AFTERSHOCK UNPUBLISHED CHAPTER PDF

Since the earlier times of documented seismological observations, it was noticed that an earthquake usually a large one was followed by a sequence of many smaller earthquakes, originating in the epicentral region; the first, larger, earthquake is called the mainshock , or main shock or main event , and the following, smaller, earthquakes are called aftershocks. These sequences, and their spatial and temporal distributions, depend on the characteristics of the mainshock and on the physical properties and the state of stress, strain, temperature, etc. Thus, the simple definition of aftershock as an earthquake occurring after a mainshock and in its epicentral region, although implying some causal relation with the mainshock, is partly semantic and largely circumstantial. Indeed, smallish earthquakes that constitute the background seismicity occur all the time in a seismic region in the absence of large events, and continue occurring whether or not a large earthquake occurs, so that not all earthquakes occurring in the region after a mainshock are necessarily aftershocks. If aftershocks are a result of the occurrence of the mainshock, then they should be related in a physical way with its rupture process.

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Since the earlier times of documented seismological observations, it was noticed that an earthquake usually a large one was followed by a sequence of many smaller earthquakes, originating in the epicentral region; the first, larger, earthquake is called the mainshock , or main shock or main event , and the following, smaller, earthquakes are called aftershocks.

These sequences, and their spatial and temporal distributions, depend on the characteristics of the mainshock and on the physical properties and the state of stress, strain, temperature, etc.

Thus, the simple definition of aftershock as an earthquake occurring after a mainshock and in its epicentral region, although implying some causal relation with the mainshock, is partly semantic and largely circumstantial. Indeed, smallish earthquakes that constitute the background seismicity occur all the time in a seismic region in the absence of large events, and continue occurring whether or not a large earthquake occurs, so that not all earthquakes occurring in the region after a mainshock are necessarily aftershocks.

If aftershocks are a result of the occurrence of the mainshock, then they should be related in a physical way with its rupture process. Kisslinger qualitatively defines three kinds of aftershocks: Class 1 is those occurring on the ruptured area of the fault plane or on a thin band around it.

Class 2 is events that occur on the same fault but outside of the co-seismic ruptured area. Class 3 is events occurring elsewhere, on faults other than the one ruptured by the mainshock; these events, whether in the same region or not, will not be considered here as aftershocks, but rather will be classified as triggered earthquakes.

The number of aftershocks decreases with time after the mainshock according to the modified Omori relation Utsu, , , as. Commonly, p ranges from 0. Among other reasons why aftershock identification is important, we can mention the following few examples.

Aftershocks can give important information about the rupture area; also, from estimations of co-seismic slip on the fault plane by inversion of seismic waves, several authors have found that aftershocks are scarce in areas of maximum slip and concentrate around their edges Dreger et al. Aftershocks can also yield information about the properties of the epicentral region Knopoff et al.

Since aftershocks can be large enough to contribute to the damage particularly after structures have been debilitated by a mainshock , it is important to estimate the hazard associated with aftershock activity Felzer et al. It has been proposed that, since aftershock activity depends among other factors on the stress status of the region, there is research on whether some characteristics of the aftershock activity from intermediate-sized earthquakes can be useful as precursory data for large earthquake hazard estimation e.

Jones, ; Keilis-Borok et al. Finally, for some studies concerned with large earthquakes, aftershocks can be considered as noise, and have to be eliminated from the catalogs e. In order to use or eliminate aftershocks it is first necessary to identify them.

Other methods include recognizing some statistical property e. Our method includes some of the above mentioned techniques used to discard events which cannot be aftershocks, and then proceeds to identify aftershocks based on the physical model of a rupture plane and on recognized statistical relationships. An early unsophisticated application of the rupture plane model, which proved that this principle of aftershock identification was feasible, was part of an unpublished MSc.

We work with seismic catalogs containing occurrence time days , hypocentral x East , y North , and z up , and, optionally, horizontal and vertical location uncertainties u h and u v , all these in kilometers. If location uncertainties are not included in the catalog, optional horizontal and vertical uncertainties are assigned equally to all events. Any events occurring before the event with the largest magnitude M max , the mainshock, are eliminated.

All spatial coordinates are then referred to those of the mainshock. The extent of the aftershock area depends on the energy, i. A first rough spatial discrimination, based on an average of the empirical magnitude M vs.

Next, a spatial clustering analysis, where events separated by no more than a given critical distance r , of the order of hundreds of meters to a few kilometers, depending on the spatial coverage of the catalog, are considered to be related, is used to eliminate events which do not relate to the mainshock or to other possible aftershocks.

When aftershock occurrence shows gaps comparable to those characterizing the background seismicity, we can consider that the aftershock activity is, if not ended, at least scarce enough to be comparable to the background activity and can no longer be distinguished from it. Events occurring after a critical gap are discarded from the possible aftershocks. Next, plane fitting is carried out iteratively; at each iteration, a plane that passes through the mainshock hypocenter is fitted to all remaining events, through a genetic scheme described below, and fit outliers events too far away from the plane are discarded.

