Station Corrections Research Articles

Reliability of earthquake location procedures in heterogeneous areas: synthetic tests in the South Western Alps, Italy

The accuracy of procedures for locating earthquakes and tomographically imaging 3-D velocity structures, is influenced by factors such as network geometry, errors in picking seismic phases, modeling errors and velocity model uncertainties. The main purpose of this study is to investigate the importance of different error sources through synthetic experiments. The synthetic data sets were generated for a realistic 3-D velocity model of the South Western Alps adopting a precise raytracing technique. The actual station configuration and recorded seismicity during the passive seismological experiment Geofrance3D-Alps, conducted between August 1996 and February 1997, were used. The station configuration corresponded to an enhancement of the regional networks operating in the area, with the addition of temporary stations. In addition, synthetic data sets were corrupted by applying different error schemes. Furthermore, to assess the locating capabilities of the regional networks, synthetic data sets were constructed for synthetic events placed in correspondence to the nodes of a regular three dimensional (3-D) grid. We mimicked the actual data management scheme for synthetic data processing: we re-located the earthquakes, performed an inversion for a minimum 1-D velocity model with station corrections and, then, ran a complete 3-D velocity model inversion. One of the main results, confirming earlier studies was that earthquake locations in strongly heterogeneous areas, obtained using a regional 1-D model, can have significant errors. Even a minimum 1-D model with station corrections does not significantly improve the accuracy of parameter locations in the South Western Alps. For high precision locations, a 3-D model is needed. However, a 3-D model cannot be reconstructed from noise contaminated data recorded by a sparse regional network.

A focused look at the Alpine fault, New Zealand: Seismicity, focal mechanisms, and stress observations

The Alpine fault is the Pacific‐Australian plate boundary in the South Island of New Zealand. This study analyzes 195 earthquakes recorded during the 6 month duration of the Southern Alps Passive Seismic Experiment (SAPSE) in 1995/1996 and two ML 5.0 earthquakes and aftershocks in 1997, which occurred close to the central part of the Alpine fault. Precise earthquake locations are derived by simultaneous inversion for hypocenter parameters, a one‐dimensional velocity model, and station corrections. Together with focal mechanisms calculated using a first motion and amplitude ratio method, these results provide a picture of the seismotectonics in the central South Island over a 6 month period. Moment tensor inversions of three earthquakes provide an independent means of comparison to the focal mechanisms derived using the amplitude/first motion method. To validate our observations over time, we compare the SAPSE seismicity with the seismicity recorded by the New Zealand National Seismic Network (NZNSN) and a local network at Lake Pukaki east of the Southern Alps (6 months versus 8 years). Our study indicates that the Alpine fault releases elastic strain seismically from the surface down to 10–12 km depth between Milford Sound in the south and the Hope fault in the north. The seismicity rate of the Alpine fault is low but comparable to locked sections of the San Andreas fault, with large earthquakes expected. Seismicity decreases north of Bruce Bay at the Alpine fault and within a triangular region along the Alpine fault located between the Hope and Porters Pass fault zones. We interpret this as the result of deformation distributed on the Alpine fault and the Hope and Porters Pass fault zones. The base of the seismogenic zone is fairly uniform at 12 km ± 2 km over large parts of the South Island. The high Alps region has a shallower base of the seismogenic zone, indicating localized elevated temperatures east of the Alpine fault. Most of the study region deforms under a uniform stress field with a maximum principal horizontal shortening direction of 110°–120°, similar to geodetic observations and plate motions. This confirms that the region is not undergoing strain partitioning. The earthquake data show that the deformation away from the Alpine fault is distributed on mainly NNE trending thrust faults and strike‐slip transfer faults with a maximum seismogenic depth of 12 km.

