INVSP gathers of local earthquake seismograms: an approach for modelling the upper crustal P and S velocity structure

Transcription

1 Geophys. J. Int. (2006) 166, doi: /j X x FAST TRACK PAPER INVSP gathers of local earthquake seismograms: an approach for modelling the upper crustal P and S velocity structure V. G. Krishna Scientist, /2, West Marredpally, Secunderabad , India. v gopalak@yahoo.com Accepted 2006 March 15. Received 2006 March 5; in original form 2005 January 3 SUMMARY A new approach is presented here for obtaining INVSP gathers of local earthquake seismograms in regions with adequate seismogenic depth range. The inverse vertical seismic profiling (INVSP) geometry is in principle quite similar to the up-hole surveys of shallow seismic exploration in which shots fired at various depths in boreholes are recorded on the surface at a fixed offset. Similarly, local earthquake seismograms of selected events at different hypocentral depths can be gathered into Constant offset INVSP sections for various stations of a mobile seismograph network. They can be modelled for P and S velocity structure to upper crustal depths at each of the stations in the network with the aid of synthetic seismogram computations using appropriate source mechanisms of the earthquakes used. An INVSP gather, obtained to a depth range of about 10 km at a station with 28 km constant offset, is modelled for the upper crustal P and S velocity structure in the Koyna-Warna seismic region in the western Indian shield to demonstrate this approach. GJI Seismology 1 INTRODUCTION Mobile seismograph networks in seismogenic regions provide a significantly large database of local earthquake seismograms. Record sections, similar to those obtained in seismic refraction profiling, are assembled for an average source depth (Krishna et al. 1999; Krishna, in preparation, 2006a,b) using the seismograms of earthquakes of nearly equal source depths (varying within their error limits) and similar source mechanisms (e.g. strike-slip or normal) and recorded at seismograph stations in a narrow azimuth range (varying within degrees). Specifically, the regions with adequate seismogenic depth range potentially offer a further possibility to obtain the local earthquake seismogram gathers in the inverse vertical seismic profiling (INVSP) geometry. These INVSP gathers especially illuminating the upper crustal structures, possibly to depths beyond the seismogenic depth range, are in principle quite similar to those acquired in up-hole surveys by shooting in boreholes at various depths and recording on the surface (Galperin 1985). Thus seismograms of a number of earthquakes of similar source mechanisms, with considerable depth range extending into the upper crust, all recorded by the same seismograph station lying within a narrow azimuth range from various epicentres and at nearly equal epicentral distances (varying within 3 4 km), can be assembled into a constant (or an average) offset INVSP gather for each recording site (Fig. 1a). Key words: INVSP gathers, Koyna-Warna seismic region, local earthquake seismograms, mobile seismograph networks, synthetic seismograms, upper crustal P and S velocities. Stroujkova & Malin (2000) assembled the seismograms from the Long Valley caldera in California for a few stations of the seismograph network in the region in the INVSP geometry (the authors refer as VSP), in a limited depth range of km. They modelled the arrival times and moveouts of the phases in the INVSPs by ray tracing. Traveltime and synthetic seismogram modelling of the INVSP gathers as presented here over a larger depth range of 2 10 km, reveal the P and S wave velocity structure of the upper crust beneath various stations of the mobile seismograph network. The INVSP technique is proposed here as a new tool for exploring the upper crustal structures using the local earthquake seismograms and illustrated in the Koyna-Warna seismic region of the Deccan volcanic province (DVP) in the western Indian shield. The INVSP record sections are modelled with the aid of reflectivity synthetic seismograms as shown in the following by refining the P and S velocity models as well as the source mechanisms of the selected earthquakes. 2 EXPERIMENT AND THE DATA SET A state-of-the-art digital seismograph network of 20 stations was deployed during to provide a reliable database for studying the Earth s structure and the earthquake processes in the Koyna region (Rai et al. 1999). Local, regional, and teleseismic events were recorded by 24-bit REFTEK/PASSCAL recorders equipped 148 C 2006 The Author

