Short-period Events Research Articles

An explanation of induced magnetic variations at Sabhawala, India

Induction vectors calculated for short-period events such as “bays” and “storm sudden commencements” (SSCs) indicate that the conductor causing the anomaly lies to the south of Sabhawala. The nature of vertical transfer functions obtained from spectral analysis of storm-time variations further confirms the presence of such a conductor. The nature of maximum ( G p) and minimum ( G 1) response functions suggests that a two-dimensional conductor south of Sabhawala is responsible for anomalies in Z. Invoking a structural model of the Earth's crust beneath the Himalayas, in keeping with accepted orogenic theories, we propose that one structure capable of producing the desired conducting path is a rise of the asthenosphere south of the Himalayan range. Another important factor contributing to the induced variations at Sabhawala is due to a sedimentary trough running parallel to the Himalayas, again to the south. These views are confirmed by a two-dimensional model calculation. It is found that the undulation of the asthenosphere alone does not reproduce the nature of the anomalies in H and Z.

Design of Planetary Cams for Short Period Events

A cam mechanism is described in which a planetary cam operates in sequence a number of equally spaced radial translating followers, the lift and return of the followers taking place during a small angular rotation of the arm which carries the planetary cam. Equations for the synthesis of the planetary cam are developed, along with those for the sliding velocity between cam and follower. A method for allowing small variations in timing is described and some experimental results are given for a number of cams operating two different types of translating follower.

Short-period nonseismic tilt perturbations and their relation to episodic slip on the San Andreas Fault in central California

Data from four tilt meters and several creep meters along the San Andreas fault in central California 30 km south of Hollister were selected to investigate nonseismic short-period tilt perturbations of minutes to hours duration. The group of short-period tilt amplitude changes with residual deflections was chosen because many nonseismic creep events have similar durations. Although each of the four tilt meters recorded many short-period events, the distribution in space and time suggests that most of these events are due to causes other than slip on the San Andreas fault, such as local strain release. Only at one station used in this investigation (MEL) were creep-related tilt changes observed. A quasi-static dislocation model was constructed to predict the tilt changes associated with propagating slip. Comparing predicted to observed tilt wave forms and amplitudes and comparing the tilt event data to creep event data suggest that (I) a tilt amplitude change of 10 -8-10 -8 rad associated with nonseismic creep events will be observed only if the tilt meter is less than 0.5-1.0 km from the fault, (2) the vertical extent of the slip zone associated with these creep events is greater than about 0.5 km but perhaps not more than a couple of kilometers, (3) nonseismic episodic slip on this part of the San Andreas fault does not appear to propagate uniformly over more than a couple of kilometers, and (4) the model presented in this paper requires 0.5-1.0 mm of dip-slip displacement to reproduce the residual deflection in the creep-related tilt wave forms observed.