Earthquakes, very similar to lightning, can strike the identical place once more. This level was tragically pushed dwelling in Cianjur in West Java, when it was struck by a lethal earthquake on 21 November 2022. The same space was closely impacted by an earthquake on 28 March 1879. The identical fast area in West Java was once more struck on 14 January 1900, impacting the close by city of Sukabumi. Subsequently, information of the place an earthquake has occurred up to now, and the way giant it was, is essential. This varieties a worthwhile enter into trendy seismic hazard maps. These assist to find out how buildings and different crucial infrastructure should be safely constructed in a selected area. Dependable data of previous earthquakes for the aim of seismic hazard might be acquired from instrumental data. However on a geological time scale, the size of the instrumental document is brief (~120yrs). It’s, subsequently, supplemented by proof of earthquakes that occurred earlier than devices have been obtainable which might be preserved within the geological and/or written historic document.

Harm within the Chinese language district of Cianjur because of an earthquake on 28 March, 1879. (CCL within the Southeast Asian and Carribean Pictures Assortment of KITLV)

Such inputs are important to grasp seismic hazard in Indonesia. It’s among the many world’s most densely populated international locations and can also be very tectonically lively. This makes it significantly susceptible to frequent earthquakes that may have main socioeconomic impacts, as we noticed in Palu on Sulawesi in 2018. Whereas the instrumental and geological document has been and continues to be investigated in Indonesia, little work has been executed to systematically re-evaluate historic earthquakes within the archipelago for shut to 3 a long time. This contains finding and analysing written descriptions of shaking results throughout earthquakes.

For these causes, my co-authors Phil Cummins, Aron Meltzner, and I explored the historic earthquake catalogue for Indonesia in our current examine revealed within the Bulletin of the Seismological Society of America. We referred to as our database the Gempa Nusantara database (additionally obtainable through GitHub) which is assortment of felt shaking depth, or macroseismic depth, observations for 1,200 earthquakes within the area of Indonesia spanning 4 centuries from 1546 till 1950. The identify of our database which suggests “Earthquakes of the Indonesian Archipelago” in Bahasa Indonesia is derived from a mixture of the Indonesian phrases for “earthquake” (gempa bumi, shortened colloquially to gempa) and “archipelago” (nusantara).

Determine 1: Color stuffed circles show every of the 7380 macroseismic observations in Gempa Nusantara with color bar indicating severity of shaking (inset: location of Indonesia in pink).

In our examine, we sought major documentary sources from the colonial interval in Indonesia. This was pushed by the truth that many trendy catalogues of historic earthquakes for Indonesia might be affected by inconsistencies that stem from a reliance on unvetted sources and/or the unwitting repetition of errors in earlier catalogues. The truth is, many of those catalogues are by-products of a multi-lingual historic earthquake catalogue compiled by a German earth scientist, Arthur Wichmann. His work covers two intervals: previous to 1857 and from 1858 to 1877. However these trendy catalogues typically fail to seek the advice of the first sources utilized by Arthur Wichmann, nor do they add new data from unused major sources equivalent to historic newspapers.

The documental materials we examined included these obtainable to Arthur Wichmann equivalent to colonial Dutch journals and official stories of post-earthquake harm reconnaissance. We added to this assortment, new data gathered from different beforehand unused sources equivalent to colonial Dutch newspapers and transcriptions of official correspondence by the Vereenigde Oostindische Compagnie or VOC (also called the Dutch East India Firm). Just like the Wichmann compilation, and all trendy catalogues derived from it, the sources for our examine are largely derived from colonial sources, with one exception. Info on earthquakes from such sources diminish drastically throughout World Battle II, when Indonesia was occupied by Japan. Nonetheless, we have been in a position to fill this hole by looking archives of native Indonesian newspapers from this era.

To categorise the severity of shaking throughout earthquakes within the absence of devices we use an depth scale referred to as the European Macroseismic Scale (EMS-98). This allowed us to transform the descriptive observations we collected to numeric knowledge. The EMS-98 scale is much like the comparatively well-known Modified Mercalli Depth (MMI) scale. The totally different colour-coded circles displayed in Determine 1 point out weak (cool colors) versus sturdy (heat colors) shaking for all 1,200 earthquakes within the Gempa Nusantara database. Very damaging shaking is displayed as darkening reds.

