The Great MacQuarie Ridge Earthquake of 1989: Introduction

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95437/1/grl4839.pd

Rupture extent of the 1978 Miyagi‐Oki, Japan, earthquake and seismic coupling in the northern Honshu Subduction Zone

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95645/1/grl4114.pd

Rupture process of the March 3, 1985 Chilean earthquake

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95610/1/grl3237.pd

Some Aspects of Energy Balance and Tsunami Generation by Earthquakes and Landslides

— Tsunamis are generated by displacement or motion of large volumes of water. While there are several documented cases of tsunami generation by volcanic eruptions and landslides, most observed tsunamis are attributed to earthquakes. Kinematic models of tsunami generation by earthquakes — where specified fault size and slip determine seafloor and sea-surface vertical motion — quantitatively explain far-field tsunami wave records. On the other hand, submarine landslides in subduction zones and other tectonic settings can generate large tsunamis that are hazardous along near-source coasts. Furthermore, the ongoing exploration of the oceans has found evidence for large paleo-landslides in many places, not just subduction zones. Thus, we want to know the relative contribution of faulting and landslides to tsunami generation. For earthquakes, only a small fraction of the minimum earthquake energy (less than 1% for typical parameter choices for shallow underthrusting earthquakes) can be converted into tsunami wave energy; yet, this is enough energy to generate terrible tsunamis. For submarine landslides, tsunami wave generation and landslide motion interact in a dynamic coupling. The dynamic problem of a 2-D translational slider block on a constant-angle slope can be solved using a Green's function approach for the wave transients. The key result is that the largest waves are generated when the ratio of initial water depth above the block to downslope vertical drop of the block H 0 /W sin δ is less than 1. The conversion factor of gravitational energy into tsunami wave energy varies from 0% for a slow-velocity slide in deep water, to about 50% for a fast-velocity slide in shallow water and a motion abruptly truncated. To compare maximum tsunami wave amplitudes in the source region, great earthquakes produce amplitudes of a few meters at a wavelength fixed by the fault width of 100 km or so. For submarine landslides, tsunami wave heights — as measured by b , block height — are small for most of the parameter regime. However, for low initial dynamic friction and values of H 0 /W sin δ less than 1, tsunami wave heights in the downslope and upslope directions reach b and b /4, respectively.Wavelengths of these large waves scale with block width. For significant submarine slides, the value of b can range from meters up to the kilometer scale. Thus, the extreme case of efficient tsunami generation by landslides produces dramatic hazards scenarios.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43233/1/00024_2003_Article_2424.pd

Asperity distributions and large earthquake occurrence in subduction zones

Ruff, L.J., 1992. Asperity distributions and large earthquake occurrence in subduction zones. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 61-83.Plate tectonics and the seismic gap hypothesis provide the framework for long-term earthquake forecasting of plate boundary earthquakes. Unfortunately, detailed examination reveals that earthquake recurrence times and rupture length vary between successive earthquake cycles in the same subduction zone. Furthermore, larger coseismic slip is commonly associated with larger rupture length. Hence, large earthquake occurrence in subduction zones is characterized by variability in: (1) recurrence times, (2) rupture length, and (3) coseismic slip. These facts, plus many other observations, indicate that there are significant spatial variations in the "strength" of the plate interface. One simple description of these variations and their role in the earthquake cycle is the asperity model, where the large strong regions of the plate interface are called asperities, and the large earthquakes occur when the large asperities break. The asperity model of earthquake occurrence is able to qualitatively explain several features of large plate boundary earthquakes. To go beyond general qualitative notions. I pose the following scientific test: are the observed asperity distributions and a simple model of their interaction self-consistent with the above three observed features of large earthquake occurrence?The distribution of the major asperities along plate boundary segments has now been determined for several subduction zones. Rupture process studies of adjacent large and great earthquakes have provided reliable estimates of the along-strike asperity lengths and separations for several adjacent asperities in the Kurile Islands, Colombia, and Peru subduction zones. The simplest mechanical model for asperity interaction is to idealize two adjacent asperities as frictional sliders that are connected by main springs to the upper plate, by a coupling spring to each other, and maintain frictional contact with a conveyer belt (the lower plate) that moves with a constant velocity. An "earthquake" occurs when the net force on the asperity frictional slider reaches some specified level. The failure force and spring constants are determined by the observed asperity distribution and simple models of elastic interaction. Two different macroscopic failure criteria are used. This simple mechanical model displays a remarkable range of behavior from simple to complex. When the two asperities are identical in all their properties, sequences of identical "earthquakes" are produced. For the more realistic case of non-identical asperities, "earthquake" sequences show great variety. Using system variables from the observed asperity distributions, the "earthquake" sequences typically display: (1) variable recurrence times, (2) variable rupture length, i.e. a combination of single-asperity and double-asperity failures, and for one of the failure criteria (3) larger coseismic slip for double-asperity failures. Statistical summaries of thousands of simulated "earthquake" sequences for asperity pairs in the Kuriles, Colombia, and Peru subduction zones are broadly consistent with the observed features of large earthquake occurrence in these subduction zones.The main conclusion is that the asperity model provides a self-consistent explanation for: fault zone heterogeneity, the rupture process, and recurrence times and rupture mode of large earthquake sequences via a simple model for adjacent asperity interaction. In addition, a conclusion independent of any particular model for fault zone heterogeneity is that simple deterministic models of fault zone interaction can explain complex patterns of large earthquake occurrence in subduction zones.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29833/1/0000180.pd

