摘要:

To investigate whether or not lithospheric slabs descending along subducting margins penetrate below the 670‐km seismic discontinuity, we have analyzed 4040PandPKIKPtravel times from 14 intermediate‐ and deep‐focus earthquakes in the northwestern Pacific and obtained estimates of the near‐source anomaly as a function of position on the lower focal hemisphere. Station and ellipticity corrections are applied, and any component of the anomaly explicable by hypocenter mislocation is removed by orthogonalization of the residual vector with respect to the location parameters; the resulting residual sphere is smoothed and interpolated by a stochastic filtering scheme to average out observational errors and the effects of small‐scale heterogeneities far from the source. The smoothed residual spheres for six Sea of Okhotsk earthquakes deeper than 500 km are dominated by NE‐SW trending troughs of negative anomalies having strikes and dips similar to the seismic zone at these depths; the troughs are bounded on the NW and SE by parallel ridges of positive anomalies, with peak‐to‐trough amplitudes averaging about 1.5 s. As the focal depth decreases, the pattern translates to the NW, so that for hypocenters near 200 km the negative trough has been replaced by a positive ridge. The axis of the anomaly pattern for two Sea of Japan deep‐focus earthquakes is rotated counterclockwise from Okhotsk, consistent with the nearly N‐S strike of the Japan seismic zone. These correlations, and the fact that one nearby earthquake not within a subduction zone shows very little anomaly, suggest that the residual sphere anomalies are caused primarily by slab heterogeneity. Forward modeling experiments corroborate this conclusion. The thermal disturbance of the mantle is calculated for an assumed flow field by a finite difference algorithm, and from it a model ofPvelocity heterogeneity is constructed; theoretical travel time residuals are computed by tracing rays through this three‐dimensional structure, and the event is relocated and the residual sphere smoothed using the same station set and by the same algorithm applied to the observations. To obtain a good fit to the deep‐focus Okhotsk data requires penetration by the Kuril‐Kamchatka slab to depths of at least 900–1000 km. Penetration to much greater depths is consistent with the data. This conclusion agrees with theSwave results of Jordan (1977); it implies the circulation of at least some upper mantle material into the deep mantle and argues against a rigorously stratified upper mantle/low

ISSN:0148-0227    

DOI:10.1029/JB089iB05p03031

年代:1984

数据来源: WILEY

摘要:

Main shocks of the earthquake sequences that occurred on the Parkfield section of the San Andreas fault in central California in 1922, 1934, and 1966 are characterized by southeast rupture expansion over the same 20‐ to 30‐km‐long section of the fault. Whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10%, the seismic moment for 1966 is 20% greater than for the earlier events to within a precision of 20%. The Parkfield area seismicity, in general, seems well described by a recurring moderate size characteristic earthquake, repeating the same epicenter, magnitude, seismic moment, rupture area, and southeast direction of rupture expansion. An unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the Parkfield earthquakes in 1857, 1881, 1901, 1922, 1934, and 1966 represent a strictly periodic process. Assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period, the next characteristic Parkfield earthquake should occur between 1983 and

ISSN:0148-0227    

DOI:10.1029/JB089iB05p03051

年代:1984

数据来源: WILEY

摘要:

Data from a network of seismometers, a tiltmeter and the pattern of summit inflation between eruptions have made qualitative eruption forecasting possible on Kilauea volcano for over 20 years. This paper formulates quantitative forecasting by calculating the probability of an eruption based on current levels of tilt, tilt rate, seismicity, and fortnightly tide. Plots of eruption probability as a function of various parameters are derived using a set of 29 eruptions during 1959–1979. The method tests the precursory significance of various parameters over different time scales and determines probabilities by comparing data before eruptions with data values generally. Tilt level is an eruption precursor significant to better than 99.9% when averaged over any interval from 1 to 20 days. Tilt rate is a precursor significant to better than 99.999%, but noise in the data requires that it be averaged over 30 days or more. Earthquakes are a short‐term precursor significant to better than 99% when averaged over 1–10 days for larger earthquakes near the summit caldera and over 5–20 days for very small earthquakes within the caldera. The fortnightly modulation of tides influences eruption probability and has a precursory significance of 97%. Probabilities are independent of the time elapsed since the last eruption. The eruption probability, when tested against the eruption record from which it was derived, is significant to the 99.98% level when compared with random guessing. Ongoing 1‐day, 7‐day, and 30‐day forecasts are calculated by computer at the Hawaiian Volcano Observatory and supplement the qualitative interpretation of geo

ISSN:0148-0227    

DOI:10.1029/JB089iB05p03059

年代:1984

数据来源: WILEY

摘要:

