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Distributed acoustic sensing (DAS) sees increased utilization in the seismological community in recent years and various applications are investigated for the usage of DAS in different branches of seismology.Strong-motion seismology uses records of earthquakes of engineering concern (MW>4.5) with hypocentral distances within few hundreds of kilometers. This demands dense networks over a wide area and installation of typical strong-motion instruments (accelerometers) can be achieved quickly and at a reasonable budget, compared to other network types. For DAS, installation and operation are more involved, and deployment is very still limited. Consequently, DAS recordings of nearby large events are still very unlikely and rare compared to accelerometers.On September 18, 2022, a shallow earthquake sequence with a MW 6.9 mainshock struck near Chishang (Taiwan) and was recorded by DAS in Hualien city, appr. 100 km north. Shaking of the mainshock and several aftershocks were noticeable in Hualien, though not damaging with PGA recorded at 0.28 m/s^2 nearby the DAS site. The DAS campaign was originally conceptualized as a test suite with different fiber installations: including buried, within a gutter (as in commercial fiber installation) and loose within a basement. The test site is in an urban area affected by surface rupturing during the 2018 Hualien earthquake.The presented recordings provide not only an unprecedented insight how strong-motion appears on DAS but also how effective different installation techniques are for this kind of event. The waveforms are also compared to records of a collocated broadband seismometer and an accelerometer 1 km away., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Distributed Acoustic Sensing (DAS) is used to record high-spatial resolution strain-rate data. For ground motion observation, the DAS data can be converted from strain rate to acceleration or velocity by array-based measurements with coherent plane waves. DAS provides an opportunity to map high-resolution shaking patterns near faults. We installed collocated geophones and optical fiber in Hualien City (a very seismically active area in Taiwan) from the end of January to the end of February in 2022. Earthquakes with magnitudes (Mw) between 3.2 and 5.4 have been recorded. These records illustrate the typical magnitude-distance dependence of ground-motion but also show saturation for higher magnitudes and/or at shorter distances (e.g for an earthquake of Mw 5.2 earthquake recorded at 100 km). For frequency-based analyses, clipped signals on DAS result in challenges not present in classical instruments (seismometers). The upper limit in dynamic range of seismometers results in easily identifiable trapezoidal signals. The dynamic range of DAS interrogators is limited by gauge length, sampling frequency, and wrapped phase in the interferometric phase demodulation. We observe that clipped DAS signals not only affect time series but also contaminate their spectra on all frequencies, due to the random nature of clipping in DAS—contrasting to the flat plateaus in clipped time series on seismometers. Therefore, the identification of the start and end points of clipped DAS records poses a major challenge, which we aim to resolve with a neural network. This approach enhances the efficiency for quality control of massive DAS datasets., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The aim of the study is to analyze the existing ground motion prediction equations (GMPEs) applicable in Albania for horizontal peak ground acceleration (PGA) and spectral acceleration (SA). We present four sets of GMPEs evaluated for Albania using a dataset of instrumental strong-motion records from the November 26, 2019, Mw6.4 Durres earthquake. Besides the mainshock, the models are tested for foreshock and aftershock events of magnitude Mw>5.0. The GMPEs from Boore et al., (2014), Chiou and Youngs (2014), and Kotha et. al., (2020), provide a better fit to the ground-motion attenuation observed for the mainshock at distances up to 400 km, while Akkar et. al., (2014) fit better at a short distance. Along with the seismological aspects of the earthquake, we give an overview of the damages and social consequences, considering the engineering seismology source of data known as “Did You Feel It?” (DYFI) and the 1360 DYFI online citizen responses collected after the Durres event. The DYFI data are higher than instrumentation data, better represented using SA at 0.3s. By integrating the instrumentally observed data with the macroseismic observed data, we explore the importance of human response to the concept of ground shaking, demonstrating how this type of information can indeed reduce the gap in regions with a