Aseismic Area Research Articles

Macroseismic survey of Salsk (Russian platform) earthquake of 22 May 2001

The Salsk, 22 May 2001, earthquake occurred within the Russian platform in almost aseismic area. Its magnitude (MS = 4.6) is the largest instrumentally recorded one in the region. This makes the event of a crucial importance in hazard evaluation. Because of poor instrumental data, the earthquake was studied by macroseismic survey. This also gives possibility to evaluate directly impact of the earthquake on the hazard in terms of intensity. Area covered by the survey includes 36 datapoints in the frames of 110 over 170 km. The interpretation of data obtained results in location of macroseismic epicenter and depth, evaluation of its maximum observed and epicentral intensities, and assessment of intensity attenuation.

Quaternary volcanic front at the junction of the South-west Japan Arc and the Ryukyu Arc

The Hohi volcanic zone, a 70 by 40 km volcanic graben on Kyushu Island at the junction of the South-west Japan Arc and the Ryukyu Arc, has been formed since 6 Ma to the present. Quaternary volcanism, producing centers along a NE–SW trending line, the volcanic front, started at about 1.5 Ma. This volcanic front crosses the E–W alignment that was built by the Pliocene volcanoes of the Hohi volcanic zone. The Quaternary volcanic front is parallel to the depth contours of the present deep-seismic plane, and is perpendicular to the current direction of subduction of the Philippine Sea plate. The age of formation of the volcanic front coincides with the time when several other tectonic events occurred, such as the counter-clockwise shift from NNW to WNW in relative convergence direction of the Philippine Sea plate. The volcanic rocks erupted after 1.5 Ma are more widely distributed, with a larger total volume in the denser seismic zone than in the aseismic zone. This suggests that the volcanism on the volcanic front is closely related to seismicity at about 120 km depth. The active seismic area may have accelerated to generate a partially molten zone beyond the dragged hydrated peridotite layer above the downgoing slab in comparison to the aseismic area.

A model for the 1995 Kozani-Grevena seismic sequence

Accurate locations of aftershocks, fault plane solutions for the main shock and for a large number of aftershocks as well as geographical distribution of macroseismic intensities led to a reliable estimation of the fault parameters and to a better understanding of the rupture process for the 1995 Kozani-Grevena destructive earthquake. The corresponding fault has a length L = 30 km, a width w = 10 km, strikes in an ENE direction (N65 °E), dips to NNW and the mean displacement on the fault during the generation of this earthquake is about 50 cm. Aseismic preshock slip in an aseismic area of the central part of the fault induced high tectonic stress in the rest of the fault. This stress reached the foreshock barrier's strength near the shallow western boundary of the aseismic area, where the two largest foreshocks occurred, and then the ultimate mainshock barrier's strength near the deep eastern boundary of the aseismic area where the mainshock occurred. From the focus of the mainshock, which was located in the deepest and northernmost part of the fault, the rupture propagated both up-dip and bilaterally and terminated vertically at a depth of about 4 km (blind fault) and horizontally at the eastern and western end of the aftershock zone. The vertical termination of the rupture not at the surface but at some depth is attributed to properties of the uppermost part of the crust or to slipping on minor shallow faults, while the horizontal east and west termination to the intersection of this fault with other faults which strike in a NE direction (geometrical barriers).

The Meade County, Kentucky, Earthquakes of January and March 1990

Abstract The series of earthquakes that occurred in Meade County, Kentucky, in January and March of 1990, are the first events in an otherwise aseismic area. First motions recorded for the three largest events in January suggest a predominantly strike-slip mechanism with a pressure axis of trend = 275° and plunge = 14° , and a tension axis of trend = 178° and plunge = 31°. A comparison between the mechanisms for the Meade county events and those shown in Taylor et al. (1989) for the southeastern Illinois area, indicates a continuity in the regional deviatoric stress from southeastern Illinois into north-central Kentucky. This finding conflicts with that previously suggested by Ault et al. (1985) who proposed that the two areas were in regions of differing stress regimes.

Are Active Mid-Plate Fault Zones of the Central United States Interconnected?

Abstract It is generally assumed that faults in mid-plate regions, such as central and eastern United States (CEUS), unless historically active, are inactive and/or lack a potential for large earthquakes. The Meers fault in Oklahoma, located in an historically aseismic area, is a spectacular exception to this rule. Paleoseismic studies of this fault show that the most recent large event occurred about 1200 years ago, had a magnitude of more that MS = 7 or 7.5, a surface rupture of 40 km length, and several meters of net displacement on a major left-lateral fault (Ramelli and Slemmons, in press). At least four fault zones in other parts of CEUS indicate that the Meers faulting event is not unique, including: (1) New Madrid epicentral region with three events of about MS = 8 in 1811 and 1812 with surface faulting and deformation (Russ, 1982), (2) Washita Valley fault (Cox and VanArsdale, 1986), (3) Kentucky River fault (VanArsdale, 1986), and possibly (4) faulting near Pierre, South Dakota (Nichols and Collins, 1987). Moreover, such midplate active faults are not unique, for there are at last four similar examples of historical seismogenic faulting (McCue and others, 1987) with earthquakes of up to 6.9 magnitude. Most active fault zones can be dearly demonstrated to be parts of branching or interconnected tectonic systems, implying transfer of stress and strain along zones of deformation. We speculate that tectonically active mid-plate fault zones may also be parts of much longer, interconnected active systems, rather than isolated “hot spots” of activity resulting solely from the response of local crustal flaws to a regional stress field.

