Post-earthquake dizziness syndrome following the 2016 Kumamoto earthquakes, Japan.

This study aimed to examine the types and causes of dizziness experienced by individuals after a major earthquake. This cross-sectional study enrolled healthy participants who experienced the 2016 Kumamoto earthquakes and their aftershocks. Participants completed a questionnaire survey on their symptoms and experiences after the earthquakes. The primary outcome was the occurrence of dizziness and the secondary outcome was the presence of autonomic dysfunction and anxiety. Among 4,231 eligible participants, 1,543 experienced post-earthquake dizziness. Multivariate logistic regression analysis revealed that age (≥21, P < .001), female sex (P < .001), floor on which the individual was at the time (≥3, P = .007), tinnitus/ear fullness (P < .001), anxiety (P < .001), symptoms related to autonomic dysfunction (P = .04), and prior history of motion sickness (P = .002) were significantly associated with the onset of post-earthquake dizziness. Thus suggesting that earthquake-related effects significantly affect inner ear symptoms, autonomic function, and psychological factors. Earthquake-induced disequilibrium may be further influenced by physical stressors, including sensory disruptions induced by earthquake vibrations, changes in living conditions, and autonomic stress. This study increases our understanding of human equilibrium in response to natural disasters.

Introduction

Major earthquakes are associated with an increased prevalence of psychiatric morbidities [1,2], sleep disorders [3,4], and dizziness [5-10]. The Kumamoto earthquakes on April 14 and 16, 2016 (Fig 1, moment magnitude = 9.0; Shindo = 7) included several high-magnitude vibrations and aftershocks without secondary disasters. Several months after the initial earthquake, significant outbreaks of dizziness were reported over a large area surrounding the earthquake epicenter [8-10].

Fig 1

Map of Kumamoto, 2016.

Blue Xs indicate the epicenters. Magenta to yellow colors indicate earthquake size (Shindo data). Information was reprinted from the Japan Meteorological Agency (https://www.jma.go.jp/jma/indexe.html) under a CC BY license with permission from the Japan Meteorological Agency, original copyright 2016.

Although several reports have described post-earthquake dizziness [5-10], its characteristic symptoms remain undefined. After the Tohoku earthquake on March 11, 2011 (moment magnitude = 9.0; Shindo = 7) and its sequelae (a tsunami and the Fukushima nuclear disaster), Nomura et al. defined the characteristic symptoms of post-earthquake dizziness as post-earthquake dizziness syndrome (PEDS) [11]. Healthy individuals (those without vestibular diseases) with PEDS experienced illusory body swaying (lasting <1 minute after the earthquake). Psychological stress and mismatched visual/somatosensory inputs that induce autonomic dysfunction reportedly cause PEDS [11]. Although studies have reported vertigo in some individuals with PEDS after the Tohoku earthquake, the influence of major earthquakes and repetitive aftershocks without a tsunami, as well as the influence of the Fukushima nuclear disaster on post-earthquake dizziness, remains unexamined.

Previous studies on conditions similar to PEDS described its symptoms as “getting one’s sea legs” on a multiday sea voyage [12-14] or the wind-induced motion of a tall building [15]. In those studies, researchers stated that the mechanisms for those phenomenon are likely to be explained by the sensory conflict theory/postural instability theory [12-15]. For example, on a ship in the sea, the deck exhibits oscillatory motion simultaneously in six degrees of freedom. The primary power of these oscillations is in the same frequency range (0.1–0.4 Hz) as the oscillations that characterize standing body sway. The onset of motion by the ship leads to a reduction in subjective body stability and balance performance, and individuals tend to exhibit consistent changes in subjective awareness and postural performance [14]. Munafo et al. and Stoffregen et al. concluded that those effects were caused by conscious awareness within the ecological analyses of perception and action, similar to the postural instability theory [12-14]. Although a tall building facing wind typically vibrates in the range of 0.063–1 Hz, random low-frequency building vibration could cause wave interference that is difficult to anticipate. Therefore, an individual counters the natural adjustments that are required to maintain an appropriate posture [15]. Walton et al. concluded that those effects are caused by motion sickness via sensory conflict among visual, vestibular, and proprioceptive sensations [15]. In addition, Stoffregen et al. speculated that the duration of exposure was directly related to the intensity of symptoms experienced, and that the magnitude of movement may also be related to symptom severity [13].

