Michael Antolik

The 1994 Java tsunami earthquake: Slip over a subducting seamount

On June 2, 1994, a large subduction thrust earthquake (Ms 7.2) produced a devastating tsunami on the island of Java. This earthquake had a number of unusual characteristics. It was the first recorded large thrust earthquake on the Java subduction zone. All of the aftershock mechanisms exhibit normal faulting; no mechanisms are similar to the main shock. Also, the large tsunami and the relatively low energy radiated by the main shock have led to suggestions that this earthquake might have involved slow, shallow rupture near the trench, similar to the 1992 Nicaragua earthquake. We first relocate the main shock and the aftershocks. We then invert long-period surface waves and broadband body waves to determine the depth and spatial distribution of the main shock slip. A dip of 12°, hypocenter depth of 16 km and moment of 3.5×l020 N m (Mw 7.6) give the best fit to the combined seismic data and are consistent with the plate interface geometry. The source spectrum obtained from both body and surface waves has a single corner frequency (between 10 and 20 mHz) implying a stress drop of ∼0.3 MPa. The main energy release was preceded by a small subevent lasting ∼12 s. The main slip occurred at ∼20 km depth, downdip and to the NW of the hypocenter. This area of slip is collocated with a prominent high in the bathymetry that has been identified as a subducting seamount. We interpret the Java earthquake as slip over this subducting seamount, which is a locked patch in an otherwise decoupled subduction zone. We find no evidence for slow, shallow rupture. No thrust aftershocks are expected if the entire locked zone slipped during the main shock, but extension of the subducting plate behind the seamount would promote normal faulting as observed. It seems probable that such a source model could also explain the size and timing of the observed tsunami.

Rupture Process of the 26 January 2001 Mw 7.6 Bhuj, India, Earthquake from Teleseismic Broadband Data

We investigate the rupture process of the 26 January 2001 Bhuj, India, M w 7.6 earthquake through inversion of teleseismic broadband body waves. This earthquake ranks as one of the most important recent events due to its occurrence within a stable continental interior, where such events are rare. The Bhuj earthquake occurred on a moderately dipping blind thrust fault within an ancient failed rift. About 70% of the seismic moment released in the earthquake was confined to a very small area (∼375 km2) surrounding the hypocenter and at depths below 12 km. The static stress drop of the Bhuj earthquake is anomalously high (∼20 MPa). The source time history of the event indicates very rapid onset to the moment release and most likely high slip velocities within the deep asperity. This suggests that some of the damage near the epicenter may have been caused by anomalously high-frequency ground motions. The teleseismic data also indicate the presence of a second area of large slip in the shallow part of the Bhuj fault, although the depth extent of this shallow large-slip area is not resolved. Comparisons of the predicted ground motions with observed intensities suggest that substantial slip occurred in the upper 10 km of the fault in order to explain the distribution of high intensities to the west and northwest of the fault. The upper surface layers near the Bhuj fault consist of unconsolidated, low-rigidity sediments and alluvium. The upper ∼ 10 km of the Bhuj fault may therefore be in a conditionally stable region that normally deforms through aseismic creep and can sustain seismic rupture only when dynamically stressed by rupture of the high-strength deep asperity. We suggest that this deep asperity may be related to a lithologic anomaly of ultramafic composition.

The 14 November 2001 Kokoxili (Kunlunshan), Tibet, Earthquake: Rupture Transfer through a Large Extensional Step-Over

The 14 November 2001 Kokoxili, Tibet, earthquake ( M w 7.8) ruptured ∼400 km of the western Kunlun fault in northern Tibet. We apply two inversion methods to P and SH waves recorded by the Global Seismographic Network to recover the spatial and temporal history of the rupture process. The observed surface rupture consists of two strike-slip segments, offset by an extensional step-over. Little surface rupture was observed in the graben system in this step-over, which is approximately 45 km long and over 10 km wide. Our results imply that the rupture did not jump this large gap, but that the rupture was continuous through the graben. The earthquake began with a small strike-slip subevent, presumably along the westernmost of the two segments. This was followed 5 sec later by a subevent of about the same magnitude but having an oblique mechanism with a large normal-faulting component. The likely location for this subevent is within the graben. This oblique-slip event probably enabled transfer of the rupture onto the main Kunlun fault. The main moment release began ∼18 sec after rupture initiation and propagated over 350 km eastward along the Kunlun fault. Slip on this segment was very heterogeneous, averaging ∼2 m for the first 200 km, followed by a sharp increase to a maximum of 7.5 m within the next 50 km, and then a rapid decline. The average rupture velocity along the main segment was high (∼3.6 km sec -1 ) and probably exceeded the local shear-wave velocity. The M w 7.8 Kokoxili earthquake had a longer surface rupture and faster average rupture velocity, radiated more energy, and probably had a lower average fracture energy than the November 2002, M w 7.9 Denali fault (Alaska) earthquake. Evidence suggests that the velocity of the rupture front dropped significantly after passing the point of maximum slip, implying a large difference in frictional strength between the two ends of the fault. Online material : Waveform fits to teleseismic body waves.

