Santi Pailoplee

Deterministic and probabilistic seismic hazard analyses in Thailand and adjacent areas using active fault data

Seismic hazards in Thailand and adjacent areas were analyzed mainly on the basis of geological fault data. We identified 55 active fault zones using remote-sensing data on earthquake source parameters derived from both active fault data and earthquake catalogues. We selected strong ground-motion attenuation models by comparing the application of several candidate models with strong ground-motion data recorded in Thailand. Both deterministic (DSHA) and probabilistic (PSHA) approaches were used—DSHA for the design of critical construction and PSHA for the design of non-critical construction. We also applied two frequency-magnitude models in the PSHA approach: the exponential magnitude distribution model and the characteristic earthquake model. The seismic hazard results obtained using the deterministic and probabilistic approaches are not equivalent. The resulting DSHA map reveals extremely high seismic hazard levels in some areas of Thailand and in surrounding countries, while the PSHA map reveals a seismic hazard distribution similar to that of the DSHA but with lower seismic hazard levels. The areas of high seismic hazard include countries neighboring Thailand, such as Myanmar, Laos, Vietnam, and Indonesia (Sumatra Island), and areas within Thailand itself, primarily those areas in northern, western, and southern Thailand that are dominated by active fault zones.

Earthquake frequency-magnitude distribution and fractal dimension in mainland Southeast Asia

The 2004 Sumatra and 2011 Tohoku earthquakes highlighted the need for a more accurate understanding of earthquake characteristics in both regions. In this study, both the a and b values of the frequency-magnitude distribution (FMD) and the fractal dimension (DC) were investigated simultaneously from 13 seismic source zones recognized in mainland Southeast Asia (MLSEA). By using the completeness earthquake dataset, the calculated values of b and DC were found to imply variations in seismotectonic stress. The relationships of DC-b and DC-(a/b) were investigated to categorize the level of earthquake hazards of individual seismic source zones, where the calibration curves illustrate a negative correlation between the DC and b values (Dc = 2.80 - 1.22b) and a positive correlation between the DC and a/b ratios (Dc = 0.27(a/b) - 0.01) with similar regression coefficients (R2 = 0.65 to 0.68) for both regressions. According to the obtained relationships, the Hsenwi-Nanting and Red River fault zones revealed low-stress accumulations. Conversely, the Sumatra-Andaman interplate and intraslab, the Andaman Basin, and the Sumatra fault zone were defined as high-tectonic stress regions that may pose risks of generating large earthquakes in the future.

MAPPING ASPERITIES ALONG THE SAGAING FAULT ZONE, MYANMAR USING b-VALUE ANOMALIES

In this study, the b-values of frequency-magnitude earthquake distributions were mapped spatially along the sagaing fault zone (SFZ), central Myanmar. Three sub-datasets of the complete earthquake catalogue were tested in order to ensure the applied assumption. Using the present-day dataset (1980–2010), two areas of low b-values, which are prospective potential earthquake sources, were identified at the Naypyidaw-Mandalay and southwestern part of Myitkyina in the central and northern part of the SFZ, respectively. To assess the possible earthquake magnitudes, the b-values were mapped in the cross-section dimension along the SFZ. The obtained areas of low b-values, referred to as the fault asperity regions, conformed to those illustrated in the surface map. The asperitys sizes, examined from specific low b-values of ≤ 0.65 and ≤ 0.060 were quantitatively estimated and empirically converted to the potential earthquake magnitudes. This analysis revealed three prospective areas surrounding the Myitkyina regions capable of generating earthquakes in the future with a possible magnitude of 8.6 Richter. The contribution of effective mitigation plans are, therefore, urgently needed for Myanmar and the adjacent area.