2 INVSP gathers of local earthquake seismograms 149 Figure 1. (a) Schematic diagram showing a stratified upper crustal model and the Constant offset INVSP geometry. Seismograms of local earthquakes at various hypocentral depths (thick dots) as recorded by a common seismograph station (thick downward arrow) at nearly equal epicentral distances are gathered and displayed at their depths (see Fig. 3). Ray paths of the upgoing (dashed) direct wave (pi or si, for a source in the ith layer), and the downgoing (continuous) primary reflections (pipj or sisj, for a source in the ith layer and a reflection on the jth interface) are shown (only a few ray codes are illustrated by labelling; p2, p2p4, p4p4). A number of other phases including conversions and various multiples are also possible as described in the text. Interfaces 1 4 are in between the upper crustal layers, and the interface T is the base of the Deccan Traps. Note that the reflected phases from sources at varying depths sample each interface at different points thus extending the subsurface coverage. (b) Ray paths of the phases pipfpj or sisfsj (see Table 2) for a source in the second layer (i = 2) and reflection on the free surface (f) followed by a reflection on the interface 1 (j = 1) and reaching the free surface. (c) Ray paths of the phases pipjpfpt or sisjsfst (see Table 2) for a source in the second layer (i = 2) and reflection on the interface 1 (j = 1) followed by a reflection within the first layer of the Deccan Traps (i.e. reflection on the free surface and the interface T) before terminating on the free surface. with three-component short-period sensors and GPS timing system. Fig. 2 shows some of the well-located earthquake epicentres and the ten seismograph stations in the initial deployment of the network in the Koyna-Warna seismic region. Local earthquakes recorded by at least six stations were used for mapping the seismogenic faults in the region. Statistical analysis of the data set of about 400 local earthquakes by Rai et al. (1999) reveals well-constrained estimates of the epicentral locations and the hypocentral depths for various events in the depth range extending to about 10 km of the seismogenic upper crust. The seismicity pattern in the region delineated by them starts as a single seismic tract in the north trending NE SW, further branching into two distinct zones with NW SE trend in the south (see inset in Fig. 2). The focal mechanisms obtained by Sharma (2000) in the Koyna-Warna seismic region distinctly reveal leftlateral strike-slip faulting in the north rapidly changing to normal faulting in the south. In the present study well-located earthquakes Figure 1. (Continued.) in the region are used with their hypocentral depth errors (ERZ) of 0.2 km. The epicentral data of the selected earthquakes are given in Table 1 for the seismograph station WR. A constant offset of 28 km is considered for obtaining the INVSP gather that is further modelled for the P and S velocity structure beneath the station. 3 THE INVSP GATHERS The significance of the INVSP technique for exploring the upper crustal structures has been realized using local earthquake seismograms, for a range of hypocentral depths extending to about 10 km, acquired at various stations from the seismograph network in the Koyna region. A sufficient number of seismograms with good S/N ratio from well-located earthquakes are available at various stations satisfying the following conditions: (i) the hypocentral depths of the selected earthquakes cover a considerable depth range, to at least 10 km in the upper crust, (ii) the epicentral distances of the station considered from all the earthquakes are nearly equal (varying within 3 4 km), (iii) the station lies in a narrow azimuth range (station azimuths varying within degrees) with respect to the epicentres of all the earthquakes, (iv) the source mechanisms of all the earthquakes considered are similar (e.g. strike-slip or normal), and (v) the earthquakes are of comparable magnitudes. The conditions (ii) (v) ensure viability of the 1-D velocity structures derived from traveltime and amplitude modelling of the INVSP gathers. The traveltime variations, if any, due to differences in 3-D velocity structure, may not be significant. Similarly, significant amplitude variations may not be caused by slight variations in the source parameters and the slightly different, but comparable, magnitudes of the earthquakes selected for the INVSP gathers. Krishna et al. (1999) presented an approach to assemble record sections of local earthquake seismograms at various epicentral

3 150 V. G. Krishna Figure 2. Location map of the Koyna-Warna seismic region showing some of the well-located epicentres and the seismograph stations (station WR seismograms are displayed here) for the INVSP gathers. The inset shows the seismicity pattern in the region. Table 1. Koyna-Warna earthquakes (EQ) data for the seismograms gather in the INVSP geometry. Station WR (Average offset used: 28.0 km). EQ No. Epicentre Station Epicentral Focal Magnitude Strike (deg) Dip (deg) Rake (deg) lat. long. azimuth distance depth ERZ (Mcoda) Initial Final Initial Final Initial Final (deg) (deg) (km) (km) (km) Strike, Dip, and Rake: Initial (after Sharma, 2000) and Final (present study). Note that the final set of source parameters are obtained after refining the P and S velocity models. distances for an average common source depth, the hypocentral depths of various events being nearly equal (varying within their error limits). A similar approach is used to assemble the record sections in the INVSP geometry for an average common offset of a station, using seismograms of local earthquakes of various hypocentral depths, the epicentral distances to the station being nearly equal (varying within 3 4 km). The INVSP geometry is similar to that shown in Fig. 1(a). Ray paths of the prominent upgoing and the downgoing waves, as are designated here, from the earthquake sources at varying depths in the upper crust and recorded by a seismograph station at a constant offset are shown in this figure, and the prominent ray codes considered are explained in Table 2. The upgoing waves essentially include