Coseismic Strain and the Transition to Surface Afterslip Recorded by Creepmeters near the 2004 Parkfield Epicenter
Coseismic Strain and the Transition to Surface Afterslip Recorded by Creepmeters near the 2004 Parkfield Epicenter
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2004 年帕克菲尔德震中附近蠕变仪记录的同震应变和向地表后滑的转变
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发表时间：
2005
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通讯作者：
R. Bilham
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作者：
R. Bilham
2004 年 9 月 28 日当地时间 11 点 15 分发生的 6 级帕克菲尔德地震对地震前发生严重地表滑移的观念可能是致命的打击。 1966年，在前一周就有传闻称沿断层带出现地表裂缝，主震前11.6小时一根水管破裂。这带来了希望：如果有位移传感器来测量表面滑移，则可能会发生大量表面滑移。在 2004 年帕克菲尔德主震发生前的几天、几小时、几分钟内，位移传感器就位，但什么也没发生。根据跨越断层的八个蠕变仪的数据，断层仍然有效地锁定在每个传感器的检测水平（6-20μm）（Langbein等，2005，本期）。表面滑移的缺失伴随着该区域钻孔膨胀计和剪切应变计阵列上不存在显着应变，并且围绕 2004 年震中的 GPS 阵列上不存在任何异常位移。尽管在一些钻孔应变仪上看到的震前应变信号表明震中应变并非完全与即将发生的地震无关，但这些信号接近仪器噪声水平，并对其未来的可靠检测提出了新的挑战。
与没有震前断层位移相比，蠕变仪都记录了主震发生时的突然同震右旋偏移。该台阶对应于断层带的右旋剪切，但在没有表面破裂的情况下，早期后滑过程中的蠕变计充当引伸计，测量应变而不是断层蠕变。 Work Ranch 的蠕变计偶然位于断层带的对面和偏心位置，这为了解地表断层的发展提供了深入的了解。
美国自 1966 年地震以来，地质调查局 (USGS) 在帕克菲尔德维护了一个蠕变计阵列，增加了……
The Mw 6 Parkfield earthquake at 11:15 local time, 28 September 2004 has dealt a possibly fatal blow to the notion that significant surface slip precedes earthquakes. In 1966, anecdotal reports of surface fissures along the fault zone had been reported in the preceding week, and a water pipe fractured 11.6 hours before the mainshock. This offered hope that substantial surface slip may have occurred had displacement sensors been in place to measure it. In the days, hours, and minutes prior to the 2004 Parkfield mainshock, displacement sensors were in place, and nothing happened. According to data from eight creepmeters crossing the fault, the fault remained effectively locked at the detection level of each sensor (6-20 μm) (Langbein et al. , 2005, this issue). This absence of surface slip was accompanied by an absence of significant strain on the borehole dilatometer and shear-strainmeter array in the region, and by the absence of any unusual displacements on the GPS array embracing the 2004 epicenter. Although preseismic strain signals seen on some borehole strainmeters suggest that epicentral strain was not completely indifferent to the pending earthquake, the signals are close to instrumental noise levels and pose new challenges for their reliable future detection.
In contrast to the absence of preseismic fault displacement, the creepmeters all recorded an abrupt coseismic dextral offset at the time of the mainshock. The step corresponds to dextral shear of the fault zone, but in the absence of surface rupture, the creepmeters during early afterslip acted as extensometers, measuring strain rather than fault creep. The serendipitous location of creepmeters at Work Ranch both across and eccentric to the fault zone has provided insight into the development of surface faulting.
The u.s. Geological Survey (USGS) has maintained a creepmeter array in Parkfield since the 1966 earthquake, increasing …