Fault properties, rheology and interseismic deformation in Southern California from high-precision space geodesy

This dissertation presents the collection and processing of dense high-precision geode- tic data across major faults throughout Southern California. The results are used to inform numerical models of the long-term slip rate and interseismic behavior of these faults, as well as their frictional and rheological properties at shallow depths. The data include campaign surveys of dense networks of GPS monuments crossing the faults, and Interferometric Synthetic Aperture Radar (InSAR) observations from ENVISAT. Using a Bayesian framework, we first assess to what extent these data constrain relative fault slip rates on the San Andreas and San Jacinto faults, and show that the inferred parameters depend critically on the assumed fault geometry. We next look in detail at near-field observations of strain across the San Jacinto fault, and show that the source of this strain may be either deep anomalous creep or a new form of shallow, distributed yielding in the top few kilometers of the crust. On the San Andreas fault, we show that this type of shallow yielding does occur, and its presence or absence is controlled by variations in the local normal stress that result from subtle bends in the fault. Finally, we investigate shallow creep on the Imperial fault, and show that thanks to observations from all parts of the earthquake cycle it is now possible to obtain a strong constraint on the shallow frictional rheology and depth of the material responsible for creep. The results also suggest activity on a hidden fault to the West, whose existence has been previously suggested but never confirmed

A pathway to the automated global assessment of water level in reservoirs with Synthetic Aperture Radar (SAR)

Global measurements of reservoir water levels are crucial for understanding Earth’s hydrological dynamics, especially in the context of global industrialization and climate change. Although radar altimetry has been used to measure the water level of some reservoirs with high accuracy, it is not yet feasible unless the water body is sufficiently large or directly located at the satellite’s nadir. This study proposes a gauging method applicable to a wide range of reservoirs using Sentinel–1 Synthetic Aperture Radar data and a digital elevation model (DEM). The method is straightforward to implement and involves estimating the mean slope–corrected elevation of points along the reservoir shoreline. We test the model on six case studies and show that the estimated water levels are accurate to around 10% error on average of independently verified values. This study represents a substantial step toward the global gauging of lakes and reservoirs of all sizes and in any location where a DEM is available.Nanyang Technological UniversityPublished versionThe National Institute of Education (NIE) at Nanyang Technological University (NTU) Grant # SUG–NAP (3/19 EP) funded this research

Active convergence of the India‐Burma‐Sunda plates revealed by a new continuous GPS network

The Rakhine (Arakan)‐Bangladesh megathrust, along which the Indian and Burma plates collide, is assumed by some to be inactive/aseismic due to the lack of notable interplate earthquakes in the modern instrumental catalog. However, geological and historical evidence of the great 1762 Arakan earthquake suggest the megathrust can produce M~8 events that could adversely affect the lives of millions of people in the region. To investigate the seismogenic potential and determine the slip budget of the megathrust, we first need to solve for India‐Burma‐Sunda relative plate motions. We present a new set of 24 GPS velocities (2011–2017) from the Myanmar‐India‐Bangladesh‐Bhutan continuous GPS network. We use the new velocities and those from previously published studies to explore the geometries and slip rates of three major faults (Rakhine‐Bangladesh megathrust, Churachandpur‐Mao Fault, and Sagaing Fault) that accommodate the India‐Burma‐Sunda plate motion. Our results suggest that the three major faults we studied are likely fully coupled; the modern shortening rate across the Burma plate is 12–24 mm/year, while the total dextral shear rate is 25–32 mm/year. The possibly fully coupled shallow megathrust, and splay faults that may sole into it, are geodetically invisible while they are not slipping. However, we can identify the transition from coupling to steady creep on the deeper extension of the megathrust; we use this to show active oblique India‐Burma convergence and to map along‐strike and along‐dip variations in dip‐slip and strike‐slip motion. This implies that the megathrust is currently accumulating strain which will eventually be released in earthquakes.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Sibling earthquakes generated within a persistent rupture barrier on the Sunda megathrust under Simeulue Island

A section of the Sunda megathrust underneath Simeulue is known to persistently halt rupture propagation of great earthquakes, including those in 2004 (Mw 9.2) and 2005 (Mw 8.6). Yet the same section generated large earthquakes in 2002 (Mw 7.3) and 2008 (Mw 7.4). To date, few studies have investigated the 2002 and 2008 events, and none have satisfactorily located or explained them. Using near-field InSAR, GPS, and coral geodetic data, we find that the slip distributions of the two events are not identical but do show a close resemblance and largely overlap. We thus consider these earthquakes “siblings” that were generated by an anomalous “parent” feature of the megathrust. We suggest that this parent feature is a locked asperity surrounded by the otherwise partially creeping Simeulue section, perhaps structurally controlled by a broad morphological high on the megathrust.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

