Near-source Strong Ground Motions Research Articles

Near-Source Strong Pulses During Two Large MJMA 6.5 and MJMA 7.3 Events in the 2016 Kumamoto, Japan, Earthquakes

Extremely large ground motions with strong pulses were observed near source faults during two large MJMA 6.5 and MJMA 7.3 events of the 2016 Kumamoto earthquakes in Japan. To investigate the mechanisms for generation of near-source strong pulses during both events, we first performed strong ground motion simulations in a broadband frequency range between 0.2 and 10 Hz using the empirical Green’s function method. For both the M6.5 and M7.3 events, strong motion generation area (SMGA) source models were prepared to simulate the ground motions, which were able to reproduce well the characteristics of the observed ground motions in and around the source areas. We also conducted ground motion simulations based on hypothetical simple source models to study the effect of rupture directivity on near-source strong ground motions. The principal findings regarding rupture directivity effects on near-source strong ground motions are as follows: (1) During the M6.5 event, the forward and upward rupture directivities from two strike-slip SMGAs caused two distinct strong pulses in the fault normal and parallel components, respectively. (2) During the M7.3 event, the upward rupture directivity along the fault dip direction from the strike-slip SMGA, including a small normal-slip, caused a strong pulse in both the fault parallel and normal components.

Strong ground motion characteristics observed in the 13 June 2011 Mw6.0 Christchurch, New Zealand earthquake

This paper examines the salient characteristics of near-source strong ground motions recorded from the 13 June 2011 Mw6.0 Christchurch, New Zealand earthquake. 26 strong motions were recorded within Rrup=30km of the source, with maximum geometric mean horizontal shaking exceeding peak accelerations and velocities of PGA=1.5g and PGV=100cm/s, respectively. The polarity of strong motion recordings with Rrup<6km illustrate the complexity of the seismic source, supporting the two-fault model of Beavan et al. [4]. Directivity in the observed ground motions in eastern and central Christchurch is dominated by the secondary rupture of a right-lateral ENE-trending fault plane, which produces polarization in a predominantly NS direction. As seen in previous events in the Canterbury earthquake sequence, significant basin-generated surface waves led to strong long period ground motions with relatively long significant durations. The installation of strong motion instruments in the Port Hills following the 22 February 2011 Mw6.2 Christchurch earthquake resulted in five new observed ground motions on rock or shallow soil sites, relative to past events, with the PARS and GODS stations on top of the causative faults recording very large amplitudes, inferred as a result of site response and topographic effect coupling. Significant nonlinear site response is evident at several locations in eastern Christchurch, resulting in truncation of acceleration amplitudes, with liquefaction-induced cyclic mobility clearly evident at two locations. Pore pressure recordings at five locations in eastern Christchurch illustrate that significant excess pore water pressure (EPWP) generation in a Mw5.3 event, which occurred 80min prior to the Mw6.0 event, resulted in residual EPWP's that had a measurable influence on high frequency ground motion amplitudes at soft soil sites in the latter event. Finally, the observed ground motion amplitudes are compared with empirical models for response spectra and significant duration.

Strong Motion Simulation of the M8.0 August 15, 2007, Pisco Earthquake; Effect of a Multi-Frequency Rupture Process

We investigated the broadband frequency (0.05-30 Hz) radiation characteristics of the August 15, 2007, Mw8.0 Pisco, Peru, earthquake by simulating the near-source strong ground motion recordings in Parcona city (PCN) and Lima city (NNA). A source model of this earthquake obtained from long-period teleseismic waveforms and InSar data shows two separate asperities, which is consistent with the observation of two distinct episodes of strong shaking in strong motion recordings. We constructed a source model that reproduces near-source records at low frequency (0.05-0.8 Hz) as well as high frequency (0.8-30 Hz) bands. Our results show that the aforementioned teleseismic source model is appropriate for simulating near-source low frequency ground motion. Our modeling of the PCN record in the broad-frequency band indicates that a very strong high frequency radiation event likely occurred near the hypocenter, which generated a large acceleration peak within the first episode of strong shaking at PCN. Using this “broadband frequency” source model we simulated the strong ground motion at Pisco city and obtained accelerations as large as 700 cm/s2and velocities as high as 90 cm/s, respectively, which may explain the heavy damage occurring in the city.

