN NaN NaN
TRF AK 146.5 201 -19.8 NaN NaN NaN
KTH AK 150.1 214 -19.0 NaN NaN NaN
CHUM AK 173.3 240 -14.9 NaN NaN NaN
CAST AK 197.6 226 -19.3 NaN NaN NaN
PPD AK 198.2 60 -15.0 NaN NaN NaN
H21K TA 199.0 305 -14.3 NaN NaN NaN
8
Supplementary Table S5: Time picks for the 2015 event. HF = HF start time. LF = LF start
time. Both times are listed relative to the peak HF signal, which is estimated using a triangle fit
to the 2–8 Hz filtered seismogram (e.g., Figure 1b). The differential time LF-HF is the delay in
the LF onset relative to the HF onset. The time LF-HF-10 includes a 10-second correction in LF
that is based on Figure S9.
stations distance azimuth HF LF LF-HF LF-HF-10
(km) (◦) (s) (s) (s) (s)
MDM AK 27.1 131 -20.9 8.5 29.4 19.4
PS07 PS 27.8 40 -21.8 NaN NaN NaN
I23K TA 32.8 276 -23.2 -10.1 13.1 3.1
F5MN XV 35.8 223 -27.3 2.9 30.1 20.1
F4TN XV 39.4 216 -29.7 4.8 34.5 24.5
F3TN XV 45.1 211 -31.7 5.0 36.7 26.7
TCOL TA 46.8 126 -25.7 4.0 29.6 19.6
COLA IU 46.8 126 -24.8 5.8 30.6 20.6
FTGH XV 48.5 189 -25.4 5.5 31.0 21.0
F6TP XV 50.6 237 -28.1 3.4 31.5 21.5
F2TN XV 51.0 206 -27.4 11.2 38.6 28.6
F1TN XV 57.0 204 -29.5 9.9 39.4 29.4
POKR TA 57.8 90 -26.2 -7.0 19.2 9.2
FPAP XV 60.2 200 -29.7 2.0 31.8 21.8
NEA2 AK 61.9 198 -28.0 4.9 32.9 22.9
F7TV XV 62.4 242 -28.6 9.4 38.0 28.0
FNN1 XV 66.6 204 -34.3 5.5 39.8 29.8
FAPT XV 66.7 198 -31.8 0.9 32.7 22.7
CCB AK 66.8 142 -26.5 12.6 39.1 29.1
FNN2 XV 71.2 212 -30.8 2.5 33.3 23.3
F8KN XV 71.8 237 -27.3 4.9 32.2 22.2
WRH AK 77.3 159 -28.7 3.9 32.6 22.6
H24K TA 87.8 24 -30.4 6.1 36.5 26.5
H23K TA 88.5 333 -30.7 -5.2 25.5 15.5
PS06 PS 95.7 329 -30.9 NaN NaN NaN
MLY AK 98.3 265 -31.5 -4.0 27.5 17.5
PS08 PS 108.7 126 -31.9 NaN NaN NaN
BWN AK 109.9 196 -34.5 -9.7 24.7 14.7
HDA AK 113.9 133 -32.7 1.7 34.4 24.4
PPD AK 152.9 72 -32.5 21.7 54.1 44.1
MCK AK 155.4 185 -39.7 7.6 47.3 37.3
I21K TA 155.8 274 -42.0 -1.8 40.2 30.2
BPAW AK 159.1 225 -40.2 -0.6 39.6 29.6
J25K TA 166.1 108 -40.4 -4.3 36.0 26.0
RND AK 191.4 183 -45.1 9.5 54.6 44.6
9
−100 0 100 200 300 400 500
SII.AK.BHZ (200, 997 km)
OHAK.AT.BHZ (198, 912 km)
KDAK.II.BHZ_00 (196, 842 km)
Q19K.TA.BHZ (203, 735 km)
P19K.TA.BHZ (203, 652 km)
CNP.AK.BHZ (193, 636 km)
HOM.AK.BHZ (196, 627 km)
BRLK.AK.BHZ (192, 608 km)
BRSE.AK.BHZ (191, 608 km)
O20K.TA.BHZ (202, 596 km)
CAPN.AK.BHZ (196, 500 km)
FIRE.AK.BHZ (191, 449 km)
SSN.AK.BHZ (195, 420 km)
SKN.AK.BHZ (203, 377 km)
CUT.AK.BHZ (195, 312 km)
TRF.AK.BHZ (204, 202 km)
FAPT.XV.HHZ (198, 66 km)
FNN1.XV.HHZ (203, 66 km)
NEA2.AK.BHZ (198, 62 km)
FPAP.XV.HHZ (200, 60 km)
F1TN.XV.HHZ (204, 57 km)
F2TN.XV.HHZ (206, 51 km)
Time (s)
2015−09−12 03:23:32 + 600.00 s; F2TN max −8.72e−01 nm / sec at t = 43.5 s BHZ BHZ_00 HHZ [ nm / sec, −−] event 20150912032512711 (2015−09−12, M2.6, −148.7, 65.1, z = 15.6 km)
22 / 22 seismograms (22 stations) ordered by input, norm −−> (sin D)^−0.50
Supplementary Figure S1: Record section of vertical component velocity seismograms, filtered
20–50 s, for the Mw 3.8 2015 VLFE. Amplitudes have been corrected for geometric spreading
of surface waves. The record section shows all stations within an azimuthal sector (here, 190◦ to
210◦), such that the waveforms would be expected to be similar, since the source-station paths
are similar. By cross-correlating all waveforms for this earthquake, we estimate a group velocity
of 3.5 km/s.
