Fault slip rates from repeating microearthquakes on the
Chihshang fault, eastern Taiwan
Hui-Hsuan Chen and Ruey-Juin Rau
（陳卉瑄、饒瑞鈞）
Department of Earth Sciences, National Cheng Kung University
ABTRACT
Seismological studies of Chihshang fault reveal quasi-periodic repeating
microearthquakes with short recurrence interval (1.1 to 3.6 yr). 190 repeating
earthquakes with waveform cross-correlation coefficient larger than 0.95 are used to
define 32 sequences (Fig.1), 33% of them belonging to quasi-periodic sequences and
67% for aperiodic sequences. Nearly 10﹪ of the 2018 events in the 1992 – 2003
earthquake catalog exhibit repeated behavior. Those quasi-periodic repeating
microearthquakes are distributed in the northern portion of fault zone with depth of
14 – 22 km, where are away from or in the edge of the aftershock areas of the larger
events occurred in 1992, 1995, and 2003 (Fig. 2). Slip rates derived from 14
quasi-periodic repeating sequences show that the repeating sequences are separated
into two major clusters in the vertical direction, whose recurrence-derived slip rates
gradually increasing from 3.9 to 4.8 cm/yr at the depth of 14 -17 km to 2.7 to 6.0
cm/yr at the deeper portion of the fault (18 –22 km at depth) (Fig. 3). The maximum
slip rates was found in the edge of 1995 aftershock zone, where the rate is 6 cm/yr at
21 km depth. The repeating sequences derived slip rates for northern portion are
consistent with surface deformation (4.0 cm/yr), but slip deficit of 2 cm/yr occurred in
the southern part. This may imply the fault behavior changes from creeping to locked
from north to south. The temporal distribution of cumulative seismic moment for
aperiodic sequences shows large fluctuations, which may be attributed to major
earthquake activities in 1995, but the pattern for quasi-periodic sequences reveals a
regular manner (Fig. 4). Using scaling relations among recurrence intervals and
seismic moment of quasi-periodic repeating sequences, the repeating earthquakes of
Chihshang shows a pattern that is similar to that observed in repeating events along
the San Andreas Fault near Parkfield, suggesting the mechanisms for the two features
may be similar (Fig. 5). That is, small-sized repeating earthquakes nucleate on a weak
asperity at the edge of larger locked asperity, therefore the slip rate for the small
events can reflect the same slip rates on fault. The large slip deficit (2.0 cm/yr)
observed in the southern part of quasi-periodic sequence corresponds to the location
of M6.5 earthquake occurred in Dec. 2003.
Fig.1. Examples of vertical component waveforms recorded at station CHK.
Cross-correlation coefficient is indicated on the left, and the date are shown on the
right.
B
A
A
B
Fig. 2. The upper panel showing the map view of Chihshang area. The box delineates
the experiment region used to search for repeating earthquakes. Location of repeating
sequences, indicated with rectangles, are shown with respect to background seismicity
from 1992 – 2003 (gray dots). Three major earthquakes are represented by red stars.
1992 M5.4, 1995 M5.3, and 2003 M6.5 aftershocks are shown as red, green, and blue
circles separately. The lower panel shows fault-parallel cross section (profile A-B)
for the relative locations between three main shocks (stars) and repeating earthquakes
(rectangles).
Fig. 3. Fault parallel section (profile A-B in Fig.1) showing repeating earthquakes
derived slip rate compared with surface measurements. Location of quasi-periodic
repeating sequences are plotted as color-filled rectangles, corresponding to different
slip rates. Background seismicity, 1992 M5.4 aftershock, and 1995 M5.3 aftershock
are shown as gray, red, and green circles individually. Black line in upper panel
showing surface measurements and red stars showing deep slip rates calculated from
recurrence intervals of quasi-periodic repeating sequences.
cumulative seismic moment
2.51E+22
2.01E+22
1.51E+22
1.01E+22
5.06E+21
96
19
98
19
94
19
96
19
98
19
04
20
94
19
92
19
02
20
92
19
90
19
00
20
90
19
seismic moment
6.00E+19
2.00E+23
1.50E+23
1.00E+23
5.01E+22
6.00E+19
04
20
02
20
00
20
Fig. 4. Upper panel shows temporal variations of cumulative seismic moment release
for aperiodic (rectangles) and periodic (circles) repeating sequences. Lower panel
shows seismic moment release by M>4 events in 1992 – 2003 catalog in the
Chihshang area.
2.0
Chihshang
Nadeau
Ellosworth
log Tr
1.5
1.0
Bakun
nor. Chihshang
0.5
0.0
-0.5
13
15
17
19
21
23
25
27
log M0
Fig. 5. Log repeating sequence recurrence time versus seismic moment normalized to
the loading rate of 2.3 cm/yr, which is the same with Parkfield’s case. Red symbols
show data for repeating events in California with long-term geodetic slip rate of 2.3
cm/yr. Black line is the least square fit to the data in California. Black crosses show
repeating earthquakes in Chihshang with assumed tectonic loading of 3.7 cm/yr.
Black triangles represent normalized result corresponding to loading rate of 2.3 cm/yr.