limited number of near-field stations. Therefore, this study makes a major contribution to the selection of GMPEs and provides a useful reference for the logic tree structure in subsequent seismic hazard assessment on both national and regional scales., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The Taiwan Milun fault zone located at the boundary between the Eurasian and Philippine Sea plates. This fault slips frequently and produced large earthquakes, as for example the Mw6.4 Hualien earthquake (6 February 2018). We map and observe the fault zone and its behavior at depth by high spatial resolution dynamic strain sensing with optical fiber. In 2021-2022, we drilled and cored the fault, and deployed a 3D multi-cross-fault fiber array comprising a borehole loop with a depth of 700 m (Hole-A, Hanging wall site, crossing the fault at depth), a surface array crossing the fault rupture zone using commercial fiber, and a second borehole loop of 500m fiber (Hole-B, Footwall site). The high spatial resolution from distributed acoustic sensing (DAS) and the retrieved core combined with geophysical logs allow us to characterize the structure on meter-scale. Within the Milun fault zone, we identified a 20-m wide fault core comprised of gray and black gouge in the core sample. DAS strain-rate records associated with the same depth as the fault core show a distinct amplification. The amplification ratio of 2.5-3 is constant as for all types of events (local, teleseismic ), when compared to DAS channels at larger depth, related to a consolidated rock material. Although the fault gouge is narrow, the nature of the amplification in strain is due to its strong material contrast from fault gouge. This result may shed the light on the understanding of fault-zone dynamics in terms of remote earthquake triggering and near-fault ground motion., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The 2018 February 6 M_w 6.3 Hualien earthquake caused severe localized damage in Hualien City, located 20 km away from the epicentre. The damage was due to strong (>70 cm s⁻¹) and sharp (duration ∼2.5 s) velocity pulses. The observed peak ground-motion velocity in Hualien City symmetrically decays with distance from the nearby Milun fault. Waveforms observed on the opposite sides of the fault show reversed polarity on the vertical and N–S components while the E–W component is almost identical. None of the published finite-fault slip models can explain the spatially highly localized large velocity pulses. In this study, we show that an M_w 5.9 strike-slip subevent on the Milun fault at 2.5 km depth, rupturing from north to south at ∼0.9Vs speed, combined with site effects caused by surficial layers with low S-wave speed, can explain the velocity pulses observed at the dense strong-motion network stations. This subevent contributes only 25 per cent of the total moment of the 2018 Hualien earthquake, suggesting that a small local slip patch near a metropolis can dominate the local hazard. Our result strongly suggests that seismic hazard assessments should consider large ground-motion variabilities caused by directivity and site effects, as observed in the 2018 Hualien earthquake.

Despite a moderate magnitude, M_w = 6.4, the 5 February 2016 Meinong, Taiwan, earthquake caused significant damage in Tainan City and the surrounding areas. Several seismograms display an impulsive S-wave velocity pulse with an amplitude of about 1 m s-1, which is similar to large S-wave pulses recorded for the past several larger damaging earthquakes, such as the 1995 Kobe, Japan, earthquake (M_w = 6.9) and the 1994 Northridge, California, earthquake (M_w = 6.7). The observed PGV in the Tainan area is about 10 times larger than the median PGV of M_w = 6.4 crustal earthquakes in Taiwan. We investigate the cause of the localized strong ground motions. The peak-to-peak ground-motion displacement at the basin sites near Tainan is about 35 times larger than that at a mountain site with a similar epicentral distance. At some frequency bands (0.9 - 1.1 Hz), the amplitude ratio is as large as 200. Using the focal mechanism of this earthquake, typical “soft” and “hard” crustal structures, and directivity inferred from the observed waveforms and the slip distribution, we show that the combined effect yields an amplitude ratio of 17 to 34. The larger amplitude ratios at higher frequency bands can be probably due to the effects of complex 3-D basin structures. The result indicates that even from a moderate event, if these effects simultaneously work together toward amplifying ground motions, the extremely large ground motions as observed in Tainan can occur. Such occurrences should be taken into consideration in hazard mitigation measures in the place with frequent moderate earthquakes.