Seismographs near Mount St. Helens Volcano recorded an earthquake swarm lasting nearly 2 months prior to the catastrophic May 18, 1980, eruption. The earthquakes are divided into four classes based on station CPW (Δ = 116 km) seismogram characteristics: (1) events with Sv:P amplitude ratio>3 and dominant frequency>3 Hz; (2) events with Sv:P ratio between 1 and 3 and dominant frequency>2 Hz; (3) events similar to characteristic 2 but with a strong (probably surface wave) phase just after theSphase; and (4) events with frequencies between 1 and 2 Hz lacking a clearSphase. The seismicity pattern for each of the four classes is unique, and we assume each group of events with similar wave‐forms represents a common physical process, depth, source area, or fault orientation. Maximum likelihood b values range between 0.97±0.05 and 1.81±0.09 for the four groups. We calculate solid earth stress and strain tides at the average hypocentral depth of 4 km. We also calculate stress and strain tides induced by ocean loading; their amplitudes are typically 20–40% those of the solid earth tides at the location of Mount St. Helens. A weak but significant correlation exists between the latter two classes of events and the tides for a time interval of about 5 days preceding the first onset of volcanic tremor and about 5 days thereafter. The polarity of the correlation is opposite for the two classes of events. In each case, the phase of the correlation changes systematically with time, the changes coinciding with the onset of tremor on March 31 and with a pronounced decrease in earthquake energy release rate on April 3. There are no significant correlations between the tides and the number of events or energy release of these two classes of earthquakes during any other interval between March 20 and May 18, 1980. The first two classes of events show no evidence of significant tidal correlation at any time during the study

ISSN:0148-0227    

DOI:10.1029/JB089iB05p03075

年代:1984

数据来源: WILEY

摘要:

Geodetic measurements made landward of the Nankai Trough, site of great subduction zone thrust earthquakes in 1854 and 1946, provide a uniquely detailed picture of the strain buildup process, supply constraints on the mechanism of strain accumulation, and allow for improved estimates of earthquake recurrence. Provided the two most recent movement cycles are similar, the observations, dating from about 1890, may be used to reconstruct a single complete deformation cycle (coseismic strain release, postseismic transients, interseismic strain accumulation). Very complete leveling and tidal gage data indicate that postseismic deformation extends more than 300 km inland from the plate boundary, persists for at least 30 years, and shows a clear tendency to become longer wavelength with increasing time. The transient movements have two timescales. The first, of about a year or less, corresponds to deformation, largely uplift, concentrated close to the coseismic fault, and is most easily explained by aseismic slip or very localized deformation downdip of the earthquake rupture plane. The second, longer, timescale is associated with a diffusion‐like spread of the deformation further landward, an effect qualitatively similar to that first predicted by Elsasser to be an expected consequence of faulting in an elastic plate overlying a viscoelastic asthenosphere. Cumulative uplift since 1890 correlates well with the distribution of uplifted marine terraces, although average post‐1890 tilt rates exceed late Quaternary and Holocene averages by at least a factor of three. Because of the nonlinearity of strain buildup and the significant permanent deformation, simple recurrence calculations typically overestimate the true interval between great earthquakes by a factor of 2 to a factor of 3. Strict application of the time‐predictable model, assumed correct, overcomes these difficulties provided the cumulative transient deformation and the proportion of permanent deformation per cycle can be esti

ISSN:0148-0227    

DOI:10.1029/JB089iB05p03087

年代:1984

数据来源: WILEY

摘要:

Frequently repeated surveys of a local network near Tokyo provide considerable detail on the horizontal strain changes immediately adjacent to the rupture zone of theM= 8.1 Kanto earthquake. Changes in the three independent tensor strain components are determined for 35 epochs during 1916–1980 to a precision of about ±1.5 μstrain. No significant shear or elongational strain changes preceded the earthquake; a change in dilational strain is marginally significant. Postseismic movements are large, accumulating to 40% or more of the coseismic strain drop. They have at least two time scales. A distinct short‐term transient lasting less than a year is explained well by slip or localized deformation that occurs downdip of the coseismic rupture plane and has a time constant of about 3 months. More complex, longer‐period postseismic movements continue for at least another 10 years. Their precise duration, as well as the transition to presumably steadier interseismic deformation, is obscured by notable irregularities in the strain buildup at Mitaka. As a result, neither the interseismic strain rate nor the occurrence time of the next Kanto earthquake can be reliably estimated from these obser

ISSN:0148-0227    

DOI:10.1029/JB089iB05p03102

年代:1984

数据来源: WILEY

摘要:

A seismic structural section across the southern Queen Charlotte transform fault zone, which separates the oceanic Pacific plate from the continental America plate off western Canada, has been determined from an offshore‐onshore refraction experiment. Two explosion profiles, one parallel and one perpendicular to the fault strike, were recorded on three ocean bottom seismographs (OBS) and seven land‐based seismographs (LBS). Assuming lateral homogeneity along the parallel profile recorded over oceanic crust at one OBS, one‐dimensional amplitude modeling produced a velocity model with the characteristics of standard ocean crust. The partially reversed perpendicular profile recorded at two OBS's and one good quality LBS extended across the fault zone, which in bathymetric cross section shows two linear fault escarpments separated by a flat terrace. An initial velocity structure, provided by time term analysis of the complete data set, was modified by ray tracing until the travel time data for the three stations were satisfied by one seismic structural section. This model shows three distinctive crustal blocks (the oceanic, terrace, and continental blocks) separated by two major, crustally pervasive faults, the outer and inner Queen Charlotte faults. The rock units composing the terrace block have lower velocities at equivalent depths than those of the blocks on either side. This may have resulted from deformation associated with oblique convergence and/or the effects of a jump in the position of the active Queen Charlotte fault from its outer to inner positions about 1–0.5 Ma ago. Depth to the base of the crustal section increases from 12 to 18 km below sea level across the terrace transition zone, representing an eastward dip of about 20°. The seismic model, after conversion to a density model, agrees well with the gravity anomaly along the profile. The interpreted structural section is consistent with recent tectonic models that require a component of convergence along the transform fa

