Foreshock Occurrence Research Articles

On the space-time pattern formation of the earthquake strain field

The dynamical strain field of a lithospheric plate is described by a simple mathematical model of two factors which denote the visco-elasticity and plasticity of the media. This model consists of two cooperative equations of diffusion and advection type which denote the dislocation flow along the fault and the visco-elastic behaviour of the plate, respectively. This is a generalization of the models previously proposed for describing the stress and strain field of the plate. Model study shows that the local concentration of the elastic strain in certain regions is caused as a result of “diffusive instability” and that space and time patterns of the strain field are created by cooperative interactions of these two effects. Numerical simulations also show that creep motions or crustal movements can be accompanied with concentration of the elastic strain, which is consistent with the field observation. The predicted pattern of the strain field is compared with the actual seismicity prior to large earthquakes, and the generation of the seismically quiet and active zones and the occurrence of foreshocks are explained.

Foreshocks (1966-1980) in the San Andreas system, California

The spatial and temporal distributions of seismicity preceding moderate (ML ≧ 5.0) main shocks in the San Andreas fault system in California have been analyzed to recognize and characterize the patterns of foreshock occurrence. Of 20 main shocks in the San Andreas system, 7, or 35 per cent, have been preceded by immediate foreshock sequences that included events within 1 day and 5 km of the main shocks. A possible correlation of the rate of foreshock occurrence with type of faulting was found such that none of the four main shocks with reverse faulting had foreshocks while 44 per cent of the strike-slip earthquakes had foreshocks. Some enhanced seismic activity was also observed at relatively large distances from the main shock (13 to 30 km) 1 to 5 days before 40 per cent of the main shocks but this activity cannot be clearly distinguished from the background seismicity. Of the seven immediate foreshock sequences, only two had the swarm-like appearance of the class II foreshocks defined by Mogi. The other foreshock sequences appear to be single events (sometimes with their own aftershocks) preceding the respective main shocks. Four of these sequences are spatially correlated with distinct physical discontinuities in their faults between the hypocenters of the foreshock and main shock, and similar discontinuities may also be associated with the other sequences. The durations of the foreshock sequences were found to decrease as the depths of the main shocks increase from 3 to 11 km, which has been interpreted as a dependence on stress. To account for this stress dependence of the duration and the presence of discontinuities, a model for foreshocks occurrence is presented. This model proposes that foreshocks may represent a process of delayed multiple rupture and that the delay between occurrence of foreshock and main shock might represent the time needed for static fatigue to break the stronger rock at the discontinuity in the fault.

A test of foreshock occurrence in the central Aleutian Island Arc

AbstractA statistical method for estimating the occurrence of foreshocks to larger earthquakes using local seismicity data is applied to the catalog of earthquakes from a local seismograph network near Adak, Alaska. Foreshocks are defined in a restricted sense as earthquakes preceding larger earthquakes within a critical time calculated from the mean time between successive earthquakes in the volume surrounding the target event. The threshold of completeness of the earthquake catalog is estimated as duration magnitude Md2.2. A low-magnitude cutoff on the catalog of Md2.5 was used to minimize potential bias from variations in weather and instrumentation condition. About 6 to 10 per cent of 51 main shocks of mb ≧ 4.5 in the Adak thrust zone were preceded by detectable earthquakes satisfying this statistical definition for foreshocks. As many as 14 percent of mb ≧ 5.0 earthquakes may have been preceded by detectable earthquakes satisfying this definition. An estimate of foreshock occurrence using as background the seismicity rate in the 4 months preceding larger earthquakes indicates a slightly higher rate of occurrence. The volume around the target events which showed foreshock activity is used to infer the size of a zone of preparation for larger earthquakes in the Adak area. The data are consistent with a zone of preparation whose size is a linear function of target-event magnitude. The size of the zone of preparation ranges from 20 to 45 km in radius for body-wave magnitudes 4.0 to 5.7.

