Jer-Ming Chiu

Basement seismicity beneath the Andean precordillera thin-skinned thrust belt and implications for crustal and lithospheric behavior

Data from a digitally recording seismic network in San Juan, Argentina, provide the first images of crustal scale basement faults beneath the Precordillera. This seismicity is near the boundary between the Precordillera (a thin-skinned thrust belt) and the Sierras Pampeanas (a region of thick-skinned basement deformation), two seismically active tectonic provinces of the Andean foreland. The seismicity data support models for this region in which crustal thickening, rather than magmatic addition or thermal uplift, plays the dominant mountain building role. The Precordillera seismicity occurs in three segments distributed north to south. The southern segment is an area of diffuse activity extending across the Precordillera and eastward into the Sierras Pampeanas that shows no patterns in map or cross section. The northern and central segments have well-defined dipping planes that define crustal scale faults extending from 5 to 35 km depth. It is clear from the relative fault geometries that the overlying Precordillera is not simply related to the basement activity. The seismicity here may result from reactivation of an ancient suture between the Precordillera and Pampeanas terranes or be occurring in basement of unknown affinity west of the suture. The seismicity provides the first constraints on basement fault geometries, and we present models integrating this information with the surface geology. These basement faults may have been responsible for the 1944 Ms 7.4 earthquake that destroyed the city of San Juan. The imaging of these faults suggests that seismic risk estimates for San Juan made on the basis of surface geologic studies may be too low.

Attenuation of high-frequency seismic waves beneath the central Andean Plateau

Observed patterns of high-frequency seismic wave attenuation suggest that near 22°S, the upper mantle structure beneath the central Andean plateau changes along strike to the south. Contrasting regions of high and low upper mantle seismic wave attenuation beneath the plateau are identified based on striking azimuthal variations in the character and frequency content of P and S waves propagating beneath the plateau to a portable seismic network deployed in Jujuy Province, Argentina (24°S, 65°W). Ray paths from intermediate depth earthquakes located north and northwest of the network transmit seismic waves with a higher frequency content than ray paths from earthquakes at similar depths and distances but located west and south of the network. The estimated apparent Q values fall into two categories: Qp >500 and Qs >350 for the high-Q paths, and Qp <350 and Qs < 200 for the low-Q paths. In addition, Sn phases from regional crustal earthquakes in the Subandean foreland fold-thrust belt to the north propagate efficiently to the Jujuy network, while Sn is not observed from foreland earthquakes located at similar distances to the south of the network. These observations combined with data from previously reported wave propagation studies suggest that south of about 22° S, the upper mantle beneath the plateau and its adjacent foreland thrust belt is more highly attenuating than the upper mantle farther north. Forward modeling of the Q measurements made at Jujuy indicates that the observations can be explained either by a variable thickness high-Q upper plate beneath the plateau, or by a thin, variable width, very low-Q zone in the asthenospheric wedge above the subducted slab. This lateral variation in upper mantle structure coincides with two physiographically distinct segments of the central Andean plateau and its adjacent foreland thrust belt to the east: the Bolivian Altiplano and Subandean ranges in the north and the Argentine Puna and Santa Barbara system in the south. We interpret the north-south change in upper mantle attenuation and the corresponding changes in physiography, topography, and tectonic style at the surface to be due to a mantle lid that is thicker beneath the Altiplano and the Subandean belt than beneath the Puna and the Santa Barbara ranges.

