Travel times of Pn -waves in the Aegean and surrounding area

Summary. Based on accurately located 23 very shallow earthquakes ( h = 1–14 km) in northern and central Greece by portable networks of seismic stations and by the joint epicentre method, the travel times of the P_n -waves from the foci of these earthquakes to the sites of 54 permanent stations in the Balkan region have been determined. The travel times of P_n -waves in the central and eastern part of the area (eastern Greece, south-eastern Yugoslavia, the Aegean Sea, Bulgaria, southern Romania, western Turkey) fit a straight line very well with the P_n velocity equal to 7.9 ± 0.1 km s^-1. On the contrary, the travel times of P_n -waves to stations in the western part of the area (Albania, western Greece) do not fit this curve because the P_n -waves travelling to these stations are delayed by more than 1 s due to the thicker crust under the Dinarides–Hellenides mountain range. Time delays for P_n -waves have been calculated for each permanent station in the Balkan area with respect to the mean travel-time curve of these waves in the central and eastern part of the area. Corrections of the travel times for these delays contribute very much to the improvement of the accuracy in the location of the shallow earthquakes in the Aegean and surrounding area.     

Source mechanism of the deep Colombian earthquake of 1970 July 31 from the free oscillation data

Summary. The method proposed by Mendiguren to determine the source parameters from free oscillation data is applied to the 1970 July 31 deep Colombian earthquake. The results indicate a source propagating horizontally for about 150 km along the lithosphere and cutting across its width. The slab behaves as a guide for source propagation. The horizontal propagation velocity is determined as 3.8 km/s. The intensity of the source grew proportionately to the second power of the propagation distance. This rate of source intensity growth may be interpreted either by a fan-shaped fault model or by a cone-shaped volume source. The average slip and stress drop are estimated as 360 cm and 300 bar for the fault model. For the volume source model the transformational shear strain and stress are estimated as 11 × 10⁻⁵ and 160 bar. There is no evidence of a double couple radiation preceding the P origin time. It is shown that the isotropic and deviatoric components of the moment tensor cannot be uniquely resolved when only observations of a single mode are available. It is observed that, from a statistical basis, the available ₀ S_n data for Colombian shock can be equally well explained by a pure deviatoric source model or by a source model including an isotropic component. Numerical experiments indicate that the inclusion of higher mode data does not change this situation. But, on the other hand, numerical experiments show that the available data and the scheme used for the inversion would not result in a solution including an artificial implosive component if the actual source were pure deviatoric. If the departure from a pure deviatoric source is produced by noise, it has to be non-random, as it could be produced by lateral heterogeneities not included in the inversion scheme.     

Three-dimensional VP and VP /VS models of the upper crust in the Friuli area (northeastern Italy)

3-D images of P velocity and P - to S -velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthquake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The best-fitting V _P and V _P / V _S 1-D models were computed before the 3-D inversion. V _P was measured on two rock samples representative of the investigated upper layers of the Friuli crust. The tomographic V _P model was used for modelling the gravity anomalies, by converting the velocity values into densities along three vertical cross-sections. The computed gravity anomalies were optimized with respect to the observed gravity anomalies. The crust investigated is characterized by sharp lateral and deep V _P and V _P / V _S anomalies that are associated with the complex geological structure. High V _P / V _S values are associated with highly fractured zones related to the main faulting pattern. The relocated seismicity is generally associated with sharp variations in the V _P / V _S anomalies. The V _P images show a high-velocity body below 6 km depth in the central part of the Friuli area, marked also by strong V _P / V _S heterogeneities, and this is interpreted as a tectonic wedge. Comparison with the distribution of earthquakes supports the hypothesis that the tectonic wedge controls most of the seismicity and can be considered to be the main seismogenic zone in the Friuli area.     

