Abstract
Palu City is composed of sedimentary rocks so that it has a high level of vulnerability when an earthquake occurs. The earthquake that occurred on September 28, 2018, 25 km north of Donggala with a magnitude of 7.4 Mw, had caused a tsunami that struck Palu City, Sigi Regency, Donggala Regency, and Parigi Mouthong Regency, Central Sulawesi Province. This earthquake also caused liquefactions in the Petobo and Balaroa areas, Palu City, and Jono Oge in Sigi Regency. In this study, the authors used microtremor data at 8 points measured before and after the earthquake. Processing using the Horizontal to Vertical Spectra Ratio method shows that the predominant frequency and amplification factor changes due to changes in local rock conditions. The analysis also shows the value of soil vulnerability index (Kg) mostly increased after the earthquake. Besides, the elliptic curve inversion method shows that before the earthquake event, the value of the S wave velocity (Vs30) in layer one was between 157.1-211.4 m/sec with a layer thickness between 65.3-90.6 meters, while after the earthquake, the velocity becomes 156-214.6 m/sec with a layer thickness ranging between 66-99.2 meters. This indicates that the area consists of soft and medium soils, which subsequently increased sediment thickness in most areas after the earthquake.
Highlights
In the region of geology, the Palu area consists of alluvium deposits and coastal deposits (Qap) consisting of gravel, sand, mud, limestone in the shallow river, delta, and marine environments [1]
The Sulawesi region is an area with a complex structure
The description of faults in the Sulawesi region has been found by several researchers with various methods
Summary
In the region of geology, the Palu area consists of alluvium deposits and coastal deposits (Qap) consisting of gravel, sand, mud, limestone in the shallow river, delta, and marine environments [1]. Most of the depressed areas of Palu City are clay, mud, and sand as alluvial deposits FIGURE 1 The presence of these sediments causes magnifying of the amplification factor at the ground surface when an earthquake occurs. Earthquake that shook the Donggala Regency with a magnitude 7.4 (Mw) caused an underwater landslide observed by aerial photography This landslide caused the tsunami to Palu City, Sigi Regency, Donggala Regency, and Parigi Mouthong Regency, Central Sulawesi Province. The area affected by the liquefaction was 34.5 Ha and a circumference of 2.5 km, mostly houses, and roads This incident is a phenomenon of loss of soil strength due to earthquake vibration. The characteristics of the microtremor signal will be identical when measured at different times [6] This case will be different when an area experiences significant subsurface structural changes. This research is expected to provide an explanation related to the influence of local geological conditions on recorded microtremor signals
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