9:45 AM - 10:10 AM
[AHW23-04] Groundwater in the area where ground flow with long displacement occurred due to the 2018 Sulawesi Island Earthquake in Indonesia
★Invited Papers
Keywords:groundwater, earthquake, flow of the ground, fault
The earthquake that struck Sulawesi, Indonesia on September 28, 2018 caused flow of the ground with long displacement in several areas in Palu City. A large amount of groundwater also spouted out with the earthquake. Similar damage has occurred in the past in this area, and it is called "Nalodo". For the purpose of elucidating the mechanism of damage and disaster prevention in the future, JICA has been studying it as part of the investigation of " Project for Development of Regional Disaster Risk Resilience Plan in Central Sulawesi in the Republic of Indonesia", and the authors have participated in it. So, we would like to report based on the study report by JICA1).
This area is the triple junction of the three plates and rotates clockwise. This causes a Palu-Koro left strike-slip fault, the displacement rate of which is very fast. A long and narrow pull-apart sedimentary basin is formed between the faults from north-northwest to south-southeast. The Palu River runs from south to north in the center, and gently sloping grounds of 1 to 3% spreads from the river to both sides. There is a hot spring near the area where Nalodo occurred.
Due to the earthquake, surface soil with a length of 1 to 3 km and a width of several hundred meters slipped and flowed toward the downstream side on the gently sloping ground in one area on the west side and four areas on the east side. According to interviews with several residents in and near Narodo, (1) a large vertical shaking and swelling occurred at the same time as the horizontal shaking of the earthquake ended, and (2) groundwater spouted and the ground started to flow at the same time as the horizontal shaking ended or within about 2 minutes. There was also testimony that hot water spouted out in some sites.
In the area where Nalodo occurred, spring belts crossed in the north-south direction before the earthquake. In order to study the origin of this spring, groundwater was sampled from the spring belts and others to test water quality. However, several months have passed since the earthquake, and the groundwater spouted by the earthquake has not been collected. Water was sampled at 29 locations, including springs in Nalodo, boring holes, a hot spring resort, deep wells, shallow wells, and rivers. A total of 37 samples were collected. Of these, 2 samples of spring water in the hot spring resort and 2 samples of deep wells are hot water with a temperature above 40 ° C. When the results were shown by the relationship between the oxygen isotope ratio δ18O and the hydrogen isotope ratio δD (δ diagram), they were plotted on the meteoric water line except for three points. On the other hand, the water of the hot spring is plotted to the right of the meteoric water line, and if it is extended, it will be between the “Arima-type” slab-related deep-seated fluid and the paleo-seawater (Yasuhara, personal communication). The other two points were spring water from a Nalodo and water from a deep well (120 m), which were plotted between the hot spring water and the meteoric water line.
It is thought that the groundwater spouted just after the shaking generated long-displacement flow. There are three possible eruption mechanisms: (1) the shallow ground is liquefied and groundwater is ejected, (2) the shallow groundwater flowing from the mountains spouted out, and (3) the pressurized deep groundwater in the fault fracture zones erupted. However, although various ground surveys and analyzes have been conducted, it is complicated and no conclusion has been reached yet.
Reference 1) JICA : Project for Development of Regional Disaster Risk Resilience Plan in Central Sulawesi in the Republic of Indonesia / Japanese support committee for liquefaction - landslide (inland) / Technical Report / (in progress).
This area is the triple junction of the three plates and rotates clockwise. This causes a Palu-Koro left strike-slip fault, the displacement rate of which is very fast. A long and narrow pull-apart sedimentary basin is formed between the faults from north-northwest to south-southeast. The Palu River runs from south to north in the center, and gently sloping grounds of 1 to 3% spreads from the river to both sides. There is a hot spring near the area where Nalodo occurred.
Due to the earthquake, surface soil with a length of 1 to 3 km and a width of several hundred meters slipped and flowed toward the downstream side on the gently sloping ground in one area on the west side and four areas on the east side. According to interviews with several residents in and near Narodo, (1) a large vertical shaking and swelling occurred at the same time as the horizontal shaking of the earthquake ended, and (2) groundwater spouted and the ground started to flow at the same time as the horizontal shaking ended or within about 2 minutes. There was also testimony that hot water spouted out in some sites.
In the area where Nalodo occurred, spring belts crossed in the north-south direction before the earthquake. In order to study the origin of this spring, groundwater was sampled from the spring belts and others to test water quality. However, several months have passed since the earthquake, and the groundwater spouted by the earthquake has not been collected. Water was sampled at 29 locations, including springs in Nalodo, boring holes, a hot spring resort, deep wells, shallow wells, and rivers. A total of 37 samples were collected. Of these, 2 samples of spring water in the hot spring resort and 2 samples of deep wells are hot water with a temperature above 40 ° C. When the results were shown by the relationship between the oxygen isotope ratio δ18O and the hydrogen isotope ratio δD (δ diagram), they were plotted on the meteoric water line except for three points. On the other hand, the water of the hot spring is plotted to the right of the meteoric water line, and if it is extended, it will be between the “Arima-type” slab-related deep-seated fluid and the paleo-seawater (Yasuhara, personal communication). The other two points were spring water from a Nalodo and water from a deep well (120 m), which were plotted between the hot spring water and the meteoric water line.
It is thought that the groundwater spouted just after the shaking generated long-displacement flow. There are three possible eruption mechanisms: (1) the shallow ground is liquefied and groundwater is ejected, (2) the shallow groundwater flowing from the mountains spouted out, and (3) the pressurized deep groundwater in the fault fracture zones erupted. However, although various ground surveys and analyzes have been conducted, it is complicated and no conclusion has been reached yet.
Reference 1) JICA : Project for Development of Regional Disaster Risk Resilience Plan in Central Sulawesi in the Republic of Indonesia / Japanese support committee for liquefaction - landslide (inland) / Technical Report / (in progress).