Abstract
The Haiyuan fault is a major seismogenic fault in north-central China where the 1920 Haiyuan earthquake of magnitude 8.5 occurred, resulting in more than 220000 deaths. The fault zone can be divided into three segments based on their geometric patterns and associated geomorphology. To study paleoseismology and recurrent history of devastating earthquakes along the fault, we dug 17 trenches along different segments of the fault zone. Although only 10 of them allow the paleoearthquake event to be dated, together with the 8 trenches dug previously they still provide adequate information that enables us to capture major paleoearthquakes occurring along the fault during the past geological time. We discovered 3 events along the eastern segment during the past 14000 a, 7 events along the middle segment during the past 9000 a, and 6 events along the western segment during the past 10000 a. These events clearly depict two temporal clusters. The first cluster occurs from 4600 to 6400 a, and the second occurs from 1000 to 2800 a, approximately. Each cluster lasts about 2000 a. Time period between these two clusters is also about 2000 a. Based on fault geometry, segmentation pattern, and paleoearthquake events along the Haiyuan fault we can identify three scales of earthquake rupture: rupture of one segment, cascade rupture of two segments, and cascade rupture of entire fault (three segments). Interactions of slip patches on the surface of the fault may cause rupture on one patch or ruptures of more than two to three patchs to form the complex patterns of cascade rupture events.
Highlights
In recent years, due to disaster threat on human caused by major earthquakes, it is increasingly desired that scien tists should offer definite information on seismic hazard to public
After the December 2004 Sumatra Mw9.1 earthquake, China Earthquake Administration held a meet ing on future earthquake trends in mainland China in early 2005, and all the researchers attending the meeting ac cepted that major events would occur on the NorthSouth seismic zone in central China in view of the geodynamic context
As the range and degree of strain release reaches the maximum, mean while the strainaccumulation areas become reduced to the least range with the maximum accumulation degree, dy namic instability takes place nearby highgradient zones of strain
Summary
Due to disaster threat on human caused by major earthquakes, it is increasingly desired that scien tists should offer definite information on seismic hazard to public. Following the assumed re currence model, in 1978 Japanese scientists inferred that a M7 or greater shock should occur at any time in Tokai [Mogi, 1981; Matsumura, 1997; Science..., 2007] Such an event, did not appear more than 30 years since . Geodynamics & Tectonophysics 2014 Volume 5 Issue 2 Pages 387–399 est variation +45 % ~ –72 % [Schwartz, Coppersmith, 1984; Bakun, Lindh, 1985; Shearer, 1958; BenZion et al, 1993] This observation led to the characteristic earth quake model and a forecast in 1984 that claimed a M6 shock would happen in Parkfield before the year 1993. With hindsight to previous earthquakes, it was stated that some abnormal phenomena appeared before earth quakes, which were summarized as “seismic precursors” Such claimed precursors did not occur in subsequent events, instead other distinct phenomena were observed. Of this paper, these issues are addressed based on our laboratory experiments and field observa tions
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