Long-period Body Waves Research Articles

SEISMIC CHARACTERISTICS OF EARTHQUAKES ALONG THE OFFSHORE EXTENSION OF THE WESTERN TRANSVERSE RANGES, CALIFORNIA

The short- and long-period body waves of five moderate (M_L 5 to 6) earthquakes that took place in the nearshore region of southcentral California between 1969 and 1981 are modeled to obtain the source parameters. Both regional Pnl waveforms and teleseismic P waveforms are used in this analysis. Four of the events are thrust earthquakes with strikes that rotate southwestward as the epicenters move southward; the fifth is a strike-slip event occurring on a fault subparallel to the San Andreas fault. The focal mechanisms for all five events are consistent with known faults in the source localities and suggest that the regional stress field is compressional in a north-south direction. At least three events have complex source time histories, suggesting that complex sources are common even for events of this size. The most exotic event started with a low stress drop subevent, ruptured bilaterally, and concluded with strong asperity ruptures at each end. Since Pnl waveforms for thrust events are particularly sensitive to crustal thickness, regionalized crustal models were developed. The crustal thickness appears to increase southward but may be a function of the distance of the event from shore rather than of latitude. Basin and range paths yield the thinnest crustal models, while paths crossing the Colorado Plateau and Rockies yield the thickest. We also examined amplitude variations with station and path. In general, the amplitudes seem to be controlled by the station site rather than the path.

The 25 November 1988 Saguenay, Quebec, earthquake: Source parameters and the attenuation of strong ground motion

The Saguenay earthquake of 25 November 1988 occurred close to the southern margin of the Saguenay Graben in southern Quebec. It was caused by almost purely dip-slip faulting centered at a depth of 26 km with a P axis oriented northeast-southwest. This faulting mechanism is similar to those of the larger historical earthquakes in eastern North America, but the focal depth is substantially greater than all but one of these events. The seismic moment estimated from regional PnI waves and teleseismic long-period body waves is 5 × 10^(24) dyne-cm., corresponding to a moment magnitude of 5.8. The source duration of the earthquake is estimated to be 1.8 sec, corresponding to a stress drop of 160 bars, which is not significantly higher than the average stress drop of 120 bars estimated from previous large earthquakes in eastern North America. In order to simultaneously match the recorded ground motion amplitudes of strong-motion acceleration, strong-motion velocity, and teleseismic short-period and long-period body waves, it is necessary to use a source function having a complex shape that implies the presence of asperities and larger local stress drops. The large set of strong-motion recordings of the Saguenay earthquake has been used to validate a procedure for estimating strong ground motion attenuation based on a simple wave propagation model. The most important feature of the recorded strong motions is that their peak amplitudes do not decay significantly with distance inside 120 km, but then decay abruptly beyond 120 km. Profiles of recorded accelerograms with absolute times indicate that at distances beyond 64 km the peak ground motions are due to strong postcritical reflections from velocity gradients in the lower crust. The principal shear-wave arrivals and the variation of their peak amplitudes with distance were reproduced in synthetic seismograms generated using a regional crustal structure model. The critical distances for the postcritical reflections were short because of the deep focal depth of the event, causing the elevation of ground motion amplitudes out to 120 km. Similar studies of earthquakes in other regions of eastern North America indicate that the strength of the postcritical reflections, and the distance ranges over which they are dominant, are controlled by the focal depth and crustal structure. Regional variations in crustal structure thus give rise to predictable regional variations in strong ground motion attenuation.

The velocity structure and heterogeneity of the upper mantle

In a particular region a smoothed and averaged representation of the velocity structure in the upper mantle can be derived from long-period body wave and higher-mode surface wave observations. The vertical resolving power of such techniques is limited by the relatively long wavelengths used. In contrast, short-period studies show considerable complexities in postulated one-dimensional models which are unresolvable in the long-period band. However, these may be associated with the mapping of lateral heterogeneity into a vertical profile. To assess the extent to which lateral heterogeneity in the mantle can be resolved, we have attempted to model the features seen on recordings of earthquakes in the Indonesia/New Guinea region at large-aperture (400–1000 km) arrays of portable seismic recorders. With horizontal heterogeneity scales of the order of 300–500 km and velocity perturbations up to 2%, the amplitude deviations from those predicted for a stratified model can be considerable and fit in well with the pattern of observations. For long-period waves such laterally heterogeneous structures have only a modest effect, with weak inter-mode coupling resulting from the heterogeneity. As a result, a one-dimensional model may still be a useful approximation for long-period studies, but will not be adequate for broad-band or short-period work.

