Flowchart summarizing the different steps composing a the data selection and b the stress tensor inversion from focal mechanisms. A new paradigm for interpreting stress inversions from focal. Origin and main features of the tector software system. Adaptive momenttensor joint inversion of clustered.
Stress orientations obtained from earthquake focal mechanisms. Vincecronin adraftprimeronfocalmechanismsolutionsforgeologists preface inrecentyears,ihavedevotedanincreasingamountofmyseeminglydecreasingresearchtimetothestudyof. Characteristics of focal mechanisms in chile subduction. A refined methodology for stress inversions of earthquake. Moment tensor inversion for the focal mechanism of the 12 earthquakes of the dongfang hainan earthquake swarm occurred from june to august 1992 with nearsource broadband data recorded by a temporal smallaperture network consisting of dcs302 digital threecomponent accelerographs. The method of gephart and forsyth 1984 usually provides more accurate estimates of stress orientation, especially for highquality data sets, but its confidence regions are in most cases too large. Focal mechanisms are released after an earthquake to show what type of earth movement produced the earthquake. The estimate for hr is more robust and produces a bias toward the stress rate of approximately 40%. Iterative joint inversion for stress and fault orientations. Fault plane solutions for the arava earthquake cluster, plotted in lower hemisphere stereographic projections. If the spatial mean stress and the stress rate are not aligned, the average focal mechanism failure mechanism should yield a stress estimate from stress inversions, approximately halfway between the two. The aftershock decay was very fast, and the focal mechanism of the strongest event with magnitude of 4.
Geological society, london, special publications, 212. Corresponding python code requires matplotlib, numpy, scipy and os modules. Focal mechanisms pacific northwest seismic network. The results of this control study show that focal mechanisms can be inverted to find the best stress tensor, but the resolution is decreased unless the fault planes can be picked a priori. Simulated focal mechanisms are made from slickenside data and used to provide a control study for the focal mechanism inversion algorithms. Home impressum legal information privacy statement how we.
Because focal mechanisms have fault plane ambiguity, an additional criterion is needed to select the fault planes used in the inversion. The main differences between existing stress inversion methods are related to the inversion algorithm and handling the fault plane ambiguity. The inversion procedure finds a reduced stress tensor, expressed in three euler angles and a shape factor, and a set of fault planes. Focal mechanisms and stress field in the atotsugawa fault area. Vince cronin a primer on focal mechanism solutions for geologists 312010 page 4 ing the vertical component at each station, we evaluate whether the first motion detected at that station was an up motion, a down motion, or no apparent signal at the expected time. If such information is missing, and faults and auxiliary nodal planes are interchanged, the stress inversions can produce inaccurate results. Tector tectonic analysis and stress inversion of fault slips, brittle structures and earthquake mechanisms. Matlab package for stress inversion from focal mechanisms. The stress estimates obtained by the focal mechanism. Focal mechanisms enter strike, dip and slip and see the resulting focal mechanism.
To perform stress tensor inversion, a number of focal mechanisms with different orientations are. Moment tensor inversion for the focal mechanism of the. Stressinverse is a matlab or python software package for an iterative joint inversion for stress and fault orientations from focal mechanisms. Synthetic tests reveal that pore pressure variations can cause apparent changes in the retrieved stress ratio r relating the magnitude of the intermediate principal stress with respect to the maximum and minimum principal stresses. Mar 06, 2019 satsi spatial and temporal stress inversion is a modified version of michaels jgr 1984, 1987 code that inverts focal mechanism data for a spatially andor temporally varying stress field. The program requires as input a gis point dataset of earthquake locations containing strike, dip. The focal mechanism of an earthquake is represented by a beachball diagram. Teleseismic waveform inversions can provide a focal mechanism for most earthquakes of magnitude larger than about 5.
These are typically shown using a socalled beachball diagram. The 2nd and 3rdorder stress patterns obtained as the result of formal stress inversion of lustrated focal mechanism solutions reveal many interesting complications in the stress eld. New insights into structural interpretation and modelling d. The inversion results show that the average misfit is small enough to satisfy the assumption of homogeneity in the focal mechanism data and that the 95% confidence regions of. The resolution can also be increased by including constraints on the magnitude of the tangential traction on the fault om author. We created a new tool, 3d focal mechanisms 3dfm, for viewing earthquake focal mechanism symbols three dimensionally. Msatsi is a matlab software package that allows performing the stress inversion using earthquake focal mechanisms. Aug 24, 2012 focal mechanisms are released after an earthquake to show what type of earth movement produced the earthquake.