10
(a) (b)
0.000
0.006
0.012
0.018
0.024
0.030
ln(V
R_m
ax /
VR
)
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VR
)
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3.80
3.80
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3.80
3.803.80
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3.803.80 3.80
3.80
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20150912032512711 | Model tactmod | Best depth 21.2 ± 4.2 km
0.00
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ln(V
R_m
ax /
VR
)
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3.60
3.60
3.70
3.70
3.70
3.70 3.703.70
3.70
3.70
3.80 3.80
3.80
3.80
3.80
3.80
3.80
3.90
20160114190410727 | Model tactmod | Best depth 16.7 ± 4.6 km
(c) (d)
0.000
0.003
0.006
0.009
0.012
0.015
ln(V
R_m
ax /
VR
)
0.000
0.003
0.006
0.009
0.012
0.015
ln(V
R_m
ax /
VR
)
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25 30
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25 30
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25 30
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25 30
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25 30
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25 30
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25 30
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25 30
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25 30
Depth, km
3.503.50
3.50
3.50
3.50
3.50 3.50
3.50
3.50
3.50
3.50
3.50
3.503.50 3.50
3.50
3.50
3.60
20130312073950214 | Model tactmod | Best depth 23.3 ± 2.6 km
0.000
0.007
0.014
0.021
0.028
0.035
ln(V
R_m
ax /
VR
)
0.000
0.007
0.014
0.021
0.028
0.035
ln(V
R_m
ax /
VR
)
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25
Depth, km
0
20
40
60
80
100
VR
(gr
ay)
10 15 20 25
Depth, km
3.203.20
3.20
3.203.20 3.20
3.20
3.20
3.20
3.30
3.30
3.30 3.40
3.40
3.40
3.40
20151031025635572 | Model tactmod | Best depth 24.1 ± 2.6 km
Supplementary Figure S2: Grid search over depth for four events in this study. For each depth,
the moment tensor inversion allows the magnitude and orientation to vary. The red arrow is the
AEC catalog depth derived from arrival times. The white arrow is the depth derived from the
moment tensor inversion. (a) Grid search over depth for the 2015 event; the best-fitting depth is
21 ± 4 km. (b) Grid search over depth for the 2016 event; the best-fitting depth is 17 ± 5 km.
(c) Grid search over depth for the 2013 event; the best-fitting depth is 23±3 km. (d) Grid search
over depth for the 2015-10-31 Mw 3.4 earthquake; the best-fitting depth is 18 ± 3 km. This
earthquake is used for comparison in Figure S6. See Silwal (2018) for full results.
11
F1TN
F2TN
FPAP
F3TN
FNN1
NEA2FNN2
FAPT
F4TN
FTGH
F5MN
F6TP
F7TV
F8KN
I23K
BWN
MDM
WRH
CCB
TCOL
PS07
POKRBPAW
MCK
HDA
PS08
PS06
I21K
RND
H24K
TRF
KTH
CHUMPS09
CAST
H21KPPD
MDM
PS07
I23K
FTGH
TCOLCOLA
F6TP
NEA2
POKR
F8KN
H23K H24K
MLY
HDA
J25K
b 2016-01-14 earthquakea 2015-09-12 very low frequency earthquake (VLFE) F6TP
I23K
BWN
WRH
CCB
TCOL
PS07
POKR
BPAW
MCK
HDA
RND
H24K
TRF
CHUM
MDM
PS07
I23K
FTGH
TCOL
F6TP
POKR
F8KN
H23K
H24K
MLY
HDA
J25K
Vertical Radial Transverse
200 s100 s
Supplementary Figure S3: Source mechanisms and waveform fits for the 2015 very-low-
frequency earthquake (VLFE) (a) and the 2016 VLFE+earthquake (b). The beachball is a lower-
hemisphere projection of the P-wave radiation pattern. The subset of waveform fits show the
observations (black) in comparison with the modeled seismograms (red). See Silwal (2018) for
full results.
12
0 5 10 15 20 25 30 35−0.1
0
0.1
0.2
0.3
Time, s
S(t
)
(a) 2015 event (shading fills 84%, dur = 9.8 s)
0 5 10 15 20 25 30 35−0.5
0
0.5
1
Time, s
Cum
ulat
ive
sum
of S
(t)
2015 event (int S(t) = 0.97)
0 5 10 15 20 25 30 35−1
0
1
2
Time, s
S(t
)
(b) 2016 event (shading fills 71%, dur = 1.2 s)
0 5 10 15 20 25 30 35−0.5
0
0.5
1
1.5
Time, s
Cum
ulat
ive
sum
of S
(t)
2016 event (int S(t) = 1.46)
Supplementary Figure S4: Estimated source time functions for the 2015 and 2016 events, using
seismograms filtered with f ≥ 1 Hz. Each pair of plots shows the estimated source time function
(top)—including a shaded portion that represents our interpretation of the source duration—as
well as a cumulative (integrated) version of the source time function (bottom). (a) Source time
function S(t) for the 2015 very-low-frequency earthquake. The estimated duration of ∼10 s is
represented by the shaded maximum pulse, which fills 84% of the integrated area of S(t). The
magnitude of Mw 3.8 estimated from long-period waveforms (Figure S3b) matches the estimated
moment from high-frequency waveforms. (b) Source time function S(t) for the 2016 earthquake,
estimated using the stations shown in Figure S5. The estimated duration of ∼1 s is represented
by the shaded maximum pulse. The magnitude of Mw 3.7 estimated from long-period wave-
forms (Figure S3a) matches the estimated moment from high-frequency waveforms after the
main pulse, indicated by∫
S(t) ≈ 1 (red curve). We attribute the later, lower-amplitude pulses
in S(t) to noise. See Figure S13 for a source time function estimated using higher frequency
waveforms.
