Efficiency of earthquake forecast models based on earth tidal correlation with background seismicity along the Tonga–Kermadec trench

The correlation between Earth’s tides and background seismicity has been suggested to become stronger before great earthquakes and weaker after. However, previous studies have only retrospectively analyzed this correlation after individual large earthquakes; it thus remains vague (i) whether such variations might be expected preceding future large earthquakes, and (ii) the strength of the tidal correlation during interseismic periods. Therefore, we retrospectively investigated whether significant temporal variations of the tidal correlation precede large interplate earthquakes along the Tonga–Kermadec trench, where Mw 7-class earthquakes frequently occurred from 1977 to 31 December 2020. We evaluated a forecast model based on the temporal variations of the tidal correlation via Molchan’s error diagram, using the tidal correlation value itself as well as its rate of change as threshold values. For Mw ≥ 7.0 earthquakes, this model was as ineffective as random guessing. For Mw ≥ 6.5, 6.0, or 5.5 earthquakes, the forecast model performed better than random guessing in some cases, but even the best forecast only had a probability gain of about 1.7. Therefore, the practicality of this model alone is poor, at least in this region. These results suggest that changes of the tidal correlation are not reliable indicators of large earthquakes along the Tonga–Kermadec trench. Graphical Abstract

Strong earthquakes in Azerbaijan for historical and contemporary periods (conceptual review)

Based on the results of the study of literary and archival primary sources, the paper clarifies the available information about catastrophic and destructive earthquakes in Azerbaijan for the historical period with a magnitude of M≥6. Among the strong historical earthquakes in Azerbaijan there are the following: earthquakes in 427, the Ganja (Goygol in 1139), the Ganja in 1235, the East Caucasian in 1667 (± 1 year), the Mashtaga (1842), numerous Shamakhi earthquakes (1192, 1667, 1668, 1669, 1828, 1859, 1868, 1872, 1902), Ardebil (1924), Lankaran (1913), Caspian earthquakes (957, 1812, 1842, 1852, 1911, 1935, 1961, 1963, 1986, 1989, 2000), which caused both changes in the relief of the Earth’s surface, and the destruction of buildings and numerous human casualties. The background seismicity was investigated based on the results of continuous instrumental observations for the period 1902—2019. Seismic processes are unevenly distributed on the territory of Azerbaijan. Existing catalogs of seismic events have been investigated. The behavioral changes in seismicity parameters have been studied. An overview conceptual analysis of two main methods for assessing seismic hazard is given: probabilistic and deterministic, which have found their wide application in recent decades. In conclusion, the most important and general tasks of future seismological studies are emphasized, which are to be performed in the next decades.

Seismological studies in the Altai-Sayan mountain region

The paper provides a brief overview of seismological studies in the Altai-Sayan mountain region. The de-velopment of a network of seismological stations and experiments with temporary stations in the epicen-tral zones of large earthquakes is described. It is shown that the background seismicity of the region is or-dered over time into structures with a hierarchy in the rate of occurrence. Large earthquakes in some cases occur in places that do not match with the areas of increased background seismicity. Major earthquakes in Eastern Tuva (Busingol, Belin-Biy-Khem, etc.) occur as shifts and rotations of blocks near rift depressions. Large earthquakes of the Western Sayan Ridge and the Academician Obruchev Ridge (Tuvan First and Second earthquakes, Sayan earthquake) are associated with faults transverse to these structures and are the result of the uneven extension of blocks of the Tuva hollow and the Tuva highlands to the north. Stud-ies in the Altai Mountains found that after a long period (about 10 years) of the aftershock process of the Chui earthquake dominating the seismicity, a period of seismic activation of adjacent (60-80 km) and dis-tant (within a radius of approximately 260-280 km) structures occurred. The center of seismic activity shifted from the epicenter of the 2003 Chui earthquake to the epicenter of the 2019 Aigulak earthquake. Experimental work with powerful vibrators has determined the capabilities of a network of seismological stations in vibroseismic monitoring of the Earth's crust.