Spatial distribution of focal mechanisms for interplate and intraplate earthquakes associated with the subducting Pacific plate beneath the northeastern Japan arc: A triple‐planed deep seismic zone

The northeastern Japan arc is located in one of the most seismically active subduction zones in the world. In this study, we relocated hypocenters and determined focal mechanisms of small earthquakes (M≤5) beneath the arc in order to investigate in detail the stress distribution in and around the descending oceanic plate. In the hypocenter relocation we adopted a “source region station correction method” in which station corrections vary with hypocenter locations. We have developed a new focal mechanism determination technique named “master solution method,” which is analogous to the “master event method” in hypocenter determination. We applied the method to P and SH wave amplitude data to obtain 1106 focal mechanism solutions. From the new mechanism solutions and relocated hypocenters we found that there occur both low‐angle thrust fault (LT) type and downdip compression (DC) type earthquakes at depths from 40 to 70 km near the aseismic front; the DC type events are underlying the LT‐type events. Almost all the earthquake clusters are composed of LT‐type events. The western limit of the region where LT‐type events have occurred is subparallel to the trench axis, although it undulates considerably; it delineates the westernend of the active region of interplate seismicity. Furthermore, we found that normal fault (NF) type events also occur at depths from 70 km in the upper plane of the double‐planed deep seismic zone, which is characterized mainly by DC‐type event. These NF‐type events are distributed only in a thin uppermost portion of the slab close to the plate boundary. Below these, in the lower plane, are downdip extension (DE) type events. This result indicates that the deep seismic zone in the northeastern Japan arc is not double‐planed but triple‐planed, even beneath the land area, which cannot be explained by any simple models.

Source Specific Station Corrections for Regional Phases at Fennoscandian Stations

The IASPEI91 global travel-time curves are used as the default for event location at the Prototype International Data Center (PIDC). In order to improve event location, a 1-D Baltic travel-time model was implemented at the PIDC in 1997 for locating events using regional phases from Fennoscandian stations. Where a single model is insufficient for characterizing the regional geology, path-dependent corrections, or Source Specific Station Corrections (SSSCs), are more appropriate for event locations. We have developed SSSCs for regional phases at the Fennoscandian stations by interpolating travel times through different 1-D models. SSSCs for stations NRIS and SPITS are also derived, given the fact that paths from both stations to high latitude events are within the Fennoscandia regionalization as Baltic.

Crustal structure and local seismicity in Colombia

Using P-wave travel time data from local seismicity, the crustal structure ofthe central and southern part of Colombia was determined. A very stableand narrow range of possible velocity models for the region was obtainedusing travel time inversion. This range of models was tested with earthquakelocations to select the best velocity model. The 1D velocity modelproposed has five layers over a halfspace, with interfaces at depths of 4,25, 32, 40 and 100 km and P-wave velocities of 4.8, 6.6, 7.0, 8.0, 8.1and 8.2 km/sec, respectively. According to this model the Moho lies at32 km depth on average. For P-waves, the station corrections range from–0.62 to 0.44 sec and for S-wave they range from –1.17 to 0.62 sec.These low variations in station residuals indicate small lateral velocitychanges and therefore the velocity model found should be well suited forearthquake locations and future starting model for 3D tomography studies.Using this new velocity model, the local earthquakes were relocated. Theshallow seismicity, 80 km, shows two subduction zones in the country: theCauca subduction zone with a strike of N120°E, dip of 35°and thickness of 35 km, and the Bucaramanga subduction zone which has,for the northern part, a strike of N103°E, dip of 27° andthickness undetermined and, for the southern part, a strike ofN115°E, dip of 40° and thickness of 20 km. Based ondifferences of thickness of brittle crust in the subducted slab and spatialdistribution of the seismicity, the Cauca and Bucaramanga subduction zonesseem to represent independent processes. The Cauca subduction seems tobe connected to the process of the Nazca plate being subducted under theNorth Andes Block. In the Bucaramanga subduction zone, the transitionbetween southern and northern parts and changes in geometry of the slabseem to be gradual and there is no evidence of a tear in the slab, howeverthe local seismicity does not allow us to determine which plate or plates arebeing subducted. The Bucaramanga nest appears to be included into thesubducted slab.

