The SuGAr, and tectonics of the Sumatra subduction zone

SuGAr station locations, colored by their installation data.  This is Figure 2 of Feng et al., 2015

SuGAr station locations, colored by their installation data. 
This is Figure 2 of Feng et al., 2015

 

The SuGAr is a 60-station continuous GPS network designed to monitor tectonic processes along the Sumatra subduction zone. It was initiated in 2002 by Caltech, and is now maintained by EOS and LIPI (the Indonesian Institute of Sciences). We have added many new stations to the network since 2010.

The Sunda megathrust has generated a large number of earthquakes in the last decade, so this network has given us a lot to work on.  Some highlights of the things we've been working on are below. 

 

A geodetic earthquake catalog for Sumatra

Earthquakes recorded by the SuGAr are marked by stars.  Their coseismic offsets, and postseismic time series if evident, are provided in Feng et al., 2015.

Earthquakes recorded by the SuGAr are marked by stars.  Their coseismic offsets, and postseismic time series if evident, are provided in Feng et al., 2015.

The SuGAr has recorded so many events in the last decade that it is very difficult to separate the signals for individual earthquakes, their postseismic signatures, or interseismic deformation. In this study Lujia developed a technique to self-consistently and robustly separate these processes. Her study documents the deformation for 30 earthquakes which occurred within or outside the SuGAr network from August 2002 through the end of 2013, providing estimates of both vertical and horizontal coseismic offsets associated with 1 M9.2, 3 M8, 6 M7, 19 M6, and 1 M5.9 earthquakes, as well as postseismic decay amplitudes and times associated with 9 M > 7 earthquakes and 1 M6.7 earthquake. Her paper is a wealth of information on Sumatran earthquakes, and we hope will spark many new modeling studies.

Feng, L., E. M. Hill, P. Banerjee, I. Hermawan, L. L. H. Tsang, D. H. Natawidjaja, B. W. Suwargadi, and K. Sieh; A unified GPS-based earthquake catalog for the Sumatran plate boundary between 2002 and 2013; J. Geophys. Res., doi:2014JB011661, 2015.

 

Searching for slow slip events in the SuGAr time series

Slow slip events (SSEs) have been observed in GPS time series for many subduction zones worldwide, but not in decade-long GPS time series from the Sumatran GPS Array (SuGAr). An outstanding question has been whether SSEs have simply not occurred on the Sunda megathrust, or whether they have been obscured by the prodigious number of earthquakes and their ensuing postseismic deformation within the time of geodetic observation. After carrying out the above study, Lujia was able to remove all known tectonic signals in order to search for evidence of SSEs. The residual time series are essentially flat at the centimeter scale. The lack of evidence for events may reflect SSEs occurring at a magnitude, location, or time scale that renders them undetectable with the current resolution of the SuGAr, that the properties of this megathrust are not conducive to SSEs, or because the megathrust is in an active period of the earthquake cycle.

Feng, L., E. M. Hill, P. Elosegui, Q. Qiu, I. Hermawan, P. Banerjee, and K. Sieh;
Hunt for slow slip events along the Sumatran subduction zone in a decade of continuous GPS data;
In press, J. Geophys. Res., 2015.

 

An unexpectedly shallow tsunami earthquake - the Mw 7.8 Mentawai earthquake

Qiu Qiang helping with a tsunami field survey following the 2010 Mentawai earthquake

Qiu Qiang helping with a tsunami field survey following the 2010 Mentawai earthquake

The Mw 7.8 October 2010 Mentawai, Indonesia, earthquake was a “tsunami earthquake,” a rare type of earthquake that generates a tsunami much larger than expected based on the seismic magnitude. It produced a locally devastating tsunami, with runup commonly in excess of 6 m. We examined this event using a combination of high-rate GPS data from the SuGAr and a tsunami field survey. The GPS showed relatively small displacements, while the field survey showed maximum tsunami runup of >16 m: This combination only be explained by high fault slip at very shallow depths (<6 km), far from the islands and close to the oceanic trench. Inelastic uplift of trench sediments likely contributed to the size of the tsunami. This result challenges the conventional wisdom that the shallow tips of subduction megathrusts are aseismic. It therefore raises important questions both about the mechanical properties of the shallow fault zone and the potential seismic and tsunami hazard of the shallow region of megathrusts.

