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美国加利福尼亚州发生6.4级地震，并伴随400多次余震。当地时间7月4日，美国加利福尼亚州南部发生6.4级强震，洛杉矶、拉斯维加斯等地均有震感。报道称，这是南加州地区25年以来最强烈的地震，初步报道显示，地震并未造成重大伤亡，但有人员受伤。
（加利福尼亚地震示意图，图片来自Earthquaketrack.com）
根据美国地质勘探局网站消息，地震发生在当地时间4日上午10时33分(北京时间5日凌晨1时33分)左右，震源深度10.7公里，震中位于克恩县里奇克莱斯特市，在洛杉矶东北方大约240公里的莫哈维沙漠附近。根据该网站信息，此次地震后已经发生百余起余震。
据洛杉矶时报报道，南加州地区过去几天经历了400多次余震。当地居民感到恐慌，但专家表示近期不会再有大的地震活动。
More than 400 small earthquakes have hit a section of Southern California over the last few days.
Residents in the area have felt rattled. But experts don’t expect major seismic activity from the recent activity concentrated in the Riverside County community of Glen Avon that they are calling the Glen Avon Earthquake Swarm.
南加州地震网在一份报告中指出，从Riverside到Chino地区发生轻微地震相对常见。从过去的历史记录来看，该地区地震一般不超过4.0级。
“Swarms of small magnitude events are relatively common in this area, extending from Riverside to Chino,” the Southern California Seismic Network said in a report. “Based on past historic recordings this swarm is only expected to generate events of magnitude less than 4.0.”
The network, a project by Caltech and the U.S. Geological Survey, said the area where the quakes have occurred has “no major mapped fault but relatively abundant small seismicity indicating a local network of small fractures and faults. Earthquakes in this area are probably related to the tectonic loading of the nearby San Jacinto and San Andreas faults.”
该地区发生的轻微地震一般不高于3.0级，且不易被居民感知。但目前居民还是可以感受到一些余震。
Most of the quakes have been below magnitude 3.0 and were not noticed by residents. But several have been felt.
当地居民Viriginia Hidalgo接受媒体采访时，将近期的感受形容为“坐过山车”。而居民Joanna Lopez 每次感到余震并告诉丈夫时，丈夫都告诉她那是过卡车造成的。
“Recently, it’s been like a roller coaster. Like, I was minding my own business, and then the earth is like shaking and I don’t know what’s … going on,” resident Viriginia Hidalgo told KCBS-TV.
“It’s nonstop. They think I’m crazy. My husband is like, ‘You’re crazy. It’s a truck that’s passing by.’ No, it’s not a truck,” Joanna Lopez told KABC-TV.
据了解，加州地区已经有5年没有经历过6.0级以上的地震。专家认为这段平静期或许已经过去。研究人员发现加州地区的地震次数较之前增加了10倍。在过去十年间，他们共记录了南加州地区180万次地震，其中大部分都是小到计算机系统无法探测。而在此之前的十年里，他们只记录了18万次地震。不过这或许也与当今科学技术能够更好地记录地层运动有关。如今，科学家能够记录地震群移动的特征，并且能够识别大地震前的微弱前震。
It has been almost five years since California experienced its last earthquake of magnitude 6 or stronger. Scientists say this calm period will end, and they give warnings about seismic dangers ranging from the Big One on the San Andreas fault to catastrophes that could come from lesser-known faults, such as the Hayward or Newport-Inglewood.
Researchers also recently discovered that the state has experienced 10 times more earthquakes than previously known. They documented 1.8 million earthquakes in Southern California over the last decade, the vast majority so small that they had been undetectable to computing systems. Previously, only 180,000 earthquakes were on record for the last 10 years.
Scientists are now better able to identify previously undiscovered faults, detect patterns of moving earthquake swarms — like the one in Riverside County — and identify faint clusters of foreshocks that occur before a larger earthquake.
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