Comment on “Laboratory measurements of ultrasonic wave velocities in rocks from the Campi Flegrei volcanic system and their relation to other field data” by M. Zamora, G. Sartoris, and W. Chelini

Journal of Geophysical Research: Solid EarthVolume 105, Issue B9 p. 21537-21542 CommentariesFree Access Comment on “Laboratory measurements of ultrasonic wave velocities in rocks from the Campi Flegrei volcanic system and their relation to other field data” by M. Zamora, G. Sartoris, and W. Chelini Jose Pujol, Jose PujolSearch for more papers by this author Jose Pujol, Jose PujolSearch for more papers by this author First published: 10 September 2000 https://doi.org/10.1029/2000JB900018Citations: 4AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL References Aster, R., R. Meyer, Three-dimensional velocity structure and hypocenter distribution in the Campi Flegrei caldera, Italy, Tectonophysics, 149, 195– 218, 1988. Benz, H., B. Chouet, P. Dawson, J. Lahr, R. Page, J. Hole, Three-dimensional P and S wave velocity structure of Redoubt volcano, Alaska, J. Geophys. Res., 101, 8111– 8128, 1996. Bianchi, R., R. Casacchia, A. Coradini, A. Duncan, J. Guest, A. Kahle, P. Lanciano, D. Pieri, M. Poscolieri, Remote sensing of Italian volcanos, Eos Trans. AGU, 71, 1789– 1791, 1990. Corrado, G., I. Guerra, A. Lo Bascio, G. Luongo, R. Rampoldi, Inflation and microearthquake activity of Phlegraean Fields, Italy, Bull. Volcanol., 403, 169– 188, 1976. De Natale, G., F. Pingue, Seismological and geodetic data at Campi Flegrei (southern Italy): Constraints on volcanological models, Volcanic Seismology P. Gasparini, R. Scarpa, K. Aki, 484– 502, Springer-Verlag, 1992. Lirer, L., G. Luongo, R. Scandone, On the volcanological evolution of Campi Flegrei, Eos Trans. AGU, 68, 226– 227229, 231, 233–234, 1987. Podvin, P., I. Lecomte, Finite difference computation of traveltimes in very contrasted velocity models: A massively parallel approach and its associated tools, Geophys. J. Int., 105, 271– 284, 1991. Pujol, J., Joint hypocentral determination in media with lateral velocity variations and interpretation of the station corrections, Phys. Earth Planet. Inter., 75, 7– 24, 1992. Pujol, J., R. Aster, Joint hypocentral determination and the detection of low-velocity anomalies. An example from the Phlegraean Fields earthquakes, Bull. Seismol. Soc. Am., 80, 129– 139, 1990. Pujol, J., P. Shen, A new 3-D velocity model for the Campi Flegrei, Italy, volcanic area71st annual meeting of the Eastern Section of the Seismol. Soc. AmMemphis, 1999. Scarcella, G., Methodi di inversioni di dati sismici: Applicazioni nel Basso Tirrreno e nei Campi Flegrei, degree in physics thesis,Univ. of Naples,Naples, Italy,1984. Zamora, M., G. Sartoris, W. Chelini, Laboratory measurements of ultrasonic wave velocities in rocks from the Campi Flegrei volcanic system and their relation to other field data, J. Geophys. Res., 99, 13553– 13561, 1994. Citing Literature Volume105, IssueB910 September 2000Pages 21537-21542 ReferencesRelatedInformation

Predicting Geophysical Measurements: Testing a Combined Empirical and Model-Based Approach Using Surface Waves

There are several approaches commonly used to provide spatial corrections of geophysical data. One end-member approach is to develop model-based corrections in which the variations are based either on a priori knowledge or on geophysical inversions, such as tomography. While this approach would have the advantage of full spatial coverage, in general one would be unable to recover the full station corrections due to the inherent averaging of models. Another end-member approach, common in seismically active regions, would be to develop empirically based corrections based directly on measurements. An example of this approach would be kriging or other interpolation techniques. The advantage of this approach is a true fit to the most applicable measurements, but at the cost of severe spatial limitations. Ideally, one would like to combine the two approaches. This short note demonstrates the value of a combined empirical and model-based approach to generating accurate spatial correction surfaces using surface wave measurements and a tomography model from the Middle East and North Africa. By kriging residual values from a tomographic model, there was a significant reduction in misfit over either approach alone. In principle, kriging measurement residuals from models could improve many geophysical applications from travel times to amplitudes.

