Short-period Seismic Data Research Articles

Modern crustal seismicity in the northern Andean Precordillera, Argentina

The Precordillera in the Andean backarc region of Argentina has a long record of large and damaging earthquakes in the last century. Its northern part is not well known seismically neither its style of deformation. In this work we study modern crustal seismicity occurring in the same epicentral area of the historical 1894 earthquake. That earthquake caused deaths and severe damage even at large distances of about 500 km. Because of the unavailability of instrumental records to study this large earthquake, not many details of its seismic source are known. Our study involves broadband and short period seismic data from modern seismicity to characterise the same epicentral area. We used seismic records from the permanent Argentine network (INPRES) and from two temporary broadband experiments (CHARGE and SIEMBRA). We determine 44 seismic locations and their focal depths which are in the range between 10 and 30 km and magnitudes 1.2 ≤ M ≤ 4. In addition, we obtain 33 focal mechanism solutions for some of these earthquakes using P-wave first motions and amplitude ratios between P and S waves; for the same region, five more moderate earthquakes have been ever observed globally and reported by the Global Centroid Moment Tensor catalogue in the last 55 years. We note that epicentres are mainly distributed in the southern sector of the study region, with a relatively greater concentration in the foothills of the Frontal Andes Cordillera. Focal mechanisms show mainly reverse, and in less proportion strike-slip solutions. Overall these results agree with compression and shortening in an east-west orientation of the Andean northern Precordilleran backarc region.

Layered structure in the upper mantle across North America from joint inversion of long and short period seismic data

We estimate crustal and uppermost mantle shear velocity structure beneath 30 stations in North America by jointly inverting the high frequency scattered wavefield observed in the P wave coda, together with long period surface wave phase and group dispersion data. Several features distinguish our approach from previous such joint inversions. 1) We apply a cross-convolution method, rather than more standard deconvolution approaches used in receiver function studies, and consider both Love and Rayleigh wave dispersion, allowing us to infer profiles of radial anisotropy. 2) We generate probabilistic 1D radially anisotropic depth profiles across the whole uppermost mantle, down to ∼350 km depth. 3) The inverse problem is cast in a trans-dimensional Bayesian formalism, where the number of isotropic and anisotropic layers is treated as unknown, allowing us to obtain models described with the least number of parameters. Results show that the tectonically active region west of the Rocky Mountain Front is marked by a Lithospheric Asthenosphere Boundary and a Lehmann Discontinuity occurring at relatively shallow depths (60–150 km and 100–200 km, respectively), whereas further east, in the stable craton, these discontinuities are deeper (170–200 km and 200–250 km, respectively). In addition, in the stable part of the continent, at least two Mid-Lithospheric Discontinuities are present at intermediate depths, suggesting the existence of strong lithospheric layering, and a mechanism for lithospheric thickening by underplating of additional layers as cratonic age increases. The Moho across the continent as well as mid-crustal discontinuities in the craton are also imaged, in agreement with independent studies.

Discrimination of teleseismic events in Central Asia with a local network of short period stations

The difficult problem of distinguishing underground nuclear explosions from earthquakes at teleseismie distances was approached using short period seismic data from 6 stations in South and Central Finland. The events were nuclear tests mostly from the Semipalatinsk and Lop Nor test sites and earthquakes from adjacent areas. The magnitude range of the events was from 4.1 to 6.6. The features of the two classes of events were examined by computing spectral ratio, third moment of frequency (TMF) and complexity from P wave signals. The spectral discrimination parameters were extracted from spectra computed in 5 different ways in order to obtain all possible information even from weak events. The standard FFT spectra were computed from. raw data, after noise adaption and data adaption, from correlograms and using combinations of adaption and correlation: methods. This was done to employ not only the spectral differences of the events but also the temporal variation of energy and lack of it as a function of frequency. The optimum frequeney windows for spectral ratio and TMF were defined using stacked spectra of about 10 events from both classes. No single discriminant could classify all the events. Their performance varied significanfly for different stations, but on average the spectral discriminants had slightly higher discrimination capability than complexity. The distributions of all discriminants were studied and a group separation function was formed using an optimum set of discriminants. Instead of discriminant values their relative positions in the corresponding distributions of nuelear tests and earthquakes were used as inputs to the function. A weight for each discriminant was derived from the amount of overlap in the distributions of earthquakes and nuelear tests. All 75 events in the data set were correctly classified with the method. The testing was performed with a jack knife method to create an independent test data base

