ABSTRAKSejarah kegempaan Jawa antara tahun 1981 hingga 2014 mencatat bahwa Daerah Istimewa Yogyakarta telah dilanda gempabumi merusak sebanyak 4 kali dan ratusan gempabumi terasa. Penelitian ini bertujuan mengkaji zona percepatan tanah maksimum (PGA) dan intensitas gempabumi berdasarkan data historis kejadian gempabumi dari tahun 1981-2014. Perhitungan percepatan tanah maksimum menggunakan persamaan Kanai dan intensitas gempabumi menggunakan persamaan Wald. Penelitian ini dilakukan dengan menggunakan 231 data pengukuran mikrotremor di sekitar Kabupaten Bantul yang berasal dari BMKG serta 147 data gempabumi merusak dan terasa dari BMKG. Hasil penelitiaan menunjukkan nilai PGA di Kabupaten Bantul bervariasi dari 57,7-412,7 gal. Zonasi PGA di Kabupaten Bantul dibagi menjadi tiga zona, yaitu zona pertama dengan nilai percepatan 57,7-91 gal, zona kedua dengan nilai percepatan 92-179 gal dan zona ketiga dengan nilai percepatan >180 gal. Sedangkan berdasarkan intensitas gempa bumi, Kabupaten Bantul memiliki potensi kerusakan dalam intensitas V-VII MMI.Kata kunci: Percepatan Tanah Maksimum (PGA), intensitas gempabumi, mikrotremor, periode dominan, Kanai, Wald. ABSTRACTSeismic historical record in Java period of 1981-2014 recorded that the Special Region of Yogyakarta was hit by damaging earthquake 4 times and hundreds of small earthquakes felt. This research was aimed to assess the peak ground acceleration (PGA) zone and the earthquake intensity based on historical data of earthquake occurrence in 1981-2014. Peak Ground Acceleration was calculated using Kanai equation and earthquake intensity using Wald equation. This research was conducted by using 231 microtremor data in Bantul District that derived from BMKG and also 147 data of damaging earthquake and small earthquake felt from BMKG. The result of this study represent peak ground acceleration value in Bantul District which was varied from 57.7-412.7 gal. Peak Ground Acceleration Zonation of Bantul District was diveded by 3 zones; first zone with PGA value 57.7-91 gal, the second zone with PGA value 92-179 gal and the third zone with PGA value higher than 180 gal. While based on earthquake intensity, Bantul District has damage potential of V-VII MMI in intensity scale.

This research was aimed to know the magnetic field anomaly pattern upon Opak's fault, to identify the direction of Opak's fault and the structure of Opak's fault based on Geomagnetic data. It was conducted in the vicinity of Opak's fault in Pundong sub-district, Yogyakarta. It used magnetic method which is based on rocks magnetic susceptibility. The data were taken by G-856AX Proton Precision Magnetometer (PPM) using looping method. Data analysis was done using diurnal variation and IGRF correction, reduction to pole, upward continuation and under surface structure modelling. From magnetic field anomaly analysis, it was discovered that magnetic field anomaly on Opak's fault has -50 nT negative closure in the East, and 100 nT positive closure in the West. According to the result interpretation of 2.5D model on two slices using Mag2DC software, it can be obtained that the under surface structure of Opak's fault consist of Young Alluvial formation, Nglanggran formation, Semilir formation, Kebo Butak formation, and Wungkal Gamping with various thickness. The location of this Opak's fault is almost the same with the location of Opak's fault in Yogyakarta geology map. Opak's fault around Pundong sub-district is a normal fault directed to N 35º E. The eastern part of the fault has a relatively constant movement, while the western part is relatively moving down.

The Grindulu Fault is a one of the fault that lies alongside the Pacitan Regency, East Java. This fault is a land geological structure that is potential to be the source of earthquake disaster. The structure interpretation is carried out using gravity data by performing spectrum analysis and derivative methods to get a better understanding of the Grindulu Fault. We used gravity data from GGMplus. Spectrum analysis is performed by using Fourier transform to the bouguer anomaly. This Spectral Analysis generates a depth estimation of Grindulu Fault Zone anomaly source. Analysis of derivative methods provides information regarding the location of the fault anomaly source and information about the type of fault. Based on the result of spectral analysis, Grindulu Fault anomaly source is estimated to be at the upper limit of 160 meters and the lower limit up to 2897 meters. The Grindulu fault is interpreted to be segmented into at least 7 parts which is consisted of normal and reverse fault segmentations.

