[en] Coulomb failure criterion has been applied widely in the scope of earthquake science to explain earthquake interactions base on stress change, with the well-known method named Coulomb failure stress change ΔCFS). Preceding studies have showed: increase ΔCFS, depicted as positive stress lobes, has correlation with occurrence of following events. However in the calculation process, ratio between regional stress and earthquake stress drop would affect stress distribution. Based on preceding researches, earthquake stress drop with similar magnitude to regional stress, would give results positive stress lobes along and at the base of the fault. Those stress distribution, could explain events interaction and mechanism of earthquake. This work carries out synthetic modeling of static ΔCFS upon varying earthquake stress drop and regional stress using COULOMB3.3. In accord with preceding studies, the results show positive ΔCFS along the fault when stress drop is comparable to regional stress. And yet, positive ΔCFS would take place at the top and at the base of the fault, expanding to the center of the fault -where the hypocenter is assumed- as the stress drop reaching regional stress in magnitude. This could explain the separated clusters of aftershock in depth observed in some earthquakes. (paper)

[en] On 26 May 2006 at 22:53:59 UTC, an earthquake with moment magnitude of 6.4 occurred in Yogyakarta, Indonesia. The source of the event is still debatable. Some believe the event was caused by the reactivation of the Opak Fault which has a left-lateral type movement. Previous studies indicated there are two possibilities to explain the mechanism of the Yogyakarta earthquake. First is based on the focal mechanism from NIED (National Research Institute for Earth Science and Disaster) Japan which indicated that the event occurred in an oblique reverse slip. This model states that the complex Opak fault is a flower structure (strike-slip) type. Second is based on NEIC (National Earthquake Information Center) US which indicated that the event was caused by a pure strike-slip fault (active Opak fault). The May 26^th earthquake triggered many aftershock events around the old Opak fault. The majority of aftershock events on 3-6 June 2006 were located around 5 km east of Opak fault. It has a trendline of N45°E and lies parallel with the Opak fault. We use Coulomb Stress change to determine which type of source model fit better the aftershocks pattern. The target fault for Coulomb Stress analysis is a left lateral pure strike slip with an orientation of N45°E/90°SE. (paper)

[en] Geothermal surface exploration entails a multi-geoscientific process, which is aimed to define the geometry and characteristics of the geothermal reservoir prior to drilling. Lately, micro-seismic event monitoring is becoming a standard procedure in inferring the structure of the potential geothermal reservoir. However, a good coverage of seismic station and abundant seismic events must be fulfilled in order to map the subsurface structure. Taking advantage of the well-designed seismic station deployed at the “ARD” geothermal field prior to its first drilling, a study of micro-earthquake tomography for 3D reservoir structure is performed in this field. A seismic network of 26 stations was set up for more than eight months from August 2011 within 20 km radius from the centre of the expected reservoir. There were 637 micro-seismic events had been detected and located, which is a very high number of seismicity for a region that is not yet under geothermal development. The purpose of this study is to construct a 3D seismic velocity structure using double-difference tomography and to infer fluid properties, i.e. steam and brine, from the ratio of the P- and S-wave velocity. Double-difference tomography is used to its ability to reduce uncertainties of the model associated with picking and velocity structure. A zone with low P and S velocity anomaly as well as a low ratio of Vp/Vs, which is interpreted as the steam dominated reservoir, is observed. Below this reservoir, there is a high P and S velocity anomaly and a high ratio of Vp/Vs which may be correlate to the non-permeable rock. (paper)

[en] Understanding the physical mechanisms involved during hydraulic stimulation is a key parameter to estimate the fluid flow and permeability increase within the geothermal reservoir. An attempt to infer the mechanical behaviour of the XO geothermal reservoir during injection is performed in this study. Microseismic is a key method for monitoring the percolation of the fluid within reservoir during the injection activity. In this study, we aim to determine the 3D seismic velocity structure of the XO geothermal reservoir using microseismic tomography as well as analyse its mechanical changes due to hydraulic injection using the evolution of the microseismicity. We use the microseismic data from 13 and 16 stations deployed before and after the injection, respectively. A total of 2,827 microseismic events were recorded from 2010 to 2013, in which only 135 microearthquakes were recorded before injection and significantly increased to 2,692 events after hydraulic fracturing. To analyze the mechanical behavior of the reservoir, first the hypocenter location accuracy must improved by using a cross correlation master technique. Then hypocenter relocation as well as the velocity structure is inverted using double difference technique. We use tomographic double difference inversion to determine the structure of Vp, Vs and the ratio Vp/Vs. The results of the 3D velocity model together with the microseismic propagation are used to analyse the changes in mechanical behaviour that occur in the reservoir during and after the hydraulic injection. (paper)