The primary earthquake and repetitive aftershocks caused low frequency (0.1–3.5 Hz) horizontal and vertical linear accelerations. Therefore, the individual was aware of sensory mismatch between visual/proprioceptive sensations and vestibular function, and natural adjustments were required to maintain posture, similar to “getting one’s sea legs” [12-14] or the wind-induced motion of a tall building [15].

Thus, this study aimed to examine how major earthquakes and repetitive aftershocks influence human equilibrium. We hypothesized that PEDS after major earthquakes without secondary disasters is caused by a low-dose sensory mismatch, and that after numerous repetitive aftershocks is caused by low-dose postural instability between visual/proprioceptive sensations and vestibular function.

Materials and methods

Standard protocol approvals, registrations, and participants’ consent

This study adhered to the tenets of the Declaration of Helsinki and was approved by the Kumamoto university institutional review board (Number: 1099). All participants enrolled in the study provided written informed consent.

Participants

Participants were recruited between June 20 and July 21, 2016, using data collected from a public institution in Kumamoto city. Data from 2017 to 2021 were analyzed. Healthy participants, 10–100 years old, without head trauma who could describe their symptoms and experiences after the Kumamoto earthquakes were included in the study. Individuals who experienced vertigo/dizziness prior to the earthquake were excluded. We included 3,656 healthy individuals who experienced the Kumamoto earthquakes and aftershocks without being exposed to radioactive agents or the tsunamis that hit Kumamoto city for over 12 weeks (Tables 1 and S1). S1 Table shows the patients’ demographic information.

Table 1

Participants’ details.

People exposed to Kumamoto earthquake: 3,656	PEDS group
	People who felt the illusion sway: 1,543
	Non-PEDS group
	People who did not feel the illusion sway: 2,113

PEDS, post-earthquake dizziness syndrome.

Main measures and outcomes

Participants completed clinical questionnaires and provided information related to age, sex, PEDS symptoms, onset dates, duration of PEDS symptoms, concomitant symptoms, and individual factors. Clinical questionnaires were developed based on those used in the Tohoku earthquake [11] (S2 Table). As geo-structural building conditions may influence the intensity of the shaking experienced [16] and subsequent PEDS, we investigated the building type (wooden or iron), and the floor level on which the participants were at the time of the earthquake, to determine sensory conflict effects. We defined PEDS as the illusion of body swaying; however, participants may consider actual vibrations (aftershocks) as PEDS. Therefore, we investigated correlations between the prevalence of PEDS, geological conditions, and the number of aftershocks in each region. The primary outcome was the occurrence of post-earthquake dizziness, i.e., vertigo/dizziness, experienced by participants after an earthquake. The secondary outcomes were autonomic dysfunction and anxiety.

Statistical analyses

For the primary outcome, changes in the proportion of patients with PEDS before and after the earthquake were compared using the Chi-squared test. For secondary outcomes, PEDS-associated symptoms were assessed using multivariate logistic regression analysis with age, sex, region, building type, floors in the building, tinnitus/ear fullness, anxiety, symptoms related to autonomic dysfunction for concomitant symptoms, and prior history of motion sickness as the explanatory variables. We also estimated the odds ratios (OR) and 95% confidence intervals (CI). For multivariate logistic regression analysis, the model was created after confirming the variance inflation factor. Imputation was performed for missing values by random forest methods. As a linear relationship was speculated, Pearson’s correlation coefficients were used to examine the relationship between the rate of perceived PEDS and number of aftershocks. Power and sample size calculations were conducted before and after data collection using PS software (Ver. 3.1.6, Vanderbilt University, Nashville, TN, USA) [17]. Statistical significance was set at P < .05. Evaluations were determined as “not applicable” if the calculated sample size after data collection was found insufficient for statistical analysis. Statistical analyses were performed using GraphPad Prism version 8.0.0 for Windows (GraphPad Software, San Diego, CA, USA, www.graphpad.com).

Results

Prevalence of PEDS after the Kumamoto earthquakes

After the Kumamoto earthquake, 1,543 participants (42.2%, 95% CI: 41.7%–46.7%) experienced the illusion of their bodies swaying. Fig 2 (panels a–f) shows the frequency and temporal parameters of PEDS. PEDS was higher in female than in male participants (Fig 2A), and female participants in their 30’s were more likely to experience PEDS (73%) than those in other age groups, while male participants in their 40’s were more likely to experience PEDS (60%) than those in other age groups (Fig 2B). PEDS was most frequently described as a feeling of the ground shaking, experienced within one week of the earthquake and lasting up to several weeks in an indoor setting (Fig 2C–2F).

