Regional soil erosion in response to land use and increased typhoon frequency and intensity,Taiwan

Reservoir sedimentation data and sediment yields from Taiwanese rivers show increased soil erosion in response to both 20th century changes in land use and a more recent increase in typhoon frequency and intensity. Decadal variations of up to 5- to 20-fold in suspended-sediment rating curves demonstrate supply-limited transport and correspond to increased sediment delivery from hillslopes due to changes in land use, regional ground shaking during the Chi-Chi earthquake, and post-2000 changes in typhoon frequency and intensity. While accelerated erosion in central Taiwan after the Chi-Chi earthquake has been documented previously, our results show that periods of increased upland erosion also occurred earlier, in response to 20th century changes in land use. Analyses of rainfall records and typhoon frequency for the period 1900–2009 further point to an island-wide increase in erosion rates corresponding to increased typhoon frequency and intensity after 1990.     

Changes in hydrogeological properties of the River Choushui alluvial fan aquifer due to the 1999 Chi-Chi earthquake, Taiwan

Changes in hydrogeological properties of the River Choushui alluvial fan aquifer before and after the 1999 Chi-Chi earthquake, Taiwan, have been identified using pumping tests. Three wells, SH2, YL2 and SC2, located in a compressional zone with high coseismic groundwater levels, were tested. The threshold of the aquifer deformation with respect to transmissivity (T) is greater than that with respect to storage coefficient (S). Decreases in the post-earthquake S are approximately 60% at SH2 and SC2, indicating aquifer compression after the Chi-Chi earthquake. Changes in the post-earthquake T range from 61% increase to 0.8% decrease. Moreover, results from anisotropy analysis of T at SC2 further illustrate that normal stresses induced by the Chi-Chi earthquake have consolidated soil particles. Soil particles dilated laterally after the earthquake, resulting in an increase of the equivalent T. The changes in hydrogeological properties have a considerable influence on spatiotemporal fluid pressure and horizontal groundwater movement, resulting in different amounts of drawdown during post-earthquake pumping.     

Impacts of September 21, 1999 Chi-Chi earthquake on the characteristics of gully-type debris flows in central Taiwan

Debris flows are more frequent in central Taiwan, because of its mountainous geography. For example, many debris flows were induced by Typhoon Herb in 1996. The Chi-Chi earthquake with a magnitude of 7.3, which took place in 1999 in central Taiwan, induced many landslides in this region. Some landslides turned into debris flows when Typhoon Toraji struck Taiwan in 2001. This study investigates the characteristics of the gullies where debris flows have occurred for a comparison. Aerial photos of these regions dated in 1997 (before the earthquake) and 2001 (after the earthquake) are used to identify the occurrence of gully-type debris flows. A Geographic Information System (GIS) is applied to acquire hydrological and geomorphic characteristics: stream gradient, stream length, catchment gradient, catchment area, form factor, and geology unit of these gullies. These characteristics in different study regions are presented in a statistical approach. The study of how strong ground motion affects the debris flows occurrence is conducted. The characteristics of the debris flow gullies triggered by typhoons before and after the Chi-Chi earthquake are quantitatively compared. The analysis results show that a significant transformation in the characteristics was induced by the Chi-Chi earthquake. In general, the transformation points out a lower hydrological and geomorphic threshold to trigger debris flows after the Chi-Chi earthquake. The susceptibility of rock units to strong ground motion is also examined. The analysis of debris flow density and accumulated rainfall in regions of different ground motion also reveal that the rainfall threshold decreases after the Chi-Chi earthquake.     

Relationships between typhoon types and debris flow disasters in Taiwan

Frequent debris flow disasters caused by heavy precipitation during the annual typhoon season are some of the most serious disasters in Taiwan. This study is on the debris flow disasters associated with the typhoons that hit Taiwan between 1986 and 2004. Typhoon data and records of debris flow disasters available for Nantou and Hualien counties in Taiwan were analyzed. The paths and rainfall characteristics of typhoons were found to have a great effect on the debris flows at these locations. Accordingly, the typhoons were grouped into four major types based on their paths and related disasters. The relationships between rainfall intensity and accumulation and debris flow are discussed for the four major typhoon types. The information may form the basis for providing useful indicators for disaster management.     

