This thesis describes the active structures of Myanmar and its surrounding regions, and the earthquake geology of the main active structures. The field observation and the remote sensing measurements of surface ruptures of the Tarlay earthquake are the first study of this kind in the Myanmar region.
Introduction Introduction
In Chapter 2 of this thesis, we provide an overview of the active structures in the country of Myanmar and the surrounding region based on remote sensing analysis. We also present our InSAR analysis of the Tarlay earthquake in the sixth chapter of this thesis.
Active Tectonics and Earthquake Potential of the Myanmar region
Abstract
Introduction
Historical records show that large and destructive earthquakes have occurred in much of the region (Figure 2). Another variable in the expression of tectonic landforms is the youth of the landscape (Yeats et al., 1997).
Neotectonics of the Myanmar region
The Indoburman range
Many of the folds in the west have only been active since the late Pliocene or later. The nature of the higher Indo-Burman area does not change significantly across the domain boundary.
The Sagaing domain
The namesake of the Sagaing segment is a small city on the rift just north of the Ayeyarwady River. The northern boundary of the Sagaing segment also marks the location where the fault propagates into multiple tracks.
The Shan-Sino domain
Still further southeast, a group of left-lateral faults covers the central part of the Shan-Sino domain (Fig. 12). The northern terminus almost coincides with the western terminus of the left-sided Kyaukme fault.
Earthquakes past and future
A predominance of dextral strike-slip within the domain, in the northern part of the 2004 megathrust rupture ( Chlieh et al., 2007 ), is consistent with this interpretation. Its hypocentral depth is 180 km (Engdashi and Villasenor, 2002), clearly within the Wadati-Benioff zone of the subducting plate.
The Sagaing fault
In addition, several magnitude 6 earthquakes have occurred along the southeastern border of the northern Indoburman range. The historical record of the northern half of the Sagaing fault may also illustrate another complexity—multisegment rupture.
Shan-Sino domain
These examples and the usually moderate magnitude of earthquakes along the Shan-Sino strike-slip faults indicate that partial rupture of these faults is typical. These low estimated fault slip rates imply a recurrence interval of > 1000 years for each of the slower-slip faults.
Conclusions
However, the historical and instrumental records show that smaller earthquakes are common during partial rupture of these faults. Estimates of slip rates for the faults of the Shan-Sino domain and empirical relationships between fault length and magnitude suggest that recurrence intervals for complete rupture of these faults are typically several thousand years.
Acknowledgments
Empirical global relationships between fault length and earthquake magnitude allow us to estimate the maximum magnitude for the active faults in each of these domains. The length of these structures implies that most of them are capable of generating events larger than Mw 7.0.
Kumar (2005), Soil-landform development of a part of the fold belt along the eastern coast of Bangladesh. Green and yellow dots show epicenters of the major 20th-century earthquakes (source: Engdahl and Villaseñor, 2002).
Earthquakes and slip rate of the Southern Sagaing fault
The Sagaing fault is one of the major strike-slip faults of Southeast Asia, dividing Myanmar from north to south (Fig.1a) (Curray et al., 1979 and Le Dain et al., 1984). Rates across the central Sagaing fault are slower than the spreading rate of the Andaman Sea (Curray et al., 1982 and Kamesh Raju et al., 2004).
Active tectonics of the southern Sagaing fault and surrounding area
A comparison of these slip rate estimates shows that estimates for the central Sagaing fault (Vigny et al., 2003, Bertrand et al., 1998, and Myint Thein et al., 1991) are slower than estimates from simplified tectonic models (Curray, 1982; Meade , 2007 ; Liu and Bird, 2008 ), even if these models consider the partitioning of slip between the Indo-Burma Range and the Sagaing Fault ( Liu and Bird, 2008 ). This apparent northward decrease in velocity implies that about 2 cm/yr of the opening rate across the center of Andaman Sea spreading is shared between the Sagaing Fault and one or more other structures.
Structural overview of southern Myanmar
The absence of active strike-slip structures along the western side of the Pegu-Yoma Range suggests that mainly contraction perpendicular to the Sagaing fault is occurring there. Clear geomorphic evidence of recent activity occurs only along that section of the Mae Ping fault closest to the Sagaing fault.
Southern Sagaing fault
Aerial photography at 1:25000 and 1:50000 scales also aided our mapping along the western and eastern flanks of the Pegu-Yoma range. On the other side of the Sagaing fault, on the western Shan Plateau, strained drainages along the major intraplate Mae Ping fault (Morley, 2002) indicate recent strike-slip activity.
1930 May Pegu earthquake and 1930 December Pyu earthquake
VIII Rossi-Forel intensity zones also parallel the fault trend. Although most of the southern Sagaing fault experienced high intensity during these two events, a lower intensity section exists between 17.5°N and 18°N.
Offsets along the Pegu section
The y axis spans ∼100 km of the Sagaing fault that experienced high seismic intensities during the Pegu earthquake. This distribution is comparable in shape to the pattern of seismic intensity along the fault during the Pegu earthquake.
