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Abstract

Resonant seismogenic and systemic-cognitive prediction of seismicity The problem of predicting the place and the time of origin of the natural disasters and large earthquakes on the Earth is global for the whole world and can contribute to the preservation of many lives. The authors of the monograph present the theory of resonant seismogenesis and the author's method of automated system-cognitive forecasting of earthquakes. Physical models of mantle convection and information models of the influence of planetary gravity complexes on the formation of seismic events are considered as well. The article also gives a description of open scalable interactive intellectual on-line environment for teaching and research based on ASC - analysis and "Eidos" system. The solution of the problem of earthquake forecasting proposed by the authors of the monograph is based both on the real causes of earthquake formation under the influence of the gravitational fields of the planets, the Sun and the Moon, and on the use of a large statistical array of data of earthquakes. It is intended for seismologists, artificial intelligence specialists and anyone interested in this issue. It is of interest to scientists involved in earthquake forecasting and is recommended for publication.
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We observed a clear tidal triggering effect on earthquakes closely related to the occurrence of the 1982 South Tonga earthquake (Mw 7.5) by precisely measuring the correlation between the earth tide and earthquake occurrence using 385 shallow earthquakes of reverse-fault type in the Tonga-Kermadec subduction zone. The result of statistical analysis indicates that the tidal effect, having been predominant for the normal stress on the fault plane, concentrated in and near the future focal area for several years preceding the South Tonga earthquake. The high correlation disappeared after the earthquake. The frequency distribution of phase angles for the pre-event period exhibited a peak at the phase angle where the tidal normal stress is at its maximum to accelerate the fault slip, which indicates that the observed high correlation is not a statistical chance but physically reasonable.
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We observe tidal triggering of earthquakes by measuring the correlation between the Earth tide and earthquake occurrence. We used the times, locations, and focal mechanisms of the 9350 globally distributed earthquakes with magnitude 5.5 or larger from the Harvard centroid moment tensor catalog. The tidal stress was theoretically computed by using the Preliminary Reference Earth Model and a recent ocean tide model, NAO.99b. We considered the shear stress on the fault plane and the trace of stress tensor, J1. Defining the tidal phase angle at the occurrence time for each earthquake, we statistically tested the phase selectivity using the Schuster's method. For all the earthquakes, no significant correlation is found between the Earth tide and earthquake occurrence both for the shear stress and for J1. By classifying the data set according to fault types, however, we find a high correlation with the shear stress for reverse fault type. The correlation is particularly clear for shallow and smaller earthquakes of this type. Significant correlation with J1 also appears for larger earthquakes of reverse fault type and for shallow and larger ones of normal fault type. We find no correlation for strike-slip type. For all the cases of high correlation, earthquakes tend to occur when the tidal stress accelerates the fault slip, indicating that high correlation is not coincidental but is physically justified. This result strongly suggests that a small stress change due to the Earth tide encourages earthquake occurrence when the stress in the future focal area is near a critical condition.
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We provide an explanation why earthquake occurrence does not correlate well with the daily solid Earth tides. The explanation is derived from analysis of laboratory experiments in which faults are loaded to quasiperiodic failure by the combined action of a constant stressing rate, intended to simulate tectonic loading, and a small sinusoidal stress, analogous to the Earth tides. Event populations whose failure times correlate with the oscillating stress show two modes of response; the response mode depends on the stressing frequency. Correlation that is consistent with stress threshold failure models, e.g., Coulomb failure, results when the period of stress oscillation exceeds a characteristic time tn; the degree of correlation between failure time and the phase of the driving stress depends on the amplitude and frequency of the stress oscillation and on the stressing rate. When the period of the oscillating stress is less than tn, the correlation is not consistent with threshold failure models, and much higher stress amplitudes are required to induce detectable correlation with the oscillating stress. The physical interpretation of tn is the duration of failure nucleation. Behavior at the higher frequencies is consistent with a second-order dependence of the fault strength on sliding rate which determines the duration of nucleation and damps the response to stress change at frequencies greater than 1/tn. Simple extrapolation of these results to the Earth suggests a very weak correlation of earthquakes with the daily Earth tides, one that would require >13,000 earthquakes to detect. On the basis of our experiments and analysis, the absence of definitive daily triggering of earthquakes by the Earth tides requires that for earthquakes, tn exceeds the daily tidal period. The experiments suggest that the minimum typical duration of earthquake nucleation on the San Andreas fault system is ∼1 year.
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Since earth tides represent the largest short-period oscillatory strains in the earth, a test has been made to see if any correlation exists between the times of occurrence of earthquakes in Southern California and the tidal potential. Two tests have been made, one of them a cross-correlation. On either basis, a statistically significant sample of earthquake events gives a correlation with the tidal potential that is of the same magnitude as a random sample.