both P and S direct waves, although some converted phases and multiples are possibly present due to interaction at various interfaces along the propagation paths. Similarly, the downgoing waves are dominantly the primary P as well as S reflections from various interfaces. Again the converted phases and a variety of multiples also constitute the downgoing wavefield. The upgoing direct waves from various sources are designated as pi and si, for P and S waves, for the source in the ith layer (see Fig. 1a). A number of such waves from different source depths constitute the P and S upgoing wavefield as shown in the INVSP record section (Fig. 3) displayed as traveltime versus depth. The downgoing reflected waves leaving various sources are designated as pipj and sisj, for P and S waves, for the source in the ith layer and a reflection on the jth interface (see Fig. 1a). Again a number of such waves constitute the downgoing wavefield as shown in the INVSP record section (Fig. 3). Thus Pj, j=1 4 phase is due to various pipj reflections and Sj, j=1 4 phase is due to various sisj reflections. In the present study modelling is focussed on the prominent P and S upgoing waves as well as the primary P and S reflections from various interfaces (Pj and Sj, j=1 4 phases as shown in Fig. 3) in the

4 Table 2. Ray codes recognized and modelled from the INVSP record section. Ray code Ray paths Remarks INVSP gathers of local earthquake seismograms 151 pi upgoing direct P waves from a see Fig. 1(a) for the ray path (dashed line), source in the ith layer, terminating several pi phases constitute the P phase in Fig. 3 on the free surface si same as above for the S waves same as above for the S phase in Fig. 3 pipj downgoing P waves from a source see Fig. 1(a) for the ray path (continuous line), in the ith layer, reflected on the jth several pipj phases constitute the Pj, j=1 4 interface and reaching the free surface phases in Fig. 3 sisj same as above for the S waves same as above for the Sj, j=1 4 phases in Fig. 3 pipfpj upgoing P waves from a source see Fig. 1(b) for the ray path (dotted line), in the ith layer, reflected on the free dotted traveltime curves in the P wave window surface followed by a reflection on in Fig. 3 (in the same order as the Pj, j=1 4 the jth interface and reaching the phases) are constituted by several pipfpj phases the free surface sisfsj same as above for the S waves same as above for the sisfsj phases, dotted travel time curves in the S wave window in Fig. 3 are in the same order as the Sj, j=1 4 phases pipjpfpt downgoing P waves from a source see Fig. 1(c) for the ray path (dashed line), dashed in the ith layer, reflected on the jth traveltime curves in the P wave window interface followed by a free surface in Fig. 3 (in the same order as the Pj, j=1 4 reflection and another reflection at phases) are constituted by several pipjpfpt the base of the Deccan Traps (T) phases before terminating on the free surface sisjsfst same as above for the S waves same as above for the sisjsfstphases, dashed traveltime curves in the S wave window in Fig. 3 are in the same order as the Sj, j=1 4 phases Note that the ray codes given above are by no means exhaustive (although these are the phases modelled here), a large number of other multiples and P-to-S and S-to-P conversions are also possible from potential interfaces in the stratified upper crust. downgoing wavefield. For the purpose of illustration and modelling, only two types of multiples are considered here for both P and S waves: (i) an upgoing direct wave from the source reflected at the free surface followed by reflections at various interfaces, (see Fig. 1b, the phase designated as pipfpj and sisfsj, for P and S waves, for the source in the ith layer and reflection on the jth interface), and (ii) a downgoing wave from the source reflected at an interface and again reflected at the free surface followed by a reflection at the base of the Deccan Traps (interface T) before reaching the recording station (see Fig. 1c, the phase designated as pipjpfpt and sisjsfst, for P and S waves, for the source in the ith layer and reflection on the jth interface). All these ray codes are given in Table 2. Subsequent model computations revealed that at least these two multiples in the later arrivals are significant in constraining the velocity-depth models. In order to assemble the seismogram sections in the INVSP geometry, essentially both the upgoing P and S waves are aligned. A range of P and S velocity models within acceptable limits (of about ±10 per cent) for the upper crustal depths are considered, with the models for the nearby 1993 Latur earthquake region (Krishna et al. 1999) as the starting models. For each set of the P and S velocity models traveltimes are generated for all the source depths being considered for the chosen constant offset INVSP gather. The resulting alignment of onsets of both P and S upgoing waves are examined. Appropriate traveltime corrections are made in order to adjust only for the individual epicentral distance variations of each seismogram with respect to the constant offset chosen for the INVSP gather. Differences, if any, in 3-D velocity structure sampled, are assumed to be insignificant and not considered for the traveltime corrections. This procedure is repeated by perturbing either or both the P and S velocity models until the best possible alignment