Mansour Niazi

Source geometry and mechanism of 1978 Tabas, Iran, earthquake from well located aftershocks

Abstract Aftershocks of the September 16, 1978 Tabas earthquake located from close-in observations made during a four-week fielding of temporary stations have been analyzed for the purpose of delineating detailed source geometry of the 1978 earthquake. Spatial distribution of aftershocks and their composite focal mechanism suggest that the geometry of faulting is far from planar. Aftershocks define two prominent alignment. The southern alignment strikes E-W to WNW-ESE, whereas the northern alignment strikes in a N-S to NNW-SSE direction with an abrupt change of nearly 55–60 degrees near 33.4°N latitude. Both field observations of surface faulting pattern and systematic variation of principal directions of stress axes computed from aftershock focal mechanisms are consistent with the upthrusting and imbrication of a wedge shaped crustal block with the wedge angle of about 120 degrees. Both geological and seismological evidence suggest that the deformed zone is truncated at the southern edge by preexisting E-W fault structures. New observations may provide a partial answer to the unexplained farfield asymmetry of the long period Rayleigh wave radiation pattern recently observed for the mainshock across IDA network.

Post-earthquake observations at Dasht-e Bayāz, Iran

Abstract A program of field observations of post-earthquake characteristics has been established at the site of the devastating 1968 Dasht-e Bayāz earthquake in northeastern Iran. Data for the period 2–4 1 2 years following the main shock have been analyzed. Aftershocks are still occurring, about seven per day of magnitude over zero, and their rate of occurrence is decaying at approximately t−1.1. The aftershocks are confined to the 1968 rupture zone, concentrated near the ends with a conspicuous aseismic region near the centrally situated 1968 focus. The shocks define a steeply dipping fault plane by their distribution, this plane being consistent with the 1968 surface rupture and fault-plane solution, as well as a composite fault-plane solution of the aftershocks. Left-lateral fault creep of several mm/year seems to be occurring, as evidenced by survey monument arrays and creepmeters installed in the study.

Magnetic field observations of the Sabzevar ophiolitic complex of northeast Iran

Abstract Surface measurements of the total magnetic field intensity along three sections transverse to the structural trends of the Joghatay Mountains ophiolitic complex extended north and west of Sabzevar, northeast Iran, are analyzed. The observed variations of the magnetic field intensity are compared with the synthetic results computed for a block structure consisting of several east-west oriented vertical parallelepiped down to 10 km depth which is also in agreement with the mapped shallow structural features of the complex. Shoaling of the basement and/or Curie surface may explain the observed northward decrease of the mean field measurements. However, in this study the variations are modeled in terms of magnetic enrichment of southern blocks.

Spatial coherence of the ground motion produced by the 1979 Imperial Valley earthquake across El Centro Differential Array

Abstract Spatial coherence of digitally recorded acceleration time histories across five elements of the El Centro Differential Array recorded during the October 15, 1979 Imperial Valley, California, earthquake is studied. Recorded within a total spacing of 214 m, the three-component time histories at five stations of this linear array provide a unique set of free-field data at sites of similar geology in the near field of a strong earthquake. The observed peak ground accelerations vary by as much as 43, 13 and 40% on the E-W, N-S and vertical components. It is shown that the incoherence of high frequency waves brought about by wave scattering near the receiver is partially responsible for the observed variation. This is evident from the frequency dependent reduction of the peak cross-correlation amplitudes of the strong phase of ground shaking. The observed reductions increase from ∼ 1% below 1 Hz to 20–30% in the 1–10 Hz band and to nearly 80% above 10 Hz. Investigation of the polarization angle of P-wave coda indicates that high frequency waves (> 10 Hz) approach the recording station randomly from nearly all directions. The peak cross-correlation of the polarization angle in general is lower than that of amplitudes. The successive sampling of this angle in the low frequency band suggest a northwestward propagation of the source during the faulting process.

Irregular geometry of the Gorda subduction and deep structure of the Eel River basin determined from teleseismic P delays

Abstract Nearly 1000 observations of travel-time residuals for teleseismic P arrivals across the Humboldt Bay Seismic Network near Cape Mendocino, California, are analyzed for a study of 3-D geometry of the Gorda plate near Mendocino Triple Junction (MTJ), by the application of a three-dimensional velocity inversion procedure. The area is underlain by the subducted edge of the Gorda plate and the eastward extension of the Mendocino Fracture Zone. The observed variation of the travel-time gradient with azimuth favors either deepening of the slab southward, or southward increase of dilatational velocity across the Mendocino Fracture Zone at depths of more than 50 km. The topographically corrected mean travel-time delays at individual stations suggest that the Eel River basin is deep and tectonically controlled. Further support for this view is provided by the inversion results, which point to the possible extension of the low-velocity material through the entire crustal section. These results also suggest that from the south, the Eel River basin may be confined at depth by a steeply northward-dipping sliver of high-velocity material (possibly a detached oceanic slab). This feature may be responsible for the intense seismic activity reported beneath the eastern end of the Mendocino Fracture Zone (MFZ). Thus, the role of the Eel River basin in accommodating the relative motion between the adjacent plates, may be significant. The leading edge of the downgoing slab appears irregular and at some latitudes may not extend beyond the coast line. The estimated velocity anomalies vary by as much as 20% in the near-surface blocks and nearly half as much at depth. The pattern may result from intense tearing and deformation of the slab, producing buckling, folding and compression ridges.

Observations of explosion generated PcP spectra at near-normal incidence

Abstract Short period recordings of PcP at the SRO station ANTO have been observed at epicentral distance of 13.5° from presumed underground explosions in western Kazahk, USSR. The core reflections are narrow band (0.6 to 2.4 Hz), short duration (3 sec) signals. Comparison of these near normally incident reflections to P waveforms observed at greater distances reveals that the PcP spectra are peaked with respect to the more representative P-wave spectra. The 1.2 Hz spectral peak is also observed for PcP waves recorded at 50 degrees. Corrections for frequency independent mantle Q attnuation models only increase the high frequency deficiency of the PcP spectra at frequencies above 1.2 Hz. A plausible explanation calls for finer structural features of core-mantle boundary (CMB) than hitherto suggested. The influence of small scale lateral heterogeneities, however, cannot be completely ruled out. (Mantle-core boundary, near normal PcP reflection.)

Geomagnetic field reconnaissance in Kuwait

Abstract Nearly 150 measurements of the total magnetic field intensity were made in Kuwait in early 1988. Observation points are located primarily along major highways over lines varying in length from few to several tens of kilometers. Measurements were made with a proton magnetometer. Station spacing varied from nearly 1 to 5 km. Diurnal correction to the data was made possible by frequent excursions to the base station. Preliminary analysis of the data indicates that the field intensity undulates in SW-NE direction nearly perpendicular to the regional tectonic trend. The amplitude of these variations range approximately from 30 to 90 γ over wavelengths varying nearly from 38 to 70 km. We have made a combined inversion of the data with the unpublished Bouguer gravity map of the country along a N-S profile in the central region and find that the data are reasonably well matched with predictions of a simple two-dimensional block model. The model consists of a layer overlying a half-space. The variables of the model are the thickness and density of the surface layer and the susceptibility of the basement rock. The observed variation of the magnetic field and Bouguer anomalies for the selected profile may be accounted for either by the simultaneous northward density reduction of about 0.06 g/cm3 in the surface layer and approximately 15% increase of the basement susceptibility, or else by nearly 800 m variation of the sedimentary thickness.