Submitted to Earth, Planets and Space 1 2 Did ionospheric anomalies precede the 2016 April Kumamoto earthquake 3 sequence? 4 5 1 Kosuke Heki and 1,2 Liming He 6 7 1. Dept. Earth Planet. Sci., Hokkaido University, Sapporo, Japan 8 2. Department of Geodesy and Geomatics, School of Resources and Civil Engineering, Northeastern 9 University, Shenyang, China 10 11 Abstract 12 High seismic intensities of the foreshock (Mw6.2) and the mainshock (Mw7.0) of the 2016 April 13 Kumamoto earthquake sequence brought huge damages. Here we examine if detectable ionospheric 14 anomalies preceded these inland shallow crustal earthquakes. We analyzed changes in ionospheric 15 total electron content (TEC) using Japanese dense network of Global Navigation Satellite System 16 (GNSS) receivers. However, we did not find anomalies of the kind which we often observe before 17 larger earthquakes. For comparison, we analyzed TEC before the Mw7.8 earthquake that occurred in 18 Ecuador, South America, on the next day of the Kumamoto mainshock. We found possible 19 preseismic TEC anomalies with amplitudes and precursor times consistent with past large 20 earthquakes. These results support the empirical relationship that sizes of the preseismic TEC 21 anomalies depend on earthquake Mw and background vertical TEC, but not on maximum seismic 22 intensities. We also found that a stationary linear positive TEC anomaly, with a shape similar to 23 medium-scale traveling ionospheric disturbance, emerged to the northwest of the epicenter 24 immediately before the Kumamoto mainshock. Further studies are needed to conclude its link to the 25 earthquake. 26 27 Keywords: 2016 Kumamoto earthquake, total electron content, ionosphere, MSTID, GNSS 28 29 1. Introduction 30 Ionospheric Total Electron Content (TEC) can be derived by comparing phases of two L band 31 microwave signals from Global Navigation Satellite System (GNSS) satellites, such as Global 32 Positioning System (GPS). Heki (2011) found ionospheric electron increase above the epicenter of 33 the 2011 Tohoku-oki earthquake (Mw 9.0), Japan, starting ~40 minutes prior to the earthquake using 34 data from GEONET (GNSS Earth Observation Network), a dense array of continuous GNSS stations 35 in Japan. This paper was followed by publications of critical papers (Kamogawa and Kakinami, 36
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Submitted to Earth, Planets and Space 1
2
Did ionospheric anomalies precede the 2016 April Kumamoto earthquake 3
sequence? 4 5 1Kosuke Heki and 1,2Liming He 6
7
1. Dept. Earth Planet. Sci., Hokkaido University, Sapporo, Japan 8
2. Department of Geodesy and Geomatics, School of Resources and Civil Engineering, Northeastern 9
University, Shenyang, China 10
11
Abstract 12
High seismic intensities of the foreshock (Mw6.2) and the mainshock (Mw7.0) of the 2016 April 13
Kumamoto earthquake sequence brought huge damages. Here we examine if detectable ionospheric 14
anomalies preceded these inland shallow crustal earthquakes. We analyzed changes in ionospheric 15
total electron content (TEC) using Japanese dense network of Global Navigation Satellite System 16
(GNSS) receivers. However, we did not find anomalies of the kind which we often observe before 17
larger earthquakes. For comparison, we analyzed TEC before the Mw7.8 earthquake that occurred in 18
Ecuador, South America, on the next day of the Kumamoto mainshock. We found possible 19
preseismic TEC anomalies with amplitudes and precursor times consistent with past large 20
earthquakes. These results support the empirical relationship that sizes of the preseismic TEC 21
anomalies depend on earthquake Mw and background vertical TEC, but not on maximum seismic 22
intensities. We also found that a stationary linear positive TEC anomaly, with a shape similar to 23
medium-scale traveling ionospheric disturbance, emerged to the northwest of the epicenter 24
immediately before the Kumamoto mainshock. Further studies are needed to conclude its link to the 25
earthquake. 26
27
Keywords: 2016 Kumamoto earthquake, total electron content, ionosphere, MSTID, GNSS 28
29
1. Introduction 30