Near-source strong ground motions observed in the 22 February 2011 Christchurch earthquake

This manuscript provides a critical examination of the ground motions recorded in the near-source region resulting from the 22 February 2011 Christchurch earthquake. Particular attention is given to reconciling the observed spatial distribution of ground motions in terms of physical phenomena related to source, path and site effects. The large number of near-source observed strong ground motions show clear evidence of: forward-directivity, basin generated surface waves, liquefaction and other significant nonlinear site response. The pseudo-acceleration response spectra (SA) amplitudes and significant duration of strong motions agree well with empirical prediction models, except at long vibration periods where the influence of basin-generated surface waves and nonlinear site response are significant and not adequately accounted for in empirical SA models. Pseudo-acceleration response spectra are also compared with those observed in the 4 September 2010 Darfield earthquake and routine design response spectra used in order to emphasise the amplitude of ground shaking and elucidate the importance of local geotechnical characteristics on surface ground motions. The characteristics of the observed vertical component accelerations are shown to be strongly dependent on source-to-site distance and are comparable with those from the 4 September 2010 Darfield earthquake, implying the large amplitudes observed are simply a result of many observations at close distances rather than a peculiar source effect.

Near-source Strong Ground Motions Observed in the 22 February 2011 Christchurch Earthquake

On 22 February 2011 at 12:51 p.m. local time, a moment magnitude Mw 6.3 earthquake occurred beneath the city of Christchurch, New Zealand, causing an level of damage and human casualties unparalleled in the country's history. Compared to the preceding 4 September 2010 Mw 7.1 Darfield earthquake, which occurred approximately 30 km to the west of Christchurch, the close proximity of the 22 February event led to ground motions of significantly higher amplitude in the densely populated regions of Christchurch. As a result of these significantly larger ground motions, structures in general, and commercial structures in the central business district in particular, were subjected to severe seismic demands and, combined with the event timing, structural collapses accounted for the majority of the 181 casualties (New Zealand Police 2011). This manuscript provides a preliminary assessment of the near-source ground motions recorded in the Christchurch region. Particular attention is given to the observed spatial distribution of ground motions, which is interpreted based on source, path, and site effects. Comparison is also made of the observed ground motion response spectra with those of the 4 September 2010 Darfield earthquake and those used in seismic design in order to emphasize the amplitude of the ground shaking and also elucidate the importance of local geotechnical and deep geologic structure on surface ground motions. New Zealand resides on the boundary of the Pacific and Australian plates (Figure 1) and its active tectonics are dominated by: 1) oblique subduction of the Pacific plate beneath the Australian plate along the Hikurangi trough in the North Island; 2) oblique subduction of the Australian plate beneath the Pacific plate along the Puysegur trench in the southwest of the South Island; and 3) oblique, right-lateral slip along numerous crustal faults in the axial tectonic belt, of which the …

Near-Source Strong Ground Motion Characteristics of the 2008 Wenchuan Earthquake

The free-field strong ground motions recorded at 64 sites within 200 km from the ruptured Longmen Shan fault during the 12 May 2008 M w 7.9 Wenchuan earthquake were used to examine the near-source strong ground motioncharacteristics. The magnitude of vertical and horizontal peak ground accelerations (PGAs), frequency content, and duration of acceleration histories are analyzed. The observed peak ground accelerations at distances exceeding 60 km from the ruptured fault were significantly higher than those predicted by the Next Generation Attenuation (NGA) model, whereas, for periods of larger than 1 s, the spectral accelerations are much smaller than those of the NGA model. The same is also true for the observed peak ground velocities (PGVs). The duration of ground shaking is longer than for most earthquakes of comparable similar magnitude. For sites within 60 km from the ruptured fault, the accelerations observed on the hanging wall are larger than those observed on the footwall. The duration of ground shaking along the rupture direction (or the forward direction) is shorter than that opposite to the rupture direction (or the backward direction). There is also a systematic increase of PGA and PGV at sites located in the direction of the rupture propagation, whereas no such clear trend was found in the stations located along the backward direction. From the observed acceleration time histories at various locations, the rupture process of the 2008 Wenchuan appears to be more complicated than the focal mechanism interpreted from far-field seismic records. For example, the generation mechanism of the two distinct wave trains observed at Wolong and other stations need more detailed studies.