13
(a) (b)
-10 0 10 20 30 40 50 60 70 80
POKRZ 8.28e-05
POKRT 8.46e-05
COLAT 1.54e-04
TCOLT 1.75e-04
MDMT 1.87e-04
CCBT 1.22e-04
FTGHT 9.90e-05
-10 0 10 20 30 40 50 60 70 80
FAPTT 8.76e-05
NEA2Z 8.42e-05
NEA2T 8.56e-05
F8KNZ 7.25e-05
F8KNT 1.01e-04
I23KT 2.19e-04
Time, s Time, s
-10 0 10 20 30 40 50 60
MDMZ 3.41e-04
MDMT 8.48e-04
POKRZ 3.06e-04
POKRT 8.40e-04
FTGHT 1.85e-03
CCBT 6.89e-04
F8KNT 2.92e-03
I23KT 9.17e-04
Time, s
Supplementary Figure S5: Corresponding fits between synthetic seismograms (red) and ob-
served seismograms for the source-time functions shown in Figure S4. In order to obtain better
agreement between data and synthetics, they are bandpass filtered 1–50 s; excluding the shorter
periods leads to a source duration estimate that is slightly longer (by ∼0.5 s) than actuality. The
max amplitudes in the data are listed in cm/s for each seismogram. All stations are <100 km
from the epicenter (Tables S4 and S5). (a) The 2015 Mw 3.7 very-low-frequency earthquake.
(b) The 2016 Mw 3.7 earthquake.
14
(a) TA.I23K (32.8 km, 81.8 km) (b) TA.POKR (57.8 km, 132.2 km)
10−2
10−1
100
101
−220
−200
−180
−160
−140
−120
−100
−80
−60
Dis
plac
emen
t Pow
er S
pect
ra [1
0*lo
g10(
m2 /H
z)] (
dB)
Frequency
I23K, BHZ
2015−09−12 VLFE2015−09−12 Noise2015−10−31 EQ2015−10−31 Noise
10−2
10−1
100
101
−220
−200
−180
−160
−140
−120
−100
−80
−60
Dis
plac
emen
t Pow
er S
pect
ra [1
0*lo
g10(
m2 /H
z)] (
dB)
Frequency
POKR, BHZ
2015−09−12 VLFE2015−09−12 Noise2015−10−31 EQ2015−10−31 Noise
(c) AK.NEA2 (61.9 km, 35.3 km) (d) TA.H24K (87.8 km, 178.7 km)
10−2
10−1
100
101
−220
−200
−180
−160
−140
−120
−100
−80
−60
Dis
plac
emen
t Pow
er S
pect
ra [1
0*lo
g10(
m2 /H
z)] (
dB)
Frequency
NEA2, BHZ
2015−09−12 VLFE2015−09−12 Noise2015−10−31 EQ2015−10−31 Noise
10−2
10−1
100
101
−220
−200
−180
−160
−140
−120
−100
−80
−60
Dis
plac
emen
t Pow
er S
pect
ra [1
0*lo
g10(
m2 /H
z)] (
dB)
Frequency
H24K, BHZ
2015−09−12 VLFE2015−09−12 Noise2015−10−31 EQ2015−10−31 Noise
Supplementary Figure S6: Vertical component displacement spectra for four stations (TA.I23K,
TA.POKR, AK.NEA2, TA.H24K), showing the enhancement in low-frequency amplitudes for
the 2015-09-12 Mw 3.8 (depth 21 km) VLFE (solid black) in comparison with the 2015-10-31
Mw 3.4 (depth 25 km) earthquake (solid red). The signal spectra are calculated for a time window
spanning from the origin time to 600 seconds. The noise spectra, plotted as dashed curves, are
calculated for a time window of 600 seconds preceding the origin time. The vertical lines mark
the limits of the bandpass used for the VLFE moment tensor inversion: 20–50 s. The four
stations here are among those used within the moment tensor inversion (Figure S3a). Note that
the two events are separated by 91 km (Figure 1a), resulting in different source-station distances,
as labeled above each subplot. The depths are estimated from moment tensor inversion as 21 km
(VLFE) and 25 km (EQ). The catalog magnitudes, estimated from high-frequency waveforms,
are Ml 2.6 (VLFE) and Ml 3.5 (EQ) (Table S1).
15
a
10 11 12 13 14 15 16 17 18 19 20−1
0
1
2
3
4
5
10 11 12 13 14 15 16 17 18 19 200
20
40
60
80
100
Time (Day of January 2016)
Cu
mu
lative
Se
ism
icity
Ma
gn
itu
de
Accelerating Foreshocks
Ma
insh
ock
b
27 28 29 30 31 01 02 03 04 05 060
1
2
3
4
5
27 28 29 30 31 01 02 03 04 05 060
50
100
150
200
250
300
Time (Day of August/September 2014)
Cu
mu
lative
Se
ism
icity
Ma
gn
itu
de
No Foreshocks
Ma
insh
ock
Supplementary Figure S7: Earthquake sequences associated with (a) the Mw 3.7 2016 earthquake
and (b) a Mw 5.0 earthquake on 2014-08-31. (a) Network matched filter catalog of earthquakes
in the 2016 sequence. (top) Magnitude (Ml) vs. time for the 2016 earthquake sequence. (bot-
tom) Cumulative seismicity over time for the 2016 earthquake sequence. Note that the rate of
foreshocks is accelerating up until the time of the mainshock. Most earthquakes do not have
foreshock sequences, as seen here. (b) Same as (a), but for the 2014 earthquake.
