Aftershock sequence of the 3 December 1988 Pasadena earthquake

The Pasadena earthquake (M_L = 4.9) of 3 December 1988 occurred at a depth of 16 km, probably on the Santa Monica - Raymond fault, which is recognized as one of the most important faults in the Los Angeles basin for its potential seismic hazard. Prior to this event, no earthquake larger than magnitude 4 had been recorded since 1930 in this area. High-quality seismograms were recorded with the Pasadena very broadband (VBB) system (IRIS-TERRAscope station) not only for the mainshock but also for the aftershocks at epicentral distances of 3 to 4 km. We determined the focal mechanisms of 9 aftershocks using these data; for most of the aftershocks, the first-motion data are too sparse to determine the mechanism. We combined the first-motion data and the waveform data of P, SV, and SH waves recorded with the VBB instrument to determine the mechanism and seismic moment of nine aftershocks. The average orientations of the P and T axes of the aftershocks are consistent with the strike of the Raymond fault. The ratio of the logarithm of cumulative seismic moment of aftershocks to that of the seismic moment of the mainshock is significantly smaller than commonly observed

The Origin of the Tsunami Excited by the Earthquake – Faulting or Slumping

The first arrival of the tsunami recorded at Monterey, California, was about 10 min after the origin time of the earthquake. Using an elastic half space, we computed vertical ground displacements for many different fault models for the Loma Prieta earthquake and used them as the initial condition for computation of the tsunami in Monterey Bay. The synthetic tsunami computed for the uniform dislocation model determined from seismic data can explain the arrival time, polarity, and amplitude of the beginning of the tsunami. However, the period of the synthetic tsunami is too long compared with the observed. We tested other fault models with more localized slip distribution. None of the models could explain the observed period. The residual waveform, the observed minus the synthetic waveform, begins as a downward motion at about 18 min after the origin time of the earthquake and could be interpreted as due to a secondary source near Moss Landing. If the large-scale slumping near Moss Landing suggested by an eyewitness observation occurred about 9 min after the origin time of the earthquake, it could explain the residual waveform. To account for the amplitude of the observed tsunami, the volume of sediments involved in the slumping is approximately 0.012 km^3. Thus the most likely cause of the tsunami observed at Monterey is the combination of the vertical uplift of the sea floor due to the main faulting and a large-scale slumping near Moss Landing

Mechanism of the 1975 Kalapana, Hawaii, earthquake inferred from tsunami data

We investigated the source mechanism of the 1975 Kalapana, Hawaii, earthquake (M_S= 7.2) by modeling the tsunamis observed at three tide-gauge stations, Hilo, Kahului, and Honolulu. We computed synthetic tsunamis for various fault models. The arrival times and the amplitudes of the synthetic tsunamis computed for Ando's fault model (fault length = 40 km, fault width = 20 km, strike = N70°E, dip = 20°SE, rake = −90°, fault depth = 10 km, and slip = 5.6 m) are ∼10 min earlier and 5 times smaller than those of the observed, respectively. We tested fault models with different dip angles and depths. Models with a northwest dip direction yield larger tsunami amplitudes than those with a southeast dip direction. Models with shallower fault depths produce later first arrivals than deeper models. We also considered the effects of the Hilina fault system, but its contribution to tsunami excitation is insignificant. This suggests that another mechanism is required to explain the tsunamis. One plausible model is a propagating slump model with a 1 m subsidence along the coast and a l m uplift offshore. This model can explain the arrival times and the amplitudes of the observed tsunamis satisfactorily. An alternative model is a wider fault model that dips 10°NW, with its fault plane extending 25 km offshore, well beyond the aftershock area of the Kalapana earthquake. These two models produce a similar uplift pattern offshore and, kinematically, are indistinguishable as far as tsunami excitation is concerned. The total volume of displaced water is estimated to be ∼2.5 km^3. From the comparison of slump model and the single-force model suggested earlier from seismological data we prefer a combination of faulting and large-scale slumping on the south flank of Kilauea volcano as the most appropriate model for the 1975 Kalapana earthquake. Two basic mechanisms have been presented for explaining the deformation of the south flank of Kilauea: (1) pressure and density variation along the rift zone caused by magma injection and (2) gravitational instability due to the steep topography of the south flank of Kilauea. In either mechanism, large displacements on the south flank are involved that are responsible for the observed large tsunamis

