F. F. Evison

The precursory earthquake swarm

Abstract Prior swarms near the sites of eleven moderate to large earthquakes from diverse tectonic settings in the New Zealand and California regions yield regressions for the estimation of magnitude and time of occurrence, and thus offer a possible basis for long-range forecasting.

The precursory earthquake swarm in New Zealand: Hypothesis tests

Abstract A series of tests is being carried out with the object of determining whether the relations evident in New Zealand and Japan, between precursory swarms and major earthquakes, are of value for long‐range, synoptic earthquake forecasting. The first New Zealand test (completed in 1990) showed that clustering, both of swarms and of mainshock events, should be allowed for, and the hypothesis was reformulated accordingly. The second New Zealand test, now completed, confirms the importance of clustering. It also reveals that the applicability of the swarm/mainshock relations is strongly affected by large‐scale tectonics. Further, a simulation study shows that, at the end of the test, the performance of the swarm hypothesis relative to the Poisson model lay between what would be expected if the hypothesis and the Poisson model, respectively, were correct in general. These results support a further reformulation of the hypothesis, which is now taken to be strongly applicable in the Hikurangi and Fiordland...

Microearthquakes, geothermal activity, and structure, central North Island, New Zealand

Abstract Microearthquakes are very frequent in the Taupo Fault Belt and very infrequent in the neighbouring Taupo-Reporoa Basin and Kaingaroa Plateau; the distribution of microearthquakes does not correlate with areas of geothermal activity. Larger earthquakes follow much the same distribution pattern. They have similar characteristics to the microearthquakes in other respects, including the prominence of swarm activity in the Taupo Fault Belt. From the seismicity of the region it is inferred that under the Taupo-Reporoa Basin the shear strength is reduced to a very low level by the presence of molten and semi-molten magma extending to within a few kilometres of the ground surface, while under the Taupo Fault Belt there is strong material, interpreted as cooled magma, at about 2–6 km depth.

Earthquakes at intermediate depths in south-west New Zealand

Abstract Earthquake activity in the southern seismic region of New Zealand extends to intermediate depths. This has hitherto been suggested on the evidence of isoseismal patterns and seismogram characteristics, and is now confirmed by computation for the Lake Te Anau earthquake of 20 May 1965, for which 45 readings of P. supported by readings of p′, pP, sP, and PP, indicate a depth of 103 ± 5 km. This result adds to the reasons for classifying the region as an active margin between the New Zealand continental mass and the Tasman Sea.

Time-variable factors in earthquake hazard

Abstract The development of a model for time-variable hazard estimation involves both subjective and objective elements in eight identifiable stages; the final stage—adoption of the model for practical use—is reached only after the conclusion of a successful performance test. At the present state of knowledge such a test consists in measuring the performance of the time-variable model against that of the present static model (i.e., seismic zoning). Precursory phenomena provide a basis for the quantification of the time-variable hazard provided the data are sufficient to allow certain intermediate quantities—the probability distributions for location, magnitude and time of occurrence, the valid alarm rate, and the failure rate—to be evaluated. Similar quantification should also be attainable on the basis of various types of regularity which have been suggested in the occurrence of large earthquakes, but the details of these models have yet to be fully ascertained.

Precursory seismic sequences in New Zealand

Abstract The sequence of precursory swarm and gap has been confirmed in the local seismic activity preceding major earthquakes at Gisborne (1966), Seddon (1966), Inangahua (1968), and Hastings (1973). The magnitude of the main shock appears to be related to that of the largest swarm earthquake as well as to the precursor time. This suggests a method of prediction which might yield long-term estimates of location, magnitude, and time of occurrence.

On the reliability of precursors

Abstract The systematic study of a proposed earthquake precursor involves many choices in accommodating the hypothesis to the data base, as exemplified in the work of Mantovani et al. The reliability of the precursor is determined by testing the hypothesis against a different, usually subsequent, set of data.

