Field Survey of Tsunami Heights and Runups Following the 22 December 2018 Anak Krakatau Volcano Tsunami, Indonesia

Abstract

The 22 December 2018 Anak Karakatau tsunami in Indonesia was a rare event in that few instrumental records existed of tsunamis generated by volcanic sources before this event. The tsunami, which left a death toll of 437, is of global importance as it provides opportunities to develop knowledge on generation, propagation and coastal effects of volcanic tsunamis. Here, we report results of field surveys along the coast of the Sunda Strait, Indonesia to study tsunami wave heights and coastal damage. We surveyed 29 locations and measured ranges of tsunami runup from 0.9 to 5.2 m, tsunami heights from 1.4 to 6.3 m, flow depths from 0.2 m to 3.0 m and inundation distances from 18 to 212 m. The largest tsunami heights and concentration of damage and fatalities occurred on the western shore of Java from Tanjung Lesung to Sumur. The largest cluster of fatalities occurred at Tanjung Lesung, where more than 50 people died while attending an outdoor music being held at the shoreline. The tsunami runup and tsunami height in Tanjung Lesung were 4.0 and 2.9–3.8 m, respectively. We believe this tragedy could have been avoided if the event organizers were more aware of the hazard posed by the Anak Krakatau volcano, as it had been actively erupting for several months prior to the tsunami, and simply moved the concert stage 100 m inland. Many of the locations surveyed demonstrated a similar pattern where the majority of casualties and destruction occurred within 100 m of the coast; in several locations, lives were saved where buildings were located at least this distance inland. The significant damage and numerous deaths which occurred in Sumur, despite the moderate tsunami height of 2.3–2.5 m, can be attributed to the extremely low-lying coastal land there. Flow depth in Sumur was 0.9–2.0 m. During our field surveys, nearly one year after the event, we noted that some of the damaged buildings were being rebuilt in the same locations just 10–30 m from the shoreline. We question this practice since the new buildings could be at the same tsunami risk as those damaged in the 2018 event.

Mekanika Perambatan Gelombang Tsunami: Studi Kasus Tsunami Teluk Palu Dan Selat Sunda Tahun 2018

“…Tsunami Selat Sunda tanggal 22 Desember 2018 yang menyapu wilayah barat Propinsi Banten dan wilayah selatan Propinsi Lampung dipicu secara langsung oleh keruntuhan sebagian bodi Anak Rakata pada sisi barat Mekanika Perambatan Gelombang Tsunami: Studi Kasus Tsunami Teluk Palu dan Tsunami Selat Sunda Tahun 2018 (Grilli et al, 2019;Widiyanto et al, 2020). Peristiwa ini adalah salah satu contoh tsunami vulkanik yang bukan dipicu langsung oleh erupsi dari hasil pengamatan survei lapangan (Heidarzadeh et al, 2020b;Muhari et al, 2019). Runtuhnya sebagian volume gunung ternyata bisa juga membangkitkan gelombang tsunami (Hunt et al, 2021) yang merambat di perairan Selat Sunda dengan lebar lebih dari 100 km (diukur menggunakan kalkulator online di https://keisan.casio.com/exec/system/1224587128).…”

“…Fenomena run-up merupakan aspek dinamik dalam perambatan tsunami karena membahas kekekalan kerapatan energi dan kontinuitas fluida inkompresibel (Synolakis, 1991;Sorensen, 2006;Cholifah and Prastowo, 2017a;Heidarzadeh et al, 2020b). Fokus perhatian adalah apakah run-up pada kasus tsunami Teluk Palu dan Selat Sunda pada tahun 2018 harus mempertimbangkan efek refraksi sebagai tambahan faktor agar estimasi run-up mendekati hasil ukur run-up surveys pada tsunami Teluk Palu (Patton et al, 2019;Heidarzadeh et al, 2020b) dan Selat Sunda (Grilli et al, 2019;Muhari et al, 2019).…”

“…Hanya 3 titik pengamatan yang memenuhi 𝑹 𝐞𝐬𝐭 > 𝑹 𝐨𝐛𝐬 (over-estimates) sedangkan sisanya berlaku 𝑹 𝐞𝐬𝐭 < 𝑹 𝐨𝐛𝐬 (under-estimates). Data 𝑹 𝐨𝐛𝐬 diperoleh dari referensi yang relevan, antara lain Heidarzadeh et al (2020b), Widiyanto et al (2020 dan Muhari et al (2019).…”

“…Tsunami sebagai salah satu bencana geologi bisa dipicu oleh sumber seismik (sumber utama), misalnya gempa tektonik (Ward, 2011) dan sumber non-seismik, misalnya longsor bawah laut (Gusman et al, 2019;Pakoksung et al, 2019;Jing et al, 2020) dan erupsi gunung api (Choi et al, 2003;Fukao et al, 2018;Sandanbata et al, 2018;Muhari et al, 2019;Heidarzadeh et al, 2020b). Statistik studi kebencanaan di Indonesia (Irsyam et al, 2017) mengatakan bahwa tsunami dan gempa tektonik adalah dua bencana seismik diantara bencana kebumian dan non-kebumian yang memberikan dampak paling parah di Indonesia.…”

“…Aspek kinematik menjelaskan fenomena time delay, yaitu keterlambatan kedatangan tsunami dibandingkan dengan estimasi waktu kedatangan di lokasi tertentu berdasarkan rumusan long wave speed di mana kecepatan gelombang tsunami diketahui hanya bergantung pada kedalaman laut (Choi et al, 2003;Sorensen, 2006;Wessel, 2009;Watada et al, 2014;Wang, 2015;Prastowo et al, 2018;Jing et al, 2020). Aspek dinamik menjelaskan fenomena run-up atau tinggi amplitudo tsunami saat tsunami tiba di daerah pantai yang dangkal (Synolakis, 1991;Reymond et al, 2012;Cholifah and Prastowo, 2017a;Heidarzadeh et al, 2020b). Dua efek fisis penting, yaitu kasus pendangkalan pantai (shoaling effect) sebagai faktor eksternal dan efek refraksi gelombang sebagai faktor internal bersama diperhitungkan untuk menguji apakah estimasi run-up berbasis modifikasi Hukum Green (Cholifah and Prastowo, 2017a;Sandanbata et al, 2018) memberikan kesesuaian yang lebih baik antara estimasi dan observasi.…”