Seismic quiescence before the M 7, 1988, Spitak earthquake, Armenia

A detailed analysis of the 35  yr of seismicity between 1962 and 1997 using a gridding technique shows that the M 7, Spitak earthquake of 1988 December 7 was preceded by a quiescence anomaly that started at approximately 1984±0.5, and lasted about 5±0.5  yr, up to the main shock. This quiescence anomaly had a radius of about 20±3  km, estimated from circular areas with 75 per cent rate decrease, centred at the point of maximum significance of the anomaly. The quiescence was clearly present in the aftershock volume during the 5  yr before the 1988 main shock, but its statistically strongest expression was located 30  km NW of the epicentre. This anomaly fulfills the association rules between precursory quiescence anomalies and main shocks, even for a tight definition, and is therefore proposed as a case of precursory quiescence. The largest value of the standard deviate Z , found by random selection of samples by gridding, was Z =14 for a time window of T _w=3  yr, using a sample size of N =300 events. This makes this anomaly the strongest observed so far, and it is the first documented in an environment of continental collision. There are no false alarms exceeding in significance the precursor. The Armenian earthquake catalogue used for this study had 4600 earthquakes with M ≥ M _min=2.2 in the area bounded by 39.5° to 42°N/42.5° to 47°E. From the point of view of homogeneous reporting this is the best catalogue we have analysed so far. The limits of the data used and the density of the grid are dictated by the data, and have no influence on the results. The choice of free parameters does not influence the results significantly within the following limits: 100≤ N ≤500, 2≤ T _w≤7, 2.2≤ M _min≤2.8.     

2001 August earthquake swarm at Shadwan Island, Gulf of Suez, Egypt

The earthquake swarm that struck Shadwan Island at the entrance of the Gulf of Suez in 2001 August included 408 events. Almost all of these events (94 per cent) were microearthquakes and only 6 per cent had small measurable magnitudes  (5.0 > M _L≥ 3.0)  . Most of the earthquakes were weak and followed each other so closely in time that they could not be identified at more distant stations. The fault plane solutions of the strongest events of the swarm show almost identical focal mechanisms, predominately normal faulting with a significant sinistral strike-slip component for nodal planes trending NW–SE. A comparison with the mechanisms of the 1969 and 1972 events which took place 20 km north of the swarm epicentral region shows similarities in faulting type and orientation of nodal planes. The azimuths of T -axes determined from focal mechanisms in this study are oriented in the NNE–SSW direction. This direction is consistent with the present-day stress field derived from borehole breakouts in the southern Gulf of Suez and the last phase of stress field changes in the Late Pleistocene, as well as with recent GPS results.
The source parameters of the largest  ( M _L≥ 3.0)  events of the 2001 August Shadwan swarm have been estimated from the P -wave spectra of the Egyptian National Seismograph Network (ENSN). Averaging of the values obtained at different stations shows relatively similar source parameters, including a fault length of  0.65 ≤ L ≤ 2 km  , a seismic moment of  7.1 × 10¹²≤ Mo ≤ 3.0 × 10¹⁴ N m  and a stress drop of  0.4 ≤Δσ≤ 10  bar.     

Likelihood analysis of earthquake catalogues

We apply several classes of stochastic multidimensional models to statistical analysis of earthquake catalogues using likelihood methods. We investigate the importance of including different earthquake parameters in the model: epicentral coordinates, hypocentral depth, time limits for interearthquake interaction, and especially spatial distribution of earthquakes as well as spatial aftershock patterns. Results of this study combined with other investigations, suggest that most distributions controlling earthquake interaction have a fractal or scale-invariant form. Developed models are used for statistical analysis of several earthquake catalogues to evaluate parameters of earthquake occurrence. These parameters are shown to be similar for shallow earthquakes of different magnitude ranges and seismogenic regions, confirming self-similarity of the earthquake process. Whereas intermediate earthquakes seem to emulate the pattern of shallow earthquake occurrence, albeit at a much smaller aftershock rate, deep earthquakes differ significantly in their properties. Predictability of standard shallow earthquake catalogues has been analysed; we present evidence that for the best available catalogues the predictability is close to 10 bits per earthquake. Several synthetic earthquake catalogues have been created and processed through the likelihood inversion scheme. The results from likelihood analysis of these catalogues confirm our approach.     

A successfully stress-forecast earthquake

A M = 5 earthquake in Iceland has been successfully 'stress forecast' by using variations in time delays of seismic shear wave splitting to assess the time and magnitude at which stress-modified microcracking reaches fracture criticality within the stressed volume where strain is released. Local investigations suggested the approximate location of the forecast earthquake. We report the criteria on which this stress forecast was based.     

The fractal geometry of the surface ruptures of the 1999 Chi-Chi earthquake, Taiwan

The 1999 September 20 Chi-Chi earthquake is the largest seismic event which occurred in the island during the twentieth century. Available seismic data relative to this earthquake are of high quality, and surface ruptures identified as features associated to the Chelungpu fault can be clearly observed at the surface and precisely mapped. We calculated the fractal dimension ( D ) and b value of Gutenberg–Richter law for 6-month aftershocks of the Chi-Chi earthquake for the fault area, and find that the surface ruptures exhibit self-similar geometry only within specific ruler intervals. The D values of the surface ruptures reflect the fault slip and geometry at depth. More importantly, the small-size aftershocks seem more likely to occur within high D value and high b value areas, whereas small D value and small b value areas have a high potential for medium- and large-size aftershocks.