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Second draft version 5th October 1995 Program VELEST USER S GUIDE - Short Intro

Second draft version 5th October 1995 Program VELEST USER S GUIDE - Short Introduction E. Kissling Institute of Geophysics, ETH Zuerich This short introduction corresponds to the VELEST Version 3.1 (10.4.95) by: Edi KISSLING, Urs KRADOLFER, and Hansrudi MAURER Institute of Geophysics and Swiss Seismological Service, ETH-Hoenggerberg CH-8093 Zurich, Switzerland Phone: +41 1 633 2605; Fax : +41 1 633 1065; Telex: 823480 eheb ch E-Mail addresses: KISS@TOMO.IFG.ETHZ.CH (Internet) KRADOLFER@SEISMO.IFG.ETHZ.CH MAURER@SEISMO.IFG.ETHZ.CH 1. Purpose and history of VELEST Program VELEST is a FORTRAN77 routine that has been designed to derive 1- D velocity models for earthquake location procedures and as initial reference models for seismic tomography (Kissling 1988; Kissling et al. 1994). Originally written in 1976 by W.L. Ellsworth and S. Roecker for seismic tomography studies (under program name HYPO2D, see Ellsworth 1977; Roecker, 1978) VELEST has been modified by R. Nowack (who also gave it its current name), C. Thurber, and R. Comer who implemented (among other things) the layered- model raytracer (Thurber 1981). In 1984 E. Kissling and W. L. Ellsworth after modifications of the flow structure and implementation of several new options used it to calculate a 'Minimum 1-D model' (i.e., a well-suited 1-D velocity model for earthquake location and for 3-D seismic tomography) for Long Valley area, California (Kissling et al., 1984). Since then VELEST has been applied to local earthquake and controled-source data from several areas in California, Alaska, Wyoming, Utah, and the Alps (Kissling 1988; Kradolfer 1989; Kissling and Lahr 1991; Maurer 1993). U. Kradolfer implemented the option to use VELEST as single-event-location routine and H. Maurer reintroduced the option to use both P- and S-wave data, separately or combined. Thanks to the cleaning efforts of U. Kradolfer and of H. Maurer the code no longer suffers from quick- and-risky programming by too many authors. The current version of VELEST has been tested to correctly run under both UNIX (HP and SUN) and VMS (DEC) operating systems with (optimized) standard FORTRAN77 compilers. If you need a reference please use Kissling et al. (1994) (see reference list) or the appropriate of the ones mentioned above. In order to avoid misunderstandings it is recommended that you also mention the version of the program. 2. Simultaneous Inversion and Coupled Hypocenter-Velocity Model Problem This USER S GUIDE to VELEST is intended to provide a brief introduction on how to run and use the program to simultaneously locate earthquakes and to calculate 1-D (layered) velocity models with station corrections. As an introduction to the coupled hypocenter-velocity model problem the reader is referred to Crosson (1976), Ellsworth (1977), and Thurber (1981). The concept of a 'Minimum 1-D model' is described in detail by Kissling (1988) and Kissling et al. (1995), who also provide several examples of applications of such 1-D velocity models for both improved earthquake locations and as initial reference models for 3-D seismic tomography. Kissling et al. (1994) -after describing the effects of initial reference models on 3-D tomographic results- in an appendix provide a brief overview over the procedure to be followed to calculate a 'Minimum 1-D model' with VELEST. The following USER S GUIDE is written under the assumption that the reader is vaguely familiar with the problems and solutions described in the above mentioned publications (see reference list). Here only a brief overview will be given. Program VELEST (iteratively, i.e., "non-linearly") solves * in 'simultaneous mode': the coupled hypocenter-velocity model problem for local earthquakes, quarry blasts, and shots; for fixed velocity model and station corrections VELEST in simultaneous mode performs the Joint-Hypocenter-Determination (JHD). * in 'single-event-mode': the location problem for local earthquakes, blasts, and shots. The model consists of a (layered) 1-D velocity model and station corrections. In both modes the forward problem is solved by ray tracing from source to receiver, computing the direct, refracted, and (optionally) the reflected rays passing through the 1-D model. In both modes the inverse problem is solved by full inversion of the damped least squares matrix [AtA+L] (A= Jacobi matrix, At=transposed Jacobi matrix; L=damping matrix). Because the inverse problem is non-linear, the solution is obtained iteratively, where one iteration consists of solving both the complete forward problem and the complete inverse problem once (see Fig. 1). In 'single-event-mode' an additional singular value decomposition of the symmetric matrix AtA (in this case