Seismic triggering of atmospheric variables prior to the major earthquakes in the Middle East within a 12-year time-period of 2002–2013

Several studies have reported the increased values of surface-latent heat flux (SLHF) and rainfall events prior to the earthquakes as the seismic precursors. In order to investigate the variation of two mentioned atmospheric variables, we analyzed 39 major earthquakes in the Middle East based on the time series of NASA remote sensing data. On this basis, we observed that accumulated rainfall and SLHF of about 29 earthquakes were higher than 10 mm and 50 W/m², respectively (~75 %), over 3–23 days prior to the main shock of major earthquakes. We believed that the records of atmospheric variables are the consequence of a seismic-triggered chain including of air ionization, surface-latent heat exhalation, water vapor condensation and subordinate rainfall as the atmospheric responses to lithospheric motions. This seismic triggering in the Middle East has averagely caused to accumulated rainfall of ~35 mm and maximum SLHF of ~115 W/m² over the 3–23 days prior to 39 major earthquakes. To investigate of spatial correlation between earthquakes and atmospheric variations, we clustered 39 major earthquakes in eight seismological regions. Then, we estimated the moderate and strong correlation (R ²) between preceding times of earthquakes with magnitude of major earthquakes and their hypocenter depth equal to 0.48 and 0.68, respectively.     

Increase in basin sediment yield from landslides in storms following major seismic disturbance

The landslide area along the Tachia River catchment of central Taiwan was investigated using the remote sensing images of various typhoon and earthquake events taken from 1996 to 2004 and the sediment discharge measured at hydrometric stations. Our findings indicate that 88% of the coseismic landslides triggered by the 1999 Chi-Chi earthquake were first-time occurrences. After the Chi-Chi earthquake, 59% of the landslide area was reactivated during typhoon Toraji and 66% during typhoon Mindulle. The landslides prone to reactivation were on the formations with closely spaced discontinuities. It is suggested that further rock-mass defects in the formations were added or opened by the earthquake, and that the landslide areas continued to extend until the end of 2004. Post-1999 rainstorms delivered large amounts of colluvial sediment into the main channel, leading to a 2-fold increase in post-seismic sediment discharge while the precipitation was only half that of the pre-earthquake rate.     

Locating, monitoring, and characterizing typhoon-linduced landslides with real-time seismic signals

Landslides induced by typhoon Morakot during its passage across Taiwan on 7–9 Aug 2009 claimed more than 700 lives and caused heavy economic loss. Unlike earthquake monitoring, precise locations of landslides could not be determined in near-real time because their seismic phases are difficult to identify. Here, we show that large, damaging landslide events are characterized seismically by a distinct waveform pattern of frequent intermixes of P and S waves over a time window of several tens of seconds. The predominant frequency band during these time windows ranges from 0.5 to 5?Hz. The high-frequency content is clearly deficient relative to that of local earthquakes by about one to two orders. We also demonstrate that large landslide events can be located and monitored with algorithms specifically designed for real-time seismic applications. This near-real-time monitoring capability would be particularly useful for emergency responders and government organizations to coordinate effective relief-and-rescue operations.     

A Climatology Model for Forecasting Typhoon Rainfall in Taiwan

The continuous torrential rain associated with a typhoon often caused flood, landslide or debris flow, leading to serious damages to Taiwan. Thus, a usable scheme to forecast rainfall amount during a typhoon period is highly desired. An analysis using hourly rainfall amounts taken at 371 stations during 1989–2001 showed that the topographical lifting of typhoon circulation played an important role in producing heavier rainfall. A climatology model for typhoon rainfall, which considered the topographical lifting and the variations of rain rate with radius was then developed. The model could provide hourly rainfall at any station or any river basin for a given typhoon center. The cumulative rainfall along the forecasted typhoon track was also available. The results showed that the R² value between the model estimated and the observed cumulative rainfall during the typhoon period for the Dan-Shui (DSH) and Kao-Ping (KPS) River Basins reached 0.70 and 0.81, respectively. The R² values decreased slightly to 0.69 and 0.73 if individual stations were considered. However, the values decreased significantly to 0.40 and 0.51 for 3-hourly rainfalls, indicating the strong influence of the transient features in producing the heavier rainfall. In addition, the climatology model can only provide the average conditions. The characteristics in individual typhoons should be considered when applying the model in real-time operation. For example, the model could give reasonable cumulative rainfall amount at DSH before Nakri (2002) made landfall on Taiwan, but overestimated the rainfall after Nakri made landfall and weakened with significant reduction in convection.     