Seismicity parameters preceding moderate to major earthquakes

Seismic events reported in the bulletins of the two large arrays, LASA and NORSAR, were merged with those from the NEIS bulletin for the period 1970–1977. Using a lower cutoff of mb = 5.8, 510 ‘main shocks’ within the P range of LASA or NORSAR were selected for this period; and various seismicity trends prior to them were investigated. A search for definite foreshocks, based on a significantly short time delay to the main shock, revealed that the true rate of foreshock occurrence was less than 20%. Foreshocks are almost exclusively associated with shallow (h < 100 km) main shocks. To establish common features, a method of averaging seismicity from many regions was used to suppress the randomness of the seismic behavior of each region. This averaging shows that the seismicity level around the main shock increases somewhat for 10 days before main shocks; this feature peaks in the last 3–4 hours prior to the main shocks. The averaging also reveals that the mean magnitude of events near the main shock increases prior to main shocks but only by a few hundredths of a magnitude unit. Again by averaging, the seismicity about main shocks is shown to tend with time toward the main shock as its origin time is approached, but the average effect is small (∼10% change). By expanding or contracting each region's time scale before averaging to relate to the magnitude of the main shock, these features are enhanced. Using a new variable to track the departures from both spatial and temporal randomness, the Poisson‐like behavior of deeper seismicity (>100 km) was demonstrated. For shallow events (<100 km) this variable reveals numerous instances of clustering and spatial‐temporal seismic gaps, with little tendency toward a uniformity of behavior prior to main shocks. A statistical test of the validity of seismic precursors was performed for approximately 90 main shock regions which had sufficient seismicity. Using a five‐variable vector (interevent time, interevent distance, magnitude, epicentral distance to main shock, and depth difference relative to main shock) for each event in a ‘precursory’ time window of 500 days before the main shock and for each event in a ‘normal’ time window of 500 days before that, the null hypothesis of equal vector means between the two groups was tested. At 90% confidence level, less than 30% of the main shock regions were thus found to exhibit precursory seismicity changes.Appendices are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐007; $1.00. Payment must accompany order.

Stochastic synthesis of earthquake catalogs

A model of earthquake occurrence is proposed that is based on results of statistical studies of earthquake catalogs. We assume that each earthquake generates additional a probability that depends on time as t−(1+θ). This assumption together with one regarding the independence of branching events on adjacent branches of the event ‘tree,’ is sufficient to permit the generation of complete catalogs of earthquakes that have the same time‐magnitude statistical properties as real earthquake catalogs. If θ is about 0.5, the process generates sequences that have statistical properties similar to those for shallow earthquakes: many well‐known relations are reproduced including the magnitude‐frequency law, Omori's law of the rate of aftershock and foreshock occurrence, the duration of a recorded seismic event versus its magnitude, the self‐similarity or lack of scale of rate of earthquake occurrence in different magnitude ranges, etc. A value of θ closer to 0.8 or 0.9 seems to simulate the statistical properties of intermediate and deep shocks. A formula for seismic risk prediction is proposed,and the implications of the model for risk evaluation are outlined. The possibilities of the determination of long‐term risk from real or synthetic catalogs that have the property of self‐similarity are dim.

Some characteristics of foreshocks and their possible relationship to earthquake prediction and premonitory slip on faults

Foreshocks occur before a large fraction of the world's major (M ≥ 7.0) earthquakes. Teleseismically located events before major earthquakes from 1914 to 1973 were considered together to examine possible average temporal and spatial patterns of foreshock occurrence. Several days before the main shocks and apparently near the epicenters of them (Δ ≲ 30 km) the activity begins to increase, culminating in a final rapid acceleration of activity in the last day. The acceleration continues up to the time of the main shocks, except for a possible temporary decrease about 6 hours before them. The seismicity increases approximately as the inverse of time before main shock. This relationship is essentially unrelated to the magnitude of the main shock. The magnitude of the largest foreshock is also unrelated to the magnitude of the main shock. In addition, pairs of major events are common. Ten percent of the world's major events are preceded by other major events within 100 km and 3 months. For foreshocks within each of three sequences studied, the ratio of the amplitudes of the P and S waves were approximately the same, suggesting that the faulting mechanisms are the same for events in each sequence. By assuming an inhomogeneous fault plane on which asperities fail by static fatigue, we derived an equation for accelerating premonitory slip as a function of time, which agrees with the observed time dependence of foreshocks.

