Microseismic Monitoring and Earthquakes II Posters
Presiding: M Kendall, University of Leeds; S Shapiro, Freie Universitaet Berlin
S41D-01 0830h
Application of Coherence Collapsing Relocation Method to Induced Microseismicity Collected at European and Australian Hot Dry Rock Sites
The authors have investigated a variation of the Collapsing method. The original Collapsing method (Jones and Stewart, JGR, 1997) is a mapping method for re-locating microseismicity that utilizes information from all the events in the data set. It has a reasonably good ability to emphasize the structure of the whole seismic cloud. However the original Collapsing method cannot resolve seismic structures with comparable sizes to the spatial distribution of the residual. This limitation of the original Collapsing method can cause difficulties in interpreting the hydraulic behaviour of a hot dry rock (HDR) reservoir. As a result, mapping techniques with higher resolutions, such as multiplet analysis and clustering analysis, have typically been used to delineate finer scale seismic structure. In the version of the Collapsing method introduced here the degree of coherence among the seismic traces is incorporated into the method. The new method is referred to as the 'Coherence Collapsing' method, and it aims to selectively relocate a group of multiplets to a common point. The Coherence Collapsing method can be used in semi-realtime to refine the locations of multiplets. The Coherence Collapsing method is based on the original Collapsing method, but the parameters in the analysis are optimized from synthetic studies to improve the reliability of the location of the multiplets. The method has been applied to the microseismic data sets collected during hydraulic stimulation at the HDR project sites in Soultz, France and Cooper Basin, Australia. Comparisons between locations derived from the Coherence Collapsing method and the single event determination/JHD method reveals a more structured distribution of events and less diffuse multiplet groups. The distribution of the multiplets are correlated with the dominant flow path estimated by integrated analysis of multiplets, logging, geological data and hydraulic records, suggesting that the Coherence Collapsing method can be effectively used for on-site monitoring of the reservoir creation process.
S41D-02 0830h
Micro-seismic Study for Central Part of Saudi Arabia y
Having experience with monitoring Earthquakes plus the fact that Riyadh is yconstantly subjected to Micro-earthquakes through out the year. Riyadh city and ycentral part of Saudi Arabia are located in the Arabian Plat which is known as a yrelativity stable platform. The Institute of Astronomy and Geophysics Research at yKing Abdulaziz City for Science and Technology installed 9 earthquake portable ystations in central part of Saudi Arabia to record micro-seismicty that caused by local yfaults and natural artificial. We recorded many events in study area mostly theses yevents were not felt. We used digital digitizer recorders ( RefTek 72A) in collecting ythe data and SAISAN software for analyses. The main objective of this study is to define the active faults in Riyadh city and surrounded area that can affect the city.
S41D-03 0830h
Variations of Southern California Seismicity: Empirical Evidence and Possible Physical Causes
We investigate on the seismicity of Southern California in order to characterize its time evolution during last decades. We analyze the time series composed by the number of events per year, and the focal mechanisms of the earthquakes. The results show a statistically significant nonstationarity with a change occurred in the sixties, in both time series. The seismicity occurred before the change point is strictly linked to the San Andreas fault system; after the sixties, the seismicity appears to be more scattered in terms of focal mechanisms and location, and with a lower seismic rate. Finally, we provide a possible physical explanation of the significant nonstationarity by modeling the postseismic stress perturbation field induced by two giant earthquakes occurred in the sixties, the Chile (1960) and Alaska (1964) earthquakes. At a first order, the postseismic stress rate seems to be in agreement with the changes in seismicity observed, supporting a causality hypothesis. The model also foretells the future behavior of the trend of seismicity in Southern California. This (forward) prediction gives an important opportunity to validate the causal hypothesis of remote (and long-term) coupling between earthquakes.
S41D-04 0830h
Characteristics of Seismicity Triggered Remotely by the Mw7.9 Denali Fault Earthquake
Within seconds to minutes following the arrival of large surface waves from the Mw 7.9 Denali Fault earthquake an unusual number of small earthquakes took place near many seismic stations throughout the western US and Canada to distances exceeding 4000 km. These remotely triggered earthquake sequences persisted with Omori-like time-decaying occurrence rates for hours or days. At first glance, remotely triggered seismicity seems to behave like aftershocks, but instead of being initiated by a local mainshock the sequences are triggered by dynamic deformations of the distant large earthquake. They are difficult to study because many appear to be small, and take place when large scattered signals from the mainshock are still arriving. We study these triggered earthquakes by high-pass filtering broadband seismic recordings from a number of seismic stations in the western Cordillera, and we "calibrate" our single-station observations with data from seismicity catalogs from regional and local networks, where possible. We are seeking to determine whether the remotely triggered earthquakes are restricted to a particular type of tectonic environment and whether their temporal decay reflects "secondary" aftershocks of a rather large triggered earthquake. Or, alternatively do they represent a more "excited" state of faults more generally in the environments where they are observed to take place? Preliminary results suggest that both magmatically active and inactive (though faulted) environments were triggered, and that the triggered sequences are not simply secondary aftershocks.