16
T0 10 20 30 40 50 60 70 80
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
BWN
CCB
COLA
F1TN
F2TN
F3TN F4TN
F5MNF6TP
F7TV
F8KN
FAPT
FNN1
FNN2
FPAP
FTGHI23K
MDMNEA2
WRH
distance, km
k fo
r v(
t) =
B*(
t − t 0)k
Z0 10 20 30 40 50 60 70 80
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
BWN
CCB
COLAI23K
MDM
NEA2
WRH
F1TN
F2TN
F3TN
F4TN
F5MNF6TP
F7TV
F8KN
FAPT
FNN1
FNN2
FPAP
FTGH
distance, km
k fo
r v(
t) =
B*(
t − t 0)k
Supplementary Figure S8: Estimated exponent k (Figure S16) for stations <80 km from the
epicenter of the 2016 event. Top is for the transverse component; bottom is for the vertical
component.
17
0.2
0.0
-0.2
-0.4
0.2
0.0
-0.2
-0.4
0.2
0.0
-0.2
-0.4
0.2
0.0
-0.2
-0.4
0.2
0.0
0.6
0.4
1.0
0.8
-300 -200 -100 0 100
Time (seconds)
a
b
c
d
e
Supplementary Figure S9: A synthetic test illustrating the influence of noise levels on the delay
in the LFF onset time pick. (a) An artificial input signal that is a triangle with total width of 20
seconds; the triangle starts at time t = 0. (b) Input signal filtered 20–100 s with no noise. The
LFF time pick is 0 s. (c) Same as (b) but with ∼10% of signal-to-noise (Gaussian noise). The
LFF time pick is 8.3 s. (d) Same as (b) but with ∼20% noise; the time pick is 9.6 s. (e) Same as
(b) but with ∼40% noise; the time pick is 11.2 s. These results indicate that the LFF onset time
pick will be delayed by 8–12 s from the actual LFF onset. See Tables S4 and S5.
18
F1TN F2TN
FPAP F3TN
FNN1 NEA2
D
D D
D
U
Time (seconds) Time (seconds)
Supplementary Figure S10: Additional examples of waveforms for the 2016 event (Figures S10–
S12), similar to Figure 2b. Each subplot has four time series; the dashed line at t = 0 is the
origin time of the Mw 3.7 earthquake. The bottom is the log-scaled envelope of the 2–8 Hz
vertical component seismogram; units show the base-10 exponent of m/s (e.g., −4 is 10−4 m/s).
The top three are the east (top), north (middle), and vertical (bottom) component seismograms
(units m/s), causal-filtered 20–100 s and cut at the P arrival time for the earthquake. Our polarity
measurement for the low-frequency foreshock (LFF) is labeled. The LFF polarity for the vertical
component is listed in Table S3.
19
FNN2 FAPT
F4TN FTGH
F5MN F6TP
D
U
Time (seconds) Time (seconds)
U?
U?
U?
D?
D
D
D D
U
Supplementary Figure S11: Additional examples of waveforms for the 2016 event (Figures S10–
S12), similar to Figure 2b. Each subplot has four time series; the dashed line at t = 0 is the
origin time of the Mw 3.7 earthquake. The bottom is the log-scaled envelope of the 2–8 Hz
vertical component seismogram; units show the base-10 exponent of m/s (e.g., −4 is 10−4 m/s).
The top three are the east (top), north (middle), and vertical (bottom) component seismograms
(units m/s), causal-filtered 20–100 s and cut at the P arrival time for the earthquake. Our polarity
measurement for the low-frequency foreshock (LFF) is labeled. The LFF polarity for the vertical
component is listed in Table S3.
20
F7TV F8KN
I23K BWN
MDM
D?
Time (seconds)
Time (seconds)
U?
Supplementary Figure S12: Additional examples of waveforms for the 2016 event (Figures S10–
S12), similar to Figure 2b. Each subplot has four time series; the dashed line at t = 0 is the
origin time of the Mw 3.7 earthquake. The bottom is the log-scaled envelope of the 2–8 Hz
vertical component seismogram; units show the base-10 exponent of m/s (e.g., −4 is 10−4 m/s).
The top three are the east (top), north (middle), and vertical (bottom) component seismograms
(units m/s), causal-filtered 20–100 s and cut at the P arrival time for the earthquake. Our polarity
measurement for the low-frequency foreshock (LFF) is labeled. The LFF polarity for the vertical
component is listed in Table S3.