Iteration continues until the goodness-of-fit criterion is met successful fit or until a preset maximum number of iterations is attained unsuccessful fit. The t g a p , c , and p values can be refined using the final results of a first, tentative aftershock determination, to do a second one.

Errors are computed for the children and the N p best fits among the whole population, parents plus children, are chosen as the parents for the next generation. The process is repeated until the goodness of fit criterion is met and the process ends or until a preset number of generations is attained. Those events with.

This method has been implemented as a Matlab program, aftplane. A definite advantage of using Matlab for this algorithm is that both data and trial models are handled as matrices, so that rotating, sorting, and identifying values is done more efficiently and with less lines of code than would be possible in other programming languages like FORTRAN or BASIC. A variation of this method is used as a function by program cleancat. Thus, for mainshocks occurring at times t i , instead of the total Omori number of aftershocks Utsu, ; Ogata, The largest events in the catalog are plotted vs.

The program iterates the whole process, as many times as needed, until no more aftershocks are found. In cleancat , parameters are not set interactively, but can be easily adjusted in a list of adjustable parameters at the beginning of the code. We will now show some examples of the application of the method. The aftplane program will be used to identify fault planes and aftershocks from three mainshock-aftershock sequences from two different parts of the world featuring different tectonic environments.

The cleancat program will be used to clean the catalog of a fault system. We chose these events as illustration because, although both events have mainly strike-slip mechanisms, they have slightly different strikes and dips, so that we wanted to test whether the method could identify these small differences.

Figure 1 shows the location of the study area in souther California, USA, and its recent seismicity; the faults ruptured during the Joshua Tree and Landers earthquakes are located within the red diamond. Seismicity map of southern California showing the location of the Joshua Tree and Landers faults both within the red diamond.

Modified from Lin et al, The catalog for the Joshuea tree earthquake contained events spanning Figure 2 right shows as blue circles the shocks identified as clustering with the main event.

Joshua Tree mainshock plus acceptable aftershock candidates left and Joshua Tree mainshock plus clustered aftershock candidates right. Joshua Tree mainshock red asterisk plus aftershocks blue diamonds ; left: plan view showing The main Joshua Tree shock and the identified aftershocks are shown in figure 3 , both in a plan view left which clearly shows the resulting The values found by aftplane agree extremely well with those estimated by Velasco et al.

Figure 4 shows a cross section parallel to the fault plane, illustrating aftershock concentrations. Joshua Tree mainshock red asterisk plus aftershocks diamonds , cross section seen along azimuth The catalog for the Landers earthquake contained 49, events spanning 4, Figure 5 right shows as blue circles the 12, shocks identified as clustering with the main event.

Landers mainshock plus acceptable aftershock candidates left and Landers mainshock plus clustered aftershock candidates right. The main Landers shock and the identified 3, aftershocks are shown in figure 6 , in a cross section seen along the determined Figure 7 shows a cross section parallel to the fault plane, illustrating aftershock concentrations.

Landers mainshock red asterisk plus aftershocks blue diamonds , view along The location of the mainshock hypocenter is obscured by those of the aftershocks. Dip Figure 8 shows the location of the study area, the mainshock epicenter red star and the subsequent seismicity recorded and located by the Colima Seismic Network RESCO. Figure 9 right shows as blue circles the 7, shocks identified as clustering with the main event. Figure 11 shows a cross section parallel to the fault plane, illustrating aftershock concentrations.

To illustrate the use of program cleancat we chose the catalog covering the whole Joshua Tree-Landers fault system Figure 1 , because this is a system with many close-lying, subparallel, faults, which gives scope to the iterative aftershock recognition scheme of the program.

Figure 11 shows all events in the catalog black crosses , and identified aftershocks as yellow circles. Total processing consisted of 10 iterations which identified and eliminated 11,, 4,, 1,, 86, 94, 30, 80, 18, 49, and 1 aftershocks, respectively, for a total of 17, aftershocks. Joshua Tree- Landers fault system seismicity black crosses and identified aftershocks yellow circles. Joshua Tree-Landers fault system seismicity versus time.

The top panel shows the largest events in the period blue circles with vertical lines. The middle and bottom panels show seismicity rates, for 46 day-long time intervals, before middle and after bottom processing by cleancat, respectively; note the different vertical scales. The method has been tried on various catalogs with good results and, when aftershocks are numerous enough, good estimates of rupture planes that agree very well with those reported in the literature.

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Nava, V. Downloaded: Nava Seismology Dept. The method. Aftplane: Joshua Tree earthquake The catalog for the Joshuea tree earthquake contained events spanning Aftplane: Landers earthquake The catalog for the Landers earthquake contained 49, events spanning 4, Cleancat: whole Joshua Tree-Landers fault system To illustrate the use of program cleancat we chose the catalog covering the whole Joshua Tree-Landers fault system Figure 1 , because this is a system with many close-lying, subparallel, faults, which gives scope to the iterative aftershock recognition scheme of the program.

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