is achieved (based on visual check) for the upgoing P and S waves. This approach is found to be very successful to assemble constant offset INVSP gathers using local earthquake seismograms as the hypocentral depths of the events considered are quite reliable. The assumption that the hypocentral depths of the well-located events are accurate enough seems reasonable from the INVSP gather obtained and thus no attempt has been made here to revise these depths. Fig. 3 illustrates the INVSP gather thus obtained for the station WR (offset 28.0 km) in the study region using seismograms of local earthquakes in the hypocentral depth range of 2 10 km. It is clearly evident from this INVSP section that the P and S phases (upgoing waves) are well aligned. It can also be seen from this figure that the later arrivals (prominently consisting of the downgoing reflected wavefield both in the P and S windows; Pj and Sj, j=1 4 phases) are also well aligned, which are modelled as shown in the following. 4 MODELLING AND THE RESULTS The INVSP gather of vertical component seismograms are band pass filtered (5 25 Hz as found appropriate) and plotted at the respective hypocentral depths with amplitudes trace normalized (Fig. 3). Traveltime modelling of the P and S phases as well as the later arriving phases Pj and Sj, j=1 4, consistent with the ray geometry in the stratified upper crust shown in Fig. 1(a), yielded preliminary models

5 152 V. G. Krishna Figure 3. INVSP gather of the local earthquake seismograms for the station WR at a common offset of 28.0 km. The seismograms are plotted at their respective hypocentral depths and the amplitudes trace normalized. Computed traveltime curves for various correlated phases are shown as described in the text. The phases P, S, Pj and Sj, j=1 4 are explained in Table 2. The dotted curves for a free surface reflection preceding the full path reflection from an interface (phases pipfpj or sisfsj, see Fig. 1b), and the dashed curves for a reflection within the first layer of the Deccan Traps following reflection from an interface (phases pipjpfpt or sisjsfst, see Fig. 1c), are in the same order as the Pj and the Sj phases. of the P and S velocity structure to the upper crustal depths. A large number of P and S velocity models giving acceptable traveltime fits (based on visual check) are further tested for the relative amplitudes fit by computing reflectivity synthetic seismograms (Kind 1985; Mueller 1985). A double-couple point source with the source-time function of Brustle and Mueller (1983) is used for various computations. The set of source parameters (strike, dip, and rake) given by Sharma (2000) are initially used but further modified as necessary for each of the seismograms in order to achieve a better fit of the relative amplitudes in the synthetics. Thus by a trial and error approach, initially for refining the P and S velocity models and later for refining the source parameters, the synthetic seismogram section shown in Fig. 4 is obtained. The inferred P and S velocity models for the station WR are shown in Fig. 5 along with the starting models (LATUR). The initial and final sets of source parameters of the individual events are listed in Table 1. The reflectivity synthetics, revealing an acceptable fit (based on visual check) to the observed seismograms, are plotted for the station WR similar to the INVSP gather and shown in Fig. 4. It may be seen from the synthetic INVSP gather that in addition to the Pand S phases and the primary reflection phases Pj and Sj, j=1 4, the associated free-surface multiple reflections considered here are also well revealed. In Fig. 3, the dotted line correlations (phases pipfpj and sisfsj) are recognized as due to a free-surface reflection of a direct wave from the source, preceding the full path reflection on Figure 4. Synthetic INVSP gather of the reflectivity seismograms obtained for the station WR at a common offset of 28.0 km. P and S velocity models (shown in Fig. 5) are refined by a trial and error approach, further the source parameters are adjusted to improve the amplitudes fit. The synthetics fit presented here is by a visual check of the relative amplitudes. The seismograms are plotted at the same hypocentral depths and amplitudes are trace normalized. The direct and the later arrivals fit including those represented by the dotted and the dashed traveltime curves seem to be acceptable being consistent with the INVSP section in Fig. 3. an upper crustal interface (ray path as in Fig. 1b), while the dashed line correlations (phases pipjpfpt and sisjsfst) are recognized as due to an additional reflection in the first layer (on the interface T, the base of the Deccan Traps) near the recording station following a full path reflection from an upper crustal interface (ray path as in Fig. 1c). These two phases are also revealed in the synthetic INVSP gather for the station WR shown in Fig. 4. Therefore, it may be reasonable to believe that the synthetics simulate well the prominent phases recognized in the observed seismograms. It is possible that the thickness and the velocity structure of the first layer (Deccan Traps) may be slightly different from the model considered here. This may cause some misfit of the computed traveltimes for the dotted and dashed lines illustrated, although their trends are clearly revealed. Since the available sources are all deeper than 2 km (Table 1), the shallower structure may not be well constrained. 5 DISCUSSION AND CONCLUSIONS The INVSP technique presented here is quite promising and 1-D models of the P and S velocity structure can be obtained at several locations within the seismogenic regions deploying dense mobile seismograph networks. It is clear that narrower ranges in the conditions (ii) (v) given in Section 3 for data selection, develop the resulting INVSP gathers more comparable to those acquired in exploration seismics by up-hole surveys (INverted VSP). Nevertheless, the feasibility of adopting these conditions and constraining

