Kate Huihsuan Chen

Triggering Effect of M 4–5 Earthquakes on the Earthquake Cycle of Repeating Events at Parkfield, California

Stress perturbations influence earthquake recurrence and are of funda- mental importance to understanding the earthquake cycle and determining earthquake hazard. The large population of repeating earthquakes on the San Andreas fault at Parkfield, California, provides a unique opportunity to examine the response of the repeating events to the occurrence of moderate earthquakes. Using 187 M 0:4 to ∼1:7 repeating earthquake sequences from the High Resolution Seismic Network cat- alog, we find that the time to recurrence of repeating events subsequent to nearby M 4-5 earthquakes is shortened, suggesting triggering by major events. The triggering effect is found to be most evident within a distance of ∼5 km, corresponding to static coseismic stress changes of > 0:6 26:6 kPa, and decays with distance. We also find coherently reduced recurrence intervals from 1993 to 1998. This enduring recurrence acceleration over several years reflects accelerated fault slip and thus loading rates during the early 1990s. Online Material: Figures of event chronologies of RES, estimates of dt, and coseismic static stress changes.

Variability of repeating earthquake behavior along the Longitudinal Valley fault zone of eastern Taiwan

[1] Two major clusters of repeating earthquake sequences (RES) are found on two fault patches near the Longitudinal Valley fault (LVF) in eastern Taiwan, the Chihshang segment to the south and the Hualien segment to the north. The 25 M 2.3–4.6 RES on the Hualien segment are widely distributed in a NNE direction over a distance of 45 km, whereas on the Chihshang segment, the 30 M 2.2–3.4 RES have a shorter along-strike extent of about 15 km. The longer spatial extent of RES on the Hualien segment may indicate a wider distributed creeping zone at depth. M � 3.8 RES are observed on the Hualien segment and are characterized by quasiperiodic recurrence. Statistical analysis of the size and number of earthquakes adjacent to the RES indicates that the smaller number of large earthquakes in the vicinity may explain the quasiperiodic recurrence of RES. We argue that on the Hualien segment, M� 4 or larger earthquakes are isolated enough in space to facilitate the occurrence of repeated ruptures.

Thrust‐type focal mechanisms of tectonic tremors in Taiwan: Evidence of subduction

Unlike tectonic tremors in subduction zones and along transform faults, the hosting structure for tremors in Taiwan remains debated. Tectonic tremors in Taiwan have been discovered at ~30 km depth beneath the southern Central Range, which is a young and active collisional mountain belt. Here we provide the first evidence for the focal mechanism of tremor using moment tensor inversion in the very low frequency band, employing broadband seismograms stacked relative to the hypocentral time of tremor. The best solution corresponds to low-angle thrust faulting, suggesting the subduction of the Eurasian plate. This mechanism is consistent with strong tidal modulation of tremor activity but differs from the normal-type faulting that dominates regional shallow earthquakes. This result suggests vertical variations in the tectonic stress regime. Thrust faulting may be facilitated by a decrease in normal stress due to the buoyant roots of the mountain belt and local high fluid pressure.

A leaping, triggered sequence along a segmented fault: The 1951 ML 7.3 Hualien-Taitung earthquake sequence in eastern Taiwan

[1] As the most destructive seismic episode ever known in eastern Taiwan, the 1951 M L 7.3 Hualien - Taitung earthquake series consisted of sequential ruptures along four distinct fault segments. It provides a good opportunity to study earthquake triggering processes along an active fault at an oblique arc-continent collision boundary. This sequence initiated on 21 October 1951 with the M L 7.3 Hualien main shock and a group of M6+ aftershocks nearby. The M L 6.0 Chihshang earthquake occurred 34 days later and 100 km away from the main shock. The M L 7.3 Yuli earthquake followed 3 m later and 5 km away from the Chihshang event. Two days later, the M L 6.0 Taitung earthquake shocked a region 40 km away from the preceding M6 event and completed the sequence. The first triggered rupture outside the main shock area did not occur on the nearby Yuli fault segment but occurred 100 km away at the Chihshang fault. Calculations of static Coulomb stress change show that most of the major aftershocks were located in areas of enhanced static stress change. However, the stress transfer alone cannot explain triggering across 100 km. With the rate/state stress transfer model, we computed the temporal order of encouraged ruptures on different segments along the collision boundary. The results show that 34 days following the major shocks in Hualien, the Chihshang segment had a higher M6+ (M ≥ 6) earthquake probability due to its significantly higher (at least an order of magnitude) background seismicity rate than the other two segments. After the Chihshang event, the rate/state model predicted a higher M6+ earthquake probability in the Yuli segment, also matching the observation. In this case, the Yuli segment was triggered ahead of the Taitung segment because of its larger increase in Coulomb stress change.

Ambient tremors in a collisional orogenic belt

Deep-seated tectonic tremors have been regarded as an observation tied to interconnected fluids at depth, which have been well documented in worldwide subduction zones and transform faults but not in a collisional mountain belt. In this study we explore the general features of collisional tremors in Taiwan and discuss the possible generation mechanism. In the 4 year data, we find 231 ambient tremor episodes with durations ranging from 5 to 30 min. In addition to a coseismic slip-induced stress change from nearby major earthquake, increased tremor rate is also highly correlated with the active, normal faulting earthquake swarms at the shallower depth. Both the tremor and earthquake swarm activities are confined in a small, area where the high attenuation, high thermal anomaly, the boundary between high and low resistivity, and localized veins on the surfaces distributed, suggesting the involvement of fluids from metamorphic dehydration within the orogen.

