Preliminary result of dominant frequency and seismic amplification in Penajam Paser Utara and its surrounding regions using the HVSR method

East Kalimantan has been designated as the new capital city of Indonesia, precisely in Penajam Paser Utara and Kutai Kartanegara Regency. According to the historical records of seismic activity, there were several destructive earthquakes in East Kalimantan. Therefore, it is necessary to study the characteristics of the local site to assess the seismic hazard potential in the new capital city area and its surroundings. In this research, the Horizontal to Vertical Spectral Ratio (HVSR) method was applied by utilizing microtremor data from 21 seismometers installed in the research area. The results of the HVSR analysis obtained the dominant frequency (f0) and amplification factor (A0) from the H/V curve. Based on the H/V calculation, the highest f0 is 13.04 Hz and the lowest value is 0.58 Hz. Meanwhile, the highest value for A0 is 6.41, and the lowest value is 1.41. The results of the study show that the research area has a low to medium level of seismic hazard.


Introduction
East Kalimantan Province, specifically in Penajam Paser Utara and Kutai Kartanegara Regency, has been designated as the new capital city of Indonesia, or IKN.One of the considerations for selecting these regions was that the potential for natural disasters is relatively small, especially earthquakes.Based on USGS data, there were about 60 earthquake events with M 3-6 from 1921-2022 (Figure 1), which is a small number when compared to earthquake events that occurred in other regions in Indonesia.However, according to the historical seismic record, several destructive earthquakes have occurred in East Kalimantan, as can be seen in Figure 1.Tectonically, there are three main faults in East Kalimantan that are still active today, namely the Maratua Fault, the Paternoster Fault, and the Mangkalihat Fault.A study revealed that these faults have a potential magnitude of up to 7.0 and can impact levels VI-VII on the MMI scale [1].Therefore, it is important to study the subsurface profile and characteristics of the new capital city and its surroundings in order to know the potential for seismic hazards in this region, considering that research in this area has not been widely carried out.This research was conducted to calculate the dominant frequency (f0) value and amplification factor (A0) to determine the subsurface characteristics by utilizing single-station microtremor data and analyzing it using the HVSR method.

Geological Setting
Kalimantan Island is part of Sundaland, which is part of the Eurasian continental plate [2].The Indo-Australian plate is moving northward with a movement velocity of 7 cm/year and colliding with the Eurasian plate at a velocity of about 0.4 cm/year, moving southeast [3].
The research area is included in the Kutai Basin, which has a thick layer of sediment with a thickness of > 3 km [13].Physiographically, to the north of the Kutai Basin, it is bordered by Sangkulirang, the Bengalon Fault Zone, and the Mangkalihat High.Whereas in the south it is bordered by the Adang Fault Zone, which has served as the axis zone of the basin since the end of the Paleogene until now [14].The tectonic framework in eastern Kalimantan is influenced by regional tectonic developments through interactions between the Pacific Plate, the Eurasian Plate, and the India-Australia Plate, as well as regional tectonics in southeastern Asia [15].
The Kalimantan area is surrounded by several faults, namely the Adang Fault in West Kalimantan, the Tinjia Fault in Serawak, the Mangkalihat Fault, which is located on the east coast of Kalimantan, the Sangkulirang Fault in East Kalimantan, and the Paternoster Fault in the Makassar Strait, which is trending west-east through Paser Regency [4].
In general, the geological conditions of the research area consist of six rock formations (Figure 2), which are composed of sedimentary rocks that contain a fairly high mixture of organic materials such as carbonate, lignite, and coal [5].These six formations are: sandstones with intercalated limestone, coal, claystone, and dacite tuff that formed in a shallow marine depositional environment.f.The Balikpapan Formation is a Middle Miocene-Late Miocene formation, composed of alternations of sandstones and clays with intercalated silt, shale, limestone, and coal.g.The Kampungbaru Formation is a Late Miocene-Plio Pleistocene formation composed of quartz sandstones intercalated with clay, shale, silt, and lignite, which are generally soft and easily crushed.h.Alluvium deposits consist of gravel, sand, and silt that have been deposited in the river, swamp, delta, and beach environments.Formed during the Holocene and is the youngest formation.
The geological structure of this area is dominated by anticlinorium folds and faults, which have three directions, namely north-south, northwest-southeast, and southwest-northeast.Three types of faults are found in this area: reverse fault, normal fault, and strike-slip fault.In the late Miocene, reverse faults formed, which were then cut by a strike-slip fault.Meanwhile, normal faults formed during the Pliocene [5].

