Plane Of Shock Research Articles

Structure of a quasi-parallel, quasi-laminar bow shock

A thick, quasi-parallel bow shock structure was observed with field and particle detectors of both HEOS 1 and OGO 5. The typical magnetic pulsation structure was at least 1 to 2 earth radii thick radially and was accompanied by irregular but distinct plasma distributions characteristic of neither the solar wind nor the magnetosheath. Waves constituting the large pulsations were polarized principally in the plane of the nominal shock, therefore also in the plane perpendicular to the average interplanetary field. A separate interpulsation regime detected between bursts of large amplitude oscillations was similar to the upstream wave region magnetically, but was characterized by disturbed plasma flux and enhanced noise around the ion plasma frequency. The shock structure appeared to be largely of an oblique, whistler type, probably complicated by counterstreaming high energy protons. Evidence for firehose instability-based structure was weak at best and probably negative.

Source process and tectonic implications of the Spanish deep-focus earthquake of March 29, 1954

The source process of the deep-focus Spanish earthquake of March 29, 1954 (m_b = 7.1, h = 630 km) has been studied by using seismograms recorded at teleseismic distances. Because of its unusual location, this earthquake is considered to be one of the most important earthquakes that merit detailed studies. Long-period body-wave records reveal that the earthquake is a complicated multiple event whose wave form is quite different from that of usual deep earthquakes. The total duration of P phases at teleseismic distances is as long as 40 s. This long duration may explain the considerable property damage in Granada and Malaga, Spain, which is rather rare for deep earthquakes. Using the azimuthal distribution of the differences between the arrival times of the first, the second and later P phases, the hypocenters of the later events are determined with respect to the first event. The focus of the second event is located on the vertical nodal plane of the first shock suggesting that this vertical plane is the fault plane. This fault plane which strikes in N2°E and dips 89.1°E defines a nearly vertical dip-slip fault, the block to the west moving downwards. The time interval and spatial separation between the first and the second events are 4.3 s and 19 km respectively, giving an apparent rupture velocity of 4.3 km/s which is about 74% of the S-wave velocity at the source. A third event occurred about 8.8 s after the first event and about 35.6 km from it. At least six to ten events can be identified during the whole sequence. The mechanism of some of the later events, however, seems to differ from the first two events. Synthetic seismograms are generated by superposition of a number of point sources and are matched with the observed signals to determine the seismic moment. The seismic moments of the later events are comparable to, or even larger than, that of the first. The total seismic moment is determined to be 7 · 10^(27) dyn cm while the moments of the first and the second shocks are 2.1 · 10^(26) dyn cm and 5.1 · 10^(26) dyn cm, respectively. The earthquake may represent a series of fractures in a detached piece of the lithosphere which sank rapidly into the deep mantle preserving the heterogeneity of material property at shallow depths.

Stability of a Plane Shock Wave in Free Space and in the Presence of Rigid or Interfacial Boundaries

The attenuation of small perturbations in the shape of a plane shock wave is studied for three distinct cases — (A): when a rigid boundary (wall) parallel to the plane of the unperturbed shock is present; (B): when a perturbed interface between two different media exists in the wake of the shock; (C): when there is no boundary (free space). The solution is obtained by extension of a method due to Zaidel. It is shown that for weak shock waves the attenuation becomes very weakly dependent on the nature of the boundary. Numerical results are obtained for shock waves with initially sinusoidal perturbations and initially Gaussian perturbations. For initially sinusoidal perturbations, Case (A) shows the slowest damping (highest peak-to-peak ampli-after some time) whereas Case (B) exhibits the most rapid damping. On the other hand, an initially Gaussian perturbation shows the slowest damping (magnitude of maximum disturbance) for Case (B) and the fastest damping for Case (A). Furthermore, the Gaussian perturbation damps monatonically with time whereas the sinusoidal perturbation damps approximately in the manner of a zero-order Bessel function — i.e., in an oscillating fashion.

The focal mechanism of the Alaska earthquake of March 28, 1964, and of its aftershock sequence

Focal mechanisms have been determined for one preshock, for the main shock, and for more than 25 aftershocks of the Alaska earthquake of March 28, 1964. For the main shock a single nodal plane with a strike azimuth of 66°, dip 85° southeast, is determinable from the polarity of the P wave. This plane may be taken either as a plane normal to the fault motion (thrust faulting) or as the fault plane (dip-slip motion on a near-vertical plane). A combination of P wave first motion and S wave polarization data make possible the determination of both nodal planes in each shock studied of the aftershock sequence. One of these planes is near vertical and closely resembles the nodal plane of the main shock; the other dips 5° to 15° to the northwest or north. For earthquakes of the Kodiak Island region, the near-vertical plane has the same orientation as that of the main shock. For earthquakes to the east of Prince William Sound, this plane shows a systematic change in orientation corresponding to the change in trend of the tectonic features. Three earthquakes that have foci at increasing depths along the line of greatest flexure of the tectonic features differ from the others. The difference in character of these foci probably provides an important clue to the right interpretation of the motion in the main shock. Although the focal mechanism solutions for the shocks are subject to the same ambiguity of interpretation as that of the main shock, criteria which favor a thrust hypothesis are advanced from the interrelation of the foci. From dislocation theory it is shown that differential slip and/or a dipping thrust plane explain satisfactorily the observed vertical displacements at the surface.