Susan L. Bilek

Rigidity variations with depth along interplate megathrust faults in subduction zones

The worlds largest earthquakes occur along the contact between subducting and overriding tectonic plates in subduction zones. Rock and sediment properties near this plate interface exert important controls on the frictional behaviour of faults and earthquake rupture dynamics. An important material property to define along the plate interface is the rigidity (the resistance to shear deformation). Rigidity affects the degree of earthquake shaking generated by a given fault displacement through its influences on seismic wave speed and earthquake rupture velocity. Here we present an investigation of the relationship between the duration of earthquake rupture and source depth, which yields estimates of rigidity variation along plate interfaces in six subduction zones in the circum-Pacific region. If stress drop is assumed constant, rigidity appears to increase with depth in each seismogenic zone by a factor of ∼5 between depths of 5 and 20 km. This result is consistent with the hypothesis that ‘tsunami’ earthquakes (characterized by large slip for a given seismic moment and slow rupture velocity) occur in regions of low rigidity at shallow depths. These rigidity trends should provide an important constraint for future fault-zone and earthquake-modelling efforts.

Do subducting seamounts generate or stop large earthquakes

Seamount subduction is a common process in subduction zone tectonics. Contradicting a widely held expectation that subducting seamounts generate large earthquakes, seamounts subduct largely aseismically, producing numerous small earthquakes. On rare occasions when they do produce relatively large events, the ruptures tend to be complex, suggesting multiple rupture patches or faults. We explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault. The complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures.

Control of seafloor roughness on earthquake rupture behavior

Earthquake rupture complexity is described for three recent large underthrusting earthquakes along the Costa Rican subduction zone, the 1983 Osa, 1990 Nicoya Gulf, and 1999 Quepos events. These earthquakes occurred in regions characterized by distinctly different morphologic features on the subducting plate. The 1990 and 1999 events occurred along linear projections of subducting seamount chains and had fairly simple earthquake rupture histories. Both events are interpreted as failure of the basal contact of closely spaced isolated seamounts acting as asperities. In contrast, the 1983 event occurred along the subducting Cocos Ridge and had a complex rupture history. Comparison of rupture characteristics of these large underthrusting earthquakes with size and location of subducting features provides evidence that seamounts can be subducted to seismogenic depths and that variations in seafloor bathymetry of the subducting plate strongly influence the earthquake rupture process.

Implications of the 26 December 2004 Sumatra–Andaman Earthquake on Tsunami Forecast and Assessment Models for Great Subduction-Zone Earthquakes

Results from different tsunami forecasting and hazard assessment mod- els are compared with observed tsunami wave heights from the 26 December 2004 Indian Ocean tsunami. Forecast models are based on initial earthquake information and are used to estimate tsunami wave heights during propagation. An empirical forecast relationship based only on seismic moment provides a close estimate to the observed mean regional and maximum local tsunami runup heights for the 2004 Indian Ocean tsunami but underestimates mean regional tsunami heights at azimuths in line with the tsunami beaming pattern (e.g., Sri Lanka, Thailand). Standard forecast models developed from subfault discretization of earthquake rupture, in which deep- ocean sea level observations are used to constrain slip, are also tested. Forecast models of this type use tsunami time-series measurements at points in the deep ocean. As a proxy for the 2004 Indian Ocean tsunami, a transect of deep-ocean tsunami amplitudes recorded by satellite altimetry is used to constrain slip along four subfaults of the M 9 Sumatra-Andaman earthquake. This proxy model performs well in comparison to observed tsunami wave heights, travel times, and inundation patterns at Banda Aceh. Hypothetical tsunami hazard assessments models based on end- member estimates for average slip and rupture length (M w 9.0-9.3) are compared with tsunami observations. Using average slip (low end member) and rupture length (high end member) (M w 9.14) consistent with many seismic, geodetic, and tsunami inversions adequately estimates tsunami runup in most regions, except the extreme runup in the western Aceh province. The high slip that occurred in the southern part of the rupture zone linked to runup in this location is a larger fluctuation than expected from standard stochastic slip models. In addition, excess moment release (9%) deduced from geodetic studies in comparison to seismic moment estimates may gen- erate additional tsunami energy, if the exponential time constant of slip is less than approximately 1 hr. Overall, there is significant variation in assessed runup heights caused by quantifiable uncertainty in both first-order source parameters (e.g., rupture length, slip-length scaling) and spatiotemporal complexity of earthquake rupture.