Fig 2

PEDS frequency according to sex, age, and earthquake parameters.

a) PEDS was present in 1,543 of the 3,656 participants (42.2%). The incidence of PEDS was higher in female than male participants. b) Female participants in their 30’s were more likely to experience PEDS (73%, asterisk) than female participants of other ages. Male participants in their 40’s were more likely to experience PEDS (60%, asterisk) than male participants of other ages. c) PEDS was most frequently described as a feeling of the ground shaking. d, e) PEDS was felt within one week of the earthquake and lasted up to several weeks. f) PEDS was more commonly experienced in indoor settings. Abbreviation: PEDS, post-earthquake dizziness syndrome.

Causes of PEDS

Multivariate logistic regression analysis revealed that age (≥ 21 years, OR 3.00, 95% CI: 2.41–3.73, P < .001), female sex (OR 1.71, 95% CI: 1.45–2.01, P < .001), floors in the building (≥ 3, OR 1.35, 95% CI: 1.08–1.67, P = .007), tinnitus/ear fullness (OR 1.43, 95% CI: 1.25–1.64, P < .001), anxiety (OR 3.05, 95% CI: 2.56–3.64, P < .001), symptoms related to autonomic dysfunction (OR 1.10, 95% CI: 1.00–1.21, P = .04), and prior history of motion sickness (OR 1.22, 95% CI: 1.10–1.36, P = .002) were significantly associated with PEDS onset (Table 2). In addition, a propensity analysis adjusted for age and sex revealed that tinnitus/ear fullness (OR 1.48, 95% CI: 1.24–1.79, P < .001), anxiety (OR 3.12, 95% CI: 2.48–3.93, P < .001), symptoms related to autonomic dysfunction (OR 1.30, 95% CI: 0.89–1.43, P = .009), and prior history of motion sickness (OR 1.23, 95% CI: 1.07–1.41, P = .003) were also significant. Thus, PEDS was significantly associated with inner ear symptoms, mood symptoms, and autonomic function. In addition, being on a high floor in a building affected PEDS onset, but this relationship depended on a specific age range and sex. Region (OR 1.07, 95% CI: 0.97–1.18, P = .14) and building type (OR 0.97, 95% CI: 0.81–1.16, P = .74) had no significant association with PEDS.

Table 2

Results of the multivariate logistic regression analysis of PEDS onset.

Explanatory variables	OR	95% CI	P -value
(Intercept)	0.16	0.11–0.23	P < .001***
Age (< 21/≥ 21 years)	3.00	2.41–3.73	P < .001***
Sex (Male/Female)	1.71	1.45–2.01	P < .001***
Region (South/North/West/Center/East)	1.07	0.97–1.18	P = .14
Building type (Wooden/Iron)	0.97	0.81–1.16	P = .74
Floors of building (< 3/≥ 3)	1.35	1.08–1.67	P = .007**
Tinnitus/ear fullness	1.43	1.25–1.64	P < .001***
Anxiety	3.05	2.56–3.64	P < .001***
Symptoms related to autonomic dysfunction	1.10	1.00–1.21	P = .04*
Prior history of motion sickness	1.22	1.10–1.36	P = .002**

*P < .05

**P < .01

***P < .001.

Abbreviations: OR, odds ratio; CI: Confidence interval.

To exclude the possibility that participants considered actual vibrations (aftershocks) as PEDS, we investigated the correlation between the prevalence of PEDS, geological conditions, and the number of aftershocks in each region. The prevalence of PEDS and geological conditions or number of aftershocks in each region were not significantly correlated (R (3) = - 0.06, 2.8%, 95% CI: 12.0%–17.2%, P = .28; Fig 3; Table 3). While the correlation analysis contained only five data points and is thus limited in statistical power, visual inspection supports the idea that no relationship exists between aftershocks and PEDS.

Fig 3

Relationship between aftershock number and the likelihood of PEDS.

There was a null correlation between geological conditions/number of aftershocks and prevalence of PEDS (R (3) = - 0.06, P = .28). Abbreviation: PEDS, post-earthquake dizziness syndrome.

Table 3

Number of aftershocks and PEDS ratio per region.

Kumamoto Region	PEDS+:PEDS-	Max Shindo	Number of aftershocks
Central	595:1,031	7	467
Northern	103:83	7	340
Southern	92:66	7	461
Western	89:92	7	777
Eastern	551:820	7	441

PEDS, post-earthquake dizziness syndrome.