地震のデータとボアホールの温度のデータを使って，1999年台湾集集地震のすべりのプロセスの性格を推定した。車籠甫断層の北部では10メートルを超える大きなずれが見られた一方，南部のずれは1～2メートルとはるかに小さかった。ところが意外にも，加速度と被害は全くちがったパターンを示し，南部では激しい高周期の地面の揺れと小さな建造物への大被害が見られた。その一方，北部では加速度のレベルが比較的低く，それに伴う被害も，断層のずれの大きさから考えると軽かった。この違いは地震の際の断層の動き方の違いに帰せられる。地震の放射エネルギーと，発せられた摩擦熱を計算すれば，總エネルギーがどのように配分されたか知ることができる。この地震では，放射エネルギーは1.1x10 16J，摩擦熱は 2x10 17J という値を得た。従って地震効率は約5%ということになる。さらに，車籠甫断層の北部と南部のそれぞれについて，こうしたエネルギー・バランスを計算した。この結果，北部ははるかに大きいすべりがあったにも関わらず熱量は低く，摩擦の値の低かったことを示している。また，放射エネルギーは北部の方が大きく，地震効率は南部より高い。北部の摩擦の値の低さは，高周波のレベルが低いことや周辺地域の被害が比較的軽かったことの説明になっている。, We use seismic data and borehole temperature data to infer characteristics of the faultingprocess of the 1999 Chichi, Taiwan earthquake. Inversion of strong-motion data andassociated observations of surface faulting show that the northern portion of the Chelungpufault had very large displacements in excess of 10 m, while the southern part of the faulthad much smaller displacements of 1-2 m. In contrast the levels of accelerations anddamage show a very different pattern, with much more severe high frequency groundmotions in the south and extensive damage to small buildings. In the north the levels ofacceleration and associated damage are relatively low, considering the very large faultdisplacements. These differences in observed ground motions are attributed to differencesin dynamic fault behavior during the earthquake. Determinations of radiated seismicenergy and estimates of frictional heat produced by the earthquake are used to put togetherthe energy budget for the earthquake. We obtained a total radiated energy of 1.1x10 16 Jand frictional heat of 2x10 17 J. These values give a seismic efficiency of about 5%. Wealso looked at the energy balance separately for the northern and southern portions of theChelungpu fault. The northern part produced much lower heat, even though the slip wasmuch larger, implying a lower value of friction. Also, the northern part produced moreradiated seismic energy, so the seismic efficiency for the north is higher than for the south.The low values of friction for the northern portion, provide a good explanation for the lowlevels of high frequency radiation and the lower levels of damage in the nearby areas.

The 1909 April 14 Taiwan earthquake caused significant damage in Taipei. Most of the information on this earthquake available until now is from the written reports on its macro-seismic effects and from seismic station bulletins. In view of the importance of this event for assessing the shaking hazard in the present-day Taipei, we collected historical seismograms and station bulletins of this event and investigated them in conjunction with other seismological data. We compared the observed seismograms with those from recent earthquakes in similar tectonic environments to characterize the 1909 earthquake. Despite the inevitably large uncertainties associated with old data, we conclude that the 1909 Taipei earthquake is a relatively deep (50–100 km) intraplate earthquake that occurred within the subducting Philippine Sea Plate beneath Taipei with an estimated M_W of 7 ± 0.3. Some intraplate events elsewhere in the world are enriched in high-frequency energy and the resulting ground motions can be very strong. Thus, despite its relatively large depth and a moderately large magnitude, it would be prudent to review the safety of the existing structures in Taipei against large intraplate earthquakes like the 1909 Taipei earthquake.

We determine the finite-fault slip distribution of the 2001 Kunlun earthquake (M_s = 8.1) by inverting teleseismic waveforms, as constrained by geological and remote sensing field observations. The spatial slip distribution along the 400-km-long fault was divided into five segments in accordance with geological observations. Forward modelling of regional surface waves was performed to estimate the variation of the speed of rupture propagation during faulting. For our modelling, the regional 1-D velocity structure was carefully constructed for each of six regional seismic stations using three events with magnitudes of 5.1–5.4 distributed along the ruptured portion of the Kunlun fault. Our result shows that the average rupture velocity is about 3.6 km s^−1, consistent with teleseismic long period wave modelling. The initial rupture was almost purely strike-slip with a rupture velocity of 1.9 km s^−1, increasing to 3.5 km s^−1 in the second fault segment, and reaching a rupture velocity of about 6 km s^−1 in the third segment and the fourth segment, where the maximum surface offset, with a broad fault zone, was observed. The rupture velocity decelerated to a value of 3.3 km s^−1 in the fifth and final segment. Coseismic slip on the fault was concentrated between the surface and a depth of about 10 km. We infer that significant variations in rupture velocity and the observed fault segmentation are indicative of variations in strength along the interface of the Kunlun fault, as well as variations in fault geometry.