Foreshock Occurrence in Central California

The hypocentral regions of 29 (M ≥ 3.5) earthquakes along the San Andreas fault system in central California have been systematically examined for increased microearthquake activity preceding the main shock. Of 20 shallow (h > 9 km) main events, 10 were preceded by an apparent buildup of seismic activity. None of 9 deeper (h < 9 km) main shocks was preceded by increased activity. The early buildup of seismicity typically began days to weeks before the main shock, was followed first by a period of quiet, then by immediate foreshocks within 24 hours of the main event. Three main shocks with no significant early buildup in the level of seismicity were preceded within 24 hours by single events that, in retrospect, might also be considered foreshocks. Few of the events on the San Andreas fault system examined here were preceded by well-defined foreshock sequences like that associated with the 1975 Oroville earthquake.

Precursory seismic observations in Himachal Pradesh S.K. 1976 and Shillong plateau

Precursory changes in P-wave residuals were noticed before the earthquakes on 1 June 1969 (M=5.0) and on 15 May 1974 (M=4.8) near Shillong and another near Nurpur (Himachal Pradesh) which occurred on 23 January 1969 (M=4). For these earthquakes, decrease in P-wave velocity was observed one to two months prior to their occurrence.&#x0D; Six monthly b values in Gutenberg Richter's frequency magnitude relationship showed a decrease in the value about 15 days and 3½ months before two other earthquakes of October 1969 and January 1970 in Himachal Pradesh. The decrease in b value for foreshocks as compared to the aftershocks was also noted for another earthquake on 5 November 1968.A well marked seismicity gap has been demarcated in Himachal Pradesh on the basis of epicentral distribution obtained with a close river valley project network of observatories. This could be Indicative of a possible damaging earthquake in the region.&#x0D; The foreshock occurrence in the Himalayan region was investigated on the basis of data upto 1975. Although no relationship between the time of first foreshock and the main shock could be established, the results show that there is a time lag of about 10 hours between the observed foreshock and the main shock on several occasions. However, foreshocks could be detected much earlier in Himachal Pradesh where a close network of seismological observatories is being maintained.&#x0D;

Numerical simulation of earthquake sequences

abstract Numerical simulation of earthquake occurrence using a one-dimensional fault model demonstrates that (a) the linear behavior of the magnitude-frequency relation is not an immutable law but rather is dependent on the mechanical properties of the fault, (b) “randomness” as measured by adherence to Poissonian statistics does not preclude useful prediction by statistical means, (c) the rate of occurrence of simulated earthquakes is in good agreement with the Kolmogorov model in which seismicity is related primarily to the stored elastic energy in a fault system, and (d) the occurrence of foreshocks and aftershocks can be well explained by the occurrence of stress-induced crack nucleation.

The time distribution of the reservoir-associated foreshocks and its importance to the prediction of the principal shock

Abstract The time distribution in the occurrence of foreshocks of the Kremasta, Koyna, and Kariba large earthquakes, which are believed to be associated with artificial lakes, has been studied. The relation, n(t)dt = no(τ−t)−hdt, fits the data well. The values of the parameter h are between 1.5 and 2.6, and the durations of the foreshock sequences are of the order of several months. The possibility of using this relation to predict the time of occurrence of the principal earthquakes associated with artificial lakes is examined, and the importance of systematic investigation of the time distribution of foreshocks for controlling the earthquake risk is noted.