SEISMICITY AND TECTONICS IN JUJUY PROVINCE, NORTHWESTERN ARGENTINA

The Portable Array for Numerical Digital Analysis (PANDA) network, a digitally recorded seismic array, operated for nine months in Jujuy province of northwestern Argentina. The network was deployed along the eastern edge of the Altiplano-Puna plateau in a major N-S thrust belt that is transitional in style between the thin-skinned deformation of the Bolivian foreland to the north and basement-involved deformation of the Pampean region to the south. Teleseismic locations of crustal earthquakes in the region indicate that seismicity is associated with compressional structures found near the eastern deformation front. No crustal seismicity was detected beneath the Puna plateau to the west. Peak seismicity levels beneath the foreland occurred between 20 and 25 km depth; a sharp decrease in seismicity was observed below 25 km. An estimate of 42 km for the thickness of the Jujuy foreland crust was inferred from wide-angle Moho reflections observed on the digital seismograms. The highest concentration of crustal seismicity was located beneath Sierra de Zapla, a broad anticline immediately east of San Salvador de Jujuy. Many of the earthquakes in the 20–25 km depth range have a shallow, west dipping nodal plane as does the focal mechanism solution for a moderately large 1973 earthquake. Inversion of focal mechanism data for the orientation of principal stresses shows that maximum compression is oriented at azimuth 74°, closely paralleling both the current Nazca-South America convergence direction and the shortening direction derived from regional Quaternary fault slip data. We interpret the earthquakes as occurring on planes of weakness first produced during Cretaceous rifting and later reactivated by Neogene compressive stresses. Crustal seismicity patterns and fault plane solutions suggest the presence of a midcrustal detachment, along which significant late Cenozoic E-W shortening has occurred.

Three-dimensional elastic wave velocity structure of the Hualien region of Taiwan: Evidence of active crustal exhumation

The Hualien region of Taiwan is located at a complex transition of the boundary between the Eurasian and Philippine Sea Plates. To the southwest, the mountains of Taiwan are uplifting rapidly as a consequence of an ongoing arc-continent collision, while to the east the oceanic Philippine Sea Plate is subducting northward beneath Eurasia. We investigated the structure and dynamics of this region by analyzing seismograms of local earthquakes recorded during a deployment of the Portable Array for Numerical Data Acquisition II network. P and S wave velocity structures deduced from travel time tomography analysis show that the collisional suture to the south of Hualien is characterized by a narrow (< 10 km width), near vertically dipping zone of low velocities that extends to depths in excess of 20 km. Velocities in the Eastern Central Range west of the suture zone are significantly higher and define a feature 10–15 km wide that appears to be continuous from the near surface to depths as great as 40 km. Farther to the west beneath the Western Central Range, the velocities again decrease. Focal mechanisms of local earthquakes show that while thrust faulting is the predominate mode of deformation throughout the region, normal faulting occurs as well beneath the Eastern Central Range. Thus the rapid uplift of the mountains of Taiwan may be a result not only of compressional shortening but also of an excess of positive buoyancy. We suggest that the higher velocities and extensional mechanisms in the Eastern Central Range are caused by the ongoing exhumation of previously subducted continental crust, while the lower velocities to the west reflect continued underthrusting of the crust beneath the Eastern Central Range.

Refinement of thrust faulting models for the Central New Madrid seismic zone

Abstract We present the results of the joint relocation of events recorded during 1989–1992 by the PANDA network in the central New Madrid seismic zone. The near-surface material in the study area is a gently-dipping layer of poorly consolidated sediments with low P-wave velocity and high V p / V s (estimated values: 1.8 km s −1 and 3). The sediments are underlain by high-velocity Paleozoic rocks. Under the network the difference in sediment thickness is only 0.6 km, but because of the low velocities the location of the events using layered models is affected by errors. Application of the joint hypocentral determination (JHD) technique to a subset of 580 events shows that the single-event locations may be in error by as much as 1 km in depth, depending on where the events are located. Analysis of synthetic data generated for a realistic 3-D velocity model supports the JHD results. The analysis of synthetic data also suggests that a V p / V s ≤ 2.3 is more appropriate for the post-Paleozoic Mississippi embayment sediments. Based on the JHD locations we present a new interpretation of the seismicity, with two en-echelon SW-dipping thrust faults connected by a west-dipping thrust fault. These faults appear associated with the Reelfoot scarp and its northern extension, the Kentucky bend scarp.