a 4*4 matrix only) is performed in order to calculate the eigenvalues. Due to the intrinsic ambiguity of the inverse problem the final solutions obtained by VELEST are but a small part -and often the least significant part- of all the output of this program. VELEST has been designed to allow great flexibility in the approach and, therefore, a large number of options and control parameters must be set and properly adjusted in the process. Though default values may be obtained from the examples provided with the source code, the calculation of a Minimum 1-D model requires multiple runs with VELEST to select and test control parameters appropriate to the data set and to the problem. If you are in a hurry or if you are used to the idea that proper computer programs always provide a unique solution that is either obviously inappropriate or precise and correct, please accept the warning: To calculate a Minimum 1-D model a single or even a few VELEST runs are useless, as they normally do not provide any information on the model space (see chapt. 4)! The principal procedure for simultaneous mode (and with minor modifica- tions also for single_event_mode) is summarized in the flowchart given in Fig. 1. The main (print) output of VELEST reflects this procedure and provides detailed information about many intermediate calculation steps even within one single iteration step. It is mainly from these intermediate results that the appropriate control parameters are obtained through many runs of VELEST. Please do not use resolution calculation in simultaneous mode. There is a bug in this part that will be fixed in version VELEST 3.2 by end of 1995. Fig. 1 Overview of procedure VELEST 3. Installation of FORTRAN program VELEST: VELEST version 3.1 may be obtained for UNIX (SUN and HP) machines and for VAX/VMS (DEC) systems. Please check that you get the appropriate source code among the files - VELESTUNIXSUN.F - VELESTUNIXHP.F - VELESTVMS.F . In each case you also need the file VEL_COM.F that contains the common bloc variables. The difference between the UNIX and VMS version are the I/O - file attachments ("open" - calls) only. Thus, in the UNIX versions you will find aside of each active line for opening call an additional line beginning with "cvms" with the open calls in the VMS-system and vice versa in the VMS version are lines beginning with "cunix" and the open calls for UNIX. The only system-dependent calls are found in the two subroutines DATETIME and CPUTIMER; so if the program should be moved onto another machine, only these two small subroutines located at end of the source code have to be changed. Alternatively, delete all calls to these two routines which are not essential for the main calculations (they just provide run-time information). VELEST is currently set up to invert data in simultaneous mode from a maxi- mum of 658 earthquakes (iepmax=658) and max. 50 shots or quary blasts (inshotsmax=50) with max. 500 observations per event (maxobsperevent =500). Current setting also allow a max. number of 500 stations in the station list (istmax=500), max. 2 velocity models (itotmodels=2) with max. 100 layers per velocity model (inltot=100). If you decide to need other dimensions please only adjust the parameters at the beginning of the file VEL_COM.F and compile the source with an (optimized) F77 compiler. In addition to the source you should also get a set of files providing examples of input and output data for simultaneous mode. To each example there belong 5 types of files: Input: - *.cmn (control parameters) - *.sta (station list) - *.mod (initial velocity model) - *.cnv (local earthquake data) Output: - *.out (main print output) 4. Calculation of Minimum 1-D model (simultaneous mode) Solutions to the coupled hypocenter-velocity model problem consist of the hypocenters, the velocity model, and station corrections. Each such solution may be rated by comparing its corresponding (calculated) travel times with the measured (observed) travel times. These travel time differences are called the misfit (or residuals) of the solution and we may measure the total misfit by using any Norm. Most often the RMS (root-mean-squared)-misfit of the solution is used. Consider any possible combination of hypocenters, velocity model, and station corrections be rated by its RMS misfit for a well-posed problem that would only have one solution with minimal RMS. Such a situation could be represented by Figure 2a. In the case of the coupled problem with local earthquake data, however, such a diagram displaying the RMS misfit for various solutions most probably looks more like Figure 2b, where serveral local RMS minima occur. In such situations the solution obtained by any iterative algorithm strongly depends on the initial model and initial hypocenter locations. uploads/Litterature/ usr-guide.pdf