Station Corrections Research Articles

Seismicity characteristics of a potentially active Quaternary volcano: The Tatun Volcano Group, northern Taiwan

The Tatun Volcano Group (TVG) is located at the northern tip of Taiwan, near the capital Taipei and close to two nuclear power plants. Because of lack of any activity in historical times it has been classified as an extinct volcano, even though more recent studies suggest that TVG might have been active during the last 20 ka. In May 2003 a seismic monitoring project at the TVG area was initiated by deploying eight three-component seismic stations some of them equipped with both short-period and broadband sensors. During the 18 months observation period local seismicity mainly consisted of high frequency earthquakes either occurring as isolated events, or as a continuous sequence in the form of spasmodic bursts. Mixed and low frequency events were also present during the same period, even though they occurred only rarely. Arrival times from events with clear P-/S-wave phases were inverted in order to obtain a minimum 1D velocity model with station corrections. Probabilistic nonlinear earthquake locations were calculated for all these events using the newly derived velocity model. Most high frequency seismicity appeared to be concentrated near the areas of hydrothermal activity, forming tight clusters at depths shallower than 4 km. Relative locations, calculated using the double-difference method and utilising catalogue and cross-correlation differential traveltimes, showed insignificant differences when compared to the nonlinear probabilistic locations. In general, seismicity in the TVG area seems to be primarily driven by circulation of hydrothermal fluids as indicated by the occurrence of spasmodic bursts, mixed/low frequency events and a b-value (1.17 ± 0.1) higher than in any other part of Taiwan. These observations, that are similar to those reported in other dormant Quaternary volcanoes, indicate that a magma chamber may still exist beneath TVG and that a future eruption or period of unrest should not be considered unlikely.

Comment on ‘Banana-doughnut kernels and mantle tomography’ by van der Hilst and de Hoop

The claim by van der Hilst and de Hoop that finite-frequency (FF) inversion of seismic traveltimes does not result in measurable improvements in tomographic images is misguided, and based upon a biased selection of images in the upper mantle, where wave front healing effects are indeed small, and where our models are generally poorly resolved because we primarily used teleseismic waves that travel steeply in the upper mantle; and upon an improper application of statistics to the better-resolved anomalies in the lower mantle. If station corrections for long-period P waves are computed using ray theory, as we do, unmodelled FF effects may be responsible for slow anomalies of up to 0.3 per cent beneath very small island stations, but these effects are negligible for larger islands such as Reunion and Kerguelen. The presence of a plume beneath these islands is the most probable explanation for the observed low velocities.

P- andS-velocity images of the lithosphere-asthenosphere system in the Central Andes from local-source tomographic inversion

SUMMARY About 50 000 P and S arrival times and 25 000 values of t* recorded at seismic arrays operated in the Central Andes between 20°S and 25°S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P, S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp, Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting.

Relocation of Early and Late Aftershocks of the 2001 Bhuj Earthquake Using Joint Hypocentral Determination (JHD) Technique: Implication toward the Continued Aftershock Activity for more than Four Years

