Search

Understanding fault zone dynamics in multi-scale is important to embrace the complexity of the earthquake behavior and its natural system. However, the opportunity to map and observe the fault zone behavior at depth with high spatial resolution are rare as also the challenge itself on targeting and identifying the fault zone at depth. We placed a 3D cross-fault fiber array with a downhole loop from surface to depth of 700m for Hole-A (Hanging wall site, crossing fault at depth), after drilling and coring to a frequent slip fault, Milun fault in a plate boundary zone, which ruptured during the 6 February 2018 Mw6.4 Hualien earthquake, and resulted in severe damage to several tall buildings with tens of casualties and injuries. Then, the surface segment crosses the surface fault rupture zone using commercial fiber, and to another downhole loop of 500m fiber for Hole-B (Footwall site). The high spatial resolution from distributed acoustic sensing (DAS) allows us to characterize the fault zone feature together with the retrieved core and geophysical logs after drilling through this frequent slip zone. This 3D route includes the experiment of using commercial fiber to the future application of surface rupture zone identification for seismic hazard mitigation. The project successfully retrieved the fault core associated with Milun fault zone, which could be also seen in geophysical logs with low velocity and resistivity, and mapped using Optical Fiber Sensing technique of the downhole fiber. Within the Milun fault zone, while a 20m thick fault core with grey and black gouge was discovered, a distinct seismic feature associated with this 20m fault gouge was found by its amplification of the strain records from DAS. This amplification ratio is about 2.5-3 when compared to the channels at deeper depth related to a consolidated rock material. This amplification factor was frequency and azimuth independently, as genuinely observed from all events (e.g. local, and teleseismic earthquakes) with similar amplification factor. Our study shows that the amplification from this 20m fault gouge zone is mainly from the nature of the heterogeneous medium in elastic constant while crossing the fault zone, especially the fault core. Similar feature at surface but with wider surface rupture zone (~ 200m) was found in DAS data as well although less evidence using commercial fiber, while could be validated from the densely deployed geophones crossing the surface rupture of the 2018 Hualien earthquake. Through the depth, a high-resolution asymmetric feature of this active fault was evidenced from the downhole optical fiber and cores. This fault zone behavior would be hardly seen or confirmed without continuous viewing of the wavefields to this high spatial resolution to meter scale. Although the narrow fault gouge, the nature of its amplification in strain due to its strong material contrast from fault gouge was intriguing, and requires intensive attention to consider the contribution of the fault zone heterogeneity in the medium. This might give hints on the understanding of the observation of earthquake dynamics triggering reported worldwide after the occurrence of a mega-earthquake.

The 2013 Ruisui earthquake is the first unequivocal evidence of the seismicity activity of the Central Range Fault (CRF) in the central Longitudinal Valley in Taiwan, and hence reveals the existence of aseismic slip on the CRF. The finite-fault coseismic model obtained from the Bayesian joint inversion of GNSS and strainmeter data suggests that the rupture area is mainly distributed on a 26 km × 22 km fault plane located at the depth of 3 to 19 km with a maximum slip of about 0.5 m. A variational Bayesian independent component analysis (vbICA) technique is applied to the detrended GNSS time series to extract postseismic deformations in the near-source region. Although the afterslip distribution was not able to be well inverted due to the lack of observation on the western side of the fault plane, using rate-and-state friction rheology to simulate the surface displacements generated by the stress-driven afterslip model, we infer for the first time the existence of a shallow velocity-strengthening region on the CRF, which is capable of hosting and sustaining aseismic transient deformations over months.

In this study, we conduct a multi-disciplinary study, including geophysics, geochemistry, and geology, in an attempt to reconstruct geothermal geological model(s) at the shallow 3 km, for a project of potential geothermal power generation at the Hongchailin site in the Ilan plain of northeastern Taiwan. Our geophysical techniques include seismic imaging from natural earthquakes as well as ambient noise. Three seismic arrays deployed at different time periods in the past decade were used. We also incorporate geophysical imaging results from previous studies, in particular a series of seismic reflection profiles. For the magnetotelluric (MT) technique, two major surveys have been conducted, including 2015-2018 AMT by National Central University and 2021-2022 MT by GERD and Academia Sinica. Three test holes were drilled around the Hongchailin site in 2016-2019. Logging and on-site measurements were conducted at increment depths, including stratigraphy and rock types, P/T measurements, fractures analyses, geochemical analyses (e.g., hydrogen/oxygen/helium isotope, fluid inclusion, etc.), and so on. Based on regional geology and structures, and incorporating geophysical subsurface imaging, we reconstruct geothermal geological models in line with detailed geological cross sections at the shallow 3 km level. We interpret that there exits a shallow geothermal reservoir within the massive quartz sandstone layers (Szeleng sandstone) at 1-2 km depth with the downhole temperature of 80-100 C. Geologically, the reservoir is located at the regional Songlo syncline and its south limb; and geophysically, it corresponds to a relatively low resistivity area. Isotope results show that the cool meteoric water came from nearby higher altitude mountain area, then flew through the Szeleng sandstone, which plunges 1-2 km depth below the Ilan plain. Hot fluid is interpreted to be derived from deeper heat source and to be upflowed along a N-S trending vertical faults system near the Hongchailin area. In addition, two E-W trending major faults, identified by the seismic reflection profiles, seem to be acted as hydrothermal fluid barriers to confine the hot fluid within the reservoir area.