A userfriendly interface is provided by the faultkin software, while. The focal mechanism can be derived from observing the pattern of first motions, that is, whether the first arriving p waves break up or down. A new inversion program is presented that determines the regional stress tensor and a set of focal mechanisms simultaneously, using pwave first motions as input data. Two possible perpendicular fault planes for mechanisms calculated from firstmotion directions as well as some methods that model waveforms, there is an ambiguity in identifying the fault plane on which slip. Online copy the first reference to the tensor program is an abstract presented at the egu in 1993. These two methods typically obtain similar stress ori.
The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. Abstract stress inversions from focal mechanisms require knowledge of which nodal plane is the fault. Tensor software for fault kinematic analysis and stress tensor inversion. Stressinverse is a matlab or python software package for an iterative. Stress inversion of regional seismicity in the sea of marmara. These two methods typically obtain similar stress orientations for similar focal mechanism data sets. Pdf iterative joint inversion for stress and fault. We have determined 151 high quality focal mechanism solutions for.
The stress inversion using earthquake focal mechanisms allows estimating. A matlab package for stress inversion combining solid classic methodology, a new simplified user handling, and a visualization tool by patricia martinezgarzon, grzegorz kwiatek, michele ickrath, and marco bohnhoff online material. Regarding the faulting mechanism of an earthquake with magnitude up. The matlab code needs just the matlab kernel and no toolboxes are required. Satsi spatial and temporal stress inversion is a modified version. Focal mechanism determination and identification of the fault. The software and synthetic focal mechanism catalogs used in this work can be obtained. Mean errors of the principal stress directions lefthand plot and of the shape ratio righthand plot as a function of the number of focal mechanisms. Satsi spatial and temporal stress inversion is a modified version of michaels jgr 1984, 1987 code that inverts focal mechanism data for a spatially andor temporally varying stress field. The second test indicated that, even though the focal mechanism of clustered microseismic events can have variations of focal mechanism parameters, the joint inversion method has a better convergent rate and yields an average focal mechanism reliably, but the individual inversion often converges to local minima. Pdf sensitivity of stress inversion of focal mechanisms. Slip vectors are indicated by closed circles for normal faulting, and open circles for reverse faulting.
Focal mechanisms are derived from a solution of the moment tensor for the earthquake, which itself is estimated by an analysis of observed seismic waveforms. The number of inversions for a mechanism is equal to the number of links in the mechanism. The fault plane in each focal mechanism is selected to minimize the slip misfit angle. Gifs for select segments included as optional download. A primer on focal mechanism solutions for geologists. Feb 06, 2018 inversion of a mechanism is obtained by fixing different links of the mechanism, one at a time. These are released after an earthquake to illustrate the type of faulting produced the earthquake. Stress tensor inversion from fault kinematic indicators and focal mechanism data. The reference frame of stress filed in central chile subduction is determined by the fmsi focal mechanism stress invevsion method with 222 focal mechanism solutions before the 2010 maule earthquake 22, the azimuths and dips of principal stress axes. In the case of a faultrelated event it refers to the orientation of the fault plane that slipped and the slip vector and is known as a faultplane solution. The stress tensor inversion also suggests local extensional stress field at a. This tool operates within the environmental systems research institute esri gis software arcscene 9. References related to the stress inversion methodology.
The inversion finds the least complex stress field model that is consistent with the data. However, the uncertainty estimates are often quite different, with the con. We investigate the sensitivity of stress inversion from focal mechanisms to pore pressure changes. Stress orientations obtained from earthquake focal. The new focal mechanism data and their analysis presented here allow and facilitate a revisiting of the tectonic interpretations of the stress eld patterns.
Software to download usgs earthquake hazard program. References related to the stress inversion methodology implemented in wintensor angelier, j. The 1982 swarm focal mechanisms are very similar to the solutions of the 2001 sequence. Sensitivity of stress inversion of focal mechanisms to pore pressure. Smith, free software helps map and display data, eos trans. Simultaneous focal mechanism and stress tensor inversion 271 9 events iz 2 events fig. Antelope and focal mechanism estimation from waveform. Simultaneous focal mechanism and stress tensor inversion. Pdf sensitivity of stress inversion of focal mechanisms to. A refined methodology for stress inversions of earthquake focal. To do this, we tested both methods on a suite of synthetic focal mechanism data sets containing random errors.
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