We employed layered model joint hypocentral determination (JHD) with station corrections to improve location identification for the 26 January, 2001 Mw 7.7 Bhuj early and late aftershock sequence. We relocated 999 early aftershocks using the data from a close combined network (National Geophysical Research Institute, India and Center for Earthquake Research Institute, USA) of 8–18 digital seismographs during 12–28 February, 2001. Additionally, 350 late aftershocks were also relocated using the data from 4–10 digital seismographs/accelerographs during August 2002 to December 2004. These precisely relocated aftershocks (error in the epicentral location<30 meter, error in the focal depth estimation < 50 meter) delineate an east-west trending blind thrust (North Wagad Fault, NWF) dipping (~ 45°) southward, about 25 km north of Kachchh main land fault (KMF), as the causative fault for the 2001 Bhuj earthquake. The aftershock zone is confined to a 60-km long and 40-km wide region lying between the KMF to the south and NWF to the north, extending from 2 to 45 km depth. Estimated focal depths suggest that the aftershock zone became deeper with the passage of time. The P- and S-wave station corrections determined from the JHD technique indicate that the larger values (both +ve and -ve) characterize the central aftershock zone, which is surrounded by the zones of smaller values. The station corrections vary from −0.9 to +1.1 sec for the P waves and from −0.7 to +1.4 sec for the S waves. The b-value and p-value of the whole aftershock (2001–2004) sequences of Mw ≥ 3 are estimated to be 0.77 ± 0.02 and 0.99 ± 0.02, respectively. The p-value indicates a smaller value than the global median of 1.1, suggesting a relatively slow decay of aftershocks, whereas, the relatively lower b-value (less than the average b-value of 1.0 for stable continental region earthquakes of India) suggests a relatively higher probability for larger earthquakes in Kachchh in comparison to other stable continental regions of the Indian Peninsula. Further, based on the b-value, mainshock magnitude and maximum aftershock magnitude, the Bhuj aftershock sequence is categorized as the Mogi's type II sequence, indicating the region to be of intermediate level of stresses and heterogeneous rocks. It is inferred that the decrease in p-value and increase in aftershock zone, both spatially as well as depth over the passage of time, suggests that the decay of aftershocks perhaps could be controlled by visco-elastic creep in the lower crust.

An ML Scale for the La Paz-Los Cabos Region, Baja California Sur, Mexico

We have developed a local magnitude scale to quantify the size of earthquakes in the La Paz–Los Cabos region, Baja California Sur, a seismically active region with little seismic history reported. For this, we used digital data from 501 local and regional earthquakes recorded by temporary networks that were deployed in the region from October 1998 to December 2003. A multiple linear regression analysis applied to 3328 average zero-to-peak synthetic Wood–Anderson amplitudes produced the following results: a −log A attenuation function for the region, the local magnitude for each earthquake, and a magnitude correction for each station. Regarding the attenuation function, our best fit to the regressed data is −log A ;eq (1.2052 ± 0.0177) log ( r /100) + 0.0016 ± 0.0003) ( r − 100) + 3, where r is the hypocentral distance. The ground-motion attenuation represented by this relationship is similar to the attenuations reported for the Peninsular Ranges of Baja California and for the southern California region. In addition, the station magnitude corrections obtained from our regression analysis range from −0.46 to 0.23. As it should be expected, positive station corrections correlate well with hard-rock sites, whereas negative corrections correlate with soft or sedimentary rock sites. The combined use of Richter’s definition of magnitude with the attenuation function and station corrections determined here give reliable and homogeneous estimations of the local magnitude.

Modeling of the 21 May 1997 Jabalpur Earthquake in Central India: Source Parameters and Regional Path Calibration

The primary objective of this study is to model regional and far-regional seismograms recorded from the 21 May 1997 Jabalpur earthquake ( M w 5.8) in central India to calibrate the propagation path in and around the Indian subcontinent. To accomplish this goal, we first established its source parameters ( δ = 65°, λ = 68°, and φ = 70°, h = 35 km) and source complexity, which consisted of two sources with a total seismic moment of 5.88 × 10 24 dyne cm and a source 1 to source 2 moment ratio of 1:4 by modeling teleseismic P and depth phases. Source parameters were independently verified by modeling near-regional seismograms of three stations, namely bhpl (Bhopal), blsp (Bilaspur), and hyb (Hyderabad), by using the same source complexity established in the teleseismic modeling and station-specific wave-propagation path models. The earthquake was relocated by fixing its depth at 35 km and using travel times of teleseismic P, pP, sP phases and regional P and S phases. The International Association of Seismology and Physics of the Earth’s Interior model was used for the location with its crust replaced by the regionalized crustal model. The relocation parameters were: origin time, 22h51m30.80sec ± 0.83 sec; latitude, 23.083° N; longitude, 80.091° E; h = 35 km. With this relocation, source-specific station corrections were established that, in turn, were used to relocate its largest aftershock. Next, we analyzed the composition of regional phases to understand how they were excited, especially how up-going and down-going seismic waves evolved and interfered as a function of distance. This understanding and the newly derived source information were used to calibrate station-specific paths to other far-regional and upper-mantle stations. To this end, we modeled amplitude and travel times of different phases, such as the P nl , S , SmS , Sm ( up ) S , and surface waves observed at far-regional stations (5° ≤ Δ ≤ 25°). This study established far-distance wave-propagation models for paths toward CHTO (Chiang Mai, Thailand), LSA (Lhasa, China), and AAK (Ala-Archa, Kyrgyzstan). We also calibrated a wave-propagation model for a path from northwest India to station NIL (Nilore, Pakistan) using seismograms recorded from an earthquake near Pokhran, the nuclear test site in India.