Surface wave dispersion and Earth structure in south-eastern China

Summary. A reconnaissance study of crust and mantle structure in southeastern China was made using surface waves confined to that region from recent earthquakes. Data from the WWSSN stations ANP arid SEO, and from the digital stations TATO and MAT, were used to measure fundamental-mode group velocities of Love and Rayleigh waves over nine paths in south-eastern China, an area which has been technically quiet since the early Cenozoic. Crustal structure in this region is typical of stable continents, but shear-wave velocities in the uppermost mantle are low for a continent, 4.45 km s⁻¹ or less. Other seismological data support this observation.     

Earthquake intensity attenuation pattern in the United States

Summary. Attenuation of earthquake intensities with epicentral distance was studied by analysing the intensity data for 39 earthquakes in the United States. Attenuation of MM intensity ( I ) with distance (Δ km) follows a simple relation of the type log I = log I ₀ - m Δ, where I ₀ is the intensity at the epicentre and m is a constant. Slope m is found to be inversely proportional to the square of the focal depth. Intensity attenuation pattern in the United States in general can be represented by a unified relation I/I ₀= exp [-(0.8999/ h ²+ 0.0014)Δ] where 16km ≤ h ≤ 60km. Intensities were calculated with the help of this equation and a good agreement with the observed intensities were found. A comparative study has also been made between the attenuation relations applicable to India and the United States.     

Analysis of the crustal velocity structure of the British Isles using teleseismic receiver functions

The onshore crustal and upper mantle velocity structure of the British Isles has been investigated by teleseismic receiver function analysis. The results of the study augment the dense offshore and sparse onshore models of the velocity structure beneath the area. In total almost 1500 receiver functions have been analysed, which have been calculated using teleseismic data from 34 broadband and short-period, three-component seismic recording instruments. The crustal structure has primarily been investigated using 1-D grid search and forward modelling techniques, returning crustal thicknesses, bulk crustal V_p / V_s ratio and velocity-depth models. H −κ stacking reveals crustal thicknesses between 25 and 36 km and V_p / V_s ratios between 1.6 and 1.9. The crustal thicknesses correlate with the results of previous seismic reflection and refraction profiles to within ±2 km. The significant exceptions are the stations close to the Iapetus Suture where the receiver function crustal thicknesses are up to 5 km less than the seismic refraction Moho. This mismatch could be linked to the presence of underplated magmatic material at the base of the crust. 1-D forward modelling has revealed subcrustal structures in northern Scotland. These correlate with results from other UK receiver function studies, and correspond with the Flannan and W-reflectors. The structures are truncated or pinch out before they reach the Midland Valley of Scotland. The isolated subcrustal structure at station GIM on the Isle of Man may be related to the closure of the Iapetus Ocean.     

Experimental study of the anisotropy of longitudinal and transverse waves from local earthquake records

Summary. The paper gives the results of a study of the anisotropy of seismic wave velocities within the Ashkhabad test field in Central Asia. The anisotropy was studied by analysing variations in the values of apparent velocities of first arrivals for epicentral distances ranging from 30 to 130 km and by analysing the delays (Δ t_s1-_s2 ) between the arrival times of shear waves with different polarizations.
The velocities of P -waves vary with azimuth from 5.3 to 6.27 km s^-1 and the velocities of S -waves vary from 3.15 to 3.5 km s^-1.
The delay times Δ t_S1 - _S2 depend on the direction of the propagation. The character of the variation of the propagation velocity of the longitudinal wave, the presence of two differently polarized shear waves S 1 and S 2 propagating at different velocities, and the character of the distribution of Δ t_S1 - _S2 on the stereogram suggest that the symmetry of the anisotropic medium is close to hexagonal with a nearly horizontal symmetry axis coinciding with the direction of maximal velocity. The azimuth of the symmetry axis of the medium is 140° and coincides with the direction of geological faults.     