Discussion

This study examined the occurrence and possible mechanisms of PEDS after major earthquakes. Our findings revealed a significantly increased prevalence of dizziness after major earthquakes. Specifically, the results from our sample, drawn from the general population, indicate that PEDS occurs after major earthquakes. The prevalence of PEDS in our study was slightly lower than that previously reported [11]. The main outcomes of previous studies [6,11] were influenced by secondary disasters (i.e., tsunami and the Fukushima nuclear disaster), whereas the main outcomes of our study focused exclusively on the effects of major earthquakes. Therefore, since secondary disasters presumably cause mental stress, this may have consequently increased the reported prevalence of PEDS in these studies. Earthquake-induced mental stress may contribute to phobic disorders [18], which may subsequently affect vestibular function and elicit the illusory feeling of swaying. Honma et al. [6] reported a positive correlation between stabilometric parameters and State-Trait Anxiety Inventory scores in individuals who experienced the Tohoku earthquake; however, this correlation was absent in individuals who did not experience the earthquake [6]. The questionnaire survey findings demonstrated that anxiety likely contributed to the observed increase in PEDS prevalence.

To investigate other causes of PEDS after the Kumamoto earthquakes, we examined motion sickness using the sensory conflict theory/postural instability theory as described in the Introduction [12-15,19,20]. The primary earthquake and repetitive aftershocks caused low frequency vibrations (0.1–3.5 Hz), which may cause motion sickness. We observed that age (≥ 21), sex (female), and floors in the building (≥ 3), as well as visual and somatosensory symptoms such as tinnitus/ear fullness, anxiety, autonomic symptoms (i.e., sweating abnormalities, digestive difficulties, urinary problems, vision problems), and prior history of motion sickness, resulted in an increased incidence of PEDS. Thus, after the Kumamoto earthquake, individuals were aware of sensory conflict between visual/proprioceptive sensations and vestibular function, and natural adjustments or adaptations were required in order to maintain posture. This was similar to the phenomenon of “getting one’s sea legs” on a multiday sea voyage [12-14] or wind-induced motion of a tall building [15]. In addition, after the earthquake, individuals experienced numerous repetitive aftershocks for several months. One explanation of our results is that numerous repetitive aftershocks caused low-level motion sickness via the postural instability theory [13] and sensory conflict theory [15].

Additionally, our findings demonstrate that the prevalence of PEDS did not directly correspond to the actual vibrations (i.e., repetitive aftershocks). The epicenter of the Kumamoto earthquakes was onshore, whereas that of the Tohoku earthquakes was offshore. Generally, normal faults (vertical direction) cause onshore earthquakes, whereas strike-slip and normal faults (vertical and horizontal directions) cause offshore earthquakes [21]. Thus, the mechanical relationship between vibration types and the development of PEDS after the Kumamoto earthquakes may have differed from that after the Tohoku earthquakes [6]. These findings and those of previous studies suggest that a higher-order information mismatch between the visual or proprioceptive systems and vestibular systems caused by a major earthquake can produce spatial disorientation, dizziness, and more pronounced autonomic symptoms [19,20]. It is similar to somatic sensations, such as those caused by startling stimuli, and often induces an autonomic reflex, termed as the startle response [22] or phobic postural vertigo [23].

In summary, we speculate that an earthquake-induced disequilibrium is further influenced by physical stressors, including sensory disruptions instigated by earthquake vibrations, changes in living conditions, and autonomic stress.

This study was limited by the fact that psychological stress was not fully investigated. Investigating the impact of post-disaster psychological stress on healthy individuals with PEDS would help verify the causes of post-earthquake dizziness. In addition, we cannot make any strong statements about the mechanistic cause mediating our results (sensory conflict/postural stability) as our study was not designed to test any one theory.

Conclusions

Exposure to major earthquakes and aftershocks induced post-earthquake dizziness in a significant percentage of individuals. Our results indicate that post-earthquake dizziness may be due to sensory conflicts/postural instability mediated by vestibular dysfunction, autonomic dysfunction, and/or psychological factors. Our findings can facilitate the management of dizziness experienced during or after disasters. Future studies should identify ways to mitigate autonomic dysfunction and prevent post-earthquake dizziness.

Patients’ demographic information.

(DOCX)

Click here for additional data file.

PEDS questionnaire.