21
0 10 20 30 40 50
F5MN
4.20e-02 cmAz= 7
Dist=22.9
F1TN
1.82e-02 cmAz= 126
Dist= 5.3
F8KN
9.48e-03 cmAz= 286
Dist=33.8
F7TV
1.82e-02 cmAz= 305
Dist=33.5
F6TP
9.45e-03 cmAz= 324
Dist=25.6
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.5
1.0
1.5
2.0
2.5
0 5 10 15 20 25 30 35 40
0.0
0.1
0.2
0.3
0.00
0.05
0.10
0.15
0.20
0 5 10 15 20 25 30
-5
0
5
X 1
0-4
2016011419041072
F8KN HHT
JAN 14 (014), 2016
19:04:10.727
-20 -15 -10 -5 0 5 10
(a)
(b)
(c)
(d)
Time, s Time, s
Time, sTime, s
cu
mu
lative
mo
me
nt
mo
me
nt ra
tem
om
en
t ra
tecu
mu
lative
mo
me
nt
Supplementary Figure S13: Source time function estimation for the 2016 event using high-
frequency (f ≤ 4.5 Hz), transverse-component waveforms at five stations within 35 km of the
epicenter: F1TN, F5MN, F6TP, F7TV, F8KN. The data were manually aligned such that the
mainshock initiated at t = 30 s. Synthetic seismograms were calculated using a standard 1D
model for central Alaska (tactmod). (a) Cumulative moment function (top) and its derivative,
the moment rate function (bottom). (b) Zoom-in of (a) showing the first 30 seconds. (c) Com-
parison of recorded seismograms (black) and synthetic seismograms (red) calculated using the
moment rate function in (a). Text labels show the station azimuth, epicentral distance, and peak
amplitude. (d) Transverse displacement seismogram at station F8KN; here t = 0 is the main-
shock origin time. The high-frequency peaks are correlated with the small peaks in the moment
rate function (b). The ramp shape from 7 s to 10 s is partially caused by the near-field term of
the mainshock SH wave, which begins with the mainshock P arrival.
22
−50 0 50
0
2
4
6BPAW [117.5 km]
−50 0 50
0
2
4
6BWN [63.7 km]
−50 0 50
0
2
4
6CCB [59.7 km]
−50 0 50
0
2
4
6CHUM [184.7 km]
−50 0 50
0
2
4
6COLA [58.1 km]
−50 0 50
0
2
4
6F1TN [10.5 km]
−50 0 50
0
2
4
6F2TN [5.2 km]
−50 0 50
0
2
4
6F3TN [6.7 km]
−50 0 50
0
2
4
6F4TN [12.4 km]
−50 0 50
0
2
4
6F5MN [18.3 km]
−50 0 50
0
2
4
6F6TP [29.1 km]
−50 0 50
0
2
4
6F7TV [39.7 km]
−50 0 50
0
2
4
6F8KN [42.6 km]
−50 0 50
0
2
4
6FNN1 [20.0 km]
−50 0 50
0
2
4
6FNN2 [26.2 km]
−50 0 50
0
2
4
6FPAP [13.7 km]
−50 0 50
0
2
4
6FTGH [11.1 km]
−50 0 50
0
2
4
6H23K [124.0 km]
−50 0 50
0
2
4
6H24K [134.5 km]
−50 0 50
0
2
4
6HDA [106.5 km]
−50 0 50
0
2
4
6I21K [147.7 km]
−50 0 50
0
2
4
6I23K [48.6 km]
−50 0 50
0
2
4
6J25K [175.7 km]
−50 0 50
0
2
4
6KTH [160.4 km]
−50 0 50
0
2
4
6MCK [111.8 km]
−50 0 50
0
2
4
6MDM [45.9 km]
−50 0 50
0
2
4
6MLY [87.3 km]
−50 0 50
0
2
4
6NEA2 [15.7 km]
−50 0 50
0
2
4
6POKR [87.2 km]
−50 0 50
0
2
4
6PPD [186.7 km]
nenana_nuc_20151022T_if0
−50 0 50
0
2
4
6PS06 [129.2 km]
−50 0 50
0
2
4
6PS07 [73.6 km]
−50 0 50
0
2
4
6PS08 [108.1 km]
−50 0 50
0
2
4
6RND [148.3 km]
−50 0 50
0
2
4
6TCOL [58.1 km]
−50 0 50
0
2
4
6TRF [155.5 km]
−50 0 50
0
2
4
6WRH [53.9 km]
Supplementary Figure S14: Envelopes of high-frequency seismograms for a normal earthquake
on 2015-10-22. No coherent signal is visible prior to the P arrival, which is denoted by the
vertical red line at t = 0. Station MDM is shown in Figure 1b.
23
−80 −60 −40 −20 0 20 40
0
2
4
6BPAW [134.2 km] k = 1.4
−80 −60 −40 −20 0 20 40
0
2
4
6CCB [62.4 km] k = 5.7
−80 −60 −40 −20 0 20 40
0
2
4
6COLA [51.6 km] k = 3.1
−80 −60 −40 −20 0 20 40
0
2
4
6HDA [111.3 km] k = 2.4
−80 −60 −40 −20 0 20 40
0
2
4
6KTH [180.3 km] k = 0.6
nenana_nuc_20120411T_if4
−80 −60 −40 −20 0 20 40
0
2
4
6MCK [132.7 km] k = 1.2
−80 −60 −40 −20 0 20 40
0
2
4
6MDM [34.0 km] k = 2.3
−80 −60 −40 −20 0 20 40
0
2
4
6MLY [85.8 km] k = 33.0
−80 −60 −40 −20 0 20 40
0
2
4
6PPD [173.3 km] k = 1.7
−80 −60 −40 −20 0 20 40
0
2
4
6PS08 [109.7 km] k = 20.4
−80 −60 −40 −20 0 20 40
0
2
4
6WRH [64.7 km] k = 3.8
Supplementary Figure S15: Envelopes of high-frequency seismograms for all stations within
200 km of the 2012 event. The vertical red lines span the high-frequency foreshock signals that
last approximately 20 s at each station. The estimated value of k, labeled above each subplot,
is based on the best-fitting curve of B(t − t0)k. For comparison with a normal earthquake, see
Figure S14. Station MDM is shown in Figure 1b.
