Submarine mass wasting and associated tsunami risk offshore western Thailand, Andaman Sea, Indian Ocean

2-D seismic data from the top and the western slope of Mergui Ridge in water depths between 300 and 2200 m off the Thai west coast have been investigated in order to identify mass transport deposits (MTDs) and evaluate the tsunamigenic potential of submarine landslides in this outer shelf area. Based on our newly collected data, 17 mass transport deposits have been identified. Minimum volumes of individual MTDs range between 0.3 km3 and 14 km3. Landslide deposits have been identified in three different settings: (i) stacked MTDs within disturbed and faulted basin sediments at the transition of the East Andaman Basin to the Mergui Ridge; (ii) MTDs within a pile of drift sediments at the basin-ridge transition; and (iii) MTDs near the edge of/on top of Mergui Ridge in relatively shallow water depths ( 1000 m) and/or comprise small volumes suggesting a small tsunami potential. Moreover, the recurrence rates of failure events seem to be low. Some MTDs with tsunami potential, however, have been identified on top of Mergui Ridge. Mass-wasting events that may occur in the future at similar locations may trigger tsunamis if they comprise sufficient volumes. Landslide tsunamis, emerging from slope failures in the working area and affecting western Thailand coastal areas therefore cannot be excluded, though the probability is very small compared to the probability of earthquake-triggered tsunamis, arising from the Sunda Trench

Evaluating the state of stress and seismic hazard in Thailand and vicinity through finite element modeling

Thailand is surrounded by seismically active plate boundary zones and large-offset crustal faults, but there is comparatively little seismic activity within its borders. On 5 May 2014, a moment magnitude (MW) 6.2 earthquake occurred in the Mae Lao district of northern Thailand. To better anticipate future seismic hazards from events like this, we model the state of stress in Thailand and its vicinity using a finite element approach. We identify northern Thailand as a region with significant stress accumulation, consistent with being caused by convergence at the Sumatra-Andaman subduction zone and southward escape of the Himalayan Orogen. The stress field in this region is compatible with the kinematics of the Mae Lao event. Our modeling also shows that the magnitude of the stress field throughout southeast Asia is particularly sensitive to the stage of the earthquake cycle at the Sumatra-Andaman subduction zone, although the Sumatra-Andaman subduction zone produces a smaller effect on the orientations of stresses in the source region of the Mae Lao earthquake. Finally, we find that the stress change effect from the Mae Lao earthquake is geographically limited and the broader region remains in nearly the same state of stress as prior to the event

Geothermal and seismic evidence for a southeastern continuation of the three pagodas fault zone into the Gulf of Thailand

Aerial photographic maps and landsat image interpretations suggest the major fault segments of the Three PagodaFault (TPF) Zone and Sri Swat Fault (SSF) Zone are oriented parallel or sub-parallel in the same NW-SE directions. The KwaeNoi River is running along the TPF in the south whereas the Kwae Yai River is running along the SSF in the north. Thesoutheastern continuation of both faults is obscured by thick Cenozoic sediments. Hence, surface lineaments cannot betraced with confidence. However, based on some interpretations of the airborne magnetic survey data, the trace of such faultsare designated to run through the western part of Bangkok and the northern end of the Gulf of Thailand. Paleo-earthquakesand the presence of hot springs along the fault zones indicate that they are tectonically active. The changes of both physicaland chemical properties of the water from Hin Dart Hot Spring and those of the surface water from a shallow well at Ban KhaoLao during the Great Sumatra–Andaman Earthquake on 26th of December 2004 clearly indicated that the southeastern continuation of the TPF is at least as far south as Pak Tho District, Ratburi. Our new evidence of the alignment of the high heatflow in the upper part of the Gulf of Thailand verified that the TPF also extend into the Gulf via Samut Songkhram Province.Studies of the seismic data from two survey lines along the Western part of the upper Gulf of Thailand acquired by BritoilPlc. in 1986, namely Line A which is approximately 60 km long, starting from Bang Khen passing through Bang Khae andending in Samut Songkhram and Line B is approximately 30 km long starting from Samut Sakon ending in Samut Song Khramsuggest that all the faults or fractures along these seismic profiles are covered by sediments of approximately 230 m thickwhich explain that the fault underneath these seismic lines is quite old and may not be active. The absent of sign or trace ofthe TPF Path to the west suggested that there is no segment of such fault along these seismic lines

Seismic activities in Kanchanaburi: Past and present

Seismic activities in Kanchanaburi Province of the western Thailand have been a major concern among the Thai publicdue to the fear that a big earthquake caused by the Three Pagodas Fault Zones (TPFZ) and the Sri Sawat Fault Zone (SSFZ), oneof the largest active fault zones in Thailand, could damage the large dams and generate a great disaster to the communities.Four hundred and thirty seven earthquakes that occurred in Kanchanaburi since 1983 have been analyzed for the time andlocation distributions along with the frequency magnitude relationship. There are no clear correlations between the epicentersof these earthquakes and the known locations of the active faults in the region. The seismic catalog used in this study iscomplete for Mw=3.0 for Kanchanaburi region. The analysis of G-R relationship yields a-value of 5.15 and b-value of 0.86. Adeterministic seismic hazard analysis of the TPFZ and SSFZ suggests that the characteristic earthquakes magnitudes of theTPFZ and the SSFZ are 7.3 and 7.0, respectively, with a maximum PGA of 0.31 and 0.28 g at the faults lines for the earthquakeoccurring at 15 km depth