How can the ISC location procedures be improved?

For many decades the International Seismological Centre (ISC) has used a well defined procedure to locate seismic events using first P-onsets and the Jeffreys–Bullen tables ([Jeffreys, H., Bullen, K.E., 1940. Seismological Tables. British Association for the Advancement of Science, Gray Milne Trust, London, 50 pp]) as the travel-time reference. However, during the last two decades, more accurate spherical Earth models have been published and enhanced computer capabilities make it easier to implement more sophisticated data inversion schemes. Several features that may improve the location procedure at the ISC were systematically tested using the location program HYPOSAT. The investigated features were the influence of • the usage of the spherical Earth models JB, PREM, IASP91, SP6, and AK135; • the usage of later onsets; • travel-time corrections for local crustal structure based on CRUST 5.1; • different weighting of the residuals; • reducing the amount of defining data at a late stage of the inversion process. Application of different combinations of these factors led to a reduction of the location errors for the 156 test events, of which the epicenter is known with an accuracy of less than 5 km. However, no clear rule of common factors to achieve this result could be defined. Most promising is the application of AK135 as model for travel-time calculations, applying crust specific station corrections and corrections for the reflection points of surface reflections, a combined usage of surface and core reflections, and removing data which have large residuals or do not much contribute to the solution for the last iterations.

M L Determination for Local and Regional Events Using a Sparse Network in Southwestern Germany

A method for the determination of consistent local magnitude M L values (Richter scale, or M WA) for earthquakes with epicentral distances ranging from 10 km through 1000 km is demonstrated. The raw data consists of nearly 1300 amplitude readings from a network of six digital seismographs in Baden–Wurttemberg (Southwestern Germany) during 26 months starting in 1995, later extended by another 1000 amplitude readings until 1999. Relying on most of the basics introduced by C.F. Richter a three-parameter attenuation curve (distance correction, magnitude-distance relation) for Baden–Wurttemberg and adjacent areas is presented. Station corrections are evaluated and the attenuation curve is calibrated with respect to other agencies for distances greater than 650 km. Reasonable parametrisations are discussed and meaningful error bars are attributed. Finally, a seventh station is incorporated by means of its station correction alone, without needing to update the attenuation curve.

Moment magnitude determination of local seismic events recorded at selected Polish seismic stations

The paper presents the method of local magnitude determination used at Polish seismic stations to report events originating in one of the four regions of induced seismicity in Poland or its immediate vicinity. The method is based on recalculation of the seismic moment into magnitude, whereas the seismic moment is obtained from spectral analysis. The method has been introduced at Polish seismic stations in the late 1990s but as of yet had not been described in full because magnitude discrepancies have been found between the results of the individual stations. The authors have performed statistics of these differences, provide their explanation and calculate station corrections for each station and each event source region. The limitations of the method are also discussed. The method is found to be a good and reliable method of local magnitude determination provided the limitations are observed and station correction applied.

Geophysical and geological interpretation of discrepancies in site residuals in northern Morocco: a simple qualitative approach

Abstract Surface geology, gravimetric, and heat flow data are used to explain discrepancies between M D site residuals, using the average magnitude site residuals ( AMSR j ) which are opposite of the station corrections ( cSta j ) calculated for the 25 selected short-period seismic stations of the Moroccan seismological network. M D site residuals range from −0.29 to +0.32, and appear to be the result of the complex interaction of the predominant near-station geophysical and geological parameters. With this simple qualitative approach, we found that high negative residuals are obtained for older well-consolidated Precambrian crystalline rocks with low surface heat flow, and high positive residuals are attributed to recent unconsolidated sediments with high heat flow. The explanation for the intermediate residual values remains more complex and requires more accuracy and better quality data, and may not be fully justified in this study.