Reappraisal of surface wave magnitudes in the Eastern Mediterranean region and the Middle East

There have been many attempts to improve parametric catalogues for surface wave magnitudes for earthquakes of this century, and many of these attempts have been based on empirical adjustments to homogenize and complete catalogues without recourse to the instrumental data with which these magnitudes have been calculated. Using the Prague formula with station corrections and a substantial volume of amplitude and period readings of surface waves, culled from station bulletins, we calculated uniformly the magnitude of all significant earthquakes, 1519 in all, in the Eastern Mediterranean region and in the Middle East between 1900 and 1998. We also calculated station corrections and their variation with time, and examined the effect of distance adjustment of the Prague formula on M(s) estimates. We find that the current procedure of averaging station magnitudes underestimates M(s). This underestimation depends on the variance and on the number of station magnitudes available for the calculation of M(s), which can be as large as 0.3 magnitude units or more. We also find that station corrections have a rather small overall effect on event magnitude, of less than +0.1, except when the number of observing stations is small, in which case the correction may reach +0.3 magnitude units. Event magnitudes derived from station magnitudes with distance adjustment, calculated from the original Prague formula, are on average 0.1 units larger than M(s) without distance correction. We find that for shallow events, Gutenberg's estimates are, on average, larger by 0.12 units and show a significant scatter, with a standard deviation three times the mean value. We find similar differences and scatter for surface wave magnitudes estimated by other workers and agencies.

Ocean tidal loading affecting precise geodetic observations on Greenland: Error account of surface deformations by tidal gravity measurements

Air-borne and satellite based altimetry are used to monitor the Greenland ice-cap. Since these measurements are related to fiducial sites at the coast, the robustness of the height differences depends on the stability of these reference points. To benefit from the accuracy of these methods on the centimeter level, station corrections regarding the Earth tides and the ocean tidal loading have to be applied. Models for global corrections esp. for the body tides are available and sufficient, but local corrections regarding the effect of the adjacent shelf area still have to be inferred from additional observations. Near the coast, ocean tidal loading causes additional vertical deformations in the order of 1 to 10 cm. Therefore, tidal gravity measurements were carried out at four fiducial sites around Greenland in order to provide corrections for the kinematic part of the coordinates of these sites. Starting in 1993 four stations were occupied on Greenland for a one year record each.The results show the expected strong tidal anomalies due to ocean tidal loading. The loading computations confirmed these observations, but it turned out that with global models only about 50 % of the observed effect can be explained. This means that at these stations a vertical deformation of up to ± 3.5 cm is not corrected applying these global models.

A Double-Difference Earthquake Location Algorithm: Method and Application to the Northern Hayward Fault, California

We have developed an efficient method to determine high-resolution hypocenter locations over large distances. The location method incorporates ordinary absolute travel-time measurements and/or cross-correlation P-and S-wave differential travel-time measurements. Residuals between observed and theoretical travel-time differences (or double-differences) are minimized for pairs of earthquakes at each station while linking together all observed event-station pairs. A least-squares solu- tion is found by iteratively adjusting the vector difference between hypocentral pairs. The double-difference algorithm minimizes errors due to unmodeled velocity struc- ture without the use of station corrections. Because catalog and cross-correlation data are combined into one system of equations, interevent distances within multiplets are determined to the accuracy of the cross-correlation data, while the relative lo- cations between multiplets and uncorrelated events are simultaneously determined to the accuracy of the absolute travel-time data. Statistical resampling methods are used to estimate data accuracy and location errors. Uncertainties in double-difference locations are improved by more than an order of magnitude compared to catalog locations. The algorithm is tested, and its performance is demonstrated on two clus- ters of earthquakes located on the northern Hayward fault, California. There it col- lapses the diffuse catalog locations into sharp images of seismicity and reveals hor- izontal lineations of hypocenters that define the narrow regions on the fault where stress is released by brittle failure.