Upper Mississippi embayment shallow seismic velocities measured in situ

Abstract Vertical seismic compressional- and shear-wave (P-and S-wave) profiles were collected from three shallow boreholes in sediment of the upper Mississippi embayment. The site of the 60-m hole at Shelby Forest, Tennessee, is on bluffs forming the eastern edge of the Mississippi alluvial plain. The bluffs are composed of Pleistocene loess, Pliocene-Pleistocene alluvial clay and sand deposits, and Tertiary deltaic-marine sediment. The 36-m hole at Marked Tree, Arkansas, and the 27-m hole at Risco, Missouri, are in Holocene Mississippi river floodplain sand, silt, and gravel deposits. At each site, impulsive P- and S-waves were generated by man-made sources at the surface while a three-component geophone was locked downhole at 0.91-m intervals. Consistent with their very similar geology, the two floodplain locations have nearly identical S-wave velocity (VS) profiles. The lowest VS values are about 130 m s−1, and the highest values are about 300 m s−1 at these sites. The shear-wave velocity profile at Shelby Forest is very similar within the Pleistocene loess (12 m thick); in deeper, older material, VS exceeds 400 m s−1. At Marked Tree, and at Risco, the compressional-wave velocity (VP) values above the water table are as low as about 230 m s−1, and rise to about 1.9 km s−1 below the water table. At Shelby Forest, VP values in the unsaturated loess are as low as 302 m s−1. VP values below the water table are about 1.8 km s−1. For the two floodplain sites, the VP/VS ratio increases rapidly across the water table depth. For the Shelby Forest site, the largest increase in the VP/VS ratio occurs at ∼20-m depth, the boundary between the Pliocene-Pleistocene clay and sand deposits and the Eocene shallow-marine clay and silt deposits. Until recently, seismic velocity data for the embayment basin came from eartquake studies, crustal-scale seismic refraction and reflection profiles, sonic logs, and from analysis of dispersed earthquake surface waves. Since 1991, seismic data for shallow sediment obtained from reflection, refraction, crosshole and downhole techniques have been obtained for sites at the northern end of the embayment basin. The present borehole data, however, are measured from sites representative of large areas in the Mississippi embayment. Therefore, they fill a gap in information needed for modeling the response of the embayment to destructive seismic shaking.

Polarity Reversal of Active Plate Boundary and Elevated Oceanic Upper Mantle beneath the Collision Suture in Central Eastern Taiwan

The active collision between the Eurasia and Philippine Sea plates in eastern Taiwan has been explored from the recently determined 3D velocity images and relocated hypocenters. A north-northeast-south-southwest-trending high- velocity zone corresponding to the oceanic upper mantle is narrowly defined under- neath the collision suture from Hualien to Taitung. This elevated and hot oceanic upper mantle must have played an important role in the tectonic evolution/mountain- building process of the adjacent continental crust. A northwest-dipping seismic zone can be identified in the northern collision zone extending from the surface to 30 km depth, which can be correlated with the northern Longitudinal Valley Fault (LVF). This zone marks a transitional plate boundary separating the high VP and high VP/ VS oceanic crust to the east and the high VP and VS upper crust and low VP and low VP/VS mid-to-lower continental crust to the west. A significant amount of plate con- vergence along the suture has been accommodated by the high-angle thrusting along the northern LVF. In contrast, a southeast-dipping seismic zone can be identified extending from the surface to 25 km depth near Taitung in the southern collision zone. This zone coincides with a region of high VP and high VP/VS, suggesting that earthquakes occurred within a highly fractured or fluid-rich zone. The reverse polarity of active-plate boundary faults marks two distinguished transition boundaries, one from eastward subduction in southern Taiwan to east-west collision in the southern collision zone corresponding to the early phase of plate collision, and the other from east-west collision to northwest subduction in the northern collision zone corre- sponding to the advanced phase of plate collision. The central collision zone is creep- ing and aseismic, which can be attributed to the high heat flow and geothermal activity during an interseismic period since the 1951 Taitung earthquake.

Three‐dimensional P wave velocity structure in the New Madrid seismic zone

A three-dimensional P wave velocity model for the New Madrid seismic zone (NMSZ) has been developed using a nonlinear travel time tomography method. The inversion involved 709 earthquakes recorded by digital, three-component Portable Array for Numerical Data Acquisition stations deployed in the NMSZ over the time period 1989 through 1992. Analysis of ray coverage and inversion of a synthetic data set showed that the model has high resolution to a depth of 11 km. Low-velocity anomalies correlate with the prominent northwest and northeast trending arms of seismicity. Lowest-velocity anomalies (−8%) occur at the intersection of these arms and are associated with shallow seismicity and a high swarm rate. These low-velocity regions are interpreted to be the result of increased fluid pressure associated with structurally disrupted rocks beneath the Blytheville-Pascola arch complex. Small regions of high velocity, limited to the upper few km of the crust, correlate with gravity and magnetic anomalies and are interpreted to be igneous intrusions. High-velocity anomalies also parallel the edges of the Reelfoot rift margin and probably are associated with crystalline Precambrian rocks that form the margin of the Reelfoot rift graben. Most earthquakes are associated with low-velocity regions and avoid regions of high velocity.