Revised ML Determination for Crustal Earthquakes in Taiwan

The local magnitude scale M L is an empirically derived scale anchored to a zero magnitude reference event. Richter defined the amplitude of ground shaking at an epicentral distance of 100 km for a zero magnitude earthquake in southern California. The crustal attenuation characteristics of Taiwan result in a relatively high ML when compared to moment magnitude, Mw. We therefore define a revised ML scale for crustal earthquakes in the Taiwan region that is more consistent with Mw. Using observed peak ground shaking and Mw, we determine a new log A0 curve of ground shaking versus hypocentral distance, R, for a zero magnitude earthquake, and station correction factors. The log A0 curve determined in this study is, log A (R) 0.332 1.568 • log(R) 0.280. 0 Using this new log A0 curve and station corrections, the new ML is more consistent with Mw, with a 0.2 magnitude unit uncertainty. The new log A0 curve has a value of 2.80 at the distance of 100 km compared to the anchor point of log A0 3 at the same distance as defined by Richter for southern California. This means that the current ML estimates in Taiwan (which use Richter's definition) average 0.2 magnitude units larger than their Mw. The station correction factors also determined are large, from 0.40 to 0.52 magnitude units. The use of station corrections in routine seismic network operation in Taiwan will improve magnitude estimates. This is particularly important for smaller events when recording stations may be pre- dominantly on either hard rock or soft soil sites, which could lead to under- or overestimates of the magnitude by up to half a magnitude unit.

Earthquake relocations, fault zone geometry and constraints on lateral velocity variations using the joint hypocenter determination method in the Taiwan area

AbstractFour subsets of earthquakes recorded by an island-wide seismic network from 1991 to 2002 in the Taiwan area are relocated using the joint hypocenter determination (JHD) technique. Relatively large horizontal and vertical shifts are observed during the relocation, which indicates there may be systematic earthquake miss-locations in the Taiwan earthquake catalog due to the over-simplification of complicated earth structures to a simple 1-D velocity model. Generally, earthquakes are more clustered after the JHD relocation. Subsurface structures defined by the relocated seismicity become clearer, while they are not as clear in the initial hypocenters. Significant subsurface structures defined by the relocated seismicity are fault systems beneath the Central Mountain Range, beneath the northern Longitudinal Valley and Coastal Range, beneath the southern Longitudinal Valley and Coastal Range, and beneath the Chao-Chou fault in southern Taiwan. Another set of results from the JHD analysis,P- andS-wave station corrections, provide valuable information on the lateral velocity variations. The JHD station corrections indicate that upper crustal materials in the Central Mountain Range are characterized by higher velocity than those in the Western Foothills and Coastal Plain. The patterns of the observed JHD station corrections are also thoroughly consistent with surface geology observations. Analysis of the four clusters of earthquakes resulted in a remarkable similarity in JHD station corrections, indicating that the overlapping subsurfaces where two or more clusters of rays had traveled through were relatively shallow. These subsurfaces are responsible for the observed patterns of positive and negative JHD station corrections.

Topography of the Seismic Velocity Discontinuities beneath the Chugoku and Shikoku Districts, Southwest Japan

The first P-arrival-time data from 513 local earthquakes were analyzed to study lateral variation of the depth to the Conrad and Moho discontinuities beneath the Chugoku and Shikoku districts, southwest Japan, as well as to determine earthquake hypocenters and P-wave station corrections. The depth to the discontinuity was estimated by minimizing the travel-time residuals of more than 8700 first P arrivals observed at 55 seismic stations. The Conrad and Moho discontinuities are located within depth ranges of 15–25 km and 30–40 km, respectively. The Moho is deeper under the mountain area than under the Seto Inland Sea area, and especially deep under the Pacific Coast of the Shikoku district and the mountain area in the Chugoku district. The depth variation of the Moho is quite similar to the Bouguer gravity anomaly distribution and the lateral variations of the P-wave velocity. The deep Moho under the southern Shikoku is located at the portion in which the continental Moho under the island arc meets the oceanic Moho that is the boundary interface between the oceanic crust and the Philippine Sea (PHS) plate dipping toward the back arc. Although there are high mountains in the northern and middle Shikoku, the Moho is not so deep because subduction of the PHS plate prevents the Moho from getting deep, while the Moho is deep due to isostatic balance under the mountain area in the Chugoku district. In addition, we indicated the possibility that the upper boundary of the oceanic crust just above the high-velocity PHS plate is in contact with the deep Moho under the western Chugoku. The contact of the Moho with the oceanic crust can explain the markedly negative gravity anomaly observed in the western Chugoku and the later phase that appears just after the first P arrival from local earthquakes.

