Station Corrections Research Articles

Examination of seismicity in the central Alpine Fault region, South Island, New Zealand

Abstract We have investigated seismicity in the Alpine Fault region of the South Island of New Zealand by relocating the New Zealand National Seismograph Network (NZNSN) data for the 3‐year period March 1991 through April 1994. Through simultaneous inversion for hypocentres, one‐dimensional velocity structure, and station corrections, we obtained velocity and station parameters appropriate for earthquake location in this region. The 3‐year period of relocated seismicity revealed nine earthquakes per year of magnitude 2.5–3.6 along the Alpine Fault. These earthquakes extend to 10 km depth, and the Alpine Fault provides a northwestern boundary to a region of diffuse seismicity extending 70 km to the southeast, with a base gradually deepening from 10 km along the Alpine Fault to 20 km at its southeastward extent. Within the diffuse seismicity there are numerous small clusters which include: the M 5.8 March 1992 Wilberforce River sequence; continued aftershocks of the M 6.1 June 1984 Godley River earthquake; and other clusters that have their largest events ranging from M 3.2 to M 4.6. The June 1994 Mw 6.7 Arthur's Pass earthquake occurred within the diffuse seismicity region, but there was no associated cluster in our 3‐year sample of background seismicity.

A local-magnitude scale for the western Great Basin-eastern Sierra Nevada from synthetic Wood-Anderson seismograms

AbstractWe have computed synthetic Wood-Anderson seismograms for over 1100 arrivals at 10 three-component, broadband digital stations in the UNR western Great Basin-eastern Sierra Nevada network. These represent all the available records from local earthquakes over magnitude 3.5 between 1990 and June of 1993, plus selected events of smaller magnitude. There were 77 events ranging in magnitude from 2.2 to 5.9, including four events over magnitude 5. The distances considered ranged from 15 to 600 km, with the best-represented range being from 30 to 450 km. We invert these measurements to determine distance and station corrections appropriate for a local-magnitude scale, constrained by Richter's original definition that an earthquake of ML = 3 will cause a 1-mm zero to peak deflection of the Wood-Anderson seismogram at 100 km from the epicenter. The results between 30 and 450 km were essentially independent of choice of curve-fitting parameters. In the 30- to 500-km distance region, the smooth distance-correction curves were very similar to that determined by Richter (1958), which is still used for southern California earthquakes. We propose to use Richter's distance-correction curve in reporting amplitude magnitudes from our digital network.

Body-wave velocities in the circum-Adriatic region

The paper presents body-wave velocities in the crust and upper mantle of the circum-Adriatic region obtained by analyzing travel times of regional phases of 419 selected earthquakes with epicenters in the central part of the External Dinarides. The studied region was divided into five areas and two kinds of velocity models were determined for each of them. The first kind ( inter-area models) gives average velocities that characterize the zone between and including the epicentral region and the area in question, while the second kind ( intra-area models) presents velocities within the respective area. The locations of hypocenters considerably improved when the new models were used instead of the standard one for the Balkan region. The large-scale variation of average velocities obtained by inter-area modeling was found to be relatively small both in the crust and in the upper mantle. On the other hand, the magnitude and distribution of station corrections and the resulting intra-area velocities point to relatively pronounced lateral velocity variation within some of the studied regions. The obtained velocities in most cases do not differ significantly when compared to the values reported in other studies. The most important exceptions occur in the epicentral area (central part of the External Dinarides) where the P-wave velocity in the upper crust is much higher then elsewhere, while the velocity in the upper mantle is found to be lower than that determined by DSS experiments. There is also an indication of pronounced variation of the Poisson ratio in this area.

