Delineation of physical factors that contribute to earthquake triggering is a challenging issue in seismology. We analyze hydrological modulation of seismicity in Taiwan using groundwater level data and GNSS time series. In western Taiwan, the seismicity rate reaches peak levels in February to April and drops to its lowest values in July to September, exhibiting a direct correlation with annual water unloading. The elastic hydrological load cycle may be the primary driving mechanism for the observed synchronized modulation of earthquakes, as also evidenced by deep earthquakes in eastern Taiwan. However, shallow earthquakes in eastern Taiwan (<18 km) are anticorrelated with water unloading, which is not well explained by either hydrological loading, fluid transport, or pore pressure changes and suggests other time-dependent processes. The moderate correlation between stacked monthly trends of large historic earthquakes and present-day seismicity implies a modestly higher seismic hazard during the time of low annual hydrological loading.

We examined the performance of Taiwan earthquake model (TEM) PSHA2015. We retrospectively forecasted the seismic activities of the 2018 Hualien, Taiwan, sequence and proposed recommendations for the next generation of seismic hazard assessment for Taiwan. We confirmed the model credibility of the area sources by comparing with locations of the Hualien sequence and seismicity during the period from 2012 to 2016. We also concluded similar forecasting reliability for the smoothing model, which could be incorporated in the next generation of probabilistic seismic-hazard analysis (PSHA) as a branch of logic tree. To examine the performance of the seismogenic structure source model, we evaluated rupture probability of the Milun fault, which ruptured during the Hualien sequence. The TEM PSHA2015 model forecasts its rupture probability in 50 yrs as 53%, and the Brownian passage time (BPT) model forecasts 80%. The BPT model, considering also time-dependency rupture probability, is suggested for future hazard assessment, especially for the seismogenic structure sources with records of their last ruptures. This earthquake sequence has raised the importance of short-term seismic hazard assessment, which provides a basis for response after a devastating earthquake and/or forecasting consequence.

The southernmost portion of the Ryukyu Trench closed to Taiwan island is a potential region to generate 7.5 to 8.7 tsunami earthquakes by shallow rupture. The fault model for this potential region dips 10o northward with rupture length of 120 km and width of 70 km. The earthquake magnitude estimated by fault geometry is Mw 8.15 with 8.25 m average slip as a constrain of earthquake scenario. The heterogeneous slip distributions over rupture surface are generated by stochastic slip model, the slip spectrum with k-2 decay in wave number domain, and they are consistent with above identical seismic conditions. The results from tsunami simulation illustrate that the propagation of tsunami waves and the peak wave heights largely vary in response to the slip distribution. The wave phase changing is possible as the waves propagate, even under the same seismic conditions. The tsunami energy path is not only following the bathymetry but also depending on slip distribution. The probabilistic distributions of peak tsunami amplitude calculated by 100 different slip patterns from 30 recording stations reveal the uncertainty decreases with distance from tsunami source. The highest wave amplitude for 30 recording points is 7.32 m at Hualien for 100 different slips. Comparing with stochastic slips, uniform slip distribution will be extremely underestimated, especially in near field. In general, uniform slip assumption only represents the average phenomenon so that it will ignore possibility of tsunami wave. These results indicate that considering effect of heterogeneous slip distribution is necessary for assessing tsunami hazard and that can provide more information about tsunami uncertainty for a more comprehensive estimation.

To assess the seismic hazard with temporal change in Taiwan, we developed a new approach which combines both the Brownian passage time (BPT) model and the coulomb stress change, and implements the seismogenic source parameters by the Taiwan Earthquake Model. The BPT model was adopted to describe the rupture recurrence intervals of specific fault sources together with the time elapsed since the last fault rupture to derive their long-term rupture probability. We also evaluated the short-term seismicity rate change based on the static coulomb stress interaction between seismogenic sources. By considering these time-dependent factors, our new combined model, relative to a time-independent model, suggests an increased long-term seismic hazard in the vicinity of active faults along the western coastal plain and the Longitudinal Valley, where active faults have short recurrence intervals and long elapsed time since their last ruptures and/or short-term elevated hazard levels right after the occurrence of large earthquakes due to the stress triggering effect. The stress enhanced by the 6 February 2016 M-L 6.6 Meinong earthquake also significantly increased the rupture probabilities of several neighboring seismogenic sources in southwestern Taiwan and raised the hazard level for the near future. Our approach drew upon the advantage of incorporating both long-and short-term models to provide time-dependent earthquake probability constraints. This new model provides more insight than any other models for Taiwan. Thus, it offers decision makers and public officials an adequate basis for rapid evaluations of consequent hazards and for responses to future emergency scenarios such as victim relocation and sheltering.

