Ya-Ju Hsu

Frictional afterslip following the 2005 Nias-Simeulue earthquake, Sumatra

Continuously recording Global Positioning System stations near the 28 March 2005 rupture of the Sunda megathrust [moment magnitude (Mw) 8.7] show that the earthquake triggered aseismic frictional afterslip on the subduction megathrust, with a major fraction of this slip in the up-dip direction from the main rupture. Eleven months after the main shock, afterslip continues at rates several times the average interseismic rate, resulting in deformation equivalent to at least a Mw 8.2 earthquake. In general, along-strike variations in frictional behavior appear to persist over multiple earthquake cycles. Aftershocks cluster along the boundary between the region of coseismic slip and the up-dip creeping zone. We observe that the cumulative number of aftershocks increases linearly with postseismic displacements; this finding suggests that the temporal evolution of aftershocks is governed by afterslip.

Deformation and Slip Along the Sunda Megathrust in the Great 2005 Nias-Simeulue Earthquake

Seismic rupture produced spectacular tectonic deformation above a 400-kilometer strip of the Sunda megathrust, offshore northern Sumatra, in March 2005. Measurements from coral microatolls and Global Positioning System stations reveal trench-parallel belts of uplift up to 3 meters high on the outer-arc islands above the rupture and a 1-meter-deep subsidence trough farther from the trench. Surface deformation reflects more than 11 meters of fault slip under the islands and a pronounced lessening of slip trenchward. A saddle in megathrust slip separates the northwestern edge of the 2005 rupture from the great 2004 Sumatra-Andaman rupture. The southeastern edge abuts a predominantly aseismic section of the megathrust near the equator.

Preseismic Deformation and Coseismic Displacements Associated with the 1999 Chi-Chi, Taiwan, Earthquake

The destructive 1999 Chi-Chi, Taiwan, earthquake ( M w 7.5) produced an approximately 100-km-long surface rupture, mostly along the previously recognized north-south-trending Chelungpu fault. Preseismic deformation in central Taiwan is realized from annually repeated Global Positioning System (GPS) data acquired during the 1992-1999 period. The total WNW-ESE shortening rate in the vicinity of the epicentral region, that is from the west coast to the western boundary of the Central Range, is up to 25 mm/yr. The crustal deformation before the Chi-Chi earthquake was essentially a uniaxial compressional strain of 0.36 μstrain/yr in the direction of 114°. The GPS measurements taken 0.2-2.7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements. Horizontal movements of 1.1-9.1 m in the NW-NNW directions are observed on the hanging wall (eastern side) of the fault. There is a northward-increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50° toward the east along the fault strike. In contrast, much smaller SE-SEE movements of 0.1-1.5 m are found on the footwall (western side) of the fault. The GPS data show 2.4-10.1 m of total horizontal offsets across the Chelungpu fault. Vertical offsets of 1.2-4.4 m with the eastern side up are also observed along the surface rupture. The uplift on the hanging wall decreases rapidly toward the east. It becomes subsidence at Sun Moon Lake and in the Puli-Wushe area. The stations on the footwall show subsidence of 0.02-0.26 m. The width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north. Manuscript received 13 October 2000.

Fault geometry and slip distribution of the 1999 Chi-Chi, Taiwan earthquake imaged from inversion of GPS data

GPS measurements of coseismic displacements from the 1999, Chi-Chi, Taiwan earthquake are modeled using elastic dislocation theory. We find that a single fault plane cannot fit the data, but rather a curved fault surface consisting of multiple segments dipping 20–25° best fits the observations. The model fault exhibits reverse and left-lateral slip on a 75 km long N-S trending segment and reverse and right-lateral slip on a 25 km E-W trending segment at the northern end of the rupture. The 21° dipping E-W segment is inconsistent with previous interpretations of high angle tear faulting.