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In this paper we test the southern California network earthquake catalog and the world-wide earthquake catalog for a fortnightly tidal periodicity and find none. In studies that test the hypothesis of tidal triggering of earthquakes, it is usually necessary to resolve the tidal shear stress onto the plane of the fault and in the direction of the slip vector to determine if the stress is compatible with the fault motion (Heaton, 1975, 1982). This analysis requires accurate focal mechanisms and knowledge of the focal plane on which slip occurred for as many earthquakes as possible to improve the statistical sampling of the data set. However, if we consider the fortnightly tide, which is a simple amplitude modulation, focal mechanism information is not required. Our analysis assumes that we have a large data base of earthquakes with varying mechanisms. The alignment of the slip vector with the tidal shear stress will vary from earthquake to earthquake. However, if tidal triggering of earthquakes does occur, more earthquakes should be triggered out of the total population when the tidal stress is large than when it is small. If the largest semi-diurnal tide is normalized to an amplitude of 1.0, the fortnightly tide would have an amplitude of 0.17 on this scale and a period of about 2 weeks (Munk and MacDonald, 1960). Therefore, the effect on the occurrence of earthquakes by the tidal shear stress will be modulated with a 2 week period. Considering the fortnightly tide allows us to utilize entire catalogs of data to test the tidal triggering hypothesis without knowledge of the focal mechanisms.
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We have conducted a series of laboratory simulations of earthquakes using granite cylinders containing precut bare fault surfaces at 50 MPa confining pressure. Axial shortening rates between 10−4and 10−6 mm/s were imposed to simulate tectonic loading. Average loading rate was then modulated by the addition of a small-amplitude sine wave to simulate periodic loading due to Earth tides or other sources. The period of the modulating signal ranged from 10 to 10,000 s. For each combination of amplitude and period of the modulating signal, multiple stick-slip events were recorded to determine the degree of correlation between the timing of simulated earthquakes and the imposed periodic loading function. Over the range of parameters studied, the degree of correlation of earthquakes was most sensitive to the amplitude of the periodic loading, with weaker dependence on the period of oscillations and the average loading rate. Accelerating premonitory slip was observed in these experiments and is a controlling factor in determining the conditions under which correlated events occur. In fact, some form of delayed failure is necessary to produce the observed correlations between simulated earthquake timing and characteristics of the periodic loading function. The transition from strongly correlated to weakly correlated model earthquake populations occurred when the amplitude of the periodic loading was approximately 0.05 to 0.1 MPa shear stress (0.03 to 0.06 MPa Coulomb failure function). Lower-amplitude oscillations produced progressively lower correlation levels. Correlations between static stress increases and earthquake aftershocks are found to degrade at similar stress levels. Typical stress variations due to Earth tides are only 0.001 to 0.004 MPa, so that the lack of correlation between Earth tides and earthquakes is also consistent with our findings. A simple extrapolation of our results suggests that approximately 1% of midcrustal earthquakes should be correlated with Earth tides. Triggered seismicity has been reported resulting from the passage of surface waves excited by the Landers earthquake. These transient waves had measured amplitudes in excess of 0.1 MPa at frequencies of 0.05 to 0.2 Hz in regions of notable seismicity increase. Similar stress oscillations in our laboratory experiments produced strongly correlated stick-slip events. We suggest that seemingly inconsistent natural observations of triggered seismicity and absence of tidal triggering indicate that failure is amplitude and frequency dependent. This is the expected result if, as in our laboratory experiments, the rheology of the Earth's crust permits delayed failure.
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ABSTRACT Tidal effects on seafloor microearthquakes have been postu- lated, but the search has been hindered by a lack of continuous long-term data sets. Making this observation is further complicated by the need to distinguish between Earth and ocean tidal influences on the seafloor. In the summer of 1994, a small ocean-bottom seis- mograph array located 402 microseismic events, over a period of two months, on the summit caldera of Axial volcano on the Juan de Fuca Ridge. Harmonic tremor was also observed on all instru- ments, and Earth and ocean tides were recorded on tiltmeters in- stalled within the seismometer packages. Microearthquakes show a strong correlation with tidal lows, suggesting that faulting is oc- curring preferentially when,ocean loading is at a minimum. The
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Tidal forces cause crustal stress changes that might be expected to trigger earthquakes but previous analyses searching for tidal periodicities in earthquake catalogs have yielded generally negative or inconclusive results. This study examines the Caltech/USGS southern California earthquake catalog from 1932 to 2003, which includes 429,886 events. We examine the data for any correlation between event occurrence and lunar phase by plotting histograms of event count versus phase as well as simple plots of event phase versus time. We analyze both the complete catalog and specified subsets of the data, including grouping by magnitude and depth. We also refine our data spatially by binning events in individual 0.5 by 0.5 degree latitude/longitude bins. Finally, we examine over 900 individual similar event clusters containing 50 or more events as defined by cross-correlation analysis. These clusters typically are less than 3 km across and include earthquakes that likely to have similar focal mechanisms and sensitivity to tidal phase. However, none of our results find a significant correlation between tidal forces and seismicity, suggesting that tidal forces do not contribute significantly to earthquake occurrence times anywhere in southern California.