Hill, E. M., J. C. Borrero, Z. Huang, Q. Qiu, P. Banerjee, D. H. Natawidjaja, P. Elosegui, H. M. Fritz, I. R. Pranantyo, L. Li, K. A. Macpherson, V. Skanavis, C. E. Synolakis, and K. Sieh; The 2010 Mw 7.8 Mentawai earthquake: Very shallow source of a rare tsunami earthquake determined from tsunami field survey and near-field GPS; J. Geophys. Res., doi:10.1029/2012JB009159, 2012.

 

The divorce of the Indian and Australian plates - the Mw 8.6 Wharton Basin earthquake

The 11 April 2012 Mw 8.6 Wharton Basin earthquake sequence was both the largest strike-slip and largest intraplate earthquake sequence that has been instrumentally recorded. The events of this sequence initiated ∼400 km to the SW of northern Sumatra, Indonesia, on the oceanic side of the Sunda megathrust, within the Wharton Basin and just west of the Investigator Fracture Zone. This location, especially with potentially deep rupture of the younger, unexpected faults, is intriguing. Scientists have long been puzzled by the nature and location of the boundary between the Indian and Australian Plates within the Indian Ocean. Because of the resistance provided by the collision of India with Eurasia far to the north, the Indian plate is moving northward ∼1 cm/yr slower than the Australian plate, which is subject to slab-pull forces along the Sunda megathrust to the east; this difference in velocity creates strain between the Indian and Australian sub-plates, and the 2012 events may have played a role in the formation of a new plate boundary between them. The events were also extraordinary for their unprecedented rupture of multiple cross faults and deep slip.  We used high-rate GPS data from the SuGAr, teleseismic data, and source time functions from broadband surface waves to show that the largest rupture was on young, WNW trending, right-lateral faults, counter to initial expectations that an old, lithospheric, NNE trending fracture zone played the primary role. We also showed that the rupture likely extended to depths of up to 60 km, into the oceanic mantle.

Hill, E. M., H. Yue, S. Barbot, T. Lay, P. Tapponnier, I. Hermawan, J. Hubbard, P. Banerjee, L. Feng, D. Natawidjaja, and K. Sieh;
The 2012 Mw 8.6 Wharton Basin sequence: A cascade of great earthquakes generated by near-orthogonal, young, oceanic-mantle faults;
J. Geophys. Res., doi:2014JB011703, 2015.

This paper was featured as an Eos Research Spotlight. 
See https://eos.org/research-spotlights/wharton-basin-earthquakes-evidence-for-a-new-plate-boundary.

 

A moderate earthquake in the middle of a seismic gap - the Mw 7.2 earthquake of the Mentawai patch

The Mentawai patch of Sumatra is a seismic gap along the Sunda megathrust that has been forecast (based on coral observations) to generate a great earthquake in coming decades. In 2007, the Mw 8.5 Bengkulu earthquake was generated by rupture of only the southern part of the patch.  In 2008, a Mw 7.2 earthquake wasgenerated by rupture of a small patch right in the middle of the patch.  Rino has been studying this small event, and asking questions such as: Why didn't this event cascade into rupture of the entire patch? and Is the patch this time breaking in pieces rather than a single giant event as has happened in the past?

Salman, R., E. M. Hill, L. Feng, S. Barbot, E. O. Lindsey;
The 2008 Mw 7.2 North Pagai earthquake sequence: Is the Mentawai patch breaking in pieces?;
In preparation.

Variable slip behavior at a structural barrier - the Banyaks section of the Sunda megathrust

<text in prep>

Morgan, P., L. Feng, E. M. Hill, L. L. H. Tsang, A. J. Meltzner, K. Sieh;
Piecemeal and time variant slip behavior at a structural discontinuity of the Sunda megathrust;
In preparation.

 

The Simeulue Saddle - A persistent barrier to earthquake rupture

<text in prep>

Morgan, P., L. Feng, E. O. Lindsey, E. M. Hill, A. J. Meltzner, K. Sieh;
Moderate Earthquakes and Persistent Rupture Barriers: a Study of the 2008 Mw 7.4 Simeulue Earthquake;
In preparation.