Microtremor analysis around Jari mud volcano, Bojonegoro

The earthquake that occurred in the Mount Pandan area in 2016 is thought to have triggered the emergence of a mudflow mixed with water and gas. The research area is in the Kendeng Zone with a complex geological structure. In this zone, there is a very fast sedimentation process so the buried material is not fully compacted. In this study, measurements were made of 6 microtremor data points around Jari Mud Volcano, Bojonegoro, East Java, Indonesia. Furthermore, the data obtained were processed using the HVSR analysis method. HVSR analysis is a method of processing microtremor signals to determine soil characteristics based on information on natural frequency values and its amplification. The result shows the natural frequency parameter and its amplification is slightly variated around Jari Mud Volcano, caused by sedimentary depth variation and/or the presence of existing rock. Based on 6 processed data, the natural frequency values range from 1.9 to 9.1 Hz. Meanwhile, the amplification value ranges from 2.6 to 4.4. Based on the amplification value, the research area is classified as low and medium amplification zones (low value dominance). This is because the amplification value is related to geomorphological conditions, in which the research area is located in the mountains and is composed of volcanic sediments.


Introduction
Microtremor is a passive seismic geophysical method.This method was developed by Aki and Kanai (1957).Initially, this method was used to determine the characteristics of sedimentary rocks associated with subsurface structures [1].Microtremor measures the natural vibrations of the ground which can come from natural sources (wind and ocean waves), as well as artificial sources (traffic, machinery, footsteps, etc.) [2].This method continues to grow, which later Nakamura (1989) developed a method to analyze HVSR (Horizontal to Vertical Spectral Ratio).In principle, HVSR analysis involves performing Fourier transform on microwave tremors to obtain the natural frequency and amplification values [1].Natural frequency is the fundamental frequency of a place in spreading vibrations or waves [2] and is related to the thickness of the sediment layer [1].Amplification is an amplification of waves that occurs when three seismic waves pass through two types of rock with different characteristics [1].This value is directly proportional to the soil susceptibility index.According to Daryono (2012), the value of the soil vulnerability index depends on geomorphological conditions of the local area which is related to rock density [3].
A mudflow mixed with water and gas occurred in the study area in 2016, after previously being preceded by an earthquake ranging from 2.5 to 4 on the Richter Scale.The cause of the eruption of mud mixed with water is thought to be due to the rupture of the mud pocket under the surface caused by pressure from below.Based on research by Santoso et al., (2018), on June 25, 2015, Badan Meteorologi, Klimatologi and Geofisika reported an earthquake with a magnitude of 4.2 M which damaged several houses around Mount Pandan.After that, more than twenty micro-earthquakes (M < 4) occurred again in this area (February 2016).A group of micro-earthquakes lies at a depth of less than 30 km below Mount Pandan [4].
Previous research study using other geophysical method has been done around this area in particular Gondang area, such as gravity method to give a proper subsurface model for making 3D model to identify subsurface using 3D inversion [5].Besides that, using magnetotelluric data to determine distribution of resistivity which is translated in the form of 2D model [6].Other geophysics methods have been also conducted arround the study area such as VLF electromagnetic [7], refraction seismic [8] and geomagnetic [9].Generally, the application of the microtremor method in the earth sector is to determine the soil vulnerability index related to earthquakes and disaster mitigation in an area.For example soil vulnerability analysis [2][4], fault zone mapping [9] and bedrock estimation [11].

Regional Geology
Kendeng Zone includes a mountain range in an east-west (T-B) direction which is located directly to the north of the Ngawi Subzone.These mountains are composed of marine sedimentary rocks that have undergone intensive deformation to form an anticlinorium (a series of small anticline hills arranged in parallel and forming a larger anticline structure).The age of sedimentary rocks in the Kendeng Formation is middle Miocene to Quaternary, with sediment types as flyash, turbidite, and land sediments [12].Figure 1 is the arrangement of rock formations that make up the area of Mount Pandan.The research area is included in the Pandan Breccia unit in the form of andesite, gray to brownish gray and yellowish brown, composed of andesite rock fragments, gravel to gravel in size, to lumps.Below the Pandan Breccia unit, there is the Kalibeng Formation in the form of marl, inserting tuff, tuffaceous sandstone, and calcarenite.The Kalibeng Formation overlaps the Kerek Formation, which is an alternating rock formation of sandstone, claystone, tuff, and marl.

3.
Methodology The data used is data from the acquisition of gravity data around the TRP mud volcano site in 2019.Data collection was carried out in 8 points gravity data and 9 points magnetic data distributed accros the study site.Figure 3 shows the microtremor data processing process.In the field acquisition data, filtering and windowing processes were carried out, which were then analyzed using the HVSR method to produce an HVSR curve.This curve is then translated into two components, namely natural frequency (X), and amplification (Y) (Figure 4).Furthermore, the data obtained is reprocessed into an image that shows the distribution of natural frequency values and amplification at each measurement station.

Results and Discussions
Microtremor recording consists of three components, two horizontal component (North-South, East-West), and the vertical component (Z) in time domain.Band-pass filter is applied to the data to pass the wave signal at a certain frequency, because the signal needed in data processing is a lowfrequency signal that ranges from 0.2 to 20 Hz [1][13].In this study, the selected frequency range is the range of 0.2 -10 Hz.Because natural low-frequency microwaves range from 0.2 -10 Hz [1].The windowing process aims to divide the wave signal, as well as select the appropriate signal to enter the next stage.Waves with large amplitude (zig-zag) are not selected because they are considered as noise  Generally, HVSR method is used to determine the vulnerability of soil and soil characteristics of an area.The analysis is applied to obtain the natural frequency and amplification around the study area.Based on the HVSR analysis that has been carried out, the results are obtained as presented in Table 2.The average values of natural frequency and amplification are chategorized as low.

Natural Frequency
The first parameter that we looking for is natural frequency that indicates the number of waves generated per unit of time.This parameter is influenced by the average velocity and thickness of the subsurface sediment.The natural frequency is inversely proportional to the sediment thickness.It means that the areas that have a fairly thick sediment thickness will tend to have a small dominant frequency value.

Amplification
The amplification parameter is affected by the wave speed.This is related to rock density, where soft rock will increase the wave amplitude which will increase the amplification value.The amplification values in the study area are classified as in Table 3.According to Daryono (2012), the seismic vulnerability index describes the level of soil vulnerability to deformation related to geomorphological conditions.Regionally, the formations that make up the study area are pandanus breccia formations composed of volcanic breccia rocks.This is thought to affect the parameter values in the research area.The average amplification value is low as presented in Table 3.

Conclusions
The natural frequency values obtained ranged from 1.9 to 9.1 Hz, and 2.6 to 4.4 for the amplification value ranges.The study area is classified as low and medium amplification zones (low dominance).The geomorphological condition of the research area is mountainous and composed of volcanic breccia rock formations.
to SESAME (2004), a minimum of 10 windows is required to conduct an HVSR analysis.The determination of the window width depends on the smallest frequency you want to get.

Figure 5 .
Figure 5. Distribution map of natural frequency values

Table 2 .
Natural frequency and amplification results of each measurement station