Seismic behavior in central Taiwan: Response to stress evolution following the 1999 Mw 7.6 Chi-Chi earthquake

Following the 1999 M_w 7.6 Chi-Chi earthquake, a large amount of seismicity occurred in the Nantou region of central Taiwan. Among the seismic activities, eight M_w ⩾ 5.8 earthquakes took place following the Chi-Chi earthquake, whereas only four earthquakes with comparable magnitudes took place from 1900 to 1998. Since the seismicity rate during the Chi-Chi postseismic period has never returned to the background level, such seismicity activation cannot simply be attributed to modified Omori’s Law decay. In this work, we attempted to associate seismic activities with stress evolution. Based on our work, it appears that the spatial distribution of the consequent seismicity can be associated with increasing coseismic stress. On the contrary, the stress changes imparted by the afterslip; lower crust–upper mantle viscoelastic relaxation; and sequent events resulted in a stress drop in most of the study region. Understanding seismogenic mechanisms in terms of stress evolution would be beneficial to seismic hazard mitigation.     

Track analysis and storm surge investigation of 2017 Typhoon Hato: were the warning signals issued in Macau and Hong Kong timed appropriately?

ABSTRACT

This study investigates the storm surge caused by Typhoon Hato, which severely affected Macau, Hong Kong, and other coastal cities in China on 23 August 2017. A typhoon and storm surge coupling model demonstrated that the maximum storm surge height reached nearly 2.5?m along the coast of Macau, while that in Hong Kong was slightly below 2?m. Furthermore, a field survey of urban flooding revealed evidence of a 2.25-m inundation in downtown Macau and a 0.55-m inundation on Lantau Island, Hong Kong, which were likely exacerbated by a combination of storm surge, heavy rainfall, and surface water runoff over a complex hilly terrain. Significant wave overtopping and runup also occurred in beach and port areas. A typhoon track analysis confirmed that several comparably strong typhoons have followed similar ESE to WNW trajectories and made landfall in the Pearl River Delta in the last few decades. Although Hato was not the strongest of these storms, its forward speed of about 32.5?km/h was remarkably faster than those of other comparable typhoons. Higher levels of storm signal warnings were issued earlier in Hong Kong than in Macau, raising questions about the appropriate timing of warnings in these two nearby areas. Our analysis of the storm’s pattern suggests that both regions’ decisions regarding signal issuance could be considered reasonable or at least cannot be simply blamed, given the rapid motion and intensification of Hato and the associated economic risks at stake.     

Analysis of a spatial distribution of landslides triggered by the 2004 Chuetsu earthquakes of Niigata Prefecture, Japan