The January 11, 2018, Mw 6.0 Bago-Yoma, Myanmar earthquake : a shallow thrust event within the deforming Bago-Yoma range

On January 11, 2018 (18:26 UTC), a Mw 6.0 earthquake occurred approximately 30 km west of the Sagaing Fault in the Bago-Yoma Range (BYR). Using a local broadband seismic network and regional seismic stations, we refine the locations and moment tensors of the earthquake sequence. We relocate 98 earthquake epicenters and determine the focal mechanism and centroid depth of the mainshock and 20 aftershocks with Mw > 4. The relocated epicenters cluster in a NW-SE direction that is consistent with the strike of the mainshock fault plane solution and the slip distribution derived from ALOS-2 interferometric synthetic aperture radar observations. Most of the aftershocks have a pure thrust focal mechanism similar to the mainshock, except for four strike-slip aftershocks. The refined locations and moment tensors of the thrust events clearly delineate a fault dipping ∼40° to the southwest at a depth range of 3–7 km, indicating that the earthquake sequence ruptured a previously unmapped, active fault. We interpret the earthquake sequence to be associated with pre-existing faults within the BYR anticlinorium. This earthquake sequence and historical seismicity indicate that the upper crust of the BYR is highly stressed, resulting in ongoing distributed deformation between the oblique Rakhine megathrust and the dextral Sagaing Fault. The seismic hazard posed by these active faults has been increasing with the development of infrastructure such as dams within the BYR. Our study highlights the need for high-resolution earthquake source parameter and strong ground motion attenuation studies for further understanding of the neotectonics of Myanmar and its related seismic hazard.Ministry of Education (MOE)National Research Foundation (NRF)Published versionThis research study was supported by the Earth Observatory of Singapore via its funding from the National Research Foundation Singapore and the Singapore Ministry of Education under the Research Centres of Excellence initiative

Surface creep rate of the southern San Andreas fault modulated by stress perturbations from nearby large events

A major challenge for understanding the physics of shallow fault creep has been to observe and model the long‐term effect of stress changes on creep rate. Here we investigate the surface creep along the southern San Andreas fault (SSAF) using data from interferometric synthetic aperture radar spanning over 25 years (ERS 1992–1999, ENVISAT 2003–2010, and Sentinel‐1 2014–present). The main result of this analysis is that the average surface creep rate increased after the Landers event and then decreased by a factor of 2–7 over the past few decades. We consider quasi‐static and dynamic Coulomb stress changes on the SSAF due to these three major events. From our analysis, the elevated creep rates after the Landers can only be explained by static stress changes, indicating that even in the presence of dynamically triggered creep, static stress changes may have a long‐lasting effect on SSAF creep rates.Published versio

Slip rate deficit and earthquake potential on shallow megathrusts

Most destructive tsunamis are caused by seismic slip on the shallow part of offshore megathrusts. The likelihood of this behaviour is partly determined by the interseismic slip rate deficit, which is often assumed to be low based on frictional studies of shallow fault material. Here we present a new method for inferring the slip rate deficit from geodetic data that accounts for the stress shadow cast by frictionally locked patches, and show that this approach greatly improves our offshore resolution. We apply this technique to the Cascadia and Japan Trench megathrusts and find that wherever locked patches are present, the shallow fault generally has a slip rate deficit between 80 and 100% of the plate convergence rate, irrespective of its frictional properties. This finding rules out areas of low kinematic coupling at the trench considered by previous studies. If these areas of the shallow fault can slip seismically, global tsunami hazard could be higher than currently recognized. Our method identifies critical locations where seafloor observations could yield information about frictional properties of these faults in order to better understand their slip behaviour.Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionThis research is supported by the National Research Foundation of Singapore and the Singapore Ministry of Education under the Research Centres of Excellence initiative, and by a Singapore NRF Investigatorship award NRF-NRFI05-2019-0009 to EH. RB acknowledges support by NSF award EAR-1801720. JM acknowledges support by NERC award NE/R00515X/1. This work comprises Earth Observatory of Singapore contribution number 337

Piecemeal Rupture of the Mentawai Patch, Sumatra : The 2008 Mw 7.2 North Pagai Earthquake Sequence