The 1997 Umbria-Marche (Italy) earthquake sequence: analysis of the data recorded by the local and temporary networks

We present some preliminary results obtained from thejoined analysis of the data collected by the permanentand the temporary networks operating in the area ofthe earthquake sequence that followed (andanticipated) the 26 September, Central Italy, mainshocks. In particular, these earthquake data haveallowed us to determine a well constrainedwave-velocity model (both P and S) with stationcorrections which demonstrated to produce robusthypocentral locations. These velocity modelswith station corrections have been used forre-locating the whole September 1997–July 1998subset of data of the permanent network, and theprevious background seismicity, starting from May1996. The focal mechanisms of the largest events werealso obtained from an analysis of the first-motionpolarities.Our results indicate that 1) the seismic activityaligns on a SE-NW trend for a total length of about50 km of extension; 2) the focal depth of theseevents is restricted to the range 0–9 km; 3) mostevents can be related to sub-parallel SW dipping faultplanes; 4) focal mechanisms of the largest shocks(ML > 4) show a coherent behaviour, withnormal fault solution on SSE-NNW striking, SW dippingplanes; 5) the space-time evolution of the activitydisplays a discontinuous mode of energy release, withdifferent episodes of activation and an apparentclustering of aftershocks at the edges of the areaswhich presumably ruptured in the main shocks.

Vibroseis recording techniques and data reduction from the Jemez Tomography Experiment

Abstract The Jemez Tomography Experiment (JTEX) is a multidisciplinary study focused on the Valles Caldera and the Jemez Mountains, New Mexico. The objectives of the project are to create a high resolution crustal model of the subsurface structure of this silicic volcanic system and to develop an interpretation of its volcanic evolution. Use of Vibroseis sources in the acquisition of refraction/wide-angle reflection seismic data provided challenges beyond conventional explosive-source data. Processing of the JTEX Vibroseis data is an involved procedure consisting of sorting, cross-correlating, filtering, and stacking numerous individual seismograms in the production of final record sections. However, excellent results (high signal-to-noise seismograms at relatively small spacings) are obtainable with coherent arrivals at source-receiver distances of more than 60 km. The primary drawback in this approach lies in the massive volume of data that is necessary to produce record sections. One benefit of the Vibroseis source used during JTEX was a method to decrease effective seismogram spacing. This technique, dubbed a “source-offset” technique, provides smaller overall seismograph station spacing by moving the Vibroseis sources during acquisition and leaving deployed seismographs stationary. After station corrections, this method effectively decreases station spacings and increases detail in resulting record sections. Various shallow crustal heterogeneities create travel-time advances and delays that affect the source-offset data differently than single-source data. Synthetic modeling demonstrates small travel-time discrepancies associated with the source-offset technique. However, the addition of traces with smaller station intervals clarifies secondary arrivals within record sections and aids in interpretation of these arrivals with a minimum amount of field effort required.

Accurate hypocentre determination in the seismogenic zone of the subducting Nazca Plate in northern Chile using a combined on-/offshore network

The coupled plate interface of subduction zones—commonly called the seismogenic zone—has been recognized as the origin of fatal earthquakes. A subset of the after-shock series of the great Antofagasta thrust-type event (1995 July 30; Mw = 8.0) has been used to study the extent of the seismogenic zone in northern Chile. To achieve reliable and precise hypocentre locations we applied the concept of the minimum 1-D model, which incorporates iterative simultaneous inversion of velocity and hypocentre parameters. The minimum 1-D model is complemented by station corrections which are influenced by near-surface velocity heterogeneity and by the individual station elevations. By relocating mine blasts, which were not included in the inversion, we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth. A study of the resolution parameters ALE and DSPR documents the importance of offshore stations on location accuracy for offshore events. Based on precisely determined hypo-centres we calculate a depth of 46 km for the lower limit of the seismogenic zone, which is in good agreement with previous studies for this area. For the upper limit we found a depth of 20 km. Our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies; the results are also in agreement with thermal studies for the Antofagasta forearc region.