(DOCX)

Click here for additional data file.

27 May 2021

PONE-D-21-13080

Post-earthquake dizziness syndrome following the 2016 Kumamoto earthquakes, Japan

PLOS ONE

Dear Dr. Miwa,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

This is a fascinating study, and I applaud your initiative in creating it under what must have been exceptionally challenging conditions. Two highly qualified Reviewers have offered their comments. In revising, please attend carefully to the comments from both Reviewers. In addition, as your study relates phenomenology to movement of the support surface, it might be helpful for you to consider one of my own articles:

Munafo, J., Wade, M. G., Stergiou, N., & Stoffregen, T. A. (2015). Subjective reports and postural performance among older adult passengers on a sea voyage. Ecological Psychology, 27, 127-143.

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Reviewer #1: This paper outlines a cross-sectional analysis of post-earthquake dizziness syndrome (PEDS) following the 2016 series of tremors in Kumamoto, Japan. Results showed that more than one third of participants reported post-earthquake dizziness. Several factors were identified as related to PEDS, including female sex, ear fullness, an age of 21 or more, and anxiety. The authors conclude that earthquakes affect inner-ear function, autonomic symptoms, and psychological factors, both directly and indirectly via changes in environmental factors.

The paper offers a useful improvement to our knowledge about post-earthquake symptoms. The principal concern is that I would like to see a greater explanation of the mechanistic link between tremors and the outcomes measured here. A possible error in reporting statistics emerged (this is potentially a minor issue). Several other improvements could be made, as described below.

The Discussion is brief and leaves the reader with some lingering questions. As in the Introduction, the hypothesis linking earthquakes with lingering dizziness surely needs to be discussed at greater length. The statement by the authors “we examined sensory conflict theory and motion sickness” is true only to a very limited extent, this could be significantly expanded. I advise the authors to consider articles on postural stability and motion sickness (e.g. Stoffregen et al. 2013; Walton et al., 2011) as there seems to be a natural link between this research and the current study.

Could the authors please check that they are correctly stating Pearson r values for their correlations? A negative correlation should be associated with a negative r value, but L137 states a positive value (note, degrees of freedom should also be added).

The authors asked participants to report “Autonomic symptoms” in their questionnaire. It must be clarified in the text what was specifically meant by “autonomic symptoms” and how they were described to participants.

It seems appropriate to reference the findings of a recent overview paper with clinical patients conducted by the main author (Miwa, 2021).

L23 delete “the”

L34 I would press the authors to say more about their hypothesis. They state a multisensory mismatch produces PEDS, but this is descriptive and not explanatory. How could a single-shot mismatch result in PEDS, or are aftershocks likely to be a necessary factor? Are there existing models that support this hypothesis?

L50 It would be valuable to state that these participants reported no head trauma brought about by the earthquake. I am assuming that those data were obtained. If they were not collected, this could be added as a limitation.

L69 “changes in the proportion of patients with PEDS … were compared by hypothesis testing” This does not specify the type of test used (e.g. t-tests), which should be included here.

L74 Could the authors specify what proportion of the responses were imputed?

L87 The number of participants in this study should be stated in the ‘Participants’ section.

L122 The authors could briefly state here what the non-significant factors were (e.g. “Building type and regional location had no significant association with PEDS”).

L133 A caveat here is that the correlation contains very few datapoints, and this should be stated by the authors in text, not a figure caption (e.g. “While the correlation analysis contained only 5 datapoints and is thus limited in statistical power, visual inspection supports the idea that no relationship exists between aftershocks and PEDS”).

In addition, Table 3 references “earthquake sickness ratio”, I assume this should be corrected to PEDS. Column 1 in Table 3 can also be removed, with ‘Kumamoto Region’ being the heading of column 2.

L135 “Despite the negative correlation between geological conditions/number of aftershocks and prevalence of PEDS” I would call this a ‘null’ correlation, not a negative one.

I advise adding significance asterisks in (e.g.) Fig 2b.

References

Miwa, T. (2020). Vestibular function after the 2016 Kumamoto earthquakes: a retrospective chart review. Frontiers in Neurology, 11.

Stoffregen, T. A., Chen, F. C., Varlet, M., Alcantara, C., & Bardy, B. G. (2013). Getting your sea legs. PLoS One, 8(6), e66949.

Walton, D., Lamb, S., & Kwok, K. C. (2011). A review of two theories of motion sickness and their implications for tall building motion sway. Wind and Structures, 14(6), 499.