Macquarie earthquake of May 23, 1989

We read with great interest the news report in Eos (June 16, 1989) on the very large Macquarie earthquake of May 23, 1989, which left U.S. scientists divided as to whether this earthquake was of strike slip or thrust type or a mixture of both.Indeed, we think we have the solution to this dilemma, since thanks to teletransmitted data from 11 Geoscope stations recording both very long‐period and broad band data, we were able to obtain a reliable preliminary mechanism less than 48 hours after this earthquake occurred. Figure 1 shows this mechanism, which is a practically pure right‐lateral strike slip, consistent with the general trend in the Macquarie ridge area as indicated from the CMT catalog for the past 10 years [Dziewonski et al., 1989], and in particular, very similar to the mechanisms of the two largest events which occurred in this area in this time period (May 25, 1981 and July 7, 1982). By moment tensor inversion of very long‐period Rayleigh waves, we obtained a moment of 2.2×1028 dyne‐cm and a centroid time of 28 s, which yields a rating for this earthquake as “average‐to‐fast” with respect to the moment/duration relation. So far, we haven't found any conclusive evidence for strong horizontal directivity, while it seems that the rupture may have extended down to (or up from) at least 40 km. These features of the solution offer a good explanation as to why no tsunami was observed for this earthquake. The confidence in our “very‐long period” solution comes from its very good compatibility with broad band data from the three teletransmitted Geoscope stations that lie within the epicentral distance range 30°− 90° from the epicenter of the Macquarie event, both in terms of mechanism as well as source duration. For shallow earthquakes the earthquake depth and the vertical dip‐slip components of faulting are poorly constrained by the very‐long period data. The addition of only a few broad band seismograms in the analysis can remove this uncertainty. Figure 2 shows two examples of fits for SH waves obtained in a combined analysis of long‐period surface waves and broad band body waves [Ekstrom, 1989]. The teleseismic SH arrival at Papeete had an amplitude of 0.9 mm. The Macquarie region lies so far south in the southern hemisphere that very few records from existing broad band digital stations appropriate for mantle P and S wave analysis will ever be available. For Geoscope (which presently counts 20 operational broad band digital stations), besides PPT (Tahiti), KIP (Hawaii) and RER (Reunion Island), which were teletransmitted (CAN, Australia,is too close), we still expect data in this distance range from stations NOU (New Caledonia), DRV (Dumont d'Urville), PAF (Kerguelen) and CRZF (Crozet Island), the last three of which will take several months to arrive because they will travel part of the way by ship. Table 1 lists the coordinates and position with respect to the epicenter of the 11 stations whose data were teletransmitted.

The New Ireland earthquake of July 3, 1985 and associated seismicity near the Pacific Solomon Sea-Bismarck Sea triple junction

A temporary network of seismographs was used to record aftershocks for several days following the M s = 7.2 earthquake of July 3, 1985 in southern New Ireland. Aftershock locations indicated a rupture ∼ 50 km long in a northwest-southeast direction, possibly on the Sapom Fault. The fault plane appears to dip toward the northeast and extends from the surface to a depth of ∼ 15 km. The focal mechanism determined from first motion data and long-period body wave synthetics is a thrust solution (strike = 320°, dip = 50°, slip = 80°) with a moment of 5−8 × 10 26 dyn cm. The average slip was estimated to be 71–107 cm and the static stress drop 10–15 bar. Focal mechanisms for other earthquakes in the region show both strike-slip and thrust solutions. The strike-slip events reflect the currently active left-lateral transform boundary between the Bismarck Sea and Pacific plates. The thrust events are part of a zone of seismicity that dips to the northeast under New Ireland and extends to a depth of over 100 km. This seismicity is believed to be a remnant of the subduction zone that existed in the region prior to a million years ago.