6 INVSP gathers of local earthquake seismograms 153 Figure 5. P and S velocity models for the upper crust beneath the station WR inferred from modelling the INVSP gather. The inferred P velocity model KOYNA I from modelling the seismic wide-angle reflection/refraction data set in the region (Krishna et al. 1989) and the P and S velocity models LATUR (used here as the starting models) in the 1993 Latur earthquake region from modelling the aftershock seismograms (Krishna et al. 1999) are also shown for comparison. reflection/refraction sections. The alternating LVLs inferred in the present study seem to be consistent with a rheological stratification of the crust inferred in this region (Krishna et al. 1989). The similarity of the inferred upper crustal velocity model, with only slight variations, with that of the nearby 1993 Latur earthquake region (Krishna et al. 1999) suggests that these models are applicable in the Koyna-Warna seismic region as well. Further developments of the INVSP technique proposed here may consider this modelling procedure for its possible implementation into an efficient inversion scheme enabling an exhaustive search of the appropriate set of model parameters (velocity and attenuation models, as well as the source parameters) and quantifying goodness of the fits obtained. However, it is beyond the scope of the present communication as it is primarily intended to introduce the INVSP gather as a new approach towards utilizing the large data sets of local earthquake seismograms from mobile seismograph networks and obtaining the upper crustal P and S velocity models. With the availability of high-quality/high-density data sets in the INVSP gathers to upper crustal depths, they may be processed by the standard seismic software packages for the VSP processing of exploration seismics thus leading to reflectivity images of the upper crust. However, in order to apply these processing techniques, the data coverage of the available earthquake hypocentres and the seismograph network density will have to be almost similar to that of the exploration VSP surveys. A large number of the 1-D velocity models obtained by the INVSP gathers at several stations of the seismograph network can further be used with advantage to construct the 3-D velocity images in the region. The INVSP technique may also find similar applications for obtaining the P and S velocity models even to the upper mantle depths in regions with adequate depth distribution of seismicity (e.g. subduction zones and regions of the intracontinental/intraplate seismicity). data selection using the currently available high-quality earthquake data sets from the modern seismograph networks certainly ensures the desirable quality of the resulting INVSP gathers as obtained here. The INVSP technique can be used more effectively with increasing availability of high-quality/high-density local earthquake data sets in the seismogenic regions. The ranges set here to various conditions for data selection may possibly be made narrower and the INVSP gathers may be obtainable for more than one constant offset at each seismograph station as larger data sets are available. The viability of the inferred models can be checked more effectively if the INVSP gathers are available for multiple offsets at each station. The P and S velocity models for the station WR, inferred by traveltime and amplitude modelling of the INVSP gather are shown in Fig. 5. The inferred models are however based on visual check of the traveltime and amplitude fits and they seem to be reasonable for the available data set. These models reveal alternating low-velocity layers (LVLs) in the upper crust at depths of km and km with velocity reduction of 5 6 per cent for P and 7 9 per cent for S waves. A large number of record sections assembled for different average source depths of the local earthquake seismograms in this region, similar to those usually acquired by seismic refraction profiling, to offsets of km also substantiate these velocity models (Krishna, in preparation, 2006b), their interpretation being reserved for a forthcoming communication. The upper crustal LVL in the Koyna region shown in Fig. 5 (model KOYNA I), is well revealed from an earlier modelling of the seismic wide-angle ACKNOWLEDGMENTS The author is thankful to both the anonymous reviewers and Dr Andrew Curtis for their encouraging reviews and constructive suggestions to improve on an earlier version of the manuscript. Prof Dr Friedemann Wenzel of the Geophysical Institute, Univ. Karlsruhe, Germany, contributed by helpful discussions. Dr S. S. Rai of NGRI made available the digital seismograms data of the local earthquakes recorded at the station WR illustrated here. 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