Seismic structure of the deep Kurile Subduction Zone

Three‐dimensional modeling of P wave travel time residuals was performed to investigate the deep slab‐related velocity structure within the Kurile subduction zone. Travel times of P waves from earthquakes at all depths within the Kurile subduction zone to World‐Wide Standard Seismograph Network and Canadian stations were measured. Instead of analyzing the travel time residuals of a single event, relative travel time residuals of P waves to common stations have been analyzed and used in this study to isolate near source contributions from the total travel time residuals. The effect of uncertainties in long wavelength heterogeneity far from the source in the lower mantle and the effect of uncertainties in the reference Earth model have been significantly reduced by analyzing relative travel time residuals. The effect of uncertainties in station corrections are eliminated by relative travel time residuals. Our measured P wave travel time data clearly indicate that high‐velocity anomalies exist in the lower mantle beneath the Kurile subduction zone but they do not appear to be caused by simple slab continuation into the lower mantle. The travel time modeling was accomplished using a fully three‐dimensional finite difference technique for wave propagation within the source region. The modeling results show that high‐velocity anomalies extend several hundred kilometers into the lower mantle but they appear to be very complicated. A large number of models consisting of simple slab continuation into the lower mantle were tested and the results show that none ofthese models was able to produce synthetics which match the observed P wave travel time data. The slab‐related velocity anomalies in the upper mantle of the Kurile subduction zone are generally consistent with the seismicity. The high‐velocity anomalies in the lower mantle below the Kurile subduction zone are broad and geometrically change from the southern to the northern Kuriles. Below the southern Kuriles, the high‐velocity anomalies appear to be subhorizontal. However, beneath the central Kuriles, there are broad, about 600‐km‐wide, high‐velocity anomalies on the continental side of the slab which extend several hundred kilometers into the lower mantle. The high‐velocity anomalies found below the northern Kuriles are broad and on both the continental and the ocean side of the slab, and extend several hundred kilometers into the lower mantle.

3-D velocity structure of the San Jacinto fault zone near Anza, California-I.Pwaves

SUMMARY Seismic arrival times from microearthquakes (ML < 4) on the San Jacinto fault near Anza, California, are used to find spatial variations in the seismic velocity that are related to the crustal structure of the fault zone. Preliminary modelling of the 1-D P-wave velocity structure of the upper 25 km of crust reveals that most of the variation in velocity is lateral rather than depth dependent. The traveltime anomalies due to lateral structure can be partially compensated for by applying station corrections, however the variance of the traveltime residuals is still 2.25 times larger than the variance of the picking error. The spatially correlated residuals show that this variance is due to localized velocity anomalies and that the data require further modelling using a 3-D velocity structure. Because the 3-D inverse problem is non-unique, smoothness constraints are applied to find the model that has the minimum structure required to fit the data to the picking error, where a smooth model is defined such that the gradient of the velocity perturbation from the original 1-D model is small. With small non-zero station corrections, a 3-D velocity model can be found that fits the data well. The structure is well resolved from 3 to 9 km depth where lateral perturbations of up to 7 per cent are determined independently of the trade-off between station corrections and poorly resolved near surface structure. The model shows a horizontal gradient with overall faster velocities in the north-east side of the fault zone. At 3–6 km depth, the signature of the fault zone is evident in the lower velocities beneath the surface trace of the fault. However, at 9 km depth. higher seismic velocities are found extending into the fault zone from the north-east block. This higher velocity region occurs where there is a distinct lack of seismicity on the fault. There is also a localized feature in the south-west of the modelled region that is more than 10 km from the main trace of the fault with velocities 3 per cent slower than average.

Initial reference models in local earthquake tomography

The inverse problem of three‐dimensional (3‐D) local earthquake tomography is formulated as a linear approximation to a nonlinear function. Thus the solutions obtained and the reliability estimates depend on the initial reference model. Inappropriate models may result in artifacts of significant amplitude. Here, we advocate the application of the same inversion formalism to determine hypocenters and one‐dimensional (1‐D) velocity model parameters, including station corrections, as the first step in the 3‐D modeling process. We call the resulting velocity model the minimum 1‐D model. For test purposes, a synthetic data set based on the velocity structure of the San Andreas fault zone in central California was constructed. Two sets of 3‐D tomographic P velocity results were calculated with identical travel time data and identical inversion parameters. One used an initial 1‐D model selected from a priori knowledge of average crustal velocities, and the other used the minimum 1‐D model. Where the data well resolve the structure, the 3‐D image obtained with the minimum 1‐D model is much closer to the true model than the one obtained with the a priori reference model. In zones of poor resolution, there are fewer artifacts in the 3‐D image based on the minimum 1‐D model. Although major characteristics of the 3‐D velocity structure are present in both images, proper interpretation of the results obtained with the a priori 1‐D model is seriously compromised by artifacts that distort the image and that go undetected by either resolution or covariance diagnostics.