A two-dimensional dislocation model for interseismic deformation of the Taiwan mountain belt

We use a Global Positioning System (GPS)-derived surface velocity field of Taiwan for the time period between 1993 and 1999 to infer interseismic slip rates on subsurface faults. We adopt a composite elastic half-space dislocation model constrained by the observed horizontal velocities projected into the direction of plate motion (306‡). The GPS data are divided into northern and southern regions and the velocities in each region are projected into single profiles. The model fault geometry includes a shallowly dipping decollement, based on the balanced geological cross-sections in the Coastal Plain and Western Foothills, and a two-segment fault representing the Longitudinal Valley Fault (LVF) in eastern Taiwan. The decollement is composed of two fault segments, one extending west under the Central Range (CR) and one extending east of the LVF, with estimated slip rates of about 35 and 80 mm/yr, respectively. The optimal geometry of decollement is subhorizontal (2‡V11‡) at a depth of 8V9 km. The inferred surface location of the western end point of dislocation in the northern profile is located 15 km east of the Chelungpu Fault, while in the southern section, it is located beneath the Chukou Fault. The elastic dislocation model successfully matches the horizontal velocity data, and predicts elastic strain accumulation in the Western Foothills that will presumably be released in future earthquakes. However, considered over multiple earthquake cycles, our model cannot explain the topography of the CR and thus fails to predict the active mountain building process in Taiwan. This failure indicates that both horizontal and vertical velocity fields require a more complex rheological model that incorporates inelastic behavior.

Focal-Mechanism Determination in Taiwan by Genetic Algorithm

We determined the focal-mechanism solutions for earthquakes with mag- nitude ML ≥4:0 that occurred in the Taiwan region between 1991 and 2005. First- motion polarities of P waves recorded at over 700 seismic stations in Taiwan were used. Because of the large number of events and stations involved, we implemented the genetic algorithm in a nonlinear global search for the focal-mechanism solutions. The algorithm was tuned and validated through synthetic tests. We finally determined the focal mechanisms of 1635 events with good qualities among 4188 earthquakes. Focal-mechanism solutions for a majority of the earthquakes display a dominant pat- tern of thrust-fault type reflecting the compressive stress field due to the plate colli- sion. Normal-fault events occurred at intermediate depths in subduction zones, which is likely the result of the bending of the subducting slabs. Strike-slip faults are also found within the Eurasia plate around the Peikang Basement High and in collision zones near Ilan where the geometry of the colliding plates is complex. Our study pro- vides a database of focal mechanisms for studying seismogenic structures and plate tectonics. This database can also be used by structural seismologists to compute syn- thetics for waveform tomography studies.

Postseismic deformation following the 1999 Chi-Chi earthquake, Taiwan: Implication for lower-crust rheology

On 1999 September 21, the Mw 7.6 Chi-Chi earthquake ruptured a segment of the Chelungpu Fault, a frontal thrust fault of the Western Foothills of Taiwan. The stress perturbation induced by the rupture triggered a transient deformation across the island, which was well recorded by a wide network of continuously operating GPS stations. The analysis of more than ten years of these data reveals a heterogeneous pattern of postseismic displacements, with relaxation times varying by a factor of more than ten, and large cumulative displacements at great distances, in particular along the Longitudinal Valley in eastern Taiwan, where relaxation times are also longer. We show that while afterslip is the dominant relaxation process in the epicentral area, viscoelastic relaxation is needed to explain the pattern and time evolution of displacements at the larger scale. We model the spatiotemporal behavior of the transient deformation as the result of afterslip on the decollement that extends downdip of the Chelungpu thrust, and viscoelastic flow in the lower crust and in the mid-crust below the Central Range. We construct a model of deformation driven by coseismic stress change where afterslip and viscoelastic flow are fully coupled. The model is compatible with the shorter relaxation times observed in the near field, which are due to continued fault slip, and the longer characteristic relaxation times and the reversed polarity of vertical displacements observed east of the Central Range. Our preferred model shows a viscosity of 0.5–1 X 10^(19) Pa s at lower-crustal depths and 5 X 10^(17) Pa s in the mid-crust below the Central Range, between 10 and 30 km depth. The low-viscosity zone at mid-crustal depth below the Central Range coincides with a region of low seismicity where rapid advection of heat due to surface erosion coupled with underplating maintain high temperatures, estimated to be between 300°C and 600°C from the modeling of thermo-chronology and surface heat flow data.

The potential for a great earthquake along the southernmost Ryukyu subduction zone

Interseismic GPS data along the Hualien-Suao coast (NE Taiwan) shows a pattern of strain accumulation that is consistent with a potential future large shallow earthquake along the southernmost Ryukyu subduction zone. The measured shortening rate parallel to the Ryukyu Trench is 80 mm/yr, about twice of the shortening rate perpendicular to the Ryukyu Trench. We invert for slip-deficit rates and the geometric configuration of the plate interface. Our preferred fault model dips 10° northward and extends about 70 km from the Ryukyu Trench to a depth of 13 km. The slip deficit rate exhibits a left-lateral motion of 78 mm/yr and a normal motion of 36 mm/yr on a 290°-trending fault. The slip rate budget of the southernmost Ryukyu subduction zone is close to the plate convergence rate, suggesting the plate interface is fully locked. Assessments of seismic hazard in this region need to consider the potential threat from M_w 7.5~8.7 tsunami earthquakes generated by shallow ruptures.