A Simple Algorithm for Local Earthquake Location Using 3D VP and VS Models: Test Examples in the Central United States and in Central Eastern Taiwan

Traditional local-earthquake location using a horizontally layered homogeneous velocity model is limited in its resolution and reliability due to the existence of frequently overlooked 3D complexity of the real Earth. During traditional 3D seismic tomography, simultaneous earthquake relocation using the resultant 3D velocity model has produced reliable earthquake locations; however, only a small subset of events are typically used and thus relocated in the inversion. The rest of the events in a catalog must then be relocated using the 3D models. The repeated calculation of travel times across 3D V P and V S models is also not efficient and not practical for a routine network earthquake location when the very time-consuming exact 3D raytracing is used. Because high-resolution earthquake data are now available from many modern seismic networks, representative high-resolution 3D V P and V S models for a region can be better determined. By taking advantage of recently available high-speed computer technology and large disk space, we implemented a simple algorithm to efficiently locate every local earthquake using the best available regional 3D V P and V S models. Once the V P and V S information for all cubic cells in a 3D grid model are determined, P and S travel times from each grid point to all seismic stations can be calculated and stored on disk files for later usage. During the iteration process for earthquake location, travel times from a trial hypocenter to all recording stations can be determined simply by a linear interpolation from those of the adjacent eight grid points available in the previously stored disk files without the need for raytracing. The iterations continue until the hypocenter adjustments at the end of the last iteration are below the given criteria and the travel-time residual, or the difference between the observed and the calculated travel times, is a minimum. Therefore, any local earthquake can be efficiently and reliably located using the available 3D velocity models. This simple location program has been applied to relocate earthquakes in the New Madrid Seismic Zone (nmsz) of the central United States and in the central eastern Taiwan region. Preliminary results in both regions reveal that earthquake hypocenters can be efficiently relocated in spite of the very significant lateral structural variations. Tests with data from Taiwan further demonstrate that the resolution of seismic tomography and the relocated seismicity is sensitive to relative distribution of seismic-network stations and background seismicity. Thus, this single-event location program can be applied to relocate all earthquakes in a seismic-network catalog and, more importantly, to allow routine earthquake location for any seismic network using the available 3D velocity models.

Anomalous Pn Waves Observed in Eastern Taiwan: Implications of a Thin Crust and Elevated Oceanic Upper Mantle beneath the Active Collision-Zone Suture

Normal Pn waves are commonly observed in Taiwan from shallow regional earthquakes at epicentral distances larger than 120 km, similar to the observations in many other continental regions. However, the critical distances to observe Pn waves for shallow eastern Taiwan earthquakes vary with azimuth corresponding to a significant variation of crustal thickness. In particular, anomalous Pn waves are commonly observed for shallow eastern Taiwan earthquakes recorded on seismic stations at epicentral distances as small as 60 km along the collision zone suture, the Longitudinal Valley. For the same event, normal Pn waves are observed at other seismic stations elsewhere on the island. The apparent velocity of the anomalous and normal Pn waves from the same event is 7.8 ± 0.15 km/sec, which is consistent with the average Pn velocity in the Taiwan area. Thus, the unusually short critical distance for Pn waves in eastern Taiwan suggests that the crust beneath the collision zone suture must be very thin and the upper mantle beneath the Longitudinal Valley must be relatively elevated compared with that beneath the other parts of Taiwan. Assuming a simple 1D layered velocity model, the Moho depth beneath the suture zone can thus be estimated at ∼23 ± 2 km. This observation is consistent with the recent report from a high-resolution 3D tomographic inversion that a narrowly confined, anomalously elevated, and north-northeast–south-southwest elongated oceanic upper mantle was imaged beneath the Longitudinal Valley from Hualien in the north to Taitung in the south (Kim et al. , 2005, 2006). Furthermore, the preceding observations may also support the interpretation that the conduction of excess heat supply from the elevated hot oceanic upper mantle into the adjacent mid- to-lower continental crust over a long period of geological time may play an important role in the crustal deformation beneath the continent, including metamorphism, thickening, and uplifting.