A revisit to P wave travel time statics at teleseismic stations

To account for local deviations from the one‐dimensional reference velocity model, the station correction formulation of Dziewonski and Anderson (1983) is extended to include the travel time variation with respect to the ray takeoff angle. The station statics are obtained through an iterative retrieval of the lower‐degree spherical harmonics normalized over the data takeoff angle range on station residual spheres, and similar event statics are introduced to decouple the effects of near‐station and near‐event heterogeneities. Except the zero degree constant term, all higher‐degree static terms are obtained on a regional rather than an individual station/event location. The P wave static corrections up to degree and order 2 are derived for 1343 stations using nearly 3 million International Seismological Center travel time residuals with respect to the iasp91 model and with event relocation and summary ray sorting. The resulting station statics account for nearly 27% of the variance of the P wave travel time residuals. Patterns of the zero‐degree statics correlate well with surface tectonic features and with previous results. At most stations over subduction zones the slowest direction of the secondorder statics, or the second‐degree in azimuth, is perpendicular to the trench.

Nonparametric description of peak acceleration as a function of magnitude, distance, and site in Guerrero, Mexico

This article describes the peak accelerations observed on a network of strong-motion accelerographs above the subduction zone in Guerrero, Mexico, as a function of magnitude and hypocentral distance. Our empirical description differs from regression analysis, however, in that it produces a table of values of peak acceleration, for selected values of M and R , and an interpolation rule to obtain peak acceleration at intermediate values. With this approach, the existence and extent of saturation of peak acceleration as magnitude increases is dictated by the data, and unlimited flexibility is allowed in the distance dependence of the peak acceleration. This nonparametric approach does not allow extrapolation beyond the range of the data, and is thus most useful when the data spans a large magnitude and distance range. After 7 yr of recording, the digital Guerrero accelerograph network has obtained enough data to test the concept. The records include events with magnitudes ranging from under 3 to 8.1, with all magnitudes recorded at distances ranging from nearly directly above the rupture to distances where acceleration has become too small to trigger the instruments. The data used in this study consist of 528 recordings (each three components) from 146 earthquakes that occurred between 1985 and 1990. All but one station are on rock sites. Since all the accelerograms are recorded by one of 30 stations, there is an average of about 18 records for each station. The results suggest that at short distances (about 14 to 25 km from the fault), as magnitude increases the peak horizontal acceleration saturates at about 20% g . However, unlike small earthquakes, peak accelerations decrease very slowly with increased distance from the rupture in a large magnitude. Station corrections for peak acceleration reduced the rms misfit by about 15% for the horizontal component, by 12% for the vertical component, and by 21% for the ratio. For some stations, horizontal corrections are close to vertical corrections, but others show very different site effects on horizonal and vertical components. The ratios of peak vertical acceleration to peak horizontal acceleration show a very weak tendency to decrease as magnitude increases, and this tendency is further reduced at most distances after correction for average site effects. The average value of this ratio is 0.70 before the station correction. After these corrections, for 55 events with known stress drop, the residual between the predicted peak acceleration and the observed peak acceleration is correlated with stress drop (Δσ). Although scattered, the observations are consistent with a theoretical prediction that Amax ∝ Δσ0.80.

Robust inversion of IASP91 travel time residuals for mantle P and S velocity structure, earthquake mislocations, and station corrections