Precursory seismicity changes prior to major earthquakes along the Sumatra-Andaman subduction zone: a region-time-length algorithm approach

In this study, we investigated the precursory seismicity changes related to the major earthquakes posed along the Sumatra-Andaman subduction zone (SASZ) using the region-time-length (RTL) algorithm. Based on the suitable RTL characteristics of r0 = 100 km and t0 = 2 years, the anomalous RTL score representing the quiescence stage mostly started 0.1–5.2 years before the subsequent major earthquake, while no activation stage was illustrated. For the spatial investigation, the RTL anomalies also clearly illustrated the location of the subsequent major earthquakes. Thus, in order to determine the prospective areas of upcoming earthquakes, the series of RTL maps calculated during the recent 5-year (2010–2014) time span was used. The obtained results reveal four risk areas along the SASZ that might pose a major earthquake in the future, namely (i) Sittwe city, western Myanmar; (ii) offshore northern Nicobar Islands; (iii) Aceh city, northernmost of Sumatra Island; and (iv) offshore western Sumatra Island. Therefore, both a tsunami hazard in the Indian Ocean and a seismic hazard in the far-field cities should be recognized urgently.

Paleoearthquake Investigations of the Mae Hong Son Fault, Mae Hong Son Region, Northern Thailand

We applied remote sensing and aerial photographic techniques to a study of the Mae Hong Son Fault (MHSF), located in the Mae Hong Son region, northern Thailand. Several fault lines are recognized in the region, trending mainly NE–SW, NW–SE, and N–S. The main morphotectonic landforms associated with the MHSF are fault scarps, offset streams, linear valleys, triangular facets, offset ridge crests, hot springs, and linear mountain fronts. A trench, a quarry, and a road cut in Caenozoic strata were used to analyze fault geometries in the area. We identified eight paleoearthquake events from trenching, quarry, and road-cut data, and from optically stimulated luminescence (OSL) and thermoluminescence (TL) dating. The OSL and TL ages of the events are: (1) 78,000 yr BP; (2) 68,000 yr BP; (3) 58,000 yr BP; (4) 48,000 yr BP; (5) 38,000 yr BP; (6) 28,000 yr BP; (7) 18,000 yr BP; and (8) 8,000 yr BP. The recurrence interval of seismic events on the MHSF appears to be ca. 10,000 years, and the slip rate was estimated as ca. 0.03–0.13 mm/yr. There is a low possibility of a large earthquake on the MHSF in the near future.

CU-PSHA: A MATLAB Software for Probabilistic Seismic Hazard Analysis

In this study, an open source MATLAB software, called CU-PSHA, is developed in order to analyze probabilistic earthquake hazards. This software aims to provide a user friendly and flexible tool for evaluating reliable earthquake hazard estimates. With the CU-PSHA, the probability of distances between the earthquake sources and the study site can be estimated. Two choices for the estimation of earthquake frequency–magnitude distribution, the exponential magnitude distribution and the characteristic earthquake models, are provided. Some strong ground–motion attenuation models are available for both shallow crustal and subduction zone earthquakes. The probability of exceedance of any individual given ground shaking value can be obtained, allowing the display of a seismic hazard curve. In addition with the supplementary MATLAB scripts, this CU-PSHA software can be employed in general seismic hazard mapping, for both ground shaking level and probability of occurrence, in any specific given time span.

Earthquake Catalogue of the Thailand Meteorological Department — A Commentary

In this study, I investigated qualitatively the earthquake catalogue of the Thai Meteorological Department (TMD), Thailand, with respect to the seismicity patterns of Thailand. The readymade relationships between the different magnitude scales were derived to allow their convenient interconversion. Earthquake declustering was performed in order to screen the main shocks from the foreshocks and aftershocks, reducing the 1998–2009 records from ~48,900 to 2,620 main events. Man-made changes in the seismicity rate were carefully checked for, but only some minor changes were found and these were not related to any network improvements. In order to assess the limit of the earthquake detection in the catalogue the criterion of the magnitude of completeness (Mc) was employed, revealing a high efficiency of earthquake detection at a low Mc (3.0–3.5 Mw), especially for the inland active fault zone that dominates in Southeast Asia. Thus, the TMDs catalogue is one of the alternative catalogues for seismicity investigation of inland earthquakes. Meanwhile for the area surrounding the Sumatra Island and Northern Myanmar, the TMDs network is sufficient only for earthquakes with a Mw > 5.4–6.0 Mw. Thus, some additional seismic recording stations are needed in the Southern and Northern parts of Thailand.