Calibration of International Monitoring System (IMS) Stations in Central and Eastern Asia for Improved Seismic Event Location

It is well documented that lateral heterogeneities in the earth can introduce serious errors in seismic event locations derived from radially symmetric earth models, in particular, for small events recorded only by sparse networks of regional stations. It should be possible to significantly improve the seismic location accuracy for such events by applying travel-time calibration information specific to the regions where the sources and stations are located. This article describes a research investigation directed toward regional travel-time calibration of 30 International Monitoring System (ims) stations in eastern Asia. For this calibration an initial 3D velocity model covering the entire region was constructed from several submodels. Seismic travel times in this complex 3D velocity model have been computed by using a raytracing algorithm based on a finite-difference approximation to the eikonal equation, which is believed to be accurate to within 0.3–0.5 sec over the regional distance range of interest. A new regional tomography algorithm has also been implemented which solves the fully nonlinear problem by iterating over linear inversion steps in which event hypocenters, model velocities, and ray paths can all be updated. Extensive testing and validation of the final tomographically refined velocity model have been performed with data from numerous ground truth (gt) explosions and earthquakes throughout the region, and it has been demonstrated that this model predicts P -wave travel times with associated root-mean-square (rms) errors on the order of 1 sec across the study region. Finally, a completely new multistation kriging method, which satisfies seismic reciprocity constraints, has been formulated and applied to derive empirical corrections to account for remaining unmodeled error, further reducing the rms error to approximately 0.7 sec. These results support the conclusion that our new 3D velocity model for this region represents a significant improvement over the default IASP91 model and will provide improved seismic location capability, in particular, for events recorded by sparse networks of regional stations. Online Material: Source-specific station correction and travel-time calibration files.

High-resolution teleseismic body-wave tomography beneath SE Romania - I. Implications for three-dimensional versus one-dimensional crustal correction strategies with a new crustal velocity model

SUMMARY The CALIXTO (Carpathian Arc Lithosphere X-Tomography) experiment offers a dense, high-quality data set to study the lithospheric/asthenospheric system underneath SE Romania, an earthquake-prone region in SE Europe. To increase the image resolution of structures in the uppermost mantle, the application of crustal traveltime corrections by a priori information before the teleseismic traveltime inversion has become a well-accepted procedure. For such a correction we present a regional 3-D crustal seismic velocity model that serves as the basis for a high-resolution teleseismic tomography (forthcoming paper by Martin et al.). Our 3-D crustal model is based on recent research in the region. We collect new results from two long-range seismic refraction lines, 3-D refraction tomography and teleseismic Ps conversions. Adding previously published models of the sediment distribution, Conrad and Moho depths, as well as crustal seismic P-wave velocities, we compile a 3-D crustal model for SE Romania. This 3-D model does not contain shallow small-scale heterogeneities (<10 km), but it reflects the large-scale structures such as variations in sediment thickness, average seismic velocities and 3-D Moho depth. It is well suited for the correction of teleseismic traveltime residuals, a prerequisite for a high-resolution teleseismic tomography study: for example, traveltime delays of up to 1.3 s are caused by the almost 20-km thick layer of sediments in the Focsani Basin. Such delays are comparable to or larger in size than the expected upper mantle traveltime residuals. We study the significance of 3-D crustal traveltime corrections relative to 1-D station corrections and show that the complex basin structures in SE Romania require a 3-D approach to reduce the smearing of crustal anomalies into the mantle. By modelling synthetic mantle structures with a slab, as it is expected for SE Romania, we also investigate how to adapt the inversion strategy, if crustal corrections are applied. Significant improvements are found by including the already corrected crustal layers in the inversion procedure, thereby enabling the inversion algorithm to project still remaining uncertainties in the less-resolved upper crustal layers. However, the fixing of the upper layers during the inversion due to the a priori knowledge of the crustal velocity anomalies clearly leads to smearing of uncorrected anomalies that are possibly located close to the crust–mantle boundary.

