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We present an interpretation of newly acquired audio-magnetotelluric data to reveal the subsurface geometry of Cimandiri Fault Zone, one of the major active faults in the western part of Java. The line section is 25 km long in a nearly north–south direction across the axes of CFZ with 24 stations of 750–1200 m spacing intervals. The 2D AMT inversion model shows two conductive zones in the southern part that may be associated to the Miocene rocks of the Southern Mountains, and a conductive zone in the northern part that is likely to be associated with Gunung Walat fold-belt. The subsurface structures of the Southern Mountains are dominated by south-dipping thrusts that may uplift the shallow marine sediments. Shallow seismicity occurred around CFZ indicating the activity of these blind thrusts.

One of the most powerful eruptions of the Holocene, known as the Samalas eruption, had a VEI of 7 and took place on Lombok, eastern Indonesia in 1257 CE. Thick tephra fall covered the entire island, and pyroclastic density currents (PDCs) up to 50 m thick buried almost half of Lombok. Until now, there has been no detailed study of the geomorphological evolution of Lombok related to this eruption, especially for the eastern part of Lombok. The main goal of this paper is therefore to reconstruct the paleo-topography on the eastern part of Lombok before the eruption of the Samalas volcano in 1257 CE, and to analyse the subsequent landscape evolution following this eruption. Data were collected from more than 1300 points in order to gather geomorphological, geological, and geochemical information for the eastern part of Lombok over the last millennium. Data were obtained from wells (which are widespread in the studied area), natural outcrops (mainly cliffs along the shoreline and the rivers), and human-made outcrops in quarries, as well as from the two-dimensional resistivity profiling (Dipole-dipole array) carried out on eastern Lombok in 2016 and 2017. We estimate that 4435 ± 5.5 × 106 m3 of pumice-rich PDC, over a surface of 171 km2, buried the eastern part of Lombok in the aftermath of the 1257 CE eruption, and as a result of this rapid pumice-rich PDC deposition there has been some aggradation of the shoreline at some points, resulting in most of the pre-1257 coral reef being buried. After several intense rainfall events, the deposited volcanic material was probably eroded during the first decades following the 1257 CE eruption by mountain streams, producing lahars. We estimate that the remaining volume of pumice-rich PDC deposits following this erosion is 625 ± 5.5 × 106 m3 over a surface of 89 km2, which is equivalent to 14% of the initial volume.

Continuous gravity monitoring as geodetic instruments for tidal variation recording has been proving its availability for more applications. The original purpose was to monitor the changes in gravity at one local, which could be correlated to the hydrological variation and geothermal reservoir changes. Most recently explored was its relation to the events of earthquakes. We have been installing the gPhone 145 since 2016 at LIPI Campus in Bandung as a preliminary observation of the instrument. In four years of installation, the gPhone has recorded the relative gravity intermittently, interrupted only by technical problems. Temperature and pressure records indicate the instrument functions properly. However, uneven recording prevented hydrological study, in which we expected to see some changes of the gravity through the years. For earthquakes’ precursor studies, additional samples of events and time-associated gravity anomalies might give more assuring results.

DOI: 10.17014/ijog.4.1.39-47The characteristic of Cimandiri Fault Zone has not been completely defined despite plenty of studies had already been accomplished. Therefore, an audio-magnetotelluric modeling was carried out. An audiomagnetotelluric survey was conducted at two parallel lines (N166oE) that intersected Cimandiri Fault Zone in Cibeber area, Cianjur. The distance between those two lines was 4.5 km and each line consisted of twenty-one stations with the distance between stations was 500 m. From the acquired forty-two apparent resistivity curves, inversion was executed to obtain two models. The models indicate layers with resistivity value of > 1000 ohm.m at about 500 m depth at both lines, which are associated to the basement layer. Columns of low resistivity zones in about the middle of each model represent fault zones as the weak zones of the area, and both models displayed them slightly dip southward as thrust faults.

The Guci and Kalipedes hot springs, in northwestern slope of Mount Slamet, Central Java appears on the contact between volcanic breccia rocks with lava flows. The hot springs is located on a structure lineament or fault, west-east trending for Kalipedes hot springs and Northwest-Southeast lineament direction for Guci hot springs. The Baturaden hot springs located at the southeast slope of Mt. Slamet appear on the contact between polimicbreccia with lava in a relatively northwest-southeast trending river valley. Breccia rocks generally have higher permeability compare to relatively impermeable lava flows. Volcanic rocks in the study area composed by olivine basalt, basaltic andesites, pyroxen andesite with fairly high potassium content or as calc alkaline basalt or andesite affinity. The presence of the main structure controlling the appearance of hot springs evidenced by radonstudies that show a high radon value to > 40 bpm/L showed high permeability that is interpreted as a weak zone due to the of the geological structure or fault activities in Kalipedes and Guci areas while in the Baturaden, the radon content is relatively low so that estimated the main control of the hotspring is permeability differences. The presence of the geological structure or the faults could be proven through magnetotelluric research. Abstrak Mata air panas Kalipedes dan Guci di lereng baratlaut Gunung Slamet, Jawa Tengah muncul pada kontak antara batuan breksi gunungapi dengan aliran lava. Mataair panas tersebut terletak pada suatu kelurusan struktur atau sesar berarah barat-timur untuk mataair panas Kalipedes dan kelurusan berarah baratlaut-tenggara untuk mataair panas Guci. Mataair panas Baturaden yang terletak di kaki tenggara G. Slamet muncul pada kontak antara breksi polimik dengan lava pada suatu lembah sungai yang berarah relatif baratlaut-tenggara. Batuan breksi umumnya memiliki tingkat kelulusan fluida yang tinggi sedangkan aliran lava relatif kedap fluida. Batuan gunungapi di daerah penelitian disusun oleh basal olivin, andesit basaltik, andesit piroksin dengan kandungan kalium cukup tinggi atau berafinitas sebagai basal atau andesit kalk alkalin. Kehadiran struktur utama pengontrol pemunculan mataair panas dibuktikan dengan penelitian gas radon yang menunjukkan nilai radon yang cukup tinggi sampai tinggi >40 dpm/L dan kelulusan fluida yang tinggi yang ditafsirkan sebagai zona lemah akibat kegiatan struktur geologi di kawasan Kalipedes dan Guci,sedangkan di daerah Baturaden nilai gas radon relatif rendah sehingga diperkirakan kontrol utama pemunculan mataair panas adalah perbedaan kesarangan batuan. Kehadiran struktur geologi atau sesar-sesar tersebut dapat dibuktikan melalui penelitian magnetotelurik.