On the linear relation between mb and Ms for discrimination between explosions and earthquakes

Summary. The statistical capability of the m _b: M _s discriminant for the discrimination of earthquake and explosion populations is examined by application of discriminant functions to a group of 83 explosions and 72 earthquakes in Eurasia. Equations are derived for the probability that an event is an earthquake or an explosion. The positive sign of DIS in the decision index equation, DIS _i = 34.3383 – 11.9569 mb _t + 7.1161 M _si , indicates that the event i is an earthquake. Its negative sign indicates that event i is an explosion. The probability of correct classification for an event, P _i , is related to its DIS _i value, by P _i = [1-exp (DIS _i )]⁻¹, where a large, positive DIS indicates a high probability that an event is an earthquake and a large, negative DIS indicates a high probability that an event is an explosion. The discrimination line M _s = 1.680 m _b– 4.825, or m _b= 0.595 M _s+ 2.872 very successfully separates the explosion population from the earthquake population. The points on this line have an equal chance of being an earthquake or an explosion; moreover, for any event, the distance parallel to the M _s-axis from the point representing that event in the m _b: M _s plane to this line is a measure of the probability for the correct classification of that event.     

Determination of Precursory Velocity Anomalies in New Zealand from Observatory Data on Local Earthquakes

The ratio of shear to compressional travel times from a local earthquake gives a good estimate of the average velocity ratio for the wave path, providing the origin time has been moderately well determined. Thus changes of velocity ratio can be identified and mapped from standard observatory data. The method is applied to the principal New Zealand earthquakes since 1964: Gisborne, 1966, M _L= 6. 2; Seddon, 1966, M _L= 6.0; Inangahua, 1968, M _L= 7.1. The results suggest that monitoring could be achieved with a station spacing of 100 km. It appears that velocity change is essentially a rapid process and that the return to normality may not begin until the earthquake is imminent.     

Ivrea seismic array: a study of continental crust and upper mantle

A seismic-array study of the continental crust and upper mantle in the Ivrea-Yerbano and Strona-Ceneri zones (northwestern Italy) is presented. A short-period network is used to define crustal P - and S -wave velocity models from earthquakes. The analysis of the seismic-refraction profile LOND of the CROP-ECORS project provided independent information and control on the array-data interpretation.
Apparent-velocity measurements from both local and regional earthquakes, and time-term analysis are used to estimate the velocity in the lower crust and in the upper mantle. The geometry of the upper-lower crust and Moho boundaries is determined from the station delay times.
We have obtained a three-layer crustal seismic model. The P -wave velocity in the upper crust, lower crust and upper mantle is 6.1±0.2 km s⁻¹, 6.5±0.3 km s⁻¹ and 7.8±0.3 km s⁻¹ respectively. Pronounced low-velocity zones in the upper and lower crust are not observed. A clear change in the velocity structure between the upper and lower crust is documented, constraining the petrological interpretation of the Ivrea-type reflective lower continental crust derived from small-scale petrophysical data. Moreover, we found a V _P/ V _S ratio of 1.69±0.04 for the upper crust and 1.82±0.08 for the lower crust and upper mantle. This is consistent with the structural and petrophysical differences between a compositionally uniform and seismically transparent upper crust and a layered and reflective lower crust. The thickness of the lower crust ranges from about 8 km in front of the Ivrea body (ARVO, Arvonio station) in the northern part of the array to a maximum of about 15 km in the southern part of the array. The lower crust reaches a minimum depth of 5 km below the PROV (Provola) station.     

Aseismic fault movement before the 1995 Kobe earthquake detected by a GPS survey: implication for preseismic stress localization?

By inversion analysis of the baseline changes and horizontal displacements observed with GPS (Global Positioning System) during 1990–1994, a high-angle reverse fault was detected in the Shikoku-Kinki region, southwest Japan. The active blind fault is characterized by reverse dip-slip (0.7±0.2  m yr⁻¹ within a layer 17–26  km deep) with a length of 208±5  km, a (down-dip) width of 9±2  km, a dip-angle of 51°±2° and a strike direction of 40°±2° (NE). Evidence from the geological investigation of subfaults close to the southwestern portion of the fault, two historical earthquakes ( M _L=7.0, 1789 and 6.4, 1955) near the centre of the fault, and an additional inversion analysis of the baseline changes recorded by the nationwide permanent GPS array from 18 January to 31 December 1995 partially demonstrates the existence of the fault, and suggests that it might be a reactivation of a pre-existing fault in this region. The fact that hardly any earthquakes ( M _L>2.0) occurred at depth on the inferred fault plane suggests that the fault activity was largely aseismic. Based on the parameters of the blind fault estimated in this study, we evaluated stress changes in this region. It is found that shear stress concentrated and increased by up to 2.1 bar yr⁻¹ at a depth of about 20  km around the epicentral area of the 1995 January 17  Kobe earthquake ( M _L=7.2, Japan), and that the earthquake hypocentre received a Coulomb failure stress of about 5.6 bar yr⁻¹ during 1990–1994. The results suggest that the 1995  Kobe earthquake could have been induced or triggered by aseismic fault movement.     

