STUDY ON PALEOEARTHQUKES ALONG THE FODONGMIAO-HONGYAZI FAULT,GANSU PROVINCE

The Fodongmiao-Hongyazi Fault is a Holocene active thrust fault, belonging to the middle segment of northern Qilianshan overthrust fault zone, located in the northeastern edge of the Tibet plateau. The Hongyapu M7(1/4) earthquake in 1609 AD occurred on it. A few paleo-seismology studies were carried out on this fault zone. It was considered that four paleoearthquakes occurred on the Fodongmiao-Hongyazi Fault between(6.3±0.6) ka BP and(7.4±0.4) ka BP, in(4.3±0.3) ka BP, in(2.1±0.1) ka BP and in 1609 AD. The occurrences of the earthquakes suggested the quasi-periodic characteristic with a quasi-periodic recurrence interval between 1 600~2 500a(Institute of Geology, State Seismological Bureau, Lanzhou Institute of Seismology, State Seismological Bureau. 1993; Liu et al., 2014). There was no direct evidence for the Hongyapu M7(1/4) earthquake in 1609 AD from trench research in the previous studies. Great uncertainty exists because of the small number of the chronology data, as a few TL and OSL measurement data and several¹⁴ C data, and it was insufficient to deduce the exact recurrence interval for the paleoearthquakes. Five trenches were excavated and cleared up respectively in the eastern segment, middle segment and western segment along the Fodongmiao-Hongyazi Fault. After detail study on the trench profiles, the sedimentary characteristics, sequence relationship of the stratigraphical units, and fault-cuts in different stratigraphical units were revealed in these five trenches. Four paleoearthquakes in Holocene were distinguished from the five trenches, and geology evidences of the Hongyapu M7(1/4) earthquake in 1609 AD were also found. More accurate constraint of the occurring time of the paleo-earthquakes since Holocene on the Fodongmiao-Hongyazi Fault is provided by the progressive constraining method(Mao and Zhang, 1995), according to amounts of ¹⁴ C measurement data and OLS measurement data of the chronology samples from different stratigraphical units in the trenches. The first paleoevent, E4 occurred 10.6ka BP. The next event, E3 occurred about 7.1ka BP. The E2 occurred about 3.4ka BP. The last event, E1 is the Hongyapu M7(1/4) earthquake in 1609 AD. Abounds of proofs for the occurrences of the events of E1, E2 and E3 were found in the trench Tc1, trench Tc2, trench Tc4 and trench Tc3, located in the eastern, middle and western segments of the Fodongmiao-Hongyazi Fault accordingly. It's considered that the events E1, E2 and E3 may cause whole segment rupturing according to the proofs for these three events found together in individual trenches. The event E4 was only found in the trench Tc5 profile in the west of the Xiaoquan village in the eastern segment of the Fodongmiao-Hongyazi Fault. The earthquake rupture characteristics of this event can't be revealed before more detailed subsequent research. The time intervals among the four paleoearthquakes are ca 3.5ka, ca 3.7ka, and ca 3.0ka. The four events are characterized by ca 3.4ka quasi-periodic recurrence interval.