24
−50 0 50
0
2
4
6BPAW [106.1 km] k = 2.1
−50 0 50
0
2
4
6BWN [56.9 km] k = 2.4
−50 0 50
0
2
4
6CAST [197.6 km] k = 3.4
−50 0 50
0
2
4
6CCB [69.0 km] k = 3.4
−50 0 50
0
2
4
6CHUM [173.3 km] k = 2.6
−50 0 50
0
2
4
6COLA [69.3 km] k = 1.9
−50 0 50
0
2
4
6F1TN [5.3 km] k = 2.8
−50 0 50
0
2
4
6F2TN [6.2 km] k = 2.0
−50 0 50
0
2
4
6F3TN [11.0 km] k = 2.7
−50 0 50
0
2
4
6F4TN [17.4 km] k = 2.7
−50 0 50
0
2
4
6F5MN [22.9 km] k = 3.8
−50 0 50
0
2
4
6F6TP [25.6 km] k = 3.8
−50 0 50
0
2
4
6F7TV [33.5 km] k = 3.0
−50 0 50
0
2
4
6F8KN [33.8 km] k = 3.6
−50 0 50
0
2
4
6FAPT [16.8 km] k = 1.7
−50 0 50
0
2
4
6FNN1 [12.5 km] k = 3.5
−50 0 50
0
2
4
6FNN2 [15.2 km] k = 4.2−50 0 50
0
2
4
6FPAP [10.5 km] k = 3.0
−50 0 50
0
2
4
6FTGH [20.1 km] k = 2.4
−50 0 50
0
2
4
6H21K [199.0 km] k = 2.1
nenana_nuc_20160114T_if4
−50 0 50
0
2
4
6H24K [143.7 km] k = 3.2
−50 0 50
0
2
4
6HDA [114.5 km] k = 1.9
−50 0 50
0
2
4
6I21K [140.7 km] k = 3.2
−50 0 50
0
2
4
6I23K [52.1 km] k = 2.5
−50 0 50
0
2
4
6KTH [150.1 km] k = 3.1
−50 0 50
0
2
4
6MCK [107.1 km] k = 1.7
−50 0 50
0
2
4
6MDM [57.3 km] k = 2.8
−50 0 50
0
2
4
6NEA2 [13.2 km] k = 2.9
−50 0 50
0
2
4
6POKR [98.6 km] k = 1.8
−50 0 50
0
2
4
6PPD [198.2 km] k = 5.1
−50 0 50
0
2
4
6PS08 [117.1 km] k = 1.9
−50 0 50
0
2
4
6RND [143.6 km] k = 1.2
−50 0 50
0
2
4
6TRF [146.5 km] k = 2.9
−50 0 50
0
2
4
6WRH [60.2 km] k = 5.6
Supplementary Figure S16: Envelopes of high-frequency seismograms for all 34 stations within
200 km of the 2016 event. The vertical red lines span the high-frequency foreshock signals that
last approximately 20 s at each station. The estimated value of k, labeled above each subplot,
is based on the best-fitting curve of B(t − t0)k. For comparison with a normal earthquake, see
Figure S14. Station MDM is shown in Figure 1b.
25
−100 0 100 200−1
0
1
2
3
4
5
6BPAW [119.9 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6BWN [62.8 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6CCB [55.2 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6CHUM [187.5 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6COLA [54.7 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6DHY [198.6 km] k = 0.1
nenana_nuc_20130312T_if2
−100 0 100 200−1
0
1
2
3
4
5
6HDA [101.9 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6MCK [109.7 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6MDM [43.6 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6MLY [92.0 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6NEA [15.0 km] k = 0.2
−100 0 100 200−1
0
1
2
3
4
5
6POKR [84.6 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6PPD [184.0 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6PS06 [132.1 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6PS08 [103.6 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6RND [146.2 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6TRF [155.5 km] k = 0.1
−100 0 100 200−1
0
1
2
3
4
5
6WAT2 [196.3 km] k = 0.0
−100 0 100 200−1
0
1
2
3
4
5
6WRH [49.4 km] k = 0.1
Supplementary Figure S17: Envelopes of high-frequency seismograms for the 2013 VLFE.
Compare with the 2015 VLFE in Figure S18. Station MDM is shown in Figure 1b.