Gempa susulan setelah gempabumi Yogyakarta Tahun 2006 memiliki hiposenter bukan di sepanjang Sesar Opak tapi cenderung di sekitar unidentified fault yang berjarak 10 – 15 km sebelah timur pegunungan Gunung Kidul. Unidentified fault tersebut berkorelasi dengan keberadaan jalur Sesar Oyo. Metode geofisika yang dapat diterapkan untuk mengidentifikasi keberadaan jalur sesar adalah metode geomagnet. Penelitian ini bertujuan untuk mengetahui pola sebaran anomali medan magnet di sekitar jalur Sesar Oyo, mengetahui susunan formasi dan jalur Sesar Oyo berdasarkan pemodelan geomagnet. Pengambilan data dilakukan menggunakan PPM dengan 35 titik pengamatan dan spasi antar titik pengamatan 1,5 km. Pengolahan data dilakukan dengan koreksi variasi harian, koreksi IGRF(International Geomagnetics Reference Field), RTP (Reduction to Pole) dan Upward Continuation. Pemodelan dilakukan dengan menganalisis anomali medan magnet yang telah direduksi ke kutub dan kontinuasi ke atas dengan ketinggian 2500 m. Hasil analisa menunjukkan rentang nilai anomali medan magnet di kawasan penelitian adalah 180 nT – 660 nT, yang menunjukkan kontras keberadaan blok sesar. Hasil pemodelan 2D menunjukkan kawasan penelitian didominasi oleh 3 formasi batuan utama yaitu batubasalt-andesitik Formasi Nglanggran, batupasir Formasi Sambipitu, dan batugamping Formasi Wonosari. Hasil pemodelan 3D menunjukkan Sesar Oyo merupakan sesar geser dengan kedalaman 150 – 300 m, jalur sesar tersebut terbagi menjadi 2 segmen yaitu dengan arah N120°E sepanjang 5,8 km dan N160°E dengan panjang 2,5 km.

Data Vs30 banyak digunakan dalam berbagai persamaan empiris. Data tersebut dapat diperoleh dari data mikrotremor dan model topografi USGS. Penelitian ini bertujuan untuk menghitung rasio antara Vs30 mikrotremor dan Vs30 USGS. Data yang digunakan sebanyak 11 titik pengukuran mikrotremor di Kecamatan Jetis dan model topografi yang diperoleh dari website USGS. Vs30 mikrotremor dihasilkan dari model ground profile dengan nilai misfit terendah. Hasil perhitungan menunjukkan Vs30 berdasarkan data mikrotremor di Kecamatan Jetis bervariasi dari 145,22 – 251,24 m/s. Vs30 USGS di Kecamatan Jetis bervariasi dari 243,07 – 384,35 m/s.  Rasio Vs30 mikrotremor dan Vs30 USGS bervariasi dari 0,49 – 0,97. Hal ini menunjukkan bahwa data Vs30 USGS tidak berbeda jauh dengan hasil Vs30 data mikrotremor.Kata kunci: Vs30, Mikrotremor, Ground Profile Vs30 data are widely used in various empirical equations. Vs30 data can be obtained from microtremor data and topographic model from USGS. This study aimed to calculate the ratio between Vs30 from microtremor and Vs30 from USGS. The study used 11 points of microtremor data in Jetis and topographic model was obtained from USGS website. Vs30 from microtremor were generated from ground profile models with the lowest misfit values. The calculations show that Vs30 based on microtremor data in Jetis ranged from 145.22 - 251.24 m/s. Vs30 from USGS in Jetis ranged from 243.07 - 384.35 m / s. Ratio of Vs30 from microtremor and Vs30 from USGS vary from 0.49 - 0.97. This value indicates that the Vs30 data from USGS does not vary much with Vs30 data from microtremor.Keywords:Vs30, Microtremor, Ground Profile