The Chihshang fault in eastern Taiwan, characterized by 3 cm of surface slip per year, is one of the most rapidly creeping thrust faults known in the world (Angelier et al., 2000; Lee et al., 2003). Other than a recent ML 6.4 earthquake on the Chihshang fault on 10 December 2003, and a ML 6.0 earthquake in 1951, no significant earthquakes are known to have occurred on this fault. Strike-slip aults that produce both large earthquakes and near surface creep have been studied (i.e., the Hayward fault in central California). They are believed to have zones of locked and creeping fault at depth and only creep near the surface (Savage and Lisowski, 1993; Bürgmann et al., 2000). In contrast, the Chihshang fault is exhibits mainly by reverse slip and may, therefore, have greater overall strength. However, due to limited geodetic coverage and difficulties in obtaining deformation information from Interferometric Synthetic Aperture Radar (InSAR) in the area (Hsu and Bürgmann, 2006), the picture of slip at depth on the Chihshang fault is poorly resolved, and the distribution of locked and creeping fault at depth is, therefore, difficult to determine. A technique for inferring fault slip rate at depth using repeating microearthquakes has recently been developed (Nadeau and McEvilly, 1999; Nadeau and Johnson, 1998) and has been applied in a variety of tectonic settings. Because the Chihshang fault has many small quakes and is creeping, it may also generate repeating earthquakes. In this study, we search for repeating earthquakes and characterize their behavior in space and time, and use this information to develop a conceptual model of the Chihshang fault.
We used 3387 earthquakes from the Central Weather Bureau Seismic Network (CWBSN) catalog in our analysis. The events occurred in the Chihshang fault region between 01 January 1991 and 9 December 2003, the day before the 6.4 ML earthquake. The relatively sparse, one-sided coverage of the CWBSN stations in the Chihshang area coupled with poor noise characteristics, complicate the task of defining complete and accurate repeating event sequences in the region. To resolve this problem, we use the threshold criteria in S-P differential time and waveform similarity to identify repeating earthquakes in the Chihshang area.
Using time waveform similarity criteria, we identify 113 repeating events in the Chihshang fault zone, about 3% of the M1.9 to 5.4 earthquakes from the study period. They are organized into 31 repeating sequences with magnitudes ranging from 1.9 to 3.7. Fig. 1 shows examples of the waveforms for one repeating sequence. Locations of the 31 repeating sequences are shown in Fig. 2. All the sequences occur between depths of 7 to 23 km, and tend to concentrate on the northern section of the fault segment.
To study the spatial distribution of fault-slip rates, we first compute the sum of slip loads for each sequence site. These sums were then divided by the corresponding sums of recurrence intervals for the sequences, giving the average deep slip rate surrounding each sequence during the study period. The results of deep slip rates are shown in Fig. 2. The slip rate estimates range from 2.0 - 7.9 cm/yr with an average of 4 cm/yr. The coefficient of variation in the slip rates is 0.33. The highest slip rates (above 6 cm/yr) are found on the deep portion of the fault, between 18 and 20 km. Slip rates of less than 3 cm/yr are found above 15 km depth. However, overall slip rates estimates do not reveal a systematic depth-dependent behavior. In the upper panel of Fig. 2, we compare the along-strike variation of deep slip rates with surface observations. The GPS data shown by black dots are a combination of the average uplift rate from 1985-1994 leveling data and horizontal deformation rate from 1992-1999 GPS data (Liu and Yu, 1990; Yu and Kuo, 2001). The InSAR data (Hsu and Bürgmann, 2006) and creepmeter data (Lee et al., 2003) are projected into along-dip direction assuming a fault dip angle of 60° at near surface. The above surface measurements are consistent with the deep repeating earthquake slip rates (4 cm/yr).
The Chihshang repeating sequences are concentrated on the northern half of the Chihshang fault zone at 7 - 23 km depth. Their average deep slip rate, 4 cm/yr, are consistent with three types of geodetic measurements at surface. We, therefore, infer the contrasting deep fault slip behavior from north to south on the Chihshang fault. The northern segment is likely creeping and southern segment is likely locked with higher earthquake potential.
(Kate Chen has been a visiting scholar in BSL since March 2006)
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