Ionospheric Total Electron Content (TEC) can be derived by comparing phases of two L band 31
microwave signals from Global Navigation Satellite System (GNSS) satellites, such as Global 32
Positioning System (GPS). Heki (2011) found ionospheric electron increase above the epicenter of 33
the 2011 Tohoku-oki earthquake (Mw 9.0), Japan, starting ~40 minutes prior to the earthquake using 34
data from GEONET (GNSS Earth Observation Network), a dense array of continuous GNSS stations 35
in Japan. This paper was followed by publications of critical papers (Kamogawa and Kakinami, 36
2013; Utada and Shimizu, 2014; Masci et al, 2015) and immediate rebuttals to them (Heki and 37
Enomoto, 2013; 2014; 2015). 38
Heki and Enomoto (2015) demonstrated that similar electron increases preceded eight past 39
earthquakes with Mw 8.2 or more. They also reported that the anomalies started about 20/40 minutes 40
before Mw 8/9 earthquakes, and that the changes in vertical TEC (VTEC) rate depended on Mw as 41
well as background absolute VTEC. Similar changes often occur due to geomagnetic activities, but 42
Heki and Enomoto (2015) demonstrated that they are not frequent enough to account for the 43
observed preseismic anomalies. The latest paper (He and Heki, 2016) revealed that both TEC 44
decrease and increase emerge simultaneously in different regions. Their 3-D structure showed that 45
positive and negative electron density anomalies line up from the epicenter along the geomagnetic 46
field. Such geometry resembles to the ionospheric response to positive surface charges as Kuo et al. 47
(2014) showed using numerical simulation. 48
The foreshock (Mw6.2) and the mainshock (Mw7.0) of the 2016 Kumamoto earthquake sequence 49
occurred on April 14 12:26 UT, and April 15 16:25 UT, with the temporal separation of ~28 hours. 50
Their shallow epicentral depths caused high seismic intensities (7 in the Japan Meteorology Agency 51
scale), and caused lots of damages to buildings and roads. Indeed, the peak seismic intensities of the 52
two earthquakes were comparable to the Mw9.0 2011 Tohoku-oki earthquake, the largest recorded 53
earthquake in Japan. According to Heki and Enomoto (2015), no preseismic TEC changes should 54
have preceded such earthquakes with Mw≤7. However, the series of study relied on cases of 55
interplate thrust earthquakes in deep-sea trenches, and its validity for shallow inland earthquakes 56
needs confirmation. Here we look for TEC anomalies immediately before these earthquakes using 57
GEONET data. We also examine, for comparison, the existence of preseismic TEC anomalies of the 58
2016 April Ecuador earthquake (Mw7.8), which occurred on the next day of the Kumamoto 59
mainshock. 60
61
2. Data and Results 62
2-1. Two Kumamoto earthquakes 63
We converted the raw data from ~1200 ground stations of GEONET to slant TEC (STEC). We 64
extracted two hour potions of the STEC time series including the two earthquakes. We selected data 65
with two GPS satellites closest to the local zenith in Kyushu. To highlight the difference from the 66
2011 Tohoku-oki earthquake case, we employed the same procedure as Heki (2011), i.e. we assumed 67
that the temporal change of the vertical TEC (VTEC) obeys a quadratic polynomial of time, and 68
defined the departure from the reference as the anomaly. In determining the reference curves, we 69
normally exclude a certain time window. Heki and Enomoto (2015) showed that preseismic 70
anomalies do not start earlier than 20 minutes before interplate events with Mw8 or less. Here we set 71
up somewhat longer time windows, ranging from -30 minutes to +20 minutes of the earthquakes. 72
The VTEC anomalies at three epochs, 20 minutes, 10 minutes, and immediately before earthquakes 73
are shown in Fig. 1. He and Heki (2016) showed that positive TEC anomalies appear at altitudes 74
~200 km in the area at the equator side (south in the northern hemisphere) of the epicenter, but we do 75
not see them in Fig.1. In spite of similar maximum seismic intensities, the Kumamoto earthquakes 76
do not show anomalies comparable to those before the 2011 Tohoku-oki earthquake. In Fig.1f, one 77
can recognize weak positive anomalies running NNW-SSE. This will be discussed later in Section 78