On 6 February 2016 (UTC 19:57), the Meinong earthquake with Richter magnitude (M-L) 6.6 struck southern Taiwan and caused hundreds of damaged buildings, resulting in 117 casualties. We investigated the relationship between the damaged buildings and the ground motion in the forms of peak ground acceleration (PGA), peak ground velocity (PGV), pseudo-spectral acceleration (SA) at 0.3s (SA0.3), 1.0 s (SA1.0), and shaking duration to identify which ground motion parameter most represents building damage. PGV and SA1.0 present better correlation with consequent damage. The Intensity converted from PGV presents better correlation to the damage than PGA. We disaggregated the TEM PSHA2015 hazard contribution to the Meinong earthquake damage region (Southern Taiwan) from different seismic source typologies to clarify the seismic source contributing to the hazard. The hazards contributed by the Meinong earthquake were 16, 26, and 23% for PGA, SA0.3, and SA1.0, respectively. The predicted seismic hazard source areas were 38, 61, and 75% for PGA, SA0.3, and SA1.0, respectively, for the PSHA with a return period of 475 years. This result indicates that the 2016 Meinong earthquake did partially diminish the seismic hazard potential in southern Taiwan. However, more than about 80% of the seismic hazard potential, especially the fault sources were not yet released. These values suggest that the seismic hazard potential in southern Taiwan remains high regardless of the 2016 Meinong earthquake.

Ground motions from normal-faulting earthquakes are generally considered to be smaller than those of strike-slip and thrust events. On 11 April 2011 a crustal normal-faulting earthquake [the Fukushima earthquake (M_w 6.6)] occurred in Eastern Japan. The peak ground acceleration (PGA) observed was considerably higher than the predictions of several ground-motion prediction equations (GMPEs), which were derived mainly from thrust or strike-slip earthquakes. In northeast Taiwan, the tectonic structure of the Ryukyu Arc and the Okinawa Trough typically entail normal-faulting earthquakes. Because of the normal-faulting earthquakes relevance to ground motions and nuclear power plant sites in northeast Taiwan, we evaluated the impact of the ground motion of normal-faulting earthquakes in offshore northeast Taiwan using a newly constructed attenuation relationship for PGA and pseudo-spectral acceleration (Sa). We collected 832 records from 13 normal-faulting earthquakes with focal depths of less than 50 km. The moment magnitude (M_w) of the 13 events was between 4 - 6. The Sa and PGA of normal-faulting earthquakes offshore northeast Taiwan determined with the newly constructed attenuation relationship were higher and lower, respectively, than those obtained using attenuation equations commonly used in the Taiwan subduction zone.

The earthquake slip distribution self-similarity is investigated in this study. We complied finite fault slip models for earthquakes in the Taiwan orogenic belt and global earthquakes to determine the slip distribution self-similarity. Forty-one earthquakes (19 Taiwan earthquakes and 22 global earthquakes) in the M-w = 4.6 - 8.9 magnitude range were examined. The fault slip exhibited self-similar scaling between the rupture slip and area. The average area ratio (R-s) and slip ratio (R-d) follows a scaling of R-s =10(a-n(Rd)) Slip self-similarity implies that a fault rupture exhibits fractal behavior. The scaling exponent can be considered as a measure for the roughness degree of the slip distribution on the fault surface. This study suggests that the slip distribution for large earthquakes (M-w > 7.0) tends to have a more homogeneous slip. Scaling exponents can provide insight into earthquake rupture mechanics and the scaling of heterogeneous slips on the fault surface provides a basis for ground motion simulation for a finite fault for an earthquake scenario, particularly for near-fault motion.