Data
This research used microtremor data from 21 seismometers installed in the IKN area and its surroundings, covering several districts and cities, including Penajam Paser Utara, Kutai Kartanegara, Balikpapan, Samarinda, and Kutai Barat, which can be seen in Figure 2. The recording data consists of three broadband components: north-south horizontal components, east-west horizontal components, and vertical components.The data was recorded in 2021, with a recording duration of 3 days for each station.

Method
Data processing was carried out using Geopsy software to obtain the HVSR curve.The HVSR method was first introduced by Nogoshi and Iragashi (1971) and made widespread by Nakamura.The HVSR method is a simple method that can describe soil characteristics by calculating the spectral ratio of the horizontal component to the vertical component [6].The HVSR method can also explain the phenomenon of the surface layer resonance frequency.The HVSR calculation can be expressed in the following equation: where FNS, FEW, and Fz are the components of the Fourier spectra in the north-south, east-west, and vertical, respectively.
The recorded microtremor data in this study is continuous, with a recording time of 3 days.In data processing, all microtremor signals were plotted to see the comparison and consistency of the data by making comparisons between 24-hour data and 1-hour data from each station.In the selection of 1-hour data, the data was cut at a quiet time, in the range of 23.00 p.m. to 02.00 a.m.This is done to reduce the noise that may be recorded on seismometers caused by human activities, animals, vehicles, etc.In general, the H/V curves obtained from 24-hour data and 1-hour data are quite similar.
The result of the HVSR calculation is a H/V curve that contains the dominant frequency value (f0) and amplification factor (A0) of each station.Based on these two parameters, the seismic hazard potential in the research area can then be determined.To extract the microtremor signal, a bandpass filter with a frequency range of 0.5-15 Hz is used, with a time window length of 60 seconds, an STA/LTA value range of 0.2-2.5, and a measurement duration of 1 hour.

