Can the Up-dip Limit of Frictional Locking on Megathrusts be Detected Geodetically?

The updip limit of the seismogenic zone of megathrusts is poorly understood. The relative absence of observed microseismicity in such regions, together with laboratory studies of friction, suggests that the shallow fault is mostly velocity strengthening, and likely to creep. Inversions of geodetic data commonly show low to zero coupling at the trench, reinforcing this view. We show that the locked, down-dip portion of the megathrust creates an up-dip stress shadow that prevents the shallow portion of the fault from creeping at a significant rate, regardless of its frictional behavior. Our models demonstrate that even if the shallowest 40% of the fault is frictionally unlocked, the expected creep at the fault tip is at most 30% of the plate rate, often within the uncertainties of surface geodetic measurements, and below current resolution of seafloor measurements. We conclude that many geodetic models significantly underestimate the degree of shallow coupling on megathrusts, and thus seismic and tsunami hazard.

Almeida, R., E. O. Lindsey, K. Bradley, J. Hubbard, R. Mallick, E. M. Hill, Can the up-dip limit of frictional locking on megathrusts be detected geodetically? Quantifying the effect of stress shadows on near-trench coupling, Geophys. Res. Lett., 45, doi:10.1029/2018GL077785, 2018.

Survey GPS observations of tectonic motion in central and western Myanmar

Since 2016, I have been leading a series of GPS surveys across Myanmar; so far we have re-visited more than 80 existing monuments that were first installed and surveyed up to 18 years ago, and we have constructed more than 30 new monuments. Initial velocities are now available for approximately half the sites.

 

Our project's goals are the measurement of interseismic strain and slip partitioning between the Sagaing fault, Arakan megathrust, and oblique faults in the Arakan ranges and central Myanmar basin. Preliminary results confirm deep locking on the Sagaing fault near Mandalay but suggest that the Kabaw fault is less active than previously proposed (eg. by Steckler et al., 2016).

 

This project is funded by the Earth Observatory of Singapore (EOS) and supported by the Myanmar Earthquake Committee (MEC) and the Myanmar Survey Department. 

Monitoring extreme rates of subsidence in southeast Asian cities with InSAR

Worldwide, more than 700 million people live in low elevation coastal areas that may be subject to the future effects of a rising sea level - and 75% of these people (more than 500 million) are in Asia.

In addition, many of the most densely populated coastal areas are subsiding quickly due to the over-extraction of groundwater, greatly accelerating the rate of local sea level rise. Since 1970, coastal areas around Bangkok have subsided by more than 70 cm, and parts of Jakarta (the world's third largest metropolitan area) have subsided by several meters.

We are processing InSAR data from a number of large coastal cities throughout southeast Asia with the aim of monitoring this issue and informing local policy to reduce the future effects of this subsidence.

"Moderate" megathrust earthquakes observed with InSAR

Earthquakes that occur on a subduction zone megathrust offshore are some of the most difficult to image geodetically. Fortunately, in Sumatra the presence of islands directly above the megathrust enables the use of traditional land- and satellite-based geodetic methods to better image and understand these events.

We imaged two moderate-size (Mw 7.4 and Mw 7.2) events that occurred in 2008 using ALOS-1 InSAR, and found that the high spatial detail resolved by the satellite data enabled a better understanding of the properties of the megathrust. 

We found that the 2008 Mw 7.4 Simeulue earthquake occurred in the same location as a similar-sized event in 2002; this area is of particular interest because it formed the boundary between the 2004 Mw 9.2 and 2005 Mw 8.6 great Sumatra earthquakes. We imaged the slip of these events in detail using InSAR, GPS, and coral uplift observations and showed that this area likely hosts a single small asperity within a wider creeping zone that acts as a barrier to the large events. This creep may be caused by the presence of a geometric complexity related to subduction of an oceanic fracture zone. 

Morgan, P., L. Feng, A. J. Meltzner, E. O. Lindsey, L. L. H. Tsang and E. M. Hill, Sibling earthquakes generated within a persistent rupture barrier on the Sunda megathrust under Simeulue Island, Geophys. Res. Lett., 44, 2159-2166, doi:10.1002/2016GL071901, 2017.

Farther south, we studied the 2008 Mw 7.2 North Pagai earthquake with a combination of ALOS-1 InSAR and GPS observations. Based on the excellent resolution afforded by these near-field data, we showed that the postseismic slip must have partially overlapped with the coseismic slip area. This phenomenon is surprising, but well explained by models of rate-and-state-dependent friction, and the observations place some constraints on the values of the frictional properties.

Salman, R., E. M. Hill, L. Feng, E. O. Lindsey, P. Banerjee, I. Hermawan, D. H. Natawidjaja, Partial rupture of the Mentawai patch: The 2008 Mw 7.2 North Pagai earthquake sequence, Geophys. Res. Lett., 122,  doi:10.1002/2017JB014341, 2017.

An unmapped fault in the western Imperial Valley, southern California

We use a dense map of interseismic velocities across the Imperial fault and surrounding areas from GPS and InSAR (Envisat) to evaluate the rate of interseismic loading and along-strike variations in surface creep. We compare the data to models of the earthquake cycle with rate- and state-dependent friction and find that the data are inconsistent with a high (>30 mm/yr) slip rate on the Imperial fault. Thus, we investigate the possibility that an extension of the San Jacinto − Superstition Hills fault system through the town of El Centro may accommodate a significant portion of the slip previously attributed to the Imperial fault. Models including this additional fault are in better agreement with the available observations, suggesting that the long-term slip rate of the Imperial fault is lower than previously suggested, and that there may be a significant unmapped hazard in the western Imperial Valley. 