Crust and upper mantle shear velocities from controlled sources

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A two ship refraction profile was undertaken on the Australian continental shelf during the Banda Sea geophysical program, carried out by the Woods Hole Oceanographic Institution, the Scripps Institution of Oceanography and the Geological Survey of Indonesia. S waves originating close to the sea bottom were observed to distances of up to 1150 km at an array of stations in northern Australia.
These observations are interpreted as implying S mantle velocities of 4.60 km s^-1 from a depth of 45 km to a depth of 76 km and 4.72 km s^-1 below a depth of 76 km.
Ratios of the P and S travel times (V_p/V_s) have been determined to be 1.74 in the crust rising to a value of greater than 1.79 below a velocity discontinuity at a depth of 200 km. It is inferred that this high value arises because the effect of temperature is greater for S than for P .
Using the data from this and other studies in the shield region of Northern Australia it has been found that the _S travel times are significantly less than predicted by the Jeffreys—Bullen tables.     

Modelling of short-interval silent slip events in deeper subduction interfaces considering the frictional properties at the unstable–stable transition regime

Recent high-resolution observations of crustal movements have revealed silent slip events (SSEs) with propagation velocities of around 10–15 km d⁻¹ and with intervals of 3–14 months along the deeper parts of the Cascadia and Nankai subduction zones. This study develops 2-D and 3-D models of these short-interval SSEs considering the frictional behaviour that was confirmed experimentally by Shimamoto for the unstable–stable transition regime. To represent this frictional behaviour, a small cut-off velocity to an evolution effect is introduced in a rate- and state-dependent friction law. When the cut-off velocity to the evolution effect is significantly smaller than that to a direct effect, steady-state friction exhibits velocity weakening at low slip velocities and velocity strengthening at high slip velocities. At the deeper Cascadia and Nankai subduction interfaces, the pore pressure is inferred to be high because of the dehydration of materials in the descending plate. Under conditions where the pore-fluid pressure is nearly equal to the lithostatic pressure and the critical weakening displacement is very small, short-interval SSEs with propagation velocities and slip velocities of 4–8 km d⁻¹ and  2 − 4 × 10⁻⁷  m s⁻¹, respectively, can be reproduced. The propagation velocity of short-interval SSEs is in proportion to the slip velocity. The results also show that during the nucleation process of large earthquakes, the occurrence of short-interval SSEs becomes irregular because of the accelerated slips that occur at the bottom of the seismogenic zone. Our results suggest that monitoring of short-interval SSEs might be useful for forecasting the main earthquakes.     

A confirmed earthquake in continental Antarctica

Summary. An earthquake of magnitude ( M _b) 4.5 has been located by the International Seismological Centre, near 81°S, 37°E in the continental platform of east Antarctica, about 1200 km from the coast of Dronning Maud Land, and 500 km from the Pole of Inaccessibility. The event was found by the Centre's 'search' procedure, which is undertaken with a deliberate delay of about two years. It occurred on 1982 November 4, and its position is well determined from five stations in Antarctica, and four farther afield. This is the first earthquake definitely located in the interior of the Antarctic continent, although there have been some earlier less well established claims, and other earthquakes have occurred near the coast, or associated with areas of volcanism or ice movement.     

