The 25 October 2010 Sumatra Tsunami Earthquake: Slip in a Slow Patch Susan L

GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L14306, doi:10.1029/2011GL047864, 2011 The 25 October 2010 Sumatra tsunami earthquake: Slip in a slow patch Susan L. Bilek,1 E. Robert Engdahl,2 Heather R. DeShon,3 and Maya El Hariri1 Received 20 April 2011; revised 13 June 2011; accepted 15 June 2011; published 28 July 2011. [1] Various models for the generation of tsunami earthquakes and shallow afterslip [e.g., Hsu et al., 2006; earthquakes have been proposed, including shallow Konca et al., 2008] (Figure 1). Approximately 700 km north, earthquake slip through low strength materials. Because another tsunami earthquake may have occurred along these physical fault conditions would likely affect other Simeulue Island in 1907 [Kanamori et al., 2010]. earthquakes in the same rupture zone, source properties of [3] Given that models link shallow slip in weak near‐trench other events may provide a guide to locations of tsunami materials to tsunami earthquake occurrence, an important earthquakes. The 25 October 2010 Mw = 7.8 Mentawai question is whether these fault conditions also impact the tsunami earthquake and surrounding events provide a test rupture of other events in the same area. Our efforts here of this hypothesis. We determine slip patterns for the address this question by first defining the rupture area and mainshock and relocate aftershocks, with the majority source characteristics of the 2010 event as well as relocating occurring in the near trench region. The two largest aftershocks of the event. We then compare the rupture extent magnitude aftershocks occurred within the downdip end of to source parameters computed for other regional earthquakes the mainshock rupture area and have long moment‐ in order to assess the possibility of consistent slow behavior normalized rupture duration, likely related to fault zone along specific patches of the plate interface. conditions. Several older relocated earthquakes at the northern edge of the 2010 rupture area also have long 2. Dataset duration character, suggesting both spatial and temporal consistency in the conditions needed to produce slow [4] Our source analysis of the 2010 mainshock incorporates both broadband P and SH waves recorded at teleseismic seismic processes along this margin. Citation: Bilek, S. L., – E. R. Engdahl, H. R. DeShon, and M. El Hariri (2011), The 25 October distances (30 90°) and a large suite of long period vertical 2010 Sumatra tsunami earthquake: Slip in a slow patch, Geophys. component seismograms for the Rayleigh wave (R1) signal Res. Lett., 38, L14306, doi:10.1029/2011GL047864. used in the relative source time function deconvolution. We also determine source parameters for 74 moderate magnitude events from 1990–2009 (Mw > 5.5, thrust mechanism 1. Introduction based on Global Centroid Moment Tensor (GCMT, www. [2] Tsunami earthquakes, events that produce larger tsu- globalcmt.org) solutions) using teleseismic P and SH waves. nami waves than expected given the earthquake size, have occurred in several subduction zones around the world, 3. Methods including Nicaragua (1992), Java (1994 and 2006), and Peru 3.1. Source Parameters: Mainshock (1996) [e.g., Kanamori, 1972; Kanamori and Kikuchi, 1993; Newman and Okal, 1998; Polet and Kanamori, 2000; Bilek [5] Estimates of the rupture extent are initially developed ’ and Lay, 2002]. Other characteristics of these events include using a theoretical Green s function deconvolution [e.g., Lay deficiency in high frequency seismic radiation, very long et al., 2009] with 59 R1 surface wave records that are earthquake rupture duration, and shallow slip along the plate azimuthally well distributed around the source. Point source interface. Models for these unusual events tend to require synthetic seismograms are created using normal mode slip through low strength materials at the shallow end of the summation for each distance and azimuth range represented seismogenic zone [e.g., Kanamori and Kikuchi, 1993; Bilek by the data collected using PREM [Dziewonski and Anderson, and Lay, 1999; Satake and Tanioka, 1999; Polet and 1981] for the velocity model. These synthetic seismograms are Kanamori, 2000]. A recent tsunami earthquake occurred deconvolved from the observed seismograms to remove along the Mentawai Islands offshore the island of Sumatra theoretical path effects, leaving a relative source time function (25 October 2010, 14:42:22 UTC) [Lay et al., 2011; (STF) to describe the source effects at each station [e.g., Lay Newman et al., 2011], in an area adjacent to recent great et al., 2009] (Figure 2a). We pick the onset and end of each STFs main source pulses (dt) and using the definition for the directivity parameter [G, G = cos(station_azimuth − rupture_ azimuth)/phase_velocity], solve for the best fit rupture duration, azimuth, and duration length (Figure 2a). 