Estimation of the High-Frequency Radiation of the 2000 Tottori (Japan) Earthquake Based on a Dynamic Model of Fault Rupture: Application to the Strong Ground Motion Simulation

Abstract In the present study we investigate the high-frequency (HF) radiation mechanism of the 2000 Tottori earthquake in Japan based on a 3D spontaneous fault rupture dynamic model. We generalize the model of HF radiation of a suddenly stopping circular crack (Madariaga, 1977; Boatwright, 1982) to the radiation from a general 3D rupture in a planar fault, where HF is radiated during gradual changes of rupture velocity at the rupture front. Local rupture velocity changes are expressed as the divergence of local rupture velocity vectors that are derived from gradients of rupture times from the dynamic model. Our numerical model of the Tottori earthquake indicates that rupture velocity changes are largely induced by barriers (locally stronger fault sections) across the fault plane and that HF radiation mainly originates within asperities (large stress-drop regions) in areas where the product of dynamic stress drop and rupture velocity changes is maximum. We develop a strong ground motion simulation methodology that incorporates HF radiation inferred from a dynamic fault rupture model. Using this methodology we investigate the HF radiation of the Tottori earthquake by inverting observed near-source acceleration envelopes of the earthquake. Our inversion results corroborate that HF radiation originates within asperities and show that significant HF radiation represents no more than a 20% of the total asperity area. Our results show that the incorporation of a directivity factor, on the basis of a well-defined physical rupture model to the radiation pattern leads to a significant improvement in fitting of observed ground motions. Our simulated near-source strong ground motions of the Tottori earthquake are also able to reproduce the ω -2 radiation theoretically predicted in 2D dynamic fault rupture models.

Rupture process of the 2007 Chuetsu-oki, Niigata, Japan, earthquake—Waveform inversion using empirical Green’s functions—

Abstract A waveform inversion was conducted to reveal the rupture process of the 2007 Chuetsu-oki, Niigata, Japan, earthquake using near-source strong ground motion records. Based on recent results on the re-location of the aftershocks, a southeast-dipping fault plane was assumed. Aftershock records were used as empirical Green’s functions to avoid uncertainty in subsurface structure in and around the source region for the calculation of Green’s functions. To ensure that the path and the site effects are shared between ground motions from the mainshock and those from the aftershocks, the mainshock fault plane was divided into three domains, each of which was allocated to one of the aftershocks used. Based on the results of the inversion, a major distinctive asperity was identified approximately 20 km southwest of the hypocenter, near Kashiwazaki City. A minor, rather obscure asperity was recognizable between the hypocenter and the major asperity. The rupture continued approximately 10 s. Agreement between the observed and synthetic ground motions is quite satisfactory, not only for the portions used for the inversion but also for the later phases. The rupture of the main asperity, which is close to Kashiwazaki City, may be a primary reason for the large amplitude ground motion and damage in Kashiwazaki City.

Rupture process of the 2007 Noto Hanto earthquake: waveform inversion using empirical Green’s function

Abstract A waveform inversion study was conducted to reveal the rupture process of the 2007 Noto Hanto, Japan, earthquake using near-source strong ground motion records. To avoid uncertainty in the subsurface structure in and around the source region when calculating Green’s functions, I used the aftershock records as empirical Green’s functions. To ensure that the path and the site effects are shared between the ground motions from the mainshock and those from aftershocks, the mainshock fault plane was divided into four domains, each of which was allocated to one of the aftershocks used as the empirical Green’s function events. The result of the inversion indicated that the rupture process included the break of two asperities. The first asperity, located close to the JMA hypocenter, was broken soon after the rupture started. This was followed by an extension of the rupture and the subsequent break of the second shallow asperity. The total moment of the mainshock was estimated to be 1.96E+19 N m, which is roughly consistent with the moment estimated by the F-net (1.36E+19 N m). Agreement between the observed and synthetic ground motions is quite satisfactory.