The 25 February 2008 Mw 7.2 North Pagai earthquake partially ruptured the middle section of the Mentawai patch of the Sunda megathrust, offshore Sumatra. The patch has been forecast to generate a great earthquake in the next few decades. However, in the current cycle the patch has so far broken in a sequence of partial ruptures, one of which was the 2008 event, illustrating the potential of the patch to generate a spectrum of earthquake sizes. We estimate the coseismic slip distribution of the 2008 event by jointly inverting coseismic offsets from GPS and interferometric synthetic aperture radar. We then estimate afterslip with 5.6 years of cumulative GPS displacements. Our results suggest that the estimated afterslip partially overlaps the coseismic rupture. The overlap of coseismic rupture and afterslip can be explained conceptually by a simple rate‐and‐state model where the degree of overlapping is controlled by the dynamic weakening and the critical nucleation size in the velocity‐weakening area. Comparing our rate‐and‐state model results with our geodetic inversion results, we suggest that the part of the coseismic rupture that does not overlap with the afterslip may represent a velocity‐weakening region, while the overlapping part may represent a velocity‐strengthening region.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

The mechanism of partial rupture of a locked megathrust: The role of fault morphology

Assessment of seismic hazard relies on estimates of how large an area of a tectonic fault could potentially rupture in a single earthquake. Vital information for these forecasts includes which areas of a fault are locked and how the fault is segmented. Much research has focused on exploring downdip limits to fault rupture from chemical and thermal boundaries, and along-strike barriers from subducted structural features, yet we regularly see only partial rupture of fully locked fault patches that could have ruptured as a whole in a larger earthquake. Here we draw insight into this conundrum from the 25 April 2015 Mw 7.8 Gorkha (Nepal) earthquake. We invert geodetic data with a structural model of the Main Himalayan thrust in the region of Kathmandu, Nepal, showing that this event was generated by rupture of a décollement bounded on all sides by more steeply dipping ramps. The morphological bounds explain why the event ruptured only a small piece of a large fully locked seismic gap. We then use dynamic earthquake cycle modeling on the same fault geometry to reveal that such events are predicted by the physics. Depending on the earthquake history and the details of rupture dynamics, however, great earthquakes that rupture the entire seismogenic zone are also possible. These insights from Nepal should be applicable to understanding bounds on earthquake size on megathrusts worldwide.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Subduction initiation and the rise of the Shillong Plateau

Discrepancies between geodetically and geologically estimated thrust fault slip rates are generally viewed as a methodological problem. Even when slip rate is steady over geological time, a discrepancy may exist because each method is sensitive to different deformation processes. However, this offers a tool to estimate the partitioning of convergence between footwall and hanging wall deformation, and therefore a way to discriminate among orogenic styles. Here we investigate one such discrepancy for the Shillong Plateau, a basement-cored contractional orogen within the Himalayan foreland basin. Using a regional block model to explain the modern geodetic velocity field and explicit uncertainty analysis of the geologic rates, we show that this discrepancy cannot be reconciled simply by invoking uncertainties in individual methods. Our results indicate that the Shillong Plateau is not an ongoing forward break of the Bhutan Himalayas, as was believed until recently. Instead, the observed inter-plate convergence and plateau uplift in this region may be driven primarily by an attempt of the negatively buoyant Indian passive margin lithosphere (the Surma Basin), south of the plateau, to initiate subduction. As a result, the uplift history of the plateau, which constrains the geologic rate, is significantly lower than expected given the geodetic convergence rate. We propose that this convergence is largely accommodated by the transport of the footwall into the mantle. This geodynamic scenario has important regional seismotectonic implications: (1) the cold and brittle sinking passive margin may have enabled the deep extent (∼30 km) and therefore large magnitude of the MW 8+ Shillong Earthquake of 1897; (2) the collapse of the Indian lithosphere into the mantle may have created the anomalously deep (∼20 km) Surma Basin; and (3) this subsidence may also drive accelerated post-Miocene westward propagation of the Indo-Burman Wedge. We propose that the Shillong Plateau is the only modern example of passive margin collapse, and can serve as a natural laboratory to study the earliest phase of subduction.Ministry of Education (MOE)Nanyang Technological UniversityNational Research Foundation (NRF)Published versionThis research was supported by the Earth Observatory of Singapore and the National Research Foundation Singapore and the Singapore Ministry of Education under the Research Centres of Excellence initiative, and by a Singapore National Research Foundation Investigatorship awarded to EMH (Proposal ID NRF2018NRF-NRFI001-21)