Number Of Foreshocks Research Articles

Incorporating Foreshocks in an Epidemic-like Description of Seismic Occurrence in Italy

The Epidemic Type Aftershock Sequence (ETAS) model is a widely used tool for cluster analysis and forecasting, owing to its ability to accurately predict aftershock occurrences. However, its capacity to explain the increase in seismic activity prior to large earthquakes—known as foreshocks—has been called into question due to inconsistencies between simulated and experimental catalogs. To address this issue, we introduce a generalization of the ETAS model, called the Epidemic Type Aftershock Foreshock Sequence (ETAFS) model. This model has been shown to accurately describe seismicity in Southern California. In this study, we demonstrate that the ETAFS model is also effective in the Italian catalog, providing good agreement with the instrumental Italian catalogue (ISIDE) in terms of not only the number of aftershocks, but also the number of foreshocks—where the ETAS model fails. These findings suggest that foreshocks cannot be solely explained by cascades of triggered events, but can be reasonably considered as precursory phenomena reflecting the nucleation process of the main event.

Estimation of Co-Seismic Land Deformation Due to Mw 7.3 2017 Earthquake in Iran (12 Nov 2017) using Sentinel-1 DInSAR

A strong shaking with Mw 7.3 occurred on 12th November 2017 around the Sarpol-e Zahab town in the border area between Iran and Iraq. It has a number of foreshocks and aftershocks increasing the total deformation, cumulatively. In this study, we have investigated how earth surface deformed after such a strong earthquake and its scatters. Because, the deformation inspection are indispensable for the safety of citizens and infrastructures. The best way for monitoring of surface deformation in such a big event is the SAR technique. This system can work effectively during night and day under different weather conditions. The Interferometric SAR (InSAR) allows accurate measurements of surface deformation in mm resolution. There are several methods for the application of SAR techniques and one of them is Differential InSAR (DInSAR) indicating an uplift and subsidence around epicentral area precisely. We preferred to use it for sensitive vertical displacement in the target area. The seismological data from the observatory centers indicate that the recent earthquake sourced from the NW-SE trending, northeast dipping High Zagros Reverse Fault Zone. According to the results, epicentral area has been exposed a vertical displacement with 90 cm uplift and -41 cm subsidence in the northeastern and southwestern block of the fault, respectively.

Analysis of Foreshock Sequences in California and Implications for Earthquake Triggering

We analyze foreshock activity in California and compare observations with simulated catalogs based on a branching aftershock-triggering model. We first examine foreshock occurrence patterns for isolated \(M \ge 5\) earthquakes in southern California from 1981 to 2011 and in northern California from 1984 to 2009. Among the 64 \(M \ge 5\) mainshocks, excluding 3 swarms and 3 doubles, 53 % of the rest are preceded by at least one foreshock within 30 days and 5 km. Foreshock occurrence appears correlated with mainshock faulting type and depth. Foreshock area is correlated with the magnitude of the largest foreshock and the number of foreshocks, however, it is not correlated with mainshock magnitude. We then examine the occurrence pattern of all seismicity clusters without a minimum magnitude requirement, and the possibility that they are “foreshocks” of larger mainshocks. Only about 30 % of the small clusters lead to a larger cluster. About 66 % of the larger clusters have foreshock activities, and the spatial distribution pattern is similar to \(M \ge 5\) mainshocks, with lower occurrence rates in the Transverse Range and central California and higher occurrence rates in the Eastern California Shear Zone and the Bay Area. These results suggest that foreshock occurrence is largely controlled by the regional tectonic stress field and fault zone properties. In special cases, foreshock occurrence may be useful for short-term forecasting; however, foreshock properties are not reliably predictive of the magnitude of the eventual “mainshock”. Comparison with simulated catalogs suggest that the “swarmy” features and foreshock occurrence rate in the observed catalogs are not well reproduced from common statistical models of earthquake triggering.