Reviewer #2: The examination of post-earthquake effects on perception of balance and movement is an interesting phenomenon that the authors were able to take advantage of after the Kumamoto earthquake. The data collection and analysis were well done. However, it is not clear why the authors are couching this a sensory mismatch issue. That is why is this the theoretical mechanism that is thought to account to the phenomenological data they collected. There needs to be a clear justification of why mismatch perspectives are the right model to use. Without that connection there isn't a strong justification for the research questions. Are there other alternatives that could also account for your findings?

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Reviewer #1: Yes: Séamas Weech

Reviewer #2: Yes: L. James Smart Jr.

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14 Jun 2021

Reviewer #1: This paper outlines a cross-sectional analysis of post-earthquake dizziness syndrome (PEDS) following the 2016 series of tremors in Kumamoto, Japan. Results showed that more than one third of participants reported post-earthquake dizziness. Several factors were identified as related to PEDS, including female sex, ear fullness, an age of 21 or more, and anxiety. The authors conclude that earthquakes affect inner-ear function, autonomic symptoms, and psychological factors, both directly and indirectly via changes in environmental factors.

The paper offers a useful improvement to our knowledge about post-earthquake symptoms. The principal concern is that I would like to see a greater explanation of the mechanistic link between tremors and the outcomes measured here. A possible error in reporting statistics emerged (this is potentially a minor issue). Several other improvements could be made, as described below.

The Discussion is brief and leaves the reader with some lingering questions. As in the Introduction, the hypothesis linking earthquakes with lingering dizziness surely needs to be discussed at greater length. The statement by the authors “we examined sensory conflict theory and motion sickness” is true only to a very limited extent, this could be significantly expanded. I advise the authors to consider articles on postural stability and motion sickness (e.g. Stoffregen et al. 2013; Walton et al., 2011) as there seems to be a natural link between this research and the current study.

Response: I appreciate your valuable comments. I have added the description of motion sickness by postural stability theory in the Discussion.

Could the authors please check that they are correctly stating Pearson r values for their correlations? A negative correlation should be associated with a negative r value, but L137 states a positive value (note, degrees of freedom should also be added).

Response: Thank you for your valuable comment. I have corrected it and added degrees of freedom.

The authors asked participants to report “Autonomic symptoms” in their questionnaire. It must be clarified in the text what was specifically meant by “autonomic symptoms” and how they were described to participants.

Response: Thank you for your kind advice. I have added specific examples in the main text and Table S2.

It seems appropriate to reference the findings of a recent overview paper with clinical patients conducted by the main author (Miwa, 2021).

Response: Thank you for the suggestion. I have added the information accordingly.

L23 delete “the”

Response: Thank you for the suggestion. I have deleted it accordingly.

L34 I would press the authors to say more about their hypothesis. They state a multisensory mismatch produces PEDS, but this is descriptive and not explanatory. How could a single-shot mismatch result in PEDS, or are aftershocks likely to be a necessary factor? Are there existing models that support this hypothesis?

Response: I appreciate your comment. I have added the required information to the Introduction.

L50 It would be valuable to state that these participants reported no head trauma brought about by the earthquake. I am assuming that those data were obtained. If they were not collected, this could be added as a limitation.

Response: Thank you. I have added the description regarding this in the Materials and Methods section.

L69 “changes in the proportion of patients with PEDS … were compared by hypothesis testing” This does not specify the type of test used (e.g. t-tests), which should be included here.

Response: Thank you for your comment. I have corrected it accordingly.

L74 Could the authors specify what proportion of the responses were imputed?

Response: Yes. The values of building type were imputed because some building occupants did not know to live in ironed-type buildings or wooden-type buildings.

L87 The number of participants in this study should be stated in the ‘Participants’ section.

Response: Thank you for your comment. I have stated the number of participants in the ‘Participants’ section.

L122 The authors could briefly state here what the non-significant factors were (e.g. “Building type and regional location had no significant association with PEDS”).

Response: Thank you for your comment. I have added the required information to the revised manuscript.

L133 A caveat here is that the correlation contains very few datapoints, and this should be stated by the authors in text, not a figure caption (e.g. “While the correlation analysis contained only 5 datapoints and is thus limited in statistical power, visual inspection supports the idea that no relationship exists between aftershocks and PEDS”).