1New Mexico Institute of Mining and Technology, Socorro, [6] We also use broadband body waves (P and SH) to invert New Mexico, USA. for fault finiteness, obtaining estimates of the overall rupture 2 Department of Physics, University of Colorado at Boulder, Boulder, duration and slip distribution on the fault plane (M. Kikuchi Colorado, USA. ‐ 3CERI, University of Memphis, Memphis, Tennessee, USA. and H. Kanamori, Note on teleseismic body wave inver- sion program, 2006, http://www.eri.u‐tokyo.ac.jp/ETAL/ Copyright 2011 by the American Geophysical Union. KIKUCHI). We use a grid of 25 km along strike by 40 km in 0094‐8276/11/2011GL047864 L14306 1of5 L14306 BILEK ET AL.: THE 2010 SUMATRA TSUNAMI EARTHQUAKE L14306 3.2. Source Parameters: Moderate Magnitude Events [7] For moderate magnitude earthquakes that occurred prior to the 2010 mainshock as well as 3 aftershocks, we also determine source time functions and depth estimates for each event by deconvolving a point source Green’s function from the P and SH waves, solving for the source time function at a range of depths between 3–65 km [e.g., Ruff and Miller, 1994; Bilek and Lay, 1999]. Synthetic seismograms generated for each depth are compared with the observed seismograms, with the optimal time function and depth producing the lowest misfit. In all cases these depths are compared to the relocated depths (section 3.3). Duration is measured from the final event time function as the time from onset to the first zero crossing, capturing the majority of the moment release of the event. These durations are then scaled by the cube root of seismic moment (Mo) to remove the effect of increasing duration with increasing seismic moment, and normalized by the duration of an Mw = 6.0 event [e.g., Bilek and Lay, 1999]. 3.3. Earthquake Relocations [8] Teleseismically recorded earthquakes along the Sunda system have been relocated using the Engdahl, van der Hilst, and Buland (EHB) method [Engdahl et al., 1998]. This method uses reported International Seismic Centre (ISC) and National Earthquake Information Center (NEIC) reported phases and increases location accuracy using itera- tive relocation with dynamic phase identification, variable Figure 1. Setting of the 2010 October Mw = 7.8 tsunami phase weighting, ellipticity and station patch corrections, and earthquake (red star, NEIC location, red square and focal the ak135 velocity model. Only a small percentage of depth mechanism from GCMT, www.globalcmt.org) along the phases (pP, pwP, sP) are reported to ISC/NEIC, but the subduction zone between the Australian and Sunda plates. inclusion of these phases can significantly improve depth Plate boundary from Bird [2003]. Aftershocks (defined in determination. time relative to mainshock) of the 2010 event (diamonds, [9] For earthquakes prior to 2010, we have integrated NEIC locations between 25 October and 06 December) additional depth phases derived using automated methods show significant activity to the north and updip of the main- to better constrain hypocentral depths. This frequency‐ shock location. Other significant earthquakes in the region based technique identifies depth phase onset times on both also shown, including the 1907 tsunami earthquake (purple velocity and displacement waveforms by searching for star [Kanamori et al., 2010]) and rupture areas for earth- abrupt changes in the gradient of the power spectral den- quakes Mw > 7.5 (dashed and solid colored lines [Hsu et al., sity function (see Pesicek et al. [2010] for further details). 2006; Konca et al., 2008]). For the 2010 mainshock and aftershocks, depth phases were derived using the automated method but also verified by analyst (Figure 3). Analyst verification was done because depth phases generally have low frequency con- dip direction to define moment subevents on a fault plane tent as they propagate through near surface layers where defined by the GCMT geometry. Subevent moment pulses, there is high attenuation and may be obscured in the P‐ defined with 5 overlapping triangles of 10 s duration, are wave coda or noise, making auto‐identification difficult. removed from the P and SH waveforms at various timing and Additionally, not all ISC phase picks for the 2010 sequence fault grid positions, allowing for synthetic seismograms to be have been reported due to the 2‐year lag in compiling such a constructed for each of the observations. Moment rate func- large database. For this study, it is imperative that reported tion and moment distribution at each grid point are computed, depth phases for potentially shallow and complex earth- and we choose the solution that minimizes misfit between the quakes be correct. observed and synthetic seismograms (Figure 2b). Slip is [10] We find a common depth that satisfies both arrival‐ computed from the moment distribution, using a value of based and waveform modeling depth constraints for all 20 GPa for fault rigidity, appropriate for the shallow region earthquakes along this margin (Figure 4) (see auxiliary of this fault zone based on global and regional estimates material).1 Earthquakes with multiple local minima in the from earthquake parameters [Bilek and Lay, 1999; Bilek, waveform modeling misfit function are flagged, and the 2007].

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