11 Search Results

Instrumental seismic monitoring has a long history in the Caucasus and started in 1899 when the first seismograph was installed in Tbilisi, Georgia. Much of the analog paper records from this time period are preserved in the Tbilisi archives because Georgia served as the regional data center. In the 1990s, due to the collapse of the Soviet Union and the political turmoil in the region, the analog networks and the communication between the newly formed national networks deteriorated. In Georgia, for the next 13 yr, the seismic network coverage was poor until the 2002 Tbilisi earthquake. Following this earthquake, themore » first permanent digital seismic station in Georgia was established in Tbilisi in 2003. The digital era progressively improved the ability to collect and archive data and today more than a hundred broadband seismic stations (including temporary arrays) are operating in the southern Caucasus. Until recently, the region lacked a coordinated effort to catalog all analog and digital era data collected by different countries into a single repository. As a result of collaboration between Lawrence Livermore National Laboratory, the Ilia State University, and the Republican Seismic Survey Center of Azerbaijan, a comprehensive earthquake catalog was compiled for the Caucasus and neighboring areas as part of a broader probabilistic seismic hazard assessment project. Here this project digitized Soviet-era paper bulletins, compiled a unified earthquake catalog from regional bulletins, developed 1D reference velocity model, and used it to relocate the events. The final catalog contains 16,963 events with magnitudes 3.7 and above, bringing together all the available data sets in the Caucasus region from 1900 to 2015, significantly improving locations, and generating the most complete earthquake catalog in the region, temporally and geographically.« less

Our objective is to improve the view of the seismicity in the Caucasus region using instrumental data between 1951 and 2019. To create a comprehensive catalog, we combine the bulletins of local agencies and the International Seismological Centre, and use an advanced single-event location algorithm, iLoc, to obtain better locations. We show that relocations with iLoc, using travel-time predictions from the 3D upper mantle velocity model, Regional Seismic Travel Time, improve the locations. Then, using the iLoc results as initial locations and the ground-truth events identified in the iLoc results as fix points, we apply Bayesloc, a multiple-event location algorithm,more » to simultaneously relocate the entire seismicity of the Caucasus region. Here we demonstrate that the simultaneous relocation of the seismicity with Bayesloc clarifies the location and geometry of major active structures accommodating ongoing convergence between the Arabian and Eurasian continents between the Black and Caspian Seas. Among our major findings is the confirmation of widespread seismicity in the mantle beneath the northern flank of the Greater Caucasus and central Caspian, resulting from north-dipping subduction of the Kura and South Caspian basins and the identification of a discrete band of crustal seismicity beneath the southern flank of the Greater Caucasus.« less

We characterize the attenuation structure of the Arabian Peninsula through the measurement of regional phase amplitudes. High-resolution is achieved by combining stations from global networks with national network data through the cooperative effort of several countries in the region, including Saudi Arabia, Oman, Iraq, and Kuwait. The result is an improved attenuation model of the crust and upper mantle for a broad frequency band that extends from 0.5 to 10 Hz. The observed attenuation is in accordance with various elements of earth structure, including plate boundary type, style of tectonism, thermo-tectonic age, and temperature. Emerging features from the model includemore » details in the structure along the Red Sea, and improved imaging of the southern Arabian Peninsula extending north from the Gulf of Aden. Finally, the resulting attenuation model can be employed for better magnitude estimates, in isolating tectonic and structural features, and in characterizing strong ground motion in the Arabian Peninsula.« less