Seismic wave energy inferred from long-period body wave inversion

AbstractThe seismic wave energy is evaluated for 35 large earthquakes by inverting far-field long-period P waves into the multiple-shock sequence. The results show that the seismic wave energy thus obtained is systematically less than that inferred from the Gutenberg-Richter's formula with the seismic magnitude. The difference amounts to one order of magnitude. The results also show that the energy-moment ratio is well confined to a narrow range: 10−6 < ES/Mo < 10−5 with the average of ∼5 × 10−6. This average value is exactly one order of magnitude as small as the energy-moment ratio inferred from the Gutenberg-Richter's formula using the moment magnitude. Comparing the energy-moment ratio with Δσo/2μ, where Δσo and μ are the stress drop and the rigidity, we obtain an empirical relation: ES/Mo ∼ 0.1 × Δσ0/2μ. Such a relation can be interpreted in terms of a subsonic rupture where the energy loss due to cohesion is not negligible to the seismic wave energy.

Source parameters of the 1949 magnitude 7.1 south Puget Sound, Washington, earthquake as determined from long-period body waves and strong ground motions

AbstractTeleseismic P, SH, and SV first motions and SH to SV amplitude ratios recorded at eight teleseismic receivers from the 1949 magnitude 7.1 Olympia, Washington, earthquake in combination with data from three stations at regional distances were utilized in a grid testing routine to constrain focal mechanism. Identification of the pP phase places the event at 54 km depth. Distinct pulses, assumed to be source effects, are observed in the far-field waveforms. Analysis of these pulses for directivity made possible discrimination between the fault and auxiliary planes. The plane taken to represent the fault surface strikes east-west ± 15°, dips 45° ± 15° to the north, and has nearly pure left-lateral slip. The preferred source model has an eastward propagation of 40 km. Surface reflections of successive source pulses suggest an upward component of propagation of 5 km. Bounds on the earthquake location and rupture of the 13 April event were determined using depth and source mechanism constraints from the teleseismic study and characteristics of local strong ground motion recordings. The 9-sec S-instrument trigger time seen in the Seattle acceleration recordings places the event at least 60 km from Seattle. Strong motion velocity at the Olympia Highway Test Laboratory is characterized by an impulsive and rectilinear S wave. The low amplitude of the vertical component of initial S motion suggests that either the epicenter is within 5 km of the Olympia Highway Test Laboratory for a pure incident SV wave or located along an azimuth of N159° if the wave is SH. The combined constraint of minimum distance from Seattle and the S polarization angle implied by the teleseismic data focal mechanism places the initiation of rupture 5 to 10 km north to north-northwest of the Olympia Highway Test Laboratory at 47.13°N, 122.95°W. This is approximately 20 km west of previously determined epicenters. The T axis, gently dipping to the southeast, supports other evidence that the Juan de Fuca plate dips to the southeast in a zone between segments of the plate north and south of the event's location. The fault plane's slip is taken to indicate that subduction is still active beneath Washington and that motion of the two segments is probably independent.

The earthquake of 1980 November 23 in Campania--Basilicata (southern Italy)