The Ilan Plain in northeastern Taiwan located at the southern termination of the Okinawa Trough, which is formed by the back-arc extension of the Ryukyu subduction zone. Owing to this extensional tectonic setting, the Ilan Plain shows distributed hot springs with high geothermal gradient in several places, such as the Chingshui, Jiaoxi, Longde-Lize and Hongchailin areas. These are potential locations for geothermal exploration which require a thorough investigation to understand their geothermal system and to evaluate their potential in geothermal power generation. Seismic velocity provides key information to determine properties of a geothermal system including the distribution of fluids, fractures, and porosity. This study obtained seismic velocity images of the Ilan Plain and the surrounding area by joint inversion of body wave travel times of local earthquakes and Rayleigh wave phase velocities from ambient noise correlations recorded by a temporary dense geophone array in the Ilan Plain and the Formosa Array in northeastern Taiwan. The joint inversion takes advantage of the complementary sensitivities of both data types to better constrain both shallow and deep structures. In addition, we also lift the flat earth surface assumption commonly assumed in the surface wave tomography to avoid possible biased velocity structure due to topography by adapting the surface wave sensitivity kernel to the surface topography in the inversion. The obtained image at the 1-km depth shows low Vp and Vs corresponding to unconsolidated Quaternary sediments. The deeper images at 2- to 5-km depths show a low-velocity corridor extending from the Guishan volcanic island northeastern offshore to the southwest corner of the Ilan Plain, implying that the heat source of the geothermal systems in the Ilan Plain may be related to magmatic intrusions of backarc rifting. Locally, in the shallow region of the Chingshui and Jiaoxi area and the deeper region of the Hongchailin and Longde-Lize area, the high Vp/Vs ratio with little increase in Vp and significant decrease in Vs could indicate a saturated rock with high porosity, based on the rock physics principles in geothermal environment summarized in previous study. The low Vp, Vs, and Vp/Vs ratio in the shallow region of the Hongchailin area could indicate vapor-rich high temperature environment. Our results demonstrate the potential of new seismic imaging integrating body- and surface-wave data for geothermal exploration.

Data and slip inversion scripts for submitted paper: Interplay between seismic and aseismic deformation on the Central Range fault during the 2013 Mw 6.3 Ruisui earthquake (Taiwan) (in revision), This research is supported by the Ministry of Science and Technology (Taiwan) grant MOST 111-2116-M-001-018.

In observational seismology, the long-period background noise level in horizontal seismometer components is always higher than the vertical component at the same location. It is believed that the horizontal long-period noise is mostly attributed to ground tilt, yet the authentic tilt recordings of background noise in the seismic frequency band (0.01–20 Hz) have not been recorded successfully. We demonstrate the use of a newly developed optical-based rotation sensor, blueSeis-3A, to correct for the tilt effect in a horizontal seismometer. A station with a horizontal long-period (1 < T < 100 s) background noise level higher than the global new high-noise model was chosen, where a seismometer, a tiltmeter, and a rotation sensor were collocated. We first confirmed that the horizontal recordings from a tiltmeter and a seismometer were visually consistent, although it was unclear if tilt contamination existed in both recordings. It became evident until the ground tilt recorded by a rotation sensor was found to be waveform-matched with them, indicating that both sensors were contaminated by tilt. The spectrum of tilt records showed that ground tilts were mostly at period bands >1 s. By subtracting tilt from seismometer records, the horizontal background noise levels could be improved, approximately to the resolution level of the rotation sensors. Our results demonstrate that collocating rotation sensors with seismometers is a feasible way to improve the quality of the long-period background noise level in seismometer horizontal components when rotation sensors are sufficiently sensitive.

Taiwan is a hotbed of seismic and orogeny activity and this is the basis of the research of Dr Hsin-Hua Huang, Earth Structures Laboratory, Institute of Earth Sciences, Academia Sinica, Taiwan. He has illuminated detailed crustal and slab structures that have furthered understanding of Taiwan orogeny, incorporated ambient noise data to resolve high-resolution imaging of the volcanic and fault structure in northern Taiwan and conducted research that led to the discovery of multi-layering magma beneath the Yellowstone supervolcano in the US. The ultimate goal of his diverse studies is to achieve an improved imaging of the structure of the Earth and establish better understanding of the orogenic tectonic process and assess and mitigate natural hazards, such as volcanoes and earthquakes. In their latest project, Huang and the team are using the seismic wave interferometry method, combined with Taiwan's existing real-time signal transmission Taiwan Broadband Seismic Network (BATS) and the Central Weather Bureau Seismic Network (CWB24), to develop an automated real-time seismicity change detection system. The idea is that by measuring changes in the transmission velocity of seismic waves in the crust, physical parameters such as the degree of rock compression or changes in porosity can be linked and discussed. The researchers have performed an extensive review of knowledge about the current stage of fault zone failure and healing processes,the advantages of seismic noise interferometry and the need for Taiwan fault zone monitoring and related seismic interferometry research. In order to understand the temporal changes in crustal structure and the behaviour of velocity changes across faults, a long-term and stable real-time seismic velocity change monitoring system is necessary.