Lindsey, E. O. and Y. Fialko, Geodetic constraints on frictional properties and earthquake hazard in the Imperial Valley, southern California, J. Geophys. Res. Solid Earth, 121, doi:10.1002/2015JB012516, 2016.

Line of Sight Displacement from ALOS-2 Interferometry: Mw 7.8 Gorkha Earthquake and Mw 7.3 Aftershock

A challenge for traditional InSAR has been its limited spatial and temporal coverage, especially for very large events whose dimensions exceed the typical swath width of 70 – 100 km. This problem is addressed by the ScanSAR mode of ALOS-2 with a swath width of 350km. We present InSAR line-of-sight displacement data from ALOS-2 covering the Mw 7.8 Gorkha, Nepal earthquake and its Mw 7.3 aftershock that were acquired within one week of each event. The data are made freely available and we encourage their use in models of the fault slip and associated stress changes.

 

Lindsey, E. O., R. Natsuaki, X. Xu, M. Shimada, M. Hashimoto, D. Melgar and D. T. Sandwell, Line of Sight Displacement from ALOS-2 Interferometry: Mw 7.8 Gorkha Earthquake and Mw 7.3 Aftershock, Geophys. Res. Lett., 42, doi:10.1002/2015GL065385, 2015.

 

Galetzka, J., D. Melgar, J. F. Genrich, J. Geng, S. Owen, E. O. Lindsey, X. Xu, Y. Bock, J.- P. Avouac, L. B. Adhikari, B. N. Upreti, B. Pratt-Sitaula, T. N. Bhattarai, B. P. Sitaula, A. Moore, K. W. Hudnut, W. Szeliga, J. Normandeau, M. Fend, M. Flouzat, L. Bollinger, P. Shrestha, B. Koirala, U. Gautam, M. Bhatterai, R. Gupta, T. Kandel, C. Timsina, S. N. Sapkota, S. Rajaure, N. Maharjan, Slip pulse and resonance of Kathmandu basin dur- ing the 2015 Mw 7.8 Gorkha earthquake, Nepal imaged with space geodesy. Sciencedoi:10.1126/science.aac6383, 2015.

 

Physics Today: An InSAR look at the Nepal earthquake.

Localized and distributed creep along the southern San Andreas Fault

InSAR data from Envisat (2003-2010) reveal pervasive shallow creep along the southern-most 50 km of the San Andreas fault. Creep is localized on a well-defined fault trace only in the Mecca Hills and Durmid Hill areas, while elsewhere creep appears to be distributed over a 1-2 kilometer-wide zone surrounding the fault. We show that the degree of strain localization is correlated with variations in the local fault strike, suggesting that creep localization is controlled by normal stress.

 

Lindsey, E. O., Y. Fialko, Y. Bock, D. T. Sandwell and R. Bilham, Localized and distributed creep along the southern San Andreas Fault, J. Geophys. Res., 119, doi:10.1002/2014JB011275, 2014.

 

 

Interseismic strain localization in the San Jacinto fault zone

Using a combination of new campaign GPS observations and InSAR data, we present an updated interseismic velocity map across the San Jacinto fault in the Anza seismic gap. The data reveal a 2–3 kilometer wide shear zone deforming at a rate that exceeds the background strain rate by more than a factor of two. We show that a deep fault zone with a shear modulus reduction of at least a factor of 2.4 would be required to fully explain the geodetic strain rate, assuming the locking depth is 15 km. Two alternative possibilities include fault creep at a substantial fraction of the long-term slip rate within the region of deep microseismicity, or a reduced yield strength within the upper fault zone leading to distributed plastic failure during the interseismic period.

Lindsey, E. O., V. J. Sahakian, Y. Fialko, Y. Bock, S. Barbot, and T. K. Rockwell, Interseismic strain localization in the San Jacinto fault zone, Pure Appl. Geophys., 171, 

doi:10.1007/s00024-013-0753-z, 2013.

 

Geodetic Slip Rates in the Southern San Andreas Fault System

Geologic measurements of the long-term slip rate on the southernmost San Andreas fault (SAF) suggest a value of 14-19 mm/yr, while inversions of geodetic data typically suggest a higher rate of 22-25 mm/yr. We conducted a suite of inversions using recent geodetic data, and find that several simplifying assumptions made in earlier geodetic models may be the cause of the apparent disagreement. The results demonstrate that the observed degree of material heterogeneity in Southern California is not enough to cause a significant bias in the geodetic slip rates. However, the models are extremely sensitive to the assumed fault geometry. In particular, models in which the SAF dips 60 degrees northeast fit both the geodetic and geologic data better than models with a vertical fault.

Lindsey, E. O. and Y. Fialko, Geodetic Slip Rates in the Southern San Andreas Fault System: Effects of Elastic Heterogeneity and Fault Geometry, J. Geophys. Res., 118,  doi:10.1029/2012JB009358, 2013.

 

Eric Lindsey - elindsey@ntu.edu.sg

Earth Observatory of Singapore

Nanyang Technological University

N2-01c-36, 50 Nanyang Avenue

Singapore 639798

© 2019 by Eric Lindsey

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