Details of the aftershock process following the 30 September 1977 Uinta Basin, Utah, earthquake

On 30 September 1977, a magnitude 4.5 ML earthquake occurred in a relatively aseismic area about 135 km east of Salt Lake City, Utah, near the boundary between the southern flank of the Uinta Mountains and the Uinta Basin. Aftershocks were recorded with a temporary local network for 15 days after the main earthquake, and 173 hypocenters were located using these data. Computed standard errors for the hypocenters are less than 0.5 km horizontally and 0.75 km vertically. Projections in plan and cross-section show that the aftershocks occurred in three clusters, which together define a roughly rectangular zone that strikes N10°E and dips 45°ESE. The zone extends about 2.5 km NE-SW, 3.7 km NW-SE, and is 1 km thick. The time-space history of the aftershocks indicates that, following the largest aftershock (11 October 1977, ML 4.0), the rupture zone was extended in the up-dip direction into a zone that was previously seismically inactive. Composite focal-mechanism solutions for the aftershocks indicate normal faulting; one nodal plane of each solution corresponds to the planar zone defined by the aftershock hypocenters.

The Late Cretaceous-Cenozoic history of western Baluchistan Pakistan—the northern margin of the Makran subduction complex

Summary The study area lies some 400 km N of the Makran coast, and extends westwards for some 450 km from the Chaman Fault, a major sinistral transform fracture zone which marks the suture between the Eurasian and Indian plates. It lies to the north of the Makran ranges of Tertiary flysch, and flanks the southern margin of a stable, aseismic area known as the Dashi-i-Margo block. The Makran region as a whole has been interpreted as an accretionary prism of Late Cretaceous to Holocene age, resulting from the northward subduction of oceanic lithosphere. The geological record of the study area is consistent with such an interpretation, for the period Late Cretaceous-Palaeocene at least. Rapid northward subduction during the Late Cretaceous gave rise to profuse andesitic volcanism on the sub-parallel, ENE-trending arc massifs of the Chagai Hills and Ras Koh geanticlines. The waning of this volcanism during the Maestrichtian coincided with the accumulation and deformation of flysch sediments in a trench which extended from Mirjawa to the south of Ras Koh; also with thick forearc sedimentation in the Dalbandin Trough, between the geanticlines. In late Palaeocene times, deformed flysch—interpreted as a subduction complex—was elevated to form a structural high in the Mirjawa range. In the western Ras Koh, the flysch, along with ultramafic slices, became emplaced against the arc massif to form a new landmass. This emplacement may have been a result of a late Palaeocene collision between the northward-migrating Indian continental plate and the continental lithosphere of the Dasht-i-Margo block, the Ras Koh flysch belt taking the brunt of the initial impact. The principal forearc faults have regular, arcuate traces which, in the south-west, parallel the structural grain of the Mirjawa flysch belt. Movement on the faults influenced sedimentation in the forearc, particularly during the Eocene and Miocene. There is evidence of tensional transform movement in Eocene times. The forearc faults were especially active during the Pliocene. Movement across them during this period was compressional and associated with major folding, particularly in the Dalbandin Trough. The trend of this folding may imply dextral transform displacement within the trough.

A rare earthquake sequence in the Kobuk Trench, northwestern Alaska

abstract Four moderate earthquakes in the magnitude of 4 to 5 range occurred in the Kobuk Trench of northwestern Alaska during October 1980. This is a characteristically aseismic area. All foci were of shallow depth, and focal mechanism solutions from the three largest events are consistent in indicating right-lateral displacement on a fault trending WNW-ESE. This would be in agreement with mapped faults of Quaternary age in the area.

Structure and permeability: Geologic controls on induced seismicity at Nurek reservoir, Tadjikistan, USSR

Induced seismicity at Nurek, Tadjik S.S.R., is occurring within the upper plate of the Ionakhsh thrust, which passes at depth directly beneath the reservoir. Within the immediate vicinity of the reservoir one can contrast two distinct areas, one seismic and the aseismic. Both consist of rocks that can be shown to have a highly anisotropic permeability due to alternating permeable and impermeable strata. Water flow is favored along bedding while essentially prohibited across bedding. However, because the two areas differ in structure, they also differ in their vertical component of permeability, as a direct result of the different attitudes of the bedding. In the aseismic area, vertical fluid flow is blocked by a syncline that cups the base of the reservoir. In the seismic area, a plunging anticline that crops out beneath the reservoir allows water to flow to depth in the high-permeability direction-along bedding. Because effective stress changes at depth are dependent on permeability characteristics, knowledge of subsurface permeability or permeability anisotropy can serve as a means by which to predict the distribution of induced seismic activity. Thus, while regional geologic studies may indicate the presence of lithologies that are conducive to the transmission of pore-water pressure, access ofmore » water to these units depends on the structural arrangement beneath the reservoir, on a very local scale.« less