Using both P and S arrival time information, 41,108 events in the International Seismological Centre (ISC) catalog for the years 1964 to 1987 are relocated relative to the IASP91 velocity model. The mean absolute horizontal relocation is 7.7 km and the mean absolute depth relocation is 9.1 km, The mean absolute origin time shift is 1.2 s. The relocation procedure increased the P residual standard deviation slightly from 2.3 s to 2.5 s while decreasing the S residual standard deviation from 6.8 s to 6.1 s. When plotted as bottoming point averages, the resulting IASP91 P and S arrival time residuals show coherent patterns as a function of geographic location. An iterative lp residual norm minimization algorithm is used to estimate the set of P and S velocity variations as well as the earthquake relocation and seismographic station parameters which best explain the travel time residuals. The procedure is robust in that extremely large travel time residuals, which are common in the ISC data, do not unduly influence the velocity estimates. Both the P and S models of lateral heterogeneity contain prominent circum‐Pacific low velocities, 1% to 2% perturbations, underlying the back arc basins between 35 and 200 km depth. This ring of negative deviations continues into the depth interval 200–400 km. The continental cratons are underlain by high‐velocity anomalies with maximum amplitudes of 2%. Iceland and the Azores are underlain by low‐velocity mantle material that extends down to at least 400 km. The Benioff zones are only intermittently imaged as 1–2% high‐velocity regions in the uppermost 400 km. They are best resolved in the P velocity variations. Both the P and S velocity models contain a circum‐Pacific ring, beneath the Benioff zones, of 1–2% positive velocity deviations in the depth range 660–870 km. Coherent high‐velocity features are seen below South America, Borneo, Tonga‐Fiji, the Marianas, and the northern Kuriles. The anomalies beneath South America and Borneo extend into the 870–1070 km depth range. Below depths of 1270 km for P variations and 1070 km for S variations the amplitude of the heterogeneity has decreased significantly. It is only in the lowermost mantle, 2670 km to the core‐mantle boundary, that the level of P heterogeneity rises significantly above the estimated noise level. In this depth range a partial ring around the Pacific basin is observed, although this.

Lithospheric structure models applied for locating the Romanian seismic events

The paper presents our attempts made for improving the locations obtained for local seismic events, using refined lithospheric structure models. The location program (based on Geiger method) supposes a known model. The program is run for some seismic sequences which occurred in different regions, on the Romanian territory, using for each of the sequences three velocity models: 1) 7 layers of constant velocity of seismic waves, as an average structure of the lithosphere for the whole territory; 2) site dependent structure (below each station), based on geophysical and geological information on the crust; 3) curves deseribing the dependence of propagation velocities with depth in the lithosphere, characterizing the 7 structural units delineated on the Romanian territory. The results obtained using the different velocity models are compared. Station corrections are computed for each data set. Finally, the locations determined for some quarry blasts are compared with the real ones.

Deep, low-frequency microearthquakes in or around seismic low-velocity zones beneath active volcanoes in northeastern Japan

Abstract Precise hypocenter relocations have been made for shallow intraplate microearthquakes beneath northeastern Japan by applying source-region station corrections. The relocated hypocenter distribution shows that most of the events are confined to the upper 15 km of the crust which forms the brittle seismogenic zone in this volcanic arc. It also shows that exceptionally deep microearthquakes, well below the base of the seismic-aseismic boundary, occur at twelve locations of northeastern Japan. They occur in the lowermost crust or in the uppermost mantle, where rheological properties of rocks are presumed to be in the ductile regime. All the 153 events presently detected in the lowermost crust or in the uppermost mantle have anomalously low predominant frequencies (1–5.5 Hz) both for P and S waves, in contrast to those (8–20 Hz) of normal shallow events in the brittle seismogenic zone. The magnitudes of these events are small (M ⩽ 2.2). Moreover, all of these events are located beneath active volcanoes and/or near P wave low-velocity zones in the uppermost mantle. The proximity of these events to the active volcanoes and other anomalous features suggest that these deep low-frequency events are generated by magmatic activity of mantle diapirs in the uppermost mantle.

Seismic Velocity Structure in the Crust and Upper Mantle beneath Northern Japan.