Source history of the 1905 great Mongolian earthquakes (Tsetserleg, Bolnay)

Two great Mongolian earthquakes, Tsetserleg and Bolnay, occurred on 1905 July 9 and 23. We determined the source history of these events using body waveform inversion. The Tsetserleg rupture (azimuth N60°) correspond to a N60° oriented branch of the long EW oriented Bolnay fault.
Historical seismograms recorded by Wiechert instruments are digitized and corrected for the geometrical deformation due to the recording system. We use predictive filters to recover the signals lost at the minute marks.
The total rupture length for the Tsetserleg earthquake may reach up to 190 km, in order to explain the width of the recorded body waves. This implies adding 60 km to the previously mapped fault. The rupture propagation is mainly eastward. It starts at the southwest of the central subsegment, showing a left lateral strike-slip with a reverse component. The total duration of the modelled source function is 65 s. The seismic moment deduced from the inversion is 10²¹ N m, giving a magnitude   M _w = 8  .
The nucleation of the Bolnay earthquake was at the intersection between the main fault (375 km left lateral strike-slip) and the Teregtiin fault (N160°, 80 km long right lateral strike-slip with a vertical component near the main fault). The rupture was bilateral along the main fault: 100 km to the west and 275 km to east. It also propagated 80 km to the southeast along the Teregtiin fault. The source duration was 115 s. The moment magnitude Mw varies between 8.3 and 8.5.
The nucleation and rupture depths remain uncertain. We tested three cases: (1) nucleation and rupture depth limited to the seismogenic zone; (2) nucleation in the seismogenic zone and rupture propagation going to the base of the crust and (3) nucleation within the crust–upper mantle interface and rupture propagation within the upper mantle.     

A three-dimensional radially anisotropic model of shear velocity in the whole mantle

We present a 3-D radially anisotropic S velocity model of the whole mantle (SAW642AN), obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on Non-linear Asymptotic Coupling Theory (NACT). The model is parametrized in level 4 spherical splines, which have a spacing of ∼ 8°. The model shows a link between mantle flow and anisotropy in a variety of depth ranges. In the uppermost mantle, we confirm observations of regions with   V_SH > V_SV   starting at ∼80 km under oceanic regions and ∼200 km under stable continental lithosphere, suggesting horizontal flow beneath the lithosphere. We also observe a   V_SV > V_SH   signature at ∼150–300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast-spreading segments. In the transition zone (400–700 km depth), regions of subducted slab material are associated with   V_SV > V_SH   , while the ridge signal decreases. While the mid-mantle has lower amplitude anisotropy (<1 per cent), we also confirm the observation of radially symmetric   V_SH > V_SV   in the lowermost 300 km, which appears to be a robust conclusion, despite an error in our previous paper which has been corrected here. The 3-D deviations from this signature are associated with the large-scale low-velocity superplumes under the central Pacific and Africa, suggesting that   V_SH > V_SV   is generated in the predominant horizontal flow of a mechanical boundary layer, with a change in signature related to transition to upwelling at the superplumes.     

ScSp observed on North Island, New Zealand: implications for subducting plate structure

Seismic phase conversions provide important constraints on the layered nature of subduction zone structures. Recordings from digital stations in North Island, New Zealand, have been examined for converted ScS ‐to‐ p ( ScSp ) arrivals from deep (>150 km) Tonga–Kermadec earthquakes to image layering in the underlying Hikurangi subduction zone. Consistent P ‐wave energy prior to ScS has been identified from stations in eastern and southern North Island, where the subducted plate interface is at a depth of between 15 and 30 km. Two ScS precursors are observed. Ray tracing indicates that the initial precursor ( ScSp ₁) corresponds to conversion from the base of an 11–14 km thick subducting Pacific crust. The second precursor is interpreted as a conversion from the top of the subducting plate. The amplitude ratio, ScSp ₁: ScS , increases from 0.10 to 0.19 from northern to southern North Island. This is within the range expected from a simple first‐order velocity discontinuity at an oceanic Moho. A 1–2 km thick layer of low‐velocity sediment at the top of the subducting plate is required to explain the remaining ScSp waveform. Our results imply that the abnormally thick Hikurangi–Chatham Plateau has been subducting beneath New Zealand for at least 2.9 Myr, thus explaining the high uplift rates observed across eastern North Island.     