S41D-05 0830h
Numerical Modelling of the Triggering of Microseismicity in Enhanced Geothermal Systems Using a Discrete Fracture Network Approach
Heat extraction from deep engineered fractured formations is currently under investigation at the Soultz sous Forêts site with the support of the European Commission. The challenge is to develop a reservoir at great depth and to circulate a fluid in order to recover heat and produce electricity. The pilot project evolved toward a three well system at 5 km in depth with temperatures close to 200 C. Massive hydraulic tests performed to develop the reservoir have shown from the recorded micro-seismic signature, that fractures can easily be re-activated. The discussion now focusses on the hydraulic significance of the shear failure mechanism, considered as the source of the accoustic emissions. To improve our understanding of these coupled hydrau-mechanical processes, a numerical model was presented [1], based on a 3D random description of fracture networks. Local flow rules along equivalent 1D channels connecting the fractures can account for (i) a normal closure versus effective stress law together with (ii) a dilatant behaviour during shearing motion when a Mohr-Coulomb failure criterion is met. The purpose of the present work is to simulate injection tests in some synthetic fracture network using power law distributions for the fracture size, and to analyse the spatio-temporal growth of the sheared zones. Assuming that this process is analogue to the triggering of the microseismicity, we then perform an evaluation of the so called SBRC reservoir characterisation method [2] stating that the spatial position of the triggering front in an homogeneous isotropic poroelastic medium with a hydraulic diffusivity Dh is at time t given by √4 π Dh t. We conclude to its validity, although it is found sensitive to the hypothesis of critically stressed pre-existing fractures. The connectivity of the sub-set of subcritically oriented fractures plays a major role in the succes of a stimulation treatment and controls an equivalent macro-cohesion behaviour at the reservoir scale. The examples also confirm that the migration rate of the seismic front is not affected by the irreversible changes in hydraulic parameters experienced by the fractures in between the injection zone and the failure front. The DFN numerical procedure is shown appropriate to discuss the occurence of delayed seismic events, triggered in the far field after the injection phase has terminated. This direct approach seems well suited to predict the change in permeability resulting from a stimulation treatment and gives pressure estimates to properly stimulate a network made of several fracture sets, accounting from a given anisotropic stress tensor. [1] Bruel, D., (2002) Impact of induced thermal stress during circulation tests in an engineered fractured geothermal reservoir. Oil & Gas Science and Technology - Rev. IFP, 57, no. 5, 459-470 [2] Shapiro, S.A., Royer, J. and Audigane, P., (1999), Large scale in situ permeability tensor of rocks from induced microseismicity. Geophysical Journal International, 137, 207-213.
S41D-06 0830h
Effects of Topography on Earthquake Ground Motions Observed During the 2004 Parkfield Earthquake
During recent earthquakes such as 1994 Northridge, 1999 Izmit, 1999 Chi-Chi events, unusually large ground motions were recorded near the crests of ridges. Some concentrated patterns of structural damage relative to topography were also observed. These observations have suggested that surface topography significantly affect ground motions. However, the effects of topography often mix with the effects of path and geologic conditions. In order to properly quantify the topographic effects, crest motions must be evaluated relative to comparable free-field motions under same site conditions. The September 28, 2004 Parkfield earthquake (Mw 6) supplied an unprecedented opportunity of studying the topographic effects. It is an anticipated event that came 15 years later than predicted date. Very densed instruments had been installed in this region to capture the anticipated event. A large number of records are available from instruments installed on hills close to the epicenter. We will study the effects of topography on strong ground motions in this work. The main purpose of this study is to isolate the influence of the topographic effects from the effects of wave propagation, geologic amplification, as well as soil-structure interaction.
S41D-07 0830h
Analysis of 2003 SAROS Earthquake Sequence, North Eastern Aegean Region
In this study, we analyzed the aftershock activity following MW=5.8, 6th July 2003 Saros earthquake. The activity occurred along the axis of the Saros gulf between the depths of 7 km and 20 km. on the continuation of the North Anatolian Fault Zone (NAFZ) in the Gulf of Saros. Focal mechanism of the mainshock and largest aftershocks are almost pure right lateral strike slip with minor normal faulting. Strike directions are aligned with the axis of Saros depression and it's on continuation of the North Anatolian Fault System and the North Eastern Aegean Trough. Normal faulting is aligned with the diffuse earthquake activity at the south. Contrasting with that diffuse activity we observed sequence at the north which aligns through the deepest trough of the Saros depression.