3.2. 79
80
81
Fig. 1. (a-c) VTEC anomalies at 20 minutes (a), 10 minutes (b) and immediately (c) before the 82
2016 Kumamoto foreshock. We used GPS satellites 1 (circle) and 11 (triangle). The black star 83
represents the epicenter. (d-f) VTEC anomalies before the Kumamoto mainshock drawn using 84
GPS satellites 19 (circle) and 6 (triangle). We used the same color scheme as the Fig. 3 in Heki 85
(2011) for the 2011 Tohoku-oki earthquake. 86
87
Fig. 2 shows the STEC time series at five stations in Kyushu for the two Kumamoto earthquakes. 88
There we selected stations with SIP located to the south of the epicenters. Although the reference 89
curves were estimated to fit the 50 minutes excluding the time interval shown with red bars at the top, 90
they overlap with the observed data throughout the interval, suggesting small anomalous behaviors 91
of the ionospheric electrons before and after the earthquakes. From Figs. 1 and 2, we conclude that 92
the 2016 April Kumamoto earthquake sequence were not associated with ionospheric TEC 93
anomalies like those shown for larger earthquakes in Heki and Enomoto (2015). We found a small 94
amplitude linear-shaped anomaly to the north of the epicenter, which will be discussed in Section 95
3.2. 96
97
98
99
Fig. 2. Slant TEC changes over two hour intervals including the foreshock (a) and the 100
mainshock (b) of the Kumamoto earthquakes, drawn using GPS satellites 1 and 19, respectively. 101
The maps show the epicenters (black stars), GNSS stations (red circles), and sub-ionospheric 102
point (SIP) trajectories calculated assuming thin ionosphere at altitude of 300 km. The observed 103
STEC changes were modeled assuming that VTEC changes obey quadratic functions of time. In 104
estimating the reference curves, we excluded the time shown with the red bars at the top, i.e. 105
from 30 minutes before earthquake to 20 minutes after earthquake. 106
107
2-2. The Ecuador earthquake 108
The Mw7.8 2016 Ecuador earthquake occurred at 23:58 UT, April 16, ~31.5 hours after the 109
Kumamoto mainshock, as an interplate thrust earthquake between the Nazca and the South American 110
Plates. The epicenter lies beneath the Pacific coast of northern Ecuador, at the depth ~20 km. This 111
earthquake does not have any causal relationship with the Kumamoto earthquakes. Nevertheless, it 112
would be meaningful to compare their preseismic ionospheric signals to understand the difference 113
coming from the two factors, Mw and background VTEC. We downloaded the GNSS raw data taken 114
at two stations in Ecuador, QUI4 (Quito) and RIOP (Riobamba), from the UNAVCO data archive 115
(www.unavco.org/data/). We then converted STEC to VTEC following He and Heki (2016). 116
Because of the small number of available GNSS stations, it is not appropriate to draw figures like 117
Fig.1. In Fig.3, we plot the VTEC time series. They show overall decrease in this period because the 118
time window includes the local sunset. We obtained the reference curves using polynomial of time 119
with degrees-8 excluding the time interval within ±20 minutes of the earthquake (red line at the top 120
of Fig.3). For a relatively small earthquake like this, it is difficult to constrain the positive bending of 121
VTEC using Akaike’s Information Criterion as done for larger earthquake in Heki and Enomoto 122
(2015). Nevertheless, the observed VTEC curves show clear positive anomalies starting 15-20 123
minutes before the earthquake, demonstrating clear differences from the Kumamoto cases (Fig.2). 124
125
126
127
Fig. 3. VTEC time series of three station-satellite pairs recording possible preseismic TEC 128
increases of the 2016 Ecuador earthquake. Red horizontal bars indicate the excluded period in 129
defining the reference curves. The map shows the SIP trajectories calculated assuming 300 km 130
for ionospheric height. Along the trajectories, we show hourly time marks (small circles), the 131
earthquake occurrence times (squares), and the precursor starting times (triangles). The yellow 132
star shows the epicenter. Because the Sat.7-Quito curve overlaps with the Sat.30-Quito curve, we 133