Teleseismic slowness-azimuth station corrections for the International Monitoring System Seismic Network

Abstract The prototype International Data Center (PIDC) in Arlington, Virginia, has been developing and testing software and procedures for use in the verification of the Comprehensive Test Ban Treaty. After three years of operation with a global network of array and three-component stations, it has been possible to characterize various systematic biases of those stations that are designated in the Treaty as part of the International Monitoring System (IMS). These biases include deviations of azimuth and slowness measurements from predicted values, caused largely by lateral heterogeneity. For events recorded by few stations, azimuth and slowness are used in addition to arrival-time data for location by the PIDC. Corrections to teleseismic azimuth and slowness observations have been empirically determined for most IMS stations providing data to the PIDC. Application of these corrections is shown to improve signal association and event location. At some stations an overall systematic bias can be ascribed to local crustal structure or to unreported instrumental problems. The corrections have been applied in routine operation of the PIDC since February 1998.

Influence of source radiation patterns on globally observed short-period magnitude estimates (mb)

Abstract We investigated the effect of the radiation pattern on station and network magnitude estimates (mb) by using centroid-moment-tensor (CMT) solutions published by the seismological group at Harvard University. The magnitude data used in this study were P amplitudes provided by the Prototype International Data Center (PIDC). When relating the station magnitude residuals to the calculated relative radiation factors, we found a small but clearly observable dependency. By compensating for this effect after inverting for station corrections and new event magnitudes, we found that the variance of the station magnitude residuals was reduced by about 2.6%. In parallel, we estimated a set of mean station corrections with and without corrections for the radiation pattern. Similarly, we recomputed the average network magnitudes (mb), which in some cases differed by up to 0.15 magnitude units from the uncorrected average network mb values. In conclusion, we found that the effect of the source radiation pattern on the amplitudes used for mb estimation on a global scale is relatively small compared to effects from other factors. Only about 24% of the predicted radiation pattern signal can be found in the observed data. However, when best possible mb values are needed (e.g., to investigate magnitude relations or station corrections for specific ray paths), compensating for the radiation pattern will help to reduce the scatter.

Joint focal mechanism determination with source-region station corrections using short-period body-wave amplitude data

AbstractWe have developed a method that simultaneously determines focal mechanism solutions of many small earthquakes and source-region station corrections for short-period body-wave amplitudes by inverting amplitude data of P, SH, and SV waves, together with P-wave polarity data. The observed seismic waveform includes the effects of site amplification and attenuation along its ray path in addition to the radiation pattern of earthquake source. The amplitude of seismograms at frequencies higher than a few Hertz is extremely sensitive to heterogeneous structure near the ground surface. Consequently, we need to know in detail the effects of site amplification and attenuation in order to estimate focal mechanisms by using short-period waveforms. However, at present, we do not know the detailed crustal structure with a resolution necessary for this estimation. In the present study, we assume that P- and S-wave attenuation factors along ray paths from hypocenters to each station can be expressed as a function of hypocentral distance, backazimuth, and incident angle. Based on this assumption, we determined focal mechanism solutions of many earthquakes and the coefficients in the function for each station simultaneously, by using P-, SH-, and SV-wave amplitudes and P-wave polarities. We applied the present method to 170 aftershocks of the 1996 Onikobe earthquake (M 5.9), which occurred in the central part of northeastern Japan. We obtained focal mechanism solutions of many microearthquakes whose mechanism solutions could not be determined by using P-wave polarity data alone. P axes of almost all the obtained focal mechanism solutions are horizontal and oriented in the east-west direction. T axes are, on average, near vertical at the shallowest depth. As the depth approaches 5 km, the T axes become horizontal and then gradually become near vertical again.

Station correction analysis for surface-wave magnitudes of New Zealand earthquakes

Station terms and standard errors are presented for 345 world-wide stations used in the determination of surface-wave magnitudes of 190 selected New Zealand earthquakes over the period 1901-1993 [1]. These will facilitate the estimation of surface-wave magnitudes of other earthquakes in the New Zealand region. The station terms and the residuals from the linear model used to estimate them are both found to be weakly related to the mean distance from the earthquakes recorded by each station. The horizontal and vertical components at a given site are treated as separate stations. The station term for the horizontal component tends to exceed that for the vertical component at mean distances in the 20°-40° range.