Near-fault ground motion is a key to understanding the seismic hazard along a fault and is challenged by the ground motion prediction equation approach. This paper presents a developed stochastic-slip-scaling source model, a spatial stochastic model with slipped area scaling toward the ground motion simulation. We considered the near-fault ground motion of the 1999 Chi-Chi earthquake in Taiwan, the most massive near-fault disastrous earthquake, proposed by Ma et al. (2001) as a reference for validation. Three scenario source models including the developed stochastic-slip-scaling source model, mean-slip model and characteristic-asperity model were used for the near-fault ground motion examination. We simulated synthetic ground motion through 3D waveforms and validated these simulations using observed data and the ground-motion prediction equation (GMPE) for Taiwan earthquakes. The mean slip and characteristic asperity scenario source models over-predicted the near-fault ground motion. The stochastic-slip-scaling model proposed in this paper is more accurately approximated to the near-fault motion compared with the GMPE and observations. This is the first study to incorporate slipped-area scaling in a stochastic slip model. The proposed model can generate scenario earthquakes for predicting ground motion.

The Taiwan Earthquake Model (TEM) has had a new probabilistic seismic-hazard analysis (PSHA) model for determining the probability of exceedance (PoE) of the ground motion over a specified period in Taiwan. We conducted several tests to investigate the adequacy of the seismic source parameters adopted by the TEM in 2015 (TEM PSHA2015). The observed maximal peak ground accelerations (PGAs) of the M-L >4: 0 mainshocks from 1993 to 2015 were used to test the predicted PGA from areal and subduction zone sources based on a time-independent Poisson distribution. This comparison excluded the observations from the 1999 Chi-Chi earthquake, because this was the only earthquake associated with the identified active fault in the testing period. We used tornado diagrams to analyze the sensitivities of these source parameters to the ground-motion values of the PSHA. This study showed that the predicted PGA for a 63% PoE in the 23-year period corresponded to observations, confirming the applicability of the parameters to areal and subduction zone sources. We adopted disaggregation analysis to determine the contribution of each seismic source to the hazard for six metropolitan cities in Taiwan. Sensitivity tests on the seismogenic structure parameters indicated that slip rate and maximum magnitude are the dominant factors in the TEM PSHA2015. For densely populated faults in southwestern Taiwan, the assessed hazard level is more sensitive to the maximum magnitude of earthquakes than it is to the slip rate of active faults, creating concern for the possibility of larger earthquakes due to multiple-segment ruptures in this area, which has not yet been considered in the TEM PSHA2015. The source category disaggregation also suggested a seismic hazard over long periods in northern Taiwan due to subduction zone sources.

The Taiwan Earthquake Model (TEM) was established to assess the seismic hazard and risk for Taiwan by considering the social and economic impacts of various components from geology, seismology, and engineering. This paper gives the first version of TEM probabilistic seismic hazard analysis for Taiwan in these aspects. We named it TEM PSHA2015. The model adopts the source parameters of 38 seismogenic structures identified by TEM geologists. In addition to specific fault source-based categorization, seismic activities are categorized as shallow, subduction intraplate, and subduction interplate events. To evaluate the potential ground-shaking resulting from each seismic source, the corresponding ground-motion prediction equations for crustal and subduction earthquakes are adopted. The highest hazard probability is evaluated to be in Southwestern Taiwan and the Longitudinal Valley of Eastern Taiwan. Among the special municipalities in the highly populated Western Taiwan region, Taichung, Tainan, and New Taipei City are evaluated to have the highest hazard. Tainan has the highest seismic hazard for peak ground acceleration in the model based on TEM fault parameters. In terms of pseudo-spectral acceleration, Tainan has higher hazard over short spectral periods, whereas Taichung has higher hazard over long spectral periods. The analysis indicates the importance of earthquake-resistant designs for low-rise buildings in Tainan and high-rise buildings in Taichung.

