Penelitian ini bertujuan untuk mengetahui karakteristik dinamis kondisi geologi lokal diantaranya frekuensi resonansi (fo) dan indeks kerentanan seismik (Kg) serta nilai percepatan getaran tanah (PGA) menurut formulasi Kannai. Pengukuran mikrotremor dilakukan pada masing-masing titik pengukuran selama 30 menit dengan menggunakan Digital Portable Seismograph. Dari analisa data diperoleh nilai frekuensi resonansi (fo) yang relatif tinggi berkisar antara 24,4 hingga 48,14 Hz dan nilai indeks kerentanan seismik (Kg) yang relatif rendah berkisar antara 0,1 hingga 4,8. Hal ini berkaitan erat dengan kondisi geologi lokal berupa batuan yang masif, sehingga daerah dengan frekuensi resonansi tinggi dan indeks kerentanan yang rendah relatif stabil secara seismik. Nilai PGA di daerah kajian berkisar antara 0,25 hingga 0,36 g. Nilai PGA di daerah kajian menunjukkan variasi yang tidak terlalu mencolok yang menunjukkan nilai PGA lebih banyak dipengaruhi oleh input nilai periode dominan di daerah kajian.
As moderate devastating earthquake that impacted the economic loss was about $ 1.68 million US, the 2018 Kalibening earthquake quite shocked where the faulting mechanism still unconfirmed. There was no reference indicating the existence of the faulting zone exactly on the earthquake sequence. We take a benefit from seismic temporary network with the density ±2 km, which deployed while a macroseismic survey in purposing to evaluate the 2018 Kalibening earthquake sequence. Earthquake relocation using double difference combined with cross-correlation and determining earthquake moment tensor inversion solution using near source seismograms were applied. We suggested the deformation scheme of this earthquake with a thrust faulting with 307.5 / 28.8 / 118.5 (Strike / Dip / Rake) as a result from mainshock and supported by aftershock moment tensor solutions. This parameter consistent to aftershocks relocation results which formed a lineation trending NW-SE appropriate with Strike = 307.5. The cross-section exhibits aftershocks pattern which elongated deeper and formed a slope from SW to NE approximately fit to Dip = 28.8. The results from investigating the background seismicity in Banjarnegara Region using combined catalog (ISC-USGS-BMKG) compared to BJI (single station) showed the sparse and the lacking of InaTEWS seismic network configurations. Finally impacted to losses in earthquake cataloging, leads the low area coverages around Banjarnegara region. Together with our results, we called for the revision of earthquake hazard assessment for Central Java province, especially in Banjarnegara region.
Java Island has an interesting tectonic system to study, especially since this region is formed from plate collisions that cause the formation of subduction zones. Because of the uniqueness, earthquakes often occur in this region, which one of them became the attention of researchers, namely the Tasikmalaya earthquake on December 15, 2017. The purpose of this study is to analyze the earthquake recording signals and also the pattern of focal mechanisms of the Tasikmalaya earthquake from processing result of JISView (BMKG) devices that have been validated with comparators from GFZ (Germany) and USGS (United States) websites. The method used is to make an analysis of the wave signal caught by the recorder station and then identify the results of the parameters generated on the calculation of the device and match the data obtained with data tectonic structure around the area of the earthquake zone. Based on the results of data processing, the intensity value of earthquake signals obtained from the processing in accordance with the data released by BMKG agencies and earthquakes from the depth of 90-110 km which occurs due to the shift of the fault, with the type of fault is strike-slip fault (dextral), where the cause of the earthquake comes from the subduction zone (benioff zone in the deep background).
Aluvium merupakan fitur geologi yang memiliki sifat rentan terhadap pengaruh gempabumi. Daerah Porong dan sekitarnya tempat semburan Lumpur Sidoarjo (Lusi) terjadi merupakan daerah dataran yang ditutupi oleh endapan aluvium Delta Brantas, sehingga daerah ini merupakan zona lemah yang rentan terhadap pengaruh gempabumi. Hal ini diperkuat dengan adanya sesar Watukosek di daerah tersebut. Dengan tujuan untuk membuktikan hal tersebut maka dilakukan observasi seismik dengan cara memasang 71 stasiun pengamat gempabumi temporal yang tersebar di daerah Sidoarjo dan sekitarnya. Hasil analisis Horizontal Vertical Spectral Ratio (HVSR) terhadap data seismik diperoleh sebaran frekuensi natural bawah permukaan lebih rendah di daerah Lusi yaitu 0,4Hz. Hasil analisis juga mengungkap bahwa di daerah tersebut mempunyai amplifikasi tanah sebesar 5,2 dan tingkat kerentanan tanah sebesar 56, lebih tinggi dibandingkan dengan daerah lain di sekitarnya. Karena letaknya di zona lemah, maka berimplikasi bahwa Lusi menjadi sensitif terhadap gangguan luar misalnya dampak kejadian gempabumi menjadi lebih besar pada daerah ini. Alluvium is a geological feature characterized by high risk vulnerability influenced by the earthquakes. Porong and surrounding areas where the eruption of Lumpur Sidoarjo’s (Lusi) occurred are areas covered by alluvium sediment of Brantas Delta, as consequences this area is a weak zone characterized by high risk vulnerability as well. This is also supported by the present of Watukosek fault system in this area. To proved, we deployed 71 temporary seismic stations distributed in and around Sidoarjo area. The Horizontal Vertical Spectral Ratio (HVSR) analysis revealed that the natural frequency in Lusi area is about 0.4Hz, this is lower than other part areas. The analysis also revealed that this area has soil amplification about 5.2 and soil vulnerability index about 56, these are higher compared with other part areas. These results support that this area is a weak zone. Because of its location in a weak zone, this implies that Lusi became sensitive to external perturbation for example the earthquake events would have greater impact to this area.
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