The origin of the tsunami excited by the 1906 San Francisco earthquake

Lawson et al. (1910) reported a tsunami observed at Fort Point in San Francisco Bay (Fig. la) during the 1906 San Francisco earthquake (M = 8¼). This observation is curious because the San Francisco earthquake is generally believed to be a strike-slip earthquake for which tsunamis are not usually expected

The origin of the tsunami excited by the 1989 Loma Prieta Earthquake — Faulting or slumping?

We investigated the tsunami recorded at Monterey, California, during the 1989 Loma Prieta earthquake (M_W=6.9). The first arrival of the tsunami was about 10 min after the origin time of the earthquake. Using an elastic half space, we computed vertical ground displacements for many different fault models for the Loma Prieta earthquake, and used them as the initial condition for computation of tsunamis in Monterey Bay. The synthetic tsunami computed for the uniform dislocation model determined from seismic data can explain the arrival time, polarity, and amplitude of the beginning of the tsunami. However, the period of the synthetic tsunami is too long compared with the observed. We tested other fault models with more localized slip distribution. None of the models could explain the observed period. The residual waveform, the observed minus the synthetic waveform, begins as a downward motion at about 18 min after the origin time of the earthquake, and could be interpreted as due to a secondary source near Moss Landing. If the large scale slumping near Moss Landing suggested by an eyewitness observation occurred about 9 min after the origin time of the earthquake, it could explain the residual waveform. To account for the amplitude of the observed tsunami, the volume of sediments involved in the slumping is approximately 0.013km^3• Thus the most likely cause of the tsunami observed at Monterey is the combination of the vertical uplift of the sea floor due to the main faulting and a large scale slumping near Moss Landing

The 1909 Taipei earthquake—implication for seismic hazard in Taipei

The 1909 April 14 Taiwan earthquake caused significant damage in Taipei. Most of the information on this earthquake available until now is from the written reports on its macro-seismic effects and from seismic station bulletins. In view of the importance of this event for assessing the shaking hazard in the present-day Taipei, we collected historical seismograms and station bulletins of this event and investigated them in conjunction with other seismological data. We compared the observed seismograms with those from recent earthquakes in similar tectonic environments to characterize the 1909 earthquake. Despite the inevitably large uncertainties associated with old data, we conclude that the 1909 Taipei earthquake is a relatively deep (50–100 km) intraplate earthquake that occurred within the subducting Philippine Sea Plate beneath Taipei with an estimated M_W of 7 ± 0.3. Some intraplate events elsewhere in the world are enriched in high-frequency energy and the resulting ground motions can be very strong. Thus, despite its relatively large depth and a moderately large magnitude, it would be prudent to review the safety of the existing structures in Taipei against large intraplate earthquakes like the 1909 Taipei earthquake

Slip distribution and tectonic implication of the 1999 Chi‐Chi, Taiwan, Earthquake

We report on the fault complexity of the large (M_w = 7.6) Chi‐Chi earthquake obtained by inverting densely and well‐distributed static measurements consisting of 119 GPS and 23 doubly integrated strong motion records. We show that the slip of the Chi-Chi earthquake was concentrated on the surface of a ”wedge shaped” block. The inferred geometric complexity explains the difference between the strike of the fault plane determined by long period seismic data and surface break observations. When combined with other geophysical and geological observations, the result provides a unique snapshot of tectonic deformation taking place in the form of very large (>10m) displacements of a massive wedge‐shaped crustal block which may relate to the changeover from over‐thrusting to subducting motion between the Philippine Sea and the Eurasian plates