We have applied an inverse method to P and S wave arrival time data observed at 52 seismic stations for 349 local earthquakes in order to estimate a three-dimensional (3-D) velocity structure beneath northern Japan. The method is characterized by a simultaneous determination of the two-dimensional depth distributions of velocity boundaries, 3-D velocity distribution and station corrections as well as hypocenter parameters of earthquakes. The Moho discontinuity under eastern Hokkaido is in-clined from a depth of 32 km beneath the Hidaka Mountains to 20 km beneath the Konsen Plateau, while the thick crust with a thickness of 30-36 km distributes widely in western Hokkaido and the Tohoku region. The dip direction of the Moho in eastern Hokkaido is approximately perpendicular to the range of the Hidaka Mountains. The upper boundary of the subducting Pacific plate distributes in the depth range from 50 to 170 km in the surveyed area and the dip angle of the plate boundary in eastern Hokkaido (the Kurile arc) is slightly larger than that in the Tohoku region (the northeastern Japan arc). Estimated P (S) wave velocities are 5.8-7.0 km/s (3.4-4.1 km/s) in the crust, 7.4-8.0 km/s (4.2-4.5 km/s) in the upper mantle, and 8.1-8.6 km/s (4.6-4.9 km/s) in the plate, respectively. The velocities in the crust beneath northern Hokkaido are lower than those beneath eastern Hokkaido and the Tohoku region. The upper mantle has velocities which gradually increase with depth, and the subducted plate contains a high velocity zone with a P wave velocity faster than 8.3 km/s. VP/VS ratios derived from P and S wave velocities generally range from 1.75 to 1.80 in the crust and upper mantle, while the ratios in the high velocity zone within the plate are less than 1.75. These results suggest that the plate is composed of two layers: the first layer is a low-velocity and high-VP/VS zone with a thickness less than 20 km, and the second layer is a thick high-velocity and low VP/VS zone. The estimated plate thickness is about 90 km in the Kurile arc and about 110 km in the northeastern Japan arc. The relocated hypocenter distribution beneath Hokkaido clearly shows the double seismic plane in the subducting plate. The upper seismic plane is located in the first layer with a low velocity and the lower seismic plane is in the second layer with a high velocity. The seismic activity in the upper seismic plane is high in the depth range from 60 to 90 km, while that in the lower seismic plane is high at depths deeper than 100 km. The double seismic plane joins at depths from 90 to 130 km.

A Method for Determination of the Three-Dimensional Seismic Velocity Structure from Local Earthquake Data.

We have developed an inverse method to estimate a three-dimensional seismic wave velocity structure using seismic arrival time data obtained from local earthquakes. The method is characterized by a simultaneous determination of a two-dimensional depth distribution of a velocity boundary, a three-dimensional velocity distribution, station corrections and hypocenters. The depth distributions of velocity boundaries and the distributions of the slowness perturbations for P and S waves are modeled by power series of latitude, longitude, and depth, This representation of the three-dimensional seismic structure is advantageous not only in reducing the number of unknown parameters, but also in analytically evaluating the boundary depths, velocities, and partial derivatives of travel times with respect to the unknown parameters. Ray tra-jectories connected between hypocenters and stations are determined by the so-called shooting method for ray tracing scheme under a simplified velocity distribution with the exact boundary depth distribution. Numerical experiments have proved that our inverse method can successfully estimate the three-dimensional velocity structure.

Velocity structure and seismicity of the Garm region, Central Asia

SUMMARY We have examined arrival-time data from two seismic networks operating within the Pamir-Tien Shan region of Central Asia in order to evaluate crustal structure and earthquake distribution in one of the most seismically active zones in the region. The data are from two networks surrounding the Soviet earthquake-prediction laboratory at Garm, Tadjikistan: a 15-station local network operated by the Soviet Complex Seismological Expedition (CSE) since the mid-l950s, and a dense telemetered network, nested within the larger CSE network, operated jointly by the US Geological Survey (USGS) and the CSE since 1975. A simultaneous inversion procedure was used to calculate l-D (layered) velocity models and station corrections independently for each of the two networks. Using a variety of data subsets and starting models, we obtained stable estimates of the velocity structure, independent of the initial velocity models. For both the CSE and the USGS network data the velocities of two upper-crustal layers proved to be indistinguishable from a model with a single layer over a half-space. For the CSE network, the best P-wave velocity model comprises an 8 km thick upper crustal layer of 5.2 km sK', overlying a 5.9 km sK' half-space. Using data from the USGS network alone, a similar velocity model was obtained, with the exception of a significantly higher velocity (V, = 6.3 km sK ') for the mid-crustal half-space. These results contradict previously published velocity models featuring anomalously low or high seismic velocities in the upper to mid-crust. Hypocentral parameters were redetermined for approximately 26 000 events occurring during the period 1975-82 using the newly obtained velocity models and station corrections. Most of the seismicity in the study area is associated with active deformation of the sedimentary rocks of the Peter the First Range. The distribution of hypocentres within the range suggests the presence of a southeastdipping zone of seismicity that can be traced updip to the Petrovsky thrust fault. Beneath this upper crustal seismicity, at a depth of 12-17 km, is an aseismic zone that may be related to the presence of low-strength evaporites at the base of the sedimentary section. This aseismic zone is in turn underlain by anomalously deep crustal seismicity, at a depth of 17-30 km. The deeper seismicity appears to be associated with a southeast-dipping feature approximately 10 km in thickness. This dipping seismic zone is interpreted as a zone of intracontinental subduction, whose surface expression is the Vakhsh thrust fault, at the frontal edge of the Peter the First Range.