On October 23, 2004, a series of powerful earthquakes with a maximum M _w = 6.6 located near the western coast of northern Honshu struck parts of northern Japan, particularly Niigata Prefecture; these earthquakes were known as the Chuetsu event. Thousands of landslides, as a secondary geotechnical hazard associated with these earthquakes, were triggered over a broad area; these landslides were of almost all types. The purpose of this study was to detect correlations between landslide occurrence with geologic and geomorphologic conditions, slope geometry, and earthquake parameters using two indexes based on Geographic Information Systems (GIS). In the study area, the landslide–area ratio (LAR), which is defined as the percentage of the area affected by landslides, was 2.9%, and the landslide concentration (LC), the number of landslides per square kilometer, was 4.4 landslides/km², which is much more than other reported cases of seismic activity with the same magnitude. This was possibly due to heavy rainfall just before the Chuetsu earthquakes. Statistical analyses show that LAR has a positive correlation with slope steepness and distance from the epicenter, while LC is inversely correlated with distance from the epicenter. The Wanazu Formation had the most concentrated landslide activity, followed by the Kawaguchi, Ushigakubi, Shiroiwa and Oyama Formations, although the Wanazu Formation occupied only 4.5% of the total area of geological units. With 8.2% of the area affected by seismic landslides, the Kawaguchi Formation had the highest LAR. It was followed by the Shiroiwa, Ushigakubi and Wanazu Formations with LAR ranging from 4.6% to 6.0%. For lots of geological subunits, landslides are more frequent in a range of slope angles between 15° and 40°. The susceptibility to landsliding of each geologic unit was thus evaluated to correlate with slope steepness. It was also noted that the effects of the earthquakes were made far worse by antecedent rainfall conditions induced by a␣typhoon, and further research emphasizing the role of antecedent rainfall was discussed.     

Meteorological triggering of earthquake swarms at Mt. Hochstaufen, SE-Germany

A growing body of evidence suggests that fluids are intimately linked to a variety of faulting processes. Yet, the particular mechanisms through which fluids and associated parameters influence the stress regime and thus the seismicity of a particular area are not well understood.We carry out a study of the spatio-temporal behavior of earthquakes, fluid-related parameters (groundwater levels) and meteorological observables (precipitation) in the swarm earthquake area of Bad Reichenhall, southeastern Germany. The small volume in which the earthquakes take place, almost yearly occurring earthquake swarms and a permanent, seismo-meteorological monitoring network, provide nearly controlled experimental conditions to study the physics of earthquake swarms and to infer characteristic properties of the seismogenic crust.In this paper we (1) describe this fairly unique study area in terms of geology, seismicity and atmospheric conditions; (2) present two cases of earthquake swarms that seem to follow above-average rainfall events; and (3) examine the observed migration of hypocenters with a simple pore pressure diffusion model.We find significant correlation of seismicity with rainfall and groundwater level increase, and estimate an average hydraulic diffusivity of D = 0.75 ± 0.35 m²/s for Mt. Hochstaufen in 2002.     

Coupling typhoon rainfall forecasting with overland-flow modeling for early warning of inundation

Taiwan suffers from an average of three or four typhoons annually, and the inundation caused by the heavy precipitation that is associated with typhoons frequently occurs in lowlands and floodplains. Potential inundation maps have been widely used as references to set up non-structural strategies for mitigating flood hazards. However, spatiotemporal rainfall distributions must be addressed to improve the accuracy of inundation forecasting for emergency response operations. This study presents a system for 24-h-ahead early warning of inundation, by coupling the forecasting of typhoon rainfall with the modeling of overland flow. A typhoon rainfall climatology model (TRCM) is introduced to forecast dynamically the spatiotemporal rainfall distribution based on typhoon tracks. The systematic scheme for early warning of inundation based on the spatiotemporal downscaling of rainfall and 2D overland-flow modeling yields not only the extent of inundation, but also the time to maximum inundation depth. The scheme is superior to traditional early warning method referring to the maximum extent and depth of inundation determined from conditional uniform rainfall. Analytical results show that coupling TRCM with an overland-flow model yields satisfactory inundation hydrographs for warning of the extent and peak time of inundation. This study also shows that the accuracy of forecasting spatiotemporal rainfall patterns determines the performance of inundation forecasting, which is critical to emergency response operations.     

The Potential of Fuzzy Multi-objective Model for Rainfall Forecasting from Typhoons

This study applies the fuzzy multi-objective approach to forecast short-term (around 24 h) typhoon rainfall, which can be implemented without much background meteorological knowledge. The physical characteristics of 40 typhoons, including route, central pressure, central velocity and cyclonic radius, were used as the data set. The fuzzy multi-objective method mined information from the database to forecast both the depth and pattern of rainfall, which were then combined to estimate a cumulative rainfall curve. The results of calibration with reference to 40 historical typhoon events and the results of validation using another five typhoon events indicate that the proposed model has the potential to forecast short-term cumulative rainfall curves if more variables can be included and more historical typhoon events can be collected to enlarge the database.     

