L. J. Shamey

Bio: L. J. Shamey is an academic researcher from Cooperative Institute for Research in Environmental Sciences. The author has an hindex of 1, co-authored 1 publications receiving 13 citations.

Source dimensions of two deep earthquakes estimated from aftershocks and spectra

Abstract: The relation the spectral corner frequency and the source dimensions, r ( P, S ) = c ( P, S ) β / ƒ is tested for two deep earthquakes under the assumption that aftershocks approximately outline the rupture area. It was found that c = 0.37 as proposed by Brune (1970) leads to radii estimates within ±20 per cent of the radii outlined by the aftershocks.

Seismic moment rates in the Utah region

Abstract: An estimate of the long-term seismicity of Utah is obtained through the calculation of seismic moment rates from available geologic data on Quaternary faulting. The recurrence rates calculated from these geologic seismic moment rates agree well with recurrence rates estimated from historical earthquake data. A return period of 80 to 190 yr for a magnitude 7.0 to 7.5 earthquake in the Utah region is predicted by historical, seismic, and geologic data. The estimation of recurrence rates from geologic data requires the use of a moment versus magnitude relationship for earthquakes characteristic of the Utah region. Spectral studies of 19 Utah earthquakes define this relationship. In addition, moment tensors and other source parameters are estimated for these earthquakes. A resultant strain tensor for Utah indicates the amount of horizontal east-west extensional strain is nearly equal to the amount of vertical compressional strain in the region.

Spatial patterns of aftershocks of deep focus earthquakes

Abstract: This paper presents precise relative relocations for aftershocks of 59 intermediate and deep earthquakes, using P, pP, and PKP arrival times read by us or reported in the literature. These aftershocks included 36 “rupture subevents,” occurring within 1 min of the initial event, and 71 “true aftershocks,” occurring later and identified statistically as being related to the initial event. We compare the relative locations with the orientations of the Wadati-Benioff zones and with the available focal mechanisms using statistical methods not previously applied to earthquake data. Surprisingly, neither true aftershocks nor rupture subevents cluster along the nodal planes of intermediate or deep earthquake focal mechanisms. Aftershocks more than 20 km from the initial events occur preferentially in the plane of the Wadati-Benioff zone, while those lying closer have isotropically distributed directions. Rupture subevents occur after the travel time of the S wave from the initial event. Larger-magnitude earthquakes generally possess rupture subevents lying farther from the initial event, whereas true aftershocks can occur 50 km or more from initial events having magnitudes as small as 5.3. Except following a few unusually large earthquakes with focal depths of about 100 km, we find no rupture subevents or true aftershocks more than about 80 km from the initial hypocenter. The existence of many aftershocks far from nodal planes does not favor models in which deep earthquake failure is simply slip along a planar fault. Rather, the aftershocks may occur in response to a general redistribution of stress caused either by the occurrence of the initial event or, possibly, by nearby continuing phase transitions. Alternatively, the failure zones of deep earthquakes may be surfaces which are frequently curved by 40° or more from the orientation of the initial nodal plane, perhaps due to inhomogeneous stress distributions.

Seismic source theory

Abstract: It is remarkable that seismology has progressed as far as it has while the knowledge in one vital area, that of the seismic source, has remained in a rather primitive state. However, if the recent research effort in the United States is any indication, this situation may be changing, because several promising lines of attack upon the problems of the seismic source have been generated during the past four years. For the sake of this review, studies of the seismic source will be divided into two basic groups, the theoretical studies which are directed primarily at formulating models of the earthquake process, and the observational studies which are directed primarily at obtaining the data necessary to either check the theoretical studies or investigate the consequences of interpreting earthquakes in terms of a specific model.

Source mechanism of the deep Colombian earthquake of 1970 July 31 from the free oscillation data

Abstract: Summary. The method proposed by Mendiguren to determine the source parameters from free oscillation data is applied to the 1970 July 31 deep Colombian earthquake. The results indicate a source propagating horizontally for about 150 km along the lithosphere and cutting across its width. The slab behaves as a guide for source propagation. The horizontal propagation velocity is determined as 3.8 km/s. The intensity of the source grew proportionately to the second power of the propagation distance. This rate of source intensity growth may be interpreted either by a fan-shaped fault model or by a cone-shaped volume source. The average slip and stress drop are estimated as 360 cm and 300 bar for the fault model. For the volume source model the transformational shear strain and stress are estimated as 11 × 10−5 and 160 bar. There is no evidence of a double couple radiation preceding the P origin time. It is shown that the isotropic and deviatoric components of the moment tensor cannot be uniquely resolved when only observations of a single mode are available. It is observed that, from a statistical basis, the available 0Sn data for Colombian shock can be equally well explained by a pure deviatoric source model or by a source model including an isotropic component. Numerical experiments indicate that the inclusion of higher mode data does not change this situation. But, on the other hand, numerical experiments show that the available data and the scheme used for the inversion would not result in a solution including an artificial implosive component if the actual source were pure deviatoric. If the departure from a pure deviatoric source is produced by noise, it has to be non-random, as it could be produced by lateral heterogeneities not included in the inversion scheme.