Teleseismic waveforms, local ground acceleration, elevation changes, surface faulting and aftershocks are used to investigate the three-dimensional geometry of fault movement in the destructive earthquake (Ms= 6.9) of 1980 November 23 in Campania—Basilicata (southern Italy). Twelve kilometres of surface faulting has been identified following this earthquake. The re-determined epicentre and focal mechanism, and the focal depth of 10 km, determined by modelling long-period teleseismic body waves, show that the hypocentre was on a downward projection of the surface faulting, and that the seismogenic normal fault was approximately planar, with a dip of 60°, from the hypocentre to the surface. Further analysis of the long-period body-waves indicates that, within 10s of the origin time of the earthquake, motion occurred on three discrete fault-segments extending for 30 km along strike. Fault rupture in the earthquake propagated predominantly towards the NW. An overall moment tensor for the earthquake is obtained from the inversion of long-period GDSN and WWSSN data, and shows that the total scalar moment of 26 × 1018 Nm is approximately double that accounted for by the fault motion in these first three subevents. We use teleseismic body-waves, locally recorded ground acceleration and aftershocks to investigate the position, timing and orientation of the additional seismic sources responsible for the remaining seismic moment. These data suggest that a fourth subevent occurred about 13 s after the first motion, approximately 20 km SE of the hypocentre of the first subevent. Two later fault ruptures also occurred, beneath the hanging wall of the earlier ruptures, about 20 and 40 s after the first motion. Long-period body waves and elevation changes are consistent with these occurring on normal faults, dipping at about 20°NE, at the base of the upper-crustal seismogenic zone. The total of six subevents that we identify for this earthquake account for almost all of the scalar moment in the overall moment tensor.

Of the May 7, 1986 Andreanof Islands Earthquake source parameters

Source characteristics of the May 7, 1986 Andreanof Islands earthquake (51.412°N, 174.830°W, NEIC) are investigated from WWSSN, GDSN and IDA records. First motions from over 60 stations determine one steeply dipping nodal plane. We constrained this nodal plane and inverted long‐period surface waves at a period of T=256 sec and determined the second nodal plane to be dip 18°, rake 116°, and strike 257°. This shallowly dipping thrust mechanism is consistent with plate motions in this region. Seismic moment from surface‐wave inversion is 1.3×1028 dyne‐cm corresponding to Mw=8.0. Amplitudes of body and surface waves from short‐period instruments yield magnitudes of and Ms=7.7. The teleseismic average P‐wave moment rate spectrum from 17 short‐ and intermediate‐period instruments is slightly lower than that of an average Mw=8.0 subduction‐zone event. We constrained the fault plane as determined above to deconvolve the first 90 secs of the long‐period body wave at 11 teleseismic stations to determine the source time function and the spatial distribution of moment release. The source time function consists of 4 moment‐releasing episodes which have a total moment release of 9.4×1027 dyne‐cm. The fault ruptured bilaterally with the largest moment releasing subevent occurring between 30‐45 sec. This subevent nucleates approximately 75‐90 km west of the determined epicenter. This region corresponds to the epicentral area of the 1957 Great Aleutian earthquake which is one of the largest earthquakes in recorded history.

Reply to comments by A. Douglas, J. B. Young, and N. S. Lyman and a note on the revised moments for Pahute Mesa tectonic release

In two earlier papers (Wallace et al., 1983, 1985), we discussed the evidence for tectonic release from underground nuclear explosions on Pahute Mesa at the Nevada Test Site (NTS) as observed in long-period body waves. It has been shown for some time that the nonisotropic component of the surface waves from most of these events could be explained by an equivalent double-couple source; namely strike-slip motion on north-striking faults.

Source processes of the 1981 Gulf of Corinth earthquake sequence from body-wave analysis

On 24 February 1981 (20 h 53 m), an offshore earthquake of magnitude M, = 6.7 occurred in the eastern part of the Gulf of Corinth, Greece. So far, it is the largest earthquake that has taken place in the area during this century. The shock caused considerable damage in coastal population centers on both sides of the Gulf. Sixteen people lost their lives and several hundred were injured, mostly by falling objects. The main shock was followed by two large aftershocks of magnitude M, = 6.4 on 2.5 February (02 h 35 m) and 4 March (21 h 58 m). These aftershocks caused additional severe damage in areas to the east of the main shock epicenter and were accompanied by surface faulting. Hypocenter parameters of the three shocks are listed in Table 1. Teleseismic long-period body-waves from the main shock on 24 February and its two principal aftershocks are studied to determine source characteristics. The stereographic projections of P-wave first motions for the three shocks are shown in Fig. 1. It is obvious that most of the available teleseismic observations are dilatational motions, indicating that the focal mechanisms are dominated by a 45” dip-slip

Source parameters of the 28 October 1983 Borah Peak, Idaho, earthquake from body wave analysis

