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| Titel |
Magnetic Properties of the Wenchuan Earthquake Slip Zone |
| VerfasserIn |
Junling Pei, Haibing Li, Zhiming Sun, Jialiang Si, Huan Wang |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250073900
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| Zusammenfassung |
| Motivated by an interest in investigating large earthquake mechanisms, the Wenchuan
earthquake Fault Scientific Drilling project (WFSD) has been launched on November 4,
2008, only 178 days after the Wenchuan earthquake struck. Large earthquakes have a
signi?cant influence on the rock magnetic records in fault slip zones. The first borehole
(WFSD-1) was drilled through 1201.15 m including Pengguan complex rocks of about 800
Ma and alternating sandstones and siltstones of Triassic age at the southern segment of
the Yingxiu-Beichuan fault (N31Ë 8’59.36", E103Ë 41’28.71"). WFSD-1 shed
light to the existence of at least 12 fault zones. The Principal Slip Zone (PSZ) of
the Wenchuan earthquake has been identified at a depth of 589.17 m to 589.28 m
(FZ590).
To understand the high magnetic susceptibility in FZ590, we sampled 6 specimens every
10 cm down from 589.05 m-depth to 589.55 m-depth. The amount of sample is typically
about 3-5 g of powder due to the limited and valuable material available. A series of rock
magnetic investigations were made, such as mass magnetic susceptibility, high-temperature
magnetic susceptibility, magnetic hysteresis loops. The mass and ferromagnetic materials
magnetic susceptibility from the 589.25 m-depth sample shows a higher peak than from other
samples, while the paramagnetic materials magnetic susceptibility shows a decrease from
589.05 to 589.55 m-depth. The k–T curves of the selected samples all display a rapid slope
increase after 380°and a marked peak occurs at about 510°in the heating curves. The
magnetic susceptibility reaches zero at about 585°. Every cooling curve shows a clear
hump between 580° and 380°, which is clearly higher than the heating curves. The
hysteresis loops show the character of closed at about 0.3 T and the low-coercivity
phases. The hysteresis parameters are plotted in a Mr/Ms versus Hcr/Hc diagram,
except the 589. 55 m-depth sample, which could not be determined due to a very
weak expression. All the samples display typical Pseudo-Single Domain (PSD)
field.
Rock magnetic data from a small amount of samples provide valuable information on the
core PSZ. The primary ferromagnetic minerals in this segment are magnetite with the PSD
grain size, which suggests that the grain size cannot be the main reason for the high magnetic
susceptibility at the PSZ. The dominant mechanism responsible for the 589.25
m-depth high magnetic susceptibility might be the production of new magnetite from
iron-containing silicates or clays (e.g. chlorite) caused by frictional heating during
earthquakes.
Keywords Wenchuan Earthquake, Yingxiu-Beichuan Fault, Slip Zone, Magnetic
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