Properties Of Aftershock Sequences Research Articles

Temporal properties of aftershock sequences of large earthquakes in Iran - Analysis of primary and secondary aftershocks of the Ezgeleh sequence

In this study, the decay of earthquake aftershock sequences of some major earthquakes in different tectonic regimes in the Iranian plateau is discussed. The studied earthquakes are Rigan [2010], Ahar-Varzaghan [2012], Goharan [2013], Sefidsang [2017] and Ezgeleh [2017]. The spatial and temporal windows are considered based on the method proposed by Gardner and Knopoff [1974] to compute decay parameters for each sequence. The decay rates of sequences were compared to well-known models to find the best fit for each sequence. The results showed that the modified Omori is the best fit for Ahar-Varzaghan and Ezgeleh sequences, for Rigan and Sefidsang sequences the modified Omori and the Kisslinger ones found as the best fits. The values of the p parameter of the Reasenberg and Shcherbakov models were larger compared to the Omori model, but the parameter of the Kisslinger model was slightly smaller compared to the Omori one. The c parameter showed an inverse relation to the threshold magnitude. The correlation between the p and c parameters and also the and the Gutenberg and Richter (G-R) parameters were investigated. In addition, we made use of a graphical method to analyze the seismic sequence of the Ezgeleh earthquake during 13 months after the main event. The graphical method was successful to estimate the occurrence of an event with an approximate magnitude of M=6.4 in the sequence.

Bayesian estimation of the Modified Omori Law parameters for the Iranian Plateau

The forecasting of large aftershocks is a preliminary and critical step in seismic hazard analysis and seismic risk management. From a statistical point of view, it relies entirely on the estimation of the properties of aftershock sequences using a set of laws with well-defined parameters. Since the frequentist and Bayesian approaches are common tools to assess these parameter values, we compare the two approaches for the Modified Omori Law and a selection of mainshock–aftershock sequences in the Iranian Plateau. There is a general agreement between the two methods, but the Bayesian appears to be more efficient as the number of recorded aftershocks decreases. Taking into account temporal variations of the b-value, the slope of the frequency-size distribution, the probability for the occurrence of strong aftershock, or larger main shock has been calculated in a finite time window using the parameters of the Modified Omori Law observed in the Iranian Plateau.

Seismotectonics of intermediate-depth earthquakes from properties of aftershock sequences

Aftershock sequences following two intermediate-depth earthquakes have been analyzed to determine their spatial and temporal distributions. The Kanaga Pass earthquake ( M b = 6.0) occurred at 105 km depth in the central Aleutian Islands. Data were provided by the Central Aleutians Seismic Network. The Iwaizumi Town earthquake ( m b = 6.4) occurred at 75 km depth, under northern Honshu, Japan. Data were provided by the network of Tohoku University. For both events the hypocenter distribution of the aftershocks agreed well with focal mechanism solutions derived by others, with down-dip compression for both. Both events were followed by numerous aftershocks that decayed with time initially according to the modified Omori relation, with p-value close to 1. In both cases, activity at a higher rate than that expected from the Omori relation began after several months, without a strong aftershock to trigger it. The Kanaga Pass activity is modeled by two Omori sequences, the second beginning 64 days after the mainshock. The p-values are 0.92 and 1.04, respectively. The corresponding b-values are 1.01 and 1.03. The Iwaizumi Town sequence is modeled by a single Omori sequence, p = 0.96, plus a constant rate of activity, 0.25 events/day, for 63 days, beginning 120.5 days after the mainshock. The b-value of the main sequence was 0.91 ± 0.14, up to the start of the linear surge, for which the b-value was 1.23 ± 0.47. No strong aftershock or event in the vicinity of the aftershock zone occurred to trigger the secondary activity, in either case. The cause of the secondary activity is not known. The two sequences occurred in subduction zones with similar properties, and in both cases previous seismicity shows a deficiency of strong earthquakes in the main thrust zone above the aftershock source volumes. A strong creep event, or silent earthquake, in a weakly-coupled interplate contact zone is a plausible explanation for the secondary sequences. The unusual setting of the Iwaizumi event, at the bottom of the upper plane of the double-planed Wadati-Benioff zone under Honshu, may play a role in that case.

Properties of aftershock sequences in southern California

The temporal behavior of 39 aftershock sequences in southern California, 1933–1988, was modeled by the modified Omori relation. Minimum magnitudes for completeness of each sequence catalog were determined, and the maximum likelihood estimates of the parameters K, p, and c, with the standard errors on each, were determined by the Ogata algorithm. The b value of each sequence was also calculated. Many of the active faults in the region, both strike slip and thrust, were sampled. The p values were graded in terms of the size of the standard error relative to the p value itself. Most of the sequences were modeled well by the Omori relation. Many of the sequences had p values close to the mean of the whole data set, 1.11±0.25, but values significantly different from the mean, as low as 0.7 and as high as 1.8, exist. No correlation of p with either the b value of the sequence or the mainshock magnitude was found. The results suggest a direct correlation of p values is with surface heat flow, with high values in the Salton Trough (high heat flow) and one low value in the San Bernardino Mountains and on the edge of the Ventura Basin (both low heat flow). The large fraction of the sequences with p values near the mean are at locations where the heat flow is near the regional mean, 74 mW/m2. If the hypothesis that aftershock decay rate is controlled by temperature at depth is valid, the effects of other factors such as heterogeneity of the fault zone properties are superimposed on the background rate determined by temperature. Surface heat flow is taken as an indicator of crustal temperature at hypocentral depths, but the effects on heat flow of convective heat transport and variations in near‐surface thermal conductivity invalidate any simple association of local variations in heat flow with details of the subsurface temperature distribution. The interpretation is that higher temperatures in the aftershock source volume caused shortened stress relaxation times in the fault zone materials, leading to a faster decay rate (higher p value). In 1967, Mogi proposed such a relation on the basis of Japanese data. Seismic velocity distributions published for southern California generally support the hypothesis, with low velocities (higher temperatures) corresponding to high p values, and vice‐versa.

Properties of aftershock sequences

Following the Inangahua earthquakes of May, 1968, the Seismological Observatory received many requests for information about aftershocks, and it became apparent that even among engineers and scientists in other fields there was considerable lack of knowledge of what to expect during an aftershock sequence. The following notes were therefore prepared, covering some of the points about which enquiries are most often received.