Complex Morphology of Subducted Lithosphere in the Mantle below the Molucca Collision Zone from Non-linear Seismic Tomography

Results of seismic studies presented in previous publications depict two opposing subducted oceanic lithospheric slabs under the Molucca region. This unique structure is related to the arc-arc collision between the Halmahera and Sangihe arcs. Recently, we have revisited the complex subduction zone structure by employing a non-linear tomographic imaging technique in which 3-D ray tracing has been implemented. We have used P- as well as S-wave arrival times from carefully reprocessed global data set. The results provide some improvements in the positioning of wave-speed anomalies. Consistent with earlier results, the new P-wave model depicts the two opposing subducted slabs of the Molucca Sea plate. The intriguing new observation is that the westward dipping slab appears to penetrate into the lower mantle by taking the form of folded slab. We envisage that the folding behavior may have been caused by the shift of the whole subduction system in the Molucca region toward the Eurasian continent due to the westward thrust of the Pacific plate combined with the large left-lateral movement of the Sorong fault. The inversion of traveltime residuals of direct S phases strongly confirms the new observation

Tomografi Gempa Bumi dan Mitigasi Bencana

Tsunami yang dibangkitkan oleh gempa bumi raksasa dengan magnitudo 9,2 yang terjadi pada tanggal 26 Desember 2004 di samudera Hindia, di dekat Aceh, telah mengakibatkan lebih dari 200.000 orang meninggal. Sementara tragedi kemanusiaan yang luar biasa tersebut masih berlangsung, gempa besar dengan magnitudo 8,7 berikutnya terjadi di dekat Nias pada tanggal 28 Maret 2005. Bencana gempa bumi dan tsunami di atas telah disusul juga oleh bencana gunungapi yang kemungkinan dipicu oleh aktivitas tektonik tersebut. Di sisi lain gempa bumi yang menimbulkan penjalaran gelombang seismik ternyata memberikan informasi penting mengenai struktur bagian dalam (interior) dari planet bumi kita. Informasi ini terkandung dalam seismogram, yaitu hasil rekaman gerakan tanah akibat suatu gempa. Dalam makalah ini hasil studi tomografi seismik (gempa bumi) yang telah penulis tekuni dipaparkan secara singkat. Dalam bagian berikut akan ditunjukkan bahwa investigasi tomografi, yaitu teknik pencitraan yang telah terlebih dahulu berhasil digunakan dalam bidang kedokteran, juga telah berhasil diterapkan untuk studi kebumian dengan sangat baik. Makalah ini secara garis besar meliputi: (i) pencitraan tomografi seismik, (ii) kontribusi penulis terkait dengan pengembangan teknik tomografi, dan (iii) riset terkait dengan mitigasi bencana. Di bagian akhir dari makalah ini akan dipaparkan secara singkat arah riset ke depan yang diperlukan dalam membantu meningkatkan keberhasilan upaya mitigasi bencana secara berkesinambungan seiring dengan sering terjadinya gempa besar dan merusak di tanah air

Estimation of Slip Distribution of the 2007 Bengkulu Earthquake from GPS Observations Using the LeastSquares Inversion Method

Continuous  Global  Positioning  System  (GPS)  observations  showed significant  crustal  displacements  as  a  result  of  the  Bengkulu  earthquake occurring on September 12, 2007. A maximum horizontal displacement of 2.11 m was observed at PRKB station, while the vertical component at BSAT station was lifted up with a maximum of 0.73 m, and the vertical component at LAIS station had subsided  "“0.97 m. Adding more constraints on the inversion for the Bengkulu earthquake slip distribution inferred from GPS observations can help solve  the  underdetermined  least-squares  inversion.  Checkerboard  tests  were performed  to  help  conduct  the  weighting  for  constraining  the  inversion.  The inversion calculation yielded an optimal value for the slip distribution by giving the smoothing constraint a weight of 0.001 and the slip constraint a weight of = 0 at the edge of the earthquake rupture area. The maximum co-seismic slip of the optimal inversion calculation was 5.12 m at the lower area of PRKB station and BSAT station. The seismic moment calculated from the optimal slip distribution was 7.14 x 1021 Nm, which is equivalent to a magnitude of 8.5