Effect of depth‐dependent shear modulus on tsunami generation along subduction zones

Estimates of the initial size of tsunamis generated by subduction zone earthquakes are significantly affected by the choice of shear modulus at shallow depths. Analysis of over 360 circum-Pacific subduction zone earthquakes indicates that for a given seismic moment, source duration increases significantly with decreasing depth (Bilek and Lay, 1998; 1999). Under the assumption that stress drop is constant, the increase of source duration is explained by a 5-fold reduction of shear modulus from depths of 20 km to 5 km. This much lower value of shear modulus at shallow depths in comparison to standard earth models has the effect of increasing the amount of slip estimated from seismic moment determinations, thereby increasing tsunami amplitude. The effect of using depth dependent shear modulus values is tested by modeling the tsunami from the 1992 Nicaraguan tsunami earthquake using a previously determined moment distribution (Ihmle, 1996a). We find that the tide gauge record of this tsunami is well matched by synthetics created using the depth dependent shear modulus and moment distribution. Because excitation of seismic waves also depends on elastic heterogeneity, it is important, particularly for the inversion of short period waves, that a consistent seismic/tsunami shear modulus model be used for calculating slip distributions.

Radiated seismic energy and earthquake source duration variations from teleseismic source time functions for shallow subduction zone thrust earthquakes

[1] Earthquake source time functions deconvolved from teleseismic broadband P wave recordings are used to examine rupture variations for 417 underthrusting earthquakes located on the interplate interface in circum-Pacific subduction zones. Moment-scaled duration of significant moment release varies with depth, with longer-duration events occurring in the shallowest 20 km of the megathrusts. The source time functions are also used to estimate radiated seismic energy. Two estimates are obtained: a simple scaled triangle substitution for the moment release history provides a minimum estimate, while integration of the time function shape provides an estimate limited only by the bandwidth of the teleseismic deconvolutions. While these energy calculations underestimate total energy, they enable systematic comparisons of rupture process within and between subduction zones. We do not find significant depth dependence for radiated energy overall, but some regions do show mild trends of increasing energy/seismic moment ratios (E/Mo) with increasing source depth that correspond to rupture duration variations in those regions. The observations of longer rupture duration at shallow depth with moderate E/Mo may be due to heterogeneous friction and structural features on the shallow plate interface. INDEX TERMS: 7215 Seismology: Earthquake parameters; 7230 Seismology: Seismicity and seismotectonics; 8164 Tectonophysics: Stresses—crust and lithosphere; KEYWORDS: radiated energy, shallow subduction, source parameters

Differences in tsunami generation between the December 26, 2004 and March 28, 2005 Sumatra earthquakes

Source parameters affecting tsunami generation and propagation for the Mw > 9.0 December 26, 2004 and the Mw = 8.6 March 28, 2005 earthquakes are examined to explain the dramatic difference in tsunami observations. We evaluate both scalar measures (seismic moment, maximum slip, potential energy) and finite-source representations (distributed slip and far-field beaming from finite source dimensions) of tsunami generation potential. There exists significant variability in local tsunami runup with respect to the most readily available measure, seismic moment. The local tsunami intensity for the December 2004 earthquake is similar to other tsunamigenic earthquakes of comparable magnitude. In contrast, the March 2005 local tsunami was deficient relative to its earthquake magnitude. Tsunami potential energy calculations more accurately reflect the difference in tsunami severity, although these calculations are dependent on knowledge of the slip distribution and therefore difficult to implement in a real-time system. A significant factor affecting tsunami generation unaccounted for in these scalar measures is the location of regions of seafloor displacement relative to the overlying water depth. The deficiency of the March 2005 tsunami seems to be related to concentration of slip in the down-dip part of the rupture zone and the fact that a substantial portion of the vertical displacement field occurred in shallow water or on land. The comparison of the December 2004 and March 2005 Sumatra earthquakes presented in this study is analogous to previous studies comparing the 1952 and 2003 Tokachi-Oki earthquakes and tsunamis, in terms of the effect slip distribution has on local tsunamis. Results from these studies indicate the difficulty in rapidly assessing local tsunami runup from magnitude and epicentral location information alone.