b-VALUE ANOMALIES ALONG THE NORTHERN SEGMENT OF THE SUMATRA–ANDAMAN SUBDUCTION ZONE: IMPLICATIONS FOR UPCOMING EARTHQUAKES

The potential areas of upcoming earthquakes were investigated along the Northern segment of the Sumatra–Andaman Subduction Zone according to the b-value of the frequency-magnitude distribution. After enhancing the completeness of the earthquake catalogue, two datasets, those recorded during (i) 1980–1994 and (ii) 1980–2003, were tested in order to verify the effective correlation between precursory b-values and the location of subsequent earthquakes. The results confirmed that areas with low b-values agreed well with the locations of the subsequent earthquakes in that region. Accordingly, the present-day dataset from 1980–2010 was carefully evaluated to determine the b-values across the region. Within this spatial investigation, three areas of low b-values and so potential hazards were found. These consisted of the (i) West coast of Myanmar, and (ii) North and (iii) South of the Nicobar Islands. From 2010–2012, a major earthquake with magnitude 7.5 mb was recorded as being generated in the region South of the Nicobar Islands. Thus, attention should be paid to the remaining two until now quiescent areas, and mitigation plans should be raised for both seismic and tsunami hazards.

Mapping of b-Value Anomalies Along the Strike-Slip Fault System on the Thailand–Myanmar Border: Implications for Upcoming Earthquakes

In order to determine the prospective areas of the forthcoming earthquake sources, the b-values of the frequency-magnitude earthquake distributions were analyzed spatially and mapped along the strike-slip fault system at the Thailand–Myanmar border. In order to constrain the relationship between the variation of b and the following hazardous earthquake, the completeness of earthquake catalogue was manipulated into two datasets for (i) 1980–2000 and (ii) 1980–2005 and the b-values mapped. Utilizing the suitable assumption of 30 fixed earthquake events, the following Mw ≥ 5.0 earthquakes illustrate a significant relation between their epicenter and the areas showing relatively low b-values. By utilizing the most recent earthquake data (1980–2015), five areas exhibiting low b-values (implying prospective earthquake sources) can be identified along the strike-slip fault system. Compared with earthquake activities evaluated previously along the strike-slip fault system, the data reveal that these five areas may potentially generate earthquakes up to 7.0Mw within the coming 50 years; the recurrence of the Mw-5.0 earthquake is about 10 years and the probabilities of the Mw-5.0 earthquake are about 40–95%, respectively. Since these prospective hazardous seismic zones are located close to cities, population centers and hydropower dams, an effective mitigation plan should be developed.

Precursory seismic quiescence along the Sumatra-Andaman subduction zone: past and present

In this study, the seismic quiescence prior to hazardous earthquakes was analyzed along the Sumatra-Andaman subduction zone (SASZ). The seismicity data were screened statistically with mainshock earthquakes of Mw ≥ 4.4 reported during 1980–2015 being defined as the completeness database. In order to examine the possibility of using the seismic quiescence stage as a marker of subsequent earthquakes, the seismicity data reported prior to the eight major earthquakes along the SASZ were analyzed for changes in their seismicity rate using the statistical Z test. Iterative tests revealed that Z factors of N = 50 events and T = 2 years were optimal for detecting sudden rate changes such as quiescence and to map these spatially. The observed quiescence periods conformed to the subsequent major earthquake occurrences both spatially and temporally. Using suitable conditions obtained from successive retrospective tests, the seismicity rate changes were then mapped from the most up-to-date seismicity data available. This revealed three areas along the SASZ that might generate a major earthquake in the future: (i) Nicobar Islands (Z = 6.7), (ii) the western offshore side of Sumatra Island (Z = 7.1), and (iii) western Myanmar (Z = 6.7). The performance of a stochastic test using a number of synthetic randomized catalogues indicated these levels of anomalous Z value showed the above anomaly is unlikely due to chance or random fluctuations of the earthquake. Thus, these three areas have a high possibility of generating a strong-to-major earthquake in the future.