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C2-002
16:10
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16:30
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Earthquake Dynamic on Geometrically Complex Faults: Lesson Learned from the 1999 Chi-Chi (Taiwan) Earthquake
Two aspects of the 1999 Chi-Chi (Taiwan) earthquake related to rupture dynamic and near-source ground motion are discussed in this paper. 1) This earthquake provided evidence of several rupture complexities attributed mainly to the asymmetric geometry of dipping faults with rupture intersecting the free-surface, such as interaction of reflected waves (coming from the free-surface of the hanging wall side) with the ongoing rupture propagation, generating trapped waves in the hanging wall and rotation of rake angle enhanced at the edge of the fault trace with considerable strike slip components. These source complexities in dipping faults promote free-surface-rupturing and extension of the rupture area resulting in large events, as well as causing hanging wall moving more than the footwall, producing amplification of the ground motion in the wedge of the hanging wall. The large and high-quality recorded data for this earthquake confirmed all these phenomena and allowed to conclude that the dynamic of the dipping faults, surface-rupture and near-source ground motion are strongly affected by their fault geometry. 2) The observed ground motion differences between the northern and southern part of the fault caused distinguished damage pattern to structures. Even though the ground motion was stronger at the northern, the most severe damages have been observed at the southern part of the fault. This was attributed to the frequency content of the ground motion. The northern part produced the strongest ground motion at low frequency, while in the southern part, the frequency content of the ground motion had the potential to excite the fundamental frequency of standard structures. The study concludes that this ground motion difference between the northern and southern part of the fault were caused by the differences in energy absorption (characterized by fracture energy) during the fault rupture dynamic. The northern part was characterized by enhanced energy absorption. Overall, the observations of this earthquake allowed to confirm that the near-source ground motion of this earthquake were essentially source dominated phenomena.
Luis Dalguer
1999 Chi-Chi Earthquake, Earthquake dynamic, Geometrical fault complexity, Near-source ground motion
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C2-001
16:30
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16:50
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WHAT CAN SURFACE SLIP DISTRIBUTIONS TELL US ABOUT FAULT CONNECTIVITY AT DEPTH?
Fault traces on the Earth’s surface are often discontinuous. In many earthquakes, however, rupture propagates across numerous fault segments, leading to the question of whether these segments are connected by a single through-going fault at depth. To help answer this question, it would be useful if the connectivity of fault segments at depth were manifested as an observable feature of the surface slip distribution, which could be mapped in the field. In the present research, I use 3D dynamic models to investigate the impact of fault connectivity at depth on the surface slip distribution in an earthquake. In particular, I compare results for faults with several coplanar segments that are disconnected by locked patches near the surface but are connected to a through-going fault at varying depths, as well as faults that are entirely disconnected or entirely connected. I find that the deeper the depth of connection between the segments, the lower the slip on the individual segments. Segments that are entirely disconnected produce roughly elliptical surface slip profiles that are almost indistinguishable from those of segments that are connected at depths below around 8 km. For segments that are connected up to shallower depths, the surface slip distributions become flatter in the middle, with steeper slip gradients near the segment edges. However, these differences are rather small, and might not be easily distinguished from slip heterogeneity induced by other factors, such as heterogeneous stress, material properties, and fault geometry. Thus, I conclude that it may be quite difficult under most circumstances to discern fault connectivity at depth from surface slip mapping unless the connection is rather shallow (1-3 km); in such cases, high slip gradients near the segment edges might be indicative of connection below the surface. The results may have implications for the predictability of earthquake size, ground motion, and seismic hazard.
