U54A-01
Heterogeneous coupling on the Sumatra megathrust constrained from geodetic and paleogeodetic measurements
Geodetic and paleogeodetic measurements of strain above the Sumatran portion of the Sunda subduction zone reveal a heterogeneous pattern of coupling along the subduction megathrust. Annual banding in coral heads provides vertical rates of deformation spanning the last half of the 20th century, and repeated GPS surveys between 1991 and 2001 and continuous measurements at GPS stations operated since 2002 provide horizontal velocities. The area of the plate interface within which the coupling is high is only a few tens of kilometers wide near the Equator but increases to a width of about 175 km farther south. The widest sections of this locked fault zone coincide with the rupture areas of four major historical Mw>8.5 interplate earthquakes. The section that ruptured during the Mw 8.7 Nias-Simeulue earthquake of 2005 released half of the moment deficit that had accumulated since its previous rupture in 1861. Farther south, beneath the Mentawai islands, overlapping ruptures of the locked fault zone produced giant earthquakes in 1797 and 1833. The accumulated slip deficit since these events is slowly reaching the amount of slip that occurred during the 1833 earthquake but already exceeds the slip that occurred during the 1797 earthquake. Thus, re-rupture of at least part of the Mentawai patch in the near future seems quite likely. In contrast, coupling is low in the Batu islands near the Equator and around Enggano island at about 5°S, where only moderate earthquakes (Mw<8.0) have occurred in the past two centuries. Temperature might influence the mode of slip along the plate interface, through its effect on the rheology of sediments at the plate interface. Other influences, such as structures on the subducting plate, may also play a role. In particular, subduction of the Investigator Fracture Zone near the Equator coincides with the relatively low coupling there.
U54A-02
Uplift and Subsidence during the September 2007 Mentawai Earthquakes from Field Observations
Rupture of the Sunda megathrust offshore southern Sumatra during the recent 16-hour sequence of MW 8.4, 7.9, and 7.0 earthquakes resulted in uplift and subsidence of the Mentawai Islands and the Sumatran mainland coast. This region last experienced rupture on the megathrust during a pair of earthquakes in 1797 and 1833. Preliminary field accounts and initial continuous GPS data suggest that all uplift associated with the September 2007 earthquakes is within the region of uplift of the great 1833 earthquake. However, the magnitude of uplift is smaller and it does not extend throughout the entire length of the older, bigger rupture. Nor does it extend farther northwestward through the northwestern half of the uplifted region associated with the great 1797 earthquake. We will present field measurements of vertical deformation for the September 2007 events, derived from in situ measurements of coral microatolls and other biological and geological markers. This spatially dense set of observations will provide much tighter constraints on the deformation pattern than the sparse cGPS measurements alone. The field observations will permit a more robust comparison with the magnitudes and extent of uplift in 1797 and 1833, which are already known from fossil microatolls. New, better-constrained inversions for slip will be discussed in the context of strain accumulation rates over the past decade to half century. We anticipate that the refined sources will still be too small in both slip amount and rupture extent to have relieved most of the potential slip that has accumulated across the 700-km-long Mentawai patch since 1833. In light of this and other evidence for pre-historical earthquake couplets and triplets along the Sunda megathrust, we argue that the September 2007 sequence might be only the beginning of a new sequence of large ruptures of the Mentawai patch.