The duration of strong shaking is particularly important for assessing building performance, potential landslides and liquefaction hazards. The results of this investigation can potentially help reduce related fatalities and economic losses. In this study, we analyzed the acceleration seismograms of the Taiwan strong motion network to characterize the strong shaking duration associated with earthquake sources, propagation paths and site effects. This study proposes a new definition for the strong shaking duration called “effective shaking duration” (ESD), which considers the amplitude and radiation energy decays. We first consider the window of a time series during which the amplitude is ≥0.01 g, and we then defined the ESD as the length of the interval of the dissipated energy within 5–95 % of the total energy during this time frame. We calculated the strong shaking duration for 495 inter-plate events with magnitudes of M L > 5.0 and focal depths <50 km in the Taiwan region from 1994 to 2012. Using a nonlinear regression procedure, we thus obtained an empirical equation for strong shaking durations. The equation is a function of earthquake magnitude, distance and site conditions, which are defined by the V s 30 value (the S-wave velocity structure of the top 30 m of the site). The results indicate that the shaking durations significantly increase with magnitude and also decrease with distance and V s 30. Compared with empirical equations from global datasets, our empirical equation is applicable to earthquakes in other regions and will produce smaller but more applicable duration values for smaller earthquakes. However, for larger events, our ESD values are comparable with those derived from other definitions (e.g., significant duration). Although the empirical relationship is mainly based on Taiwanese events, in view of the massive dataset, this empirical equation could provide important information to the global community regarding the ground shaking duration estimation in the ground motion prediction of future earthquakes.

In this study, we investigated the quasi-periodicity of large avalanches using a new modification of sandpile models, i.e., the long-range connective sandpile (LRCS) model. The LRCS model considers the random distant connection between two separated, instead of neighboring, cells and shows interesting precursory behavior before large avalanches. We explore the statistics of recurrence intervals between large events and find a strong dependence on the size L of the sandpile. In the LRCS model, the average recurrence interval W of large avalanches follows the relationship . Having the recurrence intervals of many earthquake fault systems around the world, we propose an empirical rule between those intervals and the corresponding earthquakes' magnitudes and provide evidence of the quasi-periodic behavior of natural earthquakes.

In this paper we consider the statistics of the aftershock sequence of the m = 7.65 20 September 1999 Chi–Chi, Taiwan earthquake. We first consider the frequency-magnitude statistics. We find good agreement with Gutenberg–Richter scaling but find that the aftershock level is anomalously high. This level is quantified using the difference in magnitude between the main shock and the largest inferred aftershock \( {{\Updelta}}m^{ *}. \) Typically, \( {{\Updelta}}m^{ *} \) is in the range 0.8–1.5, but for the Chi–Chi earthquake the value is \( {{\Updelta}}m^{ *} \) = 0.03. We suggest that this may be due to an aseismic slow-earthquake component of rupture. We next consider the decay rate of aftershock activity following the earthquake. The rates are well approximated by the modified Omori’s law. We show that the distribution of interoccurrence times between aftershocks follow a nonhomogeneous Poisson process. We introduce the concept of Omori times to study the merging of the aftershock activity with the background seismicity. The Omori time is defined to be the mean interoccurrence time over a fixed number of aftershocks.

The Regional Earthquake Likelihood Models (RELM) test was the first competitive comparison of prospective earthquake forecasts. The test was carried out over 5 years from 1 January 2006 to 31 December 2010 over a region that included all of California. The test area was divided into 7682 0.1°x0.1° spatial cells. Each submitted forecast gave the predicted numbers of earthquakes Nemi larger than M=4.95 in 0.1 magnitude bins for each cell. In this paper we present a method that separates the forecast of the number of test earthquakes from the forecast of their locations. We first obtain the number Nem of forecast earthquakes in magnitude bin m. We then determine the conditional probability λemi=Nemi/Nem that an earthquake in magnitude bin m will occur in cell i. The summation of λemi over all 7682 cells is unity. A random (no skill) forecast gives equal values of λemi for all spatial cells and magnitude bins. The skill of a forecast, in terms of the location of the earthquakes, is measured by the success in assigning large values of λemi to the cells in which earthquakes occur and low values of λemi to the cells where earthquakes do not occur. Thirty-one test earthquakes occurred in 27 different combinations of spatial cells i and magnitude bins m, we had the highest value of λemi for that mi cell. We evaluate the performance of eleven submitted forecasts in two ways. First, we determine the number of mi cells for which the forecast λemi was the largest, the best forecast is the one with the highest number. Second, we determine the mean value of λemi for the 27 mi cells for each forecast. The best forecast has the highest mean value of λemi. The success of a forecast during the test period is dependent on the allocation of the probabilities λemi between the mi cells, since the sum over the mi cells is unity. We illustrate the forecast distributions of λemi and discuss their differences. We conclude that the RELM test was successful in illustrating the choices required when a forecast of the location of a future earthquake is made.