Dynamic Stress Drop of Recent Earthquakes: Variations within Subduction Zones

—Stress drop is a fundamental parameter of earthquakes, but it is difficult to obtain reliable stress drop estimates for most earthquakes. Static stress drop estimates require knowledge of the seismic moment and fault area. Dynamic stress drop estimates are based entirely upon the observed source time functions. Based on analytical formulas that I derive for the crack and slip-pulse rupture models, the amplitude and time of the initial peak in source time functions can be inverted for dynamic stress drop. For multiple event earthquakes, this method only gives the dynamic stress drop of the first event. The Michigan STF catalog provides a uniform data base for all large earthquakes that have occurred in the past four years. Dynamic stress drops are calculated for the nearly 200 events in this catalog, and the resultant estimates scatter between 0.1 and 100 MPa. There is some coherent tectonic signal within this scatter. In the Sanriku (Japan) and Mexico subduction zones, underthrusting earthquakes that occur at the up-dip and down-dip edges of the seismogenic zone have correspondingly low and high values of stress drop. A speculative picture of the stress state of subduction zones emerges from these results. A previous study found that the absolute value of shear stress linearly increases down the seismogenic interface to a value of about 50 MPa at the down-dip edge. In this study, the dynamic stress drop of earthquakes at the up-dip edge is about 0.2 MPa, while large earthquakes at the down-dip edge of the seismogenic plate interface have dynamic stress drops of up to 5 MPa. These results imply that (1) large earthquakes only reduce the shear stress on the plate interface by a small fraction of the absolute level; and thus (2) most of the earthquake energy is partitioned into friction at the plate interface.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41838/1/24-154-3-4-409_91540409.pd

What controls the lateral variation of large earthquake occurrence along the Japan Trench?

The lateral (along trench axis) variation in the mode of large earthquake occurrence near the northern Japan Trench is explained by the variation in surface roughness of the subducting plate. The surface roughness of the ocean bottom near the trench is well correlated with the large-earthquake occurrence. The region where the ocean bottom is smooth is correlated with‘typical’large underthrust earthquakes (e.g. the 1968 Tokachioki event) in the deeper part of the seismogenic plate interface, and there are no earthquakes in the shallow part (aseismic zone). The region where the ocean bottom is rough (well-developed horst and graben structure) is correlated with large normal faulting earthquakes (e.g. the 1933 Sanriku event) in the outer-rise region, and large tsunami earthquakes (e.g. the 1896 Sanriku event) in the shallow region of the plate interface zone. In the smooth surface region, the coherent metamorphosed sediments form a homogeneous, large and strong contact zone between the plates. The rupture of this large strong contact causes great under-thrust earthquakes. In the rough surface region, large outer-rise earthquakes enhance the well-developed horst and grabens. As these structure are subducted with sediments in the graben part, the horsts create enough contact with the overriding block to cause an earthquake in the shallow part of the interface zone, and this earthquake is likely to be a tsunami earthquake. When the horst and graben structure is further subducted, many small strong contacts between the plates are formed, and they can cause only small underthrust earthquakes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73990/1/j.1440-1738.1997.tb00176.x.pd

SEASAT geoid anomalies and the Macquarie Ridge complex

The seismically active Macquarie Ridge complex forms the Pacific-India plate boundary between New Zealand and the Pacific-Antarctic spreading center. The Late Cenozoic deformation of New Zealand and focal mechanisms of recent large earthquakes in the Macquarie Ridge complex appear consistent with the current plate tectonic models. These models predict a combination of strike-slip and convergent motion in the northern Macquarie Ridge, and strike-slip motion in the southern part. The Hjort trench is the southernmost expression of the Macquarie Ridge complex. Regional considerations of the magnetic lineations imply that some oceanic crust may have been consumed at the Hjort trench. Although this arcuate trench seems inconsistent with the predicted strike-slip setting, a deep trough also occurs in the Romanche fracture zone.Geoid anomalies observed over spreading ridges, subduction zones, and fracture zones are different. Therefore, geoid anomalies may be diagnostic of plate boundary type. We use SEASAT data to examine the Macquarie Ridge complex and find that the geoid anomalies for the northern Hjort trench region are different from the geoid anomalies for the Romanche trough. The Hjort trench region is characterized by an oblique subduction zone geoid anomaly, e.g., the Aleutian-Komandorski region. Also, limited first-motion data for the large 1924 earthquake that occurred in the northern Hjort trench suggest a thrust focal mechanism. We conclude that subduction is occurring at the Hjort trench. The existence of active subduction in this area implies that young oceanic lithosphere can subduct beneath older oceanic lithosphere.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25729/1/0000286.pd

Introduction to subduction zones

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43121/1/24_2004_Article_BF00874621.pd

How good are our best models? Jackknifing, bootstrapping, and earthquake depth

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95433/1/eost7840.pd