26
−100 0 100 200
0
2
4
6BPAW [159.1 km] k = 0.1
−100 0 100 200
0
2
4
6BWN [109.9 km] k = 0.1
−100 0 100 200
0
2
4
6CCB [66.8 km] k = 0.1
−100 0 100 200
0
2
4
6COLA [46.8 km] k = 0.1
−100 0 100 200
0
2
4
6F1TN [57.0 km] k = 0.1
−100 0 100 200
0
2
4
6F2TN [51.0 km] k = 0.1
−100 0 100 200
0
2
4
6F3TN [45.1 km] k = 0.1
−100 0 100 200
0
2
4
6F4TN [39.4 km] k = 0.1
−100 0 100 200
0
2
4
6F5MN [35.8 km] k = 0.1
−100 0 100 200
0
2
4
6F6TP [50.6 km] k = 0.1
−100 0 100 200
0
2
4
6F7TV [62.4 km] k = 0.1
−100 0 100 200
0
2
4
6F8KN [71.8 km] k = 0.1
−100 0 100 200
0
2
4
6FAPT [66.7 km] k = 0.1
−100 0 100 200
0
2
4
6FNN1 [66.6 km] k = 0.1
−100 0 100 200
0
2
4
6FNN2 [71.2 km] k = 0.1
−100 0 100 200
0
2
4
6FPAP [60.2 km] k = 0.1
−100 0 100 200
0
2
4
6FTGH [48.5 km] k = 0.1
−100 0 100 200
0
2
4
6H23K [88.5 km] k = 0.1
−100 0 100 200
0
2
4
6H24K [87.8 km] k = 0.1
−100 0 100 200
0
2
4
6HDA [113.9 km] k = 0.1
−100 0 100 200
0
2
4
6I21K [155.8 km] k = 0.1
−100 0 100 200
0
2
4
6I23K [32.8 km] k = 0.1
−100 0 100 200
0
2
4
6J25K [166.1 km] k = 0.1
−100 0 100 200
0
2
4
6MCK [155.4 km] k = 0.1
−100 0 100 200
0
2
4
6MDM [27.1 km] k = 0.1
−100 0 100 200
0
2
4
6MLY [98.3 km] k = 0.1
−100 0 100 200
0
2
4
6NEA2 [61.9 km] k = 0.1
−100 0 100 200
0
2
4
6POKR [57.8 km] k = 0.1
−100 0 100 200
0
2
4
6PPD [152.9 km] k = 0.1
−100 0 100 200
0
2
4
6PS06 [95.7 km] k = 0.1
−100 0 100 200
0
2
4
6PS07 [27.8 km] k = 0.1
−100 0 100 200
0
2
4
6PS08 [108.7 km] k = 0.1
−100 0 100 200
0
2
4
6RND [191.4 km] k = 0.1
nenana_nuc_20150912T_if3
−100 0 100 200
0
2
4
6TCOL [46.8 km] k = 0.1
−100 0 100 200
0
2
4
6WRH [77.3 km] k = 0.1
Supplementary Figure S18: Envelopes of high-frequency seismograms for the 2015 VLFE. Sta-
tion MDM is shown in Figure 1b.
27
−0.05
0
0.05
0.1
inte
r−sta
tio
np
ha
se
co
he
ren
ce
−180
0
180
inte
r−sta
tion
ph
ase
diffe
ren
ce
ve
locity a
t X
V.F
PA
P (μ
m/s
)
−50 −45 −40 −35 −30 −25 −20 −15 −10 −5
−45 −40 −35 −30 −25 −20 −15 −10 −5
−45 −40 −35 −30 −25 −20 −15 −10 −5
−45 −40 −35 −30 −25 −20 −15 −10 −5
−0.5
0
0.5
no
rma
lize
dcro
ss−
co
rre
latio
n / N
co
mp
1/2
time relative to earthquake (s)
0.1
100
0.1
pe
rce
nta
ge
with
am
plit
ud
e
larg
er
by c
ha
nce
−3
0
3
no
rma
lize
d
cro
ss−
co
rre
latio
n / σ
all 30 stations
XV.FPAP
AK.NEA2
XV.F1TN
XV.F2TN
a
b
c
d
5
5
30
0
−30
Supplementary Figure S19: Phase coherence between mainshock and high-frequency foreshocks
of the 2016 event. (a) 1–10 Hz phase coherence between the mainshock and 4-s windows of
the foreshock arrivals. Colored curves for station pairs including the indicated stations. The
black curve averages over all available stations. (b) Averaged phase difference between the
cross-correlations. (c) Velocity seismogram at XV.FPAP. (d) 1–10 Hz cross-correlation between
the mainshock signals and the foreshocks, for individual stations (colors) and averaged over all
stations (black).
28
−2
−1
0
1
2
−25.5 to −21.5 s
de
pth
(km
)
−24.5 to −20.5 s −23.5 to −19.5 s −22.5 to −18.5 s −21.5 to −17.5 s −20.5 to −16.5 s
−2
−1
0
1
2
−19.5 to −15.5 s
de
pth
(km
)
−18.5 to −14.5 s −17.5 to −13.5 s −16.5 to −12.5 s −15.5 to −11.5 s −14.5 to −10.5 s
−2
−1
0
1
2
−13.5 to −9.5 s
de
pth
(km
)
−12.5 to −8.5 s −11.5 to −7.5 s −10.5 to −6.5 s −9.5 to −5.5 s −8.5 to −4.5 s
−2 −1 0 1 2
−2
−1
0
1
2
−7.5 to −3.5 s
distance E (km)
de
pth
(km
)
−2 −1 0 1 2
−6.5 to −2.5 s
distance E (km)−2 −1 0 1 2
−5.5 to −1.5 s
distance E (km)−2 −1 0 1 2
−4.5 to −0.5 s
distance E (km)−2 −1 0 1 2
−3.5 to 0.5 s
distance E (km)−2 −1 0 1 2
−2.5 to 1.5 s
distance E (km)p
ha
se
co
he
ren
ce
−0.1
0
0.1
Supplementary Figure S20: Phase coherence between mainshock and high-frequency foreshocks
of the 2016 event. Phase coherence is plotted as a function of foreshock location in an east-west
oriented vertical plane. This plane has zero north-south offset from the mainshock.