Seismic Velocity Structures beneath the Guntur Volcano Complex, West Java, Derived from Simultaneous Tomographic Inversion and Hypocenter Relocation

We conducted travel time tomographic inversion to image seismic velocity structures (Vp, Vs, and Vp/Vs ratio) with simultaneous hypocenter adjustment beneath the Guntur volcano complex that is located in the Garut district, West Java province, Indonesia. The Guntur volcano is one of the active volcanoes in Indonesia, although large eruptions have not occurred for about 160 years. We used volcanic and tectonic earthquakes catalog data from seismic stations deployed by Centre for Volcanology and Geological Hazard Mitigation (CVGHM). For the tomographic inversion procedure, we set grid nodes with a horizontal spacing of 2 x 2 km2 and an average vertical spacing of 2 km. Our results show low Vp, low Vs, and high Vp/Vs ratio regions beneath the Guntur crater and the Gandapura caldera at depths of 6-8 km and 7-9 km, respectively. These features can be associated with amelt-filled pore rock structure. However, a low Vp/Vs ratio and low velocities are exhibited beneath the Kamojang caldera at depths of 2-6 km that may be associated with rock with H2O-filled pores with a high aspect ratio

Estimation of S-wave Velocity Structures by Using Microtremor Array Measurements for Subsurface Modeling in Jakarta

Jakarta  is located on  a  thick sedimentary  layer that  potentially has a very  high  seismic  wave  amplification.  However,  the  available information concerning the  subsurface model and bedrock depth  is insufficient  for a seismic hazard  analysis.  In  this  study,  a  microtremor  array  method  was  applied  to estimate the geometry and S-wave velocity of the sedimentary layer. The spatial autocorrelation  (SPAC)  method  was  applied  to  estimate  the  dispersion  curve, while  the S-wave  velocity  was  estimated  using  a  genetic  algorithm  approach. The  analysis  of  the  1D  and  2D  S-wave  velocity  profiles  shows  that  along  a north-south  line,  the  sedimentary  layer  is  thicker  towards  the  north.  It  has  a positive  correlation  with  a  geological  cross section  derived  from  a borehole down to  a depth of  about 300 m. The SPT data from  the  BMKG site  were used to  verify  the  1D  S-wave  velocity  profile.  They  show  a  good agreement. The microtremor analysis  reached  the engineering bedrock  in a  range from 359  to 608  m  as  depicted by a  cross section  in  the  north-south  direction. The site class was also estimated at each site, based on the average S-wave velocity until 30 m depth. The sites UI to ISTN belong to class  D (medium soil),  while BMKG and ANCL belong to class E (soft soil)

KARAKTERISASI SENYAWA FENOLIK DARI FRAKSI METANOL BUNGA NUSA INDAH (Mussaenda erythrophylla)

Bunga nusa indah (Mussaenda erythrophylla) merupakan tanaman yang hidup di daerah tropis. Penelitian ini bertujuan untuk mengarakterisasi senyawa fenolik dari fraksi metanol pada bunga nusa indah. Penelitian ini dilakukan dengan berbagai metode pemisahan yaitu fraksinasi dan pemurnian. Isolat relatif murni diperoleh 438,9 mg berbentuk amorf. Kemurnian isolat diuji menggukanan kromatografi lapis tipis 1 dan 2 dimensi yang menunjukkan noda tunggal dan uji fitokimia yang menunjukkan positif fenolik. Spektrum ultraviolet visible (UV-Vis) dengan menggunakan pelarut metanol memberikan serapan maksimum pada panjang gelombang 221,0 nm menunjukkan adanya transisi elektronik π→π*. Pada 267,5 nm menunjukkan adanya transisi elektronik *. Spektrum infrared (IR) menunjukkan bilangan gelombang (cm-1) : 3120,82 – 3514,30 (cm-1) (OH) stretching; 1454,33 cm-1 (C=C aromatic) stretching; 1128,36 cm-1 (C-O-C) stretching; 2922,16-2858,51 cm-1 (C-H alifatic), 1730,15 cm-1 (C=O), 1583, 56 cm-1 (C=C alifatic),  827,46 cm-1 C-H alifatic. Berdasarkan hasil uji fitokimia, spektrum UV-Vis dan IR, maka isolat F2.6 diindikasi merupakan senyawa fenolik.  Kata kunci : fenolik, isolat, Mussaenda erythrophyll