moved the former slightly upward for visual clarity. 134
135
3. Discussions and conclusion 136
3-1. Preseismic TEC anomalies 137
Fig. 4a shows that the geomagnetic activity was moderately high (Dst drops ~50 nT) at the 138
occurrence times of all the three earthquakes. This might be responsible for small scale undulations 139
as seen in the TEC time series in Fig. 2, but they did not cause serious departure from reference 140
curves within the studied time windows. The three earthquakes occurred at around 21:25, 01:27 and 141
18:58 in local times. Fig.4 compares the distribution of VTEC drawn using global ionospheric maps 142
(GIM) downloaded from University of Bern (ftp.unibe.ch/aiub/). We can see that VTEC above the 143
epicenter of the Ecuador earthquake were more than twice as high as the two Kumamoto earthquake 144
cases. 145
146
147 Fig. 4. Change of the Dst index (omniweb.gsfc.nasa.gov/form/dx1.html) over the 20 days 148
period including the three earthquakes (three red vertical lines) discussed here (a). Global 149
Ionospheric Map (GIM) at the time of the foreshock (b) and the mainshock (c) of the 2016 150
Kumamoto earthquake, and the 2016 Ecuador earthquake (c). The VTEC values at the epicenters 151
(white stars) are, ~15, ~10, and ~36 TECU, respectively. 152
153
Heki and Enomoto (2015) proposed the empirical relationship among the three quantities, 154
preseismic VTEC rate changes, earthquake Mw, and background VTEC. It suggests that a larger 155
VTEC rate change tends to occur before a larger earthquake and under a larger background VTEC. 156
Fig.5 is a leftward-expanded version of Fig. 4a in Heki and Enomoto (2015). It seems obvious that 157
preseismic TEC signals are not to be seen before the 2016 Kumamoto earthquakes, whose Mw are 158
only 6.2 and 7.0 (shown in red + in Fig.5). The lack of signals before these two earthquakes suggests 159
that even shallow inland crustal earthquakes do not cause preseismic ionospheric signals much larger 160
than expected from this diagram. 161
On the other hand, positive VTEC rate changes of a little less than 2 TECU/h is expected to occur 162
before the Mw7.8 Ecuador earthquake (see contours in Fig.5). The actually observed rate change is 163
1.75 TECU/h (VTEC: 26.6 TECU) from the RIOP-Sat.30 pair (red curve in Fig.3a), which is 164
consistent with this diagram. The anomaly started ~17 minutes before the earthquake, and this is also 165
consistent with past earthquakes, shown in Fig.5a of Heki and Enomoto (2015). 166
167 168
Fig. 5. Diagram showing the dependence of the preseismic VTEC rate change (size of the 169
circle) on the background VTEC (vertical axis) and the earthquake Mw (horizontal axis), similar 170
to Fig.4a of Heki and Enomoto (2015). Riobamba-Sat.30 pair was used in adding the 2016 171
Ecuador earthquake case (small red circle in the middle). Three black crosses indicate 172
earthquakes with no significant preseismic VTEC changes detected, already reported in Heki 173
(2011). For the two Kumamoto earthquakes, we did not find precursory VTEC changes, which 174
are shown with two additional crosses in red. The rate change (TECU/h) is modeled as 3.75 Mw 175
+ 0.11 VTEC −30.6, and the contour lines show the same rate changes of 2, 4, 6, 8, and 10 176
TECU/h. Dashed parts of the contours are not substantiated by data. 177
178
3-2. MSTID-like anomaly before the mainshock 179
TEC anomalies were not clear before the Kumamoto mainshock as seen in Fig.1d-f. However, by 180
isolating GPS satellite 6 data and replacing the color scheme, a weak (up to ~1 TECU) linear-shaped 181
positive TEC anomaly emerge to extend north-northwestward from the mainshock epicenter. The six 182
panels in Fig. 6a show snapshots taken every five minutes. This anomaly starts to grow ~25 minutes 183
before the earthquake, and decay in ~10 minutes after the earthquake. 184
Its NNW-SSE elongation and relatively short wavelength (~100 km) resemble to a typical 185
night-time Medium-Scale Traveling Ionospheric Disturbance (MSTID) that occur frequently during 186
summer nights (Otsuka et al., 2011). In fact, a typical night-time MSTID occurred about two days 187