Reliable estimates of moment magnitude and source mechanism for seismic events in the Middle East can be challenging due to a small number of openly available stations, the complex tectonic setting, and regions of high attenuation. Access to high-quality waveform data from well-calibrated regional seismic stations is fundamental in producing robust and stable estimates of earthquake source parameters, particularly when measurements of absolute ground motion amplitudes are required. Earthquake source mechanism and moment magnitude are invaluable information in the assessment of seismic hazard, plate motions, and the characterization of faults and regional stress field. The expansion of the Kingdom ofmore » Saudi Arabia’s national seismic network in the past several decades provides an opportunity to develop the capabilities of routine focal mechanism and moment magnitude estimations. In this study, we use time-domain full waveform moment tensor inversion and coda envelope–derived amplitude measurements to solve for earthquake source mechanism, moment magnitudes, and their source type. Furthermore, we compared the moment magnitudes calculated from the two methods and publicly available earthquake catalogs and discuss the implications of the obtained source parameters.« less

Seismic arrays provide useful tools for regional seismic monitoring. Two small-aperture, regionally-oriented arrays, QWAR and HQAR, were deployed in Saudi Arabia and Oman in 2012 and 2016. We utilize a time-domain beampacking method, similar to frequency-wavenumber analysis, to examine the performance of the arrays in terms of slowness and azimuthal bias and event detection capabilities. Additionally, we investigate persistent ambient noise sources recorded by the arrays. We find that the arrays provide slowness vectors with biases comparable with similar-sized arrays of the International Monitoring System in other locations around the world. At QWAR, regional events of magnitude 3.0 and abovemore » are detected a majority of the time, and as magnitudes increase to 4.0 and above, the detection rate is greater than 82%. Finally, strong noise generation, primarily with slownesses characteristic of Lg waves, is found in the directions of the northern Arabian/Persian Gulf and southern Red Sea and may be a factor in event detection capabilities.« less

Earthquake magnitudes are widely relied upon measures of earthquake size. Although moment magnitude (M_w) has become the established standard for moderate and large earthquakes, difficulty in reliably measuring seismic moments for small (generally M_w<4) earthquakes has meant that magnitudes for these events remain plagued by a patchwork of inconsistent measurement scales. Because of this, magnitudes of small earthquakes and statistics derived from them can be biased. Furthermore, because small earthquakes are much more numerous than large ones, many applications, such as seismic hazard modeling, depend critically on analysis of events characterized by magnitudes other than M_w. Therefore, to assess thismore » problem, we apply coda envelope analysis to reliably determine moment magnitudes for a case study of small earthquakes from northern Oklahoma and southern Kansas. Not surprisingly, we find significant differences among M_L, m_bLg, and M_w for M ~2–4 earthquakes examined here. More troublingly, we find that relations designed to convert other magnitudes to M_w, which are relied upon for important applications such as seismic hazard analysis, often increase rather than decrease this bias for our dataset. In our case study, we find that converted magnitudes can result in a systematic bias sometimes exceeding 0.5 magnitude units, a difference that typically corresponds to a factor of ~3 in seismicity rate. Moreover, we find a correspondingly large bias in Gutenberg–Richter b-values, controlled primarily by inaccurate magnitude scaling in the conversion relationships. Although this study focuses on a relatively small geographic area, we can expect that similar issues exist with varying severity in other regions. Therefore, magnitudes of small earthquakes and their associated statistics, including seismicity rates and b-values, should be treated with caution.« less

To better characterize seismic hazard, particularly, for induced seismicity, there is an increasing interest in methods to estimate moment magnitude (M_w) for small earthquakes. M_w is generally preferred over other magnitude types, but, it is difficult to estimate M_w for earthquakes with local magnitude (M_L) <3-3.5, using conventional moment tensor (MT) inversion. The 2020 M_ww 5.7 Magna, Utah, seismic sequence provides an opportunity to illustrate and evaluate the value of spectral methods for this purpose. Starting with a high-quality seismic catalog of 2103 earthquakes (M_L<5.6), we estimate M_w using two independent spectral methods—one based on direct waves, yielding M_w,direct, andmore » the other based on coda waves, yielding M_w,coda. For the direct-wave method, we present a non-parametric (NP) inversion scheme that solves for apparent geometrical spreading, G(R), and site effects (S), similar to other NP procedures that have been used to calibrate regional M_L scales. The NP inversion is constrained using M_ws derived from MTs for nine events in the Magna sequence. We recover statistically robust and physically reasonable G(R) and S and compute M_w,direct for 635 Magna earthquakes down to M_L 0.7. For the coda-wave method, we consider two separate calibration schemes involving previous MT solutions and compute M_w,coda for 311 earthquakes down to M_L 1.0. For 280 of the events that were processed with both methods—M_w,direct and M_w,coda—are strongly correlated (r = 0.98), with a mean difference of only 0.05. We compare M_w,direct and M_w,coda with M_L and find reasonably good agreement for M_L<3.6 with the theoretically predicted relationship of M_w=(2/3)M_L+C, in which C is a regional constant. Our results imply that seismic network operators can use spectral-based M_w estimates to replace M_L estimates for events with M_L≥1.0, and possibly smaller. The main requirement is the existence of a small number of MT solutions for calibration purposes.« less

