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The vulnerability of land in an area to earthquake ground motions is one of the factors causing damage caused by the earthquakes. The city of Surabaya, which is crossed by two active fault segments, needs an assessment to reduce the risk of being affected by an earthquake that might occur. The aims of this study are (1) to find out the distribution of Seismic Site Classes, (20 to know the distribution of the value of Seismic Amplification, and (3) to know the potential of liquefaction in the city of Surabaya. Surabaya city, which is geologically dominated by alluvium deposits, consists of soft soil (SE) and medium (SD) sites based on N-SPT30 and Vs30 data. The level of soil amplification against earthquakes ranging from 1 to 4. This occurs because the physical properties of the Surabaya City soil layer are dominated by alluvium deposits. Regions with more than 2 amplification values are located around the coastline on the North and East coasts of Surabaya City. Based on the potential liquefaction index value, Surabaya City is included in the region with a high potential for liquefaction with a potential liquefaction index value of more than 5.

Referring to the Indonesian earthquake source and hazard map published by the National Earthquake Center (Pusgen) in 2017, it was stated that there were 34 active faults on the island of Java and could potentially be a source of earthquakes with earthquake strength more than the 6.5 M scale. Each potential source of earthquake needs to be carried out detailed research on each fault that is declared active so as to reduce uncertainty in the seismic hazard analysis. Due to limited funding and human resources researching, it is necessary to make research priorities based on the level of urgency. By utilizing decision making analysis techniques, priority of research is carried out by considering several aspects, such as the magnitude of the earthquake that may occured possible economic impact factors, population density and the level of importance of a region on a national and international scale. From the modeling results, it is shown that faults that have the potential to become earthquake sources that are located near large cities are a top priority for detailed research such as the Lembang, Semarang and Surabaya Faults.

The existence of the kendeng fault passing Surabaya is still the subject of study by the researchers after being declared active by the National Earthquake Centre. The Surabaya was passed by two segments of the Kendeng fault, the Surabaya segment and the Waru segment, the two segments had an earthquake potential of 6.5 M. In the context of disaster mitigation and earthquake analysis, more detailed information is needed relating to the geometry and position of the earthquake source with certainty, so that the identification of earthquake sources is needed in more detail. The magnetotelluric method is one of the geophysical methods that is able to properly identify anomalous objects in tens of kilometers depth. Kendeng fault as a potential earthquake source, one of its segments across Surabaya can be identified using the magnetotelluric method. Megnetolelluric measurements were carried out as many as 15 points in the Surabaya region in order to identify the geometry of the kendeng fault. From the 3D resistivity model, the inversion of magnetotelluric data can be identified well with the Kendeng fault that crosses the city of Surabaya, both the Waru segment and the Surabaya segment at a depth of less than 8 km

Unreinforced masonry (UM) is well known as a vulnerable structure against earthquakes. However, it remains a popular structural system for low-rise residential housing in many high-seismicity areas, particularly in developing regions due to its low cost and easy construction. In the present study, a retrofitting strategy using locally available material, bamboo strips, was proposed. In addition to its fast-growing rate, the tensile strength of bamboo is considered high, nearly comparable to its steel counterpart. A series of experimental tests were performed in this study, including the bamboo tensile test, the mortar flexural test, the diagonal compressive shear test on the masonry assemblages, and the in-plane pushover test on masonry wall specimens without and with bamboo reinforcement. The retrofitted specimens with different volumes of bamboo reinforcement were also considered. The results show that the application of bamboo reinforcement, at a proper volume, significantly increases the ultimate strength and the ductility of the masonry wall. Such results indicate that the brittle failure of UM structures can be avoided by means of bamboo retrofitting. [ABSTRACT FROM AUTHOR]

Umbulan Spring is an Indonesian groundwater spring with a historically significant discharge up to 3,500 L/s which has significantly declined in recent years, has significantly declined from 5,262 L/s since 1987. In terms of hydrogeology, this declination of discharge indicates a depletion of water resources in the groundwater basin. This study aimed to identify the cause of the depletion in the Umbulan Spring discharge by analysing the aquifer system using an integrated methodology of hydrochemical and geological approaches. Results show that groundwater that discharges via Umbulan Spring is of the same groundwater facies as that of the surrounding artesian wells, having CaMgHCO3-type water with a bicarbonate anion facies and no dominate cation facies. The range of groundwater isotopes from -7.8 to -8.2 ‰ for 18O and -46.2 to -50.5 ‰ for deuterium correspond to the local Bromo Meteoric Water Line (BMWL). Groundwater isotope correlation with the BMWL and the site elevation shows that the recharge area is located on the northern slope of the Bromo-Tengger Volcanic Complex between 1,300 – 2,300 m above sea level. The groundwater source flows through a lapilli tuff aquifer which is covered by impermeable pyroclastic flow deposits. Due to its determination as part of the same aquifer system as the artesian wells, the depletion of the Umbulan Spring discharge is related to the increasing number of artesian wells around the spring. [ABSTRACT FROM AUTHOR]