Evaluating the state of stress and seismic hazard in Thailand and vicinity through finite element modeling

Thailand is surrounded by seismically active plate boundary zones and large-offset crustal faults, but there is comparatively little seismic activity within its borders. On 5 May 2014, a moment magnitude (MW) 6.2 earthquake occurred in the Mae Lao district of northern Thailand. To better anticipate future seismic hazards from events like this, we model the state of stress in Thailand and its vicinity using a finite element approach. We identify northern Thailand as a region with significant stress accumulation, consistent with being caused by convergence at the Sumatra-Andaman subduction zone and southward escape of the Himalayan Orogen. The stress field in this region is compatible with the kinematics of the Mae Lao event. Our modeling also shows that the magnitude of the stress field throughout southeast Asia is particularly sensitive to the stage of the earthquake cycle at the Sumatra-Andaman subduction zone, although the Sumatra-Andaman subduction zone produces a smaller effect on the orientations of stresses in the source region of the Mae Lao earthquake. Finally, we find that the stress change effect from the Mae Lao earthquake is geographically limited and the broader region remains in nearly the same state of stress as prior to the event

Seismotectonics of the 2014 Chiang Rai, Thailand, earthquake sequence

On 5 May 2014, a Mw 6.2 strike-slip earthquake occurred in the Mae Lao region of Chiang Rai province in Thailand. This earthquake took place in a region of known faults and caused substantial damage and injuries, although the region had been previously identified as having a relatively low earthquake hazard. Detailed field reconnaissance and deployment of a dense, temporary, network of broadband seismometers allowed details of the damage and its relationship to seismicity to be analyzed. The aftershock sequence associated with this main shock occurs on two well-defined trends, reflecting the two potential fault planes in earthquake mechanisms for the main shock and the majority of the aftershocks. The damage area was relatively large for an event of this magnitude, but building damage was largely limited to the primary rupture region, while liquefaction and other ground failure are spatially associated with the rupture area and along regional rivers. Stress modeling, combined with the time series and pattern of aftershock activity, leads us to propose that slip near the northern termination of the main shock rupture continued slightly onto a conjugate fault, helping to trigger the distinct pattern of two discrete, conjugate trends of aftershock activity that mirror the kinematics of the main shock fault mechanism

Seismotectonics of the 2014 Chiang Rai, Thailand, earthquake sequence

On 5 May 2014, a Mw 6.2 strike-slip earthquake occurred in the Mae Lao region of Chiang Rai province in Thailand. This earthquake took place in a region of known faults and caused substantial damage and injuries, although the region had been previously identified as having a relatively low earthquake hazard. Detailed field reconnaissance and deployment of a dense, temporary, network of broadband seismometers allowed details of the damage and its relationship to seismicity to be analyzed. The aftershock sequence associated with this main shock occurs on two well-defined trends, reflecting the two potential fault planes in earthquake mechanisms for the main shock and the majority of the aftershocks. The damage area was relatively large for an event of this magnitude, but building damage was largely limited to the primary rupture region, while liquefaction and other ground failure are spatially associated with the rupture area and along regional rivers. Stress modeling, combined with the time series and pattern of aftershock activity, leads us to propose that slip near the northern termination of the main shock rupture continued slightly onto a conjugate fault, helping to trigger the distinct pattern of two discrete, conjugate trends of aftershock activity that mirror the kinematics of the main shock fault mechanism

Climatic and halokinetic controls on alluvial–lacustrine sedimentation during compressional deformation, Andean forearc, northern Chile

ACKNOWLEDGEMENTS This material is based upon work supported by Conoco-Philips, BG Group, and ENI. BHP Billiton are acknowledged for their support in the final stages of analysis and preparation of this manuscript. This manuscript benefitted greatly from reviews by Alberto Saez and Teresa Jordan. We are grateful to Rhiannon Chaloner for help with fieldwork.Peer reviewedPostprin

An assessment of uncertainties in V S profiles obtained from microtremor observations in the phased 2018 COSMOS blind trials

Site response is a critical consideration when assessing earthquake hazards. Site characterization is key to understanding site effects as influenced by seismic site conditions of the local geology. Thus, a number of geophysical site characterization methods were developed to meet the demand for accurate and cost-effective results. As a consequence, a number of studies have been administered periodically as blind trials to evaluate the state-of-practice on-site characterization. We present results from the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials, which used data recorded from surface-based microtremor array methods (MAM) at four sites where geomorphic conditions vary from deep alluvial basins to an alpine valley. Thirty-four invited analysts participated. Data were incrementally released to 17 available analysts who participated in all four phases: (1) two-station arrays, (2) sparse triangular arrays, (3) complex nested triangular or circular arrays, and (4) all available geological control site information including drill hole data. Another set of 17 analysts provided results from two sites and two phases only. Although data from one site consisted of recordings from three-component sensors, the other three sites consisted of data recorded only by vertical-component sensors. The sites cover a range of noise source distributions, ranging from one site with a highly directional microtremor wave field to others with omni-directional (azimuthally distributed) wave fields. We review results from different processing techniques (e.g., beam-forming, spatial autocorrelation, cross-correlation, or seismic interferometry) applied by the analysts and compare the effectiveness between the differing wave field distributions. We define the M index as a quality index based on estimates of the time-averaged shear-wave velocity of the upper 10 (VS10), 30 (VS30), 100 (VS100), and 300 (VS300) meters and show its usefulness in quantitative comparisons of VS profiles from multiple analysts. Our findings are expected to aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of seismic site effects