Broadband Waveform Observation of the 28 June 1991 Sierra Madre Earthquake Sequence (M_L = 5.8)

The Sierra Madre earthquake (M_I = 5.8) of 28 June 1991 occurred at a depth of about 12 km, on the Clamshell-Sawpit fault in the San Gabriel Mountains. High-quality seismograms were recorded with TERRAscope not only for the mainshock but also for the aftershocks at epicentral distances of about 16 km. We determined the focal mechanisms and seismic moments of the mainshock and 21 aftershocks by combining the waveform and first-motion data. We classified the events into five groups according to the location and waveforms recorded at PAS. Most events located within 5 km west of the mainshock are similar to the mainshock in waveform. The mechanisms thus determined are thrust mechanisms. A few events located east of the mainshock have waveforms different from the mainshock and have strike-slip mechanisms. The average Q_β values along the paths from the hypocenters of the Sierra Madre and the 3 December 1988 Pasadena earthquake (M_L = 4.9) to PAS are about 130 and 80, respectively. The stress drop of the mainshock is about 500 bars. Most of the aftershocks have stress drops between 10 and 100 bars

Slip history and dynamic implications of the 1999 Chi-Chi, Taiwan, earthquake

We investigate the rupture process of the 1999 Chi-Chi, Taiwan, earthquake using extensive near-source observations, including three-component velocity waveforms at 36 strong motion stations and 119 GPS measurements. A three-plane fault geometry derived from our previous inversion using only static data [ Ji et al., 2001 ] is applied. The slip amplitude, rake angle, rupture initiation time, and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm [ Ji et al., 2002b ]. The inversion results are validated by the forward prediction of an independent data set, the teleseismic P and SH ground velocities, with notable agreement. The results show that the total seismic moment release of this earthquake is 2.7 × 10^20 N m and that most of the slip occurred in a triangular-shaped asperity involving two fault segments, which is consistent with our previous static inversion. The rupture front propagates with an average rupture velocity of ∼2.0 km s^(−1), and the average slip duration (risetime) is 7.2 s. Several interesting observations related to the temporal evolution of the Chi-Chi earthquake are also investigated, including (1) the strong effect of the sinuous fault plane of the Chelungpu fault on spatial and temporal variations in slip history, (2) the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation, and (3) the observation that the peak slip velocity near the surface is, in general, higher than on the deeper portion of the fault plane, as predicted by dynamic modeling

Validation of the rupture properties of the 2001 Kunlun, China (M_s=8.1), earthquake from seismological and geological observations

We determine the finite-fault slip distribution of the 2001 Kunlun earthquake (M_s = 8.1) by inverting teleseismic waveforms, as constrained by geological and remote sensing field observations. The spatial slip distribution along the 400-km-long fault was divided into five segments in accordance with geological observations. Forward modelling of regional surface waves was performed to estimate the variation of the speed of rupture propagation during faulting. For our modelling, the regional 1-D velocity structure was carefully constructed for each of six regional seismic stations using three events with magnitudes of 5.1–5.4 distributed along the ruptured portion of the Kunlun fault. Our result shows that the average rupture velocity is about 3.6 km s^−1, consistent with teleseismic long period wave modelling. The initial rupture was almost purely strike-slip with a rupture velocity of 1.9 km s^−1, increasing to 3.5 km s^−1 in the second fault segment, and reaching a rupture velocity of about 6 km s^−1 in the third segment and the fourth segment, where the maximum surface offset, with a broad fault zone, was observed. The rupture velocity decelerated to a value of 3.3 km s^−1 in the fifth and final segment. Coseismic slip on the fault was concentrated between the surface and a depth of about 10 km. We infer that significant variations in rupture velocity and the observed fault segmentation are indicative of variations in strength along the interface of the Kunlun fault, as well as variations in fault geometry