Variability of impact of earthquake on debris-flow triggering conditions: case study of Chen-Yu-Lan watershed,Taiwan

This study investigates the variations in the critical conditions for debris-flow occurrence before and after the Chi-Chi earthquake in the Chen-Yu-Lan watershed, Taiwan. Topographical and rainfall parameters such as the gully gradient, drainage area, rainfall intensity, cumulative rainfall, and rainfall duration in the Chen-Yu-Lan watershed were used to analyze the conditions of debris-flow occurrence over the past 25 years. A recovery equation was proposed on the basis of rainfall parameters and used to determine the variations in the critical line of rainfall that trigger debris flow after the earthquake and to evaluate the recovery period required for the rainfall threshold of debris-flow occurrence after the earthquake to return to that before the earthquake in the watershed. The critical line for the runoff parameter versus gully gradient in the watershed was also presented.     

In search of earthquake-related hydrologic and chemical changes along Hayward Fault

Flow and chemical measurements have been made about once a month, and more frequently when required, since 1976 at two springs in Alum Rock Park in eastern San Jose, California, and since 1980 at two shallow wells in eastern Oakland in search of earthquake-related changes. All sites are on or near the Hayward Fault and are about 55 km apart. Temperature, electric conductivity, and water level or flow rate were measured in situ with portable instruments. Water samples were collected for later chemical and isotopic analyses in the laboratory. The measured flow rate at one of the springs showed a long-term decrease of about 40% since 1987, when a multi-year drought began in California. It also showed several increases that lasted a few days to a few months with amplitudes of 2.4 to 8.6 times the standard deviations above the background rate. Five of these increases were recorded shortly after nearby earthquakes of magnitude 5.0 or larger, and may have resulted from unclogging of the flow path and increase of permeability caused by strong seismic shaking. Two other flow increases were possibly induced by exceptionally heavy rainfalls. The water in both wells showed seasonal temperature and chemical variations, largely in response to rainfall. In 1980 the water also showed some clear chemical changes unrelated to rainfall that lasted a few months; these changes were followed by a magnitude 4 earthquake 37 km away. The chemical composition at one of the wells and at the springs also showed some longer-term variations that were not correlated with rainfall but possibly correlated with the five earthquakes mentioned above. These correlations suggest a common tectonic origin for the earthquakes and the anomalies. The last variation at the affected well occurred abruptly in 1989, shortly before a magnitude 5.0 earthquake 54 km away.     

Triggered/migrated seismicity due to the 2001 M w7.7 Bhuj earthquake, Western India

Paper describes triggered seismicity to 200?km distance and for a decade due to the 2001 M _w7.7 Bhuj earthquake. The Kachchh region is seismically one of the most active intraplate regions of the World due to the occurrence of two large earthquakes 1819 (M _w7.8) and 2001 (M _w7.7). Though, it has high hazard but was known to have low seismicity in view of the occurrence of fewer smaller shocks. However, the status seems to have changed after 2001. Besides the strong aftershock activity for over a decade, seismicity has spread to nearby faults in Kachchh peninsula and at several places southward for 200?km distance in Saurashtra peninsula. Beyond the rupture zone of the 2001 Bhuj earthquake, more than 40 mainshocks of M _w?~?3?C5 have occurred at 20 different locations, which is unusual. The increased seismicity is inferred to be caused by stress perturbation due to the 2001 Bhuj earthquake by viscoelastic process. In Saurashtra, over and above the viscoelastic stress increase, the transient stress increase by water table rise in monsoons seems to be affecting the timing of mainshocks and associated sequences of earthquakes.     