Detailed Distribution of Accelerating Foreshocks before a M 5.1 Earthquake in Japan

—The M 5.1 event (May 23, 1993) which occurred in one of the most active swarm areas of Japan was preceded by foreshock activity. We obtained precise hypocenters of the foreshock-mainshock-aftershock sequences with a temporary seismic network installed just above the source region twenty days before the mainshock. The foreshocks are very unique in their accelerating activity; the acceleration in the number of foreshocks enabled us to estimate the time of the mainshock with time-to-failure analysis proposed by . Although substantial snow remained in the swarm area, we quickly installed the network because the time-to-failure analysis disclosed that the mainshock was impending. The temporary network provided detailed information on both the temporal and spatial distribution of the foreshock-mainshock-aftershock sequences. Foreshocks started fifty days before the mainshock and were distributed linearly at the base of the seismogenic layer with a length of 5 km and horizontal and vertical widths of about 1 km. The temporal change of the number of foreshocks is approximated by a power law, and the time of the mainshock can be estimated by extrapolating plots of the inverse of the daily number of events. An area of seismic quiescence appeared 40 hours before the mainshock and propagated with a rate of 20 m/hour. The mainshock occurred 2 km westward from the primary foreshock area. It was located at the base of the aftershock region. This process can be interpreted as source nucleation; preslip on the fault prior to the mainshock.

Statistical features of foreshocks in comparison with other earthquake clusters

SUMMARY When earthquakes occur in a cluster, the probability that they will be foreshocks of a forthcoming significantly larger earthquake appears dependent on the magnitude differences, origin-time spans, and distances between several of the earliest events in a cluster. The earthquake catalogue of the Japan Meteorological Agency (JMA; 1926-91, MJ 2 4) is decomposed into numbers of clusters in time and space to compare statistical features of foreshocks with those of swarms and main shockaftershock sequences. Since the number of foreshocks in individual earthquake clusters is usually small, two types of data stacking are considered. First, we consider the spatial and temporal distribution of events relative to the location and time of the coming main shock. For the foreshocks, a relative doughnut-shaped pattern which converges in time to the main shock’s epicentre is seen, while this pattern is weak for swarms. Next, we consider the temporal, spatial and magnitude distributions of events relative to the time, location and magnitude of earlier events in every sequence. Then, a statistical discrimination of foreshocks from earthquakes of other types of clusters is explored. The search for significant trends of relative frequency of foreshocks in comparison with other cluster types revealed the following features of foreshocks: (1) their relative frequency is high when the inter-event time span is less than several days, but the maximum is attained at about several hours span; (2) their relative frequency is high at short distances between epicentres, especially within 10 km, and then decreases as the distance gets larger; and (3) their relative frequency increases as the magnitude increases among the earliest shocks in a cluster. The stability of the results is confirmed by finding similar trends using different cluster identification methods and different threshold magnitudes. Further, microearthquakes in the Japan University Network data file (1983-87; M 2 2) and moderate-to-large earthquakes in the International Seismological Centre world catalogue (ISC; 1979-90, Mh 2 5) are examined to see some effects of the magnitude-thresholds of these catalogues. Similar features to those obtained in the JMA catalogue are also found for these catalogues.

Seismicity associated with icebergs calving from glaciers near Mawson, East Antarctica

Icebergs produced from the termini of non‐floating or partly floating outlet glaciers at the seaward margin of the Antarctic ice sheet near Mawson (67°36'S, 62°53'E) were photographed and measured. Seismic records at Mawson are interpreted in terms of catastrophic calving of these icebergs from ice cliffs into the sea to produce low frequency seismic signals of varying amplitude and duration. It is shown how parameters of foreshocks to these ice falls could have been used to predict the approximate time and location of one of the falls. These parameters included the directions of first motions on the Mawson seismographs, the S‐P times, as well as the rate of change of smoothed event frequency data. The frequency and amplitude distributions of the foreshocks can be represented as n(t) = 180(24‐t)‐1.65 and N(A) = 225A‐1.85 where n(t) is the number of foreshocks per day, t days after the foreshocks began and N(A) is the number of events whose amplitudes were in the range A to (A+2) mm on the Mawson seismogra...