AKTIVITAS ANTIBAKTERI EKSTRAK DAUN AKAR BAMBAK (Ipomoea sp.) TERHADAP BAKTERI Staphylococcus aureus DAN Escherichia coli

Akar bambak (Ipomea sp.) merupakan tumbuhan liar yang secara tradisional dimanfaatkan sebagai obat bisul. Penelitian ini bertujuan untuk mengetahui aktivitas antibakteri ekstrak daun akar bambak terhadap S. aureus dan E. coli, serta mengetahui metabolit sekunder yang terdapat pada daun akar bambak. Ekstrak metanol daun akar bambak dipartisi dan dilakukan uji aktivitas antibakteri dengan metode difusi sumur serta uji fitokimia pada sampel yang memiliki aktivitas antibakteri terbaik. Hasil penelitian memperlihatkan daya hambat ekstrak metanol, fraksi metanol dan fraksi etil asetat terhadap E. coli pada konsentrasi 0,2 g/mL secara berturut turut sebesar 10,28; 7,30 dan 7,48 mm, serta terhadap S.aureus secara berturut-turut sebesar 11,48; 7,85 dan 8,05 mm. Sedangkan fraksi n-heksana tidak memperlihatkan adanya aktivitas antibakteri. Hal ini menunjukkan bahwa ekstrak daun akar bambak mampu menghambat pertumbuhan bakteri E. coli dan S. aureus, dimana ekstrak metanol daun akar bambak memiliki aktivitas penghambatan paling tinggi dibandingkan fraksi metanol dan etil asetat. Berdasarkan hasil uji fitokimia diketahui bahwa ekstrak metanol daun akar bambak mengandung senyawa alkaloid, steroid, polifenol dan flavonoid. Kata kunci: antibakteri, fitokimia, Staphylococcus aureus, Escherichia coli, Ipomea sp

Western Java Ambient Noise Tomography: A Preliminary Result

Western Java, Indonesia, has at least three important active faults: the Cimandiri, Lembang, and Baribis Faults, which pose a great danger for the cities near them. It is crucial to have a better understanding of shallow crustal structure to delineate active faults and deep basins in order to support seismic hazard and disaster mitigation efforts in Indonesia. In this study, we perform ambient seismic noise tomography which can give better resolution of the shallow structure beneath western Java. We have deployed a seismometer network in the western to central Java region through a research collaboration program between the Bandung Institute of Technology (ITB) and the Australian National University (ANU). We deployed 70 seismometer stations from June to January 2017 to acquire ambient seismic noise data. As the first stage of the data processing, we will focus on conducting single data preparation and cross-correlation to retrieve an estimate of Green's functions between station pairs. We also use the frequency-time analysis technique to obtain dispersion curves to measure the interstation group velocity. The group velocity is use as an input in tomography inversion. Our preliminary results show low velocity anomalies associated with sedimentary basins and a high velocity anomaly associated with the southern mountains

Preliminary Estimation of Engineering Bedrock Depths from Microtremor Array Measurements in Solo, Central Java, Indonesia

In the last decade the city of Solo, located in Central Java, Indonesia, has grown significantly and become a major city. Many industries and hotels have been built in the city and its surroundings. This study aimed to determine the engineering bedrock depths in Solo, an important parameter in seismic hazard analysis. The microtremor array method was used to obtain 1D S-wave velocity profiles and construct layer depth maps. The spatial autocorrelation (SPAC) method was used to calculate the dispersion curves, while the S-wave velocity structure was derived using a genetic algorithm (GA). The results of the S-wave velocity structure in Solo show that there are four stratigraphic layers, where the engineering bedrock depths in Solo exist within the range from 145 to 185 m. The shape of the bedrock basin is elongated in an east-west direction