H/V Curve
The results of the HVSR calculation obtained two types of H/V curves.These types of curves are characterized based on [7], namely clear peak (a), and multiple peaks (b).The variations in the H/V curve can be caused by differences in impedance contrast between sediment layers and bedrock, subsurface geological conditions, and rock compactness.The distribution of dominant frequency (f0) and amplification factor (A0) can be seen in Figure 4 and Figure 5, respectively.As shown in Figure 4, it can be seen that the research area has a dominant frequency value ranging from 0.58 to 13.04 Hz.In the eastern part of the research area, most areas have a relatively low dominant frequency of < 2.5 Hz, especially in Samboja District, Kutai Kartanegara Regency.Meanwhile, the dominant frequency value tends to increase to the west of the research area.The highest value is up to 13.04 Hz at IKN22 station, precisely in Bongan District, Kutai Barat Regency.At other stations, namely IKN12, IKN05, and IKN01 in Sepaku District, Penajam Paser Utara Regency, the dominant frequency value is relatively medium-high, ranging from 8-12 Hz.The dominant frequency is associated with sediment thickness [8], and it is also supported by the research of [12].Based on the classification of soil types by [9] (Table 1), the low dominant frequency value shows that the area has relatively deep bedrock depth.On the other hand, the areas with high dominant frequency values have relatively thin bedrock depth.
On the geological map, the eastern part of the research area is located in the Kampungbaru formation, which is the youngest formation after alluvium deposits and also located near the Mahakam Delta.This formation is composed of quartz sandstone intercalated with clay, shale, silt, and lignite, which are generally soft and easily crushed.This is presumably what causes the dominant frequency value to tend to be low.While the high dominant frequency values that dominate the western part of the research area are sedimentary layers composed of older tertiary rocks with relatively thin sediment thickness (hard sandy, hard gravel, clay, loam).This area is included in the Pamaluan formation, which is the oldest formation in the research area.Composed of sandstone with intercalations of claystone, shale, siltstone, and limestone.The amplification factor obtained at a location can be affected by differences in wave velocity associated with soil compaction.Damage to a building can be influenced by soil characteristics and building conditions [11].Typically, rocks with low compaction or soft soil will have a high amplification value.It is because the soft sediment layer can slow down the wave propagation from bedrock to the surface, so the amplification becomes high and has the potential to damage buildings during shocks like earthquakes.
Based on the amplification map above (Figure 5), it can be seen that the research area has amplification that varies from low to high levels, ranging from 1.41 to 6.41.In the south-northeast, the amplification value is in the range of 1.41-4.5, which is estimated to have a low-medium risk level.Meanwhile, the amplification value in the southwest to the northern part of the research area is at a medium-high level, ranging from 4.5 to 6.41.The amplification value is quite high at the IKN27 station, which is 6.41 Hz.In general, based on the amplification value, the seismic hazard in the research area is at a low-medium risk level.

Conclusion
The dominant frequency value in the IKN area and its surroundings varies from 0.58 to 13.04 Hz.In the western part, precisely in Samboja District, Kutai Kartanegara Regency, which will be one of the IKN locations, has a low dominant frequency value which can be assumed to have a very thick sediment layer (> 30 m) and is composed of delta sedimentation, top soil, and mud.Meanwhile, in Sepaku District, Penajam Paser Utara Regency, which will become the central government area, has a high dominant frequency.It can be assumed that the sediment layer in this area is quite thin, composed of hard rock (sandy gravel, sandy hard, clay, loam).The amplification value in the research area varies from 1.41 to 6.41.In the two districts that will become the IKN location, the amplification varies from 1.41 to 4.49.This indicates that these areas have a low to medium risk level.In other words, the IKN area is an area with low to medium potential for seismic hazards.

2 Figure 1 .
Figure 1.Historical of seismic event in East Kalimantan based on USGS catalogue 1921 -2022.The dashed line indicated the research area.The insert map shows the Kalimantan region and the eastern Kalimantan marked by red square

Figure 2 .
Figure 2. The regional geological map of the research area [16].Inverted triangles represent the measurement stations and color bars represent the rock formations

Figure 3 .
Figure 3. Two types of H/V curves.(a) clear peak, and (b) multiple peaks.The solid line indicates the average HVSR value and the dashed line indicates the deviation standard 4.2 Dominant Frequency (f0) and Amplification Factor (A0)The distribution of dominant frequency (f0) and amplification factor (A0) can be seen in Figure4and Figure5, respectively.As shown in Figure4, it can be seen that the research area has a dominant frequency value ranging from 0.58 to 13.04 Hz.In the eastern part of the research area, most areas have a relatively low dominant frequency of < 2.5 Hz, especially in Samboja District, Kutai Kartanegara Regency.Meanwhile, the dominant frequency value tends to increase to the west of the research area.The highest value is up to 13.04 Hz at IKN22 station, precisely in Bongan District, Kutai Barat Regency.At other stations, namely IKN12, IKN05, and IKN01 in Sepaku District, Penajam Paser Utara Regency, the dominant frequency value is relatively medium-high, ranging from 8-12 Hz.

Figure 4 .
Figure 4. Dominant frequency map of IKN area and its surroundings

Figure 5 .
Figure 5. Amplification map of IKN area and its surroundings