Provided that Indonesia is susceptible to very giant magnitude earthquakes and that it covers a area corresponding to the east-west extent of Europe, it was stunning that stories of damaging shaking have been few, accounting for barely 2% of all 7,380 macroseismic observations. But a handful of utmost observations exist equivalent to for an earthquake in Ambon in 1898 which accurately threw folks a number of metres and even shifted heavy cannons. In different instances, the unique supplies we sought additionally helped result in readability. For instance, an inaccurate translation from German to Dutch by Arthur Wichmann misled his contemporaries that an earthquake in 1820 within the Flores Sea made cannons “bounce” upon their gun carriages at Bulukumbu in southern Sulawesi. Materials we discovered confirmed this to be factually incorrect. Whereas the earthquake shook the cannons in questions, it didn’t produce as violent vertical shaking as was beforehand assumed.

Determine 2: Chronological variations within the completeness of macroseismic knowledge for Indonesia.

The historic document is thought to be influenced by geopolitics and socioeconomics. Provided that our supply supplies have been largely in European languages, and virtually completely from the Dutch, it was no shock that the chronologically longest written data of felt earthquakes are from areas the place the VOC established buying and selling posts, that’s, in western Java and within the Maluku area as might be seen in Determine 2. Observations from these areas enhance after political management shifted into the arms of the Dutch. This contrasts with the written document of felt earthquakes in areas recognized to be very seismically lively equivalent to Papua which remained outdoors the Dutch sphere of curiosity till the early 20^th century.

Within the Gempa Nusantara database, we documented 1,200 earthquakes. It is a considerably higher quantity compared to these recognized beforehand. In three uncommon instances, we might discriminate the causative fault strains that have been accountable for historic earthquakes in Sulawesi in 1909 and on Sumatra in 1892 and 1933. We additionally uncovered proof for an undated tsunami within the area of Palu Bay within the 1800s. A very sudden discover was the invention of extreme liquefaction in rice fields close to Batukarang on Sumatra in 1936. These press accounts from Sumatra have been reminiscent in each description and spatial extent of the catastrophic liquefaction noticed within the Palu area in 2018. We have been additionally in a position to present for the primary time that paleoseismic markers examined by paleoseismologists in coral microatolls alongside Sumatra’s west coast (Determine 3) protect solely the most important megathrust earthquakes. Smaller occasions recognized solely from the historic document can typically be hidden relying on the situation of their supply zones or are overwritten by bigger local weather indicators additionally preserved in coral microatolls.

Determine 3: Places of historic earthquakes ruptures on the Sunda Megathrust and on the Sumatran Fault (SFZ) together with coral microatolls (white and colored circles) examined up to now by paleoseismologists.

Crucially, our dataset of numerical observations allowed a easy analysis of the newest model of the 2017 Indonesian seismic hazard map (Determine 4). Utilizing observations from out dataset, we calculated how typically a sure degree of depth occurred in any given yr in 12 Indonesian cities. This was in contrast with unbiased fashions from the 2017 Indonesian seismic hazard map to foretell the identical data for those self same cities. The correspondence between observations and the hazard curves was good for some cities equivalent to Surabaya. However for others equivalent to Yogyakarta, it seems that damaging shaking happens extra incessantly than the trendy hazard curves counsel. We speculate plenty of causes for this starting from, however not restricted to, yet-to-be mapped faults and to the poorly recognized charges at which mapped faults transfer. It’s not unusual for future seismic hazard maps to be up to date as newer, extra correct knowledge turns into obtainable and we imagine the Indonesian hazard map would be the similar.

Determine 4: The 2017 Nationwide Seismic Hazard map for Indonesia.

The well-known naturalist James Hutton acknowledged, “that from what has really been, we’ve knowledge for concluding with regard to that which is to occur hereafter.” This is applicable broadly to analysis typically but in addition to the gathering, curation and collation of written documentary supplies pertinent to the examine of historic earthquakes. A lot work stays to be executed on the historic earthquake catalogue for Indonesia that faucets non-European sources, each native and regional, on the strains of current historic investigations of Indonesian manuscripts. Our work on Gempa Nusantara demonstrates how a fastidiously vetted and well-documented historic document not solely enhances research of seismic hazard however is itself an necessary standalone device for the examine of earthquake hazards. We hope it can ignite additional analysis and coverage conversations about seismic hazard in Indonesia.