Abstract:The Sidoarjo mudflow in East Java, Indonesia, has been erupting since May 29 th , 2006. The eruption has been known as the Lusi (lumpur Sidoarjo), which was previously considered as a remote seismic event consequence, but current geyser-like activities show an association with a geothermal phenomenon. A method of characterizing rare earth elements (REE) is commonly an effective tool for recognizing a geothermal system, and here it is adapted to particularly indicate the environmental origin of the Lusi mud. Results show that the Lusi hot mud is made of a porous smectite structure of a shale rock type, which becomes an ideal tank for trapping the REE, especially the light REE. Volcanic activities seem to be an important influence in the eruption; however, since there is a lack of significant isotopic evidences in the mobilization of the REE during the eruption, the chloride neutral pH water of the Lusi may hardly contain the REE. The moderate Ce and Eu anomalies found in the REE patterns of the mud strongly indicate a sea-floor basin as the most probable environment for the REE fractionation during the sedimentary rock formation, in which the weathering processes of volcanic rock origin enriched the Lusi shale with the REE.

A series of earthquakes occurred at the northern part of Lombok Island during July–September 2018 with the highest Mw7.0 5th August 2018 that caused the death of hundreds of people and ruined thousands of buildings. The earthquakes were triggered on the Flores Thrust located at the back arc zone and at only 50 km distance from the island, leading to multiple seismic hazards to Lombok and surrounding islands. The thrust could possibly be the dominant current seismic sources; however, the megathrust sources also contributed to the hazards due to the subduction between the Indo-Australia and Eurasia tectonic plates in the Nusa Tenggara region. An updated probabilistic seismic hazard analysis was, therefore, conducted on recent seismicity, detailed tectonic background, and suitable ground motion prediction equations, to determine higher seismic parameter values than the 2017 models. This means that Lombok and surrounding islands exposed to higher seismic hazards than those predicted before the earthquake events in 2018.

Abstract. Lombok Island is located in the Nusatenggara region, which is considered as one of the most active seismic parts of Indonesia. The determination of seismicity is crucial to the island, since Lombok Island is a relatively dense populated small island. Earthquakes that occurred around the island were generated by southern subduction mega-thrust and northern back-arc thrust. Mostly, distributions are concentrated on the north-west and south-east parts of Lombok Island. Probabilistic data analysis of 309 earthquake events during the period of 1973 -2017 results in the Gutenberg-Richter parameters (a and b) of 3.62 and-0.53 indicating medium to high seismic activities. The Joyner-Boore attenuation relation seems to be suitable for seismic characteristics of the island. The reconstruction of tectonic setting indicates that the Benioff zone may develop at the depths of 150 km beneath the island causing horizontal tectonic forces working within the direction of N 171 0 E -N 351 0 E. This could create a strike-slip fault on the crustal basement within the NE -SW direction.

The Lusi mud eruption in Sidoarjo of East Java has provided huge impact to the people living in the surrounding area. The eruption has been followed by ground subsidence that has changed the ground structure. An assessment method has been introduced for analyzing the hazard. The method employs a rating system to quantify the hazard parameters, in which ground deformation parameters are significant. A hazard risk index is therefore obtained by multiplying hazard potential and vulnerability parameters.

Two popular rock strength criteria, the linear Coulomb and non-linear Hoek-Brown, are widely used in underground designs. These two criteria may be applied differently depending on rock conditions. Weak rocks may have different properties compared to hard rocks. Both criteria have been applied in a current research to practically determine the applicability of the criteria in estimating the strength of weak rock masses of five shallow underground structures. Results show that both criteria are able to model the strength of the five weak rock masses, but as expected the criteria provide quite different values for each type of rocks. The strength of rock masses around underground structures depends on uniaxial compressive strength and confinement; but the linear criterion very much depends on shear characteristics of rock materials. Whereas, the non-linear criterion relies on the geological strength index (GSI). Although the GSI may have served practical descriptions for rock masses, some difficulties were found when using the GSI for very weak pyroclastic rocks. The GSI seems to provide underestimated indexes for these rock types. Estimations show that the non-linear criterion may not really exhibit curved strength envelopes rather linear in some sense, for five weak rock masses. Thus in general, when an underground structure is reasonably shallow, has a lack of confinement, and where the shear behaviour dominates rock failures, the linear criterion is more preferable than the non-linear criterion in modelling the strength of weak rock masses.