The crustal structure of Iraq was investigated through analyzing teleseismic data from 12 new seismic stations. Three seismic stations are located within the Zagros Fold-Thrust Belt, and nine are within the Mesopotamian Plain. Joint inversion of P wave receiver function and Rayleigh wave dispersion data were employed to resolve S wave velocity structure models beneath each station. Combining these models with available Moho depths within and around the study area reveals that the Moho depth is smoothly increasing from the Arabian platform toward the Zagros Mountains. Further, an exceptional deeper root is observed near the eastern edge of the Mesopotamianmore » Plain where sedimentary pile is the thickest. This root was interpreted as a structure inherited from the successive Mesozoic rifting of the NE Arabian platform enhanced by sedimentary loading and progressive subsidence. A low-velocity uppermost mantle was observed beneath the Arabian Foredeep attesting the existence of a low-strength lithospheric mantle beneath the region southwest from the Zagros deformation front. The weak rheology of the uppermost mantle may have allowed local sinking of the crust and deepening of the Moho boundary due to vertical loading and the Late Miocene lateral contraction.« less

Abstract We develop a new methodology for determining earthquake stress drop and apparent stress values via spectral ratio asymptotic levels. With sufficient bandwidth, the stress ratio for a pair of events can be directly related to these low‐ and high‐frequency levels. This avoids the need to assume a particular spectral model and derive stress drop from cubed corner frequency measures. The method can be applied to spectral ratios for any pair of closely related earthquakes and is particularly well suited for coda envelope methods that provide good azimuthally averaged, point‐source measures. We apply the new method to the 2011 Praguemore » and 2016 Pawnee earthquake sequences in Oklahoma. The sequences show stress scaling with size and depth, with the largest events having apparent stress levels near 1 MPa and smaller and/or shallower events having systematically lower stress values.« less

A comprehensive earthquake catalog was compiled for Iraq and neighboring areas as part of a broader probabilistic seismic–hazard assessment project. The Iraq Seismic Network (ISN) was established in 1976 and became operational in the early 1980s. However, recording and reporting of seismic data has been intermittent in Iraq. Hence, events were collected from various sources, including the ISN when available, International Seismological Centre (ISC), European–Mediterranean Seismological Centre, U.S. Geological Survey Centennial Catalog, Global Centroid Moment Tensor solutions, and Ambraseys’ extensive work on cataloging of instrumental era earthquakes in the Middle East (e.g., Ambraseys, 1978, 2001, 2009). We supplement these withmore » new direct moment magnitude calculations based on coda calibration technique. For many of the larger events in the catalog, more than one magnitude is available. Directly calculated moment magnitudes (M_w) were favored, followed by body–wave magnitude (mb) obtained from the ISC. Where no directly calculated M_w was available, other magnitude scales were converted to M_w using relationships compatible with the local catalog. In conclusion, the resulting earthquake catalog spans from the year 1900 until the end of 2009, covers the region bounded by 26°–40° N latitudes and 36°–51° E longitudes, and includes more than 18,000 earthquakes, of which roughly 4000 are M_w 4.0 or larger.« less