Analysis of the 23 June 2001 Mw = 8.4 Peru underthrusting earthquake and its aftershocks

On 23 June 2001, a M w = 8.4 underthrusting earthquake occurred in the southern Peru subduction zone, followed by several large aftershocks, including 26 June (M w = 6.7) and 7 July (M w , = 7.5). Broadband analyses of seismic data for the largest of these earthquakes show southeastward rupture of 180 km along the portion of the subduction zone previously ruptured in 1868 (M w 8.8-9). Moment release distributions determined are consistent with aftershock location patterns. Earthquake rupture mode varies along southern Peru, from larger multi-segment rupture in 1868 to several smaller segment ruptures in 2001, similar to rupture variation observed in northern Peru and other subduction zones. Based on models of subduction zone segment interaction, the 2001 earthquake sequence may suggest a shorter recurrence time for future earthquakes along this portion of the Peru-Chile subduction zone.

Using Earthquake Source Durations along the Sumatra–Andaman Subduction System to Examine Fault-Zone Variations

The 26 December 2004 great Sumatran–Andaman earthquake in the Indian Ocean caused extensive damage and loss of life from intense shaking and the resulting tsunami. Several studies of this earthquake suggest that portions of the fault ruptured at variable speeds, with both fast and slow rupture velocities observed along the 1200 km long rupture length. Variations in rupture velocity during the earthquake may indicate along-strike variations in megathrust frictional conditions that may influence other earthquakes along the zone. Previous work on global subduction zone systems suggests depth-dependent frictional conditions arising from heterogeneous conditions along the fault. In many of the circum-Pacific subduction zones, shallow earthquakes along the subduction megathrust have longer scaled source durations than deeper earthquakes, possibly resulting from variations in frictional conditions with depth. This study focuses on thrust mechanism earthquakes on the Sumatra– Andaman megathrust, examining aftershocks with M w >6.0 of the 2004 earthquake, as well as earthquakes that occurred in the region between 1992 and 2004. Source duration, depth, and slip distribution are determined for this set of earthquakes to explore the possibility of both along-strike and depth-dependent variations in source parameters and frictional conditions. There is evidence of depth-dependent source parameters for these events, with longer scaled durations for the shallower earthquakes, consistent with previous global studies. No temporal change is apparent in this relationship, as source parameters for previous large earthquakes are similar to those after the earthquake. Along-strike patterns suggest long-duration character for several earthquakes in the southern portion of the rupture zone but no strong evidence of slow character in the northern portion of the rupture zone. It appears unlikely that any long-term variations in the fault-zone character influenced possible slow rupture of the 2004 earthquake.

Slab pull, slab weakening, and their relation to deep intra-slab seismicity

Received 8 March 2005; revised 3 June 2005; accepted 21 June 2005; published 21 July 2005. [1] Subduction zone seismicity is highly variable. Great earthquakes occur at few subduction zones around the world, with significant variation in size and frequency of deep events. Interactions between overriding and subducting plates and slab pull strength for individual plates provide a framework for understanding these variations. Previous work suggests an inverse correlation between great earthquake moment release and the degree to which the subducted slab is connected to the surface plate. We find positive correlations between degree of plate-slab attachment and moment release from intermediate and deep earthquakes. This implies that shallow slab weakening that occurs at trenches where compressive stresses (and great earthquakes) dominate not only detaches slabs from plates, but is maintained as the slab descends, discouraging deep seismicity. Regions of low shallow moment release are consistent with extensional shallow stress regimes and undamaged slabs. Such slabs maintain mechanical strength during descent and deform seismogenically at depth. Citation: Bilek, S. L., C. P. Conrad, and C. LithgowBertelloni (2005), Slab pull, slab weakening, and their relation to deep intra-slab seismicity, Geophys. Res. Lett., 32, L14305,