ISSN:0148-0227    

DOI:10.1029/JB089iB05p03107

年代:1984

数据来源: WILEY

摘要:

A 275‐km‐long reversed refraction profile in the Oregon Cascades, two shallow earthquakes of magnitude 5 in southern Washington, a shallow earthquake of magnitude 4.6 in northern California, and a previously published analysis of the Bouguer gravity field are used to develop a crustalPwave velocity model for the Oregon Cascades. Travel time analysis of the refraction profile indicates a crustal structure characterized by surface layers withPwave velocities that vary from 2.9 to 5.2 km/s and thicknesses that vary from 2.5 to 5.0 km, upper crustal velocities of 6.1 to 6.5 km/s between the depths of 3 and 29 km, lower crustal velocities near 7.0 km/s between the depths of 29 and 44 km, and a mantle reflector at a depth of 44 km. Comparison with synthetic seismograms supports this structure and shows that the lower crustal and Mono transitions can be better modeled by continuous velocity gradients than by first‐order discontinuities. Arrival times from two shallow earthquakes in southern Washington in 1981 across 14 telemetry stations in the Oregon Cascades show an apparent velocity of 7.62 km/s. On the basis of a previously published gravity analysis, the mean north‐south component of the dip to the crust‐mantle boundary is estimated at 1° down dip to the south. The apparent velocity from the earthquakes and the dip estimate from gravity indicate that the truePnvelocity is 7.70 km/s. Arrival times from a shallow earthquake in northern California in 1978 across telemetry stations in the Washington Cascades are consistent with this model and indicate that the upper mantle velocity reaches 8.20 km/s at a depth of approximat

ISSN:0148-0227    

DOI:10.1029/JB089iB05p03121

年代:1984

数据来源: WILEY

摘要:

PLis a long‐period (20 s or more) wave train beginning just after theParrival in seismograms and continuing until the S arrival or the Rayleigh wave. The wave train can be observed at epicentral distances of about 5°–20° with instruments sensitive to these low frequencies.PLpropagates as a partially trappedP‐SVwave in the crust;Swave energy is lost to the mantle during propagation makingPLa “leaky mode.” We studyPLpropagation for a variety of earth models using the synthetic seismogram algorithm wave number integration and find that the vertical travel time in the crust is the most important parameter controllingPL's oscillation period. This period can vary by more than a factor of two between oceanic and continental paths.P‐SVleaky mode propagation includes many different modes; the low‐frequency motion termed “PL” is only the first, or fundamental mode, in this family. A second, higher‐frequency mode roughly equal in amplitude to the fundamental appears for models without a net positive velocity gradient in the crust. We use these results to match an observedPLwave train whose propagation path consisted almost entirely of the Tibetan Plateau. By considering first and secondPLmode behavior we find that the Tibetan crust is about 85 km thick, with an uncertainty of about 20 km, and possesses a significant (≥0.01 s−1) positive velocity gradient. The presence of a large, low‐velocity zone in the lower Tibetan crust thus seems unlikely. Much higher‐frequencyPLmodes also appear in the Green's functions for layer over half space models and appear to be responsible for the high‐frequencyPgphase, makingPLandPgdifferent members of the same

ISSN:0148-0227    

DOI:10.1029/JB089iB05p03135

年代:1984

数据来源: WILEY

摘要:

Deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement. Perhaps the best example is the Consortium for Continental Reflection Profiling (COCORP) Wind River line in Wyoming, which shows the Wind River thrust fault to be a strong reflector from the surface to depths of about 30 km. To identify the physical properties responsible for the seismic reflections from fault zones, we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones. These rocks are characterized by extensive ductile deformation of plastic minerals, brittle deformation of the more rigid minerals, grain size reduction, and development of a strong fabric. Average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock. Seismic anisotropy of 7% or greater is present in several mylonites dependent upon their composition and fabric. Using our data on mylonite properties and models for the crustal structure near the Wind River line, we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the COCORP Wind River reflection data. When anisotropy and the finely laminated structure of the fault zone are considered, our modeling indicates fairly strong reflections from mylonite zones in most cases. We also show that elevated pore pressure in shear zones may produce strong reflections. However, a permeability of the order of 10−16darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactiv

ISSN:0148-0227    

DOI:10.1029/JB089iB05p03153

年代:1984

数据来源: WILEY