U54A-03
Post-Earthquake Multibeam Bathymetry and Backscatter Survey of 6400 sq. km. of the Accretionary Complex / Forearc in the Area of the Main 12 Sept. 07 Earthquake Rupture Zone
When the 12 September M8.4 Southern Sumatra Earthquake hit, TGS-Nopec was carrying out a multibeam survey in the Sunda Straits, between Java and Sumatra. Working in collaboration with UTIG, OSU, BGR, and BPPT, we designed a 48 hour target-of-opportunity survey in the forearc region of the earthquake in an area where two trench perpendicular multibeam and seismic lines had previously been acquired by BGR. The objectives of the survey were to fill in an area of previously unsurveyed seafloor, and to acquire a data set for before and after comparison with the BGR data. Approx. 1100 line km of multibeam data were acquired between 17 and 20 September on a series of trench-perpendicular lines spaced 8km apart (weather limitations constrained the line orientation). Steep slopes and high backscatter seafloor result in some areas of data dropout. Using a fixed swath width and equi-distant beam spacing, bathymetric grids have been created at a range of scales that provide detailed imagery of the seafloor. The toe of slope is at ~6000m, with several normal faults cutting the seafloor on the incoming plate. The toe region of the accretionary complex shows individual folds 10-40km long, with both seaward and landward vergence. Fold wavelength is ~4km, with fold axes showing a trend slightly oblique to structures higher up on the accretionary complex. The folds in the toe region are cut by ENE trending high angle faults. 20km landward of the toe, in water depths of 5000-3000m, there is a 10km wide region of complex geomorphology and steeper slopes (locally 20 degrees and higher) showing a more degraded character with numerous slump headscarps and linear canyons 5-10km long. Although the slope is dissected by numerous slumps we observe no evidence for recent slope failure in the adjacent forearc basins. We interpret this region to be the seafloor projection of a significant thrust ramp in the accretionary complex. Landward of this area we identify 2 continuous ridges over 80km long, but that in detail show an irregular seafloor morphology. At 2500m water depth, 65km from the toe of slope, a significant geomorphic break with 10-20 degree slopes extends the length of the survey area, with water depths shoaling to 800m. We interpret this to be the surface projection of an out-of-sequence thrust. Although we identify some large debris blocks (>1km in length) in the adjacent forearc basin, the blocks appear rounded, with no obvious source or associated smaller scale debris that would suggest relatively recent emplacement. We find no clear evidence for any large recent slope failures associated with the M8.4 Earthquake or its aftershocks.
U54A-04
FIELD SURVEY AND PRELIMINARY MODELING OF THE NEAR-FIELD TSUNAMI FROM THE BENGKULU EARTHQUAKE OF 12 SEPTEMBER 2007
In the aftermath of the Bengkulu earthquake of 12 September 2007, an International Post-Tsunami Survey Team visited the near field area, along a 280-km stretch of the coast, from Ipuh in the North to Bituhan in the South. Run-up values were measured at 19 locations, averaging 2 meters, with a maximum of 4 meters and inundation reaching 500 m near Lais (3.5 deg.sS; 102 deg.E). We use a number of preliminary source models featuring various distributions of slip along the fault plane as sources in a near simulation using the MOST code. The database of surveyed points is well modeled by a uniform slip on the fault plane, and more sophisticated source models fail to substantially improve the fit. Far field modeling of the tsunami confirmed its negligible character, resulting from a contained seismic moment (5E28 dyn*cm) and the presence of shallow bathymetry (featuring a ridge at only 200 m depth) in the epicentral area.