Three-dimensionalVPandVSstructural models associated with the active subduction and collision tectonics in the Taiwan region

SUMMARY 3-D VP and VS models for the crust and upper mantle beneath the Taiwan area have been determined using selected high-resolution earthquake data from an island-wide seismic network and two local seismic arrays. Lateral structural variations in the upper crust, as also evident from surface geology, are responsible for the observed large traveltime residuals or station corrections. Prior shallow velocity information inferred from traveltime residuals and joint hypocentral determination (JHD) station corrections for the uppermost crust is essential to facilitate a reliable tomographic inversion. A finite-difference method, that is efficient and accurate for a highly heterogeneous velocity structure, is applied to calculate P- and S-wave traveltimes from the source to receiving stations. All earthquakes in the Taiwan Central Weather Bureau’s catalogue are then relocated using the resultant 3-D VP and VS models. The depth of the Moho varies significantly, especially along the east‐west direction. In the western Coastal Plain and Western Foothills the depth of the Moho is around 35 km, which deepens gradually eastward, reaches a maximum depth of ∼55 km beneath the eastern Central Mountain Range, shallows up rapidly beneath the Longitudinal Valley and Coastal Range, and merges with the thin Philippine Sea Plate offshore of eastern Taiwan. In central Taiwan, the Central Mountain Range is bounded to the east and west by two steeply westward dipping active faults from the upper crust to a depth of about 30 km. Therefore, the uplifted and thickened Central Mountain Range serves as a backstop for the converging Eurasian and Philippine Sea plates. The crust beneath the Central Mountain Range is characterized by a brittle, high-velocity and seismically active upper crust (<15 km) and a ductile, low-velocity and aseismic mid-to-lower crust (below 15 km), most probably due to the high geothermal activity from the excess heat supplied from the hot upper mantle beneath the thin oceanic crust to the east, from the surrounding hotter upper mantle beneath the thickened continental crust, and from shear heating during active collision. The collision zone in eastern Taiwan is characterized by an active and steeply eastward dipping seismic zone along a region of low VP and high VP/VS ratio near the Taitung region in southeastern Taiwan. It transforms into an active westward steeply dipping seismic zone along a transition zone between the high VP and VS oceanic crust and the low VP and VS continental crust near Hualien region in central eastern Taiwan. There is no apparent seismicity within many sedimentary basins imaged from the tomographic inversion. However, af ew basins are either bounded on one side by an active fault or are characterized by blind faults beneath. The geometry of the subduction zone in northeastern Taiwan can be clearly imaged from the relocated earthquake locations. Behind the subduction, a region of low VP and high VP/VS ratio at depths of 5 to 10 km can be identified beneath the Tatun-Chilung volcano group indicating a potential magma reservoir. Two steeply dipping linear seismic zones in the volcano region may mark the upward escape paths of the magmatic materials in the region.

3D First-Arrival Regional Calibration Model of Northern Eurasia

Seismological monitoring of the Comprehensive Test Ban Treaty (CTBT) requires detailed knowledge of travel-time characteristics of seismic phases across large areas. We use first-arrival travel times from several of Russian Deep Seismic Sounding (dss) profiles, primarily those from Peaceful Nuclear Explosions (pnes), to construct a 3D regional travel-time model of northern Eurasia. The method used, which can be viewed as a generalization of the existing regionalization techniques, is based on apparent-velocity-based spatial interpolation of the travel times picked from dss records. The resulting travel-time field is described in terms of an apparent velocity model in the 3D space of geographic coordinates and ray parameter. In the travel-time calibration, this model could be used to construct approximate source-specific station correction surfaces for any location within the region and also as a region-specific reference model to be used as a background for further calibration effort. This approach allows incremental refinement of the model as additional travel-time data become available. By converting the travel-time model into depth, a 3D regional velocity model is obtained, providing a description of the general features of the upper mantle in northern Eurasia.

