SUMMARY We present the results of a dense seismological experiment in the western part of the Gulf of Corinth (Psathopyrgos-Aigion area), one of the most active rifts in the Aegean region for which we have precise tectonic information. The network included 51 digital stations that operated during July and August 1991, covering a surface of 40 x 40 km2. Among the 5000 recorded events with ML ranging between 1.0 and 3.0, we precisely located 774 events. We obtained 148 well-constrained focal mechanisms using P-wave first motions. Of these, 60 also have mechanisms obtained by combining the P-wave first motions with the S-wave polarization directions. The observed seismicity is mainly located between 6 and 11 km depth. Most of the fault-plane solutions correspond to E-W-striking normal faulting, in agreement with the geological evidence. Most of the well-determined mechanisms indicate a nodal plane dipping 10-25" due north and a steep south-dipping plane. A similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the Corinth Rift. We discuss this evidence and the inference of a deep detachment zone, a structure where the major faults seen at the surface appear to root. A large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone. This feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes.

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A microseismic study in the western part of the Gulf of Corinth (Greece): implications for large-scale normal faulting mechanisms

Quaternary tectonics of Utah with emphasis on earthquake-hazard characterization

Using a linear inversion method applied to regional surface wave data, we have determined seismic moment tensors for 50 earthquakes in the western United States. Regional surface waves are well suited for studying moderate-sized earthquakes (4 < M < 5) located outside of local seismic networks for which first-motion solutions are difficult to obtain and seismic moment estimates are problematic due to clipping of local P and S wave recordings. We compared the inversion results to P wave first-motion solutions for 17 of the 50 events, and our comparison shows much better agreement in the orientations of the T axis than for the P axis. This could be related to the sensitivity of both methods to focal depth estimates. The surface wave results yielded a very uniform and consistent data set of stress orientations. The T axis is uniformly horizontal for all areas sampled, with E-W extension across northern California, northern Nevada, and into north central Utah; ESE-WNW extension in southern Nevada; and ENE-WSW extension in Idaho, north of the Snake River Plain. The P axis shows more variability than the T axis on local as well as regional distance scales. In the Great Basin, the results indicate a N-S change in the relative magnitude of the vertical to maximum horizontal compressive stress, With most earthquakes showing normal faulting in the Battle Mountain region and strike-slip faulting in southern Nevada. The direction of maximum horizontal compressive stress is mainly N-S in northern California and western Oregon. Our results, along with the results of Zoback et al. (1981) for central California, indicate a 60° rotation of the maximum horizontal compressive stress between northern and central segments of the San Andreas fault system. A change in stress state along the northern segment could be related to the thermal and mechanical evolution of the Pacific-North American plate boundary.

Seismic Moment tensors of western U.S. earthquakes and implications for the tectonic stress field

The contemporary seismicity of the Colorado Plateau based on seismic monitoring in the past 30 yr can be characterized as being of small to moderate magnitude, and contrary to earlier views, of a low to moderate rate of occurrence with earthquakes widely distributed. Concentrations of earthquakes have been observed in a few areas of the plateau. The most seismically active area of the Colorado Plateau is the eastern Wasatch Plateau-Book Cliffs, where abundant small-magnitude seismicity is induced by coal mining. The largest earthquakes observed to date, of estimated Richter magnitude (M L ) 5-6, have generally occurred in northern Arizona. Although very few earthquakes can be associated with known geologic structures or tectonic features in the Colorado Plateau, seismicity appears to be the result of the reactivation of pre-existing faults lacking surficial expression but favorably oriented to the tectonic stress field. The small to moderate size of the earthquakes and their widespread distribution are consistent with a highly faulted Precambrian basement and upper crust, and a moderate level of differential tectonic stress. Earthquakes in the plateau generally occur in the upper crust from the near-surface to a depth of 15-20 km, although events have been observed in both the lower crust and uppermost mantle in areas of low to normal heat flow. The latter suggests that temperatures are sufficiently low at these depths that brittle failure and hence earthquakes are still possible. The predominant mode of tectonic deformation within the plateau appears to be normal faulting on northwest- to north-northwest-striking faults with some localized occurrences of strike-slip faulting on north-west- or northeast-striking planes at shallow depths. The contemporary state of stress within the plateau is characterized by approximate northeast-trending extension in contrast to the previous belief that the plateau was being subjected to east-west tectonic compression. One area of the plateau, the eastern Wasatch Plateau and Book Cliffs, may still be characterized by compressive stresses; however, the nature of these stresses is not well understood.

Contemporary seismicity, faulting, and the state of stress in the Colorado Plateau

We show the capabilities of a downhole Distributed Acoustic Sensing (DAS) array in detecting, locating and characterizing low-magnitude earthquakes occurring in the vicinity of the Frontier Observatory for Research in Geothermal Energy (FORGE) site in Utah. 10.5 days of continuous data were acquired in a monitoring well at the FORGE geothermal site during the initial stimulation of an Enhanced Geothermal System in April-May 2019. Earthquake activity beneath Mineral Mountains, Utah also occurred within 10 km of the FORGE monitoring well. During the experiment, four events from those areas were cataloged by the University of Utah Seismograph Stations. Our processing of DAS data, including template matching, finds 82 earthquakes during that period, of which 16 are visible on the regional network. The magnitude of completeness obtained by DAS processing is better by at least M=0.5 than the dense surface array around the FORGE site. While a single vertical DAS array is limited in terms of event location due to its azimuthal ambiguity, multiple DAS wells or a combination of a downhole array with surface stations or near-surface horizontal DAS could jointly resolve locations. All detected events probably originated from the two active source areas and can be clustered into several distinct families.

Low-Magnitude Seismicity With a Downhole Distributed Acoustic Sensing Array—Examples From the FORGE Geothermal Experiment

Geometry of seismically active faults and crustal deformation within the Basin and Range-Colorado Plateau transition in Utah

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Geometry of seismically active faults and crustal deformation within the Basin and Range-Colorado Plateau transition in Utah