AbstractModeling of long-period body waves of the MS = 7.3 Borah Peak, Idaho, earthquake suggests that the earthquake was a simple rupture that nucleated at a depth of about 16 km and propagated unilaterally northwestward toward the surface. The seismic moment tensor obtained from the inversion of teleseismic body wave amplitude data agrees well with the fault plane solution obtained from short-period first motion data (strike, 138 ± 3°; dip, 45 ± 3°; rake, −60 ± 5°) and the observed surface faulting. The scalar double couple moment of 2.1 × 1026 dyne-cm obtained from the inversion is comparable to a moment of 1.4 × 1026 dyne-cm estimated from the observed surface faulting. Estimates of stress drop based on these values for the moment are 17 to 12 bars, respectively.

Evidence against a discontinuity at the top of D'

Summary. We investigate a recent suggestion of Lay & Helmberger that there is a world-wide discontinuity in 5’-wave velocity, about 250-300 km above the core-mantle boundary, with an increase of 2.5-3 per cent from above to below. Their model predicts pronounced pulse-form changes of long-period SH body waves in the epicentral distance range 95-120”, a range which has not been studied by them. Similar pulse-form changes are expected in P-wave seismograms, under the reasonable assumption of a similar increase in P velocity. Nine long-period seismogram sections of SH- and P-waves from deepfocus earthquakes below Fiji, West Brazil, the Sea of Japan, the Sea of Okhotsk and the Banda Sea are presented with practically no indication of the complications expected. Also, the decay constants of pdiff, as predicted by the new model, differ strongly from observed values. Thus, there is clear evidence against this model. The pulse-form peculiarities between mantle S and ScS in transverse-component seismograms at distances from 70” to 80°, which Lay & Helmberger interpreted as reflections from the discontinuity of their model, must have a different cause.

A body-wave analysis of the 1966 Gisborne, New Zealand, earthquake

The M l = 6.2 Gisborne earthquake of March 4, 1966 occurred along the East Coast of the North Island, New Zealand. Modeling of P and S body waves shows that the focal mechanism of this event is consistent with northwestward thrusting of the Pacific Plate beneath the North Island ( ϑ = 249°, δ = 25° and λ = 131°). The focal depth is constrained to 18 km, significantly less than the values of 25–30 km computed from local network data. Estimates of the scalar moment, source duration and stress-drop for the event are 4 × 10 24 dyne-cm, 2–3 s, and 20–120 bar, respectively. Cross-correlation errors of synthetic to observed waveforms were computed for all possible P and T axis locations and slip vector orientations and contoured on a projection of the focal sphere. The error contour at which the synthetic waveforms distinctly diverged from the observed waveforms was established by eye. The procedure shows that the analysis of long-period body waves, at least in this case, provides much better constraint on focal mechanism orientation than does first motion data alone.

The effects of tectonic release on short-periodPwaves from NTS explosions

AbstractThe first cycle (ab amplitude) of teleseismic short-period P waves from underground nuclear explosions at Pahute Mesa (NTS) show a systematic azimuthal amplitude pattern that can possibly be explained by tectonic release. The amplitudes vary by a factor of three, with diminished amplitudes being recorded at azimuths around N25°E. This azimuthal pattern has a strong sin(2φ) component and is observed, to varying degrees, for 25 Pahute Mesa events, but not for events at other sites within the NTS. Events that are known to have large tectonic release have more pronounced sin(2φ) amplitude variations. A synthesis of long-period body and surface wave investigations of tectonic release for Pahute Mesa events shows that, in general, the nonisotropic radiation is equivalent to nearly vertical, right-lateral strike-slip faulting trending from N20°W to due north. Long-period P waves at upper mantle distances demonstrate that there is a significant high-frequency component to the tectonic release. Using the long-period constraints on orientation, moment, and frequency content of the tectonic release, the expected short-period P wave effects are predicted. For models in which the downgoing P wave from the explosion triggers tectonic release within a few kilometers below the shot point, a factor of 2.5 amplitude variation with azimuth is predicted for the short-period ab amplitudes, with the lowest amplitudes expected near N25°E. Rather subtle azimuthal variations in the waveforms are expected, particulary for downward propagating ruptures, which is consistent with the absence of strong variations in the data. The occurrence of the azimuthal pattern, albeit with varying strength, for all of the Pahute Mesa events suggests a tectonic release model in which the shatterzone surrounding the explosion cavity is extended preferentially downward by driving a distributed network of faults and joints underlying the Mesa several kilometers beneath the surface. In this model, all events could have a component of tectonic release which would reflect the regional stress regime, although there may be slight spatial and temporal variations in the tectonic release contribution. Some events may trigger slip on larger throughgoing faults as well. While it is shown that tectonic release can affect teleseismic short-period signals significantly, and may contribute to the Pahute Mesa amplitude pattern, other possible explanations are considered.