Rupture processes of large deep‐focus earthquakes from inversion of moment rate functions

We determine the source time histories of five recent (1994–1996) large, deep-focus earthquakes using a method that inverts for fault slip from far-field moment rate functions. The moment rate functions are obtained through the deconvolution of multiple body wave phases using broadband records from Global Seismic Network (GSN) and GEOSCOPE stations. Tests of this method on synthetic data indicate that it is successful in determining the low-frequency rupture characteristics of deep earthquakes under a variety of complicating conditions. We find that source parameters such as average rupture velocity and stress drop are highly variable among the events studied and that some unusual characteristics exhibited by the June 9, 1994, great Bolivian earthquake are also found for other events. Comparison of the slip distributions with background seismicity and aftershock locations indicates that most of the moment release for large deep-focus earthquakes is probably occurring within the active slab interior. This provides further evidence that temperature-controlled mechanisms (such as transformational faulting) play a large role in deep earthquake faulting. Most of the events studied also show a tendency for horizontal rupture propagation, suggesting that isobaric processes may be an important factor in controlling progression of the rupture. Large gaps in the slip distributions point to possible occurrence on multiple fault planes.

Validation of Regional and Teleseismic Travel-Time Models by Relocating Ground-Truth Events

Recently developed 3D global seismic velocity models have demon- strated location improvements through independent regional and teleseismic travel- time calibration. Concurrently, a large set of high-quality ground-truth (GT) events with location accuracies 10 km or better (GT0-GT10) has been collected for Europe, the Mediterranean, North Africa, the Middle East, and western Eurasia. In this study we demonstrate event location improvements using this new data set by applying the regional and teleseismic model-based travel-time calibrations (independently and jointly). Besides relocating events using all arrivals, a subset of the GT events was also relocated using controlled station geometries generated from a constrained boot- strapping technique. This approach simulates sparse networks and reduces the effect of correlated errors to ensure valid 90% error ellipse coverage statistics. With respect to the GT events, we compared event relocations, with and without travel-time cal- ibrations, considering statistics of mislocation, error ellipse area, 90% coverage, or- igin time bias, origin time errors, and misfit. Relocations of over 1200 GT events show that Pn and/or P calibration reduced mislocation for 60%-70% of the events. Joint regional Pn and teleseismic P travel-time calibration provided the largest lo- cation improvements and achieved approximately GT5 accuracy levels. Due to cor- related errors, event locations using large numbers of stations have deficient 90% error ellipse coverage. However, the coverages derived from the model errors are appropriate for the simulated sparse regional and teleseismic networks. Our valida- tion effort demonstrates that the global model-based calibrations of Pn and teleseis- mic P travel times reduce both location bias and uncertainty over wide areas.

Compound rupture of the great 1998 Antarctic plate earthquake

The March 25, 1998, Antarctic plate earthquake ruptured a portion of the Antarctic plate more than 200 km west of its boundary with the Australian plate. The Harvard centroid moment tensor solution indicates that the earthquake was primarily a strike-slip event, but the large non-double-couple component of the moment tensor suggests considerable complexity in the rupture process. We use a finite fault inversion method to determine details of the rupture process from teleseismic body waves recorded by the Global Seismic Network. The P waves are poorly fit by one or more subevents having only a strike-slip mechanism. We find that the presence of a large oblique-normal faulting subevent located to the east of the hypocenter is necessary to improve the fit. This subevent combines with a larger strike-slip subevent to the west to comprise the main moment release in the earthquake and is the cause of the large non-double-couple component in the long-period focal mechanism. The earthquake exhibited very high slip and high stress drop compared with most interplate strike-slip events and the rupture was largely confined to the upper 15–20 km of the lithosphere. Both constituent focal mechanisms indicate that this part of the Antarctic plate is under NW-SE oriented tension, although the origin of these stresses is unknown.