A Study of Aftershocks of the 17 July 1998 Ruey-Li, Chiayi Earthquake

On 17 July 1998, an earthquake of magnitude 6.2 (M(subscript L)) occurred near Chiayi, southwestern Taiwan. Two days after the occurrence of the mainshock, a temporary digital seismic network was deployed near its epicenter for 24 days to monitor the aftershock activity. The events within and near the network were first located using a traditional single-event location and then relocated using a joint hypocentral determination (JHD) technique to determine the significance of lateral velocity variations and to improve earthquake locations. The station corrections obtained from the JHD analysis vary from 0.30 to -0.18 sec for the P-waves and from 0.51 to -0.58 sec for the S-waves. The patterns of station corrections suggest that upper crustal velocities on the eastern side of the study area are relatively higher than those on the western side. The depth cross-section of the relocated aftershocks shows a clear pattern of southeast-dipping distribution of the hypocenters. The focal mechanisms and hypocentral distribution of the relocated aftershocks reveal a close relationship between seismicity and the known faults in the study area.

CMT解と震源時間関数の迅速な自動決定とその結果の評価

Recent advances in broadband seismic observation enable us to analyze high quality waveform data just after the earthquake occurrence. An automated system for rapid determination of earthquake mechanisms for local events occurring beneath central Japan is developed with this point as background. When an earthquake occurs, its hypocentral parameters are determined automatically in several tens of seconds. In a case when an event occurring in a given area has a magnitude larger than a certain value, its source process is automatically analyzed by the use of a broadband waveform inversion technique. Considering the importance to shorten the computer process time to obtain the result, we calculate the Green's functions immediately after hypocentral parameters are calculated and when waveform data to be used in the inversion are being retrieved. The data of the Earth's response parameters, which are used in the calculation of the Green's functions, are stored on the hard disk so as to reduce the total lapse time. As a result, the CMT solution and source time function can be obtained in a few minutes after the earthquake occurrence. Inadequacy of theoretical waveform calculated by the 1-D Earth's model is corrected by introduction of station corrections to the observed arrival time and overparameterization to the source time function, where a number of double couple small sub-events are located in a certain time range. The CMT solution is represented by superimpose of moment tensors of small sub-events with very short duration. All the sub-events are assumed to occur at the same location (i. e., at the hypocenter) but to have different onset times. The source time function is also estimated from the temporal distribution of the sub-events. To evaluate the reliability of the results obtained from the present study, we compare them with the following two data sets: i) focal mechanism solutions determined by P first motion analysis, and ii) CMT solutions obtained from the other method for the same events. Our results are basically consistent with these two sets of mechanism solutions. However the agreement of the solutions is obviously better among the CMT solutions. Possible explanation for the difference between CMT and focal mechanism solutions is that the latter may be inaccurate because of insufficient push and pull first motion data, or that the total rupture may occur along the fault different from the initial rupture.

Instrumental Seismicity of the Western Alps: A Revised Catalogue

—The western Alpine regions have been instrumented since the beginning of the century, and the number of seismological stations largely increased since 1980. This dense network has allowed an important improvement in the hypocentral determination, even for low magnitude events. This condition was a good opportunity to perform a synthesis of 32 years of instrumental seismicity in the Western Alps and southeast of France (1962–1993) and to attempt an improvement of the older event location with the assistance of the more recent locations.¶The revised catalogue of seismicity is built using station corrections and regional crustal models. After the elimination of non-natural events, the catalogue is composed of 6697 events. Another improvement corresponds to the revision of magnitudes. We performed several tests to evaluate the reliability of our results location of quarry events and rock bursts, epicentral correlation with geological features, coherence in depth with interpreted seismic profile (ECORS line), Moho isobaths. A first use of this catalogue is presented for the Haute-Ubaye region in the southwestern Alps.