Source processes of the 1981 Gulf of Corinth earthquake sequence from body-wave analysis

AbstractTeleseismic long-period body waves from the 24 February 1981 Gulf of Corinth earthquake and its two principal aftershocks of 25 February (02h35m) and 4 March (21h58m) 1981 are studied to determine source characteristics. Focal mechanisms, along with observed surface fault breaks, suggest that the Corinth earthquake sequence represents normal faulting due to the N-S trending extension. Depths of the three shocks, estimated by matching synthetic seismograms to observations, are found to lie between 4 and 12 km. The azimuthal variation of observed body-wave duration indicates that the main shock is a multiple event and that the main rupture occurred about 3 to 4 sec after a relatively small foreshock and propagated toward the W-NW.Seismic moments deduced from the body-wave synthetics are 8.1 ×1025, 2.7 ×1025, and 2.2 ×1025 dyne-cm for the main, 25 February and 4 March shocks, respectively. Average final displacements and stress drops are estimated to be 37 cm and 10 bars for the main shock (for a circular fault of radius 15 km); 22 cm and 8 bars for the 25 February shock, and 18 cm and 7 bars for the 4 March shock (for circular faults of radius 11 km).The striking features of the earthquake sequence are the low stress drops of the main shock and its two principal aftershocks, and the clear eastward migration of aftershock activities. The unusually long source-time function rise times (4 sec for the main shock, 2.5 sec for both aftershocks) and low stress drops suggest an overall slow energy release during the earthquake sequence.

Observable effects of the seismic absorption band in the Earth

Summary. Two types of observable consequences of the frequency dependence of Q (the absorption band concept) are discussed: (1) The observed waveform of a long-period body wave is systematically late compared to the synthetic purely elastic response. This observation can be made after the usual travel-time anomaly has been corrected for. (2) The short-period waveform of PKIKP sampling the inner core is systematically different from that of PKPBC bottoming above the inner core. This observation can be made at distances (147°–151°) where both phases are distinct, simple and similar apart from absorption effects. The long-period phase delay is measured on SRO/ASRO records of mantle P-waves from deep events, where the observed short-period onset is used to adjust the start time for the synthetic. The delay is caused by both absorption and source effects, and the two contributions are estimated in the context of a particular type of absorption and source model. If the Q model in PREM is representative of the long-period absorption, then the solution for the high-frequency cut-off of the absorption band (2πτ1)-1 is near 0.2 Hz, but values up to 1 Hz cannot be absolutely ruled out by the data. Although this result represents the integrated effect of Qα in the mantle, for teleseismic P from deep events it should be more representative of Qα in the lower mantle. The characteristic change of waveform of short-period PKIKP with respect to PKPBC is observed on NORSAR array beams, and it can be explained by absorption on the low-frequency flank of an absorption band (or peak) in the inner core. This has the following implications: the low-frequency cut-off (2πτ2)-1 of this band is above 2Hz. The minimum Qα in the centre of the band is likely to be below 100 in the upper 200km of the inner core. At body wave frequencies between 0.5 and 2.0 Hz, Qα varies here between about 600 and 200. With respect to this absorption band, inner core velocities inferred from observed short-period arrival times correspond to the relaxed elastic moduli, the unrelaxed moduli being inaccessible to observation. The intrinsic dissipation can be in shear and/or in bulk. The essential observational difference between relaxation models of shear dissipation and some diffusion models of bulk dissipation occurs on the high-frequency side of the absorption band. Hence, it is not possible to discriminate between the two mechanisms.

