In the early morning of December 26, 2004, a great 9.1 magnitude earthquake hit the Indian Ocean, causing the most devastating tsunami of this century. The tsunami spread to the large surrounding areas, including Southeast and South Asia and East Africa [1], leaving approximately 280,000 people dead and more than 2 million displaced from their homes [2]. Indonesia's Aceh and North Sumatra provinces were the most struck by the tsunami that covered 413 km² [3]. Moreover, the deadly disaster caused an almost 24% drop in Aceh's population [4]. Nearly 150,000 people were killed, more than 20,000 were injured, and 200,000 buildings collapsed, including houses, hospitals, schools, bridges, and roads [2,3].

This catastrophe tragedy left victims in despair and shock after losing their homes and family members, being displaced, and living with uncertainty. The sudden losses took their toll in the form of traumatic feelings and psychological distress [5]. Previous empirical studies had assessed the impact of natural disasters on victims' mental health. The results showed that people suffering from such events were more likely to develop mental illnesses, such as depression, anxiety, and post-traumatic stress disorder (PTSD) [[6], [7], [8]]. Unfortunately, mental illness among survivors is prolonged because its symptoms are detected after five to ten years or even longer the following disaster [7,[9], [10], [11]].

Survivor-focused disaster studies generally inspect the symptoms of post-disaster psychological distress by estimating the risk factors associated with mental illness. However, recent studies have begun considering recovery patterns to overcome the limitation of only discussing the mental disorder symptoms [[12], [13], [14]]. They focused on post-disaster outcomes, including adaptive and maladaptive cases, by viewing the trajectory recovery patterns [15]. Studies on the longitudinal effect of post-traumatic stress disorder and depressive symptomatology indicated that around 7%–58% of survivors recovered from PTSD symptoms in 4–5.5 years. In contrast, 6%–26% remained persistent with the symptoms. The case for the delayed breakdown, with symptoms appearing six months or longer following the traumatic incident, was estimated to be between 4% and 19% [9,13,16].

The development of different outcomes of post-disaster PTSD could be rendered by trauma related to the disaster, adverse life events, and lack of support system. The job loss and drop in subjective economic status caused the maladaptive cases in Japan five years after the tsunami and the 2011 great earthquake [16]. Similarly, negative life events, including divorce, death, and workplace problems, increased delayed PTSD cases among the WTC attack survivors in the United States [12]. Three years after the Katrina Hurricane in 2005, the victims experienced more persistent PTSD than those concerned about their parents and relatives. This indicated that the support system from close family or friends is critical to remit the symptoms and the disaster's negative effects [14,17,18]. Perceived family support significantly assists adolescent survivors of the earthquake in Turkey cope with long-term psychological distress more than tangible support for 9/11 survivors [19,20]. Additionally, social support was a protective factor against delayed-onset PTSD among the Wenchuan Earthquake survivors in China [21].

Many studies have documented the long-term impact of the 2004 Indian Ocean tsunami on mental health, with a varied prevalence of post-traumatic stress disorder. 9-month surveillance reported 12% of PTSD displaced survivors in Thailand [6]. In Sri Lanka, the investigation found 56% PTSD prevalence among the displaced people six months after the tsunami, with females facing a double risk that has remained five years later [22,23]. PTSD was 12.7% among survivors aged 18 and older in India two months after the disaster and a higher rate for those injured during the tsunami [24]. Furthermore, 36.5% of the Norwegians tourist in Khao Lak, Thailand, experienced post-traumatic stress disorder two and half years after the disaster [25]. An assessment among respondents aged 15 and older found a higher PTSD score for those living in heavily damaged than moderately and undamaged areas five months after the tsunami in Sumatra, Indonesia [26].

The analysis of the PTSD trajectory recovery pattern of the survivors' mental health in the medium-term remains unclear and limited despite the findings. Understanding the pattern helps clinicians and health care practitioners provide better long-term mental health interventions after the disaster. Earlier detection and prevention could help survivors reduce persistent mental health, prevent the late-onset of psychological distress, and escalate the recovery rates. Therefore, this study aimed to analyze the risk factors of PTSD among Indian Ocean tsunami survivors in Aceh and North Sumatera provinces. It also mapped the post-traumatic stress disorder trajectory pattern of mental illness using a large survey sample in Indonesia. The study examined the medium-term consequence of mental disorders five and 18 months after the disaster. Additionally, it examined demographic characteristics, trauma exposures, counseling support, and negative life events experienced by victims to predict PTSD.

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