Preliminary study of multiple event location on the Kunming Telemetry Seismic Network

Based on the first P wave arrival time data of local earthquakes recorded in the Kunming Telemetry Seismic Network in 1982–1989, the P travel time corrections of the stations in the network were obtained by use of the parameter separation method and the multiple event location method. This set of the corrections reflects the feature of lateral inhomogeneous structure of the upper crust beneath the network to certain degree. The geographic distribution of the sation corrections has obviously regional characteristics, by which the studied area is divided into three sub-areas. In the Western Yunnan area where the stations are the most dense, except the stations of Yunxian, Shidian and Wanding in south, the station corrections are not greater than 0.15 sec. In the eastern area (to the east of Chuxiong) where the network has slightly wider station interval, most of them show obviously positive delay. In southern area, including stations of Wenshan, Simao, Jinghong, Yunxian, Shidian and Wanding, all the stations have large negative delay. The results consist with the basic feature of geologic setting in Yunnan area. The accuracy of the relocated hypocentral parameters based on the corrected travel time data has fairly improved. Therefore the station corrections can be used to the routine processing of earthquake location in the Kunming Seismic Network.

Liquid core structure and PKP station anomalies derived from PKP(BC) propagation times

PKP(BC) travel time residuals in the distance range 148.5–155.5° have been extracted from the catalogues of the International Seismological Centre and inverted to retrieve possible heterogeneities or anisotropy at the base of the liquid core, and PKP(BC) station anomalies. A binning has first been applied to the data to minimize the biases that result from the uneven sampling of the Earth by the PKP(BC) rays and from the heterogeneities in the source regions. For the liquid core structure, the data reveal weak heterogeneities, with travel time delays slightly higher near the South Pole and lower beneath southeast Asia, but most of these variations stay below the error bar level. The search for anisotropy at the base of the liquid core also leads to inconclusive results, with a weak tendency for lower velocities in the direction of the Earth's rotation axis. Station anomalies appear to be by far the major contribution to the PKP(BC) residuals. Their values are compared with the P station corrections, and with the values predicted by lower-mantle tomographic models. They show that the lower mantle makes an important contribution to the PKP residuals at medium wavelengths; these delays are generally underpredicted by tomographic models.

The P-wave velocity structure of the mantle below the Iberian Peninsula: evidence for subducted lithosphere below southern Spain

Abstract The P-wave velocity structure of the mantle below the Iberian Peninsula is investigated with the method of delay-time tomography. More than 200,000 delay-times are inverted for simultaneous estimates of P-wave velocity anomalies, event mislocation and origin time errors, and station corrections. Below the Betic-Alboran area we find a positive anomaly between 200 and 700 km depth which in shape and amplitude resembles the image of a subducted lithosphere slab. Laterally this anomaly exhibits a clear SW-NE trend. In view of the low level of spatial resolution encountered we focus in our analysis on assessing the reliability of the slab-like image. Arguments in favor of the existence of a slab structure in the upper mantle are developed from inversions of subsets of the data. The resolving power for imaging a slab below the Betic-Alboran region is investigated using sensitivity analyses with a velocity model for a subducted slab. We conclude that a slab-structure exists in the mantle below the Betic and Alboran region and that it can be imaged with sufficient reliability. We interpret the anomaly as the image of subducted lithosphere. The slab is presumably detached from the surface. We propose that subduction took place during at least part of the Oligocene followed by slab detachment in the early Miocene.

