J. Arthur Snoke

Lithospheric structure of the Chaco and Parana Basins of South America from surface-wave inversion

Surface-wave data from a portable broadband array have been used to invert for the velocity structure of the crust and upper mantle beneath the Chaco and Parana Basins of central South America. The upper-mantle velocity structure beneath the Parana Basin is cratonic in character, whereas that beneath the Chaco Basin is tectonic or asthenospheric in character. The surface-wave analysis used broadband recordings from a subset of a 14-station array deployed in a roughly east-west sawtooth arrangement along 20°S latitude, with a total E-W aperture of ∼1,400 km. Results from receiver-function analysis, as well as direct P-wave regional travel-time data, were used in the inversions to help constrain Moho depths and crust and upper-mantle velocities. S-wave structure for the intracratonic Parana Basin was determined using interstation phase and group velocities for Rayleigh waves (fundamental and first higher mode) and Love waves (fundamental mode only) based on seven events with paths which traverse the eastern Parana Basin and one event with a path across the western Parana Basin. The average Moho depth in the eastern Parana Basin is ∼42 km. The high-velocity upper-mantle lid has a maximum S-wave velocity of 4.7 km/s, with no resolvable low-velocity zone to at least 200 km depth. This cratonic velocity structure indicates the presence of a lithospheric root beneath the Parana Basin despite emplacement of the Parana plume. The limited data from the western Parana Basin are consistent with a homogeneous upper-mantle structure throughout the Parana Basin. Waveform inversion of fundamental-mode and first-higher-mode Rayleigh waves from a single subandean event was used to obtain estimates for pure-path dispersion along propagation paths through the Chaco Basin and the western half of the Parana Basin. The data were partitioned to isolate the partial-path contribution of the phase and group velocities for the Chaco Basin. The phase and group velocities from this somewhat sparse data set were inverted to obtain a velocity-depth model for the Chaco Basin. The distinguishing features of the Chaco model consist of a rather shallow Moho depth, 32 km, and low (“asthenospheric”) upper-mantle S-wave velocities, about 4.2 km/s, with velocity increasing only slightly to about 4.3km/s at 150 km depth.

85.12 - FOCMEC: FOCal MEChanism Determinations

This chapter focuses on focal mechanism determinations (FOCMEC). Focmec, performs an efficient, systematic search of the focal sphere and reports acceptable solutions based on selection criteria for the number of polarity errors and errors in amplitude ratios. The search of the focal sphere is uniform in angle, with selectable step size and bounds. The selection criteria for both polarities and amplitudes allow correction or weightings for near-nodal solutions. Applications have been made to finding best-constrained fault-plane solutions for suites of earthquakes recorded at local to regional distances, analyzing large earthquakes observed at teleseismic distances, and using recorded polarities and relative amplitudes to produce waveform synthetics. Input data can include up to a total of 500 polarities and amplitude ratios—the maximum number of input data is a compile-time option. The program Focmec produces two output files: a complete summary of information about all acceptable solutions, and a shorter summary file with one line for each acceptable solution that can be used as an input to other programs for display or further analysis. Instructions are included for compiling and running the programs, and there are two data sets with scripts and documentation for running the programs.

Structure and tectonics in the region of flat subduction beneath central Peru: Crust and uppermost mantle

Seismic waveform modeling of boundary interaction phases is used to determine the discontinuity structure of the crust in the Subandean and foreland basin regions overlying the zone of flat subduction beneath east-central Peru. The data analyzed are from intermediate-depth earthquakes (110 to 155 km) recorded on an array of three-component short-period (1 Hz) digital seismographs deployed in the epicentral region. Full use is made of both P-to-S and S-to-P converted phases in the modeling. Results from the determination of crustal structure in the Subandean and foreland basin region of east-central Peru confirm the presence of vast depositional basins comprised of low velocity sediments up to at least 8 km thick which flank the Andean orogen to the east and correlate with a substantially thickened crust atop the Brazilian shield. Crustal thickness in the foreland basin varies from about 35 km or less where sedimentary cover is minimal to 44 km in regions of maximum sedimentary deposition. There is some evidence that the crust thins slightly on the western side of the foreland basin (in the gap between basin and Subandean fold and thrust belt), but it rapidly thickens to 45–50 km beneath the Subandes proper, and to more than 50 km in the southern part of the Subandean belt. The results are consistent with, but do not require, a thick-skinned model of foreland crustal deformation similar to that found for the block faulted terrane in Argentina above the zone of flat subduction there. At least some basin formation appears to be due to block faulting, where faults may penetrate into the mantle.

A model not requiring continuous lithosphere for anomalous high-frequency arrivals from deep-focus South American earthquakes

Abstract Various workers have constructed models to explain a class of anomalous arrivals at Peruvian and Chilean stations from deep-focus South American earthquakes. These arrivals are shear waves with a later arrival time, a higher frequency content, a longer duration and a lower apparent velocity than direct S. Our models assume that there is a sufficiently sharp discontinuity at the upper interface of the descending lithospheric slab between depths of 80 and 250 km to provide efficient reflection (≈0.1) for S-waves incident from below. The observed travel times require a single S-to-S reflection at this interface if the J-B velocity-depth model is modified to allow for 7% higher velocities down to a depth of 300 km (excluding the crust). The locus of required reflection points correlates well with the upper boundary of the observed seismicity (strike and dip angles within 5°) and Q for the proposed path is consistent with the frequency content of the anomalous arrivals. Thus the existence of these arrivals requires a dipping interface down to about 250 km, but, contrary to the wave-guide model of Isacks and Barazangi, cannot be used to infer a continuous lithospheric slab down to the deep-focus earthquakes (h #62; 500 km).

Structure of the subducting Nazca Plate beneath Peru

Arrival times from intermediate-depth (110–150 km) earthquakes within the region of flat subduction beneath the subandean zone and foreland basins of east-central Peru provide constraints on the geometry and velocity structure of the subducting Nazca plate. Hypocentral locations and origin times for these events were determined using observations from a 15 station digitally recording locator array deployed in the epicentral region of eastern Peru. Observed P wave arrival times for coastal stations in Peru, some 3–6° from the epicenters, are up to 4 s early relative to predicted arrival times based on the best fit velocity-depth model used for hypocenter locations. These large negative time residuals appear to be the result of propagation paths which have long segments in the colder, higher-velocity subducting plate, P wave travel times were modeled for the effects of the slab using three-dimensional (3-D) ray tracing. Computed ray paths show that travel times to coastal stations for the eastern Peru events can be satisfactorily modeled with average velocities relative to the surrounding mantle 6% lower within the uppermost slab (assumed on the basis of other studies to be unconverted basaltic oceanic crust 6 km thick) and 8% higher within the cold uppermost mantle of the slab. Ray tracing for this plate model shows that P wave “shadow zones” can occur if the source-slab-receiver geometry results in seismic rays passing through regions in which the dip angle of the slab changes significantly. Such geometries exist for seismic waves propagating to some coastal stations from intermediate-depth earthquakes located east of the Andes. Observed first-arrival times for such cases do in fact have smaller negative residuals than those for geometries which allow for “direct” paths at similar distances. Modeling such arrivals as internally reflected waves propagating through the high-velocity part of the plate produces a significant improvement in the travel time residuals. For the slab velocities given above, we obtain a model thickness of approximately 36 km for the cold slab interior and a slight northwest component of dip in the region of subhorizontal subduction.

Stress redistribution and slow earthquakes

Abstract Earthquakes are usually assumed to result when the gradual stress buildup in a region eventually exceeds some initial local strength. There are observations which suggest that a number of mechanisms must have a significant role in stress redistribution in active earthquake regions. The stress build-up could be non-linear in time; there is evidence that this is so in the case of some major earthquakes. One mechanism that might explain non-linear strain buildup is a “slow” earthquake. This is similar to a normal earthquake except that the rupture and slip velocities are very low so that negligible energy is radiated in the normally observed seismic frequency band. The slow earthquakes studied here have been recorded by a network of Sacks-Evertson borehole strainmeters installed along the seismically active Pacific coast of Honshu (Japan). Larger slow events may have (slow) foreshocks and aftershocks in a manner similar to that for normal earthquakes. The aftershocks appear to have lower stress drops than the foreshocks. The static field changes associated with the Izu-Oshima earthquake ( m = 7) of January 1978 were sufficiently large that they were clearly recorded by the nearest three strainmeters. Interpretation of the records indicate that this earthquake was in essence a normal quake followed by a slow event of similar moment. This interpretation is corroborated by reports of ground vibration and faulting on the Izu peninsula.

Compressional deformation of island-arc lithosphere in northeastern Japan resulting from long-term subduction-related tectonic forces: finite-element modeling

Abstract Northeastern Japan experienced an approximately constant, compressional deformation during the last 5 million years resulting from the steady subduction of the Pacific plate. Because the direction of the maximum compression axis is approximately perpendicular to the strike of the island arc, 2-D finite-element modeling can be used to examine the deformation over time of the island-arc lithosphere, which, in turn, allows a test of the hypothesis that the large-scale features of topography, gravity and seismicity in northeastern Japan result from tectonic compression. The model geometry is based on heat flow data and laboratory-derived flow low, and each model run requires an assumed interplate coupling. Novel in our modeling is the ability to include erosion/deposition loading and the creation of strike-slip faults, based on a dynamically applied fracture criterion. The criterion for acceptability of a model is how well it matches observed present-day topography, gravity, and seismicity patterns. Models with the following viscosity structure are consistent with present-day observations: The long-term effective elastic thickness is 10 km in the inner arc, increasing to about 50 km near the trench. The effective elastic thickness in the inner arc is therefore much smaller than the about 30 km short-term elastic thickness estimated from seismological data. The viscosity of the lower crust is on the order of 1022 Pa s or less. Our model explains the observed positive gravity anomaly of the Kitakami range as a consequence of uplift resulting from a coupling which increases with depth along the interface between the subducting and overriding plates. The model also predicts the slip on the faults adjacent to the Backbone range, as well as the crustal shortening.

85.7 – The HYPOELLIPSE Earthquake Location Program

This chapter focuses on the HYPOELLIPSE Earthquake Location Program. The earthquake location program HYPOELLIPSE, first published in 1979 and subsequently updated with added features, was initially developed to locate crustal and subcrustal earthquakes of southern Alaska using arrival times recorded by a sparse regional seismograph network. This is a more difficult problem than locating shallow earthquakes within a dense network, which is the problem generally faced within California, so HYPOELLIPSE includes many more user-adjustable parameters than does HYPO71, the earthquake location program developed for California that is the ancestor of HYPOELLIPSE. Travel times may be computed from velocity models or travel-time tables. The program HYPOTABLE, which is included with this distribution, can create spherical-earth travel-time tables for use with HYPOELLIPSE. This allows the program to be used with stations beyond the distance at which significant travel-time errors would be introduced by a fiat-layer travel-time model. A relatively new feature of HYPOELLIPSE is the ability to work in areas with large topographic relief. Previous versions assumed that all of the stations were located at the same elevation and that small elevation differences could be accounted for by station elevation corrections. HYPOELLIPSE currently allows stations to be “embedded” within the velocity model, and will correctly compute travel time and take-off angles even for stations that are at a lower elevation than the earthquake.

85.1 Overview

This chapter presents an overview of earthquake seismology software by reflecting briefly on some of the initiatives and developments in computers and computing. The World-Wide Standardized Seismographic Network (WWSSN), in the early 1960s, provided for the first time worldwide coverage by a single network for earthquake locations, which resulted in good azimuthal and fair-to-good depth control for major earthquakes. The availability and accessibility of such data provided an impetus for the development of seismology software for handling the data and for using that data to get information about both earthquakes and Earth structure. Pioneering works using computers in seismology include those on surface-wave data inversion and those related to earthquake location—many of which used the pre-computer-developed Jeffreys–Bullen travel-time tables. These programs were developed mainly at universities and government laboratories in the United States. The seismological community has played an important role in expanding the use of personal computers (PCs) since 1981. A Committee on PCs was formed within the American Geophysical Union (AGU) in 1986 established a Working Group on Personal Computers, under their Commission on Practice, to promote sharing of seismological software among scientists worldwide. A Working Group on Personal Computers in Seismicity Studies was created in 1994 under Subcommission A of the European Seismological Commission. The activities of those and similar working groups included the organization of symposia and workshops at national and international meetings with demonstrations of existing hardware/software applications. Some of the earlier computer programs developed for seismological applications have been so widely used that they are still standards. Among these are earthquake location programs HYPO71, HYPOELLIPSE, and HYPOINVERSE; and codes for determining focal mechanisms that formed the FPFIT/FPPLOT/FPPAGE package and FOCMEC.