29
−2
−1
0
1
2
−25.5 to −21.5 s
dis
tan
ce
N (
km
)
−24.5 to −20.5 s −23.5 to −19.5 s −22.5 to −18.5 s −21.5 to −17.5 s −20.5 to −16.5 s
−2
−1
0
1
2
−19.5 to −15.5 s
dis
tan
ce
N (
km
)
−18.5 to −14.5 s −17.5 to −13.5 s −16.5 to −12.5 s −15.5 to −11.5 s −14.5 to −10.5 s
−2
−1
0
1
2
−13.5 to −9.5 s
dis
tan
ce
N (
km
)
−12.5 to −8.5 s −11.5 to −7.5 s −10.5 to −6.5 s −9.5 to −5.5 s −8.5 to −4.5 s
−2 −1 0 1 2
−2
−1
0
1
2
−7.5 to −3.5 s
distance E (km)
dis
tan
ce
N (
km
)
−2 −1 0 1 2
−6.5 to −2.5 s
distance E (km)−2 −1 0 1 2
−5.5 to −1.5 s
distance E (km)−2 −1 0 1 2
−4.5 to −0.5 s
distance E (km)−2 −1 0 1 2
−3.5 to 0.5 s
distance E (km)−2 −1 0 1 2
−2.5 to 1.5 s
distance E (km)p
ha
se
co
he
ren
ce
−0.1
0
0.1
Supplementary Figure S21: Phase coherence between mainshock and high-frequency foreshocks
of the 2016 event. Phase coherence is plotted as a function of foreshock location in map view.
This plane has zero vertical offset from the mainshock.
30
Time (ms)
Dis
tan
ce
alo
ng
fa
ult x
, cm
Time, ms
00
10
10
20
20
30
30
rate strengthening, a - b > 0
rate weakening,
a - b < 0
Fault modelLaboratory experiment
fault
viscousputty
Polycarbonate plate
viscousputty
Loading with a manual pump
(a)
(c)
fault
(b)
Rupture length (mm)
Ru
ptu
re s
pe
ed
Vr (
m/s
)
(e)
3210.5
σ (MPa)
Ru
ptu
re s
pe
ed
Vr (
m/s
)
10 15 20 30 50 70 100 150
1
5
10
50
100
500
1000
10 15 20 30 50 70 100 150
0.5
1
5
10
50
100
500
1000
(d)
Laboratory experiments
6
σ (MPa)
acc
ele
ratio
n (
ase
ism
ic)
dynamic propagation (seismic)
quasi-static
propagation
(aseismic)
acc
ele
ratio
n (ase
ism
ic)
dynamic propagation (seismic)
quasi-static
propagation
(aseismic)
Numerical simulations
Rupture length (mm)
nucleation dynamic rupture
σ = 1.58 MPa
Supplementary Figure S22: Schematic diagrams illustrating the set-up of (a) the laboratory ex-
periments of Latour et al. (2013) and (b) the numerical model of Kaneko et al. (2016). (c)
Comparison between simulation results (Kaneko et al., 2016), plotted as the blue dashed line,
with laboratory results from Latour et al. (2013), plotted as the red line and with grayscale to
show the light intensity change indicating the actively slipping zone. The curves show the po-
sition of the rupture front during a transition from quasi-static nucleation to dynamic rupture.
The rupture fronts are defined as the locations of two peak shear stresses: one within the left
rate-strengthening patch and the other within the rate-weakening patch. (d)-(e) Characteristics
of nucleation phase under different σ in numerical simulations and for 47 stick-slip events in
laboratory experiments. Observed and modeled rupture speeds increase with the rupture length.
The rupture length is defined as a distance from the edge of the rate-weakening patch at x = 11cm to the rupture front. The evolution of the rupture front under a range of σ closely matches
the laboratory results.
31
time (ms)-80 -60 -40 -20 0lo
g1
0(r
up
ture
le
ng
th (
mm
))
-1
0
1
2
3
distance along strike (cm)10 15 20 25
tim
e (
ms)
-50
-40
-30
-20
-10
0
10
time (ms)-80 -60 -40 -20 0lo
g1
0(r
up
ture
le
ng
th (
mm
))
-1
0
1
2
3
distance along strike (cm)10 15 20 25
tim
e (
ms)
-60
-40
-20
0
time (ms)-80 -60 -40 -20 0lo
g1
0(r
up
ture
le
ng
th (
mm
))
-1
0
1
2
3
distance along strike (cm)10 15 20 25
tim
e (
ms)
-60
-40
-20
0
time (ms)-80 -60 -40 -20 0lo
g1
0(r
up
ture
le
ng
th (
mm
))
-1
0
1
2
3
distance along strike (cm)10 15 20 25
tim
e (
ms)
-80
-60
-40
-20
0
f = 10-1.8(t + 31)2.5
f = 10-2.1(t + 36)2.5
f = 10-3.1(t + 57)2.7
f = 10-4.8(t + 85)3.2
σ = 0.75 MPa
σ = 0.91 MPa
σ = 1.58 MPa
σ = 2.40 MPa
A
A
B C
AB C
A
B C
A
B C
A
A
A
σ = 0.75 MPa
σ = 0.91 MPa
σ = 1.58 MPa
σ = 2.40 MPa
Supplementary Figure S23: Evolution of rupture front position (left column) and rupture
length (right column) for four simulations from Kaneko et al. (2016) for normal stresses of
σ = 0.75 MPa (top), 0.91 MPa, 1.58 MPa, and 2.40 MPa (bottom). The three stages of rupture
are labeled as A (quasi-static), B (acceleration), and C (dynamic propagation). The onsets of the
quasi-static, acceleration, and dynamic propagation phases are marked by green, black and red
dashed lines, respectively. In this figure, t = 0 is defined as the time at the end of the accel-
eration phase, which is graphically defined from Figure S22d. The magenta dashed curve is a
power function fit (B(t− t0)k), with the best-fitting power function shown by magenta text.
32