Creeping faults: Good news, bad news?

Author(s): Chen, KH; Burgmann, R | Abstract: ©2017. American Geophysical Union. All Rights Reserved. The motion of the Earths tectonic plates drive fault slip. Some faults slip in sudden movements, releasing great amounts of energy during large earthquake ruptures, while others slip in steadier movements which release energy more slowly. The latter, known as creeping faults, are believed to be less hazardous but there is mounting evidence that they are more complex than previously thought and can also pose a significant hazard. A recent review by Harris [2017] documents the earthquake potential of creeping faults in shallow continental fault zones from worldwide data. She presents a comprehensive review of prior studies; key insights into when, where, and why fault creep takes place and under which conditions creeping faults may represent high seismic hazard and suggests some directions for future research.

Earthquake School in the Cloud: Citizen Seismologists in Taiwan

It is hoped that through the cultivation of a crew of volunteer citizen seismologists, public involvement could be encouraged and the discovery and inquiry into earthquake knowledge could be promoted. These volunteers can contribute to data collection, analysis, and reporting, and have the potential to greatly improve the emergency response to earthquakes. The Citizen Seismologists in Taiwan Project (CSTaiwan) is designed to elevate the quality of earthquake science education by incorporating earthquake and tsunami stories and educational earthquake games into traditional school curricula. The project aims to build a cloud-based computing service incorporating an earthquake school (i.e., a website for online learning) where teachers can easily teach their students about earthquakes and children can learn about earthquakes in a fun environment. Here we demonstrate how students perform P -a ndS-wave picking and measure seismic intensity through an interactive learning platform, how scientists and school teachers work together, and how we create a near-realtime earthquake games competition to facilitate continuous learning while making earthquake science fun. We also develop 49 questions associated with participants’ preknowledge, attitude, and skills in earthquake sciences, called Citizen Seismological Literacy (CSL). The CSL model may serve as an example to quantify citizen’s background in earthquake sciences and could be applied as a framework for seismologists around the world who wish to approach the public for educational purposes, while considering promoting the public’s seismologic literacy.

Can slip heterogeneity be linked to earthquake recurrence

The rupture process of two M4 repeating earthquake sequences in eastern Taiwan with contrasting recurrence behavior is investigated to demonstrate a link between slip heterogeneity and earthquake recurrence. The M3.6–3.8 quasiperiodic repeating earthquakes characterized by 3 years recurrence interval reveal overlapped slip concentrations. Inferred slip distribution for each event illustrates two asperities with peak slip of 47.7 cm and peak stress drop of 151.1MPa. Under the influence of nearby M6.9 event, the M4.3–4.8 repeating earthquakes separated only by 6–87min, however, reveal an aperiodic manner. There is a distinct rupture characteristic without overlap in the slip areas, suggesting that shortening of the recurrence interval by the nearby large earthquake may change the slip heterogeneity in a repeatedly ruptured asperity. We conclude that the inherent heterogeneity of stress and strength could influence the distribution of coseismic slip, which is strongly tied to the recurrence behavior.

Influences on the location of repeating earthquakes determined from a and b value imaging

To explore where earthquakes tend to recur, we statistically investigated repeating earthquake catalogs and background seismicity from different regions (Parkfield, Hayward, Calaveras, and Chihshang Faults). We show that the location of repeating earthquakes can be mapped using the spatial distribution of the seismic a and b values obtained from the background seismicity. Molchans error diagram statistically confirmed that repeating earthquakes occur within areas with high a values (2.8–3.8) and high b values (0.9–1.1) on both strike-slip and thrust fault segments. However, no significant association held true for fault segments with more complicated geometry or for wider areas with a complex fault network. The productivity of small earthquakes responsible for high a and b values may thus be the most important factor controlling the location of repeating earthquakes. We inferred that the location of high creep rate in planar/listric fault structures might be indicated by a values of ~3 and b values of ~1.

Near‐surface versus fault zone damage following the 1999 Chi‐Chi earthquake: Observation and simulation of repeating earthquakes

We observe crustal damage and its subsequent recovery caused by the 1999 M7.6 Chi-Chi earthquake in central Taiwan. Analysis of repeating earthquakes in Hualien region, ~70 km east of the Chi-Chi earthquake, shows a remarkable change in wave propagation beginning in the year 2000, revealing damage within the fault zone and distributed across the near surface. We use moving window cross correlation to identify a dramatic decrease in the waveform similarity and delays in the S wave coda. The maximum delay is up to 59 ms, corresponding to a 7.6% velocity decrease averaged over the wave propagation path. The waveform changes on either side of the fault are distinct. They occur in different parts of the waveforms, affect different frequencies, and the size of the velocity reductions is different. Using a finite difference method, we simulate the effect of postseismic changes in the wavefield by introducing S wave velocity anomaly in the fault zone and near the surface. The models that best fit the observations point to pervasive damage in the near surface and deep, along-fault damage at the time of the Chi-Chi earthquake. The footwall stations show the combined effect of near-surface and the fault zone damage, where the velocity reduction (2–7%) is twofold to threefold greater than the fault zone damage observed in the hanging wall stations. The physical models obtained here allow us to monitor the temporal evolution and recovering process of the Chi-Chi fault zone damage.