A possible triggering mechanism for large Hawaiian earthquakes derived from analysis of the 26 June 1989 Kilauea south flank sequence

Abstract Examination of short-period seismic data from the ML = 6.1 Kilauea south flank earthquake and aftershock sequence indicates that the rupture process in large Hawaiian earthquakes is more complex than previously modeled. In contrast to the low-angle thrust solution determined for the mainshock from long-period teleseismic body waves by other workers, I find an intermediate- to high-angle reverse solution; I find, however, that focal mechanisms for coastal aftershocks of ML > 3.0 are similar to the teleseismic mechanism for the mainshock. A difference in focal mechanisms determined from short-period local-network seismic data and from long-period teleseismic data has been noted for other recent large Hawaiian earthquakes. Both the mapping of surface cracks and the focal mechanism derived from short-period seismic data for the ML = 6.6 1983 Kaoiki earthquake show strike-slip motion, whereas the centroid moment tensor solution shows low-angle thrusting. The focal mechanism calculated from short-period seismic data for the ML = 7.2 1975 Kalapana mainshock shows low-angle thrusting according to some workers, but intermediate- to high-angle reverse faulting according to others, whereas focal mechanisms calculated from long-period seismic data show low-angle thrusting. This result suggests that rupture initiation in large Hawaiian earthquakes, as represented by the short-period focal mechanisms, differs significantly from the overall rupture process, as represented by the teleseismic mechanisms. I propose that small earthquakes trigger the large-scale energy release at the bases of the volcanic edifices, the type of energy release often observed in large Hawaiian earthquakes. These triggering events may occur along rupture surfaces that differ from those along which the long-period moment release occurs and thus may represent release of a local stress concentration superposed upon the regional stress field.

Physical Science: Discrimination of Earthquakes and Explosions using Short Period Seismic Array Data

THE problem of identification of earthquakes and underground explosions on a global basis, for seismic events with body wave magnitudes (mb) below 4.7 has not been solved1,2, although many partially successful discriminants have been proposed. A positive identification parameter based solely on the short period seismic data is interesting as the equivalent long period energy detection threshold is often difficult to achieve, particularly at teleseismic distances.

Waves, weather, and ocean bottom microseisms

The latest model of the Lamont Geological Observatory Ocean Bottom Geophysical Station (OBS III), located approximately 200 km WNW of San Francisco at a depth of 3.9 km, has been in operation since May 1966. In addition to long- and short-period seismic data, this station provides data on water current direction and speed, water temperature, long- and short-period pressure variations, and horizontal and vertical accelerations at tidal periods. Twenty-three microseismic storms have been detected on the seismic and pressure sensors during the first eight months of operation (May-December 1966). The characteristics of two of these storms have been studied in detail, and the results have been compared with data from the Berkeley seismographs. Both peaks in microseismic activity appear to be related to specific weather systems, one local and one distant. Correlation between local water waves and microseismic amplitudes is observed in one case but not in the other. Particle motion amplitudes on the ocean bottom were 3 to 5 times larger than those at a near-land station (Berkeley) for the vertical component and 7 to 10 times larger for the horizontal component. Predominant periods (6 to 8 sec) were the same at both sites. Ratios of horizontal to vertical particle motion and pressure to vertical motion were measured for microseisms and compared with theoretical values for Rayleigh waves. The combination of these two ratios proves to be a very sensitive method of determining sediment properties. Coherence between pressure and vertical motion is consistently high. It is concluded that (1) the observed microseisms propagate primarily as Rayleigh waves of the fundamental mode, and (2) the thickness of the sediment layer is 0.65 km. The predominant direction of microseismic propagation during both storms was approximately perpendicular to the coastline. Study of the average phase relationship of pressure, vertical particle motion, and horizontal particle motion shows that the direction of propagation is from sea to land if fundamental mode Rayleigh waves are assumed. The microseismic energy flux was approximately 4 times larger in the oceanic structure than in the continental structure near Berkeley for both storms.