Kinematic Inversion of the 2004 M 6.0 Parkfield Earthquake Including an Approximation to Site Effects

The 2004 M 6.0 Parkfield earthquake yielded one of the largest amounts of near-source strong ground motion seismic data ever. We invert strong-motion seismograms to obtain a model for the space-time distribution of coseismic slip on the fault. To reduce noise in the inversion, we take into account local amplifications that affect each station by using records of the 1983 M 6.5 Coalinga earthquake. Site amplification correlates well with large peak ground velocities registered during the 2004 Parkfield mainshock. The inversion for a kinematic rupture model yields a nonunique solution; we therefore analyze various rupture models that explain the data equally well. Our preferred rupture model identifies a primary zone of high slip surrounding the hypocenter, where the maximum slip is 57 cm. A secondary slip area, over which contours are not well resolved, is located northwest of the hypo- center. The rupture speed is highly heterogeneous. We infer an average rupture ve- locity of 2.8 km/sec close to the hypocenter, and of 3.3 km/sec in the secondary region of large slip to the northwest of the hypocenter. By correlation of our rupture model with both microseismicity and velocity structure, we identify six patches on the fault plane that behave in seismically distinct ways. Online material: Kinematic rupture model parameters.

The slip history of the 1994 Northridge, California, earthquake determined from strong-motion, teleseismic,GPS, and leveling data

AbstractWe present a rupture model of the Northridge earthquake, determined from the joint inversion of near-source strong ground motion recordings, P and SH teleseismic body waves, Global Positioning System (GPS) displacement vectors, and permanent uplift measured along leveling lines. The fault is defined to strike 122° and dip 40° to the south-southwest. The average rake vector is determined to be 101°, and average slip is 1.3 m; the peak slip reaches about 3 m. Our estimate of the seismic moment is 1.3 ± 0.2 × 1026 dyne-cm (potency of 0.4 km3). The rupture area is small relative to the overall aftershock dimensions and is approximately 15 km along strike, nearly 20 km in the dip direction, and there is no indication of slip shallower than about 5 to 6 km. The up-dip, strong-motion velocity waveforms are dominated by large S-wave pulses attributed to source directivity and are comprised of at least 2 to 3 distinct arrivals (a few seconds apart). Stations at southern azimuths indicate two main S-wave arrivals separated longer in time (about 4 to 5 sec). These observations are best modeled with a complex distribution of subevents: The initial S-wave arrival comes from an asperity that begins at the hypocenter and extends up-dip and to the north where a second, larger subevent is centered (about 12 km away). The secondary S arrivals at southern azimuths are best fit with additional energy radiation from another high slip region at a depth of 19 km, 8 km west of the hypocenter. The resolving power of the individual data sets is examined by predicting the geodetic (GPS and leveling) displacements with the dislocation model determined from the waveform data, and vice versa, and also by analyzing how well the teleseismic solution predicts the recorded strong motions. The general features of the geodetic displacements are not well predicted from the model determined independently from the strong-motion data; likewise, the slip model determined from geodetic data does not adequately reproduce the strong-motion characteristics. Whereas a particularly smooth slip pattern is sufficient to satisfy the geodetic data, the strong-motion and teleseismic data require a more heterogeneous slip distribution in order to reproduce the velocity amplitudes and frequency content. Although the teleseismic model can adequately reproduce the overall amplitude and frequency content of the strong-motion velocity recordings, it does a poor job of predicting the geodetic data. Consequently, a robust representation of the slip history and heterogeneity requires a combined analysis of these data sets.