U54A-05
Tsunami waveform inversion of the 2007 Bengkulu, southern Sumatra earthquake
We have performed tsunami waveform inversion for the 2007 Bengkulu, southern Sumatra earthquake on September 12, 2007 (4.520°S, 101.374°E, Mw=8.4 at 11:10:26 UTC according to USGS), and found that the large slips were located on deeper part (> 20 km) of the fault plane, more than 100 km from the trench axis. The deep slip might have contributed the relatively small tsunami for its earthquake size. The largest slips more than 6 m were located beneath Pagais Islands, about 100-200 km northwest of the epicenter. The obtained slip distribution yields a total seismic moment of 3.6 × 1021 Nm (Mw = 8.3). The tsunami generated by this earthquake was recorded at many tide gauge stations located in and around the Indian Ocean. The DART system installed in deep ocean and maintained by Thai Meteorological Department (TMD) also captured this tsunami. We have downloaded the tsunami waveforms at 16 stations from University of Hawaii Sea Level Centerfs (UHSLC) and National Oceanic & Atmospheric Administrationfs (NOAA) web sites. The observed tsunami records indicate that the tsunami amplitudes were less than several tens of cm at most stations, around 1 m at Padang, nearest station to the source, and a few cm at DART station. For the tsunami waveforms inversion, we divided the source area (length: 250 km, width: 200 km) into 20 subfaults. Tsunami waveforms from each subfault (50 km × 50 km) or Greens functions were calculated by numerically solving the linear shallow-water long-wave equations. We adopted the focal mechanism of Global CMT solution (strike: 327°, dip: 12°, rake: 114°) for each subfault, and assumed a rise time of 1 min. The computed tsunami waveforms from the estimated slip distribution explain the observed waveforms at most of tide gauges and DART station. http://iisee.kenken.go.jp/staff/fujii/TsunamiTop.html
U54A-06
Real-time Tsunami Warning Operations at the NOAA West Coast/Alaska Tsunami Warning Center
The West Coast/Alaska Tsunami Warning Center (WCATWC) in Palmer, Alaska and the Pacific Tsunami Warning Center (PTWC) in Ewa Beach, Hawaii, provide tsunami warning services for a large portion of the world's coasts. The WCATWC has primary responsibility for providing tsunami detection, warnings, and forecasts to Canada, Puerto Rico, Virgin Islands, and all U.S. States except Hawaii. WCATWC also acts as back-up for the PTWC, requiring the center to constantly monitor global tsunami activities by rapidly detecting and evaluating earthquakes for their tsunamigenic potential. The Centers' goals are to issue initial messages as quickly as possible to alert those near the source to potential danger (assuming there is any), and to follow that with a reasonable forecast of impact level. With these goals in mind, a Watchstander's initial action is based entirely on estimates of tsunami potential from the earthquake's source parameters. The course of action for the first message is determined primarily by the earthquake's magnitude, location, tsunami history, tsunami travel time, estimated threat based on pre-computed models, and pre-set criteria. Supplemental messages, if necessary, are based on wave observations and forecasts generated from hydrodynamic models (which are calibrated with near real-time observations). In April 2006, the WCATWC increased staff level so that the Center can be staffed 24/7 with two watchstanders. Since then, the Center's response time for events within the primary area-of-responsibility has decreased to less than 5 minutes. In order to illustrate the WCATWC's real time tsunami warning operational environment, tsunami warning operation timelines for several tsunamigenic earthquakes - including the September 12 southern Sumatra 8.4 and the January 13 Kuril Island 8.1 earthquakes - are provided. The timelines highlight the key parameters and observations that guide tsunami warning operations chronicling the event through: 1) initial alarm, 2) earthquake analysis, 3) dissemination of information, 4) sea level observation/forecasting model calibration, and 5) supplemental message dissemination. The timelines demonstrate processing and dissemination capabilities.
U54A-07
A Guidebook to Help Coastal Sumatran Communities Prepare for Tsunamis
One way to save lives in future tsunamis in coastal Sumatran communities - where more than one million people live and where tsunamis can strike less than one half hour after the triggering earthquake - is to help these communities prepare themselves. To this end, GeoHazards International (GHI) has developed, with a team of advisors from the fields of earth science, civil engineering, emergency response management and social science, a tsunami preparedness guidebook that summarizes state-of-the-art research and worldwide experience in community tsunami preparedness. This guidebook (available at no cost on www.geohaz.org) introduces essential information about tsunamis, tsunami risk mapping, evacuation planning, community education, tsunami warning systems, and the reduction of damage that tsunamis can cause. It describes how to plan and conduct effective tsunami safety programs. Particular emphasis is placed on methods to evacuate quickly and safely all areas that could be flooded. Each section of the guidebook points to sources that provide supplementary, detailed information that may be important to particular communities. The guidebook is aimed at any person - a concerned citizen, government official, business leader, or member of a community organization - who is willing to become an advocate for local tsunami safety. Scientific expertise is not needed. GHI now seeks assistance in distributing this guidebook and in working with grassroots and international organizations to help Sumatran coastal communities use it to prepare for the next tsunami. http://www.geohaz.org