Rainfall thresholds for debris flow initiation in the Wenchuan earthquake-stricken area,southwestern China

The Wenchuan earthquake-stricken area is frequently hit by heavy rainfall, which often triggers sediment-related disasters, such as shallow landslides, debris flows, and related natural events, sometimes causing tremendous damage to lives, property, infrastructure, and environment. The assessment of the rainfall thresholds for debris flow occurrence is very important in order to improve forecasting and for risk management. In the context of the Wenchuan earthquake-stricken area, however, the rainfall thresholds for triggering debris flows are not well understood. With the aim of defining the critical rainfall thresholds for this area, a detailed analysis of the main rainstorm events was carried out. This paper presents 11 rainfall events that induced debris flows which occurred between 2008 and 2012 after the Wenchuan earthquake. The rainfall thresholds were defined in terms of mean rainfall intensity I, rainfall duration D, and normalized using the mean annual precipitation (MAP). An ID threshold and a normalized I _MAP D threshold graph could be set up for the Wenchuan earthquake-stricken area which forms the lower boundary of the domain with debris flow-triggering rainfall events. The rainfall threshold curves obtained for the study area were compared with the local, regional, and global curves proposed by various authors. The results suggest that debris flow initiation in the study area almost requires a higher amount of rainfall and greater intensity than elsewhere. The comparison of rainfall intensity prior to and after the earthquake clearly indicates that the critical rainfall intensity necessary to trigger debris flows decreased after the earthquake. Rainfall thresholds presented in this paper are generalized, so that they can be used in debris flow warning systems in areas with the same geology as the Wenchuan earthquake-stricken area.     

Future variability of droughts in three Mediterranean catchments

This study investigates the intensity change in typhoons and storm surges surrounding the Korean Peninsula under global warming conditions as obtained from the MPI_ECHAM5 climate model using the A1B series. The authors use the Cyclostationary Empirical Orthogonal Function to estimate future background fields for typhoon simulations from twenty-first-century prediction results. A series of numerical experiments applies WRF (Weather Research and Forecasting) and POM (Prinston Ocean Model) models to simulate two historical typhoons, Maemi (2003) and Rusa (2002), and associated storm surges under real historical and future warming conditions. Applying numerical experiments to two typhoons, this study found that their central pressure dropped about 19 and 17 hPa, respectively, when considering the future sea surface temperature (a warming of 3.9 °C for 100 years) over the East China Sea (Exp. 1). The associated enhancement of storm surge height ranged from 16 to 67 cm along the southern coast of the Korean Peninsula. However, when the study considered global warming conditions for other atmospheric variables such as sea-level pressure, air temperature, relative humidity, geopotential height, and wind in the typhoon simulations (Exp. 2), the intensities of the two typhoons and their associated surge heights scarcely increased compared to the results of Exp. 1. Analyzing projected atmospheric variables, the authors found that air temperatures at the top of the storm around 200 hPa increased more than those at the surface in tropical and mid-latitudes. The reduced vertical temperature difference provided an unfavorable condition in the typhoon’s development even under conditions of global warming. This suggests that global warming may not always correlate with a large increase in the number of intense cyclones and/or an increase in associated storm surges.     

Low b-value prior to the Indo-Myanmar subduction zone earthquakes and precursory swarm before the May 1995 M 6.3 earthquake

Some 455 events (m_b ⩾ 4.5) in the Indo-Myanmar subduction zone are compiled using the ISC/EHB/NEIC catalogues (1964–2011) for a systematic study of seismic precursors, b-value and swarm activity. Temporal variation of b-value is studied using the maximum likelihood method beside CUSUM algorithm. The b-values vary from 0.95 to 1.4 for the deeper (depth ⩾60 km) earthquakes, and from 0.85 to 1.3 for the shallower (depth <60 km) earthquakes. A sudden drop in the b-value, from 1.4 to 0.9, prior to the occurrence of larger earthquake(s) at the deeper depth is observed. It is also noted that the CUSUM gradient reversed before the occurrence of larger earthquakes. We further examined the seismicity pattern for the period 1988–1995 within a radius of 150 km around the epicentre (latitude: 24.96°N; longitude: 95.30°E) of a deeper event M 6.3 of May 6, 1995 in this subduction zone. A precursory swarm during January 1989 to July 1992 and quiescence during August 1992 to April 1995 are identified before this large earthquake. These observations are encouraging to monitor seismic precursors for the deeper events in this subduction zone.