Determination of earthquake energy release and ML using TERRAscope

We estimated the energy radiated by earthquakes in southern California using on-scale very broadband recordings from TERRAscope. The method we used involves time integration of the squared ground-motion velocity and empirical determination of the distance attenuation function and the station corrections. The time integral is typically taken over a duration of 2 min after the P-wave arrival. The attenuation curve for the energy integral we obtained is given by q(r) = cr^(−n)exp(−kr)(r^2 = Δ^2 + h_(ref)^2) with c = 0.49710, n = 1.0322, k = 0.0035 km^(−1), and h_(ref) = 8 km, where Δ is the epicentral distance. A similar method was used to determine M_L using TERRAscope data. The station corrections for M_L are determined such that the M_L values determined from TERRAscope agree with those from the traditional optical Wood-Anderson seismographs. For 1.5 6.5, M_L saturates. The ratio E_S/M_0 (M_0: seismic moment), a measure of the average stress drop, for six earthquakes, the 1989 Montebello earthquake (M_L = 4.6), the 1989 Pasadena earthquake (M_L = 4.9), the 1990 Upland earthquake (M_L = 5.2), the 1991 Sierra Madre earthquake (M_L = 5.8), the 1992 Joshua Tree earthquake (M_L = 6.1), and the 1992 Landers earthquake (M_w = 7.3), are about 10 times larger than those of the others that include the aftershocks of the 1987 Whittier Narrows earthquake, the Sierra Madre earthquake, the Joshua Tree earthquake, and the two earthquakes on the San Jacinto fault. The difference in the stress drop between the mainshock and their large aftershocks may be similar to that between earthquakes on a fault with long and short repeat times. The aftershocks, which occurred on the fault plane where the mainshock slippage occurred, had a very short time to heal, hence a low stress drop. The repeat time of the major earthquakes on the frontal fault systems in the Transverse Ranges in southern California is believed to be very long, a few thousand years. Hence, the events in the Transverse Ranges may have higher stress drops than those of the events occurring on faults with shorter repeat times, such as the San Andreas fault and the San Jacinto fault. The observation that very high stress-drop events occur in the Transverse Ranges and the Los Angeles Basin has important implications for the regional seismic potential. The occurrence of these high stress-drop events near the bottom of the seismogenic zone strongly suggests that these fault systems are capable of supporting high stress that will eventually be released in major seismic events. Characterization of earthquakes in terms of the E_S/M_0 ratio using broadband data will help delineate the spatial distribution of seismogenic stresses in the Los Angeles basin and the Transverse Ranges.

Tomographic inversions for mantle P wave velocity structure based on the minimization of l2 and l1 norms of International Seismological Centre Travel Time Residuals

We use International Seismological Centre (ISC) P arrival data (1964–1987) and iterative algorithms which minimize the l1 and l2 residual norms to solve simultaneously for three‐dimensional P velocity variations in Earth's mantle, source mislocations, and station corrections. We find that the maximum velocity perturbations produced by the l1 minimization (approximately ±4% in our final model) are relatively insensitive to smoothing and damping constraints. Therefore, using an l1 norm criterion allows us to keep the bias introduced to the inversion to a minimum. Among the well‐resolved features contained in both the l1 and the l2 velocity models are a fast anomaly in the lower mantle beneath the Tonga‐New Hebrides subduction zone to a depth of 1670 km and another fast anomaly beneath the Japanese island arc and eastern Asia. Continuity between these anomalies and shallower fast anomalies is not clear. A fast anomaly extending from 670 km to 2070 km depth appears beneath eastern North America, the Caribbean, and north central South America. A broad, fast anomaly appears beneath eastern Asia just above the core‐mantle boundary as well as several slow anomalies under the Pacific basin of comparable size. Both models contain a circum‐Pacific ring of 2% lower velocities in the depth range 0–200 km, associated with back arc basins. High velocities (over 2%) associated with the continental shields tend to disappear below 400 km, though a significant region of high velocity remains beneath the Siberian platform in the 400 to 670 km depth interval.

震源座標の関数としての観測点補正値を用いた震源決定方法

震源座標の関数としての観測点補正値を用いた震源決定方法