The Long-Range Connective Sandpile (LRCS) model was applied to the Italian seismicity. The Hurst exponent and the power-law slope of the frequency-size distributions for the avalanches in the LRCS model and for earthquakes in Italy are investigated. This study shows the transition of the correlation coefficient between b and H values with different calculation window length. The result shows similar behaviors in the LRCS model and the Italy catalogue. The negative correlation between b and H values can be clearly seen when an appropriate window length is employed for various time series. We suggest that the negative relationship is caused by the increasing correlation length as the system accumulates enough energy. Also the calculation window length is an important index to display the intensity of the negative correlation between these two exponents. The appropriate window length can be related to the period time for the avalanches with various sandpile and seismicity time series. editor's choice Copyright (C) EPLA, 2012

We consider implications of the Regional Earthquake Likelihood Models (RELM) test results with regard to earthquake forecasting. Prospective forecasts were solicited for M≥4.95 earthquakes in California during the period 2006–2010. During this period 31 earthquakes occurred in the test region with M≥4.95. We consider five forecasts that were submitted for the test. We compare the forecasts utilizing forecast verification methodology developed in the atmospheric sciences, specifically for tornadoes. We utilize a “skill score” based on the forecast scores λfi of occurrence of the test earthquakes. A perfect forecast would have λfi=1, and a random (no skill) forecast would have λfi=2.86×10-3. The best forecasts (largest value of λfi) for the 31 earthquakes had values of λfi=1.24×10-1 to λfi=5.49×10-3. The best mean forecast for all earthquakes was λ̅f=2.84×10-2. The best forecasts are about an order of magnitude better than random forecasts. We discuss the earthquakes, the forecasts, and alternative methods of evaluation of the performance of RELM forecasts. We also discuss the relative merits of alarm-based versus probability-based forecasts.

Acoustic emissions prior to rupture indicate precursory damage. Laboratory studies of frictional sliding on model faults feature accelerating rates of acoustic emissions prior to rupture. Precursory seismic emissions are not generally observed prior to earthquakes. To address the problem of precursory damage, we consider failure in a fiber-bundle model. We observe a clearly defined nucleation phase followed by a catastrophic rupture. The fibers are hypothesized to represent asperities on a fault. Two limiting behaviors are the equal load sharing p = 0 (stress from a failed fiber is transferred equally to all surviving fibers) and the local load sharing p = 1 (stress from a failed fiber is transferred to adjacent fibers). We show that precursory damage in the nucleation phase is greatly reduced in the local-load sharing limit. The local transfer of stress from an asperity concentrates nucleation, restricting precursory acoustic emissions (seismic activity).

We propose a generic negative correlation between power-law scaling and Hurst exponents for size/magnitude data from real and synthetic earthquakes. The synthetic earthquakes were produced from a conceptual earthquake model, the long-range connective sandpile (LRCS) model. The LRCS model is a new modification of sandpile models that considers the random distant connection between two separated cells instead of neighboring cells. We calculated the Hurst exponent H and the power-law scaling exponent B for event size data in the LRCS model. We systematically explored the relationships between these two exponents (H and B) and conclusively obtained a negative correlation between H and B. We also found this negative correlation for real earthquake data registered in the Taiwan Central Weather Bureau (CWB) catalog. This negative correlation has not been demonstrated previously for real seismicity, although it has been frequently suggested.

We analyze the Hurst exponent H and a power‐law exponent B obtained from frequency‐size distributions of avalanche events in the long‐range connective sandpile (LRCS) model and study the relation between those two exponents. The LRCS model is introduced by considering the random distant connection between two separated cells. We find that the B‐values typically reduce prior to large avalanches while the H‐values increase. Both parameters appear precursory phenomena prior to large avalanche events. Most importantly, we show that the LRCS model can demonstrate an interesting negative correlation between the B‐ and H‐values, which has been frequently implied in observations of seismicity and firstly verified in our present simulations.