Study on Potential Strong Earthquake Risks Around the Mabian Area,Southern Sichuan

Based on seismic data from the regional network for the last 34 years,we analyzed the present fault behavior of major fault zones around the Mabian area,southern Sichuan,and identified the risky fault-segments for potential strong and large earthquakes in the future.The method of analysis is a combination of spatial distribution of b-values with activity background of historical strong earthquakes and current seismicity.Our results mainly show:(1) The spatial distribution of b-values indicates significant heterogeneity in the studied area,which reflects the spatial difference of cumulative stress levels along various fault zones and segments.(2) Three anomalously low b-value areas with different dimensions were identified along the Mabian-Yanjin fault zone.These anomalies can be asperities under relatively high cumulated stress levels.Two asperities are located in the north of Mabian county,in Lidian town in western Muchuan county,and near Yanjin at the south end of the fault zone.These two areas represent potential large earthquake seismogenic sites around the Mabian area in the near future.Besides them,the third relatively smaller asperity is identified at southern Suijiang,as another potential strongearthquake source.(3) An asperity along the southwestern segment of the Longquanshan fault zone indicates the site of potential moderate-to-strong earthquakes.(4) The asperity along the segment between Huangmu town in Hanyuan county and Longchi town in Emeishan city on Jinkouhe-Meigu fault has potential for a moderate-strong earthquake.     

Resolution and significance assessment of precursory changes in mean earthquake magnitudes

The Loma Prieta earthquake (magnitude 7.0), which occurred in October 1989 in central California, was preceded by a period during which the mean magnitude of background seismicity in a small region near the eventual epicentre was abnormally low. This period may have begun as early as 1979, and it continued until mid-1988, after which the mean magnitude increased to a higher than normal value until the main earthquake. These changes were observed in the seismicity of an area 40  km in radius, centred on the Loma Prieta epicentre, and are consistent with the predictions of fracture mechanics studies. The 1988 change correlates with a reported change in long-term strain.
  A procedure has been developed for resolving such temporal changes in seismicity using CUSUM statistics. It demonstrates that the anomaly was highly significant, on the basis of analyses of two independent catalogues. There was also a significant anomaly before the 1994 Northridge earthquake.
  The hypothesis that large earthquakes are preceded by periods in which the mean magnitude of background activity is abnormally low, in the immediate vicinity of the eventual epicentre, is a tantalizing one. The analysis tool examined here may be useful for resolving such changes. Care needs to be taken, however, in routine surveillance of earthquake populations that contain large aftershock sequences.     

An evaluation of seismic hazard parameters in southern Turkey

To investigate the characteristics of earthquake hazard parameters as a means of identifying different zones of seismicity, we have compiled a catalogue of about 1850 moderate to large-sized earthquakes with magnitudes m4.0 or greater in southern Turkey for the time period from 1900 to 1990. Several methods have been applied to the earthquake catalogue to assess seismic hazard. The study area is divided into 77 overlapping cells of 2° size. Theoretical calculations were made for the prediction of maximum magnitude, intensity, b-values, strain energy release and corresponding m3 and peak ground acceleration levels for a given period of time. The resultant seismic hazard for each parameter is depicted as a contour map to indicate lateral variations in areas of seismic source. A combination and evaluation of various hazard parameters resulted in more reasonable estimates of hazard. It is found that the most hazardous seismic zones are the Rhodes and Burdur zones where the level of peak ground acceleration reaches up to 280 cm s-2 for an average return period of 100 years.