Fault-fling Effect from the Near-source Strong-Motion
Records of the Great 2008 Wenchuan Earthquake
Ming LU1

John X. ZHAO2 Ya Min ZHAO 1

1. Institute of Crustal Dynamics,
China Earthquake Administration, Beijing, China

2. GNS Science, Lower Hutt, New ZealandIntroduction•The largest horizontal permanent grounddisplacement is 5.3m and the largest verticaldisplacement is 6.2m.

•Both the main fault (Longmenshan fault) and thesecondary fault (Hanwang-Bailu fault) have arelatively shallow dipping angle towards to thenorth-west.

Wenchuan earthquake strong-motion records ：2 records within 15km
5 records within 30km

Two types of velocity pulse:
forward-directivity pulse (Somerville et al.
1997)
and fault-fling pulse (Abrahamson 2006).

The forward-directivity effect occurs only at stations on
the receiving side of the fault-rupture propagation. This velocity pulse
is typically in two directions (positive and negative) and the groundsurface displacement due to the velocity pulse reaches nearly zero after
the velocity pulse passes the recording station.

The fault-fling pulse is a result of permanent fault
displacement and this velocity pulse usuallyoccurs in one direction. The ground surface
displacement is almost constant after the faultfling pulse passes the recording site.
In practice, a near-source record may containboth types of velocity pulses.

Figure 1 presents a classicalexample of these two types ofvelocity pulse – a near-source
record obtained at Lucerne
recording station from the
1992 Landers earthquake(Mw=7.3, California, USA) ata source distance of just over
2km. This earthquake has astrike-slip focal mechanism
and the fault rupture initiatedat one end of the fault andpropagated towards to theLucerne station which islocated at the other end of thefault.

•

The fault-normal (FN)
component for this record
contains the forwarddirectivity velocity pulse as
shown in Figure 1(a) andalso contains fault-fling
effect (see the permanent
ground displacement justover 170cm, the thick line
in Figure 1(c)). The
difference between thepeak ground displacement(PGD) (just over 300cm)
and the ground permanent
displacement is due to thepositive velocities in thetime window of 14-22s.

•The fault-parallel
component contains afault-fling velocity pulse
as shown in Figure 1(b).
The velocity pulse islargely in the positive
direction which leads toa permanent grounddisplacement of justover 240cm as shown inFigure 1(c).

In the present study, we present the characteristics oftwo strong-motion records that have obvious faultfling effect and we compare the near-source records
from the Wenchuan 2008 earthquake with thosefrom the 1992 Landers earthquake (California,USA) and the 1999 Chichi, Taiwan earthquake.

A. Fault-fling effect fromthe MZQ record•Figures 3-5 show the“corrected” time histories
from the MZQ site with asource distance less than 2km.

•PGA, PGV and PGD for this record arepresented in Table 1. The processing

technique is from Boore (2001).

Figure 4 Corrected velocity
time histories of the MZQstrong-motion record(a) fault-parallel Component(b) fault-normal component(c) the vertical component

Figure 5 Correcteddisplacement time historiesof the MZQ strong-motionrecord(a) fault-parallelcomponent(b) fault-normal component(c) The vertical component2 绵竹青平 MZQ强震记录断层残余位移效应
MZQ强震记录断层残余位移效应Figure 6Response spectra of
the MZQ strongmotion
Record(a) acceleration
spectra(b) displacement
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B. Fault-fling effect fromthe SFB record•

Figure 7 shows theacceleration time histories of
the SFB record at a sourcedistance of 14 kilometres.

•The duration of strong shaking (over0.3g) is nearly 30s and is about 60s foraccelerations over 0.2g. The PGA in thefault-normal and fault-parallel direction ismoderate, only about 0.5g, and thevertical PGA is 0.56g, as shown in Table
1.

Figure 8
Corrected velocity time
histories of the SFB
strong-motion record• fault-parallel component• fault-normal component• vertical component

Figure 9 Correcteddisplacement time histories of
the SFB strong-motion record(a) fault-parallel component(b) fault-normal component(c) vertical component

c. Comparison with the large fault-fling effect of near-source
records from the 1999 Chichi, Taiwan and the 1992
Landers, California, USA, earthquakes

The 1999 Chichi, Taiwan earthquake has amagnitude of 7.6 with a reverse focal mechanism
and the TCU052 and TCU068 records are obtained
from the northern end of the Chelongpu fault on thehanging wall.
The two Chichi earthquake records selected here
have the largest PGV and PGD among the strongmotion records in the dataset of the next generationof attenuation models (NGA).
The Chichi earthquake has very similar
seismological characteristics to those of the 2008
Wenchun earthquake.

Figure 11
Comparison of the fault normal
displacement time
history
MZQ fault normal component in (a)TCU052 (EW component) record in(b)TCU068 record in (c)Figure 13
Comparison of the verticalcomponent displacement time
history
SFB fault vertical componentin (a)
TCU052 (vertical component)
record in (b)TCU068 record in (c)Thanks for your attention

Figure 12 Comparison of the fault-normalcomponent acceleration time history
MZQ fault-normal component record in (a)Lucerne record in (b);TCU052 record in (c);TCU068 in (d)This means that the damage
potential of the MZQ record for short period structures is similar to that of theLucerne record but is much larger than those of the TCU052 and TCU068
records.

Figure 14
Comparison of the SFB verticalacceleration
time history
SFB vertical component record in (a)TCU052 record in (b);TCU068 record in (c)