Evidence of tectonic release from underground nuclear explosions in long-periodPwaves

abstractIn this paper, we study the long-period body waves at regional and upper mantle distances from large underground nuclear explosions at Pahute Mesa, Nevada Test Site. A comparison of the seismic records from neighboring explosions shows that the more recent events have much simpler waveforms than those of the earlier events. In fact, many of the early events produced waveforms which are very similar to those produced by shallow, moderate-size, strike-slip earthquakes; the phase sP is particularly obvious. The waveforms of these explosions can be modeled by assuming that the explosion is accompanied by tectonic release represented by a double couple. A clear example of this phenomenon is provided by a comparison of GREELEY (1966) and KASSERI (1975). These events are of similar yields and were detonated within 2 km of each other. The GREELEY records can be matched by simply adding synthetic waveforms appropriate for a shallow strike-slip earthquake to the KASSERI observations. The tectonic release for GREELEY has a moment of 5 ՠ1024 dyne-cm and is striking approximately 340°. The identification of the sP phase at upper mantle distances indicates that the source depth is 4 km or less. The tectonic release time function has a short duration (less than 1 sec). A comparison of these results with well-studied strike-slip earthquakes on the west coast and eastern Nevada indicate that, if tectonic release is triggered fault motion, then the tectonic release is relatively high stress drop, on the order of several hundred bars. It is possible to reduce these stress drops by a factor of 2 if the tectonic release is a driven fault; i.e., rupturing with the P velocity. The region in which the stress is released for a megaton event has a radius of about 4 km. Pahute Mesa events which are detonated within this radius of a previous explosion have a substantially reduced tectonic release.

The Campania-Lucania (southern Italy) earthquake of 23 November 1980

We present the main seismological results of our study of the Campania-Lucania earthquake of 23 November 1980. A complete set of far field and local data has been analysed. From long-period body waves data we determine the fault plane solution ( φ 1 = 140° , δ 1 = 60° , φ 2 = 75° , δ 2 = 54° ), a depth of 15 km and calculate a seismic moment of 6 × 10 25 dyne cm and a source duration of 6 s. From data of a local network deployed immediately after the event we determine aftershock locations: they are aligned in a direction NW-SE that fit extremely well with the focal solution determined above. We can choose as fault plane the plane striking 140° and dipping at 60° and the event is a normal event with a large component of left-lateral strike slip. The source area evaluated from this aftershock distribution 14 km × 40 km is quite suitable for an earthquake of a seismic moment of 6 × 10 25 dyne cm.

Point-source inversion techniques

A variety of approaches for obtaining source parameters from waveform data using moment-tensor or dislocation point source models have been investigated and applied to long-period body and surface waves from several earthquakes. Generalized inversion techniques have been applied to data for long-period teleseismic body waves to obtain the orientation, time function and depth of the 1978 Thessaloniki, Greece, event, of the 1971 San Fernando event, and of several events associated with the 1963 induced seismicity sequence at Kariba, Africa. The generalized inversion technique and a systematic grid testing technique have also been used to place meaningful constraints on mechanisms determined from very sparse data sets; a single station with high-quality three-component waveform data is often sufficient to discriminate faulting type (e.g., strike-slip, etc.). Sparse data sets for several recent California earthquakes, for a small regional event associated with the Koyna, India, reservoir, and for several events at the Kariba reservoir have been investigated in this way. Although linearized inversion techniques using the moment-tensor model are often robust, even for sparse data sets, there are instances where the simplifying assumption of a single point source is inadequate to model the data successfully. Numerical experiments utilizing synthetic data and actual data for the 1971 San Fernando earthquake graphically demonstrate that severe problems may be encountered if source finiteness effects are ignored. These techniques are generally applicable to on-line processing of high-quality digital data, but source complexity and inadequacy of the assumed Green's functions are major problems which are yet to be fully addressed.