The 2001 ML 6.2 Jackson Bay earthquake sequence, South Island, New Zealand

Following the 2001 December 7 Jackson Bay earthquake (ML 6.2, MW 5.8), a temporary network of five portable seismographs was deployed in the region to record aftershock activity. Data recorded by the temporary network and nearby New Zealand National Seismological Network stations have been used to define a velocity model for the region and station corrections for each recording station. The locations of the best recorded aftershocks and the revised location of the mainshock indicate that the Jackson Bay earthquake sequence occurred 3–10 km to the east of the Alpine Fault, which is vertical in this region. A fault plane solution obtained from body‐wave modelling suggests the mainshock was primarily a reverse event (rake = 103°) centred at c. 4 km depth on a fault striking northeast‐southwest (48°) and probably dipping to the southeast (45°), which is roughly consistent with the Harvard CMT solution for this earthquake. However, on examination of the aftershock locations, such a fault plane is not clear, nor is any other. The aftershocks are located mainly in two clusters near each other at depths between 3 and 8 km and aligned approximately north‐south. Their positions are in accord with induced stress considerations and the mainshock fault plane lying between the clusters. Individual focal mechanisms for 33 aftershocks have a wide range of solutions. As a group, however, their P and T axes are reasonably well aligned and consistent with the background stress regime in the region as determined by direct inversion of P‐wave polarities. The Jackson Bay earthquake was the third thrust earthquake of magnitude >6 to occur just east of the Alpine Fault in a 7 yr period. Consideration of the mechanics of this earthquake, and the previous two, suggests that the regional stress is at a high level, in accord with the long elapsed time since a large Alpine Fault event. Although the area is small, the Jackson Bay mainshock induced a mainly positive change in Coulomb Failure Stress (CFS) on the closest section of the Alpine Fault, up to c. 0.7 MPa (7 bars).

The Local Magnitude Scale in the Korean Peninsula

A formula to determine the local magnitude (ML) following Richter’s original definition was empirically derived for the Korean Peninsula. A total of 1,644 digital seismograms from 142 Korean earthquakes that occurred from 1997 to 2000 were corrected for instrument response and convolved with the nominal Wood-Anderson torsion seismograph response to be appropriate for the original definition of ML. Then, the zero-to-peak amplitude was measured in millimeters on the synthetic Wood-Anderson seismogram. Multiple regression analysis was conducted to determine distance and station correction terms for the measured peak amplitudes. The best-fit solution for ML yielded the following formula for the Korean Peninsula:

Deep low-frequency earthquake swarm in the mid crust beneath Mount Fuji (Japan) in 2000 and 2001

Beneath Mount Fuji, the highest active volcano in Japan, deep low-frequency (DLF) earthquake activity has been monitored since the early 1980s. The DLF earthquakes occurred in the mid-crustal depth range, and burst-type activity lasting from several minutes to 30 min was detected 10 to 20 times in an ordinary year. The DLF earthquake activity increased sharply in the period from October 2000 to May 2001, showing swarm-like activity. The occurrence rate during the DLF earthquake swarm was approximately 20 times higher than the usual activity, and the wave energy released during the swarm period was twice as high as the total wave energy during the past 20 years. The DLF earthquakes in the period from 1987 to 2001 were relocated by estimating station corrections in order to reduce the effect of the change of seismic station distribution. The epicenters of most DLF earthquakes occurred in an elongated region with a long axis of about 5 km, whose center is located 2–3 km NE from the summit. A few percent of the DLF earthquakes, however, occurred around the summit area, significantly apart from the main epicenter region. The focal depths of well-located DLF events range from 10 to 20 km. During the high activity period in 2000 and 2001, most DLF events occurred within this main hypocenter area. The sharp increase of DLF earthquake activity at Mount Fuji started immediately after magma discharge and intrusion events in the Miyake-jima and Kozu-shima regions in July and August 2000. The tectonic and volcanic activity changes around the area suggest that the DLF earthquake swarm at Mount Fuji was triggered by the change of state of the deep magmatic system around Mount Fuji.