In addition, Table 3 references “earthquake sickness ratio”, I assume this should be corrected to PEDS. Column 1 in Table 3 can also be removed, with ‘Kumamoto Region’ being the heading of column 2.

Response: Thank you for your valuable comments. I have added the suggested description in the main text. In addition, I have corrected “earthquake sickness ratio” to “PEDS”, following your suggestion. In Table 3, column 1 was removed, and I added ‘Kumamoto region’ in the heading of column 2.

L135 “Despite the negative correlation between geological conditions/number of aftershocks and prevalence of PEDS” I would call this a ‘null’ correlation, not a negative one.

Response: Thank you. I have corrected accordingly.

I advise adding significance asterisks in (e.g.) Fig 2b.

Response: Thank you for the valuable advice. According to your suggestion, I have added significance asterisks in Fig 2b.

References

Miwa, T. (2020). Vestibular function after the 2016 Kumamoto earthquakes: a retrospective chart review. Frontiers in Neurology, 11.

Stoffregen, T. A., Chen, F. C., Varlet, M., Alcantara, C., & Bardy, B. G. (2013). Getting your sea legs. PLoS One, 8(6), e66949.

Walton, D., Lamb, S., & Kwok, K. C. (2011). A review of two theories of motion sickness and their implications for tall building motion sway. Wind and Structures, 14(6), 499.

Munafo, J., Wade, M. G., Stergiou, N., & Stoffregen, T. A. (2015). Subjective reports and postural performance among older adult passengers on a sea voyage. Ecological Psychology, 27, 127-143.

Reviewer #2: The examination of post-earthquake effects on perception of balance and movement is an interesting phenomenon that the authors were able to take advantage of after the Kumamoto earthquake. The data collection and analysis were well done. However, it is not clear why the authors are couching this a sensory mismatch issue. That is why is this the theoretical mechanism that is thought to account to the phenomenological data they collected. There needs to be a clear justification of why mismatch perspectives are the right model to use. Without that connection there isn't a strong justification for the research questions. Are there other alternatives that could also account for your findings?

Response: Thank you for your valuable comments. I have expanded the Discussion section and reconsidered adding the theory on motion sickness (i.e. postural instability theory and sensory conflict theory cited from Stoffregen et al. 2013; Walton et al., 2011). In this study, I speculated that both theories were associated with PEDS. I have added them in the Discussion section. (Lines 181-191)

Submitted filename: renamed_57833.docx

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5 Jul 2021

PONE-D-21-13080R1

Post-earthquake dizziness syndrome following the 2016 Kumamoto earthquakes, Japan

PLOS ONE

Dear Dr. Miwa,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Both Reviewers are pleased with your changes, but each asks for some additional clarification. I agree with Reviewer 2 that the sensory conflict theory of motion sickness is not compatible with the postural instability theory. They are mutually exclusive; so much so that the postural instability theory makes an explicit claim that sensory conflict does not even exist.

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Reviewer #1: I am confused as to why the authors include only one to two sentences on the mechanism linking earthquake with sickness (i.e. L120-126). Please be aware, many readers will not be familiar with the idea that sensory conflict or postural instability are thought to cause motion sickness. As a result, it is worthwhile to take the time and explain those ideas in some more detail here. Not too much--this is clearly not the focus of the paper--but I would expect more than the manuscript currently contains.

Indeed, the Discussion contains some of this relevant information that belongs in the Introduction. L273-297 describes previous literature and theory details that I would expect to read before the hypothesis is stated. Please consider re-phrasing this information and transposing it to the Introduction. It can then be referred to in the Discussion: How do your results fit with this literature?

Related to this point, the authors' new inclusions regarding sensory conflict theory and motion sickness were not clear. For example, "In addition, numerous repetitive aftershocks caused low dosage motion sickness via the postural instability theory/ecological approach to perception and action." (L317). There is insufficient evidence for the authors to make this claim as it is written, without qualifiers ("One explanation of our results is that..."). It is also unclear what is the 'ecological approach to perception and action' to a non-expert reader, so it seems appropriate to delete those last 6 words.

It is ultimately true that the authors cannot make any strong statements about what was the mechanistic cause for their results (sensory conflict/postural stability), as their study was not designed to test any one theory. So, they should make this point clear in their limitations. Please note, it is not a strong limitation: This is a very interesting paper, even if it cannot speak to the mechanisms at play in motion sickness.

Remove 'this correlation was not significant' (L248).

L300-301: "motion sickness": Perhaps this should this state "prior history of motion sickness"?