Offset ancient structure
Our work and theirs allow us to make a more complete interpretation of slip during the 1930 Pegu earthquake. Therefore, we suggest that these horizontal anomalies coincide with the Pegu earthquake, the most recent large earthquake in the region.
The Payagyi ancient fortress
The southern junction of the fault track and the fort wall has been destroyed by the construction of a modern south-west road. The fault trace within the ancient fort wall matches well with other tectonic features to the north and south of the fort (Fig. 2b).
Estimation of offsets
Note that the base of the fortification wall is approximately 30 cm below the ground surface west of the fault. These similarities support our restoration of the original topography of the bank near the fault trace.
Offset reconstruction
All that matters are the fort wall geometries on both sides of the fault line. This result also confirms our interpretation that the estimate of fault separation from the southern base of the fort wall is too large.
Paleoseismologic excavation in the ancient fortress
The visual reconciliation of the overall fort-wall profiles (Figs. 7d and 7e) suggests a separation of 4.8 to 5.8 m, corresponding to a fault displacement of between 5.2 and 6.3 meters. This assessment is slightly smaller than the previous assessments made by matching the top and bottom of the front wall (5-7.5 m).
Sedimentary units in the trench
Layer (a) is thicker on the northeast side of the trench than on the southwest side of the trench. The structure of charcoal indicates that it is a product of wood combustion.
Fault traces on the southern trench wall
We also hypothesize that the brick-rich layer g is associated with the destruction of the fortress. In summary, these two fault breaks indicate two or more faulting events, possibly the 1930 earthquake, after the destruction of the fort.
Discussion
Determining the date of this rift depends on one's interpretation of the age of these two units. If these hypotheses are correct, then Fault II burst during or after the destruction of the ancient fortress.
The age of ancient fortress
In summary, these two faults indicate two or more watershed events, possibly including the 1930 earthquake, following the destruction of the fortress. realm of any reason to build such a well-designed defensive structure nearby. This interpretation is not inconsistent with the radiocarbon age of organic material beneath the fort (PYG0101a), which limits the construction of the dyke to a time after AD 1220.
Late Holocene slip rate along the Southern Sagaing fault
However, their model also predicts a higher rate of slip of the Sagaing faults than other geological and geodetic estimates. Another issue is the difference between the rate of spreading in the Andaman Sea Basin (38 mm/yr) and the rate of slip observed on the Sagaing Fault (Fig. 9).
Seismic potential and behavior of the southern Sagaing fault
The pattern of surface rupture associated with the 1930 Pegu earthquake raises an interesting issue regarding the strike-slip behavior of the southern Sagaing fault. Here we favor a variant of the uniform fault slip model to explain the slip history of the southern Sagaing fault over the past 500 years.
Conclusion
Acknowledgements
Stratigraphic columns of the four pits dug through the base of the bailey (Fig. 3c). The red dotted line shows the inferred geometry of the bailey west of the fault.
Permanent upper-plate deformation in western Myanmar
In the following pages, we describe our observations of vertical deformation along the coasts of Ramree and Cheduba islands associated with the 1762 event by measuring several different sea level markers. We then discuss the possible sources and seismic parameters of the 1762 Arakan earthquake, including its earthquake magnitude and recurrence interval.
Active tectonic context
Some earlier observers suggested that uplift occurred during seismic events: Halsted (1841) noted that the height difference between each marine terrace on western Cheduba Island is identical to the amount of recent uplift there. Recently, Shishikura et al. 2009) supported this view; they suggested that the elevation of the lowest terrace on western Cheduba Island is similar to the elevation recorded by Captain Halsted.
Sea-level indicators
The main types of sea-level indicators we used include coastal erosion features (shoreline angles, seawalls and breakwater platforms) and the living position of marine organisms (coral microatolls and oysters). Below we describe the five main sea level indicators we use and their relationship to the tide datum.
Biological indicators
Along coasts with tidal ranges similar to the western Myanmar coast (tidal range ~2 m), the HLS of microatolls is between MLLW and MLWS (Kayanne et al., 2007; Kench et al., 2009). Thus, it is reasonable to suggest that the HLS of the microatolls in western Myanmar is at a height similar to the microatoll HLS in other mesotidal environments and is not higher than the level of the MLLW.
Erosional coastal features
In places where we did not find micro-atolls, we compare the height of the highest coral colony with the current MLLW. The deepest part of the notch occurs at the mean sea level (MSL) level (Pirazzoli, 1986).
Coastal emergence
Ramree Island
Agricultural activities appear to have removed most of the fossil corals and oysters from the terrace. In summary, all features indicative of the young emergence of Ramree Island are located along the west coast.
Cheduba Island
Some of these emergent coral fossils have fallen to the present wave-cut platform during the erosional retreat of the modern sea cliff. Oyster and sea-bark deposits are abundant on in situ sandstone blocks near the height of the shoreline angle of T2 (~8.5 m above MSL).
Recovering co-seismic uplift from the emergence measurements
Therefore, in the next section we first deconvolve the 1762 co-seismic rise from vertical motions that can reasonably be attributed to recent global sea level changes and interseismic deformations. The micro-atolls we surveyed near Ka-I village suggest a net rise of 3.7 ± 0.2 m, after sea level rise correction.