Long-range synoptic earthquake forecasting: an aim for the millennium

Abstract The global mapping of seismicity, and the apparent randomness of earthquake occurrence, led to the development of seismic zoning in the 1950s; this interpretation of historical seismicity by means of the stationary Poisson model is a rudimentary, but universally accepted, type of earthquake forecasting. The discovery of plate tectonics in the middle 1960s appeared to support a high degree of regularity in earthquake occurrence, as in the seismic gap hypothesis and the concept of the seismic cycle. While such regularity concepts have had some success as a basis for earthquake forecasting, they have lost support with the subsequent discovery of deterministic chaos, under which the seismogenic medium is in a state of self-organized criticality maintained by plate motion. This opens up, instead, the prospect of long-range synoptic forecasting based on precursory phenomena, as in meteorology. The onset of the long-term generation process of a large event is unpredictable in meteorology because of the ‘butterfly effect’, and in seismology because of the analogous ‘microtremor effect’. But the culmination, in a tropical cyclone or the analogous large earthquake, can be predicted by means of precursors which are part of the generation process.

LONG-TERM SEISMOGENIC PROCESS FOR MAJOR EARTHQUAKES IN SUBDUCTION ZONES

Abstract A qualitative physical process for the long-term seismogenesis of major earthquakes in subduction zones is proposed on the basis of quantitative empirical evidence that swarms, mainshocks and aftershocks are closely related phenomena. The relations, which have been identified in the comprehensive, long-term catalogues of New Zealand and Japan, represent swarms as predictors of mainshocks with respect to location, time and magnitude. Clustering of swarms and of mainshock/aftershock events is allowed for. With a database of 15 sequences of swarms, mainshocks and aftershocks, tests are being conducted with the object of refining the relations and evaluating them as a possible means of practical synoptic forecasting. Three sequences have culminated in major earthquakes since the tests began, and the systematic study now relates a total of 36 swarms with 29 mainshock/aftershock events. These empirical results strengthen and quantify the connection between swarms and major earthquakes, which several authors have demonstrated by means of numerical/physical modelling. The proposed seismogenic process includes swarms, mainshocks and aftershocks as separate event stages which are related by predictability. Interevent conditions are specified according to the Mogi criteria for the medium; cracks at which fractures subsequently occur constitute nonuniformity in the Mogi sense, and post-earthquake healing restores uniformity. Where the Gutenberg–Richter relation occurs, it is accepted as possible evidence of deterministic chaos and unpredictability; as a corollary, the process is noncyclical. The principle of scaling is held to apply except when modified by large-scale boundaries in the medium. Subduction zones and some other localities where water is abundant are indicated by the main empirical studies as favourable to the occurrence of swarms. Fluid overpressuring is therefore proposed as a mechanism for the self-triggering of swarms, and this is supported by additional examples of the predictive relations occurring in conditions of high fluid pressure, including the vicinity of large man-made reservoirs. The process can be tested by systematic studies in other subduction regions, given adequate catalogues for quantifying the algorithm. It also has implications for other tectonic environments, with swarms replaced by cognate, more protracted seismicity precursors.

The precursory earthquake swarm in Japan: hypothesis test

A formal performance test of the hypothesis that swarms are long-term precursors to major shallow earthquakes is in progress in the region of Pacific-plate subduction in Japan. The likelihood of the major earthquakes (JMA magnitude ≥6.8) that occurred in the region during the test period (1983–1998) was 5.04 times higher under the swarm hypothesis than under the stationary Poisson model; this result is inconclusive in terms of the proposed acceptance level of 20. The earthquakes were the Hokkaido-Toho-Oki earthquake (M8.1) of 4 October 1994, and the Sanriku-Haruka-Oki earthquake (M7.5) of 28 December 1994. The significance of the performance has been evaluated by a Monte Carlo study of the results since mid-1991. This shows that the Poisson model can be rejected at the 1% level, and that the results are consistent with the swarm hypothesis. The test is continuing.