Kinematics of Crustal Deformation along the Central Himalaya

Using an updated set of GPS surface velocities, the present study provides fault locking behavior and slip rate distribution of the Main Himalayan Thrust (MHT) along the central Himalaya. The two-dimensional velocity field is inverted through Bayesian inversion to estimate fault geometry and kinematic parameters of the MHT along the central Himalaya. The modeling results reveal that: (1) MHT is fully locked in the upper flat (0-9 km), partially locked along the mid-crustal ramp (15-21 km), and it is creeping in the deeper flat (> 21 km); (2) there is an insignificant slip rate of MHT along the locked-to-creeping transition zone, indicating its partially coupled/locked behavior; (3) along the deeper flat of the MHT, the estimated creeping rate is ~16.3 mm/yr, ~14.7 mm/yr, and ~14.3 mm/yr along western, central, and eastern Nepal, respectively; and (4) along the MHT on the upper crust, the modeled locking width turns out to be 97 km, 106 km, and 129 km in the western, central, and eastern Nepal, respectively. In addition, the posterior probability distribution of the locking width exhibits a bimodal Gaussian distribution coinciding with the two ramp geometry of the MHT along the western Nepal. Along the foothills of the Higher Himalaya, the inferred locking line is also aligned to the estimated maximum shear strain concentration and observed seismicity along the central Himalaya. With a general agreement to the previous geodetic results, geological estimates, and background seismicity, our findings provide a promising avenue of the contemporary crustal deformation along the Nepal Himalaya. The estimated inversion results in a Bayesian framework exhibit updated fault kinematics of the MHT and hence provides valuable inputs for seismic hazard assessment along the central Himalaya.

How Do Statistical Parameters of Induced Seismicity Correlate with Fluid Injection? Case of Oklahoma

The seismicity evolution in Oklahoma between 2010 and 2018 is analyzed systematically using an epidemic-type aftershock sequence model. To retrieve the nonstationary seismicity component, we systematically use a moving window of 200 events, each within a radius of 20 km at grid points spaced every 0.2°. Fifty-three areas in total are selected for our analysis. The evolution of the background seismicity rate μ is successfully retrieved toward its peak at the end of 2014 and during 2015, whereas the triggering parameter K is stable, slightly decreasing when the seismicity is activated. Consequently, the ratio of μ to the observed seismicity rate is not stationary. The acceleration of μ can be fit with an exponential equation relating μ to the normalized injected volume. After the peak, the attenuation phase can be fit with an exponential equation with time since peak as the independent variable. As a result, the evolution of induced seismicity can be followed statistically after it begins. The turning points, such as activation of the seismicity and timing of the peak, are difficult to identify solely from this statistical analysis and require a subsequent mechanical interpretation.

The Productivity of Cascadia Aftershock Sequences

ABSTRACT This study addresses questions about the productivity of Cascadia mainshock–aftershock sequences using earthquake catalogs produced by the Geological Survey of Canada and the Pacific Northwest Seismic Network. Questions concern the likelihood that future moderate to large intermediate depth intraslab earthquakes in Cascadia would have as few detectable aftershocks as those documented since 1949. More broadly, for Cascadia, we consider if aftershock productivities vary spatially, if they are outliers among global subduction zones, and if they are consistent with a physical model in which aftershocks are clock-advanced versions of tectonically driven background seismicity. A practical motivation for this study is to assess the likely accuracy of aftershock forecasts based on productivities derived from global data that are now being issued routinely by the U.S. Geological Survey. For this reason, we estimated productivity following the identical procedures used in those forecasts and described in Page et al. (2016). Results indicate that in Cascadia we can say that the next intermediate depth intraslab earthquake will likely have just a few detectable aftershocks and that aftershock productivity appears to be an outlier among global subduction zones, with rates that on average are lower by more than half, except for mainshocks in the upper plate. Our results are consistent with a clock-advance model; productivities may be related to the proximity of mainshocks to a population of seismogenic fault patches and correlate with background seismicity rates. The latter and a clear correlation between productivities with mainshock depth indicate that both factors may have predictive value for aftershock forecasting.

The Productivity of Cascadia Aftershock Sequences

ABSTRACT This study addresses questions about the productivity of Cascadia mainshock–aftershock sequences using earthquake catalogs produced by the Geological Survey of Canada and the Pacific Northwest Seismic Network. Questions concern the likelihood that future moderate to large intermediate depth intraslab earthquakes in Cascadia would have as few detectable aftershocks as those documented since 1949. More broadly, for Cascadia, we consider if aftershock productivities vary spatially, if they are outliers among global subduction zones, and if they are consistent with a physical model in which aftershocks are clock-advanced versions of tectonically driven background seismicity. A practical motivation for this study is to assess the likely accuracy of aftershock forecasts based on productivities derived from global data that are now being issued routinely by the U.S. Geological Survey. For this reason, we estimated productivity following the identical procedures used in those forecasts and described in Page et al. (2016). Results indicate that in Cascadia we can say that the next intermediate depth intraslab earthquake will likely have just a few detectable aftershocks and that aftershock productivity appears to be an outlier among global subduction zones, with rates that on average are lower by more than half, except for mainshocks in the upper plate. Our results are consistent with a clock-advance model; productivities may be related to the proximity of mainshocks to a population of seismogenic fault patches and correlate with background seismicity rates. The latter and a clear correlation between productivities with mainshock depth indicate that both factors may have predictive value for aftershock forecasting.