We investigate the rupture process of the 1999 Chi-Chi, Taiwan, earthquake using extensive near-source observations, including three-component velocity waveforms at 36 strong motion stations and 119 GPS measurements. A three-plane fault geometry derived from our previous inversion using only static data [ Ji et al., 2001 ] is applied. The slip amplitude, rake angle, rupture initiation time, and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm [ Ji et al., 2002b ]. The inversion results are validated by the forward prediction of an independent data set, the teleseismic P and SH ground velocities, with notable agreement. The results show that the total seismic moment release of this earthquake is 2.7 × 10^20 N m and that most of the slip occurred in a triangular-shaped asperity involving two fault segments, which is consistent with our previous static inversion. The rupture front propagates with an average rupture velocity of ∼2.0 km s^(−1), and the average slip duration (risetime) is 7.2 s. Several interesting observations related to the temporal evolution of the Chi-Chi earthquake are also investigated, including (1) the strong effect of the sinuous fault plane of the Chelungpu fault on spatial and temporal variations in slip history, (2) the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation, and (3) the observation that the peak slip velocity near the surface is, in general, higher than on the deeper portion of the fault plane, as predicted by dynamic modeling.

The Sierra Madre earthquake (M_I = 5.8) of 28 June 1991 occurred at a depth of about 12 km, on the Clamshell-Sawpit fault in the San Gabriel Mountains. High-quality seismograms were recorded with TERRAscope not only for the mainshock but also for the aftershocks at epicentral distances of about 16 km. We determined the focal mechanisms and seismic moments of the mainshock and 21 aftershocks by combining the waveform and first-motion data. We classified the events into five groups according to the location and waveforms recorded at PAS. Most events located within 5 km west of the mainshock are similar to the mainshock in waveform. The mechanisms thus determined are thrust mechanisms. A few events located east of the mainshock have waveforms different from the mainshock and have strike-slip mechanisms. The average Q_β values along the paths from the hypocenters of the Sierra Madre and the 3 December 1988 Pasadena earthquake (M_L = 4.9) to PAS are about 130 and 80, respectively. The stress drop of the mainshock is about 500 bars. Most of the aftershocks have stress drops between 10 and 100 bars.

The first arrival of the tsunami recorded at Monterey, California, was about 10 min after the origin time of the earthquake. Using an elastic half space, we computed vertical ground displacements for many different fault models for the Loma Prieta earthquake and used them as the initial condition for computation of the tsunami in Monterey Bay. The synthetic tsunami computed for the uniform dislocation model determined from seismic data can explain the arrival time, polarity, and amplitude of the beginning of the tsunami. However, the period of the synthetic tsunami is too long compared with the observed. We tested other fault models with more localized slip distribution. None of the models could explain the observed period. The residual waveform, the observed minus the synthetic waveform, begins as a downward motion at about 18 min after the origin time of the earthquake and could be interpreted as due to a secondary source near Moss Landing. If the large-scale slumping near Moss Landing suggested by an eyewitness observation occurred about 9 min after the origin time of the earthquake, it could explain the residual waveform. To account for the amplitude of the observed tsunami, the volume of sediments involved in the slumping is approximately 0.012 km^3. Thus the most likely cause of the tsunami observed at Monterey is the combination of the vertical uplift of the sea floor due to the main faulting and a large-scale slumping near Moss Landing.