Three-dimensional FEM derived elastic Green's functions for the coseismic deformation of the 2005 Mw 8.7 Nias-Simeulue, Sumatra earthquake

Using finite element models (FEMs), we examine the sensitivity of surface displacements to the location of fault slip, topography, and three-dimensional variations in elastic moduli in the context of a 2-D infinite thrust fault. We then evaluate the impact of these factors and fault geometry on surface displacements and estimates of the distribution of coseismic slip associated with the 2005 M_w 8.7 Nias-Simeulue, Sumatra earthquake. Topographic effects can be significant near the trench, where bathymetric gradients are highest and the fault is closest to the free surface. Variations in Youngs modulus can significantly alter predicted deformation. Surface displacements are relatively insensitive to perturbations in Poissons ratio for shear sources, but may play a stronger role when the source has a dilatational component. If we generate synthetic displacements using a heterogeneous elastic model and then use an elastic half-space or layered earth model to estimate the slip distribution and fault geometry, we find systematic residuals of surface displacements and different slip patterns compared to the input fault slip model. The coseismic slip distributions of the 2005 earthquake derived from the same fault geometry and different material models show that the rupture areas are narrower in all tested heterogeneous elastic models compared to that obtained using half-space models. This difference can be understood by the tendency to infer additional sources in elastic half-space models to account for effects that are intrinsically due to the presence of rheological gradients. Although the fit to surface observations in our preferred 3-D FEM model is similar to that from a simple half-space model, the resulting slip distribution may be a more accurate reflection the true fault slip behavior.

Coseismic displacements and slip distribution from GPS and leveling observations for the 2006 Peinan earthquake (Mw 6.1) in southeastern Taiwan

Since 2001, we have set up a dense geodetic network with 52 campaign-mode GPS sites and seven continuously recording GPS stations as well as six leveling routes in the Taitung area, Taiwan. Our aim was to better characterize near-fault crustal deformation of active faults at the plate suture of the Philippine Sea plate and Eurasia in southeastern Taiwan. On 1 April 2006, a moderate shallow earthquake (Mw 6.1, depth 10.8 km) occurred within this network. This earthquake resulted from rupturing of a geologically unknown or suspected fault (called the Y fault) located underneath the eastern margin of the Central Range. After removing the impacts of secular motions and postseismic slip, we estimated the coseismic displacements of the Peinan earthquake from the GPS and leveling measurements before and after the main shock. Three deformation types with distinct slip behaviors were revealed in three different regions: (1) near the epicenter—around 45 mm movement in the S-SSW direction with +20 to −20 mm vertical motion, in the northern part of the Y fault; (2) south of the epicenter across the southern part of the Y fault—approximately 35 mm in a westward movement with −60 mm subsidence (footwall side) and 40 mm in a SSW movement with at least 50 mm uplift (hanging-wall side), in the southern part of the Y fault; (3) northeast away from the epicenter—about 10 mm in a northward displacement with +15 to −10 mm vertical motions, in the Longitudinal Valley and on the western flank of the Coastal Range. This unique coseismic deformation pattern sheds new light on the characteristics of the suture zone between the Eurasian and Philippine Sea plates at the southernmost Longitudinal Valley. We used GPS and leveling measurements to invert for the fault geometry and the coseismic slip distribution. The optimal modeled fault is an 80° west-dipping fault at a depth of 0.5–20 km. The highest slip of about 0.33 m is located to the south of the hypocenter at a depth of 9–16 km. The total geodetic moment in our optimal model is 2.3 × 1018 Nt-m, which is equivalent to an earthquake of Mw 6.2. The surface coseismic displacements as well as the inferred coseismic slip distribution indicate a drastic change of slip behaviors in the middle of the Y fault. The left-lateral slippage near the hypocenter turned dramatically to reverse faulting with left-lateral component as rupturing propagated to the southern portion of the fault, suggesting that a possible right-lateral faulting occurred that coseismically cross cut the northern middle Peinanshan massif in the Longitudinal Valley.