Multifractal analysis of spatial distribution of microearthquakes in the Kanto region

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The spatial distribution of earthquakes is a fractal, which is characterized by a fractal dimension. However, if a spatial distribution has a heterogeneous fractal structure, a single value of fractal dimension [e.g. D_o (capacity dimension) or D ₂ (correlation dimension)] is not enough to characterize it. From a multifractal viewpoint, we analysed the spatial distribution of microearthquakes in the Kanto region by using a local density function. Generalized dimensions, D_q , of the spatial distribution were calculated from the slopes of generalized correlation integrals, C_q(r) versus distance r , on a log-log plot, examining the self-similarity of the spatial distribution of microearthquakes. Self-similar structures are held well at scales from 1.26 to 12.6 km. Our results suggest that the spatial distribution of microearthquakes in the Kanto region is not a homogeneous fractal structure but a heterogeneous one with generalized dimensions D₂ = 2.2 ≤ D ₃≤…≤ D _∞= 1.7. The value of D _∞, the lower limit of fractal dimension, is the fractal dimension of the most intensive clustering in the heterogeneous fractal set. The fractal dimension of the most intensive clustering of microearthquakes in the Kanto region is 1.7.     

Seismic image of a CO2 reservoir beneath a seismically active volcano

Mammoth Mountain is a seismically active volcano 200 000 to 50 000 years old, situated on the southwestern rim of Long Valley caldera, California. Since 1989 it has shown evidence of unrest in the form of earthquake swarms (Hill et al. 1990), volcanic 'long-period' earthquakes (Pitt & Hill 1994), increased output of magmatic ³He (Sorey et al. 1993) and the emission of about 500 tonnes day ⁻¹ of CO₂ (Farrar et al. 1995; Hill 1996; M. Sorey, personal communication, 1997), which has killed trees and poses a threat to human safety. Local-earthquake tomography shows that in mid-1989 areas of subsequent tree-kill were underlain by extensive regions where the ratio of the compressional and shear elastic-wave speeds V_P/V_S was about 9 per cent lower than in the surrounding rocks. Theory (Mavko & Mukerji 1995), experiment (Ito, DeVilbiss & Nur 1979), and experience at other geothermal/volcanic areas (Julian et al. 1996) and at petroleum reservoirs (Harris et al. 1996) indicate that V_P/V_S is sensitive to pore-fluid compressibility, through its effect on V_P . The observed V_P/V_S anomaly is probably caused directly by CO₂, and seismic V_P/V_S tomography is thus a promising tool for monitoring gas concentration and movement in volcanoes, which may in turn be related to volcanic activity.     

Focal mechanisms of small earthquakes in the southeastern Brazilian shield: a test of stress models of the South American plate

Focal mechanisms of small earthquakes with magnitudes of about 3 in the SE Brazilian shield are calculated using S / P amplitude ratios. Low attenuation ( Q _p from 400 to 800) in the shield upper-crustal layers allowed sharp S arrivals to be recorded up to distances of 100 km. Besides P -wave polarities, SH -wave first motions were also used to constrain the nodal-plane orientations. Normal and reverse faulting mechanisms with strike-slip components were found. The inversion of four mechanisms to estimate the stress tensor indicated a strike-slip stress regime with roughly E–W-orientated σ ₁ and N–S σ ₃. Both the orientations and the shape factor ( φ =0.7) of the inverted stress are in excellent agreement with theoretical predictions for that part of Brazil from the driving-force model of Coblentz & Richardson (1996) . Good agreement with the nature of the stress, as well as its orientation, was also found for the model of Meijer (1995) . Both of these theoretical models include spreading stresses along the continent/ocean lithospheric transition. Because the earthquakes are more than 300 km from the continental shelf they should not be affected by the local flexural forces caused by sediment load in the marginal basins. The agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses.