Stuart A. Sipkin

Estimation of earthquake source parameters by the inversion of waveform data: synthetic waveforms

Abstract Two methods are presented for the recovery of a time-dependent moment-tensor source from waveform data. One procedure utilizes multichannel signal-enhancement theory; in the other a multichannel vector-deconvolution approach, developed by Oldenburg (1982) and based on Backus-Gilbert inverse theory, is used. These methods have the advantage of being extremely flexible; both may be used either routinely or as research tools for studying particular earthquakes in detail. Both methods are also robust with respect to small errors in the Greens functions and may be used to refine estimates of source depth by minimizing the misfits to the data. The multichannel vector-deconvolution approach, although it requires more interaction, also allows a trade-off between resolution and accuracy, and complete statistics for the solution are obtained. The procedures have been tested using a number of synthetic body-wave data sets, including point and complex sources, with satisfactory results.

Rapid determination of global moment-tensor solutions

In an effort to improve data services, the National Earthquake Information Center has begun a program, in cooperation with the Incorporated Research Institutions for Seismology Data Management Center (IRIS DMC), to produce rapid estimates of the seismic moment tensor for most earthquakes with a body-wave magnitude of 5.8 or greater. Although this program is still undergoing testing and fine-tuning, an estimate of the moment tensor can usually be produced within 20 minutes of the arrival of the broadband P-waveform data from the IRIS DMC. Although the speed with which this data can be obtained varies, in many cases a solution can be obtained within two hours of the earthquake. These solutions do not vary significantly from the final solutions determined using the entire network. Because of the manner in which the data and the synthetic seismograms are aligned, the method is insensitive to the effects of timing errors, epicentral mislocation, and lateral heterogeneity in earth structure. This procedure was implemented in May 1993; the results from that month are presented here.

Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1990

Moment-tensor solutions, estimated using optimal filter theory, are listed for 114 moderate-to-large size earthquakes occurring during 1990.

Seismic radiation by magma injection: An anomalous seismic event near Tori Shima, Japan

The earthquake with a bodywave magnitude m_b=5.5, which occurred near Tori Shima, Japan, on June 13, 1984 (origin time: 0229:25.3 UT, 31.448°N, 140.036°E, depth of 10 km, m_b =5.5, M_S=5.5) is anomalous because it generated tsunamis which are disproportionately large for the magnitude of the earthquake. At Hachijo Island, 150 km from the epicenter, tsunamis were visually observed with peak-to-peak amplitude of 130 to 150 cm. Long-period seismic radiation is also anomalous. Love waves are almost absent, and Rayleigh waves are radiated with equal amplitude and phase in all directions. A simple double-couple model cannot explain these observations. With the assumption of no net volume change at the source, these data can be best explained with a compensated linear vector dipole (CLVD) with the principal tensional dipole in the vertical direction. The scalar moment of this dipole is 4×10^(24) dyn cm. Moment tensor inversions of long-period body waves and surface waves yield an almost identical solution. This CLVD source can be interpreted as horizontal fluid injection. The location of the event is in the Smith depression which is one of the nascent back arc basins just behind the Bonin arc. These basins are filled with thick sediments, and numerous young volcanoes are found near this site. Magmatic injection is most likely to occur in this tectonic environment. However, the time scale of the seismic event seems too short for magma injection to occur. A more likely mechanism involves water-magma interaction. The injection may be viewed as hydrofracturing driven by supercritical water heated by injected magma. The estimated volume of injected water is about 0.018 km^3 and that of basaltic magma is about 10% of this. This type of deformation is more efficient for tsunami generation than faulting with the same scalar moment.

Seismicity associated with the Sumatra-Andaman Islands earthquake of 26 December 2004

The U.S. Geological Survey/National Earthquake Information Center (usgs/neic) had computed origins for 5000 earthquakes in the Sumatra–Andaman Islands region in the first 36 weeks after the Sumatra–Andaman Islands mainshock of 26 December 2004. The cataloging of earthquakes of m b (usgs) 5.1 and larger is essentially complete for the time period except for the first half-day following the 26 December mainshock, a period of about two hours following the Nias earthquake of 28 March 2005, and occasionally during the Andaman Sea swarm of 26–30 January 2005. Moderate and larger ( m b ≥5.5) aftershocks are absent from most of the deep interplate thrust faults of the segments of the Sumatra–Andaman Islands subduction zone on which the 26 December mainshock occurred, which probably reflects nearly complete release of elastic strain on the seismogenic interplate-thrust during the mainshock. An exceptional thrust-fault source offshore of Banda Aceh may represent a segment of the interplate thrust that was bypassed during the mainshock. The 26 December mainshock triggered a high level of aftershock activity near the axis of the Sunda trench and the leading edge of the overthrust Burma plate. Much near-trench activity is intraplate activity within the subducting plate, but some shallow-focus, near-trench, reverse-fault earthquakes may represent an unusual seismogenic release of interplate compressional stress near the tip of the overriding plate. The interplate-thrust Nias earthquake of 28 March 2005, in contrast to the 26 December aftershock sequence, was followed by many interplate-thrust aftershocks along the length of its inferred rupture zone.

Moment tensor solutions estimated using optimal filter theory for 51 selected earthquakes, 1980–1984

Abstract The 51 global events that occurred from January 1980 to March 1984, which were chosen by the convenors of the Symposium on Seismological Theory and Practice, have been analyzed using a moment tensor inversion algorithm (Sipkin). Many of the events were routinely analyzed as part of the National Earthquake Information Centers (NEIC) efforts to publish moment tensor and first-motion fault-plane solutions for all moderate- to large-sized ( m b >5.7) earthquakes. In routine use only long-period P-waves are used and the source-time function is constrained to be a step-function at the source (δ-function in the far-field). Four of the events were of special interest, and long-period P, SH-wave solutions were obtained. For three of these events, an unconstrained inversion was performed. The resulting time-dependent solutions indicated that, for many cases, departures of the solutions from pure double-couples are caused by source complexity that has not been adequately modeled. These solutions also indicate that source complexity of moderate-sized events can be determined from long-period data. Finally, for one of the events of special interest, an inversion of the broadband P-waveforms was also performed, demonstrating the potential for using broadband waveform data in inversion procedures.

Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1984-1987

Abstract Moment-tensor solutions, estimated using optimal filter theory, are listed for 426 moderate- to large-sized earthquakes occurring from 1984 to 1987.

Moment-tensor solutions estimated using optimal filter theory : global seismicity, 1988-1989

Moment-tensor solutions, estimated using optimal filter theory, are listed for 201 moderate-to-large earthquakes occurring from 1988 to 1989.

Regional variation of attenuation and travel time in China from analysis of multple‐ScS phases

Long-period multiple-ScS waveforms from intermediate- and deep-focus earthquakes surrounding China, recorded by the Chinese Digital Seismograph Network, are used to estimate the multiple-ScS attenuation operator and differential travel times using both frequency (phase equalization and stacking) and time domain (waveform inversion) techniques. Theoretical considerations, augmented by synthetic testing, indicate that for continental regions the frequency domain approach yields a more stable estimate of whole mantle attenuation (QScS) than the time domain approach, which is better for whole mantle travel time (τScS). When teamed together, the two afford unprecedented success with continental paths. The results, 16 path-averaged estimates of QScS and τScS constitute a significant addition to the catalog of multiple-ScS studies of subcontinental mantle and provide sufficiently dense coverage to invert “path” estimates of attenuation and travel time for “regional” quantities of direct tectonic relevance. Across the frequency band of 10 to 50 mHz we obtain high (>215) values of regionalized QScS τScS the Tarim platform and the fold systems of northeastern China and low (<170) values elsewhere on the continent, defining a pattern that is well correlated with present-day levels of tectonic activity. The corresponding two-way travel time residuals with respect to the Jeffreys-Bullen (J-B) tables, ΔτScS (= τScS - 935.7 s), vary from −1.3±1.4 to +0.8±4.3 s and are typical of other “fast” continental regions. Unlike QScS, ΔτScS varies little from region to region and is correlated poorly with tectonic activity, indicating that QScS is much more sensitive to changes in the current tectonic environment. Offshore, ΔτScS for the Sea of Japan is +2.7±3.6 s and for the combined East China and Philippine seas region is +9.6±2.7 s. The latter, very large residual likely reflects the low velocities associated with back arc spreading. Throughout the study region, ΔτScS correlates well with the residuals predicted by recent upper mantle tomographic models, although the observed variance exceeds the model predictions by roughly a factor of 3.

Moment-tensor solutions estimated using optimal filter theory : global seismicity, 1991

Moment-tensor solutions, estimated using optimal filter theory, are listed for 108 moderate-to-large size earthquakes occurring during 1991.