The authors should review the the newly added material as there are several language mistakes.

Reviewer #2: You have thoughtfully addressed the initial issues raised by the reviewers - the intent of my particular comments was not to make you change your theory argument, just justify it. However, I think your revisions on this issue are reasonable. My only suggestion is to make sure to be clear that postural/ecological perspectives on motion sickness do not support sensory conflict explanations. Phenomenologically, the participants experiences might suggest conflict (which is where I think you were going originally) - but as you noted, definitive claims cannot be made at this point.

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22 Jul 2021

Reviewer #1: I am confused as to why the authors include only one to two sentences on the mechanism linking earthquake with sickness (i.e. L120-126).

Please be aware, many readers will not be familiar with the idea that sensory conflict or postural instability are thought to cause motion sickness. As a result, it is worthwhile to take the time and explain those ideas in some more detail here. Not too much--this is clearly not the focus of the paper--but I would expect more than the manuscript currently contains.

Response: Thank you for your valuable comments. I have made sure that in the revision, these concepts are explained with enough detail that readers will be sufficiently informed.

Indeed, the Discussion contains some of this relevant information that belongs in the Introduction. L273-297 describes previous literature and theory details that I would expect to read before the hypothesis is stated. Please consider re-phrasing this information and transposing it to the Introduction. It can then be referred to in the Discussion: How do your results fit with this literature?

Response: We appreciate your suggestion and I have rephrased the literature information and transposed it into the Introduction. In addition, I made sure to point them out and integrate them into our results in the discussion section. I have considered that stimulation via environment, such as an earthquake, a ship, a building facing wind, causes various degrees of motion sickness.

Related to this point, the authors' new inclusions regarding sensory conflict theory and motion sickness were not clear. For example, "In addition, numerous repetitive aftershocks caused low dosage motion sickness via the postural instability theory/ecological approach to perception and action." (L317). There is insufficient evidence for the authors to make this claim as it is written, without qualifiers ("One explanation of our results is that..."). It is also unclear what is the 'ecological approach to perception and action' to a non-expert reader, so it seems appropriate to delete those last 6 words.

Response: Thank you for your comments. I agree with the assessment made by the reviewer and have deleted the 'ecological approach to perception and action'. Further, I have added the qualifier "One explanation of our results is that..." in this sentence. My intention was to get across that the mechanisms of motion sickness were different whether it was instigated by the major earthquake or the repetitive aftershocks.

It is ultimately true that the authors cannot make any strong statements about what was the mechanistic cause for their results (sensory conflict/postural stability), as their study was not designed to test any one theory. So, they should make this point clear in their limitations. Please note, it is not a strong limitation: This is a very interesting paper, even if it cannot speak to the mechanisms at play in motion sickness.

Response: I greatly appreciate your kind advice. I have added this concern to the limitation section.

Remove 'this correlation was not significant' (L248).

Response: Thank you for the advice, I have removed this statement.

L300-301: "motion sickness":Perhaps this should this state "prior history of motion sickness"?

Response: Your suggested wording does better reflect our intended meaning. I have revised this statement throughout the manuscript.

The authors should review the the newly added material as there are several language mistakes.

Response: I apologize for this oversight. We have edited the manuscript to ensure it meets adequate English language standards.

Reviewer #2: You have thoughtfully addressed the initial issues raised by the reviewers - the intent of my particular comments was not to make you change your theory argument, just justify it. However, I think your revisions on this issue are reasonable. My only suggestion is to make sure to be clear that postural/ecological perspectives on motion sickness do not support sensory conflict explanations. Phenomenologically, the participants experiences might suggest conflict(which is where I think you were going originally) - but as you noted, definitive claims cannot be made at this point.

Response: Thank you for valuable comments. After your suggestion, I have reconsidered the mechanisms underlying earthquake-related dizziness, and concluded that the major earthquake and the repetitive aftershocks caused motion sickness via different mechanisms. I have described this in the manuscript. However, as you noted, the stated mechanism is just speculation. As such, I have also mentioned this in the limitation section.

Submitted filename: renamed_faf31.docx

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26 Jul 2021

Post-earthquake dizziness syndrome following the 2016 Kumamoto earthquakes, Japan

PONE-D-21-13080R2

Dear Dr. Miwa,

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28 Jul 2021

PONE-D-21-13080R2

Post-earthquake dizziness syndrome following the 2016 Kumamoto earthquakes, Japan

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