David D. Oglesby
dynamic rupture modeling, Fault Geometry, fault slip
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C2-014
16:50
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17:05
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A DYNAMIC RUPTURE MODEL OF THE 1999 CHI-CHI, TAIWAN, EARTHQUAKE
The September 20, 1999 (UTC) M7.6 Chi-Chi earthquake created a 100-km-long surface trace along the Chelungpu fault and produced a remarkable set of data, including near-field strong motion acceleration records and GPS static measurements. Moreover, the Taiwan Chelungpu-fault Drilling Project (TCDP) carried out some geophysical logging data, which provides an opportunity to determine the state of stress across the fault. These data can be viewed as proper constraints for earthquake dynamic modeling. Our goal is to investigate the dynamic rupture of the Chi-Chi earthquake. We construct a dynamic rupture model with spatially heterogeneous frictional behavior by using the dynamic parameters (e.g., slip-weakening distance, fault strength and dynamic stress drop) estimated from our previous kinematic study, and then investigate the mechanics of rupture and slip of this event. First, we obtain the distribution of stress change on the fault by using a finite difference method to solve the elasto-dynamic equations with spatiotemporal slip distribution, which is based on kinematic analysis of waveform inversion [Ji et al., 2003], as boundary condition. Then, we determine the constitutive relations of stress and slip to estimate the dynamic rupture parameters. The dynamic parameter estimation indicates that the slip-weakening distance at a given point on the fault is a function of final slip, with an additional relationship with depth. Next, we perform several numerical simulations of dynamic rupture process by using the finite element method and vary the obtained dynamic parameters as initial constraints. We use a standard model with homogeneous frictional behavior and initial stress condition on the fault for comparison. The standard dynamic rupture model shows a breakout phase near the free surface, directivity effect along the strike direction and a slip pattern uniformly spreading out over the fault plane. A model using homogeneous stress conditions but heterogeneous estimated slip-weakening parameters produces a rupture that dies out rapidly after nucleation. This effect may be due to very dissipative slip process associated with large slip-weakening distances. This result implies that heterogeneous stress or some other factor allows rupture in this region, or that the inferred slip-weakening distances may be higher than real life. Our on-going work is to test whether the simulated results of the dynamic rupture model will agree with the previous kinematic study if we use a spatially heterogeneous slip-weakening distance and a non-uniform initial stress condition on the fault. Exploring these parameters will give us insight into the dynamics of this earthquake.
Kuo-Fong Ma, Jolan Liao
1999 Chi-Chi Earthquake, Earthquake dynamics, Seismology
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C2-016
17:05
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17:20
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EXAMINATION OF FAULT-TO-FAULT RUPTURE TRANSFER DURING THE 2016 KUMAMOTO EARTHQUAKE IN JAPAN USING A DYNAMIC SOURCE MODEL
The 2016 Kumamoto earthquake in Japan was an inland earthquake, which composed with a series of subevents occurred along two active fault zones, i.e., the Hinagu fault zone and the Futagawa fault zone. It is considered that during the main shock the fault rupture started from the Hinagu fault zone, successively propagated toward the northeast direction to the Futagawa section. The Futagawa fault zone was separated into two parts, the Futagawa section in the northeast direction and the Uto section in the southwest direction. Almost all of the proposed heterogeneous models of the Kumamoto earthquake only considered the Futagawa section on the northeast side. However, the city of Kumamoto could have been severely damaged if the successive rupture progressed not only from the Hinagu fault zone to the Futagawa section of the Futagawa fault zone but also to the Uto section.In this study, to investigate the rupture process during the main shock of the 2016 Kumamoto earthquake, including whether and how the rupture transferred to the Futagawa section and the Uto section, we constructed several dynamic source models. In order to explain more realistic rupture process of the 2016 Kumamoto earthquake, we apllied a dynamic source model to the Futagawa fault combined with a kinematic source model for the Hinagu fault, which was proposed by Hikima (2016) based on the waveform inversion analysis. When the Futagawa fault zone is considered as a single straight fault, the rupture propagates to the Futagawa section and then traveled to the Uto section. When the Futagawa fault zone is considered as two faults with discontinuity on the same plane, the rupture does not propagate to the Uto section.
Xin Wang, Masayuki Nagano, Hiroki Karatsu, Kazuhito Hikima, Tomiichi Uetake, Tetsushi Watanabe
dynamic source model, fault-to-fault rupture transfer, Futagawa section, kinematic source model, the 2016 Kumamoto Earthquake, Uto section
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C2-011
17:20
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17:35
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MICROSEISMICITY SIMULATED ON ASPERITY-LIKE FAULT PATCHES: ON SCALING OF SEISMIC MOMENT WITH DURATION AND SEISMOLOGICAL ESTIMATES OF STRESS DROPS
Observations show that microseismic events from the same location can have similar source durations but different seismic moments, violating the commonly assumed scaling. We use numerical simulations of earthquake sequences to demonstrate that strength variations over seismogenic patches provide an explanation of such behavior, with the event duration controlled by the patch size and event magnitude determined by how much of the patch area is ruptured. We find that stress drops estimated by typical seismological analyses for the simulated sources significantly increase with the event magnitude, ranging from 0.006 to 8 MPa. However, the actual stress drops determined from the on-fault stress changes are magnitude-independent and ~3 MPa. Our findings suggest that fault heterogeneity results in local deviations in the moment-duration scaling and earthquake sources with complex shapes of the ruptured area, for some of which stress drops may be significantly (~100-1000 times) underestimated by the typical seismological methods.
Yen-Yu Lin, Nadia Lapusta
earthquake source investigation, Faut-zone dynamics, stress drop
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