before the mainshock (April 13) (Fig.6b). There is, however, a peculiar feature in the MSTID-like 188
anomaly on April 15, i.e. it is stationary in space. The difference is clear by comparing Fig.6a and 6b, 189
i.e. the one in Fig.6b propagates southwestward by ~200 m/s. The velocity of night-time MSTIDs in 190
Japan falls within 80-200 m/s (Otsuka et al., 2011), and the stagnant anomaly in Fig.6a is quite 191
exceptional for MSTID. 192
Both Night-time MSTID and preseismic TEC anomalies are considered to develop by electric 193
fields in the ionosphere. Night-time MSTID starts to develop due to polarization electric field and 194
grow with time through the Perkins instability, although it is not well understood what process 195
governs its southwestward propagation velocity (Garcia et al., 2000). On the other hand, He and 196
Heki (2016) suggested that the preseismic ionospheric anomalies could be a response to electric 197
fields made by positive charges on the surface above the epicenter. In the present case, the surface 198
electric charge before the Kumamoto mainshock may not be large enough to cause ionospheric 199
electron redistribution as seen before larger earthquakes by Heki and Enomoto (2015). However, the 200
charge might be large enough to become a seed of MSTID and make it stagnant above the epicenter. 201
Anyway, we need more observations to clarify the link. By the way, the largest “aftershock” of the 202
foreshock (Mw6.0) occurred at 15:03UT on the day of the foreshock, but we observed only typical 203
night-time MSTID propagating southwestward at that time. 204
205
3-3. Conclusions 206
We studied ionospheric anomalies immediately before the 2016 April Kumamoto earthquakes, 207
with seismic intensity comparable to the 2011 Tohoku-oki earthquake. The results can be concluded 208
as follows; 209
1) No preseismic TEC anomalies, similar to the 2011 Tohoku-oki earthquake case, were found for 210
the foreshock and the mainshock of the Kumamoto earthquake sequence. 211
2) Possible small preseismic TEC anomaly, consistent with Mw and background VTEC, was seen 212
before the Ecuador earthquake that occurred on the next day of the Kumamoto mainshock. 213
3) Existence and size of the TEC signals prior to these three earthquakes are consistent with Heki 214
and Enomoto (2015), the anomalies depend on Mw and background VTEC rather than seismic 215
intensity. 216
4) Stationary MSTID-like disturbance appeared shortly before the Kumamoto mainshock, but further 217
studies are needed to understand its link with the earthquake. 218
219
220
221
Fig. 6. Comparison of the development and movement of MSTID that appeared on April 15 222
shortly before the Kumamoto earthquake mainshock (a), and on April 13, two days before that 223
(b), shown by five-minute snap shots with GPS satellite 6 over 35 minutes interval. We drew 224
five gray lines with 100 km separation to visualize their propagation. Typical night-time 225
MSTID show southwestward movements like the example in (b). In (a), the positive crest of 226
MSTID is stagnant above the mainshock epicenter (black star). 227
228
Acknowledgements 229
We thank referees for constructive comments. The GEONET data were downloaded from the GSI 230
website (terras.gsi.go.jp). 231
232
Competing Interests 233
Here we declare that none of the authors do not have any competing interests in the manuscript. 234
235
Author Contributions 236
KH carried out the analysis of TEC before and after the Kumamoto and Ecuador earthquakes. LH 237
analyzed the night-time MSTID on non-earthquake days. Both read and approved the final 238
manuscript. 239
240
References 241
Garcia, F.J., M. C. Kelley, J. J. Makela, and C.–S. Huang (2000), Airglow observations of mesoscale 242
low-velocity traveling ionospheric disturbances at midlatitudes, J. Geophys. Res. Space Phys., 105, 243
18407-18415, doi:10.1029/1999JA000305. 244
He, L. and K. Heki (2016), Three-dimensional distribution of ionospheric anomalies prior to three 245
large earthquakes in Chile, Geophys. Res. Lett., doi:10.1002/2016GL069863, in press. 246
Heki, K. (2011), Ionospheric electron enhancement preceding the 2011 Tohoku-Oki earthquake, 247