The origins of tsunamis excited by the 1989 Lorna Prieta, the 1906 San Francisco, and 1975 Kalapana, Hawaii, earthquakes were examined in part I. Since tsunamis are mainly caused by vertical deformation under the sea-floor, the tsunami data allows us to constrain the vertical motion of the sea-floor during the earthquake and to determine the excitation mechanism of the tsunami. The first arrival of the observed tsunami from the 1989 Lorna Prieta, California, earthquake observed at Montery was about 10 min. after the origin time of the earthquake. The synthetic tsunami computed for the uniform dislocation model determined from seismic data can explain the arrival time, polarity, and amplitude of the beginning of the tsunami but the period is too long. We tested other fault models with more localized slip distribution. None of the models could explain the observed period. The residual waveform, the observed minus the synthetic waveform, begins as a downward motion at about 18 min. after the origin time of the earthquake, and could be interpreted as due to a secondary source near Moss Landing. The volume of sediments involved in the slumping is approximately 0.012 km^3. We conclude that the most likely cause of the observed tsunami is the combination of the vertical uplift of the sea floor due to the main faulting and a localized slumping near Moss Landing. The observation of tsunami excited by the 1906 San Francisco earthquake is curious because this earthquake is generally believed to be a strike-slip earthquake for which tsunamis are not usually expected. We show that the tsunami was caused by a local subsidence associated with a bend of the San Andreas fault offshore from the Golden Gate; no vertical fault motion was involved during the 1906 San Francisco earthquake. The 1975 Kalapana, Hawaii, earthquake was accompanied by large tsunamis which were well recorded at several tide-gauge stations around the Hawaii islands. To examine the source of the tsunamis associated with the earthquake, we computed synthetic tsunamis for three tide-gage stations, Hilo, Honolulu and Kahului, using various dislocation models, Hilina fault models and slump models. Crustal deformation data were used to constrain the dislocation models. We could find a combination of a dislocation model for the earthquake and a Hilina fault model which can explain the observed crustal deformation inland fairly well. However, the tsunamis computed for this combined model are too early in first arrivals and too small in amplitudes. The residual tsunamis, observed synthetic, are not very different from the observed tsunamis and can be interpreted with a slump model which involves an uplift of 100-110 cm over an area of about 2500-3000 km^2 offshore. The total volume of displaced water associated with the slumping is about 2.5~3 Km^3. The recent deployment of TERRAscope, a broadband and wide dynamic range seismic network in Southern California, provided us with a capability of recording complete waveforms of nearby earthquakes. These waveform data allow us to determine the overall faulting mechanisms, seismic moments, depths, stress drops, and the attenuation characteristics of the crust In part II. I investigated the waveforms of local earthquakes recorded at TERRAscope stations to understand the characteristics of the earthquake sequences. The Pasadena earthquake (M_L=4.9) of 3 December, 1988, occurred at a depth of 16 km, probably on the Santa Monica-Raymond fault. Prior to this event, no earthquake larger than magnitude 4 had been recorded in this area since 1930. We determined the focal mechanisms and seismic moment of 9 aftershocks by combining the first-motion data and the waveform data of P, SV, and SH waves recorded at Pasadena TERRAscope station, since the first-motion data for most of the aftershocks are too sparse to determine the mechanism. The average orientations of the P and T axes of the aftershocks are consistent with the strike of the Raymond fault. The ratio of cumulative seismic moment of the aftershocks to the seismic moment of the main shock is significantly smaller than commonly observed. The mechanisms and seismic moments of the Sierra Madre earthquake (M_L=5.8) of 28 June, 1991, sequence were determined using the same techniques that was applied in the 1988 Pasadena earthquake sequence. Most events located within 5 km west of the mainshock are similar to the mainshock in waveform. The mechanisms thus determined are thrust. Some events have high stress drops between 100 to 1000 bars; the mainshock is one of them. For other larger events, including the two largest aftershocks, the stress drops are between 10 to 100 bars. A few events located east of the mainshock have waveforms different from the mainshock and have strike-slip mechanisms. The ratio of cumulative seismic moments of the aftershocks to seismic moment of the mainshock is smaller than that of most events in California. The average Q_β values along the paths from the hypocenters of the Sierra Madre and the Pasadena earthquake to PAS are about 130 and 80 respectively. The 1992 Landers earthquake is the largest event to have occurred in Southern California since 1952. We examined the waveforms of the aftershocks recorded at PFO TERRAscope station to see the correlation of the waveform and mechanisms determined from surface wave inversion. Since the depths of the events are usually not determined very well, the amplitude ratio of surface wave to body wave was used to examine the accuracy of the depths determined with various methods. Most of the events which occurred to the south of the mainshock epicenter have similar waveforms and mechanisms. Only a few events occurred to the north of the mainshock epicenter where large slip occurred during the mainshock. These events have dissimilar waveforms and mechanisms. A near vertical distribution of the aftershock extending to a depth of 15 km, or even deeper is found at about 18 km to the south of the Landers earthquake epicenter. About 72% of the total energy of the aftershocks